Meterology Exam 4

I asserted something that may have been surprising: evaporation and condensation are occurring around you simultaneously all the time, but you often don't see the results because they're happening on the molecular level. Obvious phase changes occur when there's either "net" condensation or "net" evaporation (assuming you have some liquid water to begin with).

"Net" condensation means that the condensation rate exceeds the evaporation rate causing liquid water droplets to form. On the other hand, assuming you have some liquid water present to begin with, "net" evaporation, which means that the evaporation rate exceeds the condensation rate, causes liquid water droplets to shrink (or disappear altogether), or puddles on the ground to dry up, etc.

For starters, evaporation is the process by which liquid water molecules break the bonds with neighboring molecules and escape into the air as water vapor, and as I mentioned briefly in the last lesson, evaporation is a cooling process, for a couple of reasons. First, water molecules with the greatest kinetic energy (fastest vibrations) are most likely break the bonds with their neighbors and evaporate, which means the average kinetic energy of the remaining liquid water is reduced (because the most energetic molecules are no longer liquid).

A lower kinetic energy of the remaining water means a lower water temperature. Secondly, the breaking of bonds between liquid water molecules requires energy, and that energy comes from the surrounding air.

Water certainly has some unique properties, and it's all around us here on Earth. You may already know that a little more than two-thirds of the earth's surface is covered by water, and most water resides in the oceans.

Actually, about 97 percent of Earth's water is in the oceans, which contain an almost unimaginable amount of water -- more than a billion cubic kilometers (that's more than 100 quintillion gallons, for reference). The remaining three percent exists in ice caps near the North and South Poles, in lakes, rivers, and streams, or as groundwater (water held in soil, sand, or cracks in rocks).

Aerosols

Aerosols are essentially tiny particles suspended in the air, such as specks of dust, dirt, particles of air pollution, salt, etc. Yes, even though you can't usually see them, lots of tiny particles are suspended in the air. So, what do they have to do with cloud formation? In all of our previous discussions about evaporation rates, condensation rates, and relative humidity, water vapor had a surface to condense on to (liquid water in a container, or on the side of a cup, for instance).

To start, let's review the basic recipe for net condensation and cloud formation -- the Big MAC. M = moisture (water vapor), A = aerosols (condensation nuclei), and C = cooling (usually by lifting the air). This recipe gives us the basis for net condensation and explains how most clouds in the atmosphere form. As we'll learn later in the course, various weather features, such as cold or warm fronts, can serve as locations where air rises and cools to form clouds.

Air can also rise when it becomes buoyant and rises via convection, for example. But, some clouds require special twists on this basic recipe (and some don't form from lifting at all). Namely, we're going to look at how clouds form via "orographic lifting," how fog forms, and how "mixing clouds" such as contrails form.

So, cooling the air (decreasing its temperature) is one way to achieve net condensation. If the air cools enough (temperature decreases enough) that the evaporation rate becomes less than the condensation rate, net condensation can occur and liquid water drops can form and grow.

Another way to achieve net condensation is to increase the amount of water vapor molecules present (increase the dew point), which leads to a greater rate of condensation. If the amount of water vapor molecules increases enough (dew points increase enough) to make the condensation rate greater than the evaporation rate, then net condensation can occur and liquid water drops can form

So, any time a phase change (such as evaporation) causes water to go "up the energy staircase," energy is required to break bonds between molecules, which cools the surrounding air.

Any time a phase change (such as condensation) causes water to go "down the energy staircase," energy is released, which warms up the surrounding air.

Now, imagine a "bubble" of air that rises from the ground. Initially, the bubble has the same air density and air pressure as the surrounding air outside the bubble. But, if the bubble of air is lifted, that's no longer the case. The air inside the bubble has a higher density and pressure than its surroundings. To balance things out, the air molecules inside the bubble push out on the sides of the bubble, causing it to expand. But, molecules are doing work to cause this expansion, which costs them kinetic energy.

As a result, the air temperature in the bubble decreases, and this cooling continues as long as the parcel continues to rise and expand. If the parcel cools enough that the evaporation rate becomes slightly less than the condensation rate (relative humidity is slightly greater than 100 percent), then net condensation occurs onto cloud condensation nuclei and a cloud is born!

With these definitions under our belts, we can now understand how water cycles through the earth-atmosphere system. Essentially, liquid water in lakes, streams, rivers, and oceans evaporates into the air (additional water vapor enters the air through transpiration from plants or as evaporation of groundwater in soil, etc.).

As air rises, some water vapor condenses into cloud droplets, and when clouds grow sufficiently, water falls back to the earth (precipitation), where some gets stored as groundwater, while the rest runs off into lakes, streams, rivers, and oceans, where it can (eventually) evaporate into the air, and so on.

I should point out that the potential for evaporational cooling is greatest when a large difference between temperature and dew point exists because large differences between temperature and dew point allow for the greatest net evaporation.

As temperatures and dew points get closer, net evaporation is reduced, which yields less evaporational cooling. To really understand why this is the case, we need to explore what controls the rates of evaporation and condensation. We'll do that in the next section, as well as see why comparing evaporation rates and condensation rates is so important to weather forecasters.

