Microclimates Flashcards
(18 cards)
Define Microclimate
The distinctive climate of a small scale area. The weather variables such as rainfall, wind, temperature and humidity may be subtly different to the conditions prevailing over the area as a whole
What are the 4 distinct microclimates in the UK
1-Urban Regions
2-Upland Regions
3-Coastal Areas
4-Forest
What is an urban heat island
An urban area that is significantly warmer than its surrounding rural areas due to human activities. Under calm conditions, temperatures peak in the city centre and decrease towards the suburbs
What are the 5 main reasons urban heat islands occur
- The land surface is made of materials which absorb energy during the day and release it at night
- Increased heat due to a large number of people and industry
- Lack of vegetation leads to lack of evapotranspiration-water that plants no longer need is absorbed by the dry air by turning it into water vapour. The air provides the heat to drive the process, so during the process, the air loses heat
- Increase pollution leads to the formation of clouds and smog-causing a dome to form causing terrestrial radiation to be reflected back to earth
- Absence of winds to disperse heat and bring cooler air
Effects of Heat Islands
- The diurnal range is much smaller than in rural areas as the nighttime temperatures are higher
- The air quality is often much lower as pollutants stay close to the city leading to health issues
- Energy Consumption is higher as there may be an increase in the use of aircon, however, it may decrease energy use as less energy is needed to heat buildings in the winter
How does precipitation differ in urban micro-climates and why
Heat islands lead to increased convectional uplift, which can intensify thunderstorms, as storm cells can be refuelled by contact with the warm surfaces/air
What are the key features of an Upland Microclimate
- Temperature falls with height at a rate of 1C per 100 metres
- As altitude increases, wind speeds tend to increase as there is less shelter and the peaks are often exposed- but also depends on tunnelling effect and direction of the prevailing wind
- The leeward side of hills is much drier than the windward side. Because hills cause cloud to form, forcing air to rise. When the air rises it cools and condenses forming rain, as the air descends on the leeward side it dries and warms
Characteristics of Urban Winds
Wind speeds are lowered by the friction and the barrier of the urban canopy. The varying hights of buildings and city design creates urban canyoning, wherere the placement of buildings increase the intensity of the wind
Characteristics of a coastal region microclimate
- Annual temperature range is small due to the moderating effect of the sea, due to the seas high specific heat capacity
- The wind is generally stronger as not only is there less shelter but there is also a greater temperature gradient therefore creating stronger winds
- The sea breeze front which is where the cold sea air, meets the warmer air from the land, may have enough energy to form cumulus clouds and when the air is humid the front can trigger thunderstorms
What is the Foehn effect
A change from wet and cold conditions on one side of a mountain, to warmer and drier conditions on the other (leeward) side.
What are Katabatic winds and what do they cause
A wind that carries high-density cool air from a higher elevation down a slope under the force of gravity. The downward flow of cooling air gathers in the valley causing the colder denser air to sit beneath the warmer lighter air
It causes hills to be warmer than valleys
2 reasons when hills may be warmer than valleys
1.Temperature Inversions
- Katabatic Winds
During clear nights when there is little wind, particularly in winter. As the air cools it begins to flow downhill and gather on the valley floor- this flow of air can create katabatic winds. Often leads to the formation of fog and frost
Why does a temperature inversion occur
Occur most often when a warm, less dense air mass moves over a dense, cold air mass. This can happen when the air near the ground rapidly loses its heat on a clear night. In this situation, the ground becomes cooled quickly while the air above it retains the heat the ground was holding during the day.
What are anabatic winds
A localised wind that blows up a slope which is heated by sunshine
Explain how sea breezes form during the day
- During the day the land heats up faster than the sea, due to the seas high specific heat capacity. Causing low pressure to form over land, and therefore the air rises over the land
- This rising air can condense to produce rain over the land
- The air then descends and cools over the sea
- As air moves from areas of high pressure to areas of low pressure. The air at the surface rushes to replace the rising air lost at the land surface. This creates a wind from the sea to the land (onshore wind)
Explain a land breezes (in coastal areas) form during the night
- At night the sea is warmer than the land, due to the seas ability to retain heat. This means air rises over the sea
- The air then cools and descends over the land
- Air moves from areas of high pressure to low pressure. So the air rushes the replace the rising air lost at the surface- this creates offshore winds
A forest microclimate
- Tend to be cooler and less windy than surrounding land. Trees act as windbreaks and the incoming solar radiation is filtered by the leaves and branches
- But depends on the season, whether the forest is deciduous or evergreen and the location of the forest
Why does temperature fall with an increase in altitude
- At higher altitudes, air pressure is lower, and the temperature is directly proportional to air pressure. When pressure is lower the airs ability to hold heat is lower
- At higher altitudes, there is less ground area, these means that the air is heated by conduction less.
- Vegetation tends to decrease with altitude due to the harsher conditions and lower levels of oxygen. Sparsh vegetation means there is less protection from cold winds and decreased the ability to trap heat and therefore temperatures tend to be lower.
