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If you’ve hung around chasers, at all. ever. You’ve heard talk about Skew-Ts and hodographs. We’ll get into hodographs another time. but this is the first of a 7 part series on Skew-T diagrams.
So, what is a Skew-T? The Skew-T/Log-P (we just use skew-t, but I think log-p sounds cooler, oh well) has been with us for over 50 years as a tool used to plot and display vertical profiles through the atmosphere. At first glance, a Skew-T chart looks a bit like a piece of graph paper that has had it’s ink run, but in a few days you should be able to read them like an end user software license agreement (under promise, over deliver. under promise, over deliver).
The Skew-T axises are made up from the moist adiabatic rates, dry adiabatic rates, isobars, isotherms and saturation mixing lines. This article is going to cover the isobars. The first Skew-T image is a blank Skew-T diagram from wikipedia showing where all of the axis lines are. This image obviously isn’t giving you any information about the environment, but that’s what a blank one looks like. Big hot mess if you ask me.
Since we’re just focusing on isobars, here is the same image with just the isobars highlighted in red. Isobars are how a Skew-T indicates height in the atmosphere. Isobars are marked off by pressure in millibars (mb) or hectopascals (hPa) instead of feet or meters. You will probably hear people talk about millibars and not hectopascals, but they are effectively the same thing (1 hPa = 100 Pascals = 1 mb). I should probably note here that a real meteorologist will skewer my skew-t explanation, but for our purposes it’s fine. So, while we are talking about hPa and mb, the isobars also equate, roughly, to certain heights above ground level. You can generally translate mb to height like this.
- 1000 mb = ground level
- 900 mb = 1km above ground level
- 800 mb = 2km above ground level
- 700 mb = 3km above ground level
- 600 mb = 4km above ground level
- 500 mb = 5.5km above ground level
- 400 mb = 7km above ground level
- 300 mb = 9km above ground level
- 200 mb = 11km above ground level
We walk around at 1000 mb, LCLs or cloud bases in the 950mb range have traditionally spawned the most destructive tornadoes. We look for sheer in the 800mb to 600mb range and our overall storm motion or steering winds are usually found around 500mb. So, the thing to remember is that with Skew-Ts, isobars=height . Pretty simple so far, up next is isotherms.