Weather Weather ForecastingForecasting

Predicting Changes Predicting Changes in our Atmospherein our Atmosphere

What Causes Changes in What Causes Changes in the Weather ?the Weather ?

• Air Masses / Air MovementAir Masses / Air Movement• FrontsFronts• Pressure SystemsPressure Systems

Weather ForecastingWeather ForecastingBecause much of modern society

depends on weather conditions, knowing current weather patterns and methods used to determine the future state of the atmosphere is important.

This attempt to predictpredict the future weather is called weather weather forecastingforecasting..

Forecasting is based on probability of probability of occurrenceoccurrence.

Weather MapsWeather MapsStation modelsStation models are used to

record the atmospheric variables recorded at different locations.

These variables are:amount of cloud coveramount of cloud coverbarometric pressurebarometric pressuretemperaturetemperaturedewpointdewpointwind directionwind directionwind speedwind speed

Using IsolinesUsing IsolinesIsolinesIsolines are lines connecting

points of equal value.Weather maps use different types

of isolines:IsobarsIsobars show barometric pressure.IsothermsIsotherms show temperatures.

Synoptic Weather MapsSynoptic Weather MapsSynoptic weather mapsSynoptic weather maps provide a

synopsissynopsis, or summarysummary of weather conditions all over the United States at the indicated hour.

What is an Air Mass?What is an Air Mass?

Air massAir mass = a large body of air in the troposphere having similar pressure, wind and temperature characteristics throughout.

Air Mass CharacteristicsAir Mass CharacteristicsAir masses have characteristics that are

determined by the part of the Earth’s surface that the air mass forms over.

These areas are called source regions.source regions.

Examples: Forming over land = dry air Forming over land = dry air massmass Forming over water = moist air massForming over water = moist air massForming over equator = warm air massForming over equator = warm air mass Forming over the poles = cold air Forming over the poles = cold air massmass.

Source Source RegionsRegions

Air massesAir masses move and swirl over the surface of the Earth in different directions and at different speeds and often bump into one another.

Rain, thunderstorms, snow, tornadoes and all weather-related events can result when air masses meet.

What is a Weather What is a Weather FrontFront??

• Front = the boundary formed as two air masses meet and collide.

Types of FrontsTypes of Fronts• WarmWarm FrontFront = forms when a warm air

mass slides over a departing cold air mass. As warm air rises the temperature drops and rain or precipitation often forms.

• ColdCold FrontFront = forms as a cold air mass overtakes and moves underneath a warm air mass. The warm air is forced upward rapidly and a cold front forms. Cold fronts usually bring violent weather.

Types of Fronts (cont.)Types of Fronts (cont.)• Stationary FrontStationary Front = occurs when a cold air

mass meets a warm air mass and neither air mass moves. Precipitation sometimes forms between these two fronts.

• Occluded FrontOccluded Front = forms when two cold air masses meet and force a warm air mass between them to rise completely off the ground. The weather in an occluded front is difficult to predict because there are three air masses involved.

What is a Pressure What is a Pressure System?System?

• PressurePressure = as particles move and = as particles move and collide they exert pressure; collide they exert pressure;

• Differences in air pressure have a great Differences in air pressure have a great affect on the weather.affect on the weather.

• When particles are When particles are more densemore denselyly packed together they exert packed together they exert high high pressurepressure,,– High air pressure usually means clear, fair High air pressure usually means clear, fair

weather.weather.• When they are When they are less denselyless densely packed they packed they

exert exert less (low) pressureless (low) pressure..– Low air pressure usually means clouds and Low air pressure usually means clouds and

rainy weather.rainy weather.

High Pressure SystemHigh Pressure System• Cold air is dense and it sinks.• As the air sinks it warms up.• Warm air holds more water vapor.

– The total amount of water vapor remains the same.• As the air warms the relative humidity

decreases as droplets in the clouds evaporate.

• A high pressure system usually means fair weather as the moisture in the air is evaporated so few clouds form.

Low Pressure SystemLow Pressure System• Warm air has low density and it rises (forced

upwards by surrounding denser air)• As the air rises it cools• As the air cools the relative humidity increases

– eventually reaching the dew point– At the dew point condensation takes place and

clouds form• Low pressure systems usually form along

fronts where warm air and cold air meet• Low pressure systems cause most of the

weather in the US.• A low pressure system leads to precipitation in

the form of rain, snow, hail, sleet.

Moving FrontsMoving Fronts• When we witness a change in the

weather from day to day it is due to the movement of air masses. It is the movement and collision of air masses that causes the weather to change.

What is Severe Weather?What is Severe Weather?

• Severe weather are events that fall outside of the ordinary weather patterns.

• Severe weather can include the following:– Thunderstorms– Lightning– Tornadoes– Hurricanes– Floods– Droughts– Blizzards– Fog– Ice Storms

ThunderstormsThunderstorms• Thunderstorms are formed by rapid Thunderstorms are formed by rapid

upward movement of warm moist air.upward movement of warm moist air.• Thunderstorms can occur within a Thunderstorms can occur within a

warm moist air mass.warm moist air mass.• Thunderstorms Thunderstorms most oftenmost often occur at occur at

cold fronts.cold fronts.• Thunderstorms account for the Thunderstorms account for the

greatest number of weather-related greatest number of weather-related fatalities in the US due to the flooding, fatalities in the US due to the flooding, lightning and tornadoes that can be lightning and tornadoes that can be associated with themassociated with them..

