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MICROCLIMATE GLOBAL VEGETATION DESERT RAINFOREST DESERT Global climatic regions relate to: � Latitude � Continental location (E or W) Regional climates relate more to: � Altitude � Ocean currents, winds � Distance from sea Colder - higher altitude, polar, and more continental. FOR INSTANCE Rainforest is close to the equator NW Europe temperatures Deserts are along the tropics and in in January the interior of major continents Ice is at high latitudes Warmer - Gulf Stream takes warm water polewards Warmer - southerly, lower altitude, oceanic, Warmer - southerly, oceanic On a smaller scale, weather and climate is affected by smaller scale variations in: � Topography (relief) � Albedo � Aspect � Urban Areas � Vegetation � Moisture and humidity � Pollution, human activity MICROCLIMATE (2) Farmers alter albedo by plastic sheeting. Bare earth gains more radiant heat, but loses more heat through evaporation loss and wind at night. Prevention of frost at night can be crucial to early growth. Low lying valleys and hollows collect cold and humid air (Frost Hollow); hilltops are exposed to wind; southfacing slopes (in Europe) are warmer, with longer days, effectively, than north-facing slopes (Aspect). East or west aspect may affect rainfall or snow coverThis in Built-up areas are turn may affect vegetation, humidity, evaporation rates 2/3°C warmer than rural areas, especially at night. This is an Urban Heat Island. The Greenhouse Effect due to human pollution is not intentional... Thermal imaging of Atlanta shows the correlation of warmer temperatures and the built up area. The centre is warmest, outside the city is coolest. Bodies of water help reduce the effect. The location of the CBD and tarmac roads may be clearly seen. The effect is to warm major urban areas by 2-3°C by day and night more than rural areas. URBAN HEAT ISLAND Thermal images of Atlanta show radiant energy being absorbed during the day (above) and retained during the day (below). The roads can be seen as tarmac absorbs radiation most effectively. ATLANTA’s heat island URBAN HEAT ISLAND - REASONS RUSH HOUR TRAFFIC THROUGH A HAZE OF FUMES DOMESTIC HEATING MOTOR EXHAUSTS FACTORY & OTHER POLLUTION Human heat sources (domestic heating, cars, factories) all warm the air. Pollution by exhausts, factories and other dusts absorb radiation and prevent heat loss during the night. Dark surfaces have a low albedo. Dry surfaces reduce latent heat loss by evaporation In humid conditions, this may result in smog (a mixture of fog and smoke) which was common in pre-war London and still is in LA, Rome, Athens, Mexico City etc where surrounding hills prevent the escape of polluted air. DARK AND DRY TARMAC SURFACES SMOG RESULTS FROM POLLUTION Ice is common on exposed dark surfaces, as they lose heat rapidly overnight. Black ice is a hazard on roads.and pavements Cities designed on the grid system channel any wind along streets that contiue for many kms (wind canyons). Other cities reduce wind speed by ground level friction. Warmer cities reduce snow cover and frost frequency, advancing plant growth. URBAN CLIMATES The albedo of various surfaces in urban areas tends to be different to rural areas; tarmac is dark; glass is lighter. Reduced snow and ice cover reduces albedo. Increased pollution by traffic and other combustion tends to reduce sunshine, espcially in winter when the sun is at a low angle, passing through ,more atmosphere. Air pollutants increase condensation and cloud development and so rainfall intensity and amount. Clouds above Tripoli aided by fires A permanent haze hangs over Mexico City Denver’S ‘Brown cloud’ FROST HOLLOW 1 2 The Frost Hollow effect tends to operate when the ground surface cools, usually overnight when cloud cover is limited. It is common in mountains where snow and ice cover maintain cold surfaces for long periods, reflecting insolation and may cause cold winds down slope (eg In hollows, humidity is often high (rivers, estuaries, marsh land) and towns (usually on lower land) increase air pollution. Both tend to make fog or cloud more likely. Smog (smoke - fog) occur with bad pollution (as in Athens, LA, Mexico City, pre-war London) 3 Low-lying cloud in valleys seems like fog at ground level 4 Temperature decreases with height by 0.6°C per 100m. This can result in permanent snowcaps on mountains above forests where snow is seldom seen (here, on Cotopaxi volcano in Mexico near the equator, the snowline is at 5000m). In Britain, upland areas such as Snowdonia which range from 0-1000m above sea level, produce climates ranging from temperate maritime to the almost Arctic. � Lower temperatures cause greater soil saturation; higher altitude also tends to increase precipitation (and making it more likely to be snow rather than rain). At t higher altitudes, the growing season is shorter, frosts are more frequent and harder while winters are longer. � Agriculture is strongly affected. Some arable crops are possible at low level, on valley floors. Higher up, pasture becomes is enclosed. Above this, open moorland is used for sheep to roam, but is covered mainly with heather and other hardy plants. Besides temperature, wind speed, evaporation rates, and humidity are also affected. � Elsewhere, sensitive crops (fruit orchards, right - or vineyards) can only exist below the cold and windy upper slopes due to frost frequency in early spring. The lowest points may also be unsuitable due to the frost hollow effect. ALTITUDE � A hill farm in Snowdonia An orchard on a slope below woodland in Devon In the northern hemisphere, a southerly aspect gives effectively a higher angle of sun in the sky, and longer days. � In the southern hemisphere, a northerly aspect is warmer � The growing season is longer (by about a month for each 1°C higher in annual average temperature), � Frosts are less frequent, less severe � Maximum temperatures are higher. � ASPECT The right hand slope (above) is facing the sun, keeping it free of snow for longer. In some arid environments, shade is important, reducing temperature, humidity and evaporation rates. Shaded areas, especially if north facing, remain damper with reduced temperatures, evaporation and humidity. This also affects vegetation, soil moisture which may, in turn, affect frosts and temperature variations Isolated snow patches are likely to remain in spring on north facing slopes (in Britain) where the sun takes longer to melt the snow. Sensitive crops may nly be possible on south-facing slopes in Europe (eg vines below); the opposing slope is pasture alone. VINES VEGETATION - WOODLAND Trees reduce temperature during the day, but retain heat during the night. Temperatures are thus more even (less extreme) � Shade can be welcome in the desert, but on the forest floor, the lack of sunlight is a serious deterrent to other plants. Thick undergrowth occurs only in clearings or where old trees fall Wind speed is reduced � British forest floors with moss and marsh at ground level Evaporation are lower, especially in the day but also at night. Locally, air becomes saturated (and is not Rainforest transpiration also blown away) due to increases cloud and rainfall transpiration; this To reduce reduces evaporation. windspeed in � Humidity levels orchards remain high and (evaporation, frost constant due to and blossom loss) transpiration and low windbreaks are evaporation rates. planted. They may Mosses are common on reduce soil loss in the forest floor arable fields. � VEGETATION - OTHER PLANTS Vegetation on sand (marram grass, above) not only anchors the moving sand with its roots but also: � Reduces wind speed which stabilises sand (ripples show wind) � Increases humidity locally (cms) � Keeps temperature more even � Reduces frosts, evaporation. � Dune systems grow as a result Other plants (freshwater reeds, right) may also reduce windspeed or water current, retain sediment and allow other vegetation to colonise. Temperatures are more extreme where vegetation is absent; ice forms (left) on a bare rock surface due to rapid radiation loss overnight. By reducing light penetration to the forest floor (right) , trees are prevent the growth of competing species.