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Describing weather
13.1
What is weather?
• Atmospheric conditions, along with short-term
changes, of a certain place at a certain time.
• Change can occur within a few hours or a few days
• Example: getting caught in a rainstorm on a
sunny day vs. sunny weather for a few days in a
row
Weather variables
variable: a quantity that can change
• Used to describe weather
• Air temperature
• Air pressure
• Rainfall
• Wind speed & direction
• Humidity
• Cloud coverage
• Precipitation
Air temperature
• The measure of the average kinetic energy of molecules in
the air
Kinetic energy: energy an object has due to it’s motion
• Temperature high: high kinetic energy,
molecules are moving fast
• Temperature low: low kinetic energy,
molecules are moving slow
Air pressure
• Force that a column of air applies on the air or a surface
below it
• As altitude increases, air pressure decreases
• As altitude decreases, air pressure increases
• Air pressure is measured with a barometer
and measured in millibars (mb)
Wind
• As air moves from areas of high pressure to low pressure, it
creates wind
• Wind direction is the direction from which the wind is
blowing
• Wind is measured using an
anemometer
Humidity
• Amount of water vapor in the air
• When the humidity is high, there is more water
vapor in the air
• Skin might feel sticky
• Sweat might not evaporate from your
skin as quickly
• Measured in grams of water per cubic
meter of air (g/m3)
Relative humidity
• Amount of water vapor present in the air compared to the
maximum amount of water vapor the air could contain at that
temperature
• When air is saturated, it contains as much water vapor as possible
• Temperature determines the maximum amount of water vapor air can
contain
• Air with a relative humidity of 100% any more moisture and dew or rain
will form
• Example: sponge absorbing water
Dew point
• Temperature at which air is saturated and condensation can
occur
• When the air near the ground becomes saturated, the water
vapor in the air will condense to a liquid
• If the temperature is above 0 degrees C! dew forms
• If the temperature is below 0 degrees C! ice crystals (or
frost) form
• Higher in the atmosphere,
clouds form
Clouds and fog
• Clouds are water droplets or ice crystals
suspended in the atmosphere
• When cooling air reaches its dew point, water
vapor condenses on small particles in the air
and forms droplets
• These droplets are surrounded by thousands
of others and become visible (as clouds) when
they block and reflect light.
• Clouds that form near the Earth’s surface are
called fog
• Fog is a suspension of water droplets or ice
crystals close to or at Earth’s surface
Precipitation
• Water, in liquid or solid form, that falls from the atmosphere
• Rain: precipitation that reaches earth’s surface as water
droplets
• Snow: precipitation that reaches earth’s surface as solid, frozen
crystals of water
• Sleet: may originate as snow (snow melts as it falls through a
layer of warm air and refreezes when it passes through a layer of
below-freezing air), or it is freezing rain
• Hail: reaches earth’s surface as large pellets of ice (starts off as
a small piece of ice that is repeatedly lifted and dropped by an
updraft within a cloud, a layer of ice is added with each lifting,
when it finally becomes too heavy for the updraft to lift, it falls
to earth)
The water cycle
• Series of natural processes by which water continually moves
among oceans, land, and the atmosphere
• water vapor enters the atmosphere when water at the ocean’s surface is
heated and evaporates
• water vapor cools as it rises in the atmosphere and condenses back into
a liquid
• eventually droplets of liquid and solid water form clouds
• clouds produce precipitation, which falls to earth’s surface, continuing
the cycle