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Weather
What is weather?
The condition of the air (or
atmosphere) at a given location at
a give time
Climate
“Typical” weather in region; the average weather or
the regular variations in weather in a region over
a period of years
• includes temperature, air pressure, humidity,
days of sunshine, etc...
• drastically affected by water & mountains
– coastlines have cooler summers and warmer winters
– mountains greatly slow down winds and weather
Atmosphere
Blanket of air surrounding the earth
Air – the mixture of gases, mainly nitrogen
and oxygen, that forms the Earth’s
atmosphere
•
78% Nitrogen
•
21% Oxygen
•
1% Argon
•
1% Other stuff : Carbon dioxide, water
vapor, ozone, dust, ash, smoke, etc.
Structure of
the
Atmosphere
Lower Layers
Troposphere – the layer of the atmosphere that is
closest to the earth’s surface, where all weather
phenomenon occurs.
8 km thick at the poles
16 km thick at the equator;
(air temperature normally decreases with height)
Stratosphere - upper layer of air 8 - 16 km to
about 50 km and top of the stratosphere
contains the ozone layer; almost completely free
of clouds.
Middle and Upper layers
Mesosphere – the layer of air, 50km to 80km
where temperature begins to decrease with
height, mainly because ozone in the
stratosphere absorbs energy from the sun,
principally ultraviolet radiation
Thermosphere - where air temperatures can
exceed 1000° C (1800° F), 80km to 9600 km,
primarily due to oxygen absorbing the sun’s
energetic rays
Upper-Upper layers
Ionosphere – the region within the
atmosphere containing ionized molecules,
layers of ionized air in the atmosphere
extending from almost 60 km above the
surface of the earth to altitudes of 1000 km
and more.
Exosphere - The region beyond the
thermosphere, which extends to about
9,600 km, the outer limit of the
atmosphere.
Conditions of the Air
Temperature – amount of hotness or coldness
relative to something else
•Thermometer – an instrument that measures
relative hotness or coldness
•Dew Point temperature – The temperature at
which air becomes saturated
•Temperature scales:
1°C = 1.8°F or
1°F = 5/9 ° C
Heat transfer
• Conduction – the movement of heat from
molecule to molecule
• Convection – the movement of heat by
warm and cold currents
• Radiation – the release and transfer of
energy in wavelengths of heat and light
How is the atmosphere is heated?
Pressure
Air pressure - the downward pressure exerted by
the weight of the overlying atmosphere or the
“weight” of the atmosphere per unit AREA.
• Barometer – an instrument used to measure air
pressure
Measured in inches of
mercury in a column
• Or millibars (metric
conversion)
• Average air pressure
at sea level is 1013 millibars
Water in the atmosphere
Water Vapor
• Humidity – the amount of water vapor in
the air
• Relative humidity – the actual amount of
water vapor in the air compared to the
greatest amount the air can hold
Water in the atmosphere
Water Vapor
• Saturated – to be completely filled with
water vapor
• Psychrometer – an instrument to measure
relative humidity
• Hygrometer – an instrument used to
measure the air’s humidity
Precipitation
water or ice that condenses in the air and falls
to the ground as:
• Rain- liquid water that falls to the ground
• Snow - ice crystal flakes; water vapor in the
atmosphere that froze into ice crystals and falls
to the ground in the form of flakes
• Sleet -partially melted grains of ice
• Hail - pellets made of layers of ice and snow
• Freezing rain –rain that freezes into ice as it hits
the ground
Evaporation
the process of changing
from liquid to gas;
molecules in a liquid
state GAIN energy to
change into a gaseous
state
(latent heat energy is the
stored in molecules
through evaporation)
Evaporation
DRIVES
the water
cycle
Condensation
• the process of changing from gas to liquid;
molecules in a gaseous state that LOOSE
energy to change into a liquid state.
Clouds
• a visible mass of dense water vapor or ice
suspended in the atmosphere; formed by
surface heating & convection and/or warm
air lifted by rising over mountains
Wind
air moving at a speed fast enough to be
noticed; high pressure moves to low
pressure
•
Weather Vane – an instrument that
measures wind direction
•
Anemometer – an instrument that
measures wind speed
Air Mass
A large region of air with similar properties
throughout (ie. temperature, humidity)
Major Air Masses
•
•
•
•
•
cA – Continental Artic – dry, very cold
cP – Continental Polar – dry and cold
cT – Continental Tropical – dry and warm
mP – Maritime Polar – moist and cold
mT – Maritime Tropical – moist and warm
Synoptic Map
Station models – group of symbols
depicting weather conditions
Isobar – line of equal pressure
a. show locations of High or Low
pressures
b. close lines mean strong winds
c. lines far apart mean gentle winds
Station Model
how meteorologists
can put a lot of
information in a
small area
Water in the atmosphere
Water Vapor
• Humidity – the amount of water vapor in
the air
• Relative humidity – the actual amount of
water vapor in the air compared to the
greatest amount the air can hold
Weather Advisories
Weather WATCH – predictions about
approaching severe weather
Weather WARNING – specific severe
weather conditions have been actually
observed by a person or verified by a
computer