Note that at an altitude of 0 km (which is sea level), the air pressure is indicated to be near 1000 mb. At an altitude of approximately 2 km above sea level, the air pressure drops to around 800 mb. Likewise, when you get up to around 8 km above sea level, the air pressure drops to around 350 mb. Keep in mind that these must be understood as averages because the values do change from day to day!
As we go higher into the atmosphere the air pressure will decrease until it eventually reaches NEAR zero millibars at the edge of outer space. In today’s lab we will not be going quite that high. We will only be going up to the level where the air pressure drops to 100 mb (which is approximately 16 kilometers or about 10 miles above sea level). You can see this by looking at the 100 mb level at the top left side of the Stüve chart and how it corresponds to 16 km on the top right of the chart.
When you look at the Stüve graph you will note that in addition to the standard vertical and horizontal lines denoting height and temperature, respectively, we also have diagonal lines and curves. The diagonal lines are called DRY ADIABATS. The dashed curves are call WET ADIABATS. You can think of these as the paths that air bubbles take as they rise into the atmosphere. And just like their names suggest, a DRY air bubble (i.e., one that is not saturated and thus whose relative humidity is less than 100%) will follow the DRY ADIABATS as it rises into the sky. Similarly, a SATURATED air bubble (i.e., one whose relative humidity is 100%) will follow the curved WET ADIABATS as it rises.
As you follow these curves, you will see how the temperature of rising air bubbles change with height.
example. Let’s say you have an air bubble located at 1000 mb, and the temperature of the air bubble is 38°C. You would plot this air bubble by finding the 1000 mb horizontal line, and mark where it intersects the 38°C vertical temperature line. Of course, not every single possible temperature is going to have a vertical line. But you can see the vertical lines for 30°C and for 40°C. Therefore, you can ESTIMATE where the 38°C line would be and make your plot.
OK, now let’s assume for this example that our air bubble at 1000 mb and 38°C is NOT saturated. In other words, its relative humidity is less than 100%. We are going to lift this air bubble up to the 800 mb level. Since it is not saturated, we will follow the DRY ADIABAT. In this example, it just so happens that we have a DRY ADIABAT already on the chart that passes through our point of 38°C and 1000 mb. This is NOT the case for every possible scenario.
But for now, we can just use the visible DRY ADIABAT that happens to conveniently pass through our example point. Now lift your bubble to the 800 mb level following the DRY ADIABAT and stop.
What is the temperature of the air bubble now that it is at 800 mb? By dropping a vertical line down to the temperature axis, you can see that your air bubble has cooled to about 19°C.
As long as the air bubble stays dry, that is to say, as long as its relative humidity is less than 100%, you would follow the DRY ADIABAT to see what the temperature of your air bubble would be at any level in the atmosphere.
But we know by now that an air bubble that is rising will probably eventually cool to the point to where it can no longer “hold” all of the water vapor that may be in it. Once it reaches that point, the relative humidity will hit 100%, and condensation will begin. In other words, a cloud will form.
But where will this happen? It happens at the level in the atmosphere called the LIFTING CONDENSATION LEVEL (LCL). This is where the base of a cloud is produced by our rising air bubble.
So how do we find this LCL? First you need to know what the DEW POINT TEMPERATURE of the air bubble is at the surface where it starts rising. Remember, the dew point temperature is just another way of measuring how much water vapor is in the air. The higher the dew point, the greater amount of water vapor (i.e., moisture) is in the air.
So, let’s say the dew point temperature of our surface air bubble at 1000 mb is 24°C. To find the bubble’s approximate* LCL (i.e., where cloud base will form), you simply follow the dashed-curve WET ADIABAT that passes through the surface dew point temperature up to where it crosses the DRY ADIABAT line that passes through the temperature of the air bubble.
When you plot 24°C at 1000 mb, you will quickly note that there is no WET ADIABAT that conveniently passes through that point. BUT…you will also note that there are WET ADIABATS on either side of your plotted dew point temperature. So, you simply make your own WET ADIABAT that parallels both of these, and then you follow it!
Therefore, when you lift your “dry” air bubble on its DRY ADIABAT, and intersect that path with the WET ADIABAT that passes through your air bubble’s surface dew point temperature, you have found the approximate level in the atmosphere where it will saturate (i.e., RH=100%) and become a cloud. You have found its LCL. In our example above, the air bubble saturates at the 775 mb level. This is where the base of the cloud will be! And you can see this corresponds to a little more than 2 km above sea level.
Any further lifting of our air bubble must now follow the WET ADIABAT because it will remain saturated for the rest of its journey into the sky.
At any step of the way, we can compare the rising air bubble’s temperature to its surroundings. Thus, we can determine if the air bubble is warmer than its surroundings, or colder than its surroundings, or the same temperature as its surroundings.
If the air bubble is warmer than its surroundings, then it will continue to rise on its own much like a hot air balloon rising into the relatively cool air around it. In this scenario, we say the atmosphere at this level is UNSTABLE relative to the air bubble.
If, however, the air bubble is colder than its surroundings, then it will want to fall back to where it came from since it would be more dense than its surrounding environment. In this case, we would say the atmosphere at this level is STABLE relative to the air bubble.
If the air bubble finds itself at the same temperature as its surrounding environment, then it would tend to stop rising because it would be equally as dense as its surroundings. We would say that the atmosphere at this level is NEUTRAL relative to the air bubble.
RISING AIR BUBBLES and THUNDERSTORM POTENTIAL
Thunderstorms form whenever the atmosphere is mostly unstable. This allows air bubbles to rise through great depths in the atmosphere. And the warmer these bubbles are compared to their surrounding environment, the faster they will rise and the stronger a thunderstorm will be.
In this part of the lab, we are going to lift an air bubble in a specific environment and then determine whether thunderstorms are possible, and if they are, just how strong they will be.
First, we need to define the atmospheric environment in which air bubbles will be rising. Plot the following MORNING temperatures at each pressure level on your Stüve diagram. You may skip plotting the dew point temperatures, except for the first one, for right now. We will get back to this parameter later in the lab.
Once you have plotted your temperature points on the Stüve (called an atmospheric sounding), connect your points with straight line segments. You will then be able to see how the temperature is changing with height from the surface (1000 mb) up to 100 mb (approximately 16 kilometers above sea level).
1)Find the pressure level where the LCL is located.
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