Re: Thunder

The answer to the question of whether or not there can be thunder and lightning during a snowstorm is this. It is possible but it is quite rare. Here's why.

A thunderstorm depends on rapidly rising moist warm air to produce the effects we see. These effects are: large volumes of rain in a short period of time, strongly gusting winds, hail, and of course lightning. When conditions aren't right (surface air is too cool and dry) it is impossible for air at the surface to rise fast and high enough to create a thunderstorm cell. Basically in winter the air is not warm or moist enough to produce thunderstorm cells.

Now if you want to know more details, read on.

Think of a hot air balloon. It won't lift off until something is done to destabilize it. The pilot accomplishes this by heating the air in the balloon, making it "lighter" than the air around it. The end result is that the ballon rises. The same thing must happen to warm moist air at the ground to make it rise.

Nature can accomplish this in four different ways.

1. Warm air and cold air can collide in a front. In this case the cold air will push underneath the warm air forcing it to rise. This can trigger thunderstorms.

2. The warm air can be forced upwards when it passes over a mountain range. If the air is warm and moist this can trigger thunderstorms.

3. On a hot day the sun can dramatically warm the surface of the Earth which will in turn warm the air nearby. This doesn't happen as much in winter. The sun is not intense enough.

4. Air can be forced upwards by convergence of air at the surface (when there is a low pressure region at the surface). This effect may be too weak to trigger thunderstorms very often.

So, why don't we see vigorously rising air when it is cold enough to get falling snow? You may know that air gets colder as you go up in the atmosphere. In winter the air near the ground is cold too. Because of this the air near the ground is more stable in winter than in summer. Scientists say that stability is greater in winter because the gradient of temperature is not as steep in winter as it is in summer.

There is another important principle that you need to know about before you will really understand thunderstorms. That is the principle of latent heat. Latent heat can be understood by thinking about a common phenomenon, which you may already have discussed in your science class. When water is changed into water vapour, you have to give it some extra heat (energy). You may have experienced this when you get out of a swimming pool on a windy day. As your skin dries you feel cold. The heat that is leaving your skin is going into the water as it turns into vapour. When vapour in the atmosphere changes back into water drops or ice crystals (in clouds) this heat is released and actually warms up the air a little bit. This makes the air even more unstable and allows it to continue rising. In winter, not only is the air more stable because it is colder near the ground, but also because it is drier. There is less latent heat available to be released.

So in summary, it is very unusual to see lightning during a snow storm. This is because the winter temperature gradient is weaker. Additionally dry air near the surface in winter is much more stable than the warm moist air found at the surface in summer.

I was unable to find any reference to the frequency of lightning during snow storms in any my reference books. I have never seen lightning during a snow storm myself, but theoretically it is possible. I imagine though, that a winter storm with lightning would be very fierce. So, think about this for a while. If you don't understand some of the concepts I have discussed, ask your science teacher. He or she will be (hopefully) be thrilled that you are interested in a little extra-curricular science. If you have more questions on this topic you can send me a message at ewiebe@ocean.seos.uvic.ca

There is a Meteorology website here at the University of Victoria. It can be found at http://wikyonos.seos.uvic.ca/eos340/eos340.html . To access it you need to know the password, so send me an email and I will give it out on an individual basis.