Ch. 2 Weather

Section 1 What is Weather?Section 2 Weather Patterns

Section 3 Weather Forecasting

Section 1 What is Weather?

Refers to the state of the atmosphere at a specific time and place

Weather FactorsDescribes conditions such as air pressure, wind, temperature, and humidity, or moisture in the air.

The Sun provides almost all of Earth’s energy.Drives the Water CycleCauses wind to occur

Temperature ReviewTemperature is a measure of air molecule movementHigh temp. = Warm = air molecules move

rapidly

Low temp. = Cold = air molecules move slowly

WindMovement of air in a specific direction = from regions of higher pressure to regions of lower pressureCool air is denser so higher pressureWarm air is less dense so lower pressure

Direction can be determined using a:Wind Vane - has an arrow

that points in the direction from which wind is blowing

Wind Sock - has one open end that catches the wind, causing sock to point in the direction toward which wind is blowing

Wind speed can be measured using an:Anemometer - has rotating cups that spin

faster when the wind is strong

HumidityThe amount of water vapor present in the air

Warm air can hold more water vapor than cool air.

Water vapor condenses and sticks together when cooled

Expressed as g/m3

Relative HumidityA measure of the amount of water vapor present in air compared to the amount needed for saturation at a specific temp.

Expressed as a %

SaturationWhen relative humidity is 100%

Or, in other words, when air is holding the maximum amount of water vapor it can hold at a certain temp.

Relative Humidity & Saturation

• Air at 25°C is saturated when it contains 22 g of water vapor per cubic meter of air.

• The relative humidity is 100 percent.

Relative Humidity & Saturation

• If air at 25°C contains 11 g of water vapor per cubic meter, the relative humidity is 50 percent.

Dew PointThe temperature at which air is saturated and dew forms

The dew point changes with the amount of vapor present in the air.

When temperature drops, less water vapor can be present in the air.The vapor will condense into a liquid or

ice crystals

Because land cools so quickly, air in contact with the cold ground can reach its dew point, causing dew to form on the ground.

If the temp. is near 0 ˚C, frost will form.

Condensation forms on the outside of a glass because the cold glass brings the air next to it down to its dew pt.

Forming CloudsClouds form when warm, moist air is forced upward, expands, and cools to its dew point

Water vapor condenses around tiny particles called condensation nuclei.

Water droplets are so small that they stay suspended. Billions of these droplets form a cloud.

Ways in which air lifts and cools:

1. Rays from the Sun heat the ground and the air next to it making it buoyant. The warm air rises and cools.

2. As moist air moves over mountains, it is forced upward and cools.

3. When cool air meets warm, moist air along frontal boundaries the warm air rises and cools.

Classifying CloudsClassified mainly by shape and height

Shapes:Stratus - form layers, or smooth even

sheets in the skyFog - stratus clouds that form near the ground

Cumulus - puffy, white clouds, often with flat bases

Cirrus - thin, white feathery clouds made of ice crystals

Height: (prefixes)Cirro - high cloudsAlto - middle-elevation cloudsStrato - low clouds

Rain- or Snow-Producing Clouds

Have the word nimbus or nimbo attached to them

Sunlight usually cannot penetrate through

Ex: cumulonimbus, nimbostratus

PrecipitationWater falling from clouds

4 main types:RainSnowSleetHail

Form is determined by air temperature

RainWhen the air is warm, water vapor forms raindrops that fall as rain

Size depends on:Strength of updraft

Stronger = larger dropsRate of evaporation

If the air is dry, size can be reduced by evaporation before reaching the ground

SnowForms when the air temp. is so cold that water vapor changes directly to a solid

SleetForms when raindrops pass through a layer of freezing air near Earth’s surface

HailForms in cumulonimbus clouds when water vapor freezes in layers around a small nucleus of ice

The stronger the updraft, the larger the hailstone

Most hailstones are < 2.5 cm, but some can be as large as a softball

Causes the most damage of all forms of precipitation

Question 1The state of the atmosphere at a specific time and place is __________.

A. climateB. meteorologyC. seasonD. weather

Question 2The amount of water vapor present in the air is __________.

A. dewB. condensationC. fogD. humidity

Question 3What is the dew point?

Section 2 Weather Patterns

Air MassA large body of air that has properties similar to the part of Earth’s surface over which it developed

Can cover thousands of square km

Daily changes in Wx are due to the movement of air masses.

6 air masses of 4 diff. types affect the US:Maritime Polar (mP) - NW/NE

cool moistContinental Polar (cP) - N

cold dryMaritime Tropical (mT) - SW/SE

warm moistContinental Tropical (cT) - S

hot dry

Highs and LowsAir pressure is measured using a barometer.

Remember: Winds blow from high to low pressure areas.

