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Chapter 8

GG101 Chapter 8 Notes.docx

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James Hamilton

GG101 Chapter 8 Notes- Weather  Weather has a leading role in the vast drama played out daily on Earth’s stage.  Weather: the short-term, day-to-day condition of the atmosphere, contrasted with o climate, which is the long-term average (over decades) of weather conditions and extremes in a region.  Meteorology: the scientific study of the atmosphere; the atmosphere’s physical characteristics and motions, related chemical, physical and geologic processes, the complex linkages of atmospheric systems, and weather forecasting  Developments in supercomputing include an Earth-bound instrument network in the Automatic Weather Observation System (AWOS in Canada) and Automated Surface Observing System (ASOS; in the US), and Doppler radars are rapidly advancing the science of the atmosphere  Doppler radar detects the direction of moisture droplets toward or away from the radar, indicating wind direction and speed…part of the development of Canada’s Weather Workstation Air masses affecting North America  Each area of Earth’s surface imparts its temperature and moisture characteristics to overlying air  The effect of the surface on the air creates regional air masses with a homogenous mix of temperature, humidity, and stability  These masses of air interact to produce weather patterns- in essence, air masses are the actos in our weather drama  We classify air masses generally according to the moisture and temperature characteristics of their source regions: o Moisture: designated M for maritime(wet) and C for continental (Dry) o Temperature: (Latitude factor)- designated A (arctic), P (polar), T (tropical), E (equatorial), and AA (Antarctic)  Continental air masses originate in Siberia and northern Canada; these cA air masses originate in Siberia and northern Canada; they dominate the extreme north in winter when they cross over the North Pole and spill into the North American continent. Arctic air, colder than polar air and with a lower dew point, often forms when a high-pressure area becomes nearly stationary over northern Canada, Eastern Alaska, the Yukon or Siberia.  Continental polar (cP) air masses form only in the Northern Hemisphere and are most developed in winter and cold weather conditions. These cP air masses masses are major players in middle and high-latitude weather. The cold, dense cP air displaces moist, warm air in its path, producing lifting, cooling and condensation. An area covered by cP air in winter experiences cold, stable air, clear skies, high pressure and anticyclonic wind flow. The Southern Hemisphere lacks the necessary continental land-masses at high latitudes to create such a cP air mass  Martime Polar (mP) air masses in the Northern Hemisphere exist northwest and northeast of the North American continent over the northern oceans. Within them, cool, moist, unstable conditions prevail throughout the year. The Aleutian and Icelandic subpolar low-pressure cells reside within these mP air masses, especially in their well-developed winter pattern  Two maritime tropical (mT) air masses- the mT Gulf/Atlantic and the mT Pacific- influence North America. The humidity experienced in the East and Midwest is created by the mT Gulf/Atlantic air mass, which is particularly unstable and active from late spring to early fall o In contrast, the mT pacific is stable to conditionally unstable and generally lower in moisture content and available energy. As a result, the western United States, influenced by this weaker Pacific air mass, receives lower average precipitation than the rest of the country. Air Mass Modification  As air masses migrate from source regions, their temperature and moisture chracteristics modify and slowly take on the characteristics of the land over which they pass o For example, an mT Gulf/Atlantic air mass may carry humidity to Chicago and on to Winnipeg but gradually loses its initial characteristics of high humidity and warmth with each day’s o Modification of cP air as it moves south and east produces snowbelts that lie to the east of each of the Great Lakes o As below-freezing cP air passes over the warmer Great Lakes, it absorbs heat enrgy and moisture from the alke surfaces and becomes humidified. This enhancement produces heavy lake-effect snowfall downwind into Ontario, QC, MI, PA, and NY…some aireas receive in excess of 250cm in average snowfall a year…a couple of storms in NY serve as examples such as 2006’s lake-effect snowstown dropped 60cm of snow in a day! Atmospheric Lifting Mechanisms  For air masses to cool adiabatically (by expansion) and to reach the dew-point temperature and saturate, condense, form clouds, and perhaps produce precipitation, they must lift and rise in altitude…four lifting mech’s operate: o Convergent lifting: air flows toward an area of low pressure. All along the equatorial region, the southeast and northeast trade winds converge, forming the intertropical convergence zone (ITCZ) and areas of extensive uplift, towering cumulonimbus cloud development, and high average annual precipitation o Convectional lifting: stimulated by local surface heating, such as an urbanized area heat island, or an area of dark soil in a plowed field. The rising parcel of air continues its ascent because it is warmer and therefore less dense than the surrounding environment. Florida’s precipitation generally illustrates both these lifting mechanisms: convergence and convection. o Orographic lifting- air is forced over a barrier such as a mountain range. The lifting air cools adiabatically. Stable air forced upward in this manner may produce stratiform clouds, whereas unstable or conditionally unstable air usually forms a line of cumulus and cumulonimbus clouds. An orographic barrier enhances convectional actibity and causes additional lifting during the passage of weather fronts and cyclonic systems, thereby extracting more moisture from passing air masses.  In North America, Chinook winds are the warm, downslope airflows characteristic of the leeward side of mountains- can bring up to a 20C increase in temperature and reduced relative humidity on the receiving end  Rain Shadow: applied to dry regions leeward of mountains. East of the Cascade Range, Sierra Nevada, and Rocky Mountains, such rain-shadow patterns predominate. In fact, the precipitation pattern of windward and leeward slopes persists worldwide, as confirmed by the precipitation maps for North America o Frontal Lifting- along the leading edges of contrasting air masses. A front is a place of atmospheric discontinuity, a narrow zone forming a line of conflict between two air masses.  Cold Front: leading edge of a cold air mass- great density. Marked by temperature drop, wind shift and lowering barometric pressure mark a cold front’s advance due to lifting along the front. Squall Line- zone right along or slightly ahead of the front  Warm Front: leading edge of a warm air mass. The elading edge of an advancing warm air mass is unable to displace cooler, passive air. Instead, the warm air tends to push the cooler, underlygina ir into a characteristic wedge shape, with the warmer air sliding up over the cooler air Midlatitude Cyclonic Systems  Midlatitude Cyclone- developed from conflict between contrasting air masses. This migrating low-pressure centre with converging, ascending air spirals inward counterclockwise in the Northern Hemisphere, or inward clockwise in the Southern Hemisphere. Cyclonic motion is generated through pressure gradient force, Coriolis fo
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