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GEOG. 1710 EARTH SCIENCE LECTURE 7. ATMOSPHERIC MOISTURE AND PRECIPITATION Moisture in the atmosphere can be in a solid, liquid or gaseous state. Harry Williams, Earth Science 1 Water enters the atmosphere as WATER VAPOR (gas), via the HYDROLOGICAL CYCLE. Water is turned into water vapor by EVAPORATION (from oceans, moist soil etc.) and TRANSPIRATION (from plants); together these processes are termed EVAPOTRANSPIRATION. The change from liquid to gas requires the absorption of heat energy - LATENT HEAT OF EVAPORATION. This is usually absorbed from insolation, surrounding soil etc. It follows that as temperature increases, evaporation also increases; Harry Williams, Earth Science 2 There is a limit to how much water vapor the air can hold (absolute humidity), which depends on its temperature. If air contains less water vapor than the maximum possible it is UNSATURATED; if it contains the maximum possible for its temperature, it is SATURATED. Harry Williams, Earth Science 3 Harry Williams, Earth Science 4 E.g. air containing 15 grams per kg at 300 C is unsaturated air containing 15 grams per kg at 200 C is saturated The air at 300 C can become saturated in two ways: 1. if more water vapor evaporates into it, until its water vapor content becomes 27 g per kg; 2. if its water vapor content remains at 15 g per kg and its temperature falls to 200 C. Harry Williams, Earth Science 5 This brings us to a common measure of the air's humidity RELATIVE HUMIDITY = ratio of actual water vapor content to the maximum possible amount that could exist if the air were saturated. E.g. the air containing 15 grams per kg at 300 C has a relative humidity of 15/27 x 100% = 56%. the air containing 15 grams per kg at 200 C has a relative humidity of 15/15 x 100% = 100%. In other words, for saturated air the relative humidity is 100%; for unsaturated air the relative humidity is < 100%. Harry Williams, Earth Science 6 Because the maximum amount of water vapor air can held decreases with falling temperature, relative humidity increases as temperature falls and decreases as temperature rises. E.g. as the air containing 15 grams per kg at 300 C cools, its relative humidity increases until finally it reaches 100% at 200 C. The temperature at which air becomes saturated as it cools is called the DEW POINT TEMPERATURE. Harry Williams, Earth Science 7 Harry Williams, Earth Science 8 What happens if air is cooled to or just below its dew point? -> Condensation; the air can not hold any more water vapor so some changes back into liquid water. This process is aided by condensation nuclei - particles (dust, smoke) in the atmosphere which act as collection centers for condensing water vapor. The small water droplets typically form cloud droplets (i.e. a cloud is a mass of liquid water droplets). When the water vapor changes back into liquid water, the LATENT HEAT OF EVAPORATION is released back into the surrounding air. Harry Williams, Earth Science 9 Harry Williams, Earth Science 10 Examples of Condensation Processes Harry Williams, Earth Science 11 1. Dew and frost: at night, ground cools by longwave radiation > air in contact with ground cooled -> saturation -> condensation on exposed surfaces; if temperature > 0 -> dew; if < 0 -> frost. 2. Fog: a. Radiation fog - on a still night, a whole layer of air above the ground is chilled -> fog. b. Advection fog - warm moist air moves over a colder surface > air cools, fog forms e.g. San Francisco. c. Upslope fog - warm moist air flows up over a mountain -> adiabatic cooling -> fog. d. Evaporation fog - moisture evaporates into air that becomes saturated and fog forms – common in swampy areas. e. Valley fog – cold air sinks into valleys/low spots -> fog. Harry Williams, Earth Science 12 Advection fog Evaporation fog Harry Williams, Earth Science 13 3. Cloud: as air rises through the atmosphere it undergoes adiabatic cooling > saturation -> cloud. Why does air rise? -> Harry Williams, Earth Science 14 a. convective uplift - warm sunny day, unequal heating. b. orographic uplift - note rain shadow effect; descending air does not