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Temperature drop with altitude increase

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! :confused: 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

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!

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

There are two lapse rates to consider for aviation.

The first and most important to pilots is the DRY adiabatic lapse rate, typically about 10 degrees celcius per 3000 meters or about 1.8 degrees farehheit per 1000 feet of altitude. That is the rate you need to be aware of for most of your flying. This is the theoretical lapse rate at zero relative humidity, something which never occurs in nature but is used by meteoroligists to express a standard temperature drop with altitude.

The MOIST adiabatic lapse rate is another theoretical value of temperature change with altitude in clouds, where the relative humidity is 100%. Within clouds the lapse rate is always significantly less than the dry adiabatic lapse rate.

Happy landings, Dave!

Douglas M
Surrey, BC

the air temperature decreases by an average of 6.5°C per km (3.6°F per 1,000 ft) above the Earth’s surface and temperatures as low as -60°C (-76°F) occur at the top of the troposphere.

In January of 1943, the Civil Aeronautics Administration (of the Dept of Commerce) published the very authoritative METEOROLOGY FOR PILOTS. It was authored by B C Haynes, senior meteorologist for the U.S. Weather Bureau (CAB bulletin no. 25)

Therein it states, on pg 72, that the DRY ADIABATIC LAPSE RATE of temperature with altitude is 5.5 deg F / 1000 feet for the ideal dry adiabatic model of the atmosphere at standard sea level temperature (68 F) and pressure which then, just the same as today, was considered to be 29.92 inches Hg.

It cannot be overstressed that the dry adiabatic lapse rate for temperature I gave you is an average drawn from hundreds of thousands of observations taken from all over the globe over the past century...it is an AVERAGE. It goes on to state in this book that the temperature lapse rate is generally uniform up to about 35,000' and then the temperature remains constant (zero lapse rate) or may even increase from there on up for a distance and then begins decreasing again.

The reason they use the word "adiabatic" is that it is based on an "adiabatic" model of the earth's atmosphere....the earliest barometers were based on the "isothermal" model which assumed a constant temperature from sea level on up. The adiabatic assumes that as a given volume of air rises, it loses internal energy by doing the work of expanding against the decreasing pressure as it rises. This is much closer to reality than the isothermal and actually results in an equation that says that the atmosphere comes to an end at 102,000 feet.

In any event, some of the confusion with lapse rates is probably due to the fact that the figure of 5.5 is for the DRY rate at STP while any amount of moisture results in what they call the "wet adiabatic lapse rate" and is lower----2.5 or 3 degs F per 1000 for example.