As moist, unstable air rises, clouds often form at the altitude
where temperature and dew point reach the same value. When
lifted, unsaturated air cools at a rate of 5.4 °F per 1,000 feet
and the dew point temperature decreases at a rate of 1 °F
per 1,000 feet. This results in a convergence of temperature
and dew point at a rate of 4.4 °F. Apply the convergence rate
to the reported temperature and dew point to determine the
height of the cloud base.
Given:
Temperature (T) = 85 °F
Dew point (DP) = 71 °F
Convergence Rate (CR) = 4.4°
T – DP = Temperature Dew Point Spread (TDS)
TDS ÷ CR = X
X × 1,000 feet = height of cloud base AGL
Example:
85 °F–71 °F = 14 °F
14 °F ÷ 4.4 °F = 3.18
3.18 × 1,000 = 3,180 feet AGL
The height of the cloud base is 3,180 feet AGL.
Explanation:
With an outside air temperature (OAT) of 85 °F at the surface,
and dew point at the surface of 71 °F, the spread is 14°. Divide
the temperature dew point spread by the convergence rate of
4.4 °F, and multiply by 1,000 to determine the approximate
height of the cloud base. |
Methods by Which Air Reaches the Saturation Point
If air reaches the saturation point while temperature and dew
point are close together, it is highly likely that fog, low clouds,
and precipitation will form. There are four methods by which
air can reach the complete saturation point. First, when warm
air moves over a cold surface, the air temperature drops and
reaches the saturation point. Second, the saturation point may
be reached when cold air and warm air mix. Third, when air
cools at night through contact with the cooler ground, air
reaches its saturation point. The fourth method occurs when
air is lifted or is forced upward in the atmosphere.
As air rises, it uses heat energy to expand. As a result, the rising
air loses heat rapidly. Unsaturated air loses heat at a rate of
3.0 °C (5.4 °F) for every 1,000 feet of altitude gain. No matter
what causes the air to reach its saturation point, saturated air
brings clouds, rain, and other critical weather situations. |