Water in the Atmosphere & Weather
Humidity-Clouds-Precipitation / Air Masses-Fronts-Forecasts
Climate vs. Weather

Climate is an area’s long-term pattern of weather; An areas average temperature and
precipitation

Weather changes daily; The state of the atmosphere at a given time and place
Climate Controlled By: Latitude; Elevation; Vegetation; Prevailing winds; Nearby water; Ocean
Currents; Topography
Weather is driven by: Temperature and humidity differences between one place and another; Can be
hot or cold, wet or dry, calm or stormy, clear or cloudy
Characteristics of Water

Commonly exists in 3 states: solid, liquid, gas
Humidity

Specific humidity: actual amount of water vapor in the air at a given time & place (grams of vapor
per kilogram of air)

Warmer air can hold more water than cold air

Relative humidity: how close the air is to reaching its maximum capacity of water vapor
 expressed as a percentage
 100% means the air is saturated with water & it has reached maximum capacity

Measured with a psychrometer
 has two thermometers: dry-bulb and wet-bulb.
 The dry bulb thermometer measures the air temperature
 Air is passed through the psychrometer to evaporate moisture on the wet bulb.
 The drier the air, the greater the cooling from evaporation
 The readings are then compared to determine the relative humidity.
Dew Point


Condensation: when water changes from a gas to a liquid
Dew point: the temperature at which the air becomes saturated with water vapor & condensation
occurs
 If air cools below the dew point, water starts condensing into a liquid, forming dew or cloud
droplets
Cloud Formation
 Clouds (or fog) form only when there are condensation nuclei (like dust particles) for the water to
condense on




Air must cool below its dew point
When warm, wet air from the surface rises, it begins to cool. Eventually, the temperature drops to
its dew point, and the water vapor can condense onto the condensation nuclei
Condensation level: the atmospheric level at which condensation occurs
Clouds form in the troposphere – where temperatures decrease as altitude increases; Barrier at
the tropopause because temperatures stop decreasing, so the air will not continue to rise.
Cloud Types
 Classified by altitude and shape
Main Cloud Types

Stratus: clouds that form in layers

Cumulus: fluffy clouds with flat bases

Cirrus: high, feathery ice clouds

Nimbus: dark rainclouds
Notes: Water in the Atmosphere & Weather
Humidity, Clouds & Precipitation
Precipitation

Any form of water that falls from a cloud to Earth’s surface
Types of Precipitation

Rain



Snow
Sleet: raindrops that refroze on their way to the surface
Freezing Rain: raindrops that only freeze when they hit the surface

Hail: when frozen raindrops are bounced up & down in the cloud until they fall in a huge ball of ice
Types of Precipitation

depend on the temperature of the atmosphere, both at the surface & on the way down
Measuring Precipitation


Rain gauge: measures liquid precipitation
Measuring stick: measures frozen precipitation
Where is Precipitation?
 Equatorial low and subpolar lows (60oN and 60oS)

Rain Shadow effect: near a mountain range, the windward side gets lots of rain and the
leeward side gets little/no rain – the rain shadow