Thoughts on the ocean, the environment, the universe and everything from nearly a mile high.

Panorama of The Grand Tetons From the top of Table Mountain, Wyoming © Alan Holyoak, 2011

Wednesday, October 31, 2012

Why was the flooding from Hurricane Sandy so bad?

Flooding from Hurricane Sandy caused billions of dollars of damage to property.  Low-lying coastal areas were devastated.  Tunnels and part of the NYC subway system flooded.  Barrier islands got hammered, airport runways flooded, and the list goes on and on.

Here are some photos of the flooding along the eastern seaboard:

New Jersey

NYC subway

Atlantic City, NJ


Hoboken, NJ

Brooklyn, NY

New York

Rodanthe, North Carolina

Staten Island, NY

New York, flooded airport runway

Ground Zero Site, NYC


Subway, Hoboken, NJ

New Jersey

Why was the flooding so bad?

Six factors combined to make flooding pretty much as bad as it could be.  They are:
  1. Sea level rise
  2. Full moon and high tide
  3. Hurricane low pressure
  4. Hurricane-force winds and associated storm surge 
  5. Low-lying coastal areas
  6. Shallow sloping shoreline

First: Sea Level Rise

A report in National Geographic summarizes observations about sea level rise along the east coast of the United States.  It states that sea level rise is occurring nearly twice as fast along the east coast as the global average.  You can read more about that by clicking this link:

Sea level has been rising between Cape Hatteras, NC, and Boston, MA at the rate of 2.0-3.8 mm/year between 1950 and 2009.  If we go with a middle value of 3 mm/year, then sea level has risen about 7 inches since 1950.  That may not sound like a lot, but it becomes significant when you start looking at flood conditions.  All indications are that the rate of sea level rise is increasing as global warming progresses.

Second: High Tide

People directly affected by weather and flooding from Hurricane Sandy wouldn't have seen this, but there was a full moon on 10/29/2012.  The height of ocean tides are affected by the relative positions of the Earth, Sun, and Moon.  High tides are highest and low tides are lowest when the Earth, Sun, and Moon all line up in the same plane.  This happens when we have a full moon and a new moon.  Unfortunately, it was a full moon on 10/29, the same night Hurricane Sandy came ashore.  This means that the tides that night were already higher than normal.

Third: Low Air Pressure

A hurricane is a low pressure system.  This means that in the eye of the storm in particular and the whole storm in general has lower air pressure than high pressure systems have.  In order to understand this part of the equation you need to imagine the entire height of the atmosphere above your head.  It extends upward 100s of miles, but most of the mass of the atmosphere is in the few miles directly overhead.

The weight of the atmosphere directly overhead produces the air pressure we experience.  Interestingly, high pressure pushes down on water, causing tides to be lower than they would otherwise be.  And, vice versa, low air pressure allows tides to be higher than they would otherwise be.  How much of a difference?  A change in 1mb (millibar) of air pressure relates to up to 1cm of tidal height when high pressure pushes down on the water surface.  When air pressure is low, however, it may allow tides to be a bit higher, but it does not by itself drive tides significantly higher than predicted.

Average sea level air pressure is about 1013mb.  The air pressure in the middle of Hurricane Sandy was 946mb when it came ashore.  This ties the lowest air pressure for a hurricane making landfall this far north.  That last one was in 1938!

This means that air pressure did not mitigate tidal heights.

Fourth: Storm Surge

Storm surge is the biggest factor in coastal flooding associated with hurricanes.  The height and effect of storm surge is determined by several factors: storm intensity, tidal height, angle of waves to shorelines, presence of bays and inlets, slope of the shoreline, etc.

Here's what happens.  As a hurricane approaches shore the effects of tides are felt first.  So the first significant effects are felt as tides rise, often well above normal because of the amount of water being pushed by the storm.  Then, waves produced by the storm start coming ashore.  These tend to increase in size as time goes on.  This is because wave size is determined mainly by two factors: the strength of wind and fetch (the distance wind blows across water).

Waves produced by hurricanes can be huge because both wind velocity and fetch are massive.  Hurricane Sandy, for example, was over 1000 miles across.  And though windspeed didn't get high enough to reach more than category 1 status, the wind it produced blew over vast expanses of ocean.

So once the tide was in and Sandy came ashore, wave after wave piled up on the shore with no way for the water to get back offshore, so it was pushed farther and farther inland.  This is the water that flooded subways, tunnels, airports, etc., etc.

There are some good animations that demonstrate the combined effects of tide and storm surge.  You can view the by clicking these links:

This link shows the action of storm surge along shores with a shallow slope:
This link shows the action of storm surge along shores with a steep slope:

Fifth: Low-Lying Areas

The coastal flooding was particularly bad because the NJ, NYC area is low-lying.  This means that there was not much there to slow or stop the high storm tide (regular tide + storm surge) that Hurricane Sandy produced.

