This tutorial contains the following sections, with questions embedded throughout to help you check your understanding:
Answers to all the questions are on the next page.
Water is a remarkable substance that is important not only as a source of moisture, but also as a means of transferring energy in the atmosphere.
Water is the only common substance that is liquid at ordinary temperatures on the Earth, but it is also found in all three physical states - solid, liquid and vapor - at various places on the Earth’s surface, and sometimes in the same place at the same time (think of an iceberg floating in the ocean, with water vapor in the surrounding air).
Water is a molecule composed of two atoms of hydrogen bonded with one atom of oxygen: H20. The oxygen atom and the two hydrogen atoms share electrons in a strong covalent bond.
Water molecules as a whole have no net charge, but the oxygen end has a slight negative charge (since the electrons tend to stay on the side of the large oxygen nucleus), and the hydrogen end has a slight positive charge.
For this reason, water is referred to as a polar molecule. It has a positive end and a negative end (analogous to the north and south poles of a bar magnet). This is a very important fact about water, because it means that the molecules can bond together in hydrogen bonds. The negative (oxygen) end of one water molecule forms a slight bond with the positive (hydrogen) end of another water molecule.
Here are water molecules floating around by themselves:
Here are water molecules bonded to each other with hydrogen bonds:
This hydrogen bond is not as strong as the bonds that hold the water molecule itself together, but the attraction of water molecules to each other is a very important factor determining the properties of water.
Because of the way water molecules are arranged and can bond with each other, water has several important properties:
Since water molecules are attracted to each other by hydrogen bonding, they tend to stick together; this is called cohesion.
Because of this cohesion, water has high surface tension. An insect can walk on the surface of a pond because of surface tension: hydrogen bonds hold the molecules of water together, forming a surface “skin” that an insect can walk on (if it spreads its weight out over the surface). Surface tension is also responsible for water sticking together in small drops. In a slowly dripping faucet, a drop will get bigger and bigger, until it finally falls into the sink. This happens because the attraction of the water molecules to each other is strong, holding them together until the drop gets to be a certain size, when the pull of gravity overcomes the cohesion of the water molecules.
Water has good adhesion: water molecules are attracted to other polar molecules, and to solid surfaces. Water molecules adhere to soil particles, to fibers in a towel, and to many other substances.
Hydrogen bonding also makes capillary action possible. Capillary action is the ability of water to pull itself upward through small openings against the pull of gravity. The water molecules adhere to the surface of the material in the opening, and cohere to each other. You might think of it like this: if a mountain climber is clinging to the side of the mountain with one hand, and holding on to his climbing companion with the other, he is adhering to the mountain and cohering to his companion. Spill some water, then dip the end of a paper towel into it. What happens to the water? It climbs upward through the paper towel. This is capillary action. Capillary action helps water move upward in soil.
Water is a good solvent: water molecules can also attract ions, or charged particles. This makes it able to dissolve other substances easily.
Water has a high specific heat. Specific heat can be defined as the amount of heat required to raise the temperature of one gram of a substance 1 degree Celsius. The specific heat of water is 1 calorie/gram, much higher than the specific heat of most other substances (ammonia is an exception). The specific heat of water is about 5 times as great as that of most Earth materials, such as the iron and aluminum found in rock and soil. This means that water can absorb more solar energy without its temperature increasing. Go to the beach on a hot summer day (who said all scientific study has to be tough?), and notice the temperature of the sand and of the water. In the middle of the afternoon, the sand will feel hot and the water cool. At midnight, the sand will feel cool, and the water temperature will be about the same. The sand changes its temperature through the course of the day, but the water temperature doesn’t change much. So a large body of water has a moderating influence on climate.
Water is one of the few substances that expands when it freezes. Most substances become denser and contract when they are cooled, and expand when they are heated. Water is less dense when it is frozen, so its solid form is lighter than its liquid form; in other words, ice floats. This is great if you are a fish: you can survive in the liquid water under a cover of ice on the surface. Otherwise, ice would sink to the bottom of lakes and oceans, and it would be very hard to melt it.
The three physical states of matter that we normally encounter are solid, liquid, and gas. Water can exist in all three physical states at ordinary temperatures on the Earth’s surface.
