1. Thermal Processes
ENVI 1400 : Lecture 6

2. Incoming solar radiation
Radiation Processes
reflected solar radiation
107 W m2
Incoming solar radiation
342 W m2
Outgoing longwave radiation
235 W m2 40
Reflected by clouds, aerosol & atmosphere 30
165 77
emitted by atmosphere
Absorbed by atmosphere 67 24 78
350
back radiation
324
Reflected by surface 40 30
168 24 78
390
324
Absorbed by surface
thermals
Evapo-transpiration
Surface radiation
Absorbed by surface
ENVI 1400 : Meteorology and Forecasting

3. ENVI 1400 : Meteorology and Forecasting
Adiabatic Processes
An adiabatic process is one in which no energy enters or leaves the system.
Many atmospheric processes are adiabatic (or nearly so) – particularly those involving the vertical movement of air.
Air is a poor thermal conductor, and mixing often slow enough for a body of air to retain its identity distinct from the surrounding air during ascent.
Near-surface processes are frequently non-adiabatic.
Adiabatic Processes:
Ascent of convective plumes
Large scale lifting/subsidence
Condensation/evaporation within an airmass
Non-Adiabatic Processes:
Radiative heating/cooling
Surface heating/cooling
Loss of water through precipitation
Addition of water from evaporation of precipitation falling from above
ENVI 1400 : Meteorology and Forecasting

4. ENVI 1400 : Meteorology and Forecasting
Lapse Rate
Lapse Rate is the term given to the vertical gradient of temperature.
The fall in temperature with altitude of dry air that results from the decrease in pressure is called the Dry Adiabatic Lapse Rate = -9.8°C/km.
Dry Adiabatic Lapse Rate
Altitude
1km
9.8°C
Temperature
ENVI 1400 : Meteorology and Forecasting

5. ENVI 1400 : Meteorology and Forecasting
Condensation releases latent heat, thus saturated air cools less with altitude than dry air.
There is no single value for the saturated adiabatic lapse rate. It increases as temperature decreases, from as low as 4°C/km for very warm, tropical air, up to 9°C/km at -40°C.
Altitude
Dry Adiabatic
Lapse Rate
Saturated Adiabatic Lapse Rate
Temperature
ENVI 1400 : Meteorology and Forecasting

6. Pressure & Temperature
A column of air has pressure levels P1, P2, etc.
If the column is warmed, the air will expand and it’s density at any given level decrease.
The vertical interval between pressure levels increases, so that at any given altitude the pressure in the warmer column is greater than in the cooler.
N.B. since the total mass of air in the column is constant, the pressure at the surface does not change P0 P1 P2 P3 P4 P5
warm P5 P4 P3 P2 P1 P0
cool z
ENVI 1400 : Meteorology and Forecasting

7. ENVI 1400 : Meteorology and Forecasting
Cold-core Low intensifies with height
cool
warm H
Warm-core High intensifies with height
cool
warm L H
cool
warm
Warm-core Low weakens with height, may form a high aloft H L
cool
warm
cold-core High weakens with height, may form a low aloft
ENVI 1400 : Meteorology and Forecasting

8. ENVI 1400 : Meteorology and Forecasting
Mid-latitude low-pressure cells have colder air to the rear.
As a result, the axis of the low slopes towards the colder air
Cold low L
Warm high
Sea-level isobars
500 mb contours
ENVI 1400 : Meteorology and Forecasting

9. ENVI 1400 : Meteorology and Forecasting
High pressure cells slope towards the warmest air aloft.
The centre of the cell at 3000m may be displaced 10-15° towards the equator.
Cold low H
Warm high
Sea-level isobars
500 mb contours
ENVI 1400 : Meteorology and Forecasting

10. ENVI 1400 : Meteorology and Forecasting
The Thermal Low
Thermal lows result from the strong contrast in surface heating between land and sea
Land heats up (solar radiation) and cools down (infra-red radiation) much more rapidly than ocean  large diurnal cycle cross-coast temperature gradient
N.B. A thermal low results from fine, clear, warm weather, and thus differs from the depressions associated with cloud and bad weather.
ENVI 1400 : Meteorology and Forecasting

11. ENVI 1400 : Meteorology and Forecasting
1. Start with a horizontally uniform pressure distribution. Solar radiation starts to warm land. Air near surface is warmed by land, convection mixes warm air upwards and whole boundary layer warms.
warm
cool H
2. Air over land warms and expands. Can’t expand sideways, so column expand upwards  produces high pressure aloft. N.B. Surface pressure remains constant at this stage.
ENVI 1400 : Meteorology and Forecasting