Sixth: Shallow sloping seafloor and narrow passages between landmasses

This image of the greater NYC area shows that this highly populated area is clustered on islands and land masses separated from each other by narrow waterways.  This means that when the storm tide (tide  + surge) pushed into these areas, water stacked up and spilled more readily onto land.  This had to contribute significantly to the flooding as well.

Wrapping up

So when you combine sea level rise, high tide, low air pressure, storm surge, and local geography with a storm the size of Sandy, that's a recipe for disaster!

Tuesday, October 30, 2012

Why do hurricanes spin?

Hurricane Sandy is probably the biggest piece of weather news we've had all year (at least that people paid much attention to).  It is a massive storm that now (10-30-2012) has affected millions of people and caused billions of dollars worth of damage.  It's so big that its weather effects are being felt as far inland as Ohio and Indiana.

Hurricane Sandy, 10-28-2012 
(Image courtesy of NASA Observatory Earth)

This is NASA satellite photo (above) shows some of Hurricane Sandy, and the video below shows how the entire storm spins as air spirals toward the eye of the storm.

If you've ever wondered why hurricanes move like this, then this is your lucky day.  I'll do my best to explain why this happens.

First of all, a hurricane grows out of a tropical depression (tropical low pressure system).  A tropical depression is a weather system where sea surface temperatures are high and the air is loaded with moisture due to sea surface evaporation.  The resulting warm, moist air is extremely unstable and less dense than the air around it, so it rises into the upper atmosphere (troposphere, actually - the layer of the atmosphere right next to the Earth's surface, 3-10 miles thick).

As warm, moist air continues to rise a low pressure region forms.  This means that as air moves into the upper atmosphere it has to be replaced by air from neighboring air masses.  You can imagine a low pressure system to act like a valley or depression that neighboring air flows into.

The top image above shows a side view of a low pressure system (at least one way to imagine it).  Warm moist air rises, and that air is replaced by air from surrounding areas.  The larger and stronger a low pressure system is, the farther away it can pull air in.

The lower image shows a top view of a low pressure cell.  Imagine air in the center of the low pressure area moving up toward you and air from nearby areas flowing toward the low pressure area to replace the air that rose and moved into the upper troposphere.  Well, those arrows show what air would do if the Earth didn't spin.

Because the Earth rotates and moving air is not physically attached the surface, the Earth rotates under moving air.  Resulting physical effects, collectively called the Coriolis Effect, causes the path of air or water currents to deflect to the right in the northern hemisphere, and to the left in the southern hemisphere.

The upper image shows Coriolis Effect on air moving toward the low pressure region.  Coriolis Effect deflects the moving air to the right as it moves, in this case approaching the center of the low pressure cell.  The blue arrows in the upper figure shows how air would move if the Earth did not rotate, but the peach colored arrows show the movement of air under the influence of the Coriolis Effect.

The map below shows that most of the air moving toward the storm center ends up moving more or less parallel to the eye of the storm.  This is why there is usually little air movement at all in the eye, except upward.  This deflection to the right occurs at all distances from the eye.  The stronger the winds are, and the farther they blow, and the larger the Coriolis Effect.  This ends up making an entire hurricane spin in a counterclockwise direction (in the northern hemisphere - it's opposite in the southern hemisphere).

(Image courtesy of NASA)

The low pressure cell at the center of a hurricane is extremely powerful and pulls air in from hundreds of miles away.  Wind blowing over these long distances toward the strong low pressure cell at the eye of a hurricane deflects significantly and create a significant spiraling wind pattern.

That's it.

Tuesday, October 23, 2012

North and South - Climate change and sea ice in the Arctic and in the Antarctic

Record sea ice melting in the Arctic Ocean receives a lot of attention from the media - as far as climate change news goes - but you don't hear that much about what is happening in the Antarctic.

First of all, a quick reminder about sea ice in the Arctic Ocean:

Sea ice melt in the Arctic Ocean in 2012 smashed the previous record by 750,000 km2.  The map below shows the observed sea ice extent in Sept 2012 (white area) compared to the 1979-2000 average extent (pink line)  Wow!  

Sea ice extent is defined as the area of the sea with at least 15% sea ice cover.  The graph below shows the Arctic sea ice extent for the years 2007-2012 and the 1979-2000 average.  Sea ice melt for all individual years shown (2007-2012) have minimum sea ice extents that are significantly (statistically) less than the 1979-2000 average (dark gray line; lighter gray area is + 2 standard deviations around the 1979-2000 average).    The bottom line for the Arctic is that it is warming significantly, and much faster than even the fastest climate models developed to date.

