Could this ansewer your Q
LAPSE RATES
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This innocent section is actually one of the keys to understanding local scale weather changes.
Free Convection: this refers to air parcels that are heated initially throught contact with the surface (also called lapse rate steepening). The pavements of a town will have a very different albedo to the grass of its parks, or the surrounding fields.
Air parcels over the surface with greatest absorbing properties will be 'superheated': developing a higher temperature and lower density than the surrounding air. Density changes because the heat makes the particles move faster and more energetically meaning that there are larger spaces between them as the air mass expands.
The warmer, less dense air rises from the surface like a hot air balloon and rises up through the surrounding air. It will continue to rise as long as it remains warmer than the surrounding air. If the temperatures equalise, the air will diverge horizontally. If it is cooled below the air it will descend: one such area of subsiding air can be found in the 'eye' of a hurricane.
Forced, or mechanical convection is the vertical transfer of energy by currents in the atmosphere in response to obstructions. This takes place along fronts, such as the Polar Front where the 2 converging air masses have different temperatures and densities. It also occurs when there is an orographic barrier. This mixes air with different temperature and density and causes circulations which can lead to the formation of depressions (cyclogenesis)
The troposphere normally cools with height from the surface. This rate of cooling is known as the Environmental Lapse Rate (ELR) which is a variable rate (I often say, like my mortgage, but in fact I have a capped rate...) - an average figure is 6.5ºC per 1000m, but it varies with time, seasons, humidity (water vapour is mainly found close to the ground surface), short term changes in insolation, height etc. This usually relates to a large mass of stationary air and is the air through which other parcels will rise due to free or forced convection.
It is around 1000 times greater than the temperature change occurring horizontally due to changes in latitude, so it is a relatively rapid change in temperature.
Lapse rate steepening, also called super-adiabatic conditions occur on very hot days. The skin of air close to the earth accumulates large amounts of heat, and the temperature drops sharply above the ground. This creates extreme instability, and the development of thunderstorms. The term instability is a key one when considering lapse rates.
It is also worth noting that under certain conditions, the temperature can actually rise when moving up from the ground surface. This is known as a temperature inversion.
As a parcel of air rises up through the surrounding air it expands, as it descends it contracts. Heat is used up as the air expands. Balloons burst on reaching high levels. Descending air parcels warm as they are compressed. No heat enters or leaves the parcel, hence the term 'adiabatic'.
The rate of temperature change for unsaturated 'dry' air is 10ºC per 1000m (a fixed rate) - higher than the average ELR. This is known as the Dry Adiabatic Lapse Rate (DALR)
Rising parcels of air can reach the dew point: the height at which condensation begins to occur - also known as the cloud base. At this point, water vapour condenses and latent heat is liberated.
This rate is known as the SALR (Saturated Adiabatic Lapse Rate), and is variable depending on the amount of water vapour. A typical value for temperatures below 30 degrees Celsius is 5ºC per 1000m, increasing to 7 degrees below freezing point.
Stability occurs when the ELR is less than the DALR. Rising air will cool faster than the surrounding air and soon reach equilibrium. Air parcels will only rise due to forcing in this situation. Fair weather cumulus humilis are the only likely clouds to be created.
When the ELR is lower than both DALR and SALR, absolute stability occurs.
Absolute instability occurs when the ELR is greater than both the DALR and the SALR in the lower and middle troposphere.
Conditional instability occurs when the ELR lies between the DALR and the SALR and is a more common situation.