Air Masses Fronts and Weather PDF

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EVSC 1010 Lecture Outline Wind, Air Masses and Weather Fronts   

Wind (advection): horizontal air flow Pressure gradients: the magnitude of the pressure difference over distance, determines wind speed (directly proportional). Difference in pressure b/w high and low pressure areas. Ascending and descending air masses – development of high and low pressure systems o Rising low-pressure air creates clouds and precipitation. Air flows inward toward the low- pressure zone, creating surface winds. o Sinking high pressure air creates clear skies. Air flows outward from the highpressure zone, and also creates surface winds. o Indicate high and low pressure centers:

o o Isobars (black lines in photo): means equal pressure (iso – equal , bar – unit of pressure). Everywhere along the line is constant pressure. The closer the isobars are packed together the stronger the pressure gradient is.

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o Wind effects o Strongest winds are in the areas where pressure gradient is the greatest o Wind direction is clockwise around high pressure systems and counter-clockwise around low pressure systems o Direction of the wind is across the isobars slightly, away from the center of the high pressure system and toward the center of the low pressure system o The Coriolis effect deflects winds to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere. (Result of Earth’s rotation)

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Cyclone / Anticyclone o Cyclone: A low pressure region with an accompanying surface wind  In the Northern Hemisphere, a cyclone consists of winds spiraling counterclockwise into a low pressure region. o Anticyclone: A high pressure system with accompanying winds  An anticyclone consists of winds spiraling clockwise out from a highpressure zone. Low barometric pressure is an indicator of wet weather Air mass: a large body of air with uniform temperature & humidity at any even altitude Source regions of major air masses in North America

o o Maritime Polar o Maritime Tropical o Continental Tropical o Continental Polar o Maritime Tropical› Fronts: The boundary between a warmer air mass and a cooler one o Warm front: forms when moving warm air collides with a stationary cold mass. The moving warm air rises over the cooler dense air as the two air masses collide. Light precipitation. o Cold front: forms when moving cold air collides with stationary or slower moving warm air. The dense, cold air distorts into a blunt wedge and pushes under the warmer air. Accompanied by cumulus and cumulonimbus clouds. o Occluded front: forms when a faster moving cold mass traps a warm air mass against a second mass of cold air. Thus the warm air mass becomes trapped between two cold air masses. The faster moving cold air mass then slides beneath the warm air, lifting it completely off the ground. Precipitation occurs along both frontal boundaries o Stationary front: occurs along the boundary between two stationary air masses. Under these conditions, the front can remain in an area for several days. Warm air rises, forming conditions similar to that of a warm front. Orographic lifting and rainfall

o Orographic lift occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain. As the air mass gains altitude it quickly cools down adiabatically, which can raise the relative humidity to 100% and create clouds and, under the right conditions, precipitation.

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o Rain shadow: forms where moist air rises over a mountain range and precipitates most of its moisture on the windward side and crest of the range. The dry, descending air on the lee side absorbs moisture, forming a desert. Lie east of the CA mtn. ranges. Land-sea interactions on wind flow and precipitation o Diurnal (day-night): Sea breezes blow inland during the day and land breezes blow out to sea at night o Seasonal (monsoon): Monsoon is a seasonal wind and weather system caused by uneven heating and cooling of continents and oceans Thunderstorms – cause and lifecycle o Since warm air is lighter than cool air, warm surface air starts to rise (known as an updraft). If the air is moist, then the warm air condenses into a cumulus cloud. o Raindrops start to fall through the cloud and meanwhile, cool dry air starts to enter the cloud. Because cool air is heavier than warm air, it starts to descend in the cloud (known as a downdraft). The downdraft pulls the heavy water downward, making rain. o This cloud has become a cumulonimbus cloud because it has an updraft, a downdraft, and rain. Lightning and thunder start to occur, as well as heavy rain. The cumulonimbus is now a thunderstorm cell. o After about 30 minutes, the thunderstorm begins to dissipate. This occurs when the downdrafts in the cloud begins to dominate over the updraft. Since warm moist air can no longer rise, cloud droplets can no longer form. The storm dies out with light rain as the cloud disappears from bottom to top. o The whole process takes about one hour for an ordinary thunderstorm. Tornado: small funnel-shaped storm that protrudes from the base of a cumulonimbus cloud Hurricane (tropical cyclone): Surface air spirals inward toward a hurricane, rises through the towering wall of clouds, and then flows outward above the storm. Falling air near the storm’s center creates the eerie calm in the eye of the hurricane....