Weather Maps & Fronts

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

• Influenced by pressure systems (high and low) and fronts.

• Describes conditions such as air pressure, wind, temperature,

and the amount of moisture in the air.

• Is caused by heat energy from the sun.

Air mass

• Large body of air that has the properties similar to the part of Earth’s surface over which it develops.

• Changes in weather result from movement of air masses.

Weather Patterns

Low Pressure System

When air is heated, it becomes less dense– Therefore, it rises, leaving behind an area

where there is less air.– Called a low pressure area– Rising air carries moisture with it. As a result,

low pressure is usually associated with clouds and rain.

– Indicated by a L on a weather map.– Troughs of low pressure are important because

they sometimes lead to the development of a low pressure system. Troughs are indicated by lines (sometimes purple, black or green) with no triangles or semicircles.

High Pressure System

As air cools, it becomes more dense– It falls, creating an area where there

is more air– Called a high pressure area– Very little moisture in this air, so

usually associated with clear, sunny days.

– Indicated by a H on a weather map.

Isobars

Lines that connect points of equal atmospheric pressure.

Isobars close together mean high wind.

Isobars far apart mean light wind.

Isotherms (iso – same, therm – heat)

Lines that connect locations of equal

temperature.

Front

• A boundary between two air masses of different

density, moisture or temperature

• Cloudiness, precipitation and storms may occur at

frontal boundaries

• Four types of fronts:

cold, warm, occluded and

stationary

Cold fronts

• Cold front – shown on a map as a blue line with

triangles

• Occurs when colder air advances toward warm air.

• Cold air wedges under the warm air

• This lifts the warm air, cools it and clouds form.

• If the temperature difference is large, thunderstorms

and tornadoes may form

Warm fronts

• Warm fronts form when lighter, warmer air

advances over heavier, colder air.

• Shown as a red line with red semicircles.

Occluded fronts

• Involves three air masses of different

temperatures – cold, cool and warm

• Shown on maps as purple lines with triangles

and semicircles.

• Colder air forces the warm air upward, closing

off the warm air from the surface.

Occluded Front

Stationary Front• Occurs when a boundary between air masses stops

advancing.

• May remain in the same place for several days,

producing light wind and precipitation.

• Shown on map as alternating red and blue line. Red

semicircles point toward the cold air and blue

triangles point toward the warm air.

Stationary Front

Severe Weather

• Sinking rain - cooled air and strong updrafts of

warmer air cause the strong winds associated with

thunderstorms.

• Hail also may form as ice crystals alternately fall to

warmer layers and are lifted into colder layers

• May cause flooding, flash floods, strong winds

• Thunderstorms – occur in warm moist air masses and

along fronts.

• Winds stronger than 89km/hour – severe thunderstorm.

Severe Weather

• Lightning - Inside a cloud, liquid and ice particles

collide causing the buildup of an electrical charge.

When the charge becomes large enough, it creates a

“spark” that allows the electrons to move. The

current flows between regions of opposite electrical

charge. Lightning can occur within a cloud, between

clouds or between the cloud and the ground.

http://www.sciencekids.co.nz/videos/weather/slowmotionlightning.html

Thunder

• Results from the rapid heating of air around a bolt of

lightning.

• The extreme heat of lightning causes the air around

the lightning to expand rapidly. Then it cools quickly

and contracts.

• The rapid movement of the molecules form sound

waves

Tornado

• A violent, whirling wind that moves in a narrow path

over land.

• Wind moving in different directions (shearing)

creates a rotating column

• Ranked on the Fujita scale according to how much

damage occurs F0, F1, F2, F3, F4, F5

http://www.sciencekids.co.nz/videos/weather/tornadoes.html

Severe Weather

• Hurricane – the most powerful storm

• Large swirling low pressure system that forms over the warm

Atlantic ocean

• Must have winds of at least 119 km/hour

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

• Lose power when they reach land.

• The Saffir-Simpson Hurricane Wind Scale is a category 1 to 5

rating based on wind speed.

http://theweatherchannelkids.com/cool-clips/

Severe Weather

• Blizzards – winter storm with winds over 56 km/h, low

temperature, visibility less than 400m, with falling or

blowing snow.

• These conditions must persist for more than 3 hours.

10 min

Severe Weather Safety

• Weather Advisories - are sort of in between a WATCH and WARNING.

The expected weather condition has a pretty good chance of occurring,

even a likely chance of occurring, but typically an advisory is used for

“less” severe type of weather conditions.

• Severe weather watch – when conditions are favorable for severe

weather.

• Severe weather warning – severe weather already exists.