Weather Patterns

Weather Patterns

• Constant movement of air and moisture change the atmosphere

• This is related to the development and movement of air masses– Air Mass: large body of air with the same

properties as the surface it develops over

Air Masses

• Where the air mass develops will give it it’s characteristics.

• Over water, the air will be moist and over land it will be dry

Pressure Systems

• Molecules push down from above, creating atmospheric pressure

Pressure Systems

– Variation in atmospheric pressure determines the weather

• Areas of high pressure = descending colder air– Sinking high pressure air makes it more difficult for

air to rise and clouds to form– High pressure = good weather– Low pressure = clouds

Pressure Systems

Fronts• Front: a boundary

between two different air masses– Storms and precipitation

occur here– Air in a front will move

from high pressure to low pressure

• When two air masses with different characterizations meet, they do not blend

When Air Masses Have Different Characteristics

• The cold air mass will move under the warm air mass and the warm air will rise and winds begin

• Winds blow from high pressured air to low pressured air

Four Types Of Fronts

• Warm– Develop when a less dense warm air mass slides over departing cold air

mass• Cold

– Cold air mass moves under a warm air mass and pushes it along a steep front

– Move at twice the speed of a warm front– Creates a narrow band of violent storms

• Occluded– Result of two cool air masses merging, and forcing warm air between

them to rise– May cause strong winds and heavy precipitation

• Stationary– Pressure differences cause a warm or cold front to stop moving

• May stay in the same place for several days• Causes light wind and precipitation

Severe Weather

• These types of weather pose a danger to people and animals– They include:

• Blizzards• Thunderstorms• Tornadoes• Hurricanes

A Thunderstorm• Air speed can reach 89km per hour• Occurs inside warm moist air

masses and at fronts• Downdrafts-

– Raindrops collide with other droplets and grow larger

– Heavy raindrops fall, dragging down air

– This causes downdrafts, which make strong winds

– Lightning • Rapid uplift of air builds

electric charge in clouds• Currents of opposite electric

charge flowing in between regions makes lightning flash

• Can leap cloud to cloud or cloud to earth

– Thunder• Lightning rapidly heats air

around it, the air expands and molecules crash into eachother

– Can reach 5x the temperature of the surface of the sun

Tornadoes• Tornado – violent, whirling wind,

which moves in a narrow path over land.

• Wind shear- difference in wind direction and speed– Strong updraft will tilt wind shear and

produce rotation inside• Funnel cloud will appear• Can go up to 500km per hour• Most occur in United States

Hurricane

• Most powerful storm• Hurricane – large, swirling, low

pressure system that forms over tropical oceans– Turns heat energy in ocean into wind

• Must be 120km per hour to be classified as hurricane

• Similar to low pressure systems on land but stronger

Section Wrap Up

1) Why do high pressure areas usually have clear skies?

2) Explain how a tornado develops from a thunder storm.

3) How do two fronts form at a low pressure area? Which would bring the most severe weather?