# Pressure and Winds. Aneroid Barometer Reading Pressure.

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Pressure and Winds

Reading the wind – measuring tools Anemometer and wind vane

Weather balloon and radiosonde These carry a number of different instruments to measure different atmospheric conditions.

Pressure 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 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?  Wind is simply the movement of air molecules in the atmosphere  Visualize how water flows downhill, from higher to lower elevations, due to gravity  In the same way, air molecules flow from areas of high pressure to areas of low pressure in the atmosphere  This 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 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 knots Is there a jet stream here? At what elevation? (Data is from University of Wyoming upper air radiosonde data.)

Comparing Temperature layers and wind speeds  U of Wyoming graphs U of Wyoming graphs U of Wyoming graphs

The Jet Stream and Rossby Waves – jet streams affect weather

Coriolis Effect

Coriolis 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 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?  Surface winds are more affected by friction with earth’s surface  Geostrophic winds, like the jet stream are more affected by Coriolis Force

Factors Affecting Wind Pressure Gradient Force – High to Low pressure Coriolis Force Coriolis Force – inertia caused by Earth’s rotation Friction – only important close to surface, not at height

Effect of Coriolis Force

Effect of Friction

 Friction close to the surface offsets the effect of the Coriolis Effect.  Therefore surface winds are not geostrophic and are controlled more by the pressure gradient force

Surfaces cause changes  Since friction varies with different surfaces the effect varies  Also, what happens with increasing friction. If the wind slows down, there is less kinetic energy. Where does the energy go?  This energy can do different things which are somewhat dependent on available water.  Often the energy causes evaporation.  If water is not present, the land may be slightly heated

Ok - Physics  Obviously this is all physics. Let’s discuss this.  Remember Newton’s Laws?  Coriolis Effect is just Inertia – 1 st law  Balanced and unbalanced forces – In order for acceleration, there must be an unbalanced force.  How does this effect the weather?  Wind will accelerate or decelerate if the forces (inertia, pressure gradient, friction) are not balanced.  Friction will cause heating and evaporation of water

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 - animationanimation

Idealized Pressure Belts & Prevailing Winds Test your understanding animationanimation

Global Atmospheric Circulation Model

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 Anticyclones