The air on our planet is full of particles of moisture. These are often visible as haze, usually in calm air or on humid days. Did you know that on many days, when the air is stable, you can get above the haze and the visibility goes way up?
Click on the picture below to view it full size. You can see the haze extending to the horizon and up to a clear “haze line”. Above this line the visibility is much higher. It will be easier to see other planes at this altitude.
Calculating the top of the haze layer
The haze layer will generally be present in stable air where the temperature is above the dewpoint. At an altitude above the haze layer, the temperature will be below the dewpoint, so any moisture in the air will condense into droplets of mist of rain. Then gravity will pull these droplets down until they are back in the haze layer where they will turn back into water vapor (haze).
So the simplest way to find the top of the haze layer is to determine where the temperature and dewpoint will match.
The air is colder at altitude, but do you know how much colder? There is something called the adiabatic lapse rate, which is the rate at which the temperature drops as you climb.
It varies depending on the moisture of the air but as a rule of thumb just assume it is 2 degrees Celsius per 1000 feet of altitude. In dry air, it may be as high as 3 degrees Celsius.
So when you listen to the AWOS at sea level and it reports “temperature 12, dewpoint 6” then you can figure out the approximate top of the haze layer. That is a 6-degree difference, so we will need to climb 3000 feet.
This quick rule of thumb will let you make the haze layer calculation very quickly, just find the difference and divide it by 2 to get the altitude of the haze line.
Temperature: 10C, Dewpoint: 5C = Top of Haze Layer: 2500 feet
Temperature: 15C, Dewpoint 13C = Top of Haze Layer: 1000 feet
The next time you are flying start looking for the haze layer. It isn’t there every time, but it is there often enough (depending on where you fly).