What causes thunderstorms?

  1. Anna Blanksch

    Anna Blanksch 16
    Gold Member

    How come when it snows there is rarely lightening or thunder? Another question (not sure how much the two will relate)... When I was in Napier, New Zealand my host family said that even though they get rain, they rarely if ever have thunderstorms. Why would that be?

    Thanks for the thoughts!
     
  2. jcsd
    Earth sciences news on Phys.org
  3. turbo

    turbo 7,366
    Gold Member

    We have thunder-snow here at least several times a winter.
     
  4. You need a fair amount of warm air to build up thunder cells (those big anvil-shaped comulonimbus clouds). Once you've got these cells started, water and ice interactions start to produce charges withing the cell. Ice/snow flakes gain a positive charge and are lifted up (because they are light) to the upper portions of the cell. Meanwhile, a mushy kind of water droplet of sorts is left below with a negative charge. Once the potential difference between them is great enough, things get exciting.

    As for winter thunderstorms (thundersnows as turbo mentions) they are a product of the interaction of cold air with colder air, or the increase in cooling above lakes because of certain cold fronts. This creates updraft and allows for the building of cells and a lot of precipitation (snow and/or hail). I have lived in SE Ontario and experienced this kind of lake effect storm. You don't often get lightning and thunder, but it does happen sometimes.

    You can still get rain without lightning when thundercells aren't produced as there are many different mechanisms for cooling the air enough to produce rain.
     
  5. Greg Bernhardt

    Staff: Admin

    I live in an area where winters can be long and snowy. I can't remember the last time I heard thunder during a snow storm. Are the difference potentials usually not great enough to cause lightening during a snow storm?
     
  6. turbo

    turbo 7,366
    Gold Member

    Maine is wedged between warmer air off the Atlantic and cold arctic air masses from Canada, which is why we get thundersnow once in a while. You need a temperature differential so that warm air rises up through colder air to build charge up, like in a van de Graaf generator. We get that here when we have warm air near the ground and it gets overlain by a cold air mass.
     
  7. Greg Bernhardt

    Staff: Admin

    hmmmm ok. I live on the west side of Lake Michigan. The lake has warmer air and since weather usually travels west to east that would mean thundersnows would more likely happen on the east side of the lake right?
     
  8. turbo

    turbo 7,366
    Gold Member

    Sounds right.
     
  9. Yeah, i was in Kingston which is on the eastern end of Lake Ontario, so we would get a fair amount of the lake effect snow. Admittedly, the thundersnows were extremely rare.
     
  10. The "engine" that drives thunderstorm updrafts is latent heat release due to water vapor condensing into cloud drops. The Clausius–Clapeyron relation for water explains why water vapor can build to much higher concentrations in warm air than in cold air. In fact the relationship is exponential. In winter, storm clouds contain significantly less water than in summer because there is less water vapor in the lower atmosphere in general due to the lower temperatures. The lack of vapor to condense also means less energy to create vigorous thunderstorm updrafts. The updrafts in clouds producing winter precipitation are usually an order of magnitude slower than those in thunderstorms.

    The second ingredient you need for thunderstorms is a steep lapse rate (i.e. decreasing temperature with height) extending to a sufficient height in the atmosphere such that a saturated updraft can carry a large quantities of condensed water to an altitude where freezing takes place. Without sufficient lapse rate in the troposphere, updrafts lose their buoyancy before reaching a decent height and you get small puffy cumulus clouds rather than towering thunderheads.

    The exact mechanism isn't well understood (likely there are many), but the creation of large quantities of ice particles is needed to create an electrical charge sufficient to generate lightning discharges inside a cumulonimbus cloud. There are also other mechanisms that can create lightning without ice particles present, volcanoes for instance. Actually volcanoes create multiple kinds of lightning. The steam and ash ejected directly out of the volcano vent can become charged at the get-go due to processes not understood. But volcanoes also produce "normal" lightning because a very large towering ash cloud also contains enough water and ice to act as a cumulonimbus cloud.
     
    Last edited: May 18, 2012
  11. Well, I live on the east side of Lake Michigan and thundersnow is still incredibly rare. Even when I've heard it, it typically only occurs once or twice throughout an entire storm. This in contrast to spring and summer thunderstorms which often produce hundreds or thousands of lightning flashes.
     
  12. Chronos

    Chronos 10,053
    Science Advisor
    Gold Member

    Water molecules transfer electrical charges far more efficiently than ice crystals.
     
  13. Anna Blanksch

    Anna Blanksch 16
    Gold Member

    Wow! Great info and good follow-up questions Greg. Thanks! I love thunderstorms and would love to witness a snow thunderstorm!
     
  14. I saw a program on TV which showed scientists simulating what goes on in a thunderhead. They were investigating what sort of contact forces cause the electric field. I don't have any information other than what I remember. However, the little that I remember may be helpful.
    Suppose that there is a collision between two equal size water droplets. The two particles coalesce into one particle because of surface tension, then the vibration shakes the single particle up so that it breaks up into two water droplets.
    If the two final particles are equal size, then the two particles will be neutral. A symmetric collision results in a neutral charge.
    If the two final particles are unequal in size, the two particles will have charges that are opposite in sign but equal in magnitude. Therefore, it is asymmetric collisions that separate electric charge.
    The large droplets tend to sink due to gravity faster than the larger particles. Therefore, after a collision the two droplets are separated by gravity. This results in a potential difference between ground and air.
    This was not on the program. However, think about snow flakes. The snow flakes don't have surface tension, so that they wouldn't coalesce as well as water droplets. Furthermore, ice is hard. A coalesced pair of snowflakes would have less of a chance of breaking up. Thus, snowflakes probably generate less electric potential than water droplets.
    Maybe that is one reason lightening is more common in rain then in snow. There are fewer asymmetric collisions between snow flakes than between rain drops.
     
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