In the last issue we started talking about thunderstorms and I was forced to end the article in what you might call a “cliff hanger.” We discussed how we need a strong difference in temperature vertically in the atmosphere, with cold air located overtop of warm air. Then I went on to say that simply having a difference in temperature, no matter how strong, will not necessarily lead to thunderstorms. There are a couple more ingredients that need to be added to our atmosphere if we want it to bake up a really big storm.
The next key ingredient is water vapour, or humidity. It takes energy to evaporate water, so the more water vapour there is in the air, the more potential energy there is. To get at this energy the water vapour needs to be changed back into a liquid form – it needs to condense. As our warm air rises, it cools, and as it cools, water vapour will begin to condense. When it condenses it releases the energy it absorbed when it evaporated. This energy is released in the form of heat.
Our rising air is cooling as it rises, but not as fast as the air around it, so it continues to rise. Then condensation starts, which releases heat into the air. This makes our rising air even warmer than the air around it, so it rises even faster. Now it is starting to sound like we have everything in place for a severe storm… but not quite.
If you have air continually rising, eventually the amount of air accumulating at the top of the storm will become so great that it just has to fall back again, wiping out the storm in the process. To get around this problem we need some kind of vent at the top of the storm that takes away all the rising air that is accumulating there. We need a strong jet stream of air over top of the storm, which will help to “suck” away the accumulating air.
THE FINAL INGREDIENTS
We now have the key ingredients for a severe storm, but like any good chef, Mother Nature has additional ingredients she can use to make some storms truly awe inspiring. The first and probably most important “extra” ingredient that can be added to the mix is to have the wind change direction with altitude. Remember that the atmosphere is three-dimensional; that is, air can flow horizontally, but this horizontal direction can change as you move upwards. Why would this have an impact on our storm?
To put it in a nutshell, this change of direction can cause the developing storm to rotate. Picture what would happen if you take a rising parcel of air and push on it from the south when it is at the surface. Then as it rises a couple of thousand feet the wind switches direction and now blows from the east. Then a few thousand feet further up it is blowing from the northwest. What would happen to our rising parcel of air? It would get twisted – it would start to rotate.
Remember that if we can get air to rotate counterclockwise we have an area of low pressure. Air flows inward in a counterclockwise rotation and then is forced to move upwards. One thing we get if we can get our severe storm rotating is a small area of low pressure that helps the air to rise even more than it would without the rotation.
The second thing a rotating thunderstorm can do is to nicely separate the area of updrafts and downdrafts. This is important, since the downdrafts, even with a severe thunderstorm, will eventually cut the updraft off from its source of warm moist air. In a rotating thunderstorm, the source of warm moist air is maintained, giving these storms a long life and a lot of moisture to produce heavy rains.
There are still other items that can help contribute to severe thunderstorms, but that will have to wait until next time.