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

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

Air MovementAir Movement

WindsWinds are large horizontal movements of air near the Earth’s surface.

BreezesBreezes are smaller, local horizontal air movements.

CurrentsCurrents are vertical air movements.

Winds are named for the direction from which they come. Ex: a southwesterlysouthwesterly wind is from the south west, a sea breezesea breeze is blowing onshore from the sea.

Cause of WindsCause of WindsThe primary cause of winds are

differences in air temperature, which cause differences in air pressure.

Air always moves from high pressure Air always moves from high pressure to low pressureto low pressure.

The rate of changerate of change of air pressure between two locations is called the pressure gradientpressure gradient.

Local BreezesLocal Breezes

Local breezesLocal breezes are generated by the unequal heating of the Earth’s surface due to local conditions.

Examples:

Sea breezeSea breeze or onshore breezeonshore breeze

Land breezeLand breeze or offshore breezeoffshore breeze

Planetary Convection Planetary Convection CellsCells

Unequal heating of the surface result in density differences in the atmosphere which cause vertical air movements called vertical air movements called convection cellsconvection cells.

See diagram

Divergence ZonesDivergence Zones

In high pressurehigh pressure regions air is air is descendingdescending and as it hits the surface it spreads out.

These areas are known as zones of zones of divergencedivergence.

Divergent zones are dryDivergent zones are dry.

Convergence ZonesConvergence Zones

Air from divergent zones comes together, or converges, in the regions of low pressurelow pressure, where it rises.

These areas are known as zones of zones of convergenceconvergence.

Convergent zones are moist (wet).Convergent zones are moist (wet).

Planetary WindsPlanetary Winds

The rotation of the Earth and the unequal heating of the surface result in a series of wind belts known as the planetary wind planetary wind systemsystem.

Note: without rotation the winds would blow from the poles (high pressure) directly to the equator (low pressure).

What are Jet Streams?What are Jet Streams?Jet streamsJet streams are high-speed, high altitude are high-speed, high altitude

windswinds that have a controlling influence over the direction traveled by air masses.

These flow in a wavelike motion, instead of a straight line, and are part of the planetary convection cells.

The midlatitude jet stream is called the polar jet streampolar jet stream and has a major role in the weather of the midlatitudes.

CloudsCloudsCondensationCondensation is the process by

which gaseous water vapor changes to liquid water.

In order for condensation to take place a surface must be available, however small. This surface is called the condensation nucleicondensation nuclei.

Water vapor changing directly to a solid is called sublimationsublimation. (occurs below 0oC)

Dew and FrostDew and Frost

When water vapor condenses directly onto a cold surface, dewdew is formed.

If water vapor comes in contact with a freezing surface, the vapor will sublimate, and frostfrost will form.

How Do Clouds Form?How Do Clouds Form?

• Clouds form as humid air is cooled to the dew point.

• Condensing water vapor form tiny drops around dust particles in the atmosphere; these tiny drops are called droplets – they are so small that the slightest air movement keeps them from falling to the surface of the Earth.

• Millions of droplets together form a cloud.

• As droplets move the bump and collide into each other combining to form bigger droplets. Once the droplets reach between 0.05mm and 0.5mm they fall in the form of precipitation.

Classifying CloudsClassifying CloudsClouds are divided into four basic groups:

High - CirroCirroMiddle - AltoAltoLow - StratoStratoVertical

The word nimbusnimbus means “rainrain”, so clouds that produce rain usually have the term nimbus in their name.

Ex. Nimbostratus=low rain cloud

A cloud resting on the Earth’s surface is called fogfog.

Types of CloudsTypes of Clouds

• Cirrus Clouds (high)Cirrus Clouds (high) = = very high, very high, white, thin, whispy clouds that white, thin, whispy clouds that contain ice.contain ice.

• Cumulus Clouds (heap)Cumulus Clouds (heap) = = large puffy, large puffy, individual clouds that look like balls individual clouds that look like balls of cotton, usually fair weather clouds.of cotton, usually fair weather clouds.

• Stratus Clouds (low)Stratus Clouds (low) = = near the near the ground, gray, layers or sheet clouds ground, gray, layers or sheet clouds that usually bring precipitation.that usually bring precipitation.

What is Smog?What is Smog?

• SmogSmog is a combination of aerosols and water vapor in stagnant air.

• SmogSmog usually occurs during a temperature inversiontemperature inversion when warmer, less dense air is above a layer of colder, denser air.

PrecipitationPrecipitation

PrecipitationPrecipitation occurs when water droplets or ice crystals in a cloud grow large enough to fall.

RainRain is large water drops that fall.DrizzleDrizzle is fine water drops that

float down gently.SleetSleet is rain that freezes as it falls.SnowSnow is ice crystals that fall.HailHail is layered ice that falls.

Moisture and Energy Moisture and Energy TransferTransfer

An adiabatic temperature change is any change in the temperature of a system without any heat being added or removed.

This usually involves changes in pressure.

For example as pressure increases and air is compressed its temperature increases.

DryDry adiabaticadiabatic lapse ratelapse rateMoistMoist adiabaticadiabatic lapse ratelapse rate