Water in the Atmosphere

Essential Points 1.Water is part of the atmosphere 2.Partial pressure, vapor pressure and humidity 3.Condensation and Clouds 4.Why it Rains 5.Hail, Snow and other forms of water

Water in the Atmosphere Water vapor makes up 0-4% of the atmosphere Most variable component Residence time of a water molecule in the atmosphere = 10 days 1. Water is part of the atmosphere

Partial Pressure The pressure that would be exerted by each gas in the atmosphere if all the other gases were removed Proportional to the number of molecules present 2. Partial pressure, vapor pressure and humidity

Partial Pressure GasMolecular Weight Partial Pressure Nitrogen2878% Oxygen3221% Argon400.9% Water Vapor180-4% Carbon Dioxide ppm 2. Partial pressure, vapor pressure and humidity

Vapor Pressure 2. Partial pressure, vapor pressure and humidity

Vapor Pressure The pressure of gas necessary to keep a liquid or solid from evaporating The pressure of a substance in the atmosphere in equilibrium with a solid or liquid If partial pressure > vapor pressure, material condenses If partial pressure < vapor pressure, material evaporates 2. Partial pressure, vapor pressure and humidity

Vapor Pressure of Water 0 C32 F6 mb 10 C50 F12 mb 20 C68 F23 mb 30 C86 F42 mb 40 C104 F74 mb 50 C122 F123 mb 2. Partial pressure, vapor pressure and humidity

Vapor Pressure and Humidity The higher the vapor pressure, the more moisture the air can hold Vapor pressure increases with temperature If partial pressure (water vapor actually in air) stays constant, then – –Humidity decreases as it gets hotter –Humidity increases as it gets cooler Cooler temperatures can actually be less pleasant 2. Partial pressure, vapor pressure and humidity

Boiling Water When the vapor pressure of water = atmospheric pressure, water boils Since air pressure drops with altitude, so does the boiling point of water 2. Partial pressure, vapor pressure and humidity

Boiling Point of Water 0 km100 C212 F 5 km83C181 F 10 km73 C163 F 15 km55 C131 F 20 km40 C104 F 25 km26 C79 F 30 km14 C57 F 2. Partial pressure, vapor pressure and humidity

Partial Pressure and Vapor Pressure Partial pressure = Pressure water vapor actually exerts Vapor pressure = Pressure water vapor could exert if the atmosphere were saturated Most of the time the partial pressure is less than the vapor pressure 2. Partial pressure, vapor pressure and humidity

Adiabatic Expansion Adiabatic = No heat transfer Air warms or cools solely by expansion or compression Why? Expansion does work, energy comes from internal heat Dry air: 10 C/1000m (5.5 F/1000 feet) Moist air: 6 C/1000m (3.3 F/1000 ft) 3. Condensation and Clouds

Lapse Rate and Stability If air cools faster with altitude than 10C/km, it is unstable If air cools 6-10 C/km with altitude, it is stable for dry air but not moist air If air cools less than 6 C/km with altitude, it is stable If air is warmer at high altitude than at the surface, it is inverted and extremely stable 3. Condensation and Clouds

Inverted? If warm air rises, why is it an inversion to have warm air above cooler air? Because normally, heating at the surface keeps air at the surface warmer Thats why the troposphere is tropo Warm air above is more stable, but also less common 1. Water is part of the atmosphere

Cloud Formation Most cloud droplets require nuclei to form Most nuclei are hygroscopic (attract water) It is much easier to form water droplets than ice crystals 3. Condensation and Clouds

Why Dont Clouds Fall Down? Its a matter of Terminal Velocity – the faster things fall, the more air resistance they meet Skydiver – 50 m/sec Mouse – m/sec Cloud droplet – 1 cm/sec –The tiniest air motions keep such small particles suspended 3. Condensation and Clouds

Orographic Effects Orographic Lifting –Clouds –Precipitation Rain Shadows Katabatic Winds Chinook or Foehn Winds 3. Condensation and Clouds

Orographic Effects 3. Condensation and Clouds

Rain Forest, Washington 3. Condensation and Clouds

Rain Shadow, Nevada 3. Condensation and Clouds

Why Does it Rain? Cloud droplets are about 1/100 mm in diameter It takes over a million cloud droplets to make one raindrop Coalescence of cloud droplets? Too Slow 4. Why it Rains

The Bergeron Process Molecules are less tightly bound in liquids than in solids Therefore vapor pressure of water is greater than that of ice When ice and water droplets are present, water vapor can be saturated with respect to water droplets but supersaturated with respect to ice Therefore water can evaporate from droplets and freeze onto ice. 4. Why it Rains

Freezing Rain 4. Why it Rains

Hail Often associated with violent storms Typically 1 cm or less Can be 10 cm – capable of causing major damage Stones typically show concentric structure Probably form by repeated trips through supercooled regions 5. Hail, Snow and other forms of water

1. Water is part of the atmosphere

Record Hailstone, SD Water is part of the atmosphere

Fog Radiation: Surface cools at night by radiation Advection: Warm air carried over cool surface Upslope: Small-scale orographic clouds Steam: Warm water evaporates into cold air 5. Hail, Snow and other forms of water

Advection Fog, Labrador 5. Hail, Snow and other forms of water

Upslope Fog 1. Water is part of the atmosphere

Steam Fog 1. Water is part of the atmosphere

Frost 5. Hail, Snow and other forms of water

Ground Frost 5. Hail, Snow and other forms of water

Ice Shove 5. Hail, Snow and other forms of water

Candle Ice 5. Hail, Snow and other forms of water

Pancake Ice 5. Hail, Snow and other forms of water

Essential Points 1.Water is part of the atmosphere 2.Partial pressure, vapor pressure and humidity 3.Condensation and Clouds 4.Why it Rains 5.Hail, Snow and other forms of water