chapter 6

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Air Pressure and Winds

Chapter 6

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6.1 Wind and Air Pressure

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+Wind and Air Pressure

Wind is the result of horizontal differences in atmospheric pressure.

Atmospheric (air) pressure is the force exerted against a surface by continuous collision of gas molecules.Atmosphere is without walls but the force of

gravity prevents its escape.

+Wind and Air Pressure

The air pressure at sea level is about 1 kg/cm2.

Specifically, a column of air is 1 sq inch in cross section, measured from sea level to the top of the atmosphere would weigh 14.7 lbs.

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6.2 Measuring Air Pressure

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+Measuring Air Pressure

Air pressure is measured in millibars.1 millibar (mb) = 100 newtons/m2

1013.25 mb = sea level pressure

The mercury barometer is the standard pressure-measuring instrument. Air pressure pushes on a pool of mercury, which forces

the mercury up a tube.High pressure: The mercury goes further up the

tube. Low pressure: The mercury stays lower.

+Measuring Air Pressure

Fair weather = high pressure

Rain = low pressure

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+Measuring Air Pressure

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6.3 Pressure Changes with Altitude

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+Pressure Changes with Altitude

Air pressure is defined as the force exerted against a surface by continuous collision of gas molecules.Air pressure decreases with altitude.

Because of the decrease of pressure above youAir pressure increases with depth.

Because of the increase of pressure above you

+Pressure Changes with Altitude

Air pressure decreases about 10mb per 100m increase in elevation

Air pressure is reduced by approximately ½ for each 5 km increase in altitude.

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6.4 Why does Air Pressure Vary?

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+Why Does Air Pressure Vary?

Recall that variations in air pressure causes the wind to blow, which in turn causes changes in temperature and humidity

Water vapor affects air pressure.The amount of water vapor reduces the density of air.Cold dry air has higher pressure.Warm, dry air has higher pressure than equally warm,

moist air.

+Why Does Air Pressure Vary?

The movement of air can also cause variations in air pressure.Convergence occurs when a net airflow into a

region causing pressure to rise.Divergence results when there is a net outflow

of air from a region and surface pressure drops.

+Why Does Air Pressure Vary?

Temperature influences air pressure.

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6.5 Factors Affecting Wind

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+Factors Affecting Wind

Pressure gradient force

Coriolis force

Friction

+Factors Affecting Wind- Pressure Gradient Force

The force that generates winds results from horizontal pressure differences. Greater differences the greater the wind blows

Pressure gradient forceSpacing of the isobars indicate the amount of

pressure change occurring over a given distance.

+Factors Affecting Wind- Pressure Gradient ForceIsobars are lines connecting places of equal pressure.

If isobars are close together a steep gradient and high winds result.

If isobars are far apart, the result is a low gradient and lower wind speeds.

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+Factors Affecting Wind- Pressure Gradient ForceHow temperature differences generate

wind

Sea Breeze figure 6-11 page 169

A- Temp and pressure don’t vary horizontally

+Factors Affecting Wind- Pressure Gradient ForceB- as the sun rises the earth begins to

warm. Air above the land warms and expands creating a high pressure area (less dense air column) and the air aloft begins to flow away from the land

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+Factors Affecting Wind- Pressure Gradient ForceC- The mass transfer of air aloft creates a

surface high pressure area over the land. The surface circulation that develops from this redistribution of mass is from the sea towards the land = sea breeze.

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+Factors Affecting Wind- Pressure Gradient Force

Isobars on a surface chart: Low-pressure systems (L) that occur in the

middle latitudes are called midlatitude cyclones. Produce stormy weather

High-pressure systems (H) are called anticyclones, which tend to be associated with clearing conditions.

+Factors Affecting Wind- Coriolis Force

The Coriolis force cannot generate wind; it modifies airflow.

The Coriolis force causes all free flowing objects including wind:In the Northern Hemisphere, a deviation to the

right is due to the Earth’s rotation.In the Southern Hemisphere, the deviation is to

the left.