When water is in the vapor state, as a gas, the water molecules are not bonded to each other. They float around as single molecules.
When water is in the liquid state, some of the molecules bond to each other with hydrogen bonds. The bonds break and re-form continually.
When water is in the solid state, as ice, the molecules are bonded to each other in a solid crystalline structure. This structure is six-sided, with each molecule of water connected to four others with hydrogen bonds. Because of the way the crystal is arranged, there is actually more empty space between the molecules than there is in liquid water, so ice is less dense and therefore floats.
Each time water changes physical state, energy is involved.
In the vapor state, the water molecules are very energetic. The molecules are not bonded with each other, but move around as single molecules. Water vapor is invisible to us, but we can feel its effect to some extent, and water vapor in the atmosphere is a very important factor in weather and climate.
In the liquid state, the individual molecules have less energy, and some bonds form, break, and then re-form. On the surface of liquid water, molecules are continually moving back and forth from the liquid state to the vapor state. At a given temperature, there will be an equilibrium between the number of molecules leaving the liquid, and the number of molecules returning.
In solid water (ice) the molecules are locked together in a crystal structure: a framework. They are not moving around, and they contain less energy.
How do you make water evaporate? Here is a bowl of water. Make the water evaporate. Go ahead.
How did you make the water evaporate? You probably added heat. You might have set it out in the Sun, or possibly put it over a fire. To make water evaporate, you put energy into it. The individual molecules in the water absorb that energy, and get so energetic that they break the hydrogen bonds connecting them to other water molecules. They become molecules of water vapor. Evaporation is the change in state from liquid to vapor. In the process of evaporation, the molecule absorbs energy. This energy is latent heat. Latent means hidden, so latent heat is “hidden” in the water molecule - we can’t feel it, but it is there. Wherever that individual molecule of water vapor goes, it takes that latent heat with it. To get the molecule of water vapor to become liquid again, we have to take the energy away, that is, we have to cool it down so that it condenses (condensation is the change from the vapor state to the liquid state). When water condenses, it releases latent heat.
Now, how do you make ice melt? Here is a block of ice, water in the solid state. Make it melt. Go ahead.
Again, you probably melted the ice by adding energy. The additional energy was absorbed by the individual molecules of water, which became so energetic that they broke some of the hydrogen bonds holding the ice crystal together, and became liquid (that is, the ice melted). This energy is also latent heat, and each molecule of the liquid water is holding that latent heat. To change the liquid water back to ice, you have to take that latent heat away, or in other words, cool the water.
Water could change directly from the frozen state to the vapor state without passing through the liquid state first. This process is called sublimation. Water can also change from the vapor state to the frozen state without passing through the liquid state. This is usually called deposition, and is what you see when frost forms on grass or windows on a cold night. (Sometimes the term sublimation is used when water changes state in either direction, from solid to vapor or from vapor to solid).
The really important thing to remember is that each time water changes state, energy is absorbed or released. This energy is latent heat. Latent heat is the energy absorbed or released when a substance changes its physical state. Latent heat is absorbed upon evaporation, and released upon condensation to liquid (as in clouds). Latent heat is also absorbed when water melts, and released when it freezes.
How much heat does it take to get water to change state? If the water is at a temperature of 100 degrees C (that is, the boiling point, or 212 degrees F), it takes an additional 540 calories of heat to convert one gram of water from the liquid state to the vapor state. When the vapor converts to the liquid state, 540 calories of energy will be released per gram of water. If you are converting solid water (ice) to liquid water at 0 degrees C, it will require about 80 calories of heat to melt one gram of ice, and the 80 calories will be released when the liquid water is frozen to the solid state.
Water does not have to be at the boiling point to evaporate. If you don’t believe this, set a pan of water out in the sunshine and watch it slowly disappear. The Sun’s heat is not boiling the water, but it is evaporating it. In a given amount of water at a given temperature, some molecules of water will have more energy than others, so some molecules will be able to evaporate, while others remain in the liquid state. The lower the temperature of the water, the more energy is required for evaporation. If the water is liquid at a temperature of 0 degrees C, the latent heat of vaporization is 597 cal/g, compared to 540 cal/g at 100 degrees C. In between, at 50 degrees C, an input of 569 cal/g would be required for evaporation.
