Water & Weather

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The Hydrologic Cycle (Water Cycle)
From oceans to land and back again, the water cycle is one of nature's most remarkable processes!
Powered by energy from the Sun, the water cycle circulates impressive amounts of water from
place to place across the globe. The hydrologic cycle is relatively simple. The following chart
provides a quantitative description of how much water is cycled:

You'll notice the volume of water cycled is huge! However, if you look to the % column above, you will notice that the volume of water cycled is miniscule in comparison with how much water exists on Earth! The following chart gives a quantitative description of how much water exists on Earth:

What is Water?
Sounds like a silly question to ask, everyone knows what water is, but what about its structure
and unique properties? Some unique properties of water include its high surface tension,
relatively low density in the solid state, large heat capacity, and its capacity as a universal
solvent.

Water's molecular structure: 2 Hydrogen atoms and 1 Oxygen atom: H₂O is such that water molecules are polar, meaning they have a net negative charge on one side and a net positive charge on the other:

The positive end of one water molecule will attract to the negative end of another and the positive end of that to the negative end of yet another and so on. Like so:

Above freezing temperatures, these bonds continually form and break giving a fluid character to the entire water mass. When you are looking at water in a clogged sink or water in the ocean, you are witnessing these bonds breaking and forming on massive scales! The higher the temperature, the higher the fluidity since bonds form and break at increased rates, proportional to temperature. Technically, warm water flows more readily than cold water! Of course, at freezing temperatures (0ºC and below or 32ºF and below) water loses its fluidity since it crystallizes to ice, and we all know ice doesn't flow very well!

Water molecules form hydrogen bonds with surfaces with free electrons (negative charge). If you
look to the illustration above, you'll notice that the hydrogen atoms in water are positively charged.
Opposite polarities attract, therefore hydrogen atoms in a water molecule are attracted to surfaces
with net negative charge. A perfect example of this is one water molecule bonding to another. The hydrogen (positive) end of one water molecule will bond with the oxygen (negative) end of another
water molecule.

One would expect water to become continually denser as it gets colder. This is true, however it is
only true to a certain temperature: 4ºC. Water reaches its maximum density at 4ºC. Above and below this temperature, water decreases in density! At 4ºC, the hydrogen bonds become stronger than the liquid's tendency to shrink. It's a good thing too! If ice were the densest form of water, no one would be able to go ice skating on a frozen lake since water would freeze from the bottom up! Think of the consequences that would have on a global scale! << There's food for thought!

Since water is most dense at 4ºC, bodies of water freeze from the top down. As a result people can ice skate on frozen lakes, polar ice caps over the oceans don't sink, and ice bergs float!

The following is a graph illustrating a linear display of the relationship between temperature and
density of water at normal atmospheric pressure (1013.25mb):

(Source for chart: Dr. R.B. Howard, CSUN)

Water's Thermal Properties
Fresh water at normal atmospheric pressure boils at 100ºC (212ºF) and freezes at 0ºC (32ºF).
The boiling and freezing points of water are well known. Also well known is the fact that water
exists in gas, liquid, and solid states at prevailing temperatures on Earth. The reason water is able
to exist in all three states at moderate temperatures is because of its strong hydrogen bonds.

Water's hydrogen bonds keep water's boiling point higher than it would be without these bonds.
Generally, the higher the molecular weight of a substance, the higher its boiling point. However,
if we look at hydrogen telluride (H2Te), hydrogen selenide (H2Se), and hydrogen sulfide (H2S),
we notice that their boiling points are much lower than water (H2O) even though they are hydrogen
compounds with similar molecular structure to that of water.

Normally, molecules' (in liquid state) van der Waals forces get stronger with increased molecular
weight (e.g. H2Te, H2Se, and H2S). Relative to hydrogen bonds, van der Waals forces are weak.
Therefore, the relatively stronger hydrogen bonds in water can withstand higher temperatures
before breaking. The following chart illustrates the how unique water's boiling and freezing points
are from similar molecules (as a result of hydrogen bonding):

(This illustration was adapted from J.C. Manning's Applied Principles of Hydrology 3rd ed.)

