• Micro, meso, macro• MICROCLIMATE - near

the ground over your front yard

• MESOCLIMATE - over a field or few fields (a few square kilometers)

• MACROCLIMATE - scale of a state or country

• GLOBAL CLIMATE - over entire earth

• Climate controls– Latitude– Land and water– Ocean currents– Prevailing winds– Pressure cells– Mountain barriers– Altitude

• A MICROCLIMATE is a local atmospheric zone where the climate differs from the surrounding area. The term may refer to areas as small as a few square feet (for example a garden bed) or as large as many square miles (for example a valley).

• Microclimates exist, for example, near bodies of water which may cool the local atmosphere, or in heavily urban areas where brick, concrete, and asphalt absorb the sun's energy, heat up, and reradiate that heat to the ambient air– the resulting “URBAN HEAT ISLAND” is a kind of

microclimate• Another contributory factor to microclimate is

the slope or aspect of an area. – South-facing slopes in the Northern Hemisphere

and north-facing slopes in the Southern Hemisphere are exposed to more direct sunlight than opposite slopes and are therefore warmer for longer.

• Mountain ranges are diverters of air masses• Affect the flow of moisture-laden air and cause

rain shadows for the areas on the leeward side• Temperature decreases with the rise in height

and cold air flows downhill and settles in valleys– Air temperature is lower in such areas

• Air speed increases up the windward slope• Air speed is maximum at the crest and

minimum on the leeward side.

The effect of topography on average annual precipitation along a line running from the Pacific Ocean through central California into western Nevada.

Vertical view of changing vegetation and climate due to elevation in the central Sierra Nevada.

• An urban heat island (UHI) is a metropolitan area which is significantly warmer than its surrounding rural areas

• The temperature difference usually is larger at night than during the day, and is most apparent when winds are weak. Seasonally, UHI is seen during both summer and winter

• The main cause of the urban heat island is modification of the land surface by urban development which uses materials which effectively retain heat.

• Waste heat generated by energy usage is a secondary contributor. • As population centers grow they tend to modify a greater and greater

area of land and have a corresponding increase in average temperature. The lesser-used term “heat island” refers to any area, populated or not, which is consistently hotter than the surrounding area

• Monthly rainfall is greater downwind of cities, partially due to the UHI. • Increases in heat within urban centers increases the length of growing

seasons, and decreases the occurrence of weak tornadoes. • Increases in the death rate during heat waves has been shown to

increase by latitude due to the urban heat island effect • Decreases air quality by increasing the production of pollutants such as

ozone• Decreases water quality as warmer waters flow into area streams,

which stresses their ecosystems.

http://www.learn.londonmet.ac.uk/packages/clear/thermal/buildings/micro_climate/heat_islands.html

• Places with plants and vegetation have different thermal properties as compared to built-up and hard-surfaced unplanted areas.

• The main differences are: Plants have lower heat capacity and thermal conductivity than building materials and hard surfaces.

• Solar radiation is mostly absorbed in the leaves, so that the reflected radiation is very small.

• Rain water is absorbed in the soil. Water is later evaporated from the soil and mainly from the leaves. The evaporation rate is much higher in green areas than in unplanted, hard covered areas.

• Plants reduce the wind speed and its fluctuations near the ground.

• Compared with most land surfaces, water bodies exhibit very little change in surface temperature during the day. Water is different because:– Solar radiation can be transmitted

deep within it and then absorbed. – Heat can also pass deep within the

water by convection and mixing. – Water loses heat by evaporation. – Specific heat of water is very high

(high inertia) when compared to that of building materials.

• The relative humidity of air is also affected, since the air coming from a water body is more humid.

• Such phenomena are stronger close to water bodies, but may also affect the regional climate by creating strong air movement reaching large distances. This is mainly affected by the physical characteristics of the region, such as topography and vegetation.

• The proximity of a site to the sea or other large water bodies also affects the climatic conditions in and around the site

• Wind movement from the water body during the day, and towards it at night, is caused by temperature differences of the air close to the surfaces of the soil and water.

• Ground surface whether natural or man-made, its characteristics of reflectance, permeability and soil temperature influence the microclimate.

• Depending on the ground surface, incident radiation can be absorbed, reflected or stored and re-radiated later.

• Radiative heat gain could be decreased or increased during the daytime.

• Emissivity of a surface depends on its texture.

• During night time, rough surfaces would re-radiate the heat absorbed during the day faster than smooth surfaces.

• The diurnal variation of rainfall at any tropical location is determined by a complexity of many factors – radiational heating and

cooling, – land-sea breeze effects, – Topography, – frequency of synoptic

disturbances, etc • The simple classical scheme,

– diurnal shower activity increases over land in the afternoon and diminishes in the evening,

– clear skies predominate in the late night and morning hours.

• Contrary to the popular classical scheme, many tropical land stations do not show a rainfall maximum during the afternoon period associated with maximum surface heating.

The eastern coast has its peak rainfall at night and the west coast during the afternoon, both during the rainy and dry seasons

Convergence is enhanced in the east at night, when the land breeze meets the easterly trades.

In the afternoon, the convergence moves to the western shore when the sea breeze tries to enter the island from that direction.

The north-south orientation of the rain maximum suggests general trade wind convergence upon striking land, mostly nearly at right angles.

1.Warm air over land rises 2.Sea Breeze moves inland as a mesoscale cold front 3.Cumuli develop aloft and move seaward 4.Upper level return land breeze 5.Cool air aloft sinks over water 6.Sea Breeze (meso-cold) Front

1.Cool air over land sinks 2.Land Breeze moves out over water 3.Relatively warmer water heats air which then rises 4.Upper level return sea breeze 5.Cool air over land sinks