I've never come across this in either the Thom or Pratt books so I'm confused by it!! Why would you need to modify the dewpoint temperature and why does it deacrease by 0.5 degrees per 1000ft? I'm totally confused and would really appreciate some help! Its the only part of met I just can't get at all
It may help if we go back to the fundamentals then work from there.
The amount of water which a mass of air can absorb is determined by the air temperature. The higher the air temperature, the greater will be the amount of water that it can absorb.
The term “relative humidity” is a measure of the amount of water in a parcel of air, expressed as a percentage of the amount that it can absorb at that temperature.
When air contains the maximum possible amount of water at a given temperature, its relative humidity is 100% and it is said to be saturated. The temperature of the air at this point is called the Dew Point.
If a parcel of air moves upwards in the atmosphere the pressure acting upon it will decrease, thereby allowing it to expand. This expansion causes the air temperature to decrease. This in turn causes its humidity to increase. If the air continues to move upwards after the humidity has reached 100%, some of the water will precipitate out of the air to form very small droplets of liquid. These droplets are visible in the form of clouds. So the bottom of the clouds (the cloud base) occurs at the altitude at which the air temperature reaches its Dew Point.
As air rises in the atmosphere the decreasing air pressure allows it to expand. This expansion causes its temperature to decrease. This process is called adiabatic cooling.
The Dry Air Lapse Rate (DALR) is the rate at which unsaturated air becomes cooler as it rises in the atmosphere. The DALR is 3.0 degrees Celsius per 1000 feet altitude increase.
Condensation/evaporation processes are also affected by static pressure. It is harder for a vapour to condense into a liquid when the pressure acting upon it is reduced. So as altitude increases and pressure decreases, we need to cool the air to a lower temperature to get the water vapour to condense out of it. This means that the Dew Point decreases as altitude increases. This is the reason why as altitude increases, the dew point decreases by 0.5 degrees Celsius per 1000 feet.
So if air temperature is greater than dew point temperature, the difference between the DALR and the dew point will decrease at a rate of 3 – 0.5 = 2.5 degrees Celsius per 1000 feet.
This means that as altitude increases, if air temperature is greater than dew point, the air temperature and the dew point will converge at a rate of 2.5
degrees Celsius per 1000 feet. Clouds will form at the altitude at which the air temperature is equal to the dew point temperature.
So the cloud base can be calculated using the following equation:
Cloud base = 1000 feet x (Surface temperature – Dew point temperature) / 2.5
Inserting the data provided in this question gives:
Cloud Base = 1000 ft x (21 –7) / 2.5
So Cloud Base = 5600 feet.