From the recent discussion of condensation rates and evaporation rates, you already know what's going on when liquid water drops form and grow -- net condensation is occurring because the condensation rate is greater than the evaporation rate. But, at higher temperatures, evaporation rates increase, and with increased evaporation rates, even higher condensation rates are required for net condensation to occur.

As you know, higher condensation rates occur when the number of water vapor molecules increases, so when the air is warm, the high evaporation rates give the potential for a higher number of water molecules to remain in the vapor state without net condensation occurring. In other words, when it's warm, more water vapor molecules are needed in order for liquid water drops to form and grow. When the air is cooler, evaporation rates are decreased, meaning that fewer water vapor molecules are required for net condensation to occur.

To get an idea about why this is true, consider the following analogy. Suppose that there's a wild dance party where there are no tables and chairs (only a dance floor). As long as the energy of the dancers is high, there is no need for them to sit down and collect in groups. They simply keep bouncing around the dance floor like energetic water vapor molecules.

At some point in the evening however, some of the dancers' energy begins to wane, and they would happily sit out the next few dances if there were some tables and chairs available. Unfortunately there are none, and sitting down on the floor would be very uncool (and quite embarrassing). So, they just keep on dancing. Similarly, without sites where water molecules can easily condense (called "nucleation sites"), they are very reluctant to lose their gaseous state.

For starters, let's examine what accepting this myth really implies. By accepting this myth, we're basically treating air like a sponge, and once all the pores in the sponge get filled with water, it can't absorb any more water, so water starts dripping from the saturated sponge.

But, air isn't like a sponge. Air is also not like a hotel, which posts a "No Vacancy" sign when all of its rooms are filled with water vapor. If these ideas sound a little silly, it's because they are!

Initially, the condensation rate is small because only few water vapor molecules are present, and the probability that any one of them will come in contact with the interface between air and water is low. In fact, the evaporation rate far exceeds the condensation rate early on (net evaporation occurs).

But, as time goes on, and net evaporation continues, the air above the water contains an increasing number of water vapor molecules. As the number of water vapor molecules increases, the chance of a water vapor molecule contacting the interface between air and water and condensing back into liquid also increases, which translates to an increase in the condensation rate.

What does relative humidity tell us? It tells us how close the condensation rate is to the evaporation rate. As relative humidity nears 100 percent, the condensation rate nears the evaporation rate. Low relative humidity values mean that the evaporation rate greatly exceeds the condensation rate.

But, because relative humidity depends on the evaporation rate, which depends on temperature, relative humidity doesn't tell us how much water vapor is present in the air. For example, the relative humidity is 100 percent in both stages of our experiment above in which the condensation rate equals the evaporation rate (equilibrium), but more water vapor molecules are present in the state of equilibrium after we've increased the temperature. By itself, relative humidity is also not a good indicator of how muggy or humid the air feels to most humans.

A very tiny amount of the water in the earth-atmosphere system exists in the atmosphere (about 0.03 percent), and nearly all of it exists as water vapor. Still, the small fraction that exists as water vapor in the atmosphere is enough to fuel huge rain and snowstorms.

But, what little water vapor exists in the atmosphere at any given moment doesn't last for long, because water is regularly changing phases, and being exchanged between the earth and the atmosphere.

I'll discuss some practical applications for relative humidity shortly, but first I want to mention a little quirk about relative humidity observations. Relative humidity values calculated from standard weather instruments range from as low as near 1 percent when the evaporation rate greatly exceeds the condensation rate (a huge difference between temperature and dew point), to 100 percent when the evaporation rate equals the condensation rate (temperature and dew point are equal).

But, you already know that for net condensation to occur, the condensation rate has to be slightly greater than the evaporation rate. In other words, the temperature has to fall slightly below the dew point. But, the standard instruments that we use to make measurements are not precise enough to accurately measure the small difference between dew point and temperature when net condensation is occurring. Still, in reality, when net condensation is occurring, the dew point is ever so slightly higher than the temperature (even if we can't measure it). This leads to relative humidity values slightly greater than 100 percent within clouds, for example.

For starters, what exactly is fog? Well, fog is basically a cloud that forms at (or very near) the ground. For liquid water drops of fog to form and grow, obviously we need conditions suitable for sustained net condensation.

Cooling the air is the primary way that's accomplished in the atmosphere, and as you know, that's typically accomplished by lifting the air. But, if fog forms at (or very near) the ground, then lifting the air must not be part of the equation. How else can we cool the air?

While the focus of our discussion in this lesson has been on evaporation rates and condensation rates as they relate to net condensation and net evaporation, I want to refocus the discussion on moisture variables that meteorologists commonly measure or calculate (namely dew point and relative humidity). In this section, I'm going to focus on dew points. You may have heard dew points mentioned in weather reports or articles, but how do we interpret them? What can we do with dew points? You've encountered some of this information about dew points already in the course, but reviewing the basics and applying them to common weather situations will help you make practical use of dew points.

For starters, recall the definition of dew point: the approximate temperature to which the water vapor in the air must be cooled (at constant pressure) in order for it to condense into liquid water drops. In addition, you've also learned that (assuming air pressure doesn't change) the dew point temperature is an absolute measure of the amount of water vapor present. In other words, the higher the dew point, the more water vapor molecules in the air. The lower the dew point, the fewer water vapor molecules in the air.