produce clouds. c. frontal uplift - less dense warm air is forced to ride up over more dense cool air when they meet d. convergent uplift - air forced up by convergence from surrounding area. Harry Williams, Earth Science 15 Adiabatic processes Air warmer than surrounding air will rise (less dense). How far does air have to rise before it reaches its dew point and a cloud forms? This depends on: (i) the water vapor content of the air and (ii) the rate of adiabatic cooling. The rate of adiabatic cooling for unsaturated air is fairly constant at about 10o C per km - this is the DRY ADIABATIC RATE (DAR). The height at which the rising air reaches its dew point temperature and cloud begins to form is the LIFTING CONDENSATION LEVEL. Once the air becomes saturated it will continue to rise if it is still warmer than the surrounding air; however, it will now cool at a slower rate due to the release of the LATENT HEAT OF EVAPORATION during condensation - this rate, about 6o C per km, is the MOIST ADIABATIC RATE (MAR). The air will keep rising until it attains the same temperature as the surrounding air - the temperature of the surrounding air is called the ENVIRONMENTAL LAPSE RATE (this refers to the fact that air is normally colder with height – a typical value is about 6.4o C per 1,000 m, but this rate can vary by a few degrees from place to place and over time). Harry Williams, Earth Science 16 Harry Williams, Earth Science 17 Atmospheric Stability As mentioned previously, air warmer than its surroundings will rise because it is less dense or buoyant. Such air is said to be UNSTABLE, because it rises on its own accord. Unstable air occurs where the air's adiabatic lapse rate is less than the environmental lapse rate (i.e. the air remains warmer than the surrounding air as it rises). If the air's adiabatic lapse rate is more than the environmental lapse rate, it will be STABLE, because it will become colder than the surrounding air and stop rising. CONDITIONAL INSTABILITY occurs if the average lapse rate is between the dry and moist adiabatic lapse rates - the air will be stable if unsaturated and unstable if saturated. Harry Williams, Earth Science 18 unstable stable Harry Williams, Earth Science 19 Harry Williams, Earth Science 20 Harry Williams, Earth Science 21 Harry Williams, Earth Science 22 Instability is often apparent by the presence of clouds of pronounced vertical extent (indicating vigorous vertical air movement) e.g. cumulus type clouds. Stable air can also produce clouds if it is forced to rise (e.g. over a mountain) - in this case thin layer clouds are produced (stratiform clouds). Harry Williams, Earth Science 23 Precipitation Processes Why don't all clouds produce precipitation? - cloud droplets are so small that they are kept aloft by turbulence; the droplet must grow a lot bigger to fall to Earth. How does this occur? 1. Ice Crystal Growth: the tops of many clouds are below freezing. Supercooled cloud droplets and ice crystals can both be present. Since water vapor is more strongly attracted to ice than water, it is possible for water vapor to evaporate from the cloud droplets and condense onto the ice crystals at the same time. 2. Collision-Coalescence: As slightly larger droplets fall faster, or as droplets are swept around by turbulence, collisions occur and droplets stick together. Eventually, the drop grows large enough to fall. Harry Williams, Earth Science 24 Harry Williams, Earth Science 25 Cloud Classification: Based on shape and height. Shapes: 1. Cirriform - thin, wispy clouds composed of ice crystals; found only at high elevations. 2. Stratiform - thin sheets or layers, usually cover whole sky. 3. Cumuliform - puffy masses of cloud, non-continuous. Heights: A. High clouds - > 6000 m e.g. cirrus, cirrocumulus, cirrostratus B. Middle clouds – 2000 – 6000 m e.g. altocumulus, altostratus C. Low clouds - < 2000 m e.g. stratus, cumulus D. Towering clouds – reach from low to high – usually cumulonimbus The root "nimb" indicates that precipitation is falling from the cloud e.g. nimbostratus, cumulonimbus. Harry Williams, Earth Science 26 Harry Williams, Earth Science 27