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Weather and Climate
Weather
• The present state of the atmosphere
• Describes current conditions
• Caused by interactions of heat
energy, air pressure, winds, and
moisture
Heat Energy
• The sun’s energy that is absorbed
by the Earth is spread throughout
the atmosphere
Air Pressure
• The air pressure at any
particular point on Earth
depends on the density of the
air.
• Denser air exerts more air pressure
Winds
• Both local winds and global winds
are caused by differences in air
pressure due to unequal heating of
the atmosphere.
Moisture in the Air
• Water vapor, or moisture, in the air
is called humidity.
Humidity changes with
temperature
• Cooler temperature = less water
vapor = lower humidity
• Warmer temperature = more water
vapor = higher humidity
Relative Humidity
• The percent of moisture the air
holds relative to the amount it
could hold at a particular
temperature.
• Ex – at a certain temperature, 1kg of air can
hold 12g of water vapor. It is actually
holding 9g of moisture.What is the relative
humidity?
Relative Humidity
• 100% relative humidity means that
air is holding as much moisture as
it can hold and is saturated.
Dew Point
• The temperature at which air is
saturated and condenses
Clouds
• Clouds form when moisture in the
air condenses on small particles of
dust or other solids in the air
Cloud Classification
• Classified by shape and height
Cloud Shapes
• 3 main types of cloud
– Stratus
– Cumulus
– Cirrus
Stratus
• Smooth, gray clouds that cover the
whole sky and block the sun.
• Form at low altitudes
• Associated with light rain and drizzle
• When stratus clouds form close to the
ground the result is fog.
Cumulus
• Look like piles of cotton balls in
the sky
• Are fluffy white with flat bottoms
• Tower to great heights
• Usually indicate fair weather
Cirrus
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Feathery or fibrous clouds
Sometimes called mare’s tails
From at very high altitudes
Made of ice crystals
Can be seen in fair weather but often
indicate rain or snow is expected in
several hours.
Cloud Height
• Prefixes of cloud names describe
height of the cloud base
– Cirro = high clouds
– Alto = middle elevation
– Strato = low clouds
Shape names can also prefixes to
describe clouds
• Nimbostratus clouds bring rain and
snow
• Cumulonimbus clouds are dark at
the bottom and produce
thunderstorms.
Precipitation
• Water vapor that condenses and forms
clouds can fall to Earth as rain, sleet, snow,
or hail.
• Water that falls from the atmosphere to the
Earth is called precipitation.
Precipitation
• Air temperature determines the form
– Warm air = rain
– Cold air = snow
– Warm air then cold air = sleet
– Water droplets moving up and down in
the cloud before falling in warm air =hail