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Wind notes for lectures 7-9.docx

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Environmental Science
Tanzina Mohsin

Lecture 7 Land/Sea Breeze -Thermally induced circulation - Causes pressure differences - Often associated with day and night contrasts (diurnal variation) -Land/sea breezes arise from differential heating of the earth‟s surface on a daily (diurnal) basis. Land surfaces heats more rapidly and cools more rapidly than water surfaces. During the day, the land surface is typically warmer than the water surface. Warmer air rises and so air is rising over the land and sinking over the water. To complete the circulation air moves from the water area to the land area at the surface. This forms a cool breeze at the surface and is called a sea or lake breeze. It is particularly noticeable during the day in the summer. At the upper level there is a reverse flow with air moving out over the water. At night the opposite occurs, the water surface is warmer than the land. Air rises over the water and sinks over the land. The induced surface flow is from land to water or a land breeze. Sea breezes are strongest in the summer when the temperature contrast is the greatest. Even on seasonal scales, bodies of water warm up and cool down slower than land masses. In the summer, therefore, the average temperature of the land over the entire 24 hour period is warmer than the average sea temperature; the diurnal heating of the land mass adds to this seasonal difference to create a larger temperature contrast. Land breezes, for similar reasons, are stronger in the winter. Monsoon -Arabic word mausim meaning season. Referred to seasonal trade winds in Arabian sea -Seasonal scale thermal wind -Asian Monsoon - Large scale impacts on India and surrounding countries (South Asia) and Africa -Summer monsoon begins late may -During the winter months, the Indian Ocean is warmer than the surrounding land mass and air rises over the ocean and sinks over the land inducing a land breeze and dry conditions. During the summer, the land is warmer than the Indian Ocean. Air rises over land and sinks over the water. This induces a sea breeze. This breeze has high moisture content. As the air moves over the land, it rises, cools and water vapour condenses into water droplets (clouds) generating copious amounts of rain. The monsoon persists into the fall. Valley Breeze -Smaller scale wind -Also driven by diurnal variations in heating -During the day, land warms faster than the air and upslope winds are induced -At night, the opposite occurs and downslope winds are induced -Important features for hikers Katabatic Wind -Another mountain wind -Air passing over a glacier or ice sheet becomes very cold and dense -Cold air funnels into valleys producing cold strong winds Chinook -Winds on the lee side of mountains – foehn wind -It is a North American term -Linked to migraine headache occurrence Urban Heat Island -the warming of the urban areas and the surrounding areas -most noticeable at night and in the winter months -more intense when winds are non-existent Four factors contributing 1. Reduced evaporation  Buildings, pavement prevent evaporation  Lack of vegetation which would bring water up from the soil  More energy goes into heating the surface and less into evaporation – this enhancing the heating 2. Reduced Albedo  During winter months, snow is removed which decreases albedo from urban areas and thus more energy is absorbed 3. Heat Storage  Tall buildings create a complex geometry – canyon effect  Radiant energy is trapped by buildings  Wind is blocked, preventing convective cooling 4. Heat Generation  Industry, motor vehicles, domestic heating and air conditioning release large quantities of heat  Some types of urban pollution prevent release of excess heat Lecture 8 Anemometers Deflection Anemometer -First anemometer -Leon Battista Alberti in Italy (1450) -Connected to a spring -Compression of the spring proportional to wind speed -not good for light winds -based on wind pressure displacement Pressure Anemometer -James Lind 1774 -U-tube- wind blows into tube and pushes liquid in the tube -based on hydrostatic balance Cup Anemometer A. -Thomas Robinson 1846 -based on mechanical movement -4 cups -cup speed is one-third of the wind speed B. -Three Cup Anemometer -Canadian John Patterson 1926 -more consistent less than 3% error up to 100kph Windmill Anemometer -uses a propeller to measure wind speed -Aerovane is used to insure anemometer is pointing into the wind -mechanical movement Thermoelectric Anemometer -a wire is heated to above the ambient temperature -rate of cooling of wire is proportional to wind speed -based on wind cooling effects Laser Radar Anemometer -uses the Doppler principle to measure wind speed. Sonic Anemometer -Dr. Andreas Pflitsch -measure wind speed by how sound waves are modified by moving air Measuring Upper Level Winds -Measu
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