Local climates Flashcards
Altitude and topography effect on local climates
- Hills and mountains are important controls on climate
- Temperature and pressure fall with height
- Rate of fall of temperature varies in different parts of the world
Environmental Lapse Rate (ELR)
The rate at which air temperatures fall with
increasing altitude
– A parcel of air will rise until it cools to the same
temperature as the surrounding atmosphere
– The rate of temperature fall follows a predictable
curve
The effect of air pressure changes on an air parcel
- Air pressure decreases with height
- When air parcels ascend they expand (less pressure)
- Conversely falling air parcels are compressed (greater pressure at sea level)
Adiabatic processes
If no energy is added or removed from the
‘air parcel’…..
….the changes in pressure and
temperature in the ‘air parcel’ are termed
adiabatic processes
Why do air masses cool adiabatically when they rise?
- As air rises and pressure falls….
2… the energy for expansion comes from the air
- Air temp is a measure of energy in the air
- If energy is removed through expansion temp falls
If unsaturated air expands as it rises
air temp falls at 9.8°C km-1
Dry Adiabatic Lapse Rate (DALR)
What happens if condensation
occurs when the air parcel rises?
- Air holds a limited amount of water dependent upon its temp
- At saturation point condensation occurs
- Latent energy is released warming the air parcel
- If no water is lost from the atmosphere….
- …. parcel of air will continue to cool at a new slower rate – Saturated Adiabatic Lapse Rate
Inversions
• Where warmer air overlies cool air
• Can prevent air at ground level rising
• Subsidence inversions – descending air in
anticyclones
Wind regime in mountains can be quite different to lower levels
– Funneled through valleys/gaps
– Air compression between summits and temperature inversion above causes very rapid wind flow
Trade wind effect on mountainous areas
– Mid-latitudes influenced by the westerly winds (winds are faster aloft)
– Tropical/subtropical trade wind belts generally weakened with height
(low wind speeds in these mountain ranges)
Föhn wind/Chinook
– Warm dry wind down lee of mountains
– Caused by compression and adiabatic warming
– Onset of the wind is accompanied by sharp rise in temperature and decrease in relative humidity
Effect of topography /altitude on precipitation
The amount of moisture air can hold is strongly dependent on temperature (temperature drops with height)
Air forced to rise over mountains…
– Cools at the dry adiabatic lapse rate until dew point
– Clouds form and temperature decreases at saturated
adiabatic lapse rate
– Even if very dry region – air will eventually reach saturation
Frost Hollows
Cold air moves downhill by katabatic drainage and
can provide unusually high occurrences of frost and
low temperatures in certain locations
• These low temperatures occur when there is strong
radiational cooling
• Water vapour is a strong absorber of long-wave
radiation
– This re-emits down to the surface
– Therefore less radiational cooling occurs in moist
tropical/subtropical regions
Surface temperatures are constant because…
– High specific heat of water (energy required to increase water temperature)
– Surface layers tend to be well mixed – helps spread temperature change
– Surface energy is used for latent heat needed for evaporation (rather than specific heat)
Human influences on local climate
Shelter belts (surface roughness reduces wind speed)
– Line of trees often used to protect sensitive vegetation
– Ideal shelter belt is slightly permeable to extend wind speed reduction over a longer distance
• Urban surface is rougher than most vegetation
(roughness length)
• Urban heat island effect – hotter than surrounding rural areas
– Urban fabric strongly absorbs solar radiation
– Energy release from domestic/industrial processes and air pollution
– Greatest impact in light winds
• Urban canopy layer and urban boundary layer
• Venturi effect – winds forced to funnel through 2 buildings