Ok, let's take a look at what's happening in the Antarctic:

The map below shows the Antarctic maximum sea ice extent (white area) for 2012 compared to the 1979-2000 average sea ice extent (orange line).  2012 sea ice extent in the Arctic set a new sea ice maximum record.  The graph below the map shows the sea ice extent for 2012 compared to the 1979-2000 average.

The graph below shows average sea ice extent for the month of Sept for 1979-2000 and for selected individual years.  Interestingly sea ice extent is increasing on average around Antarctica. When we look at data of sea ice cover in recent years in the Antarctic we see that 2006, 2007, 2011, and 2012 all had higher than average sea ice extent maxima but only 2006 and 2012 maximum extents were statistically higher (different) than the 1979-2000 average. With that being said, sea ice extent did exceed the + 2 standard deviation range in 2006 and 2012.  So, what is going on in the Antarctic that is leading to increased sea ice cover?

A report by scientists at the National Snow and Ice Data Center explains very nicely what is happening with sea ice extent in the south (

Here's a brief summary of their report together with some additional information to increase clarity:
  1. Temperatures are warming in the Antarctic, just not as fast as in the Arctic (the NSIDC cites references you can refer to if you want more info on this.)
  2. Warming of the Pacific Ocean and ozone depletion over Antarctica combine to strengthen circumpolar winds.
  3. The strongest of these circumpolar winds blows east to west, and Coriolis Effect causes these winds to deflect to the left (north).  
  4. The northerly flow of air around most of Antarctica causes sea ice to be pushed farther north than usual, spreading it out and increasing sea ice extent (remember, sea ice extent = 15% ice cover or more)
It would be very interesting to know whether the total amount of sea ice being formed in the Antarctic is increasing or decreasing.  All we know right now is how the ice that is being formed is being dispersed.

In summary:

Sea ice extent around Antarctica is increasing, but it is not increasing because it is getting colder.  It is increasing because winds blowing toward the north are dispersing sea ice farther away from the Antarctic coastline than usual.  Don't forget that sea ice formation in the Antarctic winter is followed by nearly 100% sea ice melt in the Antarctic summer - this is different than in the Arctic where multiple-year sea ice has historically accumulated.  

Monday, October 8, 2012

Brief Geologic History and Zonation of the Ocean

This is reading #2 in a series that I'm developing for my future marine biology students.  Please leave a comment below if you find typos or gross inaccuracies.  Citations to references showing needed content changes are appreciated.  Thanks!


Brief Geologic History and Zonation of the Ocean

1.     Be able to provide a historical framework for major events that happened in the ocean.
2.     Be able to label all oceans and major seas on a map of the world.
3.     Be able to draw a cross-sectional image of an ocean basin, and label and define all seafloor features.
4.     Be able to describe two different ways volcanic islands are formed.
5.     Be able to list and describe all benthic zones.
6.     Be able to list and describe all oceanic zones.


The ocean is the largest life-supporting habitat on the planet.  It covers 70.9% of the Earth’s surface, has an average depth of 3700 meters, and contains over 1.3 billion km3 of living space.  In addition, the ocean is home to at least half of all known species, yet over 95% of it still remains unexplored.

Geologic History of the Ocean

The Earth is 4.5 billion years old.  It was so hot at first that water existed only as water vapor.  By 4 bya (billion years ago) the atmosphere and planet cooled enough that water vapor condensed and filled ocean basins for the first time.  This deluge also produced continental runoff that brought sediment and salts to the ocean.  Ocean salinity increased until 1.5 bya when it stabilized at concentrations we observe today.  We do not completely understand the processes that maintain ocean salinity at the current stable level.  We do know that salt is lost from water by physical and chemical processes and that salt is added to water by an influx of material from the continents.
Life evolved in the sea.  The first evidence of life appeared about 3.5 billion years ago.  The first organisms were prokaryotes, including the photosynthetic cyanobacteria.  It took cyanobacteria nearly a billion years of ongoing photosynthesis to produce enough oxygen so that it could start accumulating in the atmosphere and surface waters of the ocean.  Early oxidation of the atmosphere and ocean occurred 2.5 to 1.8 bya in what we call the great oxidation event (Fig. 2-1).  Little evidence of further increases in ocean or atmospheric oxygen occurred between 1.8 – 0.8 bya and is called the boring billion.  The first eukaryotes appeared 1.5 bya, however, making that billion years not totally boring.  Between 0.85-0.54 bya enough oxygen accumulated in the atmosphere and ocean that even the deep sea became oxygenated.  The Earth also experienced alternating hothouse and ice age conditions during that time.  Oxygen levels continued to increase, and about 530 million years ago the ocean experienced a massive proliferation of anatomically complex animal life.  This is called the Cambrian Explosion.  The oldest fossils of most modern animal body plans were produced at this time.  Oxygen concentrations in the atmosphere and ocean stabilized to modern levels a few hundred million years ago and have been the same ever since.
The size and shape of the ocean is in slow but constant flux.  Changes occur as tectonic forces create new oceanic crust in some places and drive subduction in others.  These forces push and pull continental plates around the Earth’s surface at the a few to several cm yr-1.  That’s about the same rate your fingernails grow.  Sometimes tectonic forces move the continents together creating a supercontinent like Pangea (Fig. 2-2).  When a supercontinent exists the rest of the Earth is covered by one massive ocean.  At other times, like now, the continents are dispersed around the planet surface, and the ocean is divided into several smaller basins (Fig. 2-3).

