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Chapter 18 Water in the Atmosphere 18.1 Humidity and Condensation Water Characteristics: Found in all 3 states of matter o Below 0o – Ice, snow, hail o Between 0o-100o – rain, cloud droplets o Above 100o – water evaporates into water vapor When water changes state, energy is gained or lost o Vapor to liquid – condensation dew, fog, clouds releases heat o Liquid to vapor – evaporation Absorbs heat Cooling process- leaves cool surfaces behind o Vapor to Solid – deposition Frost Reverse is sublimation (solid to vapor) Humidity Specific humidity- that actual amount of water vapor in the air o Represented by grams of water vapor per kilogram of air Capacity – The total amount of water vapor air at a given temperature can hold. o The warmer the air is, the larger its capacity Saturation – When the air is holding the maximum amount of water vapor in relation to its temperature o Ex. If the air’s capacity for water vapor is 22g/kg, and it is holding 22g, then it is said to be saturated. Relative humidity – The percentage of saturation o How close to capacity is the air? o Measured by a %. Air that has no water vapor is at 0%. Air that is at capacity is 100% o Specific Humidity / Capacity x 100 = % Relative humidity o Ex. Air’s capacity is 22g/kg It’s holding 11g 11 g/kg 22 g/kg = 0.5 0.5 x 100% = 50% The Relative humidity is 50% Condensation o As temperature drops, so does its capacity to hold water vapor o Eventually, air cools to a point where its capacity has dropped to the level of its specific humidity o The temperature at which that happens is called the dew point. o In order for dew to form, the temperature must reach the dew point, and the vapor must condense on condensation nuclei, which are tiny particles of dust, soot etc. o If there are no nuclei present for water to condense on, the air is said to be supersaturated. Dew – water vapor condensing on a cool surface Frost – water vapor deposing on a surface that’s below freezing Fog – a cloud at ground level o Advection fog- warm, moist air flows over a cool surface, cools to its dew point and condenses o Radiation fog- ground loses heat rapidly through radiation, mixes with cool air, whole layer cools to its dew point and condenses. 18.2 Clouds Types of Clouds o Classified by altitude and shape o Stratiform clouds form horizontally Stratus/strato These are layered, low clouds o Cumuloform clouds form vertically Cumulus/cumulo These are fluffy with flat bases o Clouds above 7000 meters are classified as Cirro (Cirrus) clouds. These are high altitude and feathery o Clouds from 2000-7000 meters are classified as a type of Alto cloud o Any cloud that has nimbo/nimbus in its name produces precipitation. Some Examples: o Altostratus o Nimbostratus o Stratocumulus o Cirrocumulus o Cumulonimbus o Altocumulus o Stratus o Cirrus o Cumulus Cloud Formation o The shape of a cloud will show the type of air movement that created it o Heated air will rise and cool to its dew point. o The condensation level is the altitude at which the condensation occurs o Adiabatic Lapse Rates – the rate at which air cools as it rises Dry – dry air cools at a rate of 10 degrees Celsius for every kilometer Moist – moist air cools at a rate of 5-9 degrees Celsius for every kilometer Why is this important? Moist rising air that condenses releases heat. This heat can fuel very large and dangerous clouds. Cumulonimbus Clouds o Moist air rises, cools and condenses at the condensation level. o The base of the cloud is low, but the top of the cloud can grow to enormous heights. o Unstable air is air that rises rapidly, this can create these large clouds o Stable air is air that rises slowly, this produces smaller cumulus clouds. Layered clouds o Stratiform clouds form in stable air. o Air has trouble rising in stable conditions so it will spread out 18.3 Precipitation – any form of water that falls from a cloud to earth’s surface Rain – Falling water. Raindrop size varies. Ice crystals – form by deposition (see 18.1 notes) Sleet- rain that falls through cold air and freeze Freezing Rain – rain that falls and freezes when it hits the surface Hail – layered ice. Frozen raindrop is kept aloft by winds, collides with more raindrops, falls through cloud when heavy enough to overcome winds. Can be different sizes. Measuring Precipitation Rain Gauge On average, 1 foot of snow = 1 inch of rain Where does precipitation Occur? Rising and cooling air causes precipitation The higher air rises, the more moisture it can release So…. Areas where warm, moist air rises are the places where it is most likely to rain more often. o Near equator, heat from the sun makes air rise o Storms cause air to rise and cool o Air rises up the side of a mountain, cools and forms precipitation clouds. This is the windward side of the mountain, the leeward side gets no precipitation, but plenty of wind. These winds are dry. Chinook, Santa Ana Winds Weather Modification Sometimes, it does not rain where it is needed o Cloud seeding – putting particles into clouds to produce rain