Factors Affecting Wind

1. 2 Factors Affecting Wind • We discussed the upward movement of air and its importance in cloud formation. We now turn our attention to horizontal movement - the phenomenon we call wind. • What causes air to move horizontally? Air flows from areas of higher pressure to areas of lower pressure. Because unequal heating of Earth’s surface continually generates these pressure differences, solar radiation is the ultimate driving force of wind. • Wind is controlled by a combination of forces: 1) The pressure-gradient force (PGF) 2) The Coriolis force 3) Friction • Once the air starts to move, the Coriolis force and friction come into play, but only to modify the movement - not to produce it.

2. 3 Pressure-Gradient Force: the force that generates wind • The force that generates winds results from horizontal pressure differences. • Variations in air pressure over Earth’s surface are determined from barometric readings taken at hundreds of weather stations. These pressure data are shown on surface weather maps by means of isobars. Isobars are lines connecting places of equal air pressure. The spacing of the isobars indicates the amount of pressure change occurring over a given distance and is expressed as the pressure gradient. • Thus closely spaced isobars indicate a steep pressure gradient and strong winds; widely spaced isobars indicate a weak pressure gradient and light winds. • Note that the pressure gradient force is always directed at right angles to the isobars. • The underlying cause of pressure differences observed on the daily weather map is simply unequal heating of Earth’s land-sea surface. • To illustrate how temperature differences can generate a pressure gradient and thereby create winds, we look at a common example: the sea breeze.

3. The Sea Breeze • Figure LT 6-10a shows a vertical cross section of a coastal location just before sunrise. In the present dscussion we assume that temperatures and pressures do not vary horizontally at any level. This assumption is shown by the horizontal pressure surfaces that indicate the equal pressure at equal heights. Because there is no horizontal variation in pressure (zero pressure gradient), there is no wind. • After sunrise the unequal rates at which land and water heat will initiate pressure differences and, therefore, airflow. Surface temperatures over the ocean change only slightly on a daily basis. On the other hand, land surfaces and the air above can be substantially warmed during a single daylight period. • Heating the land warms the air. The air expands and “lifts” the isobars. This warming does not by itself produce a surface pressure change. However,a given pressure surface aloft does become elevated over the land compared to over the ocean. The resultant horizontal pressure gradient aloft causes the air aloft to move from the land toward the ocean. • The warmer air aloft moves over the ocean, cools and sinks. This creates a surface high-pressure area over the ocean, where the air is collecting. Now the pressure near the ocean surface is greater than near the land surface. Thus a horizontal pressure gradient is created that causes air near the surface to move from the ocean to the land. (sea breeze). 4

4. 5 Coriolis Force: How does the Earth’s rotation modify the Wind? • Wind does not cross the isobars at right angles as the pressure-gradient force directs. • All free-moving objects, including wind, are deflected to the right of their path of motion in the Northern Hemisphere and to the left of their path of motion in the Southern Hemisphere. • The reason for this deflection is the Coriolis force: where m is the mass and u is the velocity vector of a fluid parcel, and  is the rotation vector of the Earth. • The magnitude of the Coriolis force is: where  is the latitude, f = 2 sin is called the Coriolis parameter, and u is the magnitude of the velocity. • The Coriolis force written in vector form clearly indicates that • It is directed at right angles to the direction of air flow. • It affects only wind direction, not the wind speed. • Its magnitude is affected by wind speed (the stronger the wind, the greater the deflecting force). • Its magnitude increases from zero at the Equator to a maximum at the poles. • The Coriolis force thus has the effect of deflecting air flow. It also has the effect of deflecting ocean currents.

5. 6 Coriolis effect.

6. 7 Coriolis deflection of winds blowing eastward.

7. 8 • Coriolis deflection of winds blowing in different directions: • Northern Hemisphere: All free-moving objects, including wind, are deflected to the right of their path of motion. • Southern Hemisphere: All free-moving objects, including wind, are deflected to the left of their path of motion.