How Thunderstorms How Thunderstorms FormForm

• As warm moist air is forced upward it cools and the water vapor condenses forming cumulus clouds. (These clouds can be very tall – up to 10km high)

• As the drips fall they collide and combine to form larger drops

• The falling drops create a downward movement of air that spreads out as it hits the surface causing winds– If the winds accompanying a thunderstorm

are greater than 54mph then forcasters classify it as a severe thunderstorm (these usually contain hail).

LightningLightning

HurricanesHurricanes• Hurricanes are the largest storms that occur on

Earth.• Hurricanes are very large, swirling, low pressure

systems that from over tropical waters (water temperature must be at least 80 degrees F).

• Hurricanes can last for days due to the constant supply of energy from the warm waters

• To be classified as a hurricane there must be sustained winds of greater than 74mph.– Greatest winds recorded are 236mph; Category 5

“Mitch” in Nov 1998– 1900 Galveston TX was most deadly hurricane ever with

8000 deaths.

Eye of the HurricaneEye of the Hurricane

The eyeeye of a hurricane is the relatively calm area in the middle of the storm.

The eyeeye has the lowest pressure of the storm and the eye walleye wall is where the strongest winds occur.

A storm surgestorm surge is a dome of water that can raise the water level higher than high tide where the storm lands.

How Hurricanes FormHow Hurricanes Form• Hurricanes form over warm tropical oceans

where two opposing winds meet and begin to swirl

• A low pressure forms in the middle of the swirling winds and begins rotating

• Warm moist air is forced upwards in the center• The dropping pressure in the center pulls more

air toward the center creating increasing winds and lower pressure. This cycle of increasing strength continues as long as the storm remains over warm water.

• Hurricanes weaken as they hit land because there is no longer a supply of energy (from the warm water) available.

Life Cycle of a HurricaneLife Cycle of a Hurricane• Life-cycle of a hurricane = form

first as a low pressure system, then grow into a tropical depression (<31mph winds), then grow into a tropical storm (32-74mph winds), then grow into a hurricane (>74mph winds).

• Out of the 10 tropical storms per year average, 6 will develop into hurricanes and 2 will strike the US.

Key Hurricane AlertsKey Hurricane AlertsTropical Storm WatchTropical Storm Watch: tropical storm

conditions are possiblepossible in your area w/in the next 36 hours.

Tropical Storm WarningTropical Storm Warning: tropical storm conditions are expectedexpected in your area w/in the next 24 hours.

Hurricane WatchHurricane Watch: hurricane conditions are possiblepossible in your area w/in 36 hours.

Hurricane WarningHurricane Warning: hurricane conditions are expectedexpected in your area in 24 hours or less.

Hurricane StrengthHurricane StrengthThe Saffir – Simpson ScaleThe Saffir – Simpson ScaleCategor

yAtm.

Pressure (mb.)

Wind Speed (mph)

Storm Surge (feet)

1 >980 74 – 95 4 – 52 965 -979 96 – 110 6 – 8

3 945 -964 111 – 130 9 – 12

4 920 -944 131 – 155 13 – 185 <920 >155 >18

Hurricane OriginsHurricane Origins

TornadoesTornadoes• Tornadoes are violent funnel-shaped storms with

whirling winds that move in narrow paths over land.• Tornadoes form from severe thunderstorms• Tornado producing thunderstorms involve the rapid

upward movement of warm moist air• The upward moving air begins to rotate (why the

rotation starts is still not understood)• As the speed of the rotation increases more warm

moist air is drawn into the low pressure at the center• A funnel shaped cloud extends from the bottom of

the cloud sometimes touching ground– As it touches ground it picks up dirt and debris

that give it the characteristic dark gray, black color.

• Tornadoes are one of the most destructive types of storms.

Strength of Tornadoes:Fujita ScaleStrength of Tornadoes:Fujita ScaleScale Wind Speed

(mph)Expected Damage

F0 <72 Light Damage: broken branches, billboards.

F1 72 – 112 Moderate Damage: mobile homes overturned, roofs.

F2 113 – 157 Considerable Damage: roofs off, trees snapped, uprooted.

F3 158 – 206 Severe Damage: trains overturned, cars thrown.

F4 207 – 260 Devastating Damage: houses leveled, large missles.

F5 >260 Incredible Damage: houses lifted, trees debarked, auto missles.

Tornado AlleyTornado Alley

FloodsFloods

DroughtsDroughts

BlizzardsBlizzards

FogFog

Ice StormsIce Storms

What is Climate?What is Climate?

• Climate is the state of the atmosphere of a given region averaged over time.– Climate is how hot/cold, wet/dry a place

is in general.– Climate is what it is usually like in a

place.– Example: Florida’s climate is usually

sunny and hot.

The Hydrologic CycleThe Hydrologic Cycle• The hydrologic cycle, or water cycle,

is the basis for the Earth’s weather.