Chapter 8 Air Pressure and Winds

Why does the wind blow? Differences in air pressure (caused by differences in temperatures) the wind blows in an attempt to equalize these imbalances air want to flow from high pressure to low pressure does the wind always blow directly from high pressure to low pressure? Not always… Other forces are involved Example: opening a window during the summer, opening the freezer door

Air Pressure mass of air above a given level fewer air molecules as we climb in elevation, so pressure is lower air pressure, air density, and air temperature are all interrelated (if one changes all change)

Atmospheric Pressure Similar to earth's atmosphere, the pressure at the base of this column of air results from the weight of the gases above. What happens to the pressure if the temperature remains constant but we add more air (density)?

Temperature & Elevation When two columns of air are equal in elevation and density, they are at equilibrium. Adjusting temperatures by cooling (or heating) increases (or decreases) air density. At the surface, equilibrium is maintained, but the taller column has greater upper-level pressure, and winds are generated.

Formula for Density of dry air = P/(R x T) = density P = Pressure R = specific gas constant = 287 J/KgK (dry air) T = Temperature (K) What happens to density if temperature increases? What happens to density if pressure increases? What happens to Pressure if temperature increases?

It takes a shorter column of cold (more dense) air, to exert the same surface pressure as a taller column of warm (less dense) air Atmospheric pressure decreases more rapidly with height in a cold column of air Warm air aloft is normally associated with high atmospheric pressure and cold air aloft is associated with low atmospheric pressure a horizontal difference in temperature creates a horizontal difference in pressure called a pressure gradient force (PGF) The PGF causes air to move from higher pressure toward lower pressure

To summarize: heating and cooling of a column of air can establish horizontal differences in pressure that forces air to move (PGF) net accumulation of air above the surface cause the surface air pressure to rise decrease in amount of air above a surface cause the surface pressure to fall Over large continental areas (like the Midwest) in the summer, hot surface air is marked by a surface low pressure During the winter, cold arctic air masses are accompanied by a surface high pressure

Pressure measurements millibar (mb) inches of mercury (in. Hg) hecto pascal (hPa) standard atmospheric pressure 1013.25mb = 1013.25hPa = 29.92 in. Hg

Pressure Scale & Units Many scales are used to record atmospheric pressure, including inches of mercury (Hg) and millibars (mb). The National Weather Service uses mb, but will convert to metric units of hectopascals (hPa). The conversion is simply 1 hPa = 1 mb.

Pressure Reading & Reporting Increase terrain elevation and decrease column of air above. To remove the effect of elevation, station pressure is readjusted to sea level pressure at ~10mb/100m. Isobars show geographic trends in pressure, and are spaced at 4 mb intervals.

Smoothed Isobar Maps Continental maps of station recorded sea-level pressure are often smoothed and simplified to ease interpretation. Smoothing adds error to those already introduced by error in instrument accuracy.

Smoothed Isobar Maps Continental maps of station recorded sea-level pressure are often smoothed and simplified to ease interpretation. Smoothing adds error to those already introduced by error in instrument accuracy.

Variation in Height Isobaric (constant pressure) surfaces rise and fall in elevation with changes in air temperature and density. A low 500 mb height indicates denser air below, and less atmosphere and lower pressure above. Contour lines indicate rates of pressure change.

Ridges & Troughs Upper level areas with high pressure are named ridges, and areas with low pressure are named troughs. These elongated changes in the pressure map appear as undulating waves.

Surface & 500 mb Maps Surface maps chart pressure contours, highs and lows, and wind direction. Winds blow clockwise around highs, called anticyclones. 500 mb maps reveal patterns that on average are 5600 m above the surface, where westerly winds rise and fall across ridges and troughs.

Forces that influence winds Pressure Gradient Force Coriolis Force centripetal force friction

Pressure Gradient Force (PGF) PG = ∆P/d

Pressure Gradient Force (PGF) Change in pressure per change in distance determines the magnitude of the pressure gradient force (PGF). Greater pressure changes across shorter distances creates a larger PGF to initiate movement of winds.

PGF vs. Cyclonic Winds Pressure gradient force (PGF) winds acting alone would head directly into low pressure. Surface observations of winds, such as the cyclonic flow around this low, reveal that PGF winds are deflected by other forces.

Coriolis Force = 2mΩsin apparent force due to the rotation of the Earth causes wind to deflect to the right (NH) and to the left (SH) varies with speed of object -as wind speed increases, coriolis increases varies with latitude - coriolis increases as latitude increases - coriolis = 0 at the equator - coriolis = Maximum (at the poles) Coriolis Force = 2mΩsin

Apparent Coriolis Force Figure 9.20 Earth's rotation transforms straight line motion into curved motion for an outside viewer. The Coriolis force explains this apparent curvature of winds to the right due to rotation. Its magnitude increases with wind velocity and earth's latitude.

Actual & Observed Paths Airplane travel paths have an apparent curvature, just as Coriolis forces affect winds. Again, the deflection between actual and observed paths is greater near the poles.

Geostrophic Wind Winds have direction and magnitude, and can be depicted by vectors. Observed wind vectors are explained by balancing the pressure gradient force and apparent Coriolis force. These upper level geostrophic winds are parallel to pressure contours.

Wind Speed & Pressure Contours Just as a river speeds and slows when its banks narrow and expand, geostrophic winds blowing within pressure contours speed up as contour intervals narrow, and slow as contour intervals widen.

Centripetal Acceleration & Cyclones Acceleration is defined by a change in wind direction or speed, and this occurs as winds circle around lows (cyclones) and highs (anticyclones). Centripetal force is the term for the net force directing wind toward the center of a low, and results from an imbalance between the pressure gradient and Coriolis forces.

Friction & Surface Winds Surface objects create frictional resistance to wind flow and slows the wind, diminishing the Coriolis force and enhancing the effect of pressure gradient forces. The result is surface winds that cross isobars, blowing out from highs, and in toward lows.

Surface Flow at Lows & Highs Southern Hemisphere flow paths are opposite in direction to Northern Hemisphere paths, but the same principles and forces apply.

Sensing Highs & Lows The location of high and low pressure centers are estimated by detecting surface wind direction and noting pressure, Coriolis, and friction forces. This figure illustrates the procedure when standing aloft and at the surface.

Vertical Air Motion Winds converging into a low pressure center generate upward winds that remove the accumulating air molecules. These updrafts may cause cloud formation. Likewise, diverging air molecules from a high pressure area are replenished by downward winds.