Transcript Chapter 12 WEATHER

Chapter 12
WEATHER
•Section 1 – causes of weather
•Short term variation in atmospheric
conditions are called weather.
•Climate is the long-term average of
variations in weather for a particular
place. Climate takes into account the
average of the past 30 years.
•The constant movement of air on Earth’s
surface along with the oceanic currents,
redistribute thermal energy around the
Earth. Weather is part of the constant
redistribution of Earth’s thermal energy.
•An air mass is a large volume of air that
has the same characteristics as its source
region. The source region is the area
over which the air mass formed.
•There are 5 types of air masses.
1. ARCTIC – source
region above 60N
latitude, results
in cold, dry air.
2. Continental Polar – (continental means
its occurs over land) origins are Canada
and Alaska. Results in cold, dry weather.
3. Maritime Polar – (maritime means
ocean) develops over the Northern Pacific
or Atlantic. Results in cool to mild, humid
weather.
4. Continental Tropical – develops over
Southwestern U.S. and Mexico. Results in
hot, dry weather.
5. Maritime Tropical – develops over
tropical seas, results in warm, humid
weather.
•Air mass modification – when an air mass
travels over land or water that has
different characteristics from where it
developed, it can take on some of those
characteristics.
•When an air mass undergoes
modification it exchanges thermal energy
and /or moisture with the surface over
which it travels.
•Section 2 – Weather Systems
•The direction of Earth’s winds are
influenced by Earth’s rotation. This is
called the Coriolis effect and results in
fluids and objects moving in a curved
path rather than a straight path.
•Global wind systems help equalize the
thermal energy on Earth.
•There are 3 basic wind systems.
1. Polar Easterlies – begins as cold air
sinks and turns east, usually cold, weak
and sporadic.
2. Prevailing westerlies – moves west
toward the poles. Steady winds that
move most of the weather across the U.S.
3. Trade Winds – air sinks and moves east
toward the equator. The horse latitudes
are a band of weak winds resulting from
the sinking of the trade winds.
•The areas where the wind systems meet
influence weather because of how air
masses of different temp and pressure
interact.
•Jet Stream – a narrow band of fast
moving wind in the upper atmosphere.
•The speed of the jet stream can reach
250 miles per hour.
•Fronts – when two air masses collide
Cold fronts – when cold, dense air
displaces warm air which is less dense.
Heavy precipitation and storms are
associated with cold fronts.
•Warm front – when advancing warm air
displaces cold air.
•Stationary front – when 2 air masses
meet but neither advances.
•Occluded front – when a fast moving
cold air mass pushes a warm air mass up
because there is another cold air mass in
front of it. Heavy rains and storms can
occur.
•Pressure Systems
• Low Pressure systems – associated with
rising air being replaced by air rushing in to
take its place. Characterized by cloudy
weather and rain.
• Air spins counter clockwise.
•High pressure systems – sinking air pushes
surface air out of the way. They are
associated with fair weather.
•Air spins clockwise.
•Section 3 – Gathering Weather Data
•Thermometer – measures temperature
•Barometer – measures air pressure
•Anemometer – measures wind speed
•Hygrometer – measures humididty
•A radiosonde is used to gather
information from the upper atmosphere.
It is a package of sensors and a battery
powered radio transmitter suspended
from a balloon.
•Doppler radar – measures the speed at
which precipitation moves toward or
away from a radar station. Good at
detecting rotation associated with
tornados.
•Weather satellites –
•Use infrared imagery to detect
differences in thermal energy.
•Also used to measure water vapor that
help define and identify air currents.
•Section 4 – Weather Analysis and
Prediction
•A station model is a record of weather
data for a particular site at a particular
time.
•Lines of equal pressure are called isobars.
•Lines of equal temperature are called
isotherms.
•Types of forecasting
• Digital – created by applying physical
principles and mathematics to the
atmospheric variables and making
predictions about how they will change
over time. This is the main method used
today.
•Analog – based on comparison of current
weather patterns to similar patterns from
the past. It is called analog because
forecasters look for analogous patterns
between past and current conditions.
•How good are forecasts?
•A one to three day forecast is usually
accurate for expected temperatures and
for when and how much precipitation
can be expected.