Global air circulation Mr Askew
Pressure gradient, Coriolis force and Geostrophic flow Wind is produced by different air pressure between places. Isobars indicated air pressure on a map, in some areas the isobars are close together and in others wide apart. The rate at which air pressure changes over horizontal distance as measured in direction from HP -> LP is called the pressure gradient. At any point the pressure gradient is at right angles to the isobars.
How the pressure gradient force works: Pressure gradient is described as gentle (if isobars are widely spread) or steep (close together). The pressure gradient is a force acting on the atmosphere as a result of the pressure gradient. It moves air from a HP -> LP and determines the speed of the wind. (isobars wide apart = gentle wind/ close together = strong wind. Figure 61 pg 62
The impact of Coriolis force on wind When air flows it is affected by the Coriolis force, it deflects winds and ocean currents from their initial direction. The deflections to the right in the northern hemisphere, and to the left in the southern hemisphere. Characteristics of the Coriolis force: 1. Causes a change in wind direction for all winds on the earths surface. 2. Is stronger in proportion to any increase in speed. 3. Weaker near the equator. effect/?ar_a=1
How the Coriolis force and pressure gradient affect wind Combined effects produce a geostrophic wind. (figure 63) Ferrell's law: if you stand with your back to the wind, any horizontal movement in the northern hemisphere will show a deflection to the right and in the southern hemisphere to the left. The pressure gradient remains equal throughout. The stronger the wind the stronger the Coriolis force is acting on it. The pressure gradient is always perpendicular to the isobars. When the wind reaches equilibrium, it is blowing parallel to the isobars and called geostrophic wind.
Geostrophic winds Found in the upper atmosphere. Example is the Jet stream. Nearer the earths surface friction causes the winds to blow more diagonally across the isobars. Activity 7 pg 63
Winds related to global air circulation 1. Tropical Easterlies: Blow from sub tropical high pressure areas to the equatorial low pressures. Found between 5 – 35 ⁰ latitudes. Where the winds originate: the air is usually dry and warm because of the strong descent of air. This produces fine weather sunshine. As these winds travel towards the equator they change. Absorb moisture over the oceans, warmed and become unstable = clouds form = rainfall along the equator. Disturbances in the tropical easterlies (trade winds) give rise to tropical cyclones.
Westerlies Blow from sub tropical high pressure zones to low pressure areas of the sub polar regions (60 ⁰ latitude) Initially North westerleis in the southern hemisphere and south westerly's in the northern hemisphere. The further from the equator they move, the more the direction changes because they experience a greater Coriolis force. As westerlies blow from cold to warm regions, they carry warm air to the colder regions causing the temperatures to rise The colder regions will cause the temperature of the air to drop, which may lead to condensation and precipitation.
Polar Winds Originate in HP areas near the north and south poles. Blow towards sub polar low pressure areas. Very fierce, esp within polar circles, where they develop into howling snow storms.
Air mass Characteristics Air mass: Large body of air with uniform temperature and humidity characteristics picked up from the region where it develops. Main source regions for air masses are: 1. Where the surface is geographically uniform: oceans, ice fields, deserts 2. Areas of relatively stable atmospheric conditions.
How air masses classified: The latitude where they originate. The surface which they develop affects their humidity and precipitation, for example over the sea producing wet maritime air (m) or over the land producing dry continental air (c) Table 9 pg 65 Activity 8 pg 65