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.
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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0 10 20 30 40 50 60 70 80 90
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
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
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
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
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.