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In this final tutorial on Skew-Ts, we’re going to put everything we’ve talked about together. We’re using the same Skew-T that we’ve been talking about since the start, but now we have enough information to identify, at least partially, if our storm has some of the ingredients that it needs in order to make you file an insurance claim.
The green line is still our dewpoint, the Red is the temperature and that purple line is the mixing line that got us to our LCL. Once we have the LCL plotted, we can draw a line starting at the LCL and following a moist adiabatic line. Remember that we are using a moist adiabat because our box full of dry air was pushed into the sky until it reached its condensation point and now has to be treated like a box full of wet air. Usually when we draw this line, we end up with a little triangular space that we call CIN or Convective Inhibition. You’ll usually hear weather weenies call this little box The Cap. That would be Capital T Capital C. The Cap is a bit of warm air that our box of air has to be able to push through in order to get to its Level of Free Convection or LFC. Imagine a balloon full of 80C air that gets placed into a 0C walk in freezer. That balloon will sail towards the ceiling. Now imagine that the same walk in freezer has a heater midway between the floor and the ceiling that is pumping out air that is 100C. The balloon will rise from the floor where it is 80C above ambient temperature to the level of the furnace air where now it’s 20C below ambient, making it want to sink. In order to get to the ceiling of the freezer, our balloon will need to come up with 21C worth of energy or an external force will need to push it through the heater air until it can get back to the 0C ambient temperature and start rising on its own again (yes I know the hot air would convect to the ceiling, leave it alone, it’s a thought experiment). If our box of air can’t push through The Cap, we’re not going to get much of a storm. But assuming that The Cap breaks and we make it through, our box of air will start to accelerate faster and faster towards the upper atmosphere. The yellow region starting from the LFS and going up to the Equilibrium level (EL, not illustrated) is what we call CAPE or Convective Available Potential Energy. We like CAPE. CAPE is responsible for a lot of the energy that goes into a storm.
Skew-Ts are a fairly amazing bit of graphing in my opinion. Armed with just a few bits of information you can figure out how strong a storm is likely to be, if it has the right cloud base for tornadoes, how much inhibition has to be overcome in order for the storm to initiate and a few other items that we haven’t talked about here. You can find an excellent and more indepth training, for free, from ucar here that is nice and easy to digest and will fill in a lot of blanks that I’ve left in this series.
Here is a Skew-T that has had some environmental data pasted into it. Everyday balloons are sent up that take various measurements at specific levels of the atmosphere, two of them being temperature and dewpoint. What you are looking at here is essentially the result of a balloon with a thermometer and a hygrometer being sent into the air and taking measurements at 1000mb, then 950mb, then 900mb and so on. The red line is the temperature and the green is the dew point. So at 1000mb, the temperature is 28C and the dewpoint is 20C, up at 850mb the temperature has dropped to 24C and the dewpoint has held steady at around 20C. With our environmental soundings in hand, we can now take what we know about a Skew-T and turn it into something that chasers can use. The first thing I like to find is my LCL.
The LCL, or Lifted Condensation Level, is where the base of the clouds form and is simply the intersection of the dew points mixing ratio line and the temperature. The purple line follows the mixing ratio line near 20C until it runs into the red temperature plotting at about 28C. At this point our box of air has reached it’s saturation level and has to condense in the form of a cloud. When you hear chasers talk about high based or low based storms, the LCL is usually what they are talking about. When the cloud base is around 2000m or lower, that gets most chasers into the cars and on the road. Why? we’ll cover that in the final part of this Skew-T discussion.
The wet (or sometimes called moist) adiabat is a dashed line on the Skew-T that starts at the bottom of the graph and, depending on the starting temperature, wiggles it’s way towards the north west or north east. The wet adiabat is very similar to the dry adiabat, but the rate of temperature change is different. Where the dry adiabat was changing roughly 1 C for every 100 meters, the moist rate is closer to .5 C for every 100 meters. Why the difference? Part of the reason is because as air saturates and condenses, a little bit of heat is released called latent heat. That latent heat adds to the temperature of that box of air that we’ve been talking about and slows the overall cooling rate.
Our box of air starts it’s life on the ground and follows the dry adiabatic line towards the sky. Depending on the amount of moisture in our box of air (humidity) our air will eventually get cold enough that it can no longer hold onto the water in it and it will have to condense it out (dew point). Now our box of air is still climbing, but since it has condensed it has to follow the wet adiabat.
So far, we have defined what each axis of the Skew-T represents, lets see how these things work together.
Dry Adiabats are next up. These are the first lines on the Skew-T that aren’t straight, and for what will hopefully be an obvious reason. Dry Adiabats are drawn as a solid line starting in the south east corner of the Skew-T and moving to the north west and tend to curve northward in the middle. The adiabats are an interesting bit of science.
When you take a box full of air (you’ll usually hear it called a parcel, but box is more awesome) and you lift it high above the ground, a few things happen to the air inside of it. Remembering back to the first axis of the Skew-T that we talked about (isobars), as you rise higher into the atmosphere the air pressure gets lower. You probably learned in middle school, or will learn if you aren’t there yet, that molecules pushed close together will generate more heat then molecules that are spread far apart.
So thinking back to our box, when we filled it full of air it had around 14.7 pounds per square inch of pressure on it from all sides. By the time you’ve lifted that box to 900mb that box only has 13 pounds per square inch pressing on it. At 800mb it’s 11.7 psi and at 600mb it’s down to nearly half at 8 psi! With less weight on our box of air, that air will start to expand and as it expands, it will cool.
These dry adiabat lines are showing us the rate of that cooling with height and plotting that decrease based on our starting temperature. Looking at our Skew-T then, if we started our bucket of air off at 1000mb on a day when it’s 30 C outside, and then lifted that box of air up to 600mb, that bucket of air will have cooled down to -10C or at roughly 1 C for every 100 meters. But usually we have humidity to deal with, which takes us to the moist adiabat.
OK, now we’re getting to something a little more interesting. We’ve already covered isobars and isotherms, our next axis is the saturation mixing ratio. These are dashed lines that run from the south west of the Skew-T to the north east and represent the capacity to hold water vapor in the air at altitude. In other words, when moist air rises along the dry adiabatic (we’ll learn that next), the mixing ratio lines tell us at what pressure that dry air will be forced to condense and turn into the cloud base.
But first we have to get air into the atmosphere and that starts with the dry adiabatic.