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Atmospheric Moisture Name_____________________________________Date___________________Period________ Atmospheric Moisture Background Whether in solid, liquid or gaseous form, water is the most important component of the atmosphere and essentially helps control all other aspects of weather. However, in order to truly understand the atmosphere, simply describing moisture is not enough. In meteorology, several different moisture measurements are used to determine the overall stability and characteristics of the air. All of these “moisture variables” can be broken down into two categories. The first are those that are solely dependent upon the physical amount of water contained in the air and are known as absolute measurements. Relative measurements are those variables that not only depend upon the amount of water present but also the temperature of the air. Vapor Pressure (e) and Saturation Vapor Pressure (es) The total atmospheric pressure at any time is actually the sum of many small pressures for each of the atmosphere’s components. For example, oxygen, nitrogen, etc. all exert their own individual pressures that are all added together to create the overall atmospheric pressure. Vapor pressure is the portion of total pressure contributed by water. This amount is very small which is why huge differences in water vapor content in the atmosphere yield only small fluctuations in the overall atmospheric pressure. This measure is also an absolute measurement since it is solely dependent upon the actual amount of water vapor in the air. However, there is a maximum amount of pressure that water vapor can exert in the atmosphere before the water is squeezed out into a liquid again and condenses. This limit is known as the saturation vapor pressure and is dependent upon both water content and temperature. Figure 1 illustrates this relationship between temperature and saturation vapor pressures. Notice how the water content of the air rises sharply as temperature increases. This is due to the fact that as heat energy is added, more water vapor will evaporate creating an imbalance between water trying to turn evaporate into a gas and water vapor trying to condense and return to a liquid. Therefore, the higher the heat, the higher the pressure exerted by the increased amount of vapor and therefore the higher the saturation vapor pressure. The notes Meteo. 3 also cover this aspect of the atmosphere. Figure 1: Saturation vapor pressures for water at various temperatures. Dew Point (Td) The dew point temperature is probably the most widely used moisture measurement in meteorology. It is defined as the temperature at which the moisture will condense when cooled at a constant pressure. Since warm air has a higher saturation vapor pressure than cold air, it would be expected that dew points, on average, would be higher in summer than winter. However, though the temperature of the air is a limiting value as to how high the dew point can go, the temperature of the air does not directly affect the dew point. In other words, the air temperature and the dew point temperature can change independently of each other. However, if the amount of moisture present in the air changes, then the dew point will change. The dew point, then, is a measure of the absolute amount of water vapor present in a given volume of air. Mixing Ratio (w) and Saturation Mixing Ratio (ws) Lets suppose it is possible to remove a small quantity of the atmosphere and weigh it on a scale. The mixing ratio of this air is defined as the mass of water vapor (Mv) in the sample divided by the mass of dry air (Md) in the sample: w = Mv/Md . Note that temperature does not appear in the equation at all. Again, however, there is a limit as to how much water vapor can be present in the volume of air. With mixing ratios, this limit is known as the saturation mixing ratio and is directly related to temperature. It is defined as the maximum amount of water vapor (in grams) that can be present in one kg. of dry air at any particular temperature. It is also defined to be the mixing ratio the air sample would have if it were saturated. Therefore, when the air is saturated, w = ws. These mixing ratios are usually expressed in parts per thousand. On average, 4g/kg is typical for dry air while 30-40g/kg are about the maximum value for very moist air. Relative Humidity (RH) The most common measurement of atmospheric moisture used by the general public is relative humidity. No new concepts need to be introduced here since RH is just the ratio of several of the variables already discussed. Relative humidity is defined as the amount of water vapor in the air divided by the maximum amount of vapor in the air at the current temperature. In other words, RH = w/ws x 100. Relative humidity can also be expressed in terms of pressure as the contribution made by water vapor to the total atmospheric pressure divided by the maximum pressure that water vapor can exert at the current temperature. (RH = e/es x 100) Precipitable Water (W) Precipitable water is the amount of liquid water that would result if all the water vapor in the air over a given area were to suddenly condense and fall. While not realistic, the value does have uses in some calculations and is often used in precipitation forecasts. Dew Point Depression (T - Td) and Wet Bulb Depression (T -Tw) The two final relative measures of the atmosphere’s moisture content are the dew point depression and wet bulb depression. For now, both measures are of the air’s closeness to saturation. That is, large depressions indicate that the air is relatively dry while small depressions indicate the air is moist. If T - Td or T - Tw are ever equal to zero, the air is totally saturated. To see how each measurement can be found, a sling psychrometer is used to measure the humidity of the air. This device consists of two thermometers (one of which is wrapped in a small piece of gauze) which are, in turn, attached to a handle that allows the thermometers to be swung around in circles. When measuring the humidity, the gauze of the one thermometer is wetted (therefore it is the “wet bulb”) while the other, the dry bulb, remains unchanged. As the instrument is swung in circles, the water in the gauze of the wet bulb will have a cooling effect on that thermometer and the temperature of the wet bulb will fall. The end result temperature is Tw. The dry bulb will most likely remain unchanged and is denoted as T. From these two values and some tables, the * relative humidity and dew point temperature can also be calculated. By using the equation e = es * - .35(T - Tw) the vapor pressure can also be calculated. es is the saturation vapor pressure of the wet bulb temperature. Name___________________________________Date_________________Period____________ Atmospheric Moisture Saturation Vapor Pressures for Various Temperatures* oF mb oF mb oF mb oF mb -20 .43 15 2..74 45 10.09 80 34.6 -15 .57 20 3.47 50 12.19 85 40.7 -10 .75 25 4.40 55 14.63 90 47.7 -5 .98 30 5.55 60 17.51 95 55.7 0 1.30 32 6.10 65 20.86 100 64.9 5 1.66 35 6.87 70 24.79 105 75.3 10 2.14 40 8.36 75 29.33 110 87.2 *Recall from earlier that the saturation vapor pressures of any substance are known quantities based on temperature. In this case, the vapor pressures are expressed in millibars, a common meteorological pressure unit. 1. Given: w = 7g/kg and ws = 14 g/kg Find: RH 2. Given: e = 8.36 mb and es = 17.51 mb Find: RH, T, Td o o 3. Given: T = 80 F and Tw = 70 F Find: RH, Td 4. Given: RH = 70% and es = 12.19 mb Find: Td 5. Given: RH = 70% and ws = 10g/kg Find: w 6. Given: e = 2.74 mb and es = 4.40 mb Find: RH, Td, T 7. Given: RH = 80% and e = 29.33 mb Find: T 8. When is it possible that Td will equal Tw? 9. Can Td ever exceed Tw? Explain..
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