Lecture 10

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# Lecture 10 - PowerPoint PPT Presentation

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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Presentation Transcript

### 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
• An “altimeter” is a type of aneroid barometer which is calibrated to indicate altitude.
• A “barograph” is a recording aneroid barometer.
• When reading the pressure at a certain place we are reading the “station pressure”.
• Pressure changes quickly in the vertical.
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.
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?
• The wind we experience is a result of a combination of forces:
• Coriolis Effect
• Centripetal Force
• Friction

### Let’s Play a Game!

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
• 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.

### More movies!

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 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

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
• At the surface we must now consider friction!
• Another example
Southern Hemisphere
• How would the winds flow around highs and lows in the Southern Hemisphere?
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
• 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.