Relative humidity usually ranges from just a few percent (when the evaporation rate is much larger than the condensation rate) to 100 percent, which occurs at equilibrium.

However, 100 percent is not the upper limit of relative humidity because, in reality, the condensation rate does sometimes exceed the evaporation rate slightly (that's how water droplets grow).

In practice, we can't calculate relative humidity using the equation above on the right because we can't easily determine the evaporation and condensation rates at any given time. However, we can relate evaporation and condensation rates to weather variables we can measure easily. Since we know that the condensation rate is controlled by the amount of water vapor present, and we use dew points to assess the amount of water vapor present, it stands to reason that condensation rates are connected to dew points. Indeed, higher dew points yield higher condensation rates. Meanwhile, temperature controls evaporation rates (higher temperatures yield higher evaporation rates), so relative humidity depends on dew point (which reflects the amount of water vapor present) and temperature.

I should point out, however, that we can't just substitute dew point and temperature into the equation for relative humidity above and do a simple calculation. The mathematical connections between condensation rates and dew point, and evaporation rates and temperature are too complex for that, and are beyond the scope of this course. Still, understanding the basic connections between temperature and evaporation rates, and dew point and condensation rates leads us to the following important lesson learned:

To see a real-life example, check out the graph below, which plots surface temperatures and dew points at Louisville, Kentucky on June 11, 2014.

I've highlighted a sharp drop in temperature (black line) that occurred in the middle of the afternoon, between 1500 and 1600 local time (between 3 P.M. and 4 P.M.). At 3 P.M. (15:00 on the graph), Louisville reported a temperature of 81 degrees Fahrenheit, but an hour later, the temperature was only 73 degrees Fahrenheit.

When you mix air with different properties (say, your breath, or car / airplane exhaust with the surrounding air), you will get air with different temperature and water vapor content. A mixture of warm, moist air from your breath, for example, with cooler, drier air surrounding you will yield a plume of air that is cooler with a lower concentration of water vapor than your breath, but is warmer with more water vapor than the surrounding environment.

If the new, mixed plume of air has enough water vapor so that condensation rates are greater than the evaporation rates associated with the temperature of the air mixture, a cloud will form!

All of water's phase changes actually either use energy from the surrounding air, or release energy to the surrounding air, as illustrated by the "energy staircase" diagram for ice, water, and water vapor below. Although the diagram includes all of water's possible phase changes, we're going to focus on the two of greatest interest to us for now -- evaporation and condensation.

If we start with liquid water, a few highly energetic, free-spirited water molecules can eventually break the bonds with surrounding molecules over time and escape to the vapor phase. Energy is required (600 calories per gram, to be exact) to break all the bonds to allow all the water to rather quickly evaporate and enter the gaseous phase of water vapor (the highest energy step), which cools the surrounding air.

The bottom line is that very low evaporation (and sublimation) rates associated with such low temperatures mean that contrails can have some staying power, as suggested by the multiple contrails shown in the photo on the right taken near Virginia Beach, Virginia in June, 2014. No planes were in sight at the time, but there were contrails galore from planes that had flown hours ago. Also, winds at the altitudes where planes fly are typically pretty speedy, so contrails can drift and spread out over hundreds of miles.

If you're interested in learning more about contrails, the NASA-GLOBE contrail page(link is external) is a good resource. If you search around on the Web, you may find some chatter alleging that contrails are part of some vast government conspiracy, but they're really just a result of the processes discussed in this lesson.

However, in the atmosphere, the most common way for net condensation to occur (especially for processes like cloud formation) is to cool the air. For example, in theory, clouds form when the air cools and the temperature drops to, and ever so slightly below, the dew point. Observations show that the relative humidity inside clouds is usually slightly greater than 100 percent (say, 100.2 percent as a representative value), which means the condensation rate slightly exceeds the evaporation rate.

In a cloud that forms from rapidly rising air, the rate of condensation exceeds the rate of evaporation because the rate of cooling is faster than the rate that water vapor is being removed from the air via condensation. In other words, the evaporation rate decreases more quickly than the condensation rate (which declines as liquid water drops grow and fewer water molecules are in the vapor phase), causing the condensation rate to exceed the evaporation rate (and resulting in a relative humidity slightly higher than 100 percent).

What we call "air" is really mostly empty space with tiny molecules flying around independently of each other. If we had a box filled with air, the "air" molecules (oxygen, nitrogen, carbon dioxide, etc.) would occupy a really tiny fraction of the space in the box, regardless of the temperature

In other words, no matter what the temperature is, there's always enough room for more water vapor molecules. So, the idea that colder air doesn't have enough room to hold more water vapor molecules is nonsense!

Now you can see why so little water in the earth-atmosphere system is located in the atmosphere as water vapor: water's time in the atmosphere is short before it returns to the earth. Most water exists in the oceans (or in ice sheets) because water molecules reside there for such long times before evaporating.