HighsWinds blow away from a center of high pressure

Earth’s rotation causes the wind to swirl in a CW dir.

Called anticyclones

Sinking motion in highs makes it difficult for air to rise and clouds to form

High pressure usually signals good Wx.

LowsWinds blow toward a center of low pressure

Earth’s rotation causes the wind to swirl in a CCW dir.

Called cyclones

Air rises in a low causing clouds to form.

Lows are usually associated w/ cloudy and/or rainy Wx.

FrontBoundary b/w two air masses of diff. density, moisture, or temp.

Cloudiness, ppt, and storms sometimes occur at frontal boundaries as air is lifted

4 types:Cold, Warm, Occluded, Stationary

Need to know symbols!

Cold FrontOccurs when colder air advances toward warm air

Cold air wedges under warm air like a plow with a steep front causing air to lift and cool rapidly

Generally produces heavy ppt that starts and stops abruptly

Thunderstorms common; tornadoes may form

Warm FrontOccurs when lighter, warmer air advances over heavier, colder air

Usually forms a boundary with a gentle slope causing air to lift and cool gradually

Generally produces steady ppt that lasts for hours or days

Occluded FrontOccurs when a cold air mass moves toward cool air w/ warm air b/w the two

Colder air forces warm air upward closing it off from the surface

Stationary FrontOccur when a boundary b/w air masses stops advancing

May remain in place for many days producing light wind and ppt

Severe WeatherPoses danger to people, structures, or animals

Includes:ThunderstormsTornadoesHurricanesBlizzards

ThunderstormsForm in warm, moist air masses and along fronts whenever air lifts and cools rapidly forming cumulonimbus clouds

Falling raindrops cool the surrounding air. Cool, dense air sinks forming downdrafts.

Extremely powerful storms can produce large hailstones.

Damage:Flash floodingStrong winds (>89 km/h = severe

thunderstorm)HailStrong downdraftsLightningTornadoes

LightningOccurs when rapid movement of air w/in a storm cloud causes diff. parts of the cloud to become oppositely charged

Lightning is the flow of current b/w regions of op. elec. charge.

Can occur w/in a cloud, b/w clouds, or b/w a cloud and the ground

Can reach temp. of about 30,000 ˚C

ThunderResults from the rapid heating of air around a bolt of lightning

The extreme heating of air around lightning causes it to expand and contract rapidly, which forms sound waves heard as thunder

TornadoesViolently rotating columns of air in contact with the ground

Form at the base of cumulonimbus clouds

Ranked according to the Fujita scale according to how much damage they cause (F0 - F5)

HurricanesLarge, swirling low-pressure systems that form over the warm Atlantic Ocean

Called typhoons in the Pacific Ocean and cyclones in the Indian Ocean

Must have wind speeds >199 km/h

Ranked according to the Saffir-Simpson scale according to wind speed (Category 1 - Category 5)

Damage:High windsTornadoesHigh wavesStorm surgesFloods

BlizzardsWinter storms that have winds ≥56 km/h, low temps., and visibility <400 m that persist for at least 3 hours

Watches vs. WarningsWatches - are issued when conditions are favorable for severe Wx

Warnings - are issued when severe Wx conditions already exist

Question 1Water falling from clouds is _______________.

A. Cloud cover

B. Dew

C. Fog

D. Precipitation

Question 2A large body of air that has properties similar to the area over which it develops is ______________.

A.An air mass

B.Cloud cover

C.Fog

D.Front

Question 3What is the boundary between two air masses of different density, moisture, or temperature called?

How do you classify them?

Section 3Weather Forecasting

MeteorologistA person who studies the Wx

Gathers info. about current Wx conditions using tools such as Wx balloons, Wx satellites, Doppler radar, etc. to make predictions about future Wx patterns

Collects data from 2 places:Earth’s surfaceUpper atmosphere

Makes Wx maps which are used to help predict the Wx

Uses computers to help predict the Wx

Weather MapMap that shows current conditions and is used to help predict the Wx

Includes station models, highs and lows, fronts, and isobars

Station ModelsShow Wx conditions at a specific location

on Earth’s surfaceCan include info. on up to 20 diff. Wx

conditions

Highs and LowsDrawn as circles with the word “high” or

“low” in the middle

FrontsDrawn as lines and symbols

IsobarsLines drawn to connect points of equal

pressureClosely spaced isobars indicate a large

pressure diff. over a small area, which means high winds

IsothermsLines drawn to connect points of equal

temp.

Question 1A _____________ shows the weather conditions of a specific location on Earth’s Surface.

A.Isobar

B.Isotherm

C.Station Model

D.Weather map

Question 2On a weather map, the line that connects points of equal temperature is called?

A.Berm

B.Isobar

C.Isotherm

D.Station Model

Question 3What is an Isobar?