Water, Atmospheric Moisture and Weather

Geography 101 – Physical Geography Study Guide. Water, Atmospheric Moisture and Weather (Chpts 7 and 8) Humidity and Saturation in liquid water, molecules move around (not all at same speed – temperature is a measure of average speed of molecules). At surface those with enough speed evaporate. Some water molecules in air will reach liquid and condense. When rate at which molecules evaporate and rate at which they condense are equal, air is saturated. If you heat liquid, a greater fraction have enough speed to escape – warmer water evaporates quicker. If you cool air, speed of vapor molecules decreases and molecules are more likely to stick to surface of water and condense. vapor pressure (vp) = pressure resulting from water vapor molecules in air (mb) saturation vapor pressure (svp) = amount of water vapor (measured as a vapor pressure) needed for saturation to occur (at this point the air can hold no more water and it will start to condense) Fig 7.12 relative humidity = vapor pressure / saturation vapor pressure (measured as %) dew point = temperature at which the water vapor in the air would be saturated (Fig 7.9) saturation vapor pressure depends on temperature. The colder it is, the lower the svp. If air is unsaturated there are 2 ways of bringing it to saturation: 1. cool it (keeping the amount of water vapor the same) 2. add water vapor to it (keeping the temperature the same) In 1. it would need to be cooled to the dew point. In 2. the vp would need to be increased to the svp. Low dew point = dry air High dew point = moist air If air’s wv content is constant then humidity is regulated by temperature changes (Fig 7.11): If temp. goes down, svp goes down and relative humidity increases if temp. goes up, svp goes up and relative humidity decreases latent heat – heat required to change the phase of a substance without changing its temperature latent heat of melting (heat required to melt water) latent heat of evaporation (heat required to evaporate water into vapor) latent heat of condensation (heat released by condensing water) – provides heat to air when water condenses in clouds. This heat causes air to rise and provides energy to drive thunderstorms. latent heat of freezing (heat released by freezing water into ice) Measurement of relative humidity using sling psychrometer (Fig 7.14) – uses wet-bulb and dry-bulb. Difference in temperature used to find relative humidity. Clouds Clouds form when water vapor condenses from vapor to liquid to form tiny droplets in the air. These cloud droplets grow by condensation of water vapor and by bumping into each other, colliding and joining (coalescing). In order to get clouds the air must be cooled to the dew point temperature. This normally happens when air is forced to rise. As it rises it expands. This expansion causes cooling (cloud in a bottle experiment). Compressing air causes it to heat (and the cloud evaporates). Processes that force air to rise (Fig 8.6, 8.8, 8.9, 8.10, 8.11, 8.13): convective lifting: local surface heating, warm air rises, expands and cools orographic lifting: horizontally moving air forced to rise over a mountain (topographic barrier) frontal wedging: formed when masses of warm and cold air meet. warmer, less dense air forced to rise over colder denser air. convergence of air: low pressure at surface (rising air over a surface low pressure), collision of oppositely-moving air masses (e.g. Florida peninsula) Adiabatic temperature changes (Rising and sinking air) adiabatic = without gain or loss of heat when air is compressed it heats up when air expands it cools when air rises it passes through regions of successively lower pressure, expands and cools. It cools at the dry (= unsaturated) adiabatic lapse rate (DAR) = 10°C/km. If air rises enough it will cool to the dew point and condensation can occur. This altitude is the lifting condensation level. Upon condensation water releases its latent heat, so as this heat is added to the atmosphere the air will not cool as much when it rises from here. It now cools as it rises at the moist adiabatic lapse rate (MAR). This depends on the actual amount of water vapor in the atmosphere, but is ~ 6°C/km. These adiabatic lapse rates are not the same as the actual (measured) temperature changes as you go up in the atmosphere. The actual temperature falls off at the environmental rate. Stability: If a rising parcel of air is warmer than the surrounding (environmental) air it will rise (“unstable atmosphere”) If a rising air parcel is colder than the surrounding (environmental) air it will sink (“stable atmosphere”) In a stable atmosphere it is difficult for air to rise, it must be pushed up. In this case it is not likely that clouds will form. If they do the clouds that form are thin flat clouds – stratus. In an unstable atmosphere air rises up easily and thick, buoyant clouds (cumulus) will form. Santa Ana conditions characterized by high winds, and very dry conditions. Also typically warm air. Clear skies. initiated by high pressure center over Great Basin (usually close to Salt Lake City) – forces air downslope from elevated desert plateau. If pressure is very high, winds will be strong (high pressure gradient). Winds are from NE due to flow around anticyclone (see NH wind direction around surface H). Air descends from ~ 1500m = 1.5 km. Warms by 15°C on its descent to Los Angeles. Causes its RH to be very low. Why? Severe Weather thunderstorms – highly unstable atmosphere, clouds with a lot of vertical development, latent heat release, hail formation (Fig 8.19 – 8.21) tornadoes – (Fig 8.23) waterspouts (tornado over water) tropical cyclones, hurricanes, typhoons