From the PMO Desk: Measuring Cumulus Cloud Heights While UnderwayPeggy Alander, PMO Miami
As a PMO, I have often heard how difficult it is to judge the height of the clouds while underway. The Bowditch and the International Cloud Atlas both break the atmosphere into three étages —high, middle and low. Each étage is defined by the range of levels at which a certain genera of cloud is most frequently found.
The High étage contains Cirrus, Cirrostratus, and Cirrocumulus clouds. The Middle étage contains Altocumulus, Altostratus and Nimbostratus, and the Low étage contains Cumulus, Cumulonimbus, Stratocumulus, Stratus.
The étages overlap and their limits vary with latitude. The approximate heights of the limits are described in the table below.
Experienced observers know the types and general heights of clouds, but many don’t know the magic formula for obtaining cumuliform cloud heights.
The Bowditch states that the height of the base of the clouds formed by vertical development-any type of cumulus-can be determined by psychrometer. This is possible because the height that the air must rise before condensation takes place is proportional to the difference between surface air temperature (in Fahrenheit) and the dew point (in Fahrenheit). While at sea, multiply the difference by 236 to get the height in feet. So for every degree difference between the outside air temperature (F) and the dew point (F), the air must rise 236 feet before condensation will take place. For example, if the dry-bulb temperature is 80°F, and the wet-bulb temperature is 77°F the dew point is 76°F, or 4 degrees lower than the outside air temperature. Multiply 4 X 236 = 944 feet. Easy!
For conversion in Centigrade, take the difference between the dry bulb (C) and the dew point (C) and multiply by 126.3. That will give the height of the cumuliform clouds in meters. For example, if the dry-bulb temperature is 26.8°C, and the wet-bulb temperature is 25.0°C, the dew point is 24°C or 2.8°C lower than the surface air temperature. Multiply 2.8 X 126.3 = 354 meters or 1,161 feet.
Keep in mind that this formula only works for cumuliform clouds, and that the height of the encoded low cloud height measurement in the AMVER/SEAS software is reported in feet.
Until next time....
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