From WindsportAtlanta.com: Wiki
It's a nice summer's day. You see whitecaps. You rig up and go on the water but there's just no power in the sail. You struggle to get planing. That's odd - a few months back when it was cooler you were flying in the same conditions. What just happened?
The wind's ability to power your sail depends on the density of air where you are. Warmer air, more humid air and us being at altitude in N. GA all contribute to the air being less dense in the summer and, thus, the wind having less power. There's a way to quantify these effects which also affect pilots, sky divers and even race car: density altitude. It's not a complicated concept - it's based on what many of us are already familiar with having seen anything about Mt. Everest: air density is less the higher we go. Density altitude just gives us a handy yardstick.
The density altitude is the altitude relative to standard atmospheric conditions at which the air density would be equal to the indicated air density at the place of observation. In other words, the density altitude is the air density given as a height above mean sea level. The density altitude can also be considered to be the pressure altitude adjusted for a non-standard temperature.
Pressure Altitude is the height above a standard datum plane (SDP), which is a theoretical level where the weight of the atmosphere is 29.92 "Hg (1,013.2 mb) as measured by a barometer.
Factors that affect density altitude are
- Local air pressure which varies with weather systems. For example, tropical systems are notable for lower pressures and cold fronts for higher pressures
- Air temperature. Warmer air is less dense. Thus there is a significant difference in the power of wind from a 40 degree winter day to 95 degree summer day.
- Altitude, the higher you go, the less dense the air is. Lake Lanier is approx. 1070 ft above sea level which all other factors being equal means the air is less dense here than the coast or beach.
- Humidity. More humid air is less dense - water molecules weigh less than other air gases including nitrogen and oxygen.
The baseline "sealevel" for Density Altitude is 59°F (15°C) at 29.92inHg (1013mb) and 0% humidity, essentially pressure altitude at a specific temperature and humidity. The NWS displays corrected pressure level in most reports instead of the raw to help determining pressure changes related to weather systems. A good rule of thumb is to decrease the barometer value by 1 inHg for every 1000ft of altitude increase. This means that a barometer value of 29.54 inHg at sea level would be approximately 28.54 inHg at 1000ft of elevation. This roughly equates to about a 3% change in air density per 1000 ft. ie all other things being equal, the air at Lake Lanier is about 3% less dense than the air at the beach (sea level).
The higher the density altitude - the bigger sail you'll need to have the same power for a given wind. The lower means you can use a smaller sail. Whitecaps in the summer may not be a guarantee of planing conditions while, in the winter, you seem to be pleasantly powered up.
The screenshots of a winter's day compared to a summer's day show how the air can be up to 15% more dense in the winter. Or, your 6.5 now has the power of a 7.8! Conversely, on that hot summer day, you'd need a 9.2 to have the same power as a 7.8 on a cold winter's day.
Also, since air density also lessens with altitude - roughly 3.5% - for the first thousand feet above sea level, that means the air at a higher altitude lake like Lake Lanier is less dense than at the beach. You'll need a slightly bigger sail for the same wind and weather conditions at the lake than you would at the beach.
You can use the NWS calculator to play with the numbers to see how the different factors affect the air density.
Sources and Links
- Density Altitude at Road Atlanta (proxy for Lake Lanier)
- Density Altitude at Road Atlanta Yesterday
- Historical Density Altitude at Road Atlanta (past dates)
- Weather.gov Density Altitude Calculator Requires actual pressure, temp and humidity
- Density Altitude on Wikipedia
- Pressure Altitude on Wikipedia
- Density Altitude for Pilots