Download Moisture in the Air

Moisture in the Air
Key Terms
• Evaporation – the process by which water
molecules escape into the air.
• Humidity – water vapor or moisture, in the
air.
• Relative humidity – the percent of moisture
the air holds relative to the amount it could
hold at a certain temperature.
Measuring Relative Humidity
• Psychrometer – device used to measure
relative humidity.
– Consists of 2 thermometers.
– One is covered with a moist cloth.
– When air passes over wet cloth, water
evaporates and temperature cools.
Measuring Relative Humidity
• Differences in temperature between 2 bulbs
is used to determine relative humidity
• Compare temp difference (degrees
difference) to dry thermometer temp
– Look up relative humidity in chart
Clouds
• Form when moisture
in the air condenses on
small particles of dust
or other solids in the
air.
• Mixture in which
particles of water are
suspended in air.
Cloud Formation
• Warm air can hold more moisture than cold air.
• As air rises it cools.
• Air becomes saturated with water.
– Holds maximum amount of water.
• Water vapor in air condenses  clouds form.
– Dew point – the temp. at which water vapor
condenses.
Cumulus clouds
• “Fluffy” clouds
• Flat bottom
• Usually indicate fair
weather
– Can produce
thunderstorms
Stratus Clouds
• Smooth, gray
• Cover the entire sky
• Associated with light
rain and drizzle
• When close to ground
 fog
Cirrus Clouds
•Thin and feathery, made of ice crystals
•Can be seen in fair weather
•May indicate that rain or snow will fall in several hours.
•A.k.a. – mares’ tails
Special Types of Clouds
• Cirrostratus -high-level clouds composed of ice
crystals
– can cover the entire sky
– relatively transparent, as the sun or the moon can easily
be seen through them
• Cumulonimbus – thunderclouds
• Nimbostratus - dark, low-level clouds
accompanied by light to moderately falling
precipitation
Precipitation
• Water vapor that condenses and falls to
Earth
– Ex. – rain, sleet, snow
• Formation:
– Cloud droplets increase in size by colliding and
combining with other droplets
– Droplets become to heavy, and are pulled down
by gravity
Snow
• Forms when water
vapor turns directly
into a solid
• 6-sided flakes (ice
crystals)
Hail
• Forms in
cumulonimbus clouds
• Small chunks of ice
• Formed when water
droplets hit ice pellets
in clouds and freeze
• Can be held in clouds
by winds and grow in
size before falling
Measuring Rainfall
• Rain gauge
– Straight sided
container with flat
bottom
– Collects rain over a
given period of time
– Usually measure in
mm or cm