8. 9 Geostrophic Wind • A few kilometers above the surface the effect of friction is negligible, and thus the Coriolis force is responsible for balancing the pressure-gradient force and thereby directing airflow. • To illustrate, consider an air parcel initially at rest at ‘Starting point’ in Fig LT 6-13. Since our parcel is at rest, the Coriolis force acting on it is zero; only the pressure-gradient force acts on it. Under the influence of the pressure-gradient force, which is always directed perpendicularly to the isobars, the parcel begins to accelerate directly toward the area of low pressure. As soon as the flow begins, the Coriolis force starts to act and causes a deflection to the right of the path of motion (since our parcel is in the Northern Hemisphere). As the parcel continues to accelerate, the Coriolis force intensifies. Thus, the increased speed results in further deflection. Eventually the wind turns so that it is flowing parallel to the isobars. When this occurs the pressure-gradient force and Coriolis force balance and the airflow is said to be in geostrophic balance. Winds generated by this balance are called geostrophic winds.

9. 10 Curved Flow and the Gradient Wind • Isobars are not generally straight; instead, they make broad sweeping curves and occasionally connect to form roughly circular cells of either high or low pressure. Thus winds around cells of high or low pressure follow curved paths in order to parallel the isobars. Winds of this nature, which blow at a constant speed parallel to curved isobars, are called gradient winds. • How do the Coriolis force and pressure-gradient force combine to produce gradient winds? • Fig LT 6-15a shows the gradient flow around a centre of low pressure. As soon as the flow begins, the Coriolis force causes the air to be deflected. In the Northern Hemisphere the resulting wind blows counterclockwise about a low.

10. 11 Gradient Wind - continued… • Because the Coriolis force deflects the winds to the left in the Southern Hemisphere, the flow is reversed there - clockwise around low pressure centres and counterclockwise around high pressure centres. • It is common practice to call all centres of low pressure cyclones and the flow around the cyclonic. Cyclonic flow has the same direction of rotation as Earth: counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. • Centres of high pressure are frequently called anticyclones and exhibit anticyclonic flow (opposite that of Earth’s rotation). • Whenever isobars curve to form elongated regions of low and high pressure, these areas are called troughs and ridges, respectively. The flow about a trough is cyclonic; the flow around a ridge is anticyclonic. • Referring back to Fig LT 6-15a, we see that in flow about a low pressure centre, the inward-directed pressure-gradient force is opposed by the outward-directed Coriolis force plus an outward centrifugalforce, which is generated by motion along a curved path. Thus the pressure-gradient force must exceed the Coriolis force to keep the path curved (parallel to the isobars) - that is, to produce the gradient wind. • The opposite situation exists in anticyclonic flows, where the inward-directed Coriolis force must balance the combined pressure-gradient force and centrifugal force to produce the gradient wind. • Despite the importance of the centrifugal force in establishing curved flow aloft, near the surface, friction comes into play and greatly overshadows this much weaker force.

11. 12 Surface Winds • Friction has an important effect on wind only within the first few kilometers of Earth’s surface. • Friction acts to slow the movement of air. By slowing air movement, friction also reduces the Coriolis force, which is proportional to wind speed. This alters the force balance in favour of the pressure-gradient force with the outcome that there is a movement of air at an angle across the isobars toward the area of lower pressure. • In a cyclone pressure decreases inward. Thus friction causes a net flow toward its centre. Therefore, the resultant winds blow into and counterclockwise about a surface cyclone. • In an anticyclone the oppose is true: the pressure decreases outward and thus friction causes a net flow away from the centre. Therefore, the resultant winds blow outward and clockwise about a surface anticyclone. • In whatever hemisphere, friction causes a net inflow (convergence) around a cyclone and a net outflow (divergence) around an anticyclone.

12. 13 How Winds Generate Vertical Air Flow • First consider the situation around a surface low pressure system (cyclone) in which the air is spiraling inward. Here the net inward transport of air causes a shrinking of the area occupied by the air mass, a process called horizontal convergence. Whenever air converges horizontally, it must pile up. That is, it must increase in height to allow for the decreased area it now occupies. • This process generates a “taller” and therefore heavier air column. Thus low pressure centres cause a net accumulation of air, which increases their pressure. • For a surface low to exist for very long, surface convergence must be maintained by divergence aloft at a rate equal to the inflow below. This process is shown in Fig LT 6-19.

13. 14 Vertical Air Flow - continued... • Divergence aloft can exceed surface convergence. If this happens, then the result is an intensified surface inflow and accelerated vertical motion. Thus divergence aloft can intensify storm centres as well as maintain them. • If surface convergence exceeds divergence aloft, then the result is as described above: the surface flow “fills in” and weakens the cyclone.