February 17, 2011 - Answering Your Questions!

By: Mallory Brooke
By: Mallory Brooke

As we approach the 60's and even some 70's for the Valley the end of this week, I'm taking your questions! Read on in today's blog!

We could have record breaking heat as we go through the remainder of the week, with highs in the upper 60's and nearing 70 degrees as we get into Friday. Right now the record is 68º, set back in 1955.

Since the weather is bit calmer, we can take your questions! Feel free to ask more!

**Don from New Market asks about wet bulb and dry bulb temperatures - referring back to some wintry precipitation we had a few weeks ago.

Well, we need to go over a few terms before we can start explaining what "wet-bulbing" is:
Dry bulb temperature: Actual air temperature (nothing fancy)
Wet bulb temperature: Temperature air will cool to when water is evaporated into unsaturated air
Dewpoint depression: Difference between temperature and dewpoint

When the air is unsaturated, the dewpoint will be much lower than the wet bulb temperature. As air saturates, your dewpoint increases while your temperature decreases. Evaporation is a cooling process, hence the temperature decrease. (As snowflakes fall into dry air, they evaporate, thus cooling the air). "Wet-bulbing" is the process by which the temperature drops because of evaporational cooling.

So how would you calculate a wet bulb temperature? (Much of this information is also available at www.theweatherprediction.com - it's a fabulous website).

It's called the "1/3" rule: First calculate the dewpoint depression. Divide that number by 3. Take the number and subtract it from the actual air temperature.

An example: Let's say the temperature is 37 and the dewpoint is 19. The dewpoint depression is 18. Take 18/3...we get 6. 37-6 = 31º - that would be our temperature once "wet-bulbing" had stopped. Hence we can easily go from rain, to snow or ice because of evaporational cooling.

Feel free to ask more questions about this!

Don also asked about banding structures in major snow events.

Unfortunately, this can get very technical...very fast. The easiest way to describe why banding structures occur is the combination of instability in the atmosphere in a certain area of the storm. Many times we'll see thundersnow in those significant banding structures, which means the atmosphere is unstable - very similar to what we see in the spring and summer.

**Matt asked about spring fire season and why it ends April 30th when most of our fires are in June through September.

We have both spring and fall fire seasons in Virginia. Spring is Feb 15-April 30 and I believe fall is Oct 15-Nov 30. These dates are set by the Virginia Department of Forestry.

A lot of this has to do with the amount of leaves and brush on the ground and how easily it can catch fire - especially if we've had a dry winter or summer. A dry winter leads to an enhanced spring fire season, and the same thing goes for a dry summer season and fall fire season.

Yes, wildfires are possible at all times, especially when we're dry, but these fire "seasons" deal mainly with all the extra extremely flammable and dry material that may be a forest floor at these times.

**Victoria asked about the temperatures on the First Alert Channel..

Those temperatures are not put out by WHSV. What you see on the 24/7 Channel are two forecasts from WHSV, however everything else is provided by AccuWeather. We will check into this.

However, I wouldn't trust the 20º different they are showing - it is a forecast error on their end, not ours. The temperature in Bridgewater wouldn't vary more than 2-4º from the temperature in Harrisonburg. Be sure to watch the WHSV forecasts on the 24/7 channel, or check WHSV.com, for the most accurate weather information.

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