Figure 2-1. High and low ranges of oxygen accumulation in the atmosphere (top graph), ocean surface waters (middle graph), and ocean deep waters (bottom graph). (Image modified from Holland, 2006)

Figure 2-2. Tectonic movement of contents between 250 mya and today. Pangea is the most recent supercontinent. (Image: USGS)

Figure 2-3.  Boundaries of oceans and major seas of the modern world. (Image: NOAA)
Seafloor Topography

The seafloor extends from sea level at the margins of all continents and islands down to ocean trenches more than 10,000 meters below the surface.  Figure 2-4 shows a generalized profile of the seafloor and major features associated with it.  These features include the continental shelf, shelf break, continental slope, continental rise, abyssal plain, volcanic islands, trenches, seamounts, and mid-oceanic ridges.

Figure 2-4. Generalized cross section of an ocean basin (not to scale).  1 - Continental shelf; 2 - Shelf break; 3 – Continental slope; 4 – Continental rise; 5 – Abyssal plain; 6 – Volcanic island; 7 – Trench; 8 – Seamount; 9 – Mid-oceanic ridge. (Image: ARH)

The continental shelf is the submerged edge of a continent.  Continental shelves extend a few kilometers to over 1000 kilometers in width, and the outer edge of the shelf is a few hundred to several hundred meters deep.  The continental shelf break marks the outer edge of the continental shelf.  This is where the shallow grade of the continental slope gives way to the steep grade of the continental slope.  The continental slope plunges down a few thousand meters before it reaches the continental rise.  The continental rise is the transitional area that shifts gradually from the steep grade of the continental slope to a flat abyssal plain.  The abyssal plain may be 3000-6000 meters deep and is a vast, muddy expanse covering most of the seafloor, though volcanic islands, trenches, seamounts, and mid-oceanic ridges interrupt it.
Volcanic islands form along trenches where oceanic crust subducts under another tectonic plate (Fig. 2-5).  Islands that form along trenches typically form an island arc.  A couple of examples of island arcs include the Aleutian Islands and the Marianas Islands (Fig 2-6).  Volcanoes form as subduction pushes crust material and water trapped in the sediment downward.  Heat from the mantle superheats the subducted rock and water, but since that material is now under extreme pressure the water remains liquid and facilitates the further heating of rock around it.  Lower density rock in the subducted crust becomes semi-pliable and gradually rises toward the surface.  When this superheated rock material gets close enough to the surface, pressure is reduced and the rock can transition into magma that is released during a volcanic eruption.  Ongoing or repeated magma release adds to the height of underwater volcanoes until they sometimes break the ocean surface as volcanic islands.  You may not have known this, but the tallest mountain in the world from base to peak is not Mt. Everest, it’s Moana Kea on the big island of Hawaii.  It is 10,200 m tall from base to peak; the peak of Mt. Everest is only 8,848 m above sea level.
By the way, the deepest part of the deepest trench in the ocean, the Challenger Deep of the Mariana Trench is 10,916 m deep.  That is so deep that if you put Mt. Everest (8,848 m tall) in it, its peak would still be over 2 km below the ocean surface!

Figure 2-5. Subduction of an oceanic plate and formation of an island arc volcano. (Image: ARH)

Figure 2-6. The Marianas Trench and Aleutian Trench and associated island arcs.  Island arcs include islands and seamounts. (Images: modified from Google Earth)
            Volcanic islands can also occur where a tectonic plate slides over a hot spot in the mantle where a plume of mantle material pushes through the crust toward the seafloor.  This is how the Hawaiian Island chain was formed (Fig. 2-7).  By the way, mantle plumes/hot spots can occur on land.  A mantle plume is what fuels the geysers and thermal activity in Yellowstone National Park. 