It will take a total of about 720 calories per gram to sublimate water, that is change it directly from ice at 0 degrees C, to vapor at 100 degrees C: this includes 80 calories from latent heat of fusion (melting) + 100 calories to raise the temperature of the water 100 degrees C + 540 calories to make the liquid water evaporate (latent heat of vaporization). Similarly, about 720 calories per gram will be released when water is changed directly from vapor to ice, the process called deposition.
There are several ways to describe the humidity of the air. You may not need all of them, but here they are, just in case.
Absolute humidity expresses the water vapor content of the air using the mass of water vapor contained in a given volume of air. It may be measured in grams of vapor/cubic meter of air. A problem with using absolute humidity is that an air parcel changes volume as the ambient temperature and pressure change. This means that the absolute humidity changes when the volume changes, even though the mass of water vapor has not changed.
Specific humidity measures the water vapor content of the air using the mass of water vapor for a given mass of air. It may be measured in grams of water vapor per kilogram of air. The kilogram of air measured includes the water vapor present (compare this to mixing ratio, below). Unlike absolute humidity, specific humidity doesn’t change as the air parcel expands or is compressed.
Mixing ratio also measures the water vapor content using a measure of mass, but it measures the mass of water vapor for a given mass of dry air. It may be measured in grams of water vapor per kilogram of dry air. Notice the difference between mixing ratio and specific humidity: specific humidity includes the water vapor in the air in the denominator, while mixing ratio measures water vapor per mass of dry air. Since water vapor comprises only a few percent of the mass of air, the values for specific humidity and mixing ratio are very close for a given parcel of air. Mixing ratio is not affected by changes in pressure and temperature. This is a measure commonly used by meteorologists. At a temperature of 20 degrees C, at average sea-level pressure, the saturation mixing ratio is 14 grams of water per kilogram of dry air.
Vapor pressure measures the water vapor content of the air using the partial pressure of water vapor in the air. (Pressure may be expressed using a variety of units: in pascals, in millibars, or in pounds per square inch, among others). The gases in the atmosphere exert a certain amount of pressure (about 1013 millibars at sea level). Since water vapor is one of the gases in air, it contributes to the total air pressure. The contribution by water vapor is rather small, since water vapor only makes up a few percent of the total mass of a parcel of air. The vapor pressure of the water in the air at sea level, at a temperature of 20 degrees C, is 24 millibars at saturation.
Most of these measures of humidity cannot easily be determined directly. It is actually easier to measure relative humidity.
Relative humidity: we can compare how much water vapor is present in the air to how much water vapor would be in the air if the air were saturated. For this we use relative humidity. Relative humidity is a ratio that compares the amount of water vapor in the air with the amount of water vapor that would be present in the air at saturation. One way it can be stated would be as the ratio of the actual mixing ratio to the saturation mixing ratio. Relative humidity is given as a percentage: the amount of water vapor is expressed as a percent of saturation. If 10 grams of water vapor were present in each kilogram of dry air, and the air would be saturated with 30 grams of water vapor per kilogram of dry air, the relative humidity would be 10/30=33.3%.
For example, a parcel of air at sea level, at a temperature of 25 degrees C, would be completely saturated if there were 20 grams of water vapor in every kilogram of dry air.
If this air actually contains 20 grams of water vapor per kilogram of dry air, we would say that the relative humidity is 100%.
Consider a bowl of water. We’ve already seen this bowl, and we made water evaporate from it. As you recall, to make the water evaporate, we added heat, which was absorbed by the individual molecules of water. As each molecule absorbs heat, it gets more energetic, and eventually has so much energy that it breaks the hydrogen bonds holding it to the other water molecules, leaves the liquid water, and floats off on its own, as a molecule of water vapor. In other words, it evaporates.
Even while some water molecules are evaporating, others are condensing, changing from the vapor state to the liquid state, and joining the liquid water in the bowl. At any given temperature, there will eventually be equilibrium between the number of molecules evaporating, and the number of molecules condensing. When the number of molecules evaporating balances the number of molecules condensing, we say that the air above the liquid water is saturated. The term saturation refers to the maximum amount of water that can be present as vapor in the atmosphere. If the air above the water bowl is saturated, then for every molecule of water that evaporates, another molecule condenses.
