Transcript What is weather?

TOPIC 7
What is weather?
Weather is the state or condition of the
variables of the atmosphere at any given
location for a short period of time.
What are some weather variables?
• Variables include:
– Temperature
– Air Pressure
– Wind
– Moisture conditions
– Cloud cover
– Precipitation
– Storms
Where do most weather changes
occur?
Most weather changes occur in the
troposphere (lower layer of the atmosphere)
What creates the atmospheric
changes?
Atmospheric changes are created by the
uneven heating of the Earth’s surface (land vs.
water)
What is air temperature and how is it
measured?
• Air temperature is the measure of
average kinetic energy (Topic 5)
• It is measured with a thermometer.
The units or scales can be °C
(degrees Celsius), °F (degrees
Fahrenheit) or K (Kelvin). ESRT pg.
13.
• They are modeled on maps as
isotherms (line connecting equal
temperatures)
How is the atmosphere heated?
• The atmosphere is heated by:
– Direct absorption of insolation
– Direct absorption of long wave radiation (what the
Earth gives out)
– Condensation – change of water vapor to liquid,
releasing energy to form clouds, dew, fog and frost.
– Convection Currents (Cells) – air pressure differences
causes air to move.
– Expansion and Cooling of air –
• Air expands, its temperature decreases
• Air condenses, its temperature increases
What is atmospheric pressure? How is
it measured?
• Atmospheric pressure is also called air pressure.
It is the pressure due to the weight of overlying
atmosphere pushing down on any given area.
• It is measured with an instrument called a
barometer. Units can be millibars (mb), inches
of mercury (inches) or atmospheres (atm).
What is the standard air pressure at
sea level?
The standard air pressure at sea level is 1 atm
(atmosphere), 29.92 inches or 1013.2 mb
(millibars)
How does temperature effect air
pressure?
• As the temperature of air increases, the air
pressure will decrease. This occurs because
warm air will expand and decrease its density.
• Air pressure is shown on maps by the use of
isobars (lines connecting equal pressure
values)
• This is an indirect relationship
How does water vapor affect air
pressure?
• The greater the amount of water vapor (moisture content
or absolute humidity), the lower the air density and
pressure. This is an indirect relationship.
• Each water molecule in the atmosphere replaces another
molecule of air (usually oxygen or nitrogen). A water
molecule has a mass of 18g. Oxygen (o2) has a mass of 32g
(16g x 2 molecules). Nitrogen (N2 ) has a mass of 28g (14g x
2 molecules). When the mass gets smaller, the density will
change.
How does altitude (elevation) affect
air pressure?
• As the altitude increases, the pressure
decreases (ESRT pg. 14)
• This is an indirect
relationship.