Water's Heat Capacity
The calorie is a unit of heat. Heat is a form of energy measured indirectly by measuring
temperature changes in a substance or process. The calorie is the amount of heat needed to increase temperature of 1 gram of water by 1ºC. You may also hear British Thermal Unit or
BTU used, which is also a unit of heat. However the Btu is somewhat awkward; defined as
the amount of heat necessary to increase the temperature of 1 pound of water by 1ºF.

Specific heat (specific heat capacity) is related to the calorie (and Btu) in that it is defined as the
amount of heat required to increase the temperature of a unit mass of a substance by 1º. You will
notice that it makes no reference to either the type of unit used for the mass, or the type of unit
used for the temperature. The reason for this is because specific heat is a ratio of a substance's
specific heat (represented by a number) to water's specific heat (set at 1.000). Ratios are pure
numbers, therefore a specific heat value will be the same in all systems of units.

Scientists arbitrarily gave water a specific heat of 1.000 (one) as a reference point for all other
substances. Since water is readily available and has a rather high specific heat capacity, scientists
had good reason to pick water as the reference substance!

The specific heat of water (or any substance for that matter) numerically equals its specific heat
capacity. However, "heat capacity" is not in reference to how much heat water holds. Water can
have an indefinite amount of heat added to it. There is no set limit when water (or any substance)
can no longer "hold" heat. What happens to a substance, like water, that has heat continually added
to it is temperature increase, and eventual phase change (e.g. liquid to gas).

The relatively high specific heat of water is a good thing too! If it weren't as high as 1.000, our
oceans would heat up faster since it would require fewer calories to raise the water's temperature!
If you take a peace of lead (with a specific heat of about 0.03) and add heat energy to it, it will
feel very much hotter than the same mass of water to which you add the same amount of heat
energy to!

The specific heat of water (1.000) is about 10 times greater than that of glass (0.12); 17 times
greater than that of silver (0.06), and about 33 times greater than that of mercury or lead (both with
specific heats of 0.030)! Good thing the oceans don't consist of mercury! Of course, it's a good
thing for many more reasons too! We owe a debt of gratitude to the hydrogen bonds amongst
water molecules, which play an important role in why water's heat capacity is as high as it is.

Heat of Fusion & Heat of Vaporization
Latent heat of fusion is the amount of heat per unit mass required to change a substance at its melting point completely to a liquid at the same temperature. Latent heat of vaporization is the amount of heat per unit mass necessary to change a liquid at its boiling point completely to a gas at the same temperature. We'll get back to this in the 6th paragraph, so keep these definitions in mind as you read!...

The key phase in these two definitions is, "...at the same temperature." Earlier, we learned that heat can be indirectly measured and quantified with the calorie unit (or Btu). We also learned that 1 calorie is the heat energy required to raise the temperature of 1 gram of pure water by 1ºC. If enough calories are added to a substance, in this case water, it will eventually undergo a phase change (or change of state); say from liquid to gas or solid to liquid.

We know that if we were to add, say... 25 calories to a gram of water, we should expect to see the temperature of that gram of water raise by 25ºC. However, it is only certain that this will occur if the liquid water's beginning temperature (before heat is added) is between 0ºC and 75ºC. We could be certain to expect a 25ºC temperature increase! Why is that?

Remember that 0ºC is the temperature at which pure water will begin to freeze, and 100ºC is the temperature at which it will begin to boil. What happens when water freezes (0ºC)? It goes from a liquid to solid (becomes ice). What happens when water boils? It goes from liquid to gas (becomes steam). What you are witnessing when you see water freeze or turn to steam are phase changes!

