Thursday, October 4, 2007

The Temperature & Dewpoint

Thursday morning was a foggy start for many in the midwest, Great Lakes and northeast. There were also several reports of heavy dew in the comments today.

You can be your own forecaster if you will at deciding if you have a good chance of seeing morning fog or heavy dew by checking the latest weather observation for your area before going to bed.

What you will want to do is look at the temperature and the dewpoint temperature. The closer they are, the higher the chance for fog and dew. If they are equal, that means to air is completely saturated and you will likely have total cloud cover.

There are definitely some other factors involved, namely wind.

If wind is present, it keeps the atmosphere stirred up and would inhibit fog formation.

Here is an example: If the dewpoint is 51 degrees at 10 pm, and you are expecting an overnight low of 52 -- as long as drier air doesn't move in overnight (meaning the dewpoint temperature doesn't fall) and the wind is calm, then you will probably wake up to a muggy, damp morning with dew and fog.

If you have that same scenario with a decent breeze, you will wake up to a grey morning with a deck of clouds overhead, but the wind will prevent the fog from forming most likely at the surface.

But say the low drops to 52 and drier air moves in during the night taking the dewpoint to 40 degrees, you will probably wake up to a sunny morning with just some scattered clouds.

The dewpoint temperature will never be higher than the air temperature -- that would be called super-saturation and it doesn't happen.

1 comment:

  1. Does dew contribute much to the water table over the long run? Does it decrease evaporation from the so that water already IN the soil is not touched??

    Does that make ANY sense?