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Archive for October, 2006

Constructing the adiabatic graphs in Exercise 6 seems to be the hardest part of the lab for most students. Take a few minutes to look over what you are expected to do. You will first plot the environmental lapse rate data on the graph paper provided in the lab manual. Next you are to create graphs of the adiabatic temperature change for air parcels. Once this is done, you will label where the air is cooling at the DAR and SAR, identify the level of condensation where clouds start to form, and where the air is stable and unstable. Finally, there are several questions about comparing air densities, cloud characteristics, and precipitation.

Use the sample problem below as a template for answers the questions in the exercise. My example uses one parcel while the lab activity uses two. Here we go …

For my example:

  • Ground level (altitude 0 meters) air temperature of parcel = 15o C

  • Dew Point Temperature of the air parcel = 10o C

The first step is to plot the values of the environmental lapse rate. I have not provided the data here as there’s no need to know what they are for this example. See graph,

Next I need to graph the adiabatic temperature change. To do this I need three points:

  • Point 1 – Ground level air temperature

  • Point 2 – The condensation level

  • Point 3 – The end point (highest elevation for the problem)

Point 1 is given above, 15o C at ground level so it can be plotted right away.

Point 2 is the condensation level. I’ll use the same procedure as explained in “Adiabatic Lapse Rates – Part 1”. First I determine the amount of temperature change (cooling) that will take place between the surface and the condensation level (altitude where the dew point is reached by adiabatic cooling). So, at ground level the parcel temperature is 15o C and at the condensation level it will be 10o C, so that means the air will rise and cool by 5o C (i.e., 15o C – 10o C). Now, take the amount of temperature change and multiply it by the inverted DAR to get the condensation level in meters. You should arrive at a value of 500 meters. Now plot this point on your graph, that is, plot 10o C at 500 meters. This, again, is your condensation level. Connect point 1 with point 2 using a straight line. Because we used the dry adiabatic rate to compute this portion of the graph, label the line DAR. (See graph).

Point 3 will be the temperature of the air at the highest point of our graph which in this problem is 3,000 meters (in your lab problem it’s 1200 meters). There’s a three – step process to calculate point 3:

Step 1. Calculate the change in elevation between points 2 and 3.

The change in elevation is equal to the starting elevation minus the ending elevation. The starting elevation is 500 meters because we have brought the air up to that altitude already. The ending elevation is 3,000 meters so,

Change in elevation = 500m – 3,000m = -2,500 m

Step 2. Calculate the amount of temperature change between points 2 and 3.

To calculate the amount of temperature change, or amount of cooling that will take place between points 2 and 3 we’ll take the change in elevation and multiply it by the saturated adiabatic rate (SAR) of .6oC/100m. Why the SAR? We use the SAR because the air is rising upwards from the condensation level where it reached its dew point temperature. When it reaches dew point the air is fully saturated. So,

Amount of temperature change = -2,500 m X .6oC/100m = -15o C

What does the -15oC mean? It means that the air will rise and cool by 15oC going from 500m to 3,000m.

Step 3. The last step is to calculate the new temperature.

To calculate the new temperature, that is the temperature at 3,000 meters, take the starting temperature and add the amount of temperature change found in Step 2 above. The starting temperature is the 10o C because we started at 500 meters and rose upwards to 3,000 meters. So,

10o C + (-15o C) = – 5o C

Now I’ll plot – 5o C at 3,000 meters on the graph and connect this point to the dew point temperature at the condensation level. Because I used the SAR to compute this new temperature, I’ll label this portion of the graph “SAR”. (See graph) and I have a completed graph of the adiabatic temperature change.

Finally, I’ll draw and label a line that indicates the level of condensation. This is the altitude of the dew point temperature. I’ll also label where the air is unstable and stable. The air is unstable from the surface up to 2400 meters because the parcel is warmer and less dense than the surrounding air between these two elevations. From 2400 meters to 3000 meters the air is stable because the parcel is colder than the surrounding air between these two levels. A cloud will start to form at the condensation level and grow upwards through that portion of the graph where the air is unstable. Once it reaches the elevation where the air is stable it will likely stop growing as it will resist uplift. (See graph)

You have a few questions in the exercise dealing with comparing air densities at different elevations. Air density is determined by both temperature and pressure. If I were to compare the density of my example parcel with that of the surrounding air at the surface, the parcel would be less dense. Why? Given that we are comparing the parcel to the surrounding air at the same elevation, the pressure is the same and thus temperature will determine the density. Because the parcel is warmer than the surrounding air at the surface, it is less dense.


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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 4oC.

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 13oC and a dew point temperature of 4oC, we’ll have to decrease the air temperature by 9o C to bring it to saturation. If the air is rising and cooling at the Dry Adiabatic Rate (DAR) of 1o C/100 meters and it decreases it temperature by 9oC, then multiplying the DAR by 9oC will gives us the condensation level. But not so fast…. We have to set up the DAR so that when we multiply it by 9oC 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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The saturation curve is used to help answer several questions in labs 5 and 6. As a reminder:

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.

For example, air with a temperature of 25oC is saturated with a mixing ratio of 20g/kg

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).

For example, air with a water vapor content of 5 g/kg has a dew point temperature of about 4oC.

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