Density altitude is pressure altitude corrected for nonstandard temperature. In plain language, it is how thick the air feels to the aircraft. High density altitude means thinner air. Low density altitude means denser air. The Part 107 test does not usually ask you to compute a precise density altitude with a flight computer. It asks what causes it and what the aircraft will do.
Hot, high, and humid
Three conditions push density altitude up. Heat makes air less dense. Higher elevation means less air above you, so pressure is lower before temperature even enters. Humidity makes the air less dense than dry air at the same temperature and pressure, because water vapor is lighter than the dry air it displaces. The memory line used throughout the course is hot, high, and humid. The opposite, cold, low, and dry, is low density altitude and better performance. "Only at sea level" is not a cause of high density altitude. Sea level on a cold day is where performance is usually best.
You can see the ingredients in a METAR without calling them density altitude. Temperature and dew point are the Celsius pair. Field elevation is on the sectional, in the airport data block, as feet MSL. A 30 °C afternoon at a 5,000-foot airport with a dew point close to the temperature is the classic thin-air setup. Decoding that METAR is covered on the METAR page.
What the rotors actually lose
A multirotor makes lift by accelerating air downward. In thinner air each revolution moves less mass, so the rotors produce less lift and less thrust for the same RPM. The aircraft works harder to hover, climb rate falls, and the battery is spent sooner. The course example is a hot afternoon at elevation: the drone struggles to climb and the pack drains early. The correction is to reduce payload, shorten the flight, and keep a larger reserve. It is not to climb higher to "get cooler."
The same physics is why a manufacturer's performance table matters more than a rule of thumb. The ACS expects you to use the aircraft's own data: maximum takeoff mass, temperature range, wind limit, and any density-altitude or endurance table. If the planned weight is above the table, or the table does not cover a hot high site, do not guess that the sea-level hover time still applies. Reduce the load or move the job. Weight and balance are a separate limit, covered with center of gravity.
Density altitude is not the 400-foot rule
Pilots mix these up because both use the word altitude. Section 107.51(b) limits the aircraft to 400 feet AGL, or 400 feet above a structure when you stay within 400 feet of it. That is a regulatory ceiling. Density altitude does not raise it and does not waive it. A waiver of §107.51, if you ever need one, is an operational waiver, not something LAANC grants because the grid looks tall. High density altitude is a reason you might not reach a legal altitude, not a reason to exceed it.
Weather around the number
High density altitude often arrives with other no-go clues. A hot humid afternoon is also when thunderstorms build. The mature stage of a thunderstorm, with downdrafts and outflow, is the hazardous stage. A microburst is a localized downdraft that spreads at the surface and changes wind speed and direction suddenly. Those hazards are independent of the density-altitude calculation, and they are worse for a small aircraft that is already performance-limited. Haze, smoke, and fog can also drop flight visibility below the 3-statute-mile minimum even when the density altitude is only moderate.
Before a hot-site job, read the temperature, the field elevation, the forecast wind, and the aircraft's published limits together. If climb or endurance is marginal in the table, treat the manufacturer's number as a limit. The test answer for "hot, humid, high elevation" is reduced lift and reduced performance, not "no change" and not "warm air is denser."