Download Ch 24 Weather

Chapter 24 Weather
• The state of the atmosphere at any given time or place.
• Differences in air pressure are caused by unequal heating
of Earth’s surface. The region along the equator receives
more solar energy than the poles. This creates regions
of low pressure and high pressure which creates winds.
• Air moves from areas of high pressure to areas of
low pressure = wind.
• In general, air moves from the equator to the poles.
• The processes that affect air movement also influence
weather.
Air Masses
An air mass is a large body of air in the lower
troposphere that has similar characteristics of
temperature and humidity.
• Air masses move across the earth’s surface,
causing changes to the weather.
• The boundary between air masses is called a
front (frontal system).
• Approaching fronts change the weather.
The four types of air masses that affect the
weather of North America come from six regions.
Air masses are based on where they come from:
1.
2.
3.
4.
Maritime - moist – from the ocean
Continental - dry – from the land
Tropical - warmer – from near the tropics
Polar - colder – from near the poles
An air mass usually brings the weather of its source
region:
mT – maritime tropical – higher humidity and warmer
cT – continental tropical – lower humidity and warmer
mP – maritime polar – higher humidity and cooler
cP - continental polar – lower humidity and cooler
Fronts
• A boundary that forms between two different air
masses.
• When two unlike air masses meet, density differences
usually keep the air masses separate.
• A cool air mass is dense and does not mix with the less
dense air of a warm air mass.
• No fronts in the tropics. Why? Similar air mass densities
• Air masses that don’t differ in pressure, don’t move
much.
• For a front to form, one air mass must collide with
another air mass
• Four different kinds of fronts:
1.
3.
COLD FRONT
STATIONARY FRONT
2. WARM FRONT
4. OCCLUDED FRONT
Squall Line
Cold Front
Warm front
Stationary front
Occluded front
Plotting Fronts and Precipitation
• Most weather maps mark the locations of
fronts and areas of precipitation.
– Fronts are identified by sharp changes in wind
speed, wind direction, temperature, and humidity.
– Areas of precipitation are commonly marked by
using colors or symbols.
• Light rain = green
• Snow = grey or white
• Different colors also may represent different amounts
of precipitation.
FRONTS ARE
ONLY ATTACHED
TO LOWS.
Plotting Pressure
• Lines on weather maps connect points of
equal measurement.
– Equal temperatures = isotherms
– Equal atmospheric pressures = isobars
• Closely spaced bars indicate rapid change in pressure
and high wind speeds.
• Widely spaced bars indicate gradual change in pressure
and low wind speeds.
• Isobars that form circles indicate centers of high (H) or
low (L) pressure.
Isobars
Winds
•
•
•
•
Isobars connect points of equal pressure.
Winds blow parallel to isobars.
In the U.S. winds flow clockwise around highs.
In the U.S. winds flow counterclockwise around
lows.
Mid – Latitude
Cyclones
• Last several days
(formation to dissipation)
– Areas of low pressure that
are characterized by rotating
wind that moves toward the
rising air of the central low
pressure region.
– Influence weather patterns
in the middle latitudes.
• In North America, travel 45
km/h from west to east,
spin counterclockwise.
Anticyclones
• Air sinks and flows
outward from a center of
high pressure.
• Clockwise air movement in
North America (Coriolus
Effect)
• Bring dry weather, sinking
air does not form clouds.
• If become stationary, may
create pollution problems
and/or drought.
• HIGH
OR LOW?
• CYCLONIC
OR
ANTICYCLONIC?
Severe Weather
• Any weather that may cause property damage
or loss of life.
– THUNDERSTORMS
– HURRICANES
- WINTER STORMS
- TORNADOES
WATCH:
Conditions are ideal for severe weather to develop.
WARNING:
Severe weather has been spotted or is expected
within 24 hours.
• Nowcasts
Types of Forecasts
– Use radar to focus on timing precipitation and tracking
severe weather.
• Daily forecasts
– Predict weather conditions for a 48-hr period.
• Extended forecasts – look ahead 3-7 days
• Medium range forecasts – look ahead 8-14 days
– Use computer analysis
• Long range forecasts – cover monthly and seasonal
periods
– Use computer analysis
• Accurate forecasts are good for 0 to 7 days
but accuracy decreases with each day.
Thunderstorms
• Form in cumulonimbus clouds.
• Produce intense precipitation, lightning, and strong winds.
• Typically short-lived, usually 15-30 minutes.
• Generally form along fronts on hot humid summer days.
• Three distinct stages:
– Cumulus Stage
• Warm, moist air rises; cumulus clouds form.
– Mature Stage
• Condensation continues while clouds keep building
• Heavy rain/Hail occurs and downdrafts form
– Dissipating Stage
• Downdrafts stop air currents from rising
• Storm eventually fades
Lightning and Thunder
• During thunderstorms, clouds discharge electricity in
the form of lightning.
• The upper part of the cloud  + charges
• Lower part of the cloud  - charges
• Lightning is a huge spark that travels within a cloud,
from cloud to cloud, or from cloud to the ground.
• It can travel miles from the actual storm.
• There is no such thing as “heat” lightning.
• Temperatures can reach 54,000 oF
• To find the distance to the storm, count the number of
seconds from the time of the lightning flash to when
thunder is heard. Divide this number by 5.
• The released electricity heats the air, and the air
expands rapidly which produces a loud noise – thunder
Tornadoes
•
•
•
•
•
•
•
•
•
•
Smallest, most violent, shortest-lived severe storm.
Spiral around low pressure areas.
A destructive, rotating column of air with very high wind speeds.
Forms when a thunderstorm meets high-altitude, horizontal winds
which cause the rising air in the thunderstorm to rotate.
Starts out as a rapidly spinning, funnel-shaped extension from the
base of the cumulonimbus cloud.
These narrow (100m wide) funnel shaped extensions may or may
not touch ground.
Called tornadoes when they touch the ground.
Destructive force is due to the speed of the winds in the tunnel.
Winds may reach 400 km/hr
Most deaths and injuries are from flying objects or collapsing
buildings.
Tornado Alley
Scale for
Measuring
Tornadoes
Link
Hurricanes
•
•
•
•
•
•
•
•
•
•
Tropical storms that form over warm, tropical ocean water.
The warm ocean heat increases the force of the rising air.
Winds spiral inwards toward intense low pressure.
Wind speeds are greater than 74 km/h
Consists of a series of thick cumulonimbus cloud bands
that spiral upward around the center (eye) of the storm.
Winds increase toward the eye and may reach 275 km/h
along the eyewall.
The eye is an area of calm, clear, sinking air.
Typically can reach 700 km in diameter
Dangerous due to rising sea levels and large waves
(storm surge)
Most deaths due to drowning.
Link
Saffir-Simpson Scale for Hurricanes
• Based on central pressure, wind speed, and
storm surge
• Hurricane season in the Atlantic begins
June 1st and ends November 30th
• Hurricanes are not given names, tropical
storms are given names, and they retain
their name if they develop into a hurricane.
• Tropical depression
• Tropical storm
• Hurricane
Global Weather Monitoring
• Weather stations around the
world.
– manned and automated.
– Info sent to collection centers.
– WMO (World Meteorological
Organization) (1873).
– sponsors World Weather Watch
(rapid exchange of info.)
• now part of the United Nations
1970
Air Temperature: 60 F
Dew Point Temperature: 45 F
Wind Speed: 15 mph
Wind Direction: Southwest
Precipitation/Weather: Light Rain
Atmospheric Pressure: 1008 mb
Cloud Cover: 70%
Cloud Type: Stratus