In reality, only a small fraction of water molecules are able to escape the "clutches" of the ocean to take a trip through the atmosphere as water vapor and eventually precipitation, but even this small fraction has a huge impact on weather! Clearly, water's phase changes play a key role in various aspects of the hydrologic cycle, especially evaporation (liquid to gas) and condensation (gas to liquid), so we need to explore these phase changes more in-depth.

og can form in other ways, too. For example, a moist air mass with higher concentrations of water vapor (and therefore higher condensation rates) can move over a chilly surface such as a snow pack (with low evaporation rates), leading to net condensation. You may have also seen fog forming over lakes or streams, especially on cool autumn mornings.

In such cases, the water is relatively warm, while the overlying air is cool, and such "steam fog" forms from the mixing of warm, moist air near the water surface and cooler, drier air just above. How can the mixing of air create net condensation?

Now what if someone brings in several tables surrounded by chairs into the dance party? Many of the tired dancers will immediately grab a seat around a table, changing their state from "dancers" to "sitters." In this example, each table serves as a site where dancers can easily change state, and they do so readily

In the atmosphere, aerosols (dust, dirt, particles of air pollution, salt, etc.) provide surfaces onto which water vapor can easily condense into liquid water drops. These particles are called condensation nuclei. Without condensation nuclei, relative humidity would have to reach 300 to 400 percent in order for cloud drops to grow and not readily evaporate.

What about the condensation rate? To explore the controllers of the condensation rate, let's perform a little experiment, starting with a closed, empty container filled with dry air (no water vapor molecules). Now, let's pour some water into the container and see what happens.

In time, the most energetic water molecules break the molecular bonds with their neighbors and evaporate into the space above the water, gradually increasing the number of water vapor molecules there. As time passes and as and more water molecules enter the vapor phase in the space above the water, some water vapor molecules condense back into liquid as they come in contact (by chance) with the interface between the liquid water and the air above.

So, if evaporation is a cooling process, what about its reverse -- condensation (the process by which water vapor changes to liquid)? When water vapor condenses back into water, there's a step down in energy levels, so if you're thinking that condensation is a warming process, you're correct!

Indeed, the energy used to evaporate water in the first place is never lost (a consequence of the conservation of energy), so as water vapor condenses into liquid water and bonds form between molecules, energy is released (600 calories per gram -- identical to the amount required for evaporation) to keep the energy books balanced. The release of this energy, called "latent heat of condensation," warms up the surrounding air.

Near the cold bottom half of the cup, water vapor molecules move more slowly and the rate of evaporation is reduced. When the air in contact with the cup cools enough so that the rate of evaporation is slightly less than the rate of condensation (net condensation occurs), liquid water drops form and grow.

Meanwhile, the top-half of the cup, and the thin layer of air immediately surrounding it, are warmer, leading to a higher rate of evaporation, and the rate of evaporation is greater than the rate of condensation. In other words, any microscopic water droplets that temporarily form on the top half of the cup evaporate almost immediately (because net evaporation is occurring), causing the outside of the top-half of the cup to remain dry.

At equilibrium, the temperature of the remaining water on the bottom of the container is lower than the temperature of the water that was present at the start of the experiment. That's because the most energetic water molecules evaporated, thereby lowering the average kinetic energy (in other words, the temperature) of the water left behind.

Moreover, the temperature of the remaining water equals the temperature of the "air" above the water. This state of equilibrium, where the condensation rate equals the evaporation rate, is depicted on the left below.

Of course, when dew points climb during the summer, the air can begin to feel very humid or "muggy."

Obviously, how something "feels" is somewhat subjective, but dew point can help you determine whether the air will feel uncomfortable. Recall the table below, which shows how the air feels to most humans, based on dew points.

For starters, the bonds that loosely connect water molecules in the liquid phase aren't all that strong, so occasionally, the natural vibration of water molecules breaks these bonds, resulting in evaporation.

Of course, as you know, the vibration of molecules depends on temperature: the higher the temperature, the faster the molecular vibrations, and the more likely a liquid water molecule will break free from its neighbors and evaporate into water vapor. So, that means water temperature is a major controller of the evaporation rate. Lower water temperatures yield lower evaporation rates, while higher water temperature yield higher evaporation rates.

The most obvious way to increase the dew point is to evaporate water into the air, and that explains why the highest dew points tend to be found during summer in warm, moist, maritime Tropical (mT) air masses. In the summer, mT air masses sometimes cause air with dew points above 70 degrees Fahrenheit to overtake much of the eastern United States.

On occasion in the United States (usually for short periods of time), dew points can even rise into the low 80s, but extremely rarely climb higher than that. But, the region of the world with the highest dew points is near the Persian Gulf in the Middle East, where dew points in the summer can exceed 90 degrees Fahrenheit on rare occasions. Such high dew points correspond to some of the highest water vapor concentrations on Earth!

Orographic lifting is a major "weather controller" in mountainous regions of the world, and in fact, some of the rainiest and snowiest places in the world are on the "windward" side of mountains (the side of the mountain the air ascends).

On the other hand, regions that experience persistent downsloping winds (the "leeward" or "lee" side of the mountains) tend to be quite dry, because clouds (and therefore precipitation) tend to evaporate as air flows down the mountain side and warms up.

As I mentioned before, once a water molecule evaporates into the atmosphere, it doesn't stay there for long. On average, it takes about 11 days for a molecule to evaporate into the air (or enter via transpiration or sublimation), condense into a cloud droplet, and fall back to earth as precipitation.

