Transcript Weather

Weather
Weather
Weather is the local, short term
atmospheric condition resulting from the
interaction of certain variables.
All weather occurs in the troposphere
The weather in the United States
generally moves from the southwest to
the northeast.
Atmospheric Variables
1.
2.
3.
4.
5.
There are five (5) constantly changing
factors that cause weather:
Temperature
Air Pressure
Wind
Moisture
Atmospheric Transparency
1. Temperature
The Earth’s surface heats up at
different rates due to duration and
intensity of insolation, land and water
differences and altitude.
Temperature is a measure of how cold
or hot something is.
2. Air Pressure
Caused by the weight of the air, which varies
due to the temperature, altitude and moisture
content.
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Warm air is less dense, therefore, it’s lighter.
The higher the altitude, the less air above,
therefore, less pressure.
Water vapor is lighter than dry air.
Air pressure is measured with a barometer.
At sea level, air pressure is 14.7 lbs/in2 (one
atmosphere) or 29.92 inches of mercury, or
1013.2 millibars.
3. Wind
Movement of air horizontal to the
Earth’s surface caused by differences in
air pressure.
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Wind blows from high pressure areas to
low pressure areas.
The greater the difference between the
air pressures of two areas, the higher
the wind speed.
4. Moisture
Water vapor enters the atmosphere by
a process called evapotranspiration.
Evapo ~ comes from evaporation (liquid
into gas)
 Transpiration ~ when plants release water
vapor into the atmosphere.
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5. Atmospheric Transparency
If the atmosphere was made out of
clear, colorless, gases, it would be
transparent. However, there are
aerosols in the air.
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The more aerosols, the less transparent;
resulting in an increase in insolation
absorbed or reflected by the atmosphere
and the more chance of clouds and rain.
Atmospheric Temperature
Isotherms ~ Temperature data shown on
a map by the use of isolines.
6 Factors determine temperature:
1. Amount of insolation
•
Generally, the more insolation at a
location, the warmer the temperature.
2. Amount of radiation
•
Terrestrial radiation is reradiated back
into the atmosphere and heats it.
3. Condensation
•
Change of water vapor into liquid water.
Releases a large amount of latent heat.
4. Sublimation
•
Change of water vapor into ice. Releases
a large amount of latent heat.
5. Frictional Drag:
•
Due to the Coriolis Effect, the interface
between the atmosphere and the Earth’s
surface produces frictional heat.
6. Wind
•
Heat is moved around by the wind.
Adiabatic Heating and Cooling
Due to molecular movement:
When gas expands, it’s temperature
decreases.
When gas is compressed, the
temperature increases.
This automatic change in temperature
of a gas due to expansion or
compression is Adiabatic temperature
change.
Atmospheric Moisture/
Energy of Evaporation
It takes 540 calories/gram to change liquid
water to water vapor (esrt)
When evapotranspiration occurs, the more
energetic water molecules leave first.
The molecules that are left behind are not as
energetic.
The result is a low temperature of an
evaporating liquid.
Factors that effect evaporation rate:
The higher the temperature, the faster
the rate of evaporation.
The greater the surface area of the
water, the faster the rate of evaporation.
The stronger the wind speed, the faster
the rate of evaporation.
If the air has a high moisture content
(saturated), it cannot absorb any more.
Saturation Vapor Pressure
Saturation: When air is holding the maximum
amount of water vapor it can.
Vapor Pressure: The pressure that water
vapor exerts.
Saturation Vapor Pressure: The pressure
that is exerted when the air is saturated.
Dynamic equilibrium is reached when
saturated vapor pressure is reached
(condensation equals evaporation)
Measuring Relative Humidity and
Dew Point Temperature
Sling Psychrometer: Contains an ordinary
thermometer (dry-bulb) and a thermometer
with a cloth around the bulb (wet-bulb).
When the cloth is moistened and the
psychrometer is whirled around, the wet-bulb
temp drops due to evaporation.
The amount of cooling depends on the rate of
evaporation.
Humidity
Humidity: The water vapor content in
the atmosphere.
Absolute Humidity: The weight of water
vapor in a given volume of air.
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Directly related to vapor pressure.
Air can hold more water vapor at higher
temperatures.
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Warmer temps = more humidity
Relative Humidity
Relative Humidity: The ratio of the
amount of water in the air to the
maximum amount that could be present
at that temperature.
High temperatures can hold more water.
RH is expressed as a percent.
When RH reaches 100%, the air is
saturated.
Dew Point
Dew Point Temperature: The temperatured
at which the air is saturated with moisture
(relative humidity is 100%)
A further drop in temperature would result in
condensation (rain) or sublimation (snow).
The reason is that colder air can not hold as
much water as warmer air.
Dew Point does not depend on relative
humidity; only on the amount of water vapor
in the air.
Air Masses and Fronts
Weather is caused by the interaction of Air
Masses.
Air Mass ~ Large body of air with similar
characteristics (pressure, moisture, and
temperature)
Air masses form when a large air mass sits
over a part of the Earth’s surface for a long
time.
The part of the Earth that an air mass forms
over is called a source region.
Air masses combine two
of five characteristics:
Temperature:
Arctic (A)
Polar (P)
Tropical (T)
Moisture:
Continental (c) – dry
Maritime (m) – wet
An air mass is described by its
source region . . .
Cold:
Arctic (A)
Polar (P)
Warm:
Tropical (T)
Continental:
Dry (c)
Maritime:
Wet (m)
Everything comes down to . . .
The interaction of weather variables: temperature,
air pressure, moisture content (relative humidity and
dew point), precipitation (rain, snow, hail, sleet, etc.)
wind speed/direction and cloud cover.
Air will either:
Or . . .
·Rise, expand, cool
Sink, compress,
and condense in a
heat and
low-pressure,
evaporate in a
convergent,
high-pressure,
counterclockwise
divergent,
system.
clockwise system