Figure 2-7. The Hawaiian Island chain.  The closest trench to the Hawaiian Islands is the Aleutian Trench, over 3500 km away. (Image: Google Earth)

            When a volcanic island does not reach the surface it is a seamount.  Actually, a seamount is any underwater rise that does not reach the surface.  Scientists and fishermen discovered that seamounts are often islands of high biomass and biodiversity surrounded by low biomass habitats.
The last topographic feature addressed in this section is the mid-oceanic ridge.  The mid-oceanic ridge is an undersea mountain range that exists along divergent boundaries where seafloor spreading occurs (Fig. 2-8).  The peak of mid-oceanic ridges usually rises a few thousand meters above the abyssal plain on either side of it.  The interconnected mid-oceanic ridge system constitutes the longest continuous mountain range on the planet.

Zonation of the Ocean

The ocean is divided into the benthic and pelagic zones (Fig. 2-9).  The benthic zone includes the seabed, and the pelagic zone includes the water column. 

Divisions of the Benthic Zone

            The marine benthos extends from the high tide mark of the intertidal or littoral zone to the bottom of the deepest trench.  The littoral zone benthos includes all seafloor that is covered and uncovered periodically by tidal exchange.  Littoral benthic habitats include rock to mud substrates.  The sublittoral zone extends from the bottom of the littoral zone to the continental shelf break.  Depending on water turbidity, latitude, and depth, light may reach the seafloor up to a few hundred meters deep.  This is where we find marine benthic communities including kelp forests, kelp beds, seagrass beds, turtle grass beds, and coral reefs.  The benthic zone of continental slopes is called the bathybenthic or bathyal zone.  The abyssal benthic zone includes depths of the continental rise and abyssal plains.  This is the largest benthic habitat in the ocean.  The hadal benthic zone is found only in trenches.  The deeper benthic zone is, the less we know about it.

Figure 2-8. Age of oceanic crust.  The newest crust exists at divergent boundaries at mid-oceanic ridges, and the oldest material is at trenches. (Image: NOAA)

Divisions of the Oceanic Zone

The term water column refers to all water from the surface all the way to the bottom in a particular location.  The water column of the ocean can therefore extend to more than 10,000 m in some places.  Scientists have divided the water column into divisions by depth and other factors to reduce confusion when referring to different regions of the ocean.  Keep in mind that there is no rigid line separating one division from the next, and these divisions are used only as general guidelines in discussing environments at different depths.
All water that is not part of the intertidal or littoral zone is called the oceanic zone.  The oceanic zone is divided into water that lies over continental shelves and deeper water.  The division over the shelf is called the neritic zone.  The rest of the ocean is called the pelagic zone.
The divisions of the open ocean, starting at the shoreline and moving offshore are indicated in Figure 2-9.  The uppermost horizontal layer of the oceanic zone is the epipelagic zone.  It usually extends to a depth of a few hundred meters.  This is also called the photic zone.  This zone’s maximum depth is usually defined as the depth where only 1% of surface incident solar radiation remains.  Most pelagic ocean life lives in the epipelagic zone because this is where photosynthesis can take place.

Figure 2-9. Divisions of the ocean. (Image: Wikimedia Commons)

The mesopelagic zone spans extends from the bottom of the epipelagic zone to 700-1000 m. Most solar radiation is absorbed or scattered in the epipelagic zone, but the mesopelagic zone is not entirely dark.  When you are in this zone and you look toward the surface on a sunny day you can still discern a faint glow.  There is too little light here for phytoplankton to carry out enough photosynthesis to meet their basic energy needs.  Because of this, the mesopelagic zone is also called the disphotic zone.   Many animals that live here bio luminesce and migrate vertically into the epipelagic zone each night in order to feed. 
The next layer in the water column is the bathypelagic zone. The bathylpelagic zone extends from about 1000 m to depths as deep as 4000 m.  The upper bound of this layer is defined as the depth where surface light is no longer discernible.  The lower bound of this layer generally corresponds with the lower end of the continental slope.  Some bathypelagic animals may migrate up into the mesopelagic zone to feed, but most organisms in the bathypelagic zone feed on material that drifts down from above as well as on each other. 
The abyssalpelagic zone exists below the bathypelagic zone.  This layer extends from about 4000m to the abyssal plain, usually 4000-6000 m deep.  Organisms in the bathypelagic zone make a living by consuming whatever drifts down from above, by eating each other, and by feeding on benthic organisms.
The deepest pelagic layer is hadalpelagic zone.  This zone exists only in trenches, in water as much as 10,000 m deep.  We know the least about life in the hadalpelagic zone of any ocean depth, though fish were observed in the Japan Trench via ROV in 2010 at depths approaching 8000 m. 
Every division of the ocean has its own set of challenges and opportunities for organisms that live there.  One of the goals of marine biology is to identify what those challenges and opportunities are, and then discover how marine organisms exploit them.