12. ENVI 1400 : Meteorology and Forecasting
3. Horizontal pressure gradient aloft drives a flow from over land to over ocean. H
cool
warm
cool
warm
cool H L
4. Mass of air in column over land is reduced  surface pressure falls to produce a surface low. High pressure aloft weakens, but is maintained by continued heating at surface. Surface pressure gradient drives flow from sea to land: the sea breeze.
ENVI 1400 : Meteorology and Forecasting

13. ENVI 1400 : Meteorology and Forecasting
5. When solar heating stops, pressure driven flows act to equalize pressure, restoring conditions to the initial uniform pressure field.
If land cools sufficiently at night, the reverse situation can be established.
Over large land masses there may be insufficient time over night for the sea breeze to reach regions far from the coast, and a weak surface low is maintained over night. This then deepens during the following days, and a heat low may be maintained for days or weeks, until synoptic conditions change. H L
warm
cool H L
ENVI 1400 : Meteorology and Forecasting

14. ENVI 1400 : Meteorology and Forecasting
Sea Breeze
Formation of local thermal low over land, results in the formation of a sea-breeze
In-flowing cool air from sea forms a sea-breeze front – a miniature cold front
Air ahead of the front is forced upward, contributing to the formation of cumulus.
950 mb
975 mb
1000 mb
25C
15C
ENVI 1400 : Meteorology and Forecasting

15. Pressure as an indicator of temperature
Because the depth of a layer of air increases as its temperature increases, we can use the difference in altitude between two constant pressure levels as an indicator of the mean temperature of the layer.
Charts are usually produced of the depth of the layer between 1000 and 500 mb.
The layer depth is usually quoted in deca-metres (10s of metres)
A useful rule of thumb is that for mb layer depths less than 528 dm (5280 m) any precipitation will fall as snow rather than rain.
ENVI 1400 : Meteorology and Forecasting

16. ENVI 1400 : Meteorology and Forecasting
528
546
564
SLP (mb) & thickness : 48hr forecast valid
ENVI 1400 : Meteorology and Forecasting

17. ENVI 1400 : Meteorology and Forecasting

18. ENVI 1400 : Meteorology and Forecasting
546
528
564
SLP (mb) & thickness (dm) : 36hr forecast valid
ENVI 1400 : Meteorology and Forecasting

19. ENVI 1400 : Meteorology and Forecasting
546
564
SLP (mb) & thickness (dm) : analysis valid
ENVI 1400 : Meteorology and Forecasting

20. ENVI 1400 : Meteorology and Forecasting
546
2°C
564
12°C
850 mb Temperature (2°C contours), RH (%), wind (m s-1) : analysis valid
ENVI 1400 : Meteorology and Forecasting

21. ENVI 1400 : Meteorology and Forecasting
546
564
Surface temperature (2°C contours) and SLP (mb)(5mb contours) : analysis valid
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22. ENVI 1400 : Meteorology and Forecasting
The Thermal Wind
It is commonly observed that clouds at different altitudes move in different directions  winds are in different directions.
The gradient of wind velocity (speed & direction) is called the (vertical) wind shear.
In the free air, away from surface (where friction effects complicate matters), the wind shear depends upon the temperature structure of the air.
The thermal wind is a theoretical wind component equal to the difference between the actual wind at two different altitudes.
Any two levels can be used, but unless otherwise stated the altitudes of the 1000mb and 500mb levels are usually used.
Note that the 1000mb level might be below sea level, and is usually within the boundary layer and thus influenced by friction effects at the surface.
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23. ENVI 1400 : Meteorology and Forecasting
cold
LOW
warm
500mb
HIGH
Vg(1000)
LOW
996
1000mb
HIGH
1004
1008
ENVI 1400 : Meteorology and Forecasting

24. ENVI 1400 : Meteorology and Forecasting
mb thickness
5580
5640
5700
LOW
VG1000
5640
5700
LOW VT
VG500
5760
Contours of
500 mb surface 60
5820
HIGH
120
Contours of
1000 mb surface
HIGH
180
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25. ENVI 1400 : Meteorology and Forecasting
Note that cold air is to the left of the thermal wind vector (looking along wind) in the northern hemisphere, to the right in the southern hemisphere.
The decrease in temperature towards the poles results in a westerly thermal wind in the upper atmosphere in both hemispheres.
The largest meridional temperature gradient occurs in mid-latitudes across the polar front.
The thermal wind makes up a significant component of the jet-stream, located over the upper part of the polar front.
ENVI 1400 : Meteorology and Forecasting