4.2.1 - The Atmosphere (KQ2): Weather and Wind

Weather and Climate

Weather and wind patterns are extremely important in terms of life on Earth...determining when major natural events will occur, major climatic regions, and whether or not it's a good beach day!  Many people confuse weather with climate, so what is the difference? 

Weather – What’s happening currently…a moment in time.  Weather is if it's raining, snowing, hot, or cold.

Climate – The average weather pattern of a certain area…determines the BIOME an area is divided into.  We will discuss Biomes in the next section.

So, what causes different climates with their weather patterns?  The major reason, again, is the sun, causing differences in insolation.  Insolation is the amount of solar radiation received on a surface (W/m2).  Due to the tilt of the Earth, there are different amounts of insolation for different parts of the Earth.  It is basis for the formation of the climates/biomes.  There is a diagram of this below:

So, because of this, and the fact that the inside of the Earth gives off different amounts of heat to the surface, the Earth is heated unequally.  When that happens, the air particles where there is more heat (energy) will rise, creating an area of low pressure below.  Areas of less heat will have a higher pressure.  The cooler air will rush in to fill the area left by the lower pressure.  We know this movement as wind.  Thus, air always flows from areas of high pressure (cool) to areas of low pressure (warm).  Measuring the different pressures on Earth is a major way how meteorologists make their forecasts.  We will get more into weather patterns and forecasting in 4.2.3.

There are two types of wind systems we will discuss:  Global Winds and Local Winds

Global Wind Systems

As stated before, air generally moves from the poles to the equator, this is because air flows from high pressure to low pressure.  Globally, high pressure forms when cold air sinks at the poles.  Low pressure forms when warm air rises at the equator.  BUT, the air doesn't flow in a straight line.  Why???  The answer lies in the Coriolis Effect.

The Coriolis Effect causes air to move in a curved path.  It is caused by the Earth spinning on its axis, where the Earth spins fastest at the equator, and slowest near the poles.  As air moves from the equator to the pole, it will travel east faster than the land beneath it causing the air to follow a curved path.

In the Northern Hemisphere, the Coriolis Effect causes wind to curve to the Right facing the north pole.  In the Southern Hemisphere, the Coriolis Effect causes wind to curve to the Left facing the south pole.

The global winds blow across the entire planet and generally blow at the same speed and direction.  Some are high in the atmosphere and you cannot feel them on the ground.  Each hemisphere contains 3 wind belts that divide up the planetary winds: the Trade Winds, the Westerlies, and the Easterlies.

Trade Winds

The Trade Winds blow toward the equator, an area of low pressure called Doldrums.  They are located between 0-30º north and south.  Called trade winds because many European sailors used these winds for trade.  In northern hemisphere, trade winds come from the NE, so they are called Northeast trade winds.  In the southern hemisphere, trade winds come from the SE, so they are called Southeast trade winds.  (Winds are named for the direction they travel from.)

Westerlies

The Westerlies are between 30º and 60º latitude.  The high pressure at 30º causes air to descend and blow toward the poles.  This zone of high pressure at 30º are called Horse Latitude.  In northern hemisphere, westerlies are southwest winds.  In the southern hemisphere, westerlies are northwest winds.

Easterlies

The Easterlies are located above 60º latitude (between 60-90º) at both poles.  The high pressure over the poles (caused by the sinking cold dense air) causes the air to blow away from the poles toward 60º latitude.  At the north pole, they are northeast winds since the wind comes from the northeast. (Ho Ho Ho).  At the south pole, they are southeast winds since the wind comes from the southeast

Here is a diagram of the Global Wind Patterns:

Local Wind Systems

Local winds blow over small areas.  Unlike global winds, local winds can change speed and direction frequently.  These are the winds we feel on the ground.  They are influenced by local conditions and local temperature variations.

In coastal areas, during the day, the land heats up faster than the water, so a breeze will blow from the sea to the land, called a sea breeze.  At night, the land will cool off faster than the water, so the cool land air will flow toward the warmer ocean water air, creating a land breeze, where the wind blows from the land to the sea.

In mountainous areas, during the day, the valley heats up, so the warm less dense air flows up the mountain, creating a valley breeze.  At night, the mountain will cool off faster than the valley, so the cool mountain air descends because it is more dense, creating a mountain breeze.

Rain Shadows

When air moving inland from the ocean that contains a large amount of water vapor meets the windward side of a mountain range (the side facing the wind), it rises and begins to experience adiabatic cooling. Because water vapor condenses as air cools, clouds form and precipitation falls. The presence of the mountain range causes large amounts of precipition to fall on its windward side. The cold, dry air then travels to the other side of the mountain range (the leeward side), where it descends and experiences higher pressures, which cause adiabatic heating. This air is now war and dry and process arid conditions on the leeward side forming the region called a rain shadow. 

Land Relief and Ocean Currents

Ocean currents and the land both play a major role in the variety of weather and climate on Earth.  Watch the following video from NASA to learn about the impacts.