Atmospheric Moisture

Atmospheric Moisture - Atmospheric Moisture:1 Atmospheric...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Atmospheric Moisture:1 Atmospheric Moisture Evaporation & Condensation Humidity Cooling the Air Clouds Precipitation Stability Convectional Precipitation Orographic Precipitation Evaporation & Condensation We’ve mentioned evaporation and condensation before, but let’s dig a little deeper on the topics. Imagine water in a lake. The water molecules in the liquid are bouncing around, as molecules do. Those at the surface are bouncing and some of them bounce enough to escape the water and go into the air. This is evaporation. At the same time, molecules of water vapor are in air and some of them bounce and hit the liquid water surface and join up with the liquid. This is condensation. If there isn’t much water vapor in the air, then there will be more evaporation than condensation. At some point, though, there is no room for more water vapor in the air and then we say that the air is saturated. At saturation, the amount of evaporation is the same as condensation. In terms of systems, saturation is an equilibrium situation where inputs to the air (evaporation) equal outputs (condensation). The same concepts apply to sublimation, where ice turns directly to vapor and vice versa. Evaporation: evap > cond. liquid Condensation: evap < cond liquid Humidity The water vapor content of the air is called humidity. Humidity is described in several ways. Absolute humidity is the density of the water vapor, or the mass of water in a cubic meter of air. When air heats up it expands, so the absolute humidity decreases as temperature rises. (The same amount of water is in a larger volume, so density decreases.) Relative humidity relates the amount of Atmospheric Moisture:2 RH water vapor in the air to the maximum amount that can be in the air at a particular temperature: RH = (amt. in air/max. amt.) x 100; multiplying it by 100 makes it a percentage. Since the maximum amount increases with temperature and since the maximum amount is in the denominator, there is an inverse relationship between relative humidity and temperature. If we follow relative humidity over a typical day, we find that it usually is highest at dawn, when the temperature is at its minimum and then decreases until sometime in the mid afternoon, when the temperature is highest. Then it rises until dawn again. The absolute humidity may stay the same, but the relative humidity varies with the temperature. The dew point is another measure of the water content of the air. For a given amount of water vapor in the air, there is a temperature at which the relative humidity will be 100%. This temperature is the dew point. This is a bit confusing–just remember that dew point is a temperature, but it really is a measure of the amount of water vapor in the air. As the dew point approaches the air temperature, the relative humidity increases. When the two are the same, the air is saturated. Mid. 6AM Noon 6PM Mid. Air can become saturated either by adding water vapor though more evaporation or by cooling the air or by some combination of the two. When you take a shower, you are adding water vapor to the air in the bathroom and you may saturate it. As the shower continues, there is no more room for water vapor, so condensation increases and you fog up the mirror. If you have a cold glass of ice tea on a humid day, the air next to the glass is chilled, which may bring it to saturation. Then condensation occurs and water drops form on the outside of the glass. So, we are causing condensation by saturating the air–in the bathroom by adding moisture to the air and with the drink by cooling the air next to the glass. We’ll spend a fair amount of time looking at how nature does these same things and the resulting clouds and precipitation. Atmospheric Moisture:3 Cooling the Air Nature has several ways of cooling the air and causing saturation and condensation. One is to remove energy from the air in what we call diabatic processes. Dew is like the water on our glass in that the air near the surface is cooled to the dew point, generally at night. Dew will not form on windy nights because the air is mixed up and no air is in contact with the surface long enough for the condensation to occur. If the dew point is below freezing, then frost will appear, instead of dew. Fog is essentially a cloud found at the ground surface. Diabatic processes cause most fog. Radiation fog, for example, appears when the air at night gives off enough longwave radiation to cool to the dew point. Advection fog occurs when warm, moist air moves over a cooler surface. This is common in coastal areas where warmer, moister air over the water moves over the land during the night. Clouds generally form not by the removal of energy, but by changing the volume of the air. The term adiabatic (‘not diabatic’) refers to such processes. If the same amount of energy is spread out over a greater volume, then the temperature will drop. And as temperature changes, so does the relative humidity. Adiabatic lapse rates describe the changes to the air temperature and relative