Understanding Weather and Climate 3rd Edition Edward Aguado and James E. Burt Anthony J. Vega

Chapter 4. Atmospheric Pressure and Wind Part 1. Energy and Mass Chapter 4. Atmospheric Pressure and Wind

Introduction Atmospheric pressure = force per unit area exerted by atmospheric gases Expressed in millibar or pascal Surface pressure is approximately 1000 mb (1013.25 mb) Total pressure expressed through Dalton’s Law, the sum of partial pressures exerted by individual gases Or, the weight of the overlying air (sea level weight = 14.7lbs/in2) Pressure is exerted in all directions equally, not just downward Pressure is a function of density and temperature

Vertical and Horizontal Changes in Pressure Pressure decreases with height Recording actual pressures may be misleading as a result All recording stations are reduced to sea level pressure equivalents to facilitate horizontal comparisons Compressibility of atmospheric gases causes a non-linear decrease in pressure with height Pressure will be less at P2 than at P1 due to pressure decreasing with height

The Equation of State Relationships between pressure, temperature, and density are described through the equation of state (ideal gas law) Indicates that at constant temperatures, an increase in air density will trigger a pressure increase Under constant density, an increase in temperature will be accompanied by an increase in pressure Molecular movement in a sealed container. Pressure increased by increasing density (b) or temperature (c)

Measurement of Pressure Mercury Barometers, invented by Torricelli Utilizes an inverted tube filled with mercury Height of mercury indicates downward force of air pressure Measurement frequently given as a distance Average Sea Level Pressure = 76 cm (29.92 in) of mercury Corrections to Mercury Barometer Readings Three barometric corrections must be made to ensure homogeneity of pressure readings First corrects for elevation, the second for air temperature (affects density of mercury), and the third involves a slight correction for gravity with latitude Aneroid Barometers Use a collapsible chamber which compresses proportionally to air pressure Requires only an initial adjustment for elevation

Aneroid barometer (left) and its workings (right) A barograph continually records air pressure through time

The Distribution of Pressure It is useful to examine horizontal pressure differences across space Pressure maps depict isobars, lines of equal pressure Through analysis of isobaric charts, pressure gradients are apparent Steep (weak) pressure gradients are indicated by closely (widely) spaced isobars A weather map depicting the sea level pressure distribution for March 4, 1994

Horizontal Pressure Gradients The pressure gradient force initiates movement of atmospheric mass, wind, from areas of higher to areas of lower pressure Horizontal wind speeds occur relative to the strength of the pressure gradient Horizontal Pressure Gradients Typically only small gradients exist across large spatial scales Smaller scale weather features, such as hurricanes and tornadoes, display larger pressure gradients across small areas Vertical Pressure Gradients Average vertical pressure gradients are usually greater than extreme examples of horizontal pressure gradients as pressure always decreases with altitude

A surface pressure chart

Hydrostatic Equilibrium The downward force of gravity balances strong vertical pressure gradients to create hydrostatic equilibrium Forces balance and the atmosphere is held to Earth’s surface Local imbalances initiate various up- and downdrafts The Role of Density in Hydrostatic Equilibrium Gravitational force is relative to mass A dense atmosphere experiences greater gravitational force A vertical pressure gradient must increase to offset increased gravitational force Higher temperature columns of air are less dense than cooler ones For warm (cold) air this equates to smaller (larger) vertical pressure gradients leading to hydrostatic equilibrium

Heating causes a density decrease in a column of air. The column contains the same amount of air, but has a lower density to compensate for its greater height.

Horizontal Pressure Gradients in the Upper Atmosphere Upper air pressure gradients are best determined through the heights of constant pressure due to density considerations Constant pressure surfaces of cooler air will be lower in altitude than those of warmer air Height contours indicate the pressure gradient Twice daily, heights are drawn at 60 m intervals for the 850, 700, 500, and 3000 mb levels 500 mb height contours for May 3, 1995

Upper air heights decrease with latitude

Forces Affecting the Speed and Direction of Wind The Coriolis Force Free moving objects in the atmosphere are influenced by Earth rotation A resulting apparent deflective force This causes two types of motion Translational movement, movement of an object from one place to another Caused by planet rotation Proportional to latitude Rotational movement, indicative of vorticity Also related to latitude such that it is maximized at the poles and zero at the equator Overall result is a deflection (if viewed from surface) of moving objects to the right (left) in the northern (southern) hemisphere

The deflective force also increases with speed of the moving object Coriolis deflection increases from zero at the equator to a maximum at the poles The deflective force also increases with speed of the moving object Overall force is weak Noticeable deflection only on objects with long periods of motion Takes place regardless of the direction of motion Coriolis deflection

Friction A force of opposition which slows air in motion Initiated at the surface and extend, decreasingly, aloft Important for air within 1.5 km (1 mi) of the surface, the planetary boundary layer Because friction reduces wind speed it also reduces Coriolis deflection Friction above 1.5 km is negligible Above 1.5 km = the free atmosphere

Winds in the Upper Atmosphere Upper air moving from areas of higher to areas of lower pressure undergo Coriolis deflection Air will eventually flow parallel to height contours as the pressure gradient force balances with the Coriolis force This geostrophic flow (wind) may only occur in the free atmosphere

Supergeostrophic and Subgeostrophic Flow Height contours and pressure distributions are frequently curved Air flow remains parallel to the height contours This flow is not truly geostrophic as forces are temporarily out of balance Around high pressure areas, air undergoes rapid acceleration and the Coriolis force dominates the pressure gradient force producing supergeostrophic conditions Around low pressure areas, subgeostrophic conditions occur as the pressure gradient force dominates a weaker Coriolis force Both supergeostrophic and subgeostrophic conditions result in airflow parallel to curved height contours Termed gradient flow

Supergeostrophic flow Subgeostrophic flow

Cyclones, Anticyclones, Troughs, and Ridges Global air pressure is typically divided into a number of small-scale high and low pressure areas High pressure areas, or anticyclones, have clockwise (counterclockwise) airflow in the northern (southern) hemisphere This occurs as air diverges from the high pressure areas at the surface and is deflected by Coriolis acceleration which turns air to the right (left) Characterized by descending air which warms creating clear skies Opposite conditions occur relative to low pressure areas, or cyclones Air converges toward low pressure centers, cyclones are characterized by ascending air which cools to form clouds and possibly precipitation In the upper atmosphere, ridges correspond to surface anticyclones while troughs correspond to surface cyclones

Coriolis deflection into a low pressure system in the northern hemisphere

Ridges and troughs in the northern hemisphere

Maps depicting troughs, ridges, cyclones, and anticyclones

Measuring Wind Wind direction always indicates the direction from which wind blows An azimuth indicates the degree of angle from due north moving clockwise from 0 to 360o Wind vanes are devices which indicate wind direction while anemometers record wind speed An aerovane indicates both wind speed and direction Similar devices are attached to balloons to record upper air conditions An azimuth

An aerovane

End of Chapter 4 Understanding Weather and Climate 3rd Edition Edward Aguado and James E. Burt