+Factors Affecting Wind- Coriolis Force

The Coriolis force is a deflecting force.It is always directed at right angles to the

direction of airflow. It controls only wind direction, not speed.It is affected by wind speed (the stronger the

wind, the greater the force).The Coriolis force is strongest at the poles and

is nonexistent at the equator.

+Factors Affecting Wind- Coriolis Force

Coriolis force

+Factors Affecting Wind- Friction

Remember pressure gradient force is the primary driving force for winds. Air moves from high to low concentrations but wind speeds don’t increase indefinitely.

Friction significantly influences airflow near Earth’s surface, but its effect is negligible at higher altitudes.

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6.6 Winds Aloft

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+Winds Aloft

Geostrophic flow occurs when the pressure gradient force and the Coriolis force are balanced.The wind flows parallel to the isobars.They flow in relatively straight paths.

Wind Direction is directly related to prevailing pressure patterns. So if we know direction we can establish a rough approximation of the pressure distribution.

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+Winds Aloft

Buys Ballot’s Law states that if you stand with the wind at your back, low pressure will be found on your left, high pressure on the right.

The situation is reversed in the southern hemisphere.

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+Winds Aloft

Curved airflow and the gradient wind

Curved airflow:Winds around cells

of high pressure or low pressure follow curved paths.

+Winds Aloft

Gradient winds blow at a constant speed.They are parallel to the curved isobars.Centers of low pressure are called cyclonic; winds

flow counter-clockwise in the Northern Hemisphere.A trough is the result of isobars curving to form

elongated regions of low pressure.Centers of high pressure are anticyclonic; winds flow

clockwise in the Southern Hemisphere.A ridge is the result of isobars curving to form

elongated regions of high pressure.Opposite is true in the Southern Hemisphere

+Winds Aloft

Curved airflow and the gradient wind

+Winds Aloft

In Low Pressure Centers- Figure AThe inward directed pressure gradient force is

opposed by the outward directed Coriolis force. The pressure gradient force must exceed the coriolis force to overcome the air’s tendency to continue moving in a straight line.

In High Pressure Centers- Figure BPressure gradient force and Coriolis force are not

balanced. The imbalance provides a change in direction and generates the curved flow

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6.7 Surface Winds

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+Surface Winds

Surface winds travel at an angle across isobars, toward low pressure.

+Surface Winds

Air flow crosses the isobars at varying angles depending on friction for terrain, but winds always move from high to low pressure.

In a cyclone pressure decreases inward and friction causes a new flow towards its center

In an anticyclone pressure decreases outward and friction causes a net flow away from its center

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6.8 How Winds Generate Vertical Air Motion

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+How Winds Generate Vertical Air Motion

Factors that promote vertical airflow:Friction

Increased friction causes a drop in wind speed resulting in a pileup of air upstream from the ocean to land.

Convergence can result in cloudy weather.Decreased friction causes and increase in wind

speed from the land to the ocean. Subsidence and divergence results in clearing

weather.

+How Winds Generate Vertical Air Motion

Vertical airflow is associated with cyclones and anticyclones.

+How Winds Generate Vertical Air Motion

CyclonesHorizontal convergence- net inward transport of

air causes a shrinking of the area it occupiesThis causes a pile up of air creating a dense

column which increases pressureTO compensate aloft, divergence (spreading

out) occurs at an equal rate to inflow belowRemember rising air causes bad weather

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+How Winds Generate Vertical Air Motion

AnticyclonesConvergence (pulling in) happens aloft and

general subsidence of the air column happensThis creates good, nice weather

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6.9 Wind Measurements

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+Wind Measurement

Wind direction is always measured from the direction it is coming.A North wind blows north to south.A East wind blows east to west.

A prevailing wind consistently blows more often than not from one direction.

+Wind Measurement

+Wind Measurement

A wind vane shows wind direction.

Wind speed is often measured with a cup anemometer, which has a dial that shows wind speed.

An aerovane is a combination of wind vane and anemometer.