Transcript Water in the Atmosphere

Water in the Atmosphere
Chapter 20
Forms of Water
› Water exists in the
atmosphere in 3
forms or phases:
1.
2.
3.
Gas called water vapor
Solid known as ice
Liquid
› Water changes from one
phase to the next
when heat energy is
absorbed or released
Some Vocabulary to Know
• Latent heat – heat energy that is absorbed
or released by a substance during a phase
change
• Evaporation – Most evaporation takes place
in the oceans; water vapor can also enter
the atmosphere by evaporation from lakes,
streams, ponds, and soil.
• Sublimation – process in which solid
changes directly into a gas
Humidity
• Water vapor in the atmosphere is known as
humidity
• Controlled by rates of condensation &
evaporation
• Rate of evaporation is determined by
temperature of air
• Higher temperature, higher evaporation rate
• Rate of condensation is determined by vapor
pressure
• Vapor pressure is part of the total
atmospheric pressure caused by water
vapor
Humidity Continued
• Dew point – Temperature at which the
condensation rate equals the evaporation
rate. Temperatures below dew point, water
droplets form
• Absolute humidity – mass of water vapor
contained in a given volume of air
Absolute humidity = mass of water vapor (grams)
Volume of air (cubic meters)
• Relative humidity – ratio of the actual water
vapor content of the air to the amount of
water vapor needed to reach saturation.
• Measure of how close the air is to reaching
dew point.
Measuring Humidity
• Relative humidity
can be measured
by: thin polymer
film, a
psychrometer, a
dew cell, and a hair
hygrometer.
Clouds & Fog
• Clouds are a collection of small water droplets or
ice crystals that fall slowly through the air.
Cloud Formation
Cooling Processes That Help
Form Clouds
• Four major processes that cause cooling for
clouds to form:
1. Adiabatic Cooling – temperature of an air
mass decreases as the air mass rises &
expands
2. Mixing – when 1 body of moist air mixes
with another body of moist air of a different
temp
3. Lifting – forced upward movement of air
results in cooling of air& cloud formation
4. Advective Cooling – temperature of an air
mass decreases as the air mass moves over
a cold surface
Classification of Clouds
CUMULUS CLOUDS
CHARACTERISTICS --• Puffy
• Vertical Growing Clouds
• Thick
• Looks like popcorn
• Form thunderstorms
• Cumulus – piled, heaped
STRATUS CLOUDS
CHARACTERISTICS -• Stratus - Sheet like
• Low clouds
• Layered
• Covers large areas
of the sky
• Blocks the sun
• “Covers” the earth
CIRRUS CLOUDS
CHARACTERISTICS -• Highest Clouds
• Wispy
• Feathery
• Cirrus – curly
• Made of ice crystals
CUMULONIMBUS CLOUDS
CHARACTERISTICS -• Thunderstorm Clouds
• Produce Hail,
Lightning, and
Thunder
• Anvil shaped
• Can reach into the
stratosphere
• Nimbo - Rain
Fog
• Result of condensation of water vapor in the
air.
• Near the surface of Earth
• 2 types:
– Radiation fog – results from the loss of heat by
radiation; forms from the nightly cooling of
Earth; layer of air in contact with the ground
becomes chilled below dew point;
– Advection fog – forms when warm, moist air
moves across a cold surface; forms mostly along
coasts.
Precipitation
• 4 forms of precipitation:
– Rain – liquid precipitation
– Snow – common solid precipitation
– Sleet – forms when rain falls through a
layer of freezing air near the ground
– Hail – solid precipitation in the form of
lumps of ice
Causes of Precipitation
• Coalescence – formation
of a large droplet by the
combination of smaller
droplets
• Supercooling – a
condition in which a
substance is cooled
below its freezing point,
condensation point, or
sublimation point
without going through a
change in state
Measuring Precipitation
• Meteorologists use a variety of instruments
– Rain gauge
– Doppler radar