wind system

Lines of equal pressure values are called isobars (see map above). It is standard to show isobars on maps at 4 mb intervals where 1000 mb is the base value. Notice the values of the pressures are higher near the "high" and lower near the "low". Interpreting pressure values is very straight forward, the bigger the number, the higher the pressure.

High pressure systems, also called anticyclones, have air moving clockwise and outward from the center in the Northern Hemisphere. High pressure areas are generally associated with fair, clear weather.

Low pressure systems, also called cyclones, have air moving counterclockwise and into the center in the Northern Hemisphere. Low pressure areas are generally associated with clouds, precipitation and storminess.

Wind and the Forces that Create it

Wind is a horizontal movement of air and there are also vertical motions associated with high and low pressure areas.

The underlying cause of wind flow is the uneven heating of the earth's surface. This causes pressure differences between locations which lead to the formation of wind as air flows from high pressure to low pressure areas.

On a large scale, wind functions to move excess heat from the tropics to the polar regions to try to reach an equilibrium or balance in the earth's distribution of energy.

Three Forces Affecting Wind Flow

• Pressure Gradient
• Coriolis Effect
• Friction

Pressure Gradient Force (PGF)

Pressure gradient force is created by the uneven heating of earth which results in a pressure difference between locations, this creates a force: Pressure Gradient Force (PGF)

Due to PGF, air moves from higher to lower pressure. PGF operates at right angles to the isobars. In the diagram below you can see that with pressure gradient as the only force, the wind is flowing in the same direction as the force itself.There is a strong relationship between the pressure gradient (difference in pressure between locations) and the wind speed. The greater the pressure difference between places (stronger pressure gradient), the higher the wind speed. We can see this by looking at the spacing between isobars on a weather map. Closely spaced isobars = strong (or steep) pressure gradient = strong winds. Widely spaced isobars = weak pressure gradient = weak winds Coriolis Effect - An apparent force created by Earth's rotation - Causes objects to be deflected from their path of motion. - Coriolis effect varies as a function of latitude: There is no coriolis at the equator Maximum coriolis at the poles Coriolis is also affected by the speed of the wind - faster the wind, the greater the deflection. - Operates on wind and ocean currents. The direction of deflection is... To the right in the Northern Hemisphere To the left in the Southern Hemisphere - Coriolis only influences wind direction and never the wind speed. Friction Friction slows surface wind speed and weakens the Coriolis effect. Near the surface, winds flow differently than they do in the upper levels of the atmosphere. We'll look at the differences between the upper and lower atmospheric wind flows. Upper Level Winds: The Geostrophic Wind The geostrophic wind results from a balance between PGF and Coriolis only. It flows parallel to the contours and occurs above 1 km in the atmosphere. Air flow around a curved surface: Anticyclonic flow - air flow in a clockwise direction around a high Cyclonic flow - air flow in a counterclockwise direction around a low If winds are geostrophic, air flow looks like it does in the figure below (in the Northern Hemisphere). Remember these are upper level winds found above 1000 m or 1 km above the ground. Southern hemisphere circulations will be in the opposite direction. Three Cell Model We now allow the Earth to spin introducing the Coriolis effect which complicates the pattern of circulation markedly - see the two figures below. Again, we see air rising from the equator (equatorial low) and diverging (spreading apart) aloft. The air then moves poleward in both hemispheres. As the air moves towards the poles, it begins to converge in the range of 25° and 35° north and south latitude, let's just call it 30° for now. You can see how this occurs by placing your fingers on the lines of longitude printed on a globe. As you move your hands poleward keeping your fingers on the longitude lines, you should notice that your fingers converge (move towards each other). A similar thing happens to the air. This upper air convergence, coupled with radiational cooling cause the air to subside (sink towards the earth's surface) in the subtropics(around 30° lat). As the air reaches the surface, atmospheric pressure increases forming the subtropical highs. This region is also known as the horse latitudes and is characterized by light or non-existant winds. This area was so named by the early Spanish sailors who threw their horses overboard to conserve water when their ships were becalmed in this region. After sinking back to the surface near 30° latitude, some of the air returns to the equator (creating the trade winds in each hemisphere) and some continues poleward (now at ground level), creating the westerlies in both hemispheres. Air returning to the equator converges there and causes air to rise. This area is therefore called the Intertropical Convergence Zone (ITCZ) and is associated with clouds and precipitation. This region is also known as the doldrums and is also a region of very light winds. Near 60° latitude air is forced to rise along the polar front creating clouds and precipitation. This is the region of the subpolar low and the polar front(and polar jet stream). Some air continues to the pole where the air descends (creating the polar high) and travels toward 60° latitude (winds between 60° and 90° are the polar easterlies). The top diagram you see below is the one found in your textbook. In the bottom figure, representing the same 3-cell circulation, shows a distinct circulation cell in the midlatitudes (approx. 30° - 60° latitude). This is called the Ferrel cell and isn't 100% correct, but it is often shown this way to ease student comprehension of the complicated pattern. Otherwise, the two pictures are showing the same general circulation of the atmosphere. This bottom diagram shows that at 60° the air rises (at the polar front) and some air moves to the poles and some returns aloft to the region of the subtropical highs. In actuality, there isn't really a well defined upper level return flow to complete this cell, in fact the winds are westerly at high altitudes which is inconsistent with a return flow aloft (winds would be easterly due to coriolis and they are not). Regardless, it's okay if you learn it this way. Also note in this bottom picture that the polar front is shown as a blue line with triangles on it and it "wiggles" (not straight and fixed at 60° latitude). This is a correct depiction. Also note the movement in the position of the ITCZ. This is also correct and will be discussed briefly later (and also, thoroughly in your textbook). See in the bottom diagram that there are three convection cells in each hemisphere: Hadley Cell, Ferrel Cell, and Polar Cell (Hadley and Polar are best defined) In both diagrams, you can see the following features: Pressures are: Equatorial Low, Subtropical Highs, Subpolar Lows, Polar Highs Wind Belts: Tradewinds (NE and SE), Westerlies, Polar Easterlies. Two areas with lack of wind: Doldrums (near equator) and Horse Latitudes (near 30° latitude in each hemisphere) To summarize: At 0°: Equatorial Low, Intertropical Convergence Zone (ITCZ), Doldrums 30° N and S: Subtropical High, Horse Latitudes 60° N and S: Subpolar Low, Polar Front, Polar Front Jet Stream (or just Polar Jet Stream) 90° N and S: Polar High Winds in between major pressure regions (so latitudes are approximate): 0°- 30° N = NE Tradewinds 0°- 30° S = SE Tradewinds 30° - 60° N and S = Westerlies 60° - 90° N and S = Polar Easterlies Polar Front and Polar Jet Stream The region of the polar front is a place of convergence between cold, polar air from the north with warm, tropical air from the south. Lifting occurs at the boundary leading to clouds and precipitation. The boundary is characterized by waves known as Rossby waves or longwaves (real life: subpolar low region is not a straight zone like the three cell model presents it (in the top picture), but rather, a wavy boundary like that shown in the bottom figure). Along this boundary with a large temperature difference, a strong horizontal pressure difference is created. This creates the Jet Stream, a zone of high velocity winds found high in the atmosphere. Winds can be around 200-250 mph. The region of the polar front is a zone of always changing weather. It is the location where mid-latitude cyclones begin their life cycle. There are clouds and storminess as lifting is occuring at this boundary.