Lab manual exercise 6 explored the impact of adiabatic temperature change on the stability of the air. As a refresher, here is the explanation for finding the condensation level and drawing an adiabatic graph. Realize that in this exercise the effect of the dew point lapse rate is not taken into account and thus is a simplified demonstration.

First, recall:

Determining the saturation point for a given air temperature:If you know the temperature of the air, find its value on the X-axis and move vertically to the curve. Once you intersect the curve move horizontally to the the Y-axis to determine the saturation point of the air at that temperature.

Determining dew point temperature for a given humidity:If you are given the humidity of the air as a mixing ratio, find the value on the Y-axis and move horizontally over to the saturation curve. Once you reach the curve, move vertically down to the X-axis and determine the temperature you get is the dew point temperature (temperature the air will have to be for the air to be saturated at the given humidity).

As an example, let’s say that the water vapor content of of the air is 5 g/kg and you need to determine the dew point temperature. Find 5 gm/kg on the Y-axis then move horizontally to the saturation curve, and then down to the X-axis. The temperature you find is the dew point temperature of about 4^{o}C.

Once you have the dew point temperature you need to calculate the condensation level. The **condensation level** is the *elevation* where clouds start to form. Given that condensation requires the air to be saturated, we will force the air to rise upwards, allow it to expand and cool adiabatically until the air reaches its dew point temperature. Once the air achieves its dew point temperature it will be fully saturated and the cloud will start to form by condensation. So, from the surface up to the condensation level the air is considered “dry” and from the condensation level up it will be considered “saturated”.

If the air parcel has a ground level temperature of 13^{o}C and a dew point temperature of 4^{o}C, we’ll have to decrease the air temperature by 9^{o} C to bring it to saturation. If the air is rising and cooling at the Dry Adiabatic Rate (DAR) of 1^{o} C/100 meters and it decreases it temperature by 9^{o}C, then multiplying the DAR by 9^{o}C will gives us the condensation level. But not so fast…. We have to set up the DAR so that when we multiply it by 9^{o}C we end up with the right units for the condensation level, meters:

Continue to “Adiabatic Lapse Rates – Part 2” in a subsequent posting.

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