Cloud Base Altitude = ((((temperature - dew point) / 4.5) * 1000) + measure station altitude). As a mass of air rises, particularly on warm and humid days, it expands in the lowering air pressure, causing the air mass to cool and greatly reducing the air's ability to hold moisture. At some point, the moisture in the air exceeds the value which that air mass can hold, forcing the water vapor to condense, forming clouds. The point at which when air is cooled to the temperature that it can no longer hold its moisture is called the dew point. The rate at which air cools as it rises is averaged at 5.5°F per 1000 feet; the variable is the speed of the rising air mass. The dew point also decreases at about 1.0°F over the same distance. Thus the spread between the air temperature and dew point decreases by 4.5°F per 1000 feet. The altitude at which this spread becomes zero can be calculated using the stated formula. Here is the current NOAA aviation weather. In our calculator, enter the station altitude (that is where the temperature and dew point are measured) in either feet or meters, the temperature and the dew point, in degrees C or F. Do be aware that the formula is followed directly and that the introduction of other scales is for ease of the user in etering data. Click on Calculate and the calculated but approximate cloud base altitude will be returned in both feet and meters above sea level.
You might find our Density Altitude Calculator, as well as our Pressure Conversion Table or our Pressure Altitude To Density Altitude Conversion Table pages useful.