Everything is about perspective; there’s that saying “One person’s garbage is another person’s treasure.” And so it is about air. That’s not to say that air is a treasure. Or garbage for that matter. The point is that to most people air is just air. The air might feel cold or warm, dry or wet but for the most part it doesn’t affect them, other than how they might dress for the day and that they need to continue to breath it. But to a pilot, the characteristics of the air have a significant impact on the performance of the aircraft we are flying.
When pilots think of the air, density altitude is (or should be) one of, if not the first, things that comes to mind. Density altitude is the atmospheric reality within which we operate our aircraft. I like to describe it as “the altitude at which the aircraft ‘thinks’ it is operating at.” You could easily argue that there isn’t more critical phases of a flight with respect to density altitude than takeoff and landing when the aircraft is low (to the ground) and slow (relative to cruise speed). And at takeoff the aircraft is also the heaviest it will be.
This is why density altitude is an important factor at airports. Critical aircraft performance metrics are all affected by density altitude, such as the distance required for takeoff and, for aircraft with normally aspirated engines, whether or not the aircraft can climb at a rate required to clear any obstacles. This is why calculating density altitude is a part of proper flight planning; as pilots we need to ensure that the runway is long enough to enable the aircraft to get off the ground and that the runway at our destination is both long enough for us land and then, if necessary, to take off from.
NAV CANADA started including density altitude in METAR for Canadian airports in late 2009, making things that much easier when flight planning. The density altitude is automatically calculated and included in the remarks section of the METAR whenever the density altitude is 200 ft or higher than the aerodrome elevation as noted in NAV CANADA’s Aviation Weather Services Guide [PDF]. For example, here’s a METAR from CYYC today (22-Jun-2016 1900 UTC):
METAR CYYC 221900Z 13006KT 040V220 40SM SCT068 SCT220 BKN300 22/05 A2999 RMK CU3CI1CI1 SLP154 DENSITY ALT 5100FT
The elevation of Calgary International Airport (CYYC) is 3606 feet MSL (height above mean sea level) and calculated density altitude for CYYC is 5100 feet MSL. It may not seem like a big difference but it’s enough of a difference that the takeoff rolls of heavy aircraft will be longer and the climb out will be slower, which is great for any avgeeks as planespotting will be easier since aircraft will be lower for longer.
But back to calculating density altitude. The reported density altitude for Calgary is 5100 feet. Or is it? The answer is a complicated ‘yes’ and ‘no’, leaning much more towards no than yes. Calculating the true density altitude requires four variables: altitude, pressure, temperature and humidity.
The NAV CANADA calculated density altitude that is included in the METAR above is using the dry calculation; that is, a density altitude that does not factor in the humidity. And, in fact, the calculated density altitude in the METAR remarks is also rounded. The actual dry density altitude, based on the above METAR, is 5154 feet MSL. But when we include humidity (using the dew point from the METAR) in the calculation, we get the true density altitude of 5294 feet MSL. This true density altitude, the altitude at which the aircraft will “think it is at,” is almost 200 feet higher than is reported in the METAR, a difference of almost 4%. Although this difference will likely not affect an airliner, where the margin for error is much more forgiving, that 4% difference might be catastrophic for a small [general aviation] aircraft that is close to (or at) gross weight trying to takeoff from a short runway.
If you’re interested in reading more about calculating density altitude, check out the Wikipedia article or the article “Hot, High and Heavy — The Deadly Cocktail of Density Altitude” [PDF] in the July 2012 issue of NOAA/NWS’s The Front.
UPDATE (2016-06-23): NAV CANADA responded that they use the dry calculation so as to share commonality with the United States, which also uses the dry method to determine density altitude. However, NAV CANADA said that there have been internal discussions to “review the calculations to include the dew point” so we may see that change in the future.