Transcript Dewpoint and Cloud Formation

Dew Point and Cloud
Formation
Dew point temperature
• Dew Point: The
 If the air temperature is
temperature at which
lower than the dew point
water vapor in the air
temperature, clouds form.
begins to condense and
 If the air temperature is
form clouds.
greater than the dew point
• When the dew point
temperature, no clouds
temperature and the air
form. (Clear skies!)
temperature are equal,
the air is holding as
much water vapor as
possible.
 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)
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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 • Air mass sinks, because of
increased density (cooler air =
because the air mass:
more dense)
–
–
–
–
Sinks
Compresses
Heats ( temp)
Evaporates
• 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 wetbulb 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.