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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

Lecture 10

Pressure and Wind

atmospheric pressure
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 pressure3
Atmospheric Pressure
  • An “altimeter” is a type of aneroid barometer which is calibrated to indicate altitude.
  • A “barograph” is a recording aneroid barometer.
reading pressure
Reading Pressure
  • When reading the pressure at a certain place we are reading the “station pressure”.
  • Pressure changes quickly in the vertical.
  • 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.
what happens when you open a can of coffee
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
what causes the wind
What causes the wind?
  • The wind we experience is a result of a combination of forces:
    • Pressure Gradient Force
    • Coriolis Effect
    • Centripetal Force
    • Friction
imagine that you are on the north pole
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?

coriolis effect
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.
the earth s spin
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.
coriolis at the equator
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.
curved winds
Curved Winds

Gradient Wind

winds aloft
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.
sfc vs aloft
SFC VS. Aloft
  • At the surface we must now consider friction!
  • Another example
southern hemisphere
Southern Hemisphere
  • How would the winds flow around highs and lows in the Southern Hemisphere?
wind as a force
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.
measuring wind
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.