However, it takes a lot more than 1 calorie to make water go from a liquid at 100ºC to steam at 100ºC! It also takes a lot more than 1 calorie to make water go from a solid at 0ºC to liquid at 0ºC! Hey!! There's not even a temperature change here! Why is that?!

The obvious reason is because these phase changes require more than 1 calorie of heat energy. How many calories you ask? Good question! It requires an input of 80 calories (144 Btu/lb) per gram of pure water to cause it to go from a solid at 0ºC to a liquid at the same temperature (Latent Heat of Fusion), and it requires 539 calories (970 Btu/lb) per gram of pure water to cause it to go from a liquid at 100ºC to a gas at the same temperature (Latent Heat of Vaporization)!

Let's look at a graph depicting the energy relationships as water changes state from solid to liquid to gas...

It is important to realize that this graph works both ways. In other words, if it requires 539 calories to cause 1 gram of liquid water at 100ºC to convert to steam at the same temperature, then it will require that steam at 100ºC to lose those 539 calories of heat energy if it is to revert back to liquid form again. (539 in/539 out)

The concept holds for ice and liquid water: If it requires 80 calories to cause 1 gram of ice at 0ºC to convert to liquid water at that same temperature, then it will require that liquid water at 0ºC to lose those 80 calories of heat energy if it is to revert back to solid form again. (80 in/80 out)

Okay, one last question to be sure we've got it! ...How many calories of heat energy are required to make 2 grams of ice at a temperature of -15ºC into steam at 105ºC? Click here to check your answer!

These processes of energy transformation play a major role in running Earth's weather and climate machine! Evaporation, condensation... all these processes require the addition or subtraction of latent heat energy!

Water's Viscosity
Viscosity refers to a fluid's resistance to flow. A highly viscous fluid, say ryolitic lava, will flow much slower than a fluid with low viscosity, like lemonade. For instance, honey is more viscous than water, therefore water will flow much more easily than honey will when poured. John C. Manning eloquently defines viscosity, "...as the internal friction of a fluid."

In physics, the viscosity of a liquid decreases with increasing temperature (an inverse proportion \/ /\). Vice versa, the viscosity of a gas increases with increasing temperature (a direct proportion /\ /\).

If you've ever heated honey up, you might have noticed that it flowed much faster than it did before you heated it up. This is because you lowered its viscosity by adding heat to it! Water is no different! It also has different viscosities depending on its temperature! In fact, water at 40ºC flows more readily than water at 5ºC. For every 0.5ºC (1ºF), water's viscosity will change by about 1.5%. If I heat water by 0.5ºC, its viscosity will drop by 1.5%. If I cool water by 0.5ºC, its viscosity will rise by 1.5%.

Imagine the rate of discharge in a river if the water in that river changed by a degree or two! According to Manning, a river was measured having a discharge of 18,397,044 L/day when it was at 4ºC in the winter months. The following summer, that same river was measured as having a temperature of 12ºC (8ºC warmer) discharging 23,393,722 L/day, a full 4,996,728 liters more water! Manning also writes that firefighters can use smaller diameter hoses (easier to move) and still get as much water as before by simply adding a degree or two to the water temperature! Of course, that's also a degree or two less energy the fire has to spend converting that water to steam, but the pros of mobility outweigh the cons of physics (can't get somethin' for nothin')!

Water's Compressibility
Water at temperatures common in nature has a compressibility factor of around 0.0000034, meaning that a hydrostatic pressure of 6.89 kilopascals (1lb/ sq. in) would reduce unit volume by about 0.0000034 of the original volume.

This compressibility of water is so slight we could never actually see it with our own unaided eyes. We might think water is not compressible. However, if that were the case then the oceans would be about 30 meters higher than they are now, and therefore cover an extra 5 million square kilometers of Earth!

Water's Surface Tension (Cohesion)
The cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension. The molecules at the surface do not have other like molecules on all sides of them and consequently they cohere more strongly to those directly associated with them on the surface. This forms a surface "film". Have you ever seen water bugs walking across the surface of water in a swimming pool or glassy pond? They are able to do that without sinking because of surface tension!

