Another meteorology question!
 

Hi

I'm about to take my met exam at some point in the next couple of weeks. I have so far managed to get through almost all of my revision and doing well with my practice questions. However I've had a go at question 37 in the confuser which says:

"Given a surface temperature of +21 degrees celsius and a dewpoint temperature of +7 degrees celsius, at approximately what height will the base of cumulus cloud be found?"

So far I have been learning that:

DALR = 3 degrees per 1000ft
SALR = 1.5 degrees per 1000ft
Average ELR = 2 degrees per 1000ft

So to calculate cloud base you simply calculate the altitude where the DALR temperature reaches the dew point temperature. The problem is according the confuser explanation says "dewpoint decreases by 0.5 degrees / 1000ft so DALR is modified". The nearest answer is therefore 5500 feet.

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 :uhoh:

Thanks for any info you can offer in my hour of need :)

I have always used the method:
cloud base = temperature/dewpoint spread x 400 feet

So in this case 14 x 400 = 5600 feet

Rough and ready, so I don't know if its useful, but it usually gets the answer ;)

No, it's not mentioned in the Pratt book, but you might find this useful

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.

Just remember none of this exam stuff works in practice....

Fortunately, anywhere where somebody has actually measured the temp and DP and is reporting them, they will also be reporting the actual cloudbase ;)

That's probably why they put it in the exams......You'll never see it again in real life.

I think the only relevance of this stuff is when temp is reported "same" as the DP, you need to be careful going for a flight because it won't take much for fog to form - especially in the evening as the air cools.

Fog formation is generally poorly forecast.

I use the T/D spread as a measure/check on the cloud base. See if either I or the met guy didn't make a gross mistake somewhere. Plus, a T/D spread of less than three is a warning that fog may form suddenly, or low clouds, or carb ice.

On the other hand, a T/D spread of more than ten usually means a glorious day for flying.