humidity. Recall that air temperature decreases with altitude at about 6.5°C per 1000 meters and we call this the environmental lapse rate. (This rate of 6.5°C per 1000 meters is just an average–the actual rate can vary, as we’ll see later.) Here we are only talking about the air above being cooler than the air below. The cause of the ELR is the reduction in pressure with altitude. Adiabatic lapse rates are different in that the air itself is being moved to a new location. Imagine a balloon with air in it at ground level. If we take that balloon higher up, the air around it will have lower pressure, so the balloon will expand. The same amount of air will fill a bigger volume and the temperature inside the balloon will decrease. Now imagine a cubic meter of air at the surface. If we push it up, it expands because of lower pressure. Now, that same amount of air is filling up more than a cubic meter and the temperature decreases. If we take air and push it up, it will cool at about 10°C per 1000 meters. This is called the dry adiabatic lapse rate and it is different than the environmental lapse rate because the air itself is moving. As the air continues to rise and cool, the relative humidity steadily increases. Eventually, it may become saturated (reach 100% RH). As it continues to rise, condensation occurs and clouds form. The air continues to rise and cool, but it now cools at a different rate, called the wet adiabatic lapse rate, at 6°C per 1000 meters (actually it varies, but 6°C is a reasonable average and we will use that value). The air is still rising and cooling as with the dry rate, but now condensation is happening and latent heat is being converted to sensible heat, partially offsetting the decrease. We’ll get back to these lapse rates shortly. Atmospheric Moisture:4 Clouds We have been talking about condensation in the air and this forms clouds. What has to happen is that relative humidity has to reach 100%, but that alone is not enough. Water has to condense on something, so tiny particles have to be present and the water condenses on these. These condensation nuclei are dust, soot and salts floating around in the air. If no nuclei are present, the air can become supersaturated, actually having more than 100% relative humidity. The idea behind cloud seeding is that if you add condensation nuclei to supersaturated air, then water drops will form and more rain will fall. It’s not clear how successful this is in practice, but cloud seeding is being done here in West Texas and elsewhere. Clouds can be made of water drops or ice particles. Water drops can remain liquid in clouds well below 0°C, but after about –12°C, they turn to ice. The reason for this is that ice cannot form without ice nuclei, and these only form in the atmosphere at –12°C and colder. Ice nuclei are tiny six-sided objects that are the right shape for ice crystals to develop on them. Snowflakes, with their sixsides, are an expression of this hexagonal crystal form of ice. Cloud types are based on location and shape. In terms of shape, they can be cirriform, or wispy clouds, stratiform, or layered clouds, or cumuliform, or clouds formed by rising air. High clouds are made of ice, not water, and are usually found above 6000 meters. These are mostly cirrus, which are thin and wispy cirriiform clouds. Cirrus clouds have very little water in them and do not produce precipitation. Middle clouds are mostly between 2000 and 6000 meters and here it is warm enough for liquid water. Stratus clouds are often in thick layers that can cover the whole sky. Altostratus are found at these heights. Stratus can also be low clouds, found under 2000 m. At any height, you may find cumuliform clouds, formed by rising air. Cumulus clouds are the puffy, cotton-ball types. If conditions are right, these generate thunderstorms. Nimbus is the term associated with rain, so cumulonimbus clouds are cumulus clouds producing rain and nimbostratus are stratus clouds producing rain. Precipitation Most water drops in clouds are small enough that they stay suspended in the air. They have to grow a lot in order to be heavy enough to fall. Drops may run into each other and coalesce into larger drops and this dominates rainfall in warm clouds of the Tropics and sometimes in the mid-latitudes. In colder clouds, though, snow forms by ice crystals gaining ice from the water vapor in the air Atmospheric Moisture:5 and by colliding with supersaturated water drops, then the water freezes onto the ice. As the snowflakes grow larger, they fall. If the air temperature near the ground is above freezing, the snow will turn to rain. If it is below freezing, it will remain snow. Warmer clouds will tend to produce denser snow (wet snow good for snowballs) while colder ones produce less dense ‘powder’ (good for skiing). Hail forms when ice crystals fall and may start to melt, but then are caught in an updraft and re-freeze higher up. While falling, they come in contact with more liquid water that becomes attached and freezes. Then they fall again and they can go through this process many times; each time they add a layer of ice. Most hailstones are relatively small, but