Surface tension is why raindrops are round (not tear-drop-shaped) as they fall through the air. The relatively powerful hydrogen bonds among water molecules gives water a surface tension 2-3 times higher than most common liquids! Surface tension is usually measured in dynes/cm. Water at 20°C has a surface tension of 72.8 dynes/cm compared to 465 for mercury!

Temperature again plays a role in the relative strength or weakness of water's surface tension. Increasing water's temperature will decrease its surface tension, and vice versa (inverse proportion). Another way to lower water's surface tension is to add detergent to it.

Detergents and soaps lower surface tension by rendering the hydrogen bonds much less effective among water molecules such that attraction to individual molecules is lessened. Remember that hydrogen in water molecules (as discussed earlier) attracts to the oxygen atoms of other water molecules. This attraction is muddled by detergents and soaps. In fact, if any of you read John McPhee's book, "The Control of Nature", you might remember him mentioning how locals would throw soap into a geyser to induce eruption... what they were doing was inducing eruption by reducing the water's surface tension!

Water's Capillarity (Adhesion)
It's what makes sand castles hold together beyond the angle of repose (30º), it's why soil can hold water, it's why Superman can leap tall buildings in a single bound!!...(okay, maybe not that last one).

So what is this capillarity? It is the force that results from greater adhesion of a liquid (water) to a solid surface than internal cohesion of the liquid itself and is therefore able to literally rise along vertical surfaces. Hydrogen bonds tend to form between water and the solid surface it's in contact with. The following illustration shows capillary rise of water in a small glass tube:

Again, water temperature affects capillarity: increase temperature = decrease capillarity and vice versa (inverse proportion). As we've learned, higher temperatures tend to increase rates of hydrogen bond breakage and formation.

Capillarity is important because it effectively counteracts gravity thus making it possible for soil to hold water. If water's capillarity was much lower, it would simply sink through soils leaving them dry and lifeless (indeed, there would be no soil)! Hydrogen bonds between water molecules and soil particles are strong enough to prevent gravity from pulling water to depths beyond the reach of vegetation (and water wells)!

Water as a Solvent
Water is the universal solvent. That means, it is able to put many chemical elements in solution. Fortunately, it doesn't really tend to put us in solution, but it is considered to be one of the most corrosive substances known when it comes to the inorganic world (rocks)! Karst topography is a perfect example of what water can do!

Water forms hydrogen bonds with molecules of other substances (like limestone). The individual ions of the substance are surrounded by water molecules. Perhaps the most widely used example of this involves common table salt (sodium chloride or NaCl).

A water molecule, as we've learned, is 2 positively charged hydrogen atoms and a negatively charged oxygen atom. The sodium in table salt is positively charged, and the chlorine is negatively charged. Add water. The sodium and chlorine separate becoming two ions [electrically charged particles: cation ("cat eye on") being the term used for positively charged ions, and anion ("an eye on") being the term used for negatively charged ions].

What occurs is the pairing of water molecules' oxygen atoms (negative charges) to salt molecules' sodium atoms (positive charges) and water molecules' hydrogen atoms (positive charges) to salt molecules' chlorine atoms (negative charges) resulting in literal separation of the sodium and chlorine ions....solution.

When water is evaporated away, it no longer separates these ions, and the sodium and chlorine come together again as NaCl (table salt). In fact, large bodies of water such as Pleistocene Lake Manly, the Dead Sea and Salt Lake have and, in the case of the Dead Sea and Salt Lake, continue to experience salt recrystalization as water is evaporated by energy of the Sun.

The following illustration conceptualizes this "separation" of ions:

Water's Electrical Properties
Pure water is a good insulator, meaning it does not carry an electrical current effectively. Water is dielectric (not conductive) since it is able to neutralize ions and keep them apart such as illustrated above.

