Unit 2- Module 1

Local Winds & Microclimates

Objectives Overview of local winds (sea & land breezes, valley winds)

Overview of microclimates (valley, urban, woodland)

Local Winds

Local Winds Local winds are caused by small scale temperature and pressure

differences.

They affect a much smaller geographical area compared to large scale

systems., such as depressions and tropical cyclones.

Two examples of local winds are land and sea breezes and valley

winds.

Land & Sea Breezes

Land & Sea Breezes

This is a wind system that develops over coastal areas.

It is caused by the different capacities of water and land to absorb

and retain heat from the Sun.

Land & Sea Breezes During the day, if the weather is sunny and calm, the land quickly absorbs

shortwave solar radiation and starts to warm.

Some of this heat is transferred to the air above, which starts to rise.

By contrast, over the sea, the temperature does not rise so much because water warms relatively slowly (incoming radiation can reach a depth of about 30 metres).

The temperature difference between sea and lad sets up a pressure gradient, with relatively low pressure over the land and relatively high pressure over the sea.

This causes air to blow inland as a SEA BREEZE (typically blow inland about 10 km and reach a height of 500m or so).

Typically the breeze starts in the mid-morning and strengthen during the afternoon, subsiding in the evening when the Sun’s rays are weaker.

Land & Sea Breezes

Overnight, the land cools more quickly than the sea.

This causes a reversal in the temperature and pressure gradient and a

breeze develops from the land onto the sea, known as a LAND

BREEZE.

Because the temperature difference between land and sea is not so

great at night time, the breeze is usually weaker than in the day time.

The strongest breezes occur when the temperature difference

between land and sea is most marked.

Sea Breeze (@ Day)

Land Breeze (@ Nights)

Valley Winds

Valley Winds At night, the ground surface cools.

This is particularly marked when the sky is clear because cloud cover acts as a ‘blanket’ and reduces the heat loss from radiation cooling.

In mountain regions, the ground and air is coldest over snow and ice fields.

The cold air is relatively dense and it starts to sink.

If this flow continues, a cold wind, known as a KATABATIC WIND, develops down the valley.

During the day, the wind flow is reversed. The sun’s rays heat the valley floor and slopes, causing relatively warm air to rise.

If the flow develops up the valley, it is known as an ANABATIC WIND.

Microclimates

What is a Microclimate? A microclimate is the climate in the lower few metres of the

atmosphere, over a relatively small area, for example on a valley slope, in an urban area or within a woodland area.

Local factors such as slope angle, vegetation cover and building material are all important influences.

There is no set or agreed scale at which microclimate operates. So, in a city for example, the whole urban area can be considered as having a microclimate and, at the same time, many smaller scale microclimates might exist between the individual buildings.

In places where relief has a marked climatic effect, the term ‘topoclimate’ is sometimes used to describe the conditions at a local scale.

Valley Microclimates

Valley Microclimates A number of different microclimates operate at various points

within a valley, depending on aspect, slope angle and altitude.

Because relatively cool air tends to sink downwards at night, valley bottoms are often cooler than higher slopes.

This is particularly the case in clear still conditions.

Night time radiation from the Earth’s surface escapes upwards and there are no turbulent winds to cause air at different levels to mix.

Frost hollow (low lying areas in which frost is most likely) can form on the valley floor.

Valley Microclimates The effect of frost hollows on farming is important. Plants that are

sensitive to frost have to be grown on valley slopes rather than on the valley bottom.

Within hilly and mountainous regions the different angle and aspect of slope creates different microclimates on opposite sides of valleys.

This is especially true for valleys that trend east-west because there is a marked difference in intensity of solar radiation that the north and south facing slopes receive.

In the northern hemisphere, it is the south facing slopes that receive the strongest sunshine. On these slopes, it is locations where the angle between the surface and the Sun’s rays is 90°.

The different intensity of solar radiation on valley slopes affects not only the temperature but also the moisture content of soil.

Valley Microclimates

This is because evaporation rates are at a maximum where it is the

warmest.

These factors affect natural vegetation, settlement and land use.

Woodland Microclimates

Woodland Microclimates On hot days, it is often cooler in woodland or forests.

This is because there is a distinct microclimate under the trees. Typically between 5 and 15% of incoming solar radiation is reflected by leaves and much of the rest is absorbed by the tree canopy.

It is not only the temperature that is affected. Trees provide shelter from winds so air is often still in a woodland.

Because air movement is weak, moisture from transpiration is not quickly dispersed and humidity tends to be high. At night, the effect of a tree canopy is to retain heat, causing the range of temperature inside the woodland to be less than elsewhere.

The exact microclimate within woodland depends upon the type of tree and season. Trees that provide a very dense canopy cut out most of the light, while deciduous trees in winter provide very little cover and a high proportion of light reaches the ground.

Woodland Microclimates The microclimate of woodlands affects the type of vegetation that

grows at ground level.

In general, the darker the conditions, the fewer plants will grow

(therefore there is little vegetation on the floor of trees with a

dense canopy cover).

Under deciduous trees there are often very dense vegetation in

spring and then the vegetation tends to die in high summer, when

the tree canopy is most left.

Urban Microclimates

Urban Microclimates

In urban areas, much of the land is covered by concrete, asphalt,

brink and glass.

These surfaces have very different physical properties from soil and

plants, hence they create a distinctive microclimate.

The variable height of buildings and the street pattern affect not

only air flow but also the intensity of solar radiation received at

ground level.

Some places might be in shade for much of the day while other

places might receive extra light reflected from glass buildings.

Urban Microclimates Towns and cities are generally warmer than surrounding areas, described

by a term known as URBAN HEAT ISLAND.

Urban areas also tend to have more clouds and rainfall and less windy conditions and humidity than rural areas.

The air in urban areas is more polluted than air elsewhere, resulting in less solar radiation from reaching the ground level.

In general, the larger the urban area, the more marked is the climatic contrast with the surrounding areas.

Although incoming solar radiation is deduced by dust and smog particles, this cooling effect is more than counterbalanced by factors which cause urban areas to be relatively warm.

Urban Microclimates

Brick, concrete and asphalt are good absorbers of heat. During the day, they become warm and then slowly release their heat at night (thereby preventing the temperature from falling as quickly as elsewhere). At night time, the difference between urban and rural temperatures is greatest.

Urban heat islands are also formed due to the burning of fuel for energy. Central heating systems and vehicle engines all produce heat.

Urban heat islands are most marked at times of light winds. When the winds are strong, the effect is to mix the air and reduce the urban-rural temperature difference.

The presence of tall buildings in urban areas, creates friction and reduces the speed of air movement. This causes winds to be generally lighter than rural areas.

Urban Microclimates. However, some exceptions occur when air is funnelled between tall buildings,

resulting in gusts and eddies.

Often, there is a significant difference in wind speeds between streets if some are built parallel with the prevailing wind direction and others are built at 90°.

In one case, the wind is strengthened as it is channelled down the street, whereas in streets at a right angle there might by very little wind.

Rainfall is higher in urban areas compared with surrounding regions and thunderstorms are more frequent.

The higher ground temperature in cities makes convection currents more likely and therefore increases the chance of rain.

In addition, the frictional effect of high buildings sets up air turbulence and triggers upward air movements. The rising air cools, condensation occurs and cumulus clouds build.

THE END