geography Lab exercise

Atmospheric Pressure and Wind

Unit 6a

Atmospheric Pressure • pressure = force/unit area

• surface pressure increases as weight of the column of air above increases

• pressure decreases with altitude

Measurement

• Atmospheric pressure is mostly given in millibars (mb) on weather maps

• Average sea level pressure = 1013.25 mb

• Normal range: 980 mb – 1050 mb

• Surface pressures are adjusted to sea level equivalent on most surface weather maps

Barometer • Mercurial Barometer

(Torricelli, 1643)

• Aneroid Barometer

Horizontal Pressure Variation Isobars = lines of constant pressure

Pressure Gradient

• pressure gradient: – the change in pressure across a horizontal surface

• pressure gradient force (pgf): – the force acting horizontally, tending to move air toward

the direction of low pressure

– steeper pressure gradient = greater pgf

– greater pgf = greater wind speed

Pressure Gradient

• air moves from high to low pressure

• wind is greatest where isobars are closest together (steep gradient)

• wind is least where isobars are furthest apart (low gradient)

ß pressure gradient

Temperature, Pressure, Wind • Varying surface temperatures create pressure differences

• This creates “thermally-induced” pressure gradient

• Leading to wind

Dynamically-induced Pressure • Caused by converging or diverging air • Descending air causes high pressure • Ascending air causes low pressure

HIGHLOW

Wind Measurement

• Direction: – “You name a wind from whence it came”

--Mr. Balogh – Wind Vane

• wind speed – Anemometer

Wind Compass

Wind Vane & Anemometer

Factors Influencing Wind

• Pressure Gradient Force

• Coriolis Force (Coriolis Effect)

• Surface Friction

Coriolis Force

• caused by earth’s rotation • deflects wind from its intended direction: – to the right in N. Hemisphere – to the left in S. Hemisphere

Coriolis Force • Amount of deflection increases – with wind velocity – with latitude

Coriolis Force

• creates geostrophic wind in the upper troposphere

• geostrophic wind flows parallel to isobars

Surface Friction

• Frictional resistance at surface causes lower wind speed

• This reduces coriolis force

• Resulting friction layer wind (surface wind) flows at an oblique angle to isobars

High & Low Pressure Cells (for surface winds)

• Cyclone = low pressure center – N.H.: counterclockwise inward – S.H.: clockwise inward

• Anticyclone = high pressure center – N.H.: clockwise outward – S.H.: counterclockwise outward