Monday, October 1, 2012

Water - a reading for my marine biology class

Hi -- I am developing a set of readings for my marine biology class.  The class is designed for upper division biology majors who have already had a year of introductory biology, including an introduction to ecology.

This is the first reading I have finished (including images, photos, etc.).  If you have comments or spot typos, etc., I'd appreciate it if you'd let me know by leaving a comment below.

This reading introduces students to the chemical and physical properties of water - something you MUST understand in order to get a grip on life in water.


Reading 1: Water

Learning Objectives:
1.     Be able to describe the molecular characteristics of water
2.     Be able to explain what makes water a polar molecule
3.     Be able to comment on the significance of the polar nature of water
4.     Be able to list and describe physical characteristics of water
5.     Be able to describe the density anomaly of water


You have to learn about water if you want to be a marine biologist.  Without being dramatic, it is safe to say that water is the most important and biologically valuable substance on Earth.  Water is the stuff that makes aquatic environments different than terrestrial habitats, and most people know amazingly little about water and typically take it completely for granted until they don’t have it.  For one thing, water is the only substance on Earth that exists naturally and simultaneously in its solid, liquid, and gaseous states.  It’s unique physical and chemical properties make it a vital substance at all levels of biological organization, from molecules to the global ecosystem.

Characteristics of Water

Water: A Polar Molecule

Just about everyone knows that H20 is the chemical formula for water.  Most people, however, don’t know what water is like at the molecular level and why this matters.  Elemental oxygen has an atomic number of 8.  Two of its electrons are in an inner electron orbital, but the other six electrons are in outer orbitals.  A Lewis dot diagram for oxygen shows two orbitals populated with two electrons and two orbitals containing a single electron each (Fig. 1-1).  The orbitals with one electron need one more electron each in order to be full and stable. 

Figure 1-1.  Lewis dot diagram for oxygen. Dots represent electrons. (Image: ARH)

Two hydrogen atoms form covalent bonds with one atom of oxygen to form water.  The protons in the hydrogen atoms of H2O repel each other, and you might assume and that water forms a linear molecule with oxygen in the middle and hydrogen atoms sticking out both ends, but water is not linear.  The hydrogen atoms are actually on the same side of the oxygen atom.  Here’s why.  There are pairs of electrons in oxygen’s other two orbitals.  These are called lone pair electrons.  The lone pair electrons are slightly more repulsive to each other than the protons of the hydrogen atoms are to each other.  This causes the hydrogen atoms to be displaced to one side of the oxygen atom, and the two lone pair electrons remain on the other side (Fig 1-2).  Because two of oxygen’s electron orbitals are full it forms bonds with up to only two other atoms, but if you include the lone pair electrons, water has a tetrahedral form.  The bond angles in a uniform tetrahedral are all 109.5o, but water is an unequal tetrahedral.  The bond angle between the hydrogen atoms in water is only 104.45o.  This is because the lone pair electrons repel each other more strongly than the hydrogen atoms do, forcing the hydrogen nuclei closer together than they would otherwise be. 

Figure 1-2. Tetrahedral nature, bond angle, and partial charges of water.  O = oxygen atom; H = hydrogen atom; 2e- = lone pair electrons; δ+ = partial positive charge; δ- = partial negative charge. (Image: ARH) 

The unequal distribution of electrons in water produces partial positive and partial negative charges (Fig. 1-2).  A partial positive charge is associated with each hydrogen nucleus, and a partial negative charge is associated with each of the lone pair electrons.  The existence of partial charges around a molecule produces a dipole moment.  In water this imbalance is constant so each water molecule experiences a permanent dipole moment.
The permanent dipole moment makes water a polar molecule.  One of the most significant things a polar molecule can do is form a hydrogen bond with another polar molecule.  A hydrogen bond forms when a partial positive charge on one molecule comes into close proximity to a partial negative charge on another polar molecule.  These opposite partial electrical charges attract each other and form a hydrogen bond.  Because hydrogen bonds are weak, they readily form, break, and reform.  A water molecule can form up to four hydrogen bonds at a time (Fig. 1-3).  The number of hydrogen bonds existing between water molecules and between water molecules and other things is a function of temperature.  The number and duration of hydrogen bonds between water molecules determines whether water is in its solid, liquid, or gaseous form.  Water molecules are constantly reoriented in relation to each other as hydrogen bonds break and reform.  Hydrogen bonds form and break 1011 to 1012 times per second in liquid water at 0oC, and 105 to 106 times per second in ice at 0oC.