But we know that to make water evaporate, you add heat. So what happens if we raise the temperature of the bowl and the surrounding air? More water will evaporate. For a while, the rate of evaporation will exceed the rate of condensation.
Eventually, though, the balance between evaporation and condensation will stabilize at the new temperature. Once again, evaporation will balance condensation. The air will again be saturated, but there will be more molecules of water vapor present in the air above the bowl. At the higher temperature, more water vapor will be present in the air at saturation. This is a good general rule to remember: the higher the air temperature, the more water vapor will be present in the air at saturation.
What happens if we lower the temperature? As the temperature is lowered, more water molecules return to the liquid state (condense) than evaporate.
Eventually, at the new lower temperature, there will again be a balance between the number of molecules evaporating and the number condensing. But there will be fewer molecules of water vapor in the air at the new cooler temperature than were present at higher temperatures. This rule to remember is just a restatement of the previous one: the lower the air temperature, the less water vapor will be present in the air at saturation.
Relative humidity depends on two factors: the amount of moisture available, and the temperature. So you can have a change in relative humidity in one of two ways:
When the amount of water vapor is held constant:
Let’s look more closely at how the air changes temperature when it rises or subsides. We know that warm air rises, and when it rises it becomes cooler. If you remember that, you can reason your way through a lot of meteorology.
Rising air experiences a drop in temperature, even though no heat is lost to the outside. The drop in temperature is a result of the decrease in atmospheric pressure at higher altitudes. If the pressure of the surrounding air is reduced, then the rising air parcel will expand. The molecules of air are doing work as they expand. This will affect the parcel’s temperature (which is the average kinetic energy of the molecules in the air parcel).
One of the results of the laws of thermodynamics is that there is an inverse relationship between the volume of an air parcel and its temperature. During either expansion or compression, the total amount of energy in the parcel remains the same (none is added or lost). The energy can either be used to do the work of expansion, or to maintain the temperature of the parcel, but it can’t be used for both.
If the total amount of heat in a parcel of air is held constant (no heat is added or released), then when the parcel expands, its temperature drops. When the parcel is compressed, its temperature rises.
In the atmosphere, if the parcel of air were forced to descend, it would warm up again without taking heat from the outside. This is called adiabatic heating and cooling, and the term adiabatic implies a change in temperature of the parcel of air without gain or loss of heat from outside the air parcel. Adiabatic processes are very important in the atmosphere, and adiabatic cooling of rising air is the dominant cause of cloud formation.
For the atmosphere, the drop in temperature of rising, unsaturated air is about 10 degrees C/1000 meters (5.5 deg F per 1000 feet) altitude. If a parcel of air is at 24 degrees C at sea level, and it rises to 1000 meters, its temperature will go down to 14 degrees C. If it goes up to 2000 meters, its temperature will go down to 4 degrees C.
This rate of temperature change of unsaturated air with changing altitude is called the dry adiabatic lapse rate: the rate of change of the temperature of rising or subsiding air when no condensation is taking place (we’ll talk about the condensation part shortly).
If the air subsides, it also changes temperature. It warms up, and it is warming up at the dry adiabatic lapse rate. So, if the air at 4000 meters altitude has a temperature of -10 degrees C, and it subsides to 3000 meters, its temperature will warm up to 0 degrees C. If it continues to subside, then at 2000 meters, its temperature will be 10 degrees C.
Make sure you notice that we are talking about moving air (rising or subsiding), not still air. The change in temperature of still air (that is, air that is not rising or subsiding) follows the environmental lapse rate, which varies considerably, but averages about 6.5 deg C/1000 meters (3.6 deg/1000 feet). In still air, if you went up in a hot air balloon, carrying a thermometer and taking the air temperature every 1000 meters, on average the temperature would drop 6.5 degrees C every 1000 meters. The rate of temperature change as you rise in still air is not as great as the rate of change of rising air; that is, the air parcel does not cool off as fast.
For instance, the air temperature at sea level is 28 degrees C. Climb into your balloon, release the tethers, and go up 1000 meters in the still air.
Let’s abandon the still air for the moment, and return to the air which is rising, and getting colder. Remember what happens to relative humidity when air temperature decreases? Ok then.
