Chapter 5

Weather

5.1 A. What is weather?

The state of the atmosphere at a specific time and place.

B. Weather Factors1. Air Temperature

-the higher the temperature, the faster

the air molecules are moving

2. Wind

a) air moves from regions of high pressure to regions of low pressure a. warm air- less dense- low pressure b. cool air - more dense- high pressure

b) wind speed is measured with an anemometer

c) wind direction

Wind vane -points to the

direction from which the wind came

Wind sock -points in the

direction the wind is blowing

3. Humidity-amount of water vapor in

the air Warm Air

More water vapor present

Water molecules move quickly and don’t come together

Cool Air Less water vapor

present Water molecules move

slowly and stick together to form condensation

When air is saturated, precipitation occurs

4. Relative Humidity The amount of water vapor present

in the air compared to amount needed for saturation at a specific temperature

This measurement is given as a percent

(think of it as how much water is in a sponge)

5. Dew Point The temperature at which air is saturated

with water vapor and condensation forms

This temp. changes with the amount of water vapor in the air

P.121 (figure 4)

C. Clouds 5 Steps of Cloud Formation:

1. Warm air forced upward, expands, cools2. Relative humidity increases3. When R.H. reaches 100%, air is saturated4. Water vapor condenses around small

particles such as dust and salt5. Billions of these droplets form a cloud

Cloud Classification SHAPE

Stratus- low altitude, fair weather, rain or snow

Cumulus- puffy, white, tall, fair weather, thunderstorms

Cirrus- high, thin, feathery, white, fair weather, indicates approaching storms

HEIGHT Cirro- high clouds Alto- middle clouds Strato- low clouds

Dark Clouds- “nimbo”

D. PrecipitationA. Water droplets combine and grow large

enough to fall to EarthB. Forms:

1. Rain- temp. above freezing2. Snow- temp. below freezing3. Sleet- raindrops freeze near surface4. Hail- windy conditions, ice pellets

5.2 Weather Changes

A. Air masses-develop over land with similar properties-changes occur when air masses move-four different air masses over the U.S.

1. Cold DryCanada, Northern Plains2. Hot Dry Mexico, Texas3. Cool Moist Washington, New England4. Warm Moist S. California, Florida

B. Winds1) cyclones

-winds blow in a counter-clockwise direction -stormy weather2) anti-cyclones

-winds blow in a clockwise direction-fair weather

C. Air Pressure-measured with a barometer-low pressure = cloudy weather-high pressure = fair weather

D. Fronts- boundaries between air masses

ColdFront

Cold air wedges under warm air pushing it up

Thunderstorms and tornadoes

WarmFront

Air advances over heavier, cooler air

Hours and days of wet weather

OccludedFront

Colder air sandwiches warm air

showers

Stationary Front

No advancing air

Light wind and precipitation

E. Severe Weather1. Thunderstorms

-warm-moist air masses and along fronts-rain-hail-strong wind-lightning-flash flooding

2. Blizzards

-snow fall-low temps-more than 3 hours-visibility less than 400 meters-wind more than 56 km/hr

3. Hurricanes-low pressure system-turn heat from ocean to wind-at least 119 km/hr

Atlantic Ocean- HurricanesPacific Ocean- TyphoonsIndian Ocean- Cyclones

4. Tornadoes-winds blow at different heights-winds blow in different directions-winds blow at different speeds (wind shear)-rotating column parallel to the ground-funnel cloud forms when an updraft tilts the

column of rotating air-Fujita Scale ranks the severity of the storm

5.3 Weather Forecasts

-Meteorologist-person who studies the weather-gathers information on current weather-predicts future weather

A. Measurements-temperature-air pressure-humidity-precipitation-winds

B. InstrumentsCommon-thermometer- temp.-rain gauge- rainfall-anemometer- wind speed-barometer- air pressure-psychrometer- humidity-wind vane- wind direction

Modern

-satellites-computers-Dopplar Radar

C. Weather MapsAir Pressure

a) High pressure = good weather =

b) Low pressure = poor weather =

H

L

Isobarsa. lines connecting points of equal atmospheric

pressureb. lines close together:

-large pressure difference over a small area-results in strong winds

Isotherms-lines connecting points of equal temperature

Weather Station Model -describes the

weather at one location