Figure 1-3.  Hydrogen bonding between water molecule. Dashed lines indicate hydrogen bonds. (Image: ARH)

Adhesive and Cohesive Nature of Water

Water forms hydrogen bonds with itself and with any other polar substance.  This characteristic makes water makes a universal solvent.  This means that any charged atom or polar molecule can dissolve in water.  Substances that can dissolve in water include sugars, salts, ions, etc.  Molecules lacking partial charges, also known as non-polar molecules, cannot readily dissolve in water.  The ability of water to form hydrogen bonds with other polar molecules gives it an adhesive nature.  A meniscus along the inner wall of a glass tube is an evidence of this characteristic (Fig. 1-6).   The adhesive nature of water combined with its cohesiveness also produces capillary action.

Figure 1-4. The adhesive nature of water produces a meniscus because water adheres to the inner wall of the glass cylinder. (Image: ARH)
Water is also cohesive.  This characteristic allows capillary action to occur, and it also produces the phenomenon called surface tension.  A water molecule in the middle of a mass of water can form bonds in any direction, but water molecules at the surface can form bonds only laterally along the surface and with molecules below them (Fig. 1-5).  Remember that water is tetrahedral.  When molecules at the surface bond with their neighboring water molecules the natural tendency of these bonds is to be bent, but bonds between surface water molecules are put under stress since they also bond to molecules below them.  The molecules below the surface pull down on the water molecules above them.  This downward stress makes bonds between surface water molecules flatter than normal.  This is what produces surface tension, and surface tension exists in bodies of water ranging from small droplets in the air to the entire ocean.

 Figure 1-5. Bonding directions of water at the surface and in the middle of a body of water.  The darker upper layer is where horizontal hydrogen bonds are under physical stress and create surface tension. (Image: ARH)
Surface tension can be a significant factor in aquatic systems.  Plankton, organic matter, and other substances can be trapped in or on it.  In the lab even dense objects can be supported by surface tension (Fig. 1-6).  Surface tension increases in cold water and when water contains dissolved salts.  Surface tension is weaker in warm water and when dissolved organics, fats, oils, and even floating plants are present.  A few organisms, such as water striders, live most of their life on the surface tension of water.  The thin microhabitat in and on the surface tension is called the neuston.


Figure 1-6. Dense objects supported by surface tension: a scalpel blade (left), and a pin (right). (Images: ARH)

Specific Heat of Water

Specific heat is the amount of energy in calories needed to raise the temperature of 1.0 g of a substance 1oC.  Water has an extremely high specific heat of 1.0, the highest of any common substance on Earth.  This means that water has to absorb a lot of energy to increase its temperature even a little bit.  This also means that water must lose a lot of energy before it cools even a little bit.  This tendency to be able to gain or lose large amounts of energy without much heat flux is called thermal inertia, and makes water a vital component of living things.  For example, the most abundant substance in our bodies is water, so our bodies resist thermal fluctuations and increases our ability to maintain a constant internal core temperature even when atmospheric temperature fluctuates widely.  What this means for aquatic organisms is that since they live in water their environmental temperature is extremely stable compared to that of air.  This also means that the ocean can take up a massive amount of energy in the tropics and be moved in surface currents hundreds to thousands of miles to higher latitudes, thus moderating the Earth’s climate at low and high latitudes.
In addition, water exhibits a latent heat of melting, fusion, evaporation, and sublimation.  The latent heat of melting is 79.72 cal g-1 of ice at standard temperature and pressure (STP).  This means that in order for ice at 0oC to melt into liquid water at 0oC, each gram of ice has to take up 79.72 cal of energy.  The latent heat of fusion, the reverse process to melting, means that to change liquid water at 0oC into ice at 0oC each gram of water has to lose 79.72 cal.  The latent heat of evaporation is much higher, 540 cal g-1.  This is the amount of energy that must be absorbed by liquid water at STP to convert it into water vapor.  Of course 540 cal is also released when each gram of water condenses.  Lastly, the latent heat of sublimation is 679 cal g-1 at STP.  This is the amount of energy each gram of ice has to absorb in order to release molecules of water directly as water vapor.  The latent heat of melting, fusion, evaporation, and sublimation show us that water has to gain or release large amounts of energy in order to undergo phase changes.

Density Anomaly of Water

Density is a function of an object’s mass divided by its volume.  Pure water has a density of 999.84 kg m-3 at STP and maximum density of 1000 kg m-3 at 3.98oC.  By comparison, air has a density of 1.29 kg m-3 at STP, so water is about 775 times denser than air.  As a result, water provides objects far greater buoyancy than air.  Density is therefore a significant factor for aquatic organisms. Water density is affected by temperature and salinity. 

