MICROCLIMATE

1. GLOBAL VEGETATION • Global climatic regions relate to: • Latitude • Continental location (E or W) • Regional climates relate more to: • Altitude • Ocean currents, winds • Distance from sea MICROCLIMATE DESERT RAINFOREST DESERT Colder - higher altitude, polar, and more continental. FOR INSTANCE Rainforest is close to the equator Deserts are along the tropics and in the interior of major continents Ice is at high latitudes NW Europe temperatures in January Warmer - Gulf Stream takes warm water polewards Warmer - southerly, lower altitude, oceanic, Warmer - southerly, oceanic

2. 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 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. MICROCLIMATE (2) Low lying valleys and hollows collect cold and humid air (Frost Hollow); hilltops are exposed to wind; south-facing 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 turn may affect vegetation, humidity, evaporation rates Built-up areas are 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...

3. FROST HOLLOW 1 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) Low-lying cloud in valleys seems like fog at ground level 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 Mistral in S.France). 3 4

4. 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

5. RUSH HOUR TRAFFIC THROUGH A HAZE OF FUMES MOTOR EXHAUSTS DARK AND DRY TARMAC SURFACES DOMESTIC HEATING FACTORY & OTHER POLLUTION SMOG RESULTS FROM POLLUTION URBAN HEAT ISLAND - REASONS 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.

6. Ice is common on exposed dark surfaces, as they lose heat rapidly overnight. Black ice is a hazard on roads.and pavements A permanent haze hangs over Mexico City Denver’S ‘Brown cloud’ Clouds above Tripoli aided by fires 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. Warmer cities reduce snow cover and frost frequency, advancing plant growth. 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. 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.

7. 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). A hill farm in Snowdonia An orchard on a slope below woodland in Devon ALTITUDE • 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.

8. 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. Sensitive crops may nly be possible on south-facing slopes in Europe (eg vines below); the opposing slope is pasture alone. VINES ASPECT Isolated snow patches are likely to remain in spring on north facing slopes (in Britain) where the sun takes longer to melt the snow. 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

9. Trees reduce temperature during the day, but retain heat during the night. Temperatures are thus more even (less extreme) • Wind speed is reduced • Evaporation are lower, especially in the day but also at night. Locally, air becomes saturated (and is not blown away) due to transpiration; this reduces evaporation. • Humidity levels remain high and constant due to transpiration and low evaporation rates. Mosses are common on the forest floor VEGETATION - WOODLAND 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 British forest floors with moss and marsh at ground level Rainforest transpiration also increases cloud and rainfall To reduce windspeed in orchards (evaporation, frost and blossom loss) windbreaks are planted. They may reduce soil loss in arable fields.

10. Other plants (freshwater reeds, right) may also reduce windspeed or water current, retain sediment and allow other vegetation to colonise. 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 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.