Temperature drop with altitude increase

[fdelacou]

Hello,

I'm a student pilot and I have been reading many of the books to get the flying skills in but I have found some contradictory information and I'm now more confused more than informed!
The subject of my confusion is about the temperature decrease in the troposphere. I'm getting different values depending on the source. all the values are per 1000 feet.

Cessna Pilot Center training (Kings):
2 degrees celsius
2.5 degress celsius

Pilot's handbook of aeronautical knowledge
3 degrees celsius

Does any one has an idea on the value that should be retained? The only case that I have seen these values used are for performance calculation and for clouds height determination and it seems to me that the 1 degree of separation is too significant to be considered negligeable. Of course the 2.5 is the less risky choice I guess...

Any thoughts? comments?


Frederik Delacourt

[FloridaAviator]

I preface my comments with the statement that I am not an expert on aviation weather by any stretch of the imagination. However, I do recall some of these concepts from my PPL training days ...

The phenomenon that you are describing (temperature decrease with an increase in altitude) is known as the lapse rate. There are several different kind of lapse rates to consider which is where some of your differing numbers may be coming from ...

The environmental lapse rate is the change in temperature with altitude for the stationary atmosphere (i.e. - the temperature gradient). The average value for the environmental lapse rate is 2 degrees C (3.5 degrees F) per 1000 ft. However this assumes a standard atmosphere with stable air where the temperature falls steadily with altitude. It is important to understand that it is often rare to have this steadiness in the atmosphere. The level of moisture in the atmosphere as well as other phenomena can create temperature inversion layers where the temperature of the air actually increases with altitude at times. The important point to remember is that this is always considered a rough order estimate.

A cool experiment I used in analyzing my weather data before a flight was to record the temperature, dew point, and lowest cloud layer level. The point where clouds begin to form is the point where the air is completely saturated (temperature equals dew point). If you take the difference between thte temperature and the dew point (in degrees C), divide it by the standard lapse rate of (2 degrees C), and then multiply by 1000 ft, this will give you the estimated level of the clouds AGL. Comparing this calculated value to the information in your METAR report will give you a good idea about the stability of the atmosphere (whether the standard lapse rate is in effect). More often than not, the calculated cloud layer is pretty close ... it's cool how science and math works sometimes :-)

By the way, as an aside, the other two defined lapse rates are the dry adiabatic lapse rate and the moist adiabatic lapse rate which refers to the temperature of an air mass as it moves upward. As you might expect, the dry rate refers to a dry mass of air and the moist rate refers to a moist mass of air (the value can vary drastically depending upon the moisture content of the air).

Hope this helps answer some of your questions and not confuse you even more :-) Feel free to ask for clarification if anything is unclear.

Clear Skies,
Dave

[fdelacou]

Ah OK! Now it makes sense I guess the confusion is coming from the fact that they made it sounds like the value where for standard atmosphere...

I like your reverse analysis to predict the stability and I will add it to my routine... Any particular reason you are not starting from the altitude and determining the temperature drop instead of starting with the temperature and comparing the clouds height?

Thanks for the answer!

[FloridaAviator]

Here in the United States, the number of fields that provide METAR reports vary from location to location. However, almost all locations provide a temperature and dew point measurement.

Although the standard lapse rate of 2 degrees C per 1000 ft is almost never exact, it *is* a decent estimate. With this estimated standard lapse rate along with the temperature and dew point, you can do a rough estimation of the cloud layer level at any location. Also, when flying at night, it is sometimes more difficult to gauge the height of a cloud layer. Using the above calcualtion again gives you a general idea of the conditions enroute.

I would never use this is a go/no-go criteria for VFR flight, but it should give you a general idea of the cloud layers you may encounter along your line of flight (and the progression over your flight route). For example, if the spread between temperature and dew point decreases as you get closer to your destination, you may be able to infer that the lowest cloud layer may be getting lower as you progress through your flight.

It's just another tidbit of information that goes into your overall analysis of flight conditions so you are as prepared as you can be for what might pop up mid-flight.

Bottom line, you can certainly due the calculation either direction ... I do it both ways ... it is purely a matter of personal preference :-)

Clear Skies,
Dave