I have discussed this before, but as the summer heats up, let me refresh or provide you some information on Density Altitude. When we fly in high temperature and humidity conditions we effectively increase the altitude of runway elevations and with that our stall speeds increase. All of a sudden that ever so difficult to land warbird is now a real handful off the end of the runway or that heavily loaded UAV that flew great last winter now flies like a lead brick.
Density altitude is a combination of altitude, barometric pressure, air temperature and dew point. Most pilots will check this before departing the airport as they calculate their weight and balance. As an example, one could have a situation where they fly from a 1000′ runway elevation in normal conditions, but on a 105 degree day with high barometric pressure which usually is associated with high temperature and clear skies, plus high humidity and that 1000′ runway is now nearly a mile high where the air is thin. In the Midwest our humidity, even in high temperatures, can be high as moist air is pumped in from the south and southwest. Takeoff rolls are longer, stalls are quicker and overall the performance of your airplane is heavily influenced by the density altitude.
Here are two websites you can use to determine your density altitude. The first provides weather information necessary including dew point, barometric pressure, and pressure both current and historical. You will need to provide your elevation. You can typically use your GPS.
The second is a density altitude calculator. Simply include the necessary information and you can determine the flying conditions either currently or based on the previous days data.
Take the time to determine your density altitude when weather dictates high temperatures and high humidity. As the air gets thin, your prop has less to bite into and performance suffers across the entire flight envelope. But remember, winter will return and in that dense cold, dry air, performance will be awesome.