Pressure and WindsPressure and Winds

Aneroid BarometerAneroid Barometer

Reading Pressure

Reading the wind – measuring tools Reading the wind – measuring tools Anemometer and wind vaneAnemometer and wind vane

Weather balloon and radiosondeWeather balloon and radiosonde

These carry a number of different instruments to measure different atmospheric conditions.

Pressure Changes with TemperaturePressure Changes with Temperature

In warmer air, the molecules spread apart. This means that there is lower pressure.

In colder air, the molecules are compressed, causing higher pressure.

Notice the decreasing pressure with higher altitude. This is due to less air molecules at height and less molecules above them pressing them down.

Atmospheric Pressure

Understanding Air PressureUnderstanding Air Pressure

Notice how increased weight on top increases the pressure.

Range of atmospheric pressure

Notice the dates of the highest recorded pressures.

Hurricanes are ------ pressure systems.

What is wind?What is wind?

Wind is simply the movement of air Wind is simply the movement of air molecules in the atmospheremolecules in the atmosphere

Visualize how water flows downhill, from Visualize how water flows downhill, from higher to lower elevations, due to gravityhigher to lower elevations, due to gravity

In the same way, air molecules flow from In the same way, air molecules flow from areas of high pressure to areas of low areas of high pressure to areas of low pressure in the atmospherepressure in the atmosphere

This is windThis is wind

This shows the ridges and troughs of high altitude winds. This leads to the jet stream on the next page.

For example, look at the H and L For example, look at the H and L pressure areas here.pressure areas here.

The Jet Stream

The Jet Stream is high speed wind (>65 knots) at high altitudes in the atmosphere.

Jet streams are > 65 knotsJet streams are > 65 knotsWind speed change with height (university of wyoming data, Fort Worth Oct 23, 2002)

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Wind speed in knots

Altitude (m

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Is there a jet stream here? At what elevation? (Data is from University of Wyoming upper air radiosonde data.)

Comparing Temperature layers and Comparing Temperature layers and wind speedswind speeds

U of Wyoming graphs U of Wyoming graphs

The Jet Stream and Rossby The Jet Stream and Rossby Waves – Waves –

jet streams affect weatherjet streams affect weather

Coriolis Effect

Coriolis ForceCoriolis Force

Isobars – lines of equal pressure

Surface winds blow from high pressure towards low pressure. This is called the pressure gradient force. Due to ground friction the Coriolis Force has no effect.

Geostrophic WindGeostrophic Wind

At high altitudes there is little friction and the Coriolis Force and Pressure gradient force balance each other to produce winds that blow parallel to isobars.

What is the difference?What is the difference?

Surface winds are more affected by friction Surface winds are more affected by friction with earth’s surfacewith earth’s surface

Geostrophic winds, like the jet stream are Geostrophic winds, like the jet stream are more affected by Coriolis Forcemore affected by Coriolis Force

Factors Affecting Wind

Pressure Gradient Force – High to Low pressure

Coriolis Force – inertia caused by Earth’s rotation

Friction – only important close to surface, not at height

Pressure Gradient ForcePressure Gradient Force

Pressure Gradient Force worksheet

Effect of Coriolis ForceEffect of Coriolis Force

Effect of Friction

Effect of FrictionEffect of Friction Friction close to Friction close to

the surface the surface offsets the effect offsets the effect of the Coriolis of the Coriolis Effect. Effect.

Therefore surface Therefore surface winds are not winds are not geostrophic and geostrophic and are controlled are controlled more by the more by the pressure gradient pressure gradient force force

Gradient Wind

Surfaces cause changesSurfaces cause changes

Since friction varies with different surfaces the Since friction varies with different surfaces the effect varieseffect varies

Also, what happens with increasing friction. If the Also, what happens with increasing friction. If the wind slows down, there is less kinetic energy. wind slows down, there is less kinetic energy. Where does the energy go?Where does the energy go?

This energy can do different things which are This energy can do different things which are somewhat dependent on available water. somewhat dependent on available water.

Often the energy causes evaporation. Often the energy causes evaporation. If water is not present, the land may be slightly If water is not present, the land may be slightly

heatedheated

Ok - PhysicsOk - Physics

Obviously this is all physics. Let’s discuss this.Obviously this is all physics. Let’s discuss this. Remember Newton’s Laws?Remember Newton’s Laws? Coriolis Effect is just Inertia – 1Coriolis Effect is just Inertia – 1stst law law Balanced and unbalanced forces – In order for Balanced and unbalanced forces – In order for

acceleration, there must be an unbalanced force.acceleration, there must be an unbalanced force. How does this effect the weather? How does this effect the weather? Wind will accelerate or decelerate if the forces Wind will accelerate or decelerate if the forces

(inertia, pressure gradient, friction) are not (inertia, pressure gradient, friction) are not balanced.balanced.

Friction will cause heating and evaporation of Friction will cause heating and evaporation of waterwater

Ocean currents are also caused by winds and the Coriolis Force. Follow the direction of the currents going clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. This means that, generally speaking, cold currents go along the west coasts of continents while warm currents travel along the east coasts of continents. Check this out. This is a temperature control.

Global Circulation

Because the earth spins, the winds change direction (actually the wind goes straight and the earth changes direction – Coriolis Effect- inertia.)

LINK

Global Circulation if earth didn’t spin – from high pressure cold latitude to low pressure warm latitude - animation

Idealized Pressure Belts & Prevailing WindsTest your understanding animation

Global Atmospheric Circulation Global Atmospheric Circulation ModelModel

In the Northern Hemisphere, as winds blow towards a low pressure system they circle towards the center in a counterclockwise direction. This flow is called a cyclone. Winds circle outwards from high pressure in a clockwise direction. This is called an anticyclone.

Surface High and Low Pressure Systems

Notice the flow from high(clockwise) to low (counterclockwise). The air then rises at the center of the low pressure.

At higher altitudes, the rising air diverges and flows outwards. It descends again to form a high pressure center with air pushing downwards causing high pressure.

Cyclones and AnticyclonesCyclones and Anticyclones