On the other hand, water tends to stay in liquid (or solid) form on earth for a much longer time. Liquid water molecules reside in the ocean for roughly 2,800 years before evaporating. A water molecule that ends up in a glacier may stay there for tens of thousands of years (but this happens to relatively few water molecules in the scheme of things).

Recall that on clear nights with calm or very light winds, net radiational cooling of the ground and conduction cause a significant chill to originate near the ground and spread upward slowly. Eventually, as evaporation rates decrease with declining temperatures, there may be enough water vapor present so that condensation rates exceed evaporation rates. If that's the case, then fog forms as net condensation causes liquid water drops to grow.

One striking example of such "ground fog" occurs in valley locations (often called "valley fog"). Cold dense air settles on the floor of the valley at night, and as the layer of cold air thickens, net condensation can cause dense fog throughout the valley. The image below shows an example of dense valley fog captured from above, by a webcam mounted on a communications tower on a mountain outside of State College, Pennsylvania (where Penn State's main campus is located) on the morning of October 6, 2016 just after dawn.

Remember, evaporation and condensation are occurring around you all the time, even if you can't see the results. Therefore, water molecules are impacting (condensing) and leaving (evaporating) all over the surface of the cup, but the rates of evaporation differ from the bottom half of the cup to the top half.

Recall that the cup is partially filled with cold water, which has made the bottom part of the cup relatively cold, and in turn, a thin layer of air surrounding the bottom half of the cup cools as well.

The bottom line that I want you to take away from these applications is that relative humidity is useful for assessing the difference between temperature and dew point, and for assessing how much cooling is needed for net condensation to occur (useful for predicting cloud and fog formation).

Relative humidity won't tell you how much water vapor is in the air, or give you an idea of how humid the air might feel by itself, but when you see clouds in the sky or fog near the ground, you're seeing the results of 100 percent relative humidity and net condensation!

lifting condensation level

Remember that the most common way for clouds to form is by cooling the air by lifting it until the temperature decreases to the dew point, which increases relative humidity to 100 percent, paving the way for net condensation to begin.

So, for all practical purposes, the temperature does not measurably fall below the dew point and we don't see relative humidity values greater than 100 percent reported. Since the dew point serves as a lower bound for temperature, on clear, calm nights when dew points won't change much, weather forecasters sometimes use the dew point as a guide for what the nighttime low temperature might be.

Remember that, by definition, the dew point is the approximate temperature to which the water vapor in the air must be cooled at constant pressure in order for it to condense into liquid water drops. Once the temperature falls to the dew point, relative humidity increases to 100 percent, and the measurable cooling ceases as long as dew points don't decrease further. Net condensation occurs onto condensation nuclei when there's a little extra cooling that we can't measure with a standard thermometer.

We can use these ideas to analyze what's going on in the photograph on the left, which shows something that you've probably observed before -- liquid water drops forming on the outside of a glass containing a cold beverage. This photograph shows a metal cup partially filled with cold water. The bottom half of the cup (approximately) is coated with a layer of small liquid water drops (often called "dew"), while the top half is not

So, should we believe that somehow the air near the bottom half of the cup can't "hold" any more water vapor, which caused liquid water droplets to form on the side of the glass, while the air just above can magically "hold" more water vapor (since no water drops had formed on the top part of the cup)? Absolutely not!

Ultimately, the LCL (or altitude of the cloud base) depends on surface relative humidity when air parcels are rising from the surface to form clouds. When relative humidity values are low, there's a large difference between temperatures and dew points, which means that a lot of cooling must occur before the temperature drops to the dew point (which requires lifting the air to higher altitudes).

So, the LCL will be high (cloud bases will be high) when surface relative humidity values are low. When surface relative humidity is high, the LCL will be lower (cloud bases will be at a lower altitude) because the difference between temperature and dew point is small, so not much cooling is required for the temperature to equal the dew point (not as much lifting is required).

To complete the recipe for making a cloud, we need to cool the air. As I mentioned in the last section, cooling the air is the most common way to achieve net condensation in the atmosphere. Cooling allows water vapor molecules to slow down and reduces evaporation rates. Furthermore, with water vapor molecules moving more slowly, more of them sluggishly huddle around condensation nuclei, paving the way for them to condense and form liquid cloud drops.

So, what's the most common way to cool the air until net condensation occurs? Lifting it! To understand how lifting the air causes it to cool, let's start with the understanding that air pressure is greatest near the earth's surface, and decreases with increasing altitude. The number of air molecules per unit volume decreases at higher altitudes, and this reduced air density goes along with a reduction in air pressure at higher altitudes.

I should also point out that sinking air is the enemy of clouds. Sinking "bubbles" of air encounter higher pressures at lower altitudes, which cause the bubble to compress and warm up.

So, while rising air is a common ingredient in cloud formation because it's associated with cooling, sinking air tends to evaporate clouds because it's associated with warming (and thus, higher evaporation rates and lower relative humidity).

On the other hand, if there's not enough water vapor present, and / or the temperature of the new mixture of air isn't low enough, no mixing cloud will form (which is why often, you don't see your breath, or car exhaust, etc.). Mixing clouds are usually short-lived, although contrails can last for hours or even days. The air is quite cold at the altitudes where airplanes fly, and airplane exhaust is chock full of water vapor and aerosols to serve as nucleation sites (as is your car exhaust).