Figure 1-7.  The temperature - density relationship for pure water.  Maximum density of 1000 kg m-3 occurs at 3.98oC. (Image: ARH)

Temperature affects water density as indicated in Fig. 1-7.  The density of pure liquid water increases from about 958 kg m-3 at 100oC to a maximum density of 1000 kg m-3 at 3.98oC.  Its density then declines slightly until it reaches 0oC.  When water loses enough additional heat to reach the latent heat of fusion it undergoes a phase change into its crystalline form – ice.  When this happens its density drops precipitously to 917 kg m-3.  When water is a liquid, even at 0oC, it forms and breaks hydrogen bonds so rapidly that one water molecule may be bonded to one, two, three, or even four other water molecules at a time, but the duration of those bonds is so fleeting that a crystalline structure is not produced.  When water freezes, however, enough energy is lost from the water that when bonds are formed between water molecules those bonds are retained long enough to form a 3D crystalline structure.  The crystalline structure of ice is made up of interconnected rings of six water molecules each (Fig. 1-8).  The tetrahedral bonding between water molecules in ice keeps those molecules farther away from each other on average than they are in the liquid water.  The resulting 3D lattice crystalline structure of ice includes spaces within and between 6-member rings.  These spaces make ice less dense than water.  The density difference between liquid water and ice is called the density anomaly of water.  This explains why ice floats in water.

Figure 1-8. Three-dimensional structure of ice.  Spaces within the 6-member rings and between 6-member rings make ice less dense than liquid water.  Blue balls are water molecules.  Black dashed lines are hydrogen bonds between water molecules in the foreground.  Gray dashed lines are hydrogen bonds between water molecules in the background.  Red dashed lines are hydrogen bonds between water molecules in the foreground and the background. (Image: ARH)

Water is the only commonly occurring substance on Earth that is less dense as a solid than as a liquid.  This single characteristic of water allows life, as we know it to exist on Earth.  If water were like just about everything else, denser as a solid than a liquid, ice would sink to the bottom of lakes and the ocean whenever it forms.  The ocean would become super-cooled and then filled with ice, and it would never melt.  Eventually all bodies of water of Earth would become solid masses of ice except perhaps for a thin layer of water at the surface melted by solar radiation.  But because ice floats, ice actually forms an insulating layer at the top of bodies of water, allowing water underneath to remain liquid even when the air above ice reaches temperatures far below freezing, allowing organisms below the ice to survive. 
The density of water is also affected by salinity.  The temperature-density curve for water is non-linear (Fig. 1-7).  This is not the case for salinity (Fig. 1-9).  Temperature alone can increase water density to 1000 kg m-3, but by adding salt or other solutes, water density be increased well above that.  Water can dissolve up to about 38 g of salt per 100 g of water at STP, but seawater normally contains only about 3.3-3.7 g of salt per 100g of water.

Figure 1-9. The salinity - density relationship for water at 15oC.  (Image: ARH)

The importance of water density cannot be overstated, since density differences are what keep masses of water of different densities from mixing with each other.  Figure 1-10 shows the relationship between temperature differences and the relative thermal resistance to mixing resulting from those temperature-density differences.  These data show that a difference of even a few degrees is enough to provide a significant resistance to mixing, as evidenced by the large relative thermal resistance that exists in the thermocline shown in Fig. 1-10.  These density differences are enough to keep warm surface waters of the epipelagic zone from mixing with deeper, colder water, and are enough to keep warmer inshore waters from mixing with colder offshore waters.  At the same time, density driven by temperature and salinity drive the world’s most important deep ocean current – the global conveyor belt.  This current plays a major role in regulating Earth’s climate.
Figure 1-10. Typical mid-summer temperature profile and relative thermal resistance values in a small temperate lake.  Relative thermal resistance indicates the resistance of successive layers of water to mix with each other based on resistance of freshwater at 4oC to mix with freshwater at 5oC. (Image: ARH) 


The chemical and physical characteristics of water determine how it bonds, what dissolves in it, how dense it is, and how resistant neighboring water masses are to mix with each other.  The resistance to mixing between successive layers of water limits the movement of nutrients, dissolved oxygen, plankton, and other things suspended or dissolved in the water.
In summary, by gaining a better understanding of the molecular and chemical nature of water, you can better imagine what life is like for aquatic creatures.  You can also gain a greater appreciation for how the ocean helps moderate and maintain living conditions on Earth.

Review Questions
1)   Explain what makes water a tetrahedral molecule.
2)   Explain why water is a polar molecule and why this matters.
3)   Explain the density anomaly of water and why this matters.
4)   Define the term “relative thermal resistance” and how this limits water mixing, and then comment on why this matters.