Winds

Module 6: Winds 

To get a dust storm, three ingredients are needed: 

1. Fine-grained soils (dust is made of silt and clay) 

2. A lack of vegetation or rocks sheltering the soil (deserts) 

3. High winds to low dust into the air 

Atmospheric forces make and affect wind: 

1. Gravity: pulls air down to Earth 

1. Higher pressure closer to Earth’s surface 

2. Pressure decreases rapidly with increased altitude 

3. Small elevation changes result in significant change in air pressure 

2. Pressure gradient force: air moves from high pressure to low pressure 

1. Pressure gradient: differences in air pressure 

2. Variation of pressure over space 

3. Air moves horizontally from areas of high pressure to areas of low pressure: air movement = wind 

3. Coriolis effect: rotation of the earth “deflects” path of moving air: moving air bends right in northern hemisphere, left in southern hemisphere. 

4. Friction: surfaces/surface roughness can slow winds. Friction with objects at the surface can move these objects (ex. Leaves, dust). 

Atmospheric pressure: the air has weight 

• Atmosphere that is held down by gravity exerts a force upon every surface 

• Pressure = force per unit area 

• Atmospheric pressure: the weight of all of the air above you (about 1.04 kg/cm2 or on average 1013 mbars) 

Which of the following best explains why small emissions of chlorofluorocarbons are so damaging to the ozone layer? 

• Each CFC molecule is capable of destroying many ozone molecules 

Characteristics of Low Pressure 

• Warm air rises (less dense) from surface 

• Rising air causes lower air pressure at surface 

• Regions of low pressure are also called “convergent” because air converges from surrounding high pressure regions 

Characteristics of high pressure: 

• Cool, dry (denser) air descends from troposphere 

• Descending dense air causes high pressure at the surface 

• Regions of high air are also called “divergent” because air diverges from high to surrounding low pressure regions 

Air pressure ____ with increasing elevation/altitude 

• Decreases exponentially 

A convective loop: 

Isobars: lines connecting points with the same pressure at the surface 

• H: center of high pressure 

• L: center of low pressure 

Pressure gradient: the change in pressure from one location to another 

• Pressure changes, and therefore the pressure gradient, are greatest perpendicular to the isobars 

The pressure gradient determines the force which compels the air to move from high to low pressure. This is called the pressure gradient force.  

• A large pressure gradient (isobars close together) has a large pressure gradient force. 

• A small pressure gradient has a small pressure gradient force. 

Air moves in response to the pressure gradient force to make wind: 

• Large pressure gradients make strong winds, and small pressure gradients make weak winds. 

The larger the difference in air pressure, the faster the faster the flow of air is. The closer the isobars, the faster the wind is in that area. 

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Pressure Gradient force: the force which compels the air to move from high to low pressure 

Large pressure gradients (isobars close together)  

• has a large pressure gradient force 

• Makes strong winds 

Small pressure gradient  

• has a small pressure gradient force 

• Make weak winds 

Air in the pressure gradient forces blow down the gradient from high to low pressure. 

• Their speed and direction are impacted by the Coriolis force (#3) and friction (#4) 

Land and sea breezes (also called onshore/offshore winds) 

• Daytime: air over land heats up and the sea is relatively cool (sea breeze) 

• Land = low pressure and sea = high pressure 

• Nighttime: air over land cools and the sea is relatively warm (land breeze) 

Katabatic Winds:  

Cold (dense) air over ice fields, 

• Hemmed in by mountains 

• Causes high pressure over ice 

Warmer (less dense) air over the ocean 

• Causes low pressure over ocean 

Forces: strong pressure gradient from high pressure zero over the ice cap to the low-pressure zone over the ocean. 

Gravity: strong gravitational pull of dense air at higher elevation over the ice cap down to sea level 

High Winds:  the air flow is restricted to narrow valleys so very high wind velocities occur 

• These high wind velocities can pick up glacial silt in the valleys and make dust storms 

Block Island as a Wind Farm : 

• Low pressure maritime weather systems traveling up East Coast generate large pressure gradients 

• Water surfaces have lower friction than land 

• Higher wind speeds 

• Even on calm days, land and sea breezes product some wind near the coast 

• The average CT household uses about 9,000 kWh of electricity per year 

• This pilot project would produce enough energy to support almost 14,000 homes 

• This is the first offshore wind farm in the US 

In a Saharan dust storm, high winds at the surface mobilize the dust shown in the satellite image. The dust is blowing east but the pressure gradient force is from northeast to southwest. The Coriolis and friction effects cause this change in direction. 

Atmospheric Force #3: Coriolis Effect 

Coriolis effect: Air movement is deflected due to Earth’s rotation 

• Northern hemisphere: Air deflects to the right 

• Southern hemisphere: air deflects to the left 

• As a result, wind flows parallel to the isobars. This is called a geostrophic wind. 

• The magnitude of Coriolis depends on latitude and wind speed. 

Deflection due to Coriolis effect: objects moving over the rotating Earth are deflected counterclockwise relative to the geographic grid. 

• Latitude lines are circle of different sizes 

• Larger circles (ex. The equator) require more distance to be covered than shorter circles (ex. The Arctic Circle) during the Earth’s daily rotation. 

• To “keep up”, areas closer to the equator need to spin faster than at higher latitudes.  

Deflection is least at the equator and greater at the poles: 

On a small planetoid in space, Ceres, to get gravity, you need to spin really fast so the Coriolis effect is extreme. 

Atmospheric Force #4: Friction 

• As wind flows over the surface friction reduces the speed 

• Friction also changes the direction of the geostrophic wind 

• The pressure gradient force overpowers the Coriolis effect 

• As a result, wind flows across the isobars 

Forces influencing wind direction: 

Upper atmosphere 

• At 5-6 kms above the surface 

• Influenced only by pressure gradient force and Coriolis force 

• Geostrophic winds that flow parallel to isobars 

• In Northern hemisphere – cyclones spiral counterclockwise 

• Anticyclones spiral clockwise 

• Opposite in the Southern hemisphere: cyclones spiral clockwise, anticyclones spiral counterclockwise 

Doldrums: intense heating causes air to rise, leading to little horizontal motion at times. Very calm conditions 

Sublimation: when water transitions directly from ice to vapor