Technically, given the very cold air at the altitudes where planes fly, contrails are mostly made of ice crystals, so the aerosols are really serving as "freezing nuclei" instead of condensation nuclei, but their general purpose is the same.

But, by and large, the water drops that comprise most clouds form and grow with relative humidity values just slightly greater than 100 percent (say, a few tenths of a percent greater).

That's why we see liquid water drops form in the atmosphere when the condensation rate slightly exceeds the evaporation rate associated with a given temperature.

The biggest supplier of water vapor to the atmosphere is evaporation (by far), with transpiration and sublimation making smaller contributions, and overall in the hydrologic cycle, the largest volumes of water are transferred between the earth and atmosphere via evaporation and precipitation.

The amount of water at or near the earth's surface stays relatively constant over short time-periods (say, a year), which means that average global precipitation must be roughly balanced by average global evaporation. So, over the course of a year, water that evaporates into the air as water vapor is balanced (approximately) by the water that falls back to earth as precipitation.

relative humidity

The amount of water vapor in the air at any given time is usually less than that required to saturate the air. Expressed as a percentage.

If we take our container in equilibrium and increase the temperature (depicted on the right above), what happens? The increase in water temperature causes the evaporation rate to increase and, for a time, net evaporation occurs. But, with increased evaporation, more water molecules exist in the air above the water, which in turn increases the condensation rate.

The condensation rate again increases until it equals the evaporation rate, and a new equilibrium is achieved (with greater evaporation rates and condensation rates than the original equilibrium, shown above on the right).

RH = condensation rate evaporation rate × 100%

The equation for relative humidity

You probably don't realize it, but evaporation and condensation are occurring around you simultaneously all the time! You just can't see the results because they're happening on the molecular level. Obvious phase changes occur when there's "net" condensation, meaning that the condensation rate exceeds the evaporation rate (liquid water droplets form), or if there's "net" evaporation (assuming you have some liquid water to start with), which means that the evaporation rate exceeds the condensation rate.

The evaporation of rain drops on their descent to the ground is a great example of net evaporation. Tiny raindrops end up shrinking or disappearing altogether as the rate of evaporation exceeds the rate of condensation.

Have you ever been taught that "warm air holds more water vapor than cold air," or perhaps heard it when reading or watching a story about weather? If you search around on the Web, you can find plenty of sites that explain processes like cloud formation with the idea that cold air can't hold as much water vapor as warm air.

The explanations usually go something like this: "air cools to the point where it can't hold any more water vapor, and liquid water drops form." But, don't believe everything you read on the Internet! This idea is scientific garbage, and it poorly describes what's really happening when net condensation causes liquid water droplets to form.

Moisture

The fact that moisture is required to make a cloud is no surprise, so I won't spend much time on it. By "moisture," I specifically mean water vapor. If we're going to make a cloud (which is made of water in various forms), obviously we need to have some water vapor present.

The bottom line is that the growth of liquid water droplets as "dew" on the side of your drinking cup, on blades of grass in the morning, or as cloud droplets (just as a few examples), depends on evaporation rates and condensation rates. Liquid water drops grow when net condensation occurs and not because the air just can't "hold" any more water vapor. Remember, there's always plenty of room in cold air for water vapor molecules.

The real issue is that as the temperature of the air decreases, water vapor molecules slow down and evaporation rates decrease making it possible for condensation rates to exceed evaporation rates (if enough cooling occurs). But, in order to achieve net condensation in the real atmosphere, we need another ingredient. We'll explore that on the next page, as well as discuss the overall recipe for making clouds.

In other words, in a world without a surface on which water vapor can condense, a lot of water vapor molecules need to hover near an embryonic water drop to locally boost the condensation rate and sustain net condensation, allowing the water drop to grow before it evaporates. That's because liquid water molecules have an easier time escaping (evaporating) from a tiny spherical drop than they do from a large, flat water surface.

Therefore, evaporation rates from tiny, spherical drops are much greater than evaporation rates from a flat surface of water at the same temperature. But, we never see relative humidity values in the atmosphere anywhere near 400 percent, and yet cloud drops still form and grow. So, there must be some "preferred sites" for water molecules to condense in the atmosphere.

As you know, the lifting of the air causes it to cool. When air sinks, the opposite occurs: it warms up. As winds encounter a mountain or hill and air flows up the slope, if the air cools enough, net condensation can occur and clouds can form. Some clouds that form via orographic lifting can be really spectacular, as shown in the YouTube clip below (50 sec., no narration), which shows an awesome time-lapse of clouds forming over Mount Rainier in Washington.

These particular clouds are called "lenticular clouds" because of their smooth lens-like shape, although not all clouds that form via orographic lifting are lenticular. Note in the video how the cloud forms as air blows up the mountainside and cools to the point of net condensation. Meanwhile, as air blows down the mountain on the other side, the cloud dissipates somewhat as air warms up and some cloud drops evaporate.

