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Let's say I'm planning a day trip from a low elevation home base to a warm, mountainous location. I wake up early, check the weather, do performance calculations based on current METARs, and decide to make the trip. However, when I go to return home in the late afternoon, I find the elevation and the heat make the density altitude too high for me to take off.

Using purely FAA-approved weather sources, how could this be avoided? As far as I can find, there are no forecasts that include temperature or pressure. Several non-aviation weather sites provide an hourly forecast of both of those things. Is that the best way to avoid being stuck on the ground at a high density altitude? It's obviously no substitute for a standard weather briefing, but it seems to be a necessary part of "all available information."

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You are not required to forecast temperature and pressure altitude (i.e., density altitude) to determine ahead of time whether or not your aircraft will be within its performance limitations upon your anticipated departure later in the day (using your example).

If, at the actual time you want to depart, the density altitude is too high (beyond your aircraft's performance limitations) then you can't depart.

So that you don't get stuck and inconvenienced at an airport because of your aircraft's performance limitations you can use any means available (local tv weather, commercial weather services like Foreflight or similar, etc.) to forecast/anticipate/ predict the potential density altitude that may exist at the time of your planned departure.

The issue doesn't become a safety or legal issue unless you actually depart when the density altitude (which you can calculate just prior to departure with real-time airport temperature and pressure altitude) is beyond your aircraft's performance limitations.

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The best way is to check the weather and be very knowledgeable of the effects of weather on your altimeter.

Density altitude is primarily pressure altitude corrected for temperature, but higher humidity will also raise density altitude (H2O is lighter than N2 or O2).

The major factor is temperature, so taking off in early morning, when Temps are lowest, is plan 1.

Density Altitude = Pressure Altitude + [120 × (OAT - ISA)]

Also, keep an eye on lower pressure moving into the area, which will raise the pressure altitude around 1000 feet for every inch of pressure drop (one inch could be a fairly serious storm).

Humidity effects are minor, unless the air is very warm. At 10C air can carry 1% moisture, at 40C this rises to 5%, and at 50C to 9%!

But a big part of the "forecast" is how much weight you plan to be taking off with. Leaving a few hundred pounds behind can greatly improve safety margains.

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