Weather Systems

Circulation Cells Insolation is greatest at the equator, humidity is high, the airmass is unstable. The cold air at the poles is more dense, so there is a pressure gradient of about 1 in 8,000 toward the poles. Air rising at the equator moves towards the poles down the pressure gradient.

CORIOLIS EFFECT. Earth's rotation produces a force on winds and currents. The force is perpendicular to the wind and current. It acts to the right of the flow looking downstream in the northern hemisphere. This effect is referred to as the CORIOLIS EFFECT.

Hadley Cells Convection in the tropics is deep and massive. Large amounts of latent heat are released in the towering cumulus clouds of the tropical atmosphere, providing further driving for the upward motion. At the top of the tropical troposphere (12-15 km), the atmosphere is stable and the upward motion is brought to a halt. This forces the air to diverge poleward on both sides of the equator.

Hadley Cells The further the poleward moving air gets from the equator, the larger the Coriolis force, and the air begins to deflect to the right (in the Northern Hemisphere) and left (in the Southern Hemisphere). The result (in both hemispheres) is an eastward flow of air (westerly winds). The further poleward we go, the stronger the westerlies become, till we reach the jet stream latitude.

Polar Front At the polar front the cold dense air undercuts the warm tropical air pushing it upwards, setting in motion the process that causes weather fronts

Formation of fronts Some air from the mid latitude Hadley cell runs northwards at the surface and meets cold polar air flowing South, forming a polar front which circles the planet At this boundary a sharp gradient in temperature occurs between these two air masses, each at very different temperatures.

Jet Stream The jet stream is a narrow snaking tube of strong winds associated with the polar front. It has a significant effect on the formation of fronts and and the track of depressions

Wind This all has the effect of creating areas of high and low pressure and moving the airmass to create wind. Variations in atmospheric pressure channel the wind like hills and valleys. The air tries to flow from high to low pressure The flow is affected by the Coriolis affect.

Highs and Lows In the northern hemisphere wind blows anti clockwise into a low pressure. It rises and flows outward at altitude. The wind blows clockwise at altitude into a High and the descends and flows out at the surface.

Fronts A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorlogical phenomena

Cold Fronts Cold fronts may feature narrow bands of thunderstorms and severe weather, and may on occasion be preceded by squall lines or dry lines. Cold fronts and occluded fronts generally move from west to east,

Warm Fronts Warm fronts are usually preceded by stratiform precipitation and fog. The weather usually clears quickly after a front's passage. Some fronts produce no precipitation and little cloudiness, although there is invariably a wind shift. warm fronts move poleward

Speed Because of the greater density of air in their wake, cold fronts and cold occlusions move faster than warm fronts and warm occlusions. Mountains and warm bodies of water can slow the movement of fronts.

Occluded front 1

Occluded Front

Warm Sector This is the area between a leading warm front and a trailing cold one. The cold front moves faster than the warm front, and catches it up Away from the low pressure there is a chance of good weather

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