However, the more ions in a solution, the more conductive water becomes. Saline water is more conductive than fresh water for this reason.

Water from Above
Now that we know what water is, let's talk about precipitation!

Precipitation comes in several forms. Rain, drizzle, snowflakes, snow pellets, hail and more (read more about the types of ppt on the FAQ page). We know the source of this water is mainly from the world ocean (I say ocean since it really is just one big ocean) as water is evaporated from the surface by solar energy input (latent heat of vaporization). We can look at the hydrologic cycle to see how this water is transported across Earth.

So how do we measure precipitation? There are many ways, the most common, and the way we do it here at Pierce College, is to use a standard 8" rain gauge.

Another way to measure precipitation is to use a tipping bucket rain gauge. We also utilize this instrument here at Pierce College.

But what about determining the average amount of precipitation over a larger area, like the San Fernando Valley? This involves a statistical approach, and there are 3 major methods used to determine areal averages...

Determining Average Precipitation Over Large Areas
There are many ways to interpret and analyze precipitation (ppt) data recorded at stations throughout the world. Weather folks like us aren't the only ones interested in precipitation! Engineers and hydrologists are also very interested in precipitation amounts over given areas. The National Weather Service hosts a plethora of maps and charts to aid hydrologists and engineers in their work. Precipitation frequencies, intensities, averages are just some of the aspects of ppt data sought by these men and women.

One problem facing engineers and hydrologists is area of consideration. We know we can obtain relatively accurate rain or snowfall amounts at any particular gage (also spelled gauge...go figure), but what about figuring amount that fell over a larger area? One gage will not do! In fact 100s of gages would not do!

Take for instance the hydrologist. She may need to know how much precipitation fell over a drainage basin so that assessments can be made of how much discharge will occur in a river downstream. This is important since it may be a matter of life and death for civilians living downstream! The power of a fast-moving river is incredible, able to rip buildings from their foundations like toothpicks in a pickle sandwich!

The hydrologist will never know exactly how much ppt fell in a given drainage basin: that would require there be a gage covering every square centimeter of that basin! Considering a drainage basin may very well be 200-300sq km in size, that's a lot of ugly ppt gages, and even more money! So the hydrologist needs to determine the average amount of ppt that fell in that basin.

How does she go about computing areal averages? There are at least 3 major ways of answering this question. Each has its pros and each has its cons. The following are the three methods sometimes used:

The Arithmetic Average
The easiest way to determine areal averages is to simply divide the sum of ppt recorded from all the gages in a given area by the total number of gages. For example, if there are 15 gages in a drainage basin and you want to determine the average amount of rainfall for that basin, you would add up the totals from each gage and divide that sum by 15! That's it! Arithmetic Average

The Thiessen Method
This was thought up by A.H. Thiessen (hence the name) back in the first decade of the 1900s. It is advantageous in that it allows for an uneven distribution of ppt gages (gages are not equidistant from each other). A drawback of this method, is that it assumes ppt varies linearly between stations, therefore it is not recommended as a method for determining areal averages (average over an area) in mountainous regions due to topographical affects of ppt intensities. It is, however, good for use over relatively flat and expansive areas such as the Great Plains. Thiessen Method

The Isohyetal Method
Perhaps the best method available for determining areal averages is the isohyetal method. Isohyetal comes from Greek roots-iso meaning equal and hyet meaning rain. Thus, isohyet means equal rain, or more specifically, it means a line of equal rain. An isohyetal map shows lines of equal ppt drawn the same way a topographic contour map is drawn. One line may be a linear depiction of 2cm of rainfall, and another line next to that may depict 3cm of rainfall, next to that; 4cm, then 5cm and so on.

Although the isohyetal method is the most accurate of the three methods, its accuracy depends heavily on the skill of the person drawing the isohyets. Topography should be considered when drawing up the lines, particularly in mountainous regions where rainfall tends to increase with altitude on windward slopes. Isohyetal Method

-by Steve W. Woodruff