Water in the Atmosphere I. Atmospheric Moisture Water exists on Earth in 3 forms: Liquid Solid (ice) Gas In our atmosphere, water exists mainly in its gaseous form: water vapor What is the principal source of water vapor in Earth’s atmosphere? The oceans! A. Humidity Humidity: the amount of water vapor in the atmosphere Saturated: when the air is holding all the water vapor it can at a given temperature As the air temperature increases, what happens to the amount of water vapor that volume of air can hold? Warmer air can hold more water vapor than cold air 1. Relative Humidity: ratio of the amount of water vapor in the air to the amount it can hold when saturated. Psychrometer: instrument used to measure relative humidity. Sling Electronic What does it mean to say the air is saturated? It cannot hold any more water! What happens when the air becomes saturated? Fill in the blank… The higher the relative humidity, the _______________ the chance that water vapor will condense into rain or snow. When a certain volume of air is saturated, what is its relative humidity? 100% As outside temperatures increase during the day, what happens to relative humidity? Hygrometer RH decreases (with increasing temperatures) If outside temperatures stay the same or decrease, what happens to relative humidity? RH increases (greater chance of precipitation) with decreasing temperatures 2. Specific Humidity The actual amount of moisture in the air. High Low B. Dew Point The temperature to which air must be cooled to reach saturation At any temperature lower than the dew point, water vapor begins to condense What happens during condensation? Water vapor changes to liquid water Dew: air contacts a cool surface and loses heat until it reaches saturation Frost: if dew point falls below freezing, water vapor changes directly to solid ice crystals, or frost II. Clouds Clouds are visible masses of liquid water droplets suspended in the atmosphere A. Cloud Formation Clouds form when water vapor condenses into liquid water droplets in the air In order for condensation to occur: 1. 2. air must be saturated (cooled to dew point) must have a solid surface to condense on (condensation nuclei) Condensation Nuclei: small particles in the air created by: – dust – volcanoes – factory smoke – forest fires – ocean salt Several processes may bring about the cooling necessary for clouds to form: 1. Convective Cooling Most clouds form this way Air temperatures decrease as air rises and expands Adiabatic Temperature Changes: temperature changes without the addition or removal of heat temperature changes due to rising or sinking air Warm air rises, expands and cools What happens to cool air? Cool air sinks, compresses and warms 2. Forceful Lifting Air cools as it is forced over a topographical feature (like a mountain range). 3. Temperature Changes Cold Air Warm Air Two masses of moist air with different temperatures mix 4. Advective Cooling Wind carries warm moist air over cold oceans or cold land The cold water or land absorbs heat from the air and the air cools B. Classifications of Clouds 1. Stratus Clouds low level clouds sheet-like or layered cover a large area Nimbostratus = stratus cloud with rain Altostratus = stratus formation at higher altitude 2. Cumulus Clouds puffy, piled, popcorn, or heaped form when warm moist air rises and cools flat base Cumulonimbus: cloud of great vertical development (“thunderhead”) middle altitude clouds 3. Cirrus Clouds cirrus means “curly” wispy, stringy high altitude clouds made up of ice crystals due to the low temperature and high altitude seen prior to a snowfall or rainfall III. Precipitation Any moisture that falls from the air to Earth’s surface May be liquid or solid Four main types: rain snow sleet hail 1. RAIN: forms when separate drops of water fall to the Earth from clouds 2. SNOW: forms when water vapor condenses directly into ice crystals 3. SLEET: a mixture of snow and rain; forms when rain passes through a cold layer of air and freezes into ice pellets 4. HAIL: balls or irregular lumps of ice (hailstones); usually form in cumulonimbus clouds Big Hail! Bad Hail! How does precipitation form? Clouds produce precipitation when its droplets or ice crystals become large enough to fall as rain or snow Coalescence: Droplets are carried by the updrafts and downdrafts in a cloud They collide and coalesce to form larger droplets. When the droplets become too large to be sustained on the air currents… they begin to fall as rain or snow.