Water is essential for life on our planet, and it's a critical part of many weather processes. But, it can be easy to take water, and all of its quirks, for granted. I mention "quirks" of water because water, for lack of a better phrase, is a bit "weird." For example, water is one of the rare substances that can simultaneously exist in all three phases (solid, liquid, and gas) in the same place at the same time.

Think about it: when it's raining, you have liquid rain drops and cloud droplets as well as invisible water vapor (gas) in the atmosphere. But, did you also know that many clouds also contain frozen ice crystals? That means water exists in gas, liquid, and solid form in close proximity at the same time!

Regions like the eastern U.S. (and the Persian Gulf) are prone to high dew points with mT air masses because the waters of the Gulf of Mexico (and the Atlantic Ocean near the southeast U.S. coast) are very warm in the summer, which leads to high evaporation rates. The high evaporation rates from the Gulf of Mexico lead to high concentrations of water vapor in the atmosphere and high dew points.

This high dew-point air can then spread throughout the eastern U.S. by the wind. That's right, just as the wind can bring warmer or cooler air into a region (temperature advection), it can also bring moist or dry air into a region (moist advection or dry advection, respectively). On August 21, 2017, note the general wind flow off the Gulf of Mexico and off the warm Atlantic Ocean waters into the Southeast U.S., which over a period of days had helped usher moist air into much of the eastern U.S.

The states of net evaporation and net condensation are extremely important to weather forecasters, because they have implications for cloud and precipitation formation, as well as evaporation of precipitation (and subsequent evaporational cooling) among other things.

To better understand how net evaporation and net condensation are achieved, we need to understand a bit more about what controls the evaporation rate (the number of water molecules evaporating in a given area over a given time period) and the condensation rate (the number of water vapor molecules condensing into liquid water in a given area over a given time period).

I hope that by now, you understand that relative humidity, dew point, and temperature are all closely intertwined. After all, relative humidity depends on both dew point (which is connected to condensation rates) and temperature (which is connected to evaporation rates). As the temperature nears the dew point, the evaporation rate and condensation rate become increasingly similar, and relative humidity increases. On the other hand, if the difference between temperature and dew point grows, relative humidity decreases.

To see this relationship in action, watch the short video (2:44) below, in which I discuss temperature, dew point, and relative humidity trends in State College, Pennsylvania from 00Z October 6, 2016 through 00Z October 7. From the video, you should clearly see how relative humidity changes based on trends in temperature and dew point, as well as how the changes in relative humidity impact the observed weather.

Another practical application of relative humidity is that it gives us a basic idea of whether a little or a lot of cooling is needed for net condensation to occur. If relative humidity is high (near 100 percent) very little cooling is needed in order to achieve net condensation (there's a small difference between temperature and dew point). If relative humidity is low (say, less than 50 percent), then quite a bit of cooling is needed to achieve net condensation because a large difference between temperature and dew point exists.

To see what I mean, take a look at the two simplified station models below. The station model on the left has a temperature of 85 degrees Fahrenheit and a dew point of 50 degrees Fahrenheit. The station model on the right has a temperature of 40 degrees Fahrenheit and a dew point of 35 degrees Fahrenheit.

So, once dew points creep into the middle or upper 60s, most folks start to feel like the air is "muggy" or "sticky," and when dew points climb into the 70s, most folks find the air to be truly uncomfortable and stifling. By itself, dew point is a much more useful number to gauge human comfort than relative humidity (which depends on temperature, as you know). Also, because high dew points signal a high concentration of water vapor in the atmosphere, they may signal the potential for heavy rain and flooding from intense rainfall rates if showers and thunderstorms develop.

Weather forecasters always keep tabs on dew points because they're a critical part of making and communicating weather forecasts. Dew points are useful for everything from describing comfort levels to providing a piece of the puzzle in determining whether net condensation will occur. But, forecasters can't just concentrate on dew point when it comes to assessing the potential for net condensation and cloud formation; they have to be concerned with the evaporation rate (which depends on temperature), too. As you know, we have a variable that is useful for comparing the condensation rate and the evaporation rate--relative humidity. We'll wrap up our lesson next by discussing the best uses of relative humidity and some applications to everyday weather. Read on!

As you've learned, when more water vapor molecules are in the air, the likelihood that any water vapor molecule will condense onto a surface increases. So, more water vapor molecules in the atmosphere (higher dew points) mean higher condensation rates. When fewer water vapor molecules are in the atmosphere, dew points are lower, and the likelihood that any water vapor molecule will condense onto a surface decreases. So, lower dew points mean lower condensation rates.

What constitutes "high" and "low" dew points? At the surface of the earth, the lowest dew points tend to be found during winter, in bitterly cold, dry continental Arctic (cA) air masses. In cA air masses, dew points can be well below 0 degrees Fahrenheit. On rare occasions, dew points in cA air masses in the northern United States can drop to -50 degrees Fahrenheit or lower! Dew points in cA air masses are so low because low evaporation rates over the bitterly cold ice and snow-covered grounds of polar latitudes mean that few water vapor molecules enter the air.