(You have two choices here, either the relative humidity decreases or it increases. This is a VERY IMPORTANT point for you to understand, so stop and think about it before you rush off to view the answer.)
You can maybe see what’s coming next. If the air is rising and cooling at a rate of 10 deg C/1000 meters, (5.5 deg/1000 feet), eventually, it’s going to cool off enough for the relative humidity to reach 100%, and condensation can take place. The dew point is the temperature at which the air becomes saturated and condensation takes place (note: dew point is a temperature, given in degrees C or F). The lifting condensation level is the altitude at which condensation begins (note: lifting condensation level is an altitude, given in meters or feet). You can look up at the windward sides of mountains and see where the lifting condensation level is, because that is where you will see the bases of clouds that have formed.
Here’s where it gets a bit complicated. Remember what happens when water changes state?
So, if condensation is taking place, latent heat is being released to the surrounding air. So you have two opposing trends going on at the same time within this parcel of air. It’s rising and cooling, but it’s also condensing and being warmed. Which one will win out? That is, will the air get colder, or will it get warmer?
Well, what happens is that the air will still cool off, but not as fast. If water vapor in the air is condensing, the adiabatic rate is lower. The air is only cooling off at a rate of about 5 degrees C/1000 meters (2.7 deg per 1000 feet). This is called the saturated adiabatic lapse rate (or the wet adiabatic lapse rate, or the moist adiabatic lapse rate, depending on the textbook you are using). The saturated lapse rate varies with the original temperature of the air parcel, but 5 degrees C/1000 meters is a commonly used value.
So, let’s assume a rising parcel of air reaches the lifting condensation level at 2000 meters, at a dew point temperature of 12 degrees C. At this point, clouds will form. As the air continues to rise, it will continue to decrease in temperature, but more slowly than it cooled off before condensation began.
Two parts to this question; think about both before viewing the answer.
Now that you’ve got all that, try practicing moving air up and down a mountain, to see what happens.
Ok, here’s the mountain.
It is exactly 3000 meters in elevation, rising from sea level, since it is located right on the coast. It is in the middle latitudes, and the prevailing westerly winds blow from the ocean on to the shore. The air temperature at sea level is 26 degrees C. The moving air strikes the mountain, and is forced to rise along the windward slopes until it gets to the top. Then it can subside down the leeward slopes.
The first question is, what happens to the temperature of the air as it rises up the side of the mountain?
The next question is, how much will the temperature change? To answer this, you first have to decide which lapse rate to use. We know that the air is rising, and we will also state that condensation is not taking place in the rising air (that is, the temperature of the air has not reached dew point). Therefore...
Note: Do not go on until you understand the answer to this question! Going on without thinking about how all this works will give you very bad luck on your next exam.
The next thing we need to do is give some lapse rates you can work with. Here they are:
Also, we will state that the dew point temperature for this parcel of air is 6 degrees C.
So, since we have already determined that we will be cooling the air parcel off using the dry adiabatic lapse rate, let’s see what happens to this air.
The air starts at sea level with a temperature of 26 degrees C.
Remember that 6 degrees C is dew point.
For this particular situation, 2000 meters is the altitude at which condensation takes place. This is called the lifting condensation level. Please note the difference between dew point and lifting condensation level. Dew point is a temperature and is given in degrees C or F, while the lifting condensation level is an altitude (the altitude at which dew point is reached) and is given in meters or feet.
So, as the air continues to rise, it cools to a lower temperature (because it is rising), but it doesn’t cool off as rapidly (because latent heat is being released). Therefore, the lapse rate changes.
Notice that condensation is taking place, so clouds (composed of tiny droplets of liquid water) will form, though it may or may not rain.
Once the air reaches the top of the mountain, it can begin subsiding down the leeward slopes. It subsides because it is cooler than the surrounding air.
Let’s assume that no more moisture remains in the subsiding parcel of air.
Note that the air is warmer at sea level on the leeward side of the mountain than it was when it started at sea level on the windward side. It is also drier. Do you see why the leeward sides of mountain ranges are in the “rainshadow” of the mountains? There are many rainshadow deserts as a result of this process.
Also note that we did not use the normal environmental lapse rate in this example. That’s because the environmental lapse rate applies to still air, and the air around this mountain is either rising or subsiding.