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Lecture 10. Pressure and Wind. Also called “Barometric Pressure” We measure pressure with a barometer. The first barometers were mercury barometer. The aneroid barometer is commonly used in homes. Atmospheric Pressure. Atmospheric Pressure.

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Lecture 10 l.jpg

Lecture 10

Pressure and Wind


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Also called “Barometric Pressure”

We measure pressure with a barometer.

The first barometers were mercury barometer.

The aneroid barometer is commonly used in homes.

Atmospheric Pressure


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Atmospheric Pressure

  • An “altimeter” is a type of aneroid barometer which is calibrated to indicate altitude.

  • A “barograph” is a recording aneroid barometer.


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Reading Pressure

  • When reading the pressure at a certain place we are reading the “station pressure”.

  • Pressure changes quickly in the vertical.



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Wind

  • Thus far we have looked at vertical motion, in terms of cloud development.

  • We also have horizontal motions referred to as “wind”.

  • Let’s look at what causes the wind.


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What happens when you open a can of coffee?

  • The noise you hear is due to air rushing into the can.

  • Air moves from high to low pressure.

  • Wind is nature’s attempt to balance inequalities in air pressure


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What causes the wind?

  • The wind we experience is a result of a combination of forces:

    • Pressure Gradient Force

    • Coriolis Effect

    • Centripetal Force

    • Friction






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Imagine that you are on the North Pole

You want to launch an object towards Madison, where do you aim towards?

Hint: Will Madison be in the same place by the time the object gets there?


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Coriolis Effect

  • The coriolis effect modifies the wind.

  • All free moving objects or fluids in the northern hemisphere are deflected to the right.

  • In the southern hemisphere they are deflected to the left.

  • The coriolis effect is result of the earth spinning.



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The Earth’s Spin

  • The N. Hemisphere spins counter-clockwise  coriolis deflects to the right

  • The S. Hemisphere spins clockwise  coriolis deflects to the left

  • The wind is deflected by coriolis differently in each hemisphere.



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Coriolis at the Equator

  • Coriolis is zero at the equator!

  • However, as soon as an object moves just a little off of the equator, coriolis will act on it.




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Curved Winds

Gradient Wind


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Winds Aloft

  • Winds that are not influenced by surface friction and therefore behave as geostrophic winds and gradient winds.

  • To look at winds at upper levels we look on constant pressure charts.

    • i.e. 850mb, 700mb, 500mb, 250mb charts

  • These charts are at a constant pressure level, and have contours of height.





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SFC VS. Aloft

  • At the surface we must now consider friction!

  • Another example



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Southern Hemisphere

  • How would the winds flow around highs and lows in the Southern Hemisphere?




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Wind as a Force

  • Near the surface the wind speed goes to zero as a result of friction.

  • This is why it is often more difficult on a windy day to drive a “tall” vehicle.

  • Or why it is windier on a bridge.

  • Wind exerts a force on objects, if the objects do not move the wind moves around the object.


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Measuring Wind

  • Wind speed and direction are measured.

  • Winds are named for the direction they come from.

    • Wind vanes are used to measure wind direction.

    • When the wind consistently blows from one direction, that direction is called the prevailing wind.


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