Why Cumulus Clouds Rarely Form In Summer Over Lake Okeechobee During Daylight Hours?

If cumulus clouds form on both days, on which is the cloudbase (the height of the bottom of the cloud) lower?

• This question will involve consideration of conditions observed on two different days. On Day One, the surface temperature is 25°C and the surface dewpoint is 15°C. On Day Two, the surface temperature is 25°C and the surface dewpoint temperature is 20°C. If cumulus clouds form on both days, on which of the two days is the cloudbase (i.e. the height of the bottom of the cloud) lower? Clearly explain the reasoning you used to arrive at your answer.

• Answer:

  Dew point temperature is a measure of how much the water vapor is in the air. The higher the dew point the more water vapor per unit volume there is. Cumulus clouds form from surface heating lifting the air so that it expands and cools. Air cools at the dry adiabatic lapse rate of 9.8 degrees C for every 1000 meters. If the air temperature is the same at the surface as in your problem, the air that has the most water vapor, that is the higher dew point will form cumulus clouds at lower levels since the water vapor does not have to be lifted and cooled for saturation to occur and cloud droplets to form. It becomes trickier if the temperatures are also different. For example, if the temperature on the fist day was only 17 degrees C and 15 degrees C dew point, that will produce lower cumulus than in the case of the 25 degree C temperature over 20 degrees C dew point. Again the reason is that less lifting is necessary for saturation to occur. You can use the dry adiabatic lapse rate to estimate the height of the cloud and it works well in most cases. In your example of 10 degrees cooling to get to the dew point and using 9.8 degrees per 1000 meters gives a height of approximately 1020 meters that clouds will form. on day two, where there is 5 degrees of cooling needed for saturation, the height is approximately 510 meters. Again showing that at equal temperatures the air with the most water vapor, or higher dew point will reach saturation quicker and form the cloud at a lower level.