Atmospheric Pressure and Wind Frode Stordal, University of Oslo

Transcription

1 Chapter 4 Lecture Understanding Weather and Climate Seventh Edition Atmospheric Pressure and Wind Frode Stordal, University of Oslo Redina L. Herman Western Illinois University

2 The Concept of Pressure Earth contains a number of gas molecules that exert a force on all surfaces and the amount of force exerted per unit of surface area is pressure. Pressure is measured in millibar or pascal. Sea-level pressure is about 1000 mb ( mb). Total pressure expressed through Dalton s Law, the sum of partial pressures exerted by individual gases. Pressure is exerted in all directions equally.

3 The Concept of Pressure Molecular movement in a sealed container (a). Pressure increased by increasing density (b) or temperature (c).

4 Vertical and Horizontal Changes in Pressure Pressure decreases with altitude. All recording stations are reduced to sea-level pressure in order to make horizontal comparisons. Compressibility of atmospheric gases results in a nonuniform decrease of pressure with height.

5 Vertical and Horizontal Changes in Pressure Pressure does not decrease with height in a uniform rate. It decreases most rapidly at low elevations and gradually tapers off at greater altitudes.

6 The Equation of State Pressure, temperature, and density are related to one another and their relationship can be described through the equation of state (ideal gas law). The equation of state results in the following: p = ρ R T p ρ R Pressure Density Gas constant At constant temperatures, an increase in air density will cause pressure to increase. Under constant density, an increase in temperature will also cause an increase in pressure.

7 The Distribution of Pressure It is important to view pressure differences. Pressure maps depict isobars, or lines of equal pressure. Pressure gradients depict the rate of change in pressure. They are apparent on maps by the spacing between the isobars. Steep pressure gradients are indicated by closely spaced isobars. Weak pressure gradients are indicated by widely spaced isobars. A weather map depicting the sea-level pressure distribution for March 4, 1994.

8 The Distribution of Pressure Pressure Gradients The pressure gradients provide the movement of air commonly known as wind. The strength of the pressure gradient force determines the horizontal wind speed. Horizontal Pressure Gradients Typically, small gradients exist across large areas. Concentrated weather features, such as hurricanes and tornadoes, display larger pressure gradients across small areas. Vertical Pressure Gradients Vertical pressure gradients are greater than extreme examples of horizontal pressure gradients as pressure always decreases with altitude.

9 The Distribution of Pressure Hydrostatic Equilibrium Gravity balances strong vertical pressure gradients to create hydrostatic equilibrium. Local imbalances create various up- and downdrafts p/ z = -ρg The Role of Density in Hydrostatic Equilibrium Gravitational force is proportional to mass. A dense atmosphere needs greater gravitational force to remain in balance. For warm air, this equates to smaller vertical pressure gradients leading to hydrostatic equilibrium. For cold air, this equates to larger vertical pressure gradients leading to hydrostatic equilibrium.

10 The Distribution of Pressure 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.

11 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.

12 Forces Affecting the Speed and Direction of the Wind The Coriolis Force Objects in the atmosphere are influenced by Earth s rotation. Overall, the result is a deflection of moving objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. F c = 2Ωsin(φ)v Force/mass (acceleration) Ω φ v Earth s rotation rate Latitude velocity

13 Forces Affecting the Speed and Direction of the Wind The Coriolis Force Coriolis deflection increases from zero at the equator to a maximum at the poles. The deflective force also increases with the speed of the moving object. Takes place regardless of the direction of motion.

14 Forces Affecting the Speed and Direction of the Wind The Coriolis Force

15 Forces Affecting the Speed and Direction of the Wind Friction A force of opposition which slows air in motion. Initiated at the surface and extends, decreasingly, aloft. Important for air within ~1.5 km of the surface, the planetary boundary layer. Because friction reduces wind speed, it also reduces Coriolis force. Friction above ~ 1.5 km (free atmosphere) is negligible.

16 Winds Aloft and Near the Surface Gradient Flow Upper air moving from areas of higher pressure 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.

17 Winds Aloft and Near the Surface Winds in the Upper Atmosphere 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.

18 Winds Aloft and Near the Surface Gradient Flow

19 Winds Aloft and Near the Surface Near-Surface Winds Winds near the surface slow due to friction and therefore are not parallel to the isobars. They cross the isobars. Coriolis deflection still occurs but it is reduced.

20 Anticyclones, Cyclones, Troughs, and Ridges Cyclones

21 Anticyclones, Cyclones, Troughs, and Ridges Anticyclones

22 Anticyclones, Cyclones, Troughs, and Ridges Troughs and Ridges Low and high pressure systems occur as elongated areas called troughs (low pressure) and ridges (high pressure). Pressure is distributed as cyclones and anticyclones at the surface and gradually give way to ridges and troughs in the upper atmosphere.

23 Anticyclones, Cyclones, Troughs, and Ridges Troughs and Ridges Ridges and troughs in the Northern Hemisphere.