Dew Point and Cloud Formation

Dew point temperature Dew Point: The temperature at which water vapor in the air begins to condense and form clouds. When the dew point temperature and the air temperature are equal, the air is holding as much water vapor as possible.  If the air temperature is lower than the dew point temperature, clouds form.  If the air temperature is greater than the dew point temperature, no clouds form. (Clear skies!)   Air temperature (dry-bulb) decreases with altitude, therefore, dewpoint can be reached as air rises. Warm air rises, expands, cools and condenses – forming clouds.

Psychrometer A Psychrometer is a tool used to measure humidity The difference between the wet-bulb temperature and the dry-bulb temperature is called the wet-bulb depression. The Cloud Base is the altitude of the lowest part of the cloud where the air temperature and the dewpoint temperature are equal.

Using data charts: Calculating the dewpoint and relative humidity is easy! Step 1: Get your wet-bulb and dry-bulb temperatures. (Think: ‘Sling Psychrometer’) Step 2: Figure out the wet-bulb depression (the difference between the wet-bulb & dry bulb on the psychrometer) Step 3: Use the Dewpoint Temperature or Relative Humidity chart in your ESRT by matching the columns for the dry-bulb temp and the difference between the dry-bulb & wet-bulb (depression)

For example: If your wet-bulb is 20° and your dry-bulb is 24°, your depression will be ‘4’. In the left column, find ‘24°’ On the top find ‘4’ Where they meet is your dewpoint temp: 18°!

Of course, even the simple stuff can be tricky... There are several ways to ask what the dewpoint or relative humidity is: 1.You’re given the dry-bulb & wet-bulb temps & calculate the depression. 2.You’re given the dry-bulb temp and the depression. 3.You’re given the dry-bulb temp and either the relative humidity or dewpoint temp and have to find the other.

The next exciting chart is the Cloud Base Altitude Chart... Cloud Base Altitude: Called the ‘Generalized graph for determining cloud base altitude’; this graph compares dewpoint and dry bulb temps to show the altitude at which clouds will form under certain conditions. We use this graph by comparing the dry-bulb temp (solid line) with the dewpoint (dashed line) Where they meet is the cloud base altitude.

The height of the cloud base changes regularly because temperatures change. If the temp changes, so does the dewpoint and cloud base altitude. As the dewpoint temperature drops, the cloud base altitude increases.  Cloudless days occur when the dewpoint temp is much lower than the air temperature (dry-bulb) –A high-pressure system  Fog is a cloud, with a very low cloud base altitude. So, when there is fog, it means the air temp is the same as the dewpoint. –A low-pressure system

Adiabatic Lapse Rate When a mass of air descends, the air temperature and dewpoint both increase because the air mass: – Sinks – Compresses – Heats ( temp) – Evaporates Air mass sinks, because of increased density (cooler air = more dense) Compresses because of increased air pressure Heats because...  altitude =  temp Evaporates (gets dryer) because temp increases, increasing the air’s ability to hold more water vapor!

So, we determine the base altitude at which clouds form by... Determining the dewpoint, then comparing the air temp with the dewpoint temp using the cloud base altitude chart. – Dewpoint is determined by: Finding the depression (difference between wet- bulb and dry-bulb temps) Using air temp (dry-bulb) and depression (difference) on page 12 of the ESRT to find the dewpoint and/or relative humidity.