7/20/11 Excessive Heat!

An Excessive Heat Watch is in effect. As eluded to last week, the Bermuda High has set up, extending into the central US. This high pressure will allow temperatures to rise into the 90's with an increase in the dew point. This combined will give a heat index in the Boston area of around 100 degrees. This heat will last into Saturday when a cool front from Canada will bring a temporary relief in dew points. The heat however remains building in the plains and will return next week.

One relief, albeit small will be a strong breeze to help remove moisture away from your body, but it will be a hot wind.

With such temperatures and humidity, nighttime lows will not fall far and  expect some areas to remain near 80 on Thursday night.

Heat Index Map for Thursday. Much of the eastern US will have high heat indices. Dark ORANGE = 97+

The heat index is what the temperature feels like on the body when you take the temperature and dew point together.  A high dew point means that it is harder for moisture (sweat) to evaporate off of your body and you can feel hotter and your natural cooling system will be overworked if exerted.  See example below.

Today's High Temperatures- Coolest at the Cape, Islands, Gloucester.

In for the first heat wave of 2011!

 Example:

A temp of 95 with a dewpoint of 74 on Friday will give you a Rel Humidity of 51%. In the summer that will feel very hot. Keep in mind, for example. Your body will "feel" to only cool to 74 degrees and only 50% of the sweat on your body will evaporate.(if the dew point were 58 as it were this past Sunday, you would have 29% humidity 71% of your sweat would evaporate and your body could feel like it cools to 58 in a breeze.