they can be large, even softball size. Big ones can cause considerable damage. Sleet forms when rain falls though a cold layer of air and refreezes before hitting the ground. Freezing rain is liquid when it hits, but then quickly turns to ice. Roads can be covered in ice and tree branches can break under the extra weight. Precipitation is the term we use for all forms of water falling to the ground. Atmospheric Stability Precipitation only occurs when the air rises and we are going to look at several different ways to get air to rise. But first let’s look at the concept of atmospheric stability. This refers to how easily air will rise. If air is stable, it can be pushed up, but then will drop back down. Unstable air will keep rising. A parcel of air warmer than the air around it will rise because it is less dense. It will stop rising and cooling when it is the same temperature as the air around it. This is stable air. Unstable air will always be warmer than the air around it and will keep rising. The lapse rates we talked about earlier come into play here. Let’s say that we have an environmental lapse rate of -12°C per 1000 m (recall that the value I gave before of 6.5°C was just an average–it varies.). Take a parcel of air on the ground at 30°C and push it up. It cools at the dry adiabatic rate of -10°C per 1000 m. So, at 1000 m, it will be 20°C. The surrounding air will have cooled at 12°C in that 1000 meters, so it will be 18°. The parcel is still warmer and it continues to rise. At 2000 meters, the parcel is 10° while the surrounding air is 6°. There is nothing to stop that parcel from continuing to rise, so it is unstable. Now let’s say that the ELR is 6.5°C per 1000 meters. Push up that same parcel of air and it will never be warmer than the surrounding air, so it will drop back down. This is stable air. When the ELR is higher than the DALR, the air is unstable. When ELR is lower than DALR, it is stable, most of the time at least. Atmospheric Moisture:6 Unstable 2000 m 10° 6° 1000 m 20° 18° Stable ground 30°C Parcel 30°C Surrounding Air ELR = -12°C/1000 m 20° 23.5° 30°C Parcel 30°C Surrounding Air ELR = -6.5°C/1000 m Convectional Precipitation But what if the air parcel starts out warmer than the surrounding air? Let’s look at a situation where some of the air is heated more than the air around it. Imaging a parking lot surrounded by grass. The air over the parking lot will be heated more and start to rise. So now our parcel starts out at 35°C and the surrounding air is 30°, with an ELR of 5°C per 1000 meters. At 500 meters, the parcel will be 30° and the surrounding air will be 27.5°. At 1000 meters, the parcel will be 25° and so will the surrounding air, so the parcel will stop rising. This is a stable situation. But what if the dew point for that air is 26°? The parcel will reach 26° at 900 meters, before it met the temperature of the surrounding air. Now, the parcel is rising at the WALR, at 6°C per 1000 meters, so it will continue to rise because it will always be warmer than the surrounding air. This is an unstable situation and it will result in rain. Convectional precipitation is caused by differential heating of the ground surface. If there is enough rising and enough moisture available, the rising air results in rain. Some of our summer rain is caused by this situation. Atmospheric Moisture:7 Stable Unstable Altitude, m WALR 1000 DALR ELR 0 25 30 35 Temperature, °C DALR ELR 25 30 35 Temperature, °C Orographic Precipitation Another way of getting the air to rise is to push it over mountains. This is called orographic precipitation. Imagine the Sierra Nevada in California. Relatively moist air blows off the Pacific Ocean, carried by the Westerlies and then they run into the base of the Sierra at 500 meters above sea level, where the air in our hypothetical example is 30°C. The air will be pushed up by the wind and cool at the DALR. Let’s say that dew point is reached at 2000 m. When that air reaches 2000 m, it is 15° and as it continues to rise it cools at the WALR. At the crest of the mountains, at 3000 m, it is now 9°. Now the air descends the other side of the range. Because it is going down, it is warming and descending air always changes at the DALR. When it reaches the valley floor on the other side, it has warmed to 24°C. So, on the windward side, the air cools at the DARL until it reaches the dew point, then switches to the WALR. After it passes the crest, it goes down the other side (the lee side), warming at the DALR. Because of this pattern, we find that the windward side tends to get lots of rain and the lee side gets much less. Downwind of such mountain ranges, it tends to be dry because the air has lost much of its moisture crossing the mountains. The term for this is the rainshadow effect, and it explains why we are pretty dry here on the downwind side of the Rocky Mountains. Atmospheric Moisture:8 (-6°C/1000 m) (1000 m) = -6°C 9° 3000 m (10°C/1000 m) (1500 m) = 15°C 15°C - 6°C = 9°C 9°C + 15°C = 24°C WALR DALR 15° (-10°C/1000 m) (1500 m) = -15°C 30°C - 15°C = 15°C DALR 30° 500 m 2000 m 24° 1500 m ...
View Full Document

Ask a homework question - tutors are online