You should have noticed in the video that when the relative humidity was 100 percent for an extended period of time, fog was reported. Did you notice, however, that rain was never reported? Sometimes, students assume that if they surface relative humidity is 100 percent it must be raining, but that's not necessarily true. When relative humidity is 100 percent at the surface, the temperature equals the dew point, and it's very likely that net condensation may be occurring around hygroscopic condensation nuclei suspended in the air. If net condensation occurs for a long enough period of time, the end result is essentially a cloud at (or very near) the ground. In other words, fog!

When it's raining, the relative humidity must be near 100 percent somewhere, and it is -- up in the clouds! That's where net condensation is occurring as tiny cloud drops grow. Larger rain drops that fall from the clouds actually develop from a variety of processes, but once the rain drops fall beneath the cloud, they're usually falling into an environment with relative humidity that's less than 100 percent. If the relative humidity near and above the surface is too low, most or all of the rain drops can evaporate before reaching the ground (remember that low relative humidity values indicate major net evaporation), paving the way for significant evaporational cooling, as we've already discussed. But, even when rain does reach the ground, usually some drops have evaporated partially or entirely along the way. In other words, for rain to reach the ground, relative humidity need not be 100 percent in the lowest part of the atmosphere; it just can't be too low, or else all of the rain drops will evaporate before reaching the ground.

Since relative humidity depends on both the condensation rate and the evaporation rate, it depends on both dew point and temperature. The larger the difference between dew point and temperature, the lower the relative humidity. The smaller the difference, the higher the relative humidity.

When the condensation rate equals the evaporation rate at equilibrium (the dew point equals the temperature), relative humidity is 100 percent.

There's no doubt that dew points can tell a meteorologist (or any weather-savvy person) quite a bit about moisture. But, it's probably not the most commonly cited moisture variable in weather reports. My guess is that you've heard weather forecasters mention relative humidity many times in weather broadcasts or weather-related articles.

While relative humidity is not an absolute measure of how much water vapor is present (it doesn't tell us about the concentration of water vapor in the air), it's still an extremely useful variable. Let's review a few important points you've already learned:

Why did the temperature fall during this hour? Evaporational cooling! It started to rain between 3 P.M. and 4 P.M., and during that hour, 0.13 inches of rain fell in Louisville, but as precipitation began, evaporation of rain drops cooled the air and temperatures decreased. Also note that dew points increased during the time highlighted in the graph above.

Why is that? Well, if liquid raindrops were evaporating into water vapor, that means more water vapor was present in the air, and as you may recall, higher concentrations of water vapor go along with higher dew points.

The presence of an existing surface is extremely important to achieving sustained net condensation. While I didn't explicitly say so at the time, our initial discussion about evaporation rates, condensation rates, and relative humidity was based on the existence of a flat surface of water.

Without a surface on which to condense, water vapor alone is very reluctant to condense onto itself -- so reluctant that relative humidity values near 400 percent are necessary for water molecules in the gas phase to cling to each other and grow into a detectable drop before evaporating.

While this valley is commonly known as "Happy Valley," it could have been called "foggy valley" on this particular morning, as the view from the ground on Penn State's campus around the same time as the webcam image shows. Visibility was very poor, and you can barely make out some light towers located only a couple hundred feet from the camera. The daily skycam time lapse focused on Penn State's Beaver Stadium(link is external) is worth a look, too.

You can see the fog develop overnight as the lowest part of the atmosphere cools to the point of net condensation. Then, through the morning, the fog gradually dissipates and Beaver Stadium emerges. Eventually clouds break for blue skies and a beautiful, sunny afternoon.

The warming that occurs with condensation is not easily noticeable to humans, but I bet you've noticed the impacts of evaporational cooling. When you get out of a swimming pool on a hot day, water drops on your skin begin to evaporate, which cools your skin.

You've also noticed evaporational cooling in action if you've ever felt a rush of cool air before a shower or thunderstorm arrives. Indeed, temperatures often decrease just before, and after rain arrives. That's because the smallest raindrops evaporate along their descent to the ground, which extracts energy from the surrounding air.

The "steam fog" that I just mentioned is a type of mixing cloud, meaning that it forms when warm, moist air mixes with cooler, drier air. Even if you haven't noticed steam fog over a lake or stream before, I bet you've noticed that sometimes on cold days, you can see your breath, as a fleeting cloud forms as you exhale.

You've also probably noticed that sometimes, "exhaust clouds" appear behind the tailpipes of automobiles, or contrails (short for condensation trails) form behind airplanes flying in the sky. These are all just mixing clouds.

hygroscopic

a term describing salts and other compounds that remove moisture from the air

orographic lifting

cloud formation that occurs when warm moist air is forced to rise up the side of a mountain

The condensation rate will continue to increase until it matches the evaporation rate

equlillibrum

The possible paths that water can take as it changes phases and gets transported between the earth and atmosphere make up the...

hydrologic cycle

Sublimation

the process by which ice changes directly to water vapor without becoming liquid first (you may have experienced this process if you've ever noticed that ice cubes sometimes "shrink" if left in your freezer for a long time).

Evaporation

the process by which liquid water changes to a gas (water vapor), as bonds between neighboring liquid water molecules break, and molecules escape to the air as water vapor.

Transpiration

the process by which plants release water vapor to the air (plants transport water from their roots to the leaves, where they "sweat" and the water evaporates into the air).

Condensation

the process by which water vapor changes to liquid (the reverse of evaporation).