What is weather?

Weather• Weather it is the condition of the atmosphere

at a particular time and place .

Air Masses• a body of air extending hundreds or thousands

of miles sideways and sometimes as high as the stratosphere and having nearly the same conditions of temperature and humidity.

Types of air massesHumid Air masses

Dry Air masses

HumidityIt is the amount of water vapor in the air .

Pressure• It is the weight of air pressing on a unit area.

• The difference of air pressure is due to unequal heating of the Earth.

• Warm air masses have fewer air molecules than cold air masses.

Therefore , they are less dense having lower air pressure.

Air Fronts• When air masses move across the surface of the

earth they may collide.• These collisions happen halfway between one of

the poles and the equator.• The 2 air masses don’t mix much when they collide.• Each keeps its own temperature ,humidity and

pressure.• As a result ,a boundary forms, this boundary is

called “Front”.

Types of Air Fronts

Scientists classify fronts by the characteristics and movements of the two air masses.

1-Warm FrontWhen a warm air mass moves into an

area of cooler air ,the boundary between the air is called

“Warm front”The warmer ,less dense air slides up

over the cooler denser air in a wide ,gentle slope

• As the warmer air cools ,its relative humidity increases, and the water vapor condenses into clouds.

• A warm front usually results in steady rain .

2)Cold Front• When a cold air mass moves into an

area of warmer air , the boundary between air masses is called “Cold front”

• The colder ,denser air slides under the warmer air is pushed up .

• As the warmer air cools ,the water vapor condenses and form clouds that brings precipitation

Stationary front

• The boundary between 2 air masses that are not moving against each other is called

“Stationary front”.

Some of the warmer air mixes with cooler air ,causing clouds to form .

As the clouds are pushed up ,they cool and produce light rain or snow .

Because the front is not moving ,the precipitation can last for a long time .

How fronts are represented on the

weather map?

Global wind pattern• Wind is the movement of air from an area of

higher pressure to an area of lower pressure.(Remember: Differences in air pressure is due to

uneven heating of the earth.)

Global Winds

• They are winds that blow across long distances in a predictable patterns.

• Each of these winds carry air masses from one region to another .

• Each of these winds always blows in the same direction

• Global winds don’t follow a straight path as they blow from poles toward the equator or from the equator towards the poles.

• They curve clockwise in the Northern Hemisphere and counter clockwise in Southern Hemisphere .

This curving of the winds is called “Coriolis effect”

Jet Stream• Is a band of very strong wind that blows from

east to west high in the atmosphere ,usually above 6000m .

• Jet streams from along boundaries of large air masses when warm air from the tropics meets air from the poles.

• The sudden temperature change causes a huge difference in air pressure .

• This results in wind speeds of up to 498km/hr