By Meteorologist Joe Joyce, WBZ-TV

Let the warm up begin! A warm front lies south of New England the morning. Clouds are breaking with increasing sunshine for the rest of the morning into the early afternoon. A light SW wind will help allow temps to climb into the 70’s to near 80 inland, with a cooler breeze to be found at our south facing coastlines. The warm front will begin to push through late today and tonight. Clouds will be spilling back into New England late today and tonight. The remnants of the tornado outbreak in the Plains is spilling over the ridge and will be directed into the region tonight along the warm front. The best chance of a few showers will be between 6PM and midnight tonight.

With the warm front north of the region tomorrow, SW breezes will pick up with gusts over 20. Sun filled skies and highs will be climbing to near 85-90. With the typical high temp in the mid 50’s, highs will be running close to 30 degrees above normal tomorrow. It will be suddenly summer around here with record-breaking heat likely. Unfortunately, for the thousands of runners in the Boston Marathon..the combination of record-breaking heat, sunshine, and moderate humidity will make for a situation where it will be easy to over exert your self. Less experienced runners will be the most vulnerable. The last time the marathon was in the 80’s was 2004. The warmest Boston marathon was 1976 with temps nearing 100. Runners should be fine as long as they stay within their limits. Dewpoints in the mid 50’s will make a more muggy feel…but that is a moderate humidity. There will be enough dry air to help evaporate the perspiration and help cool the body.

Air temperature and relative humidity have long been suspected of affecting the performance of marathon runners. Though these factors are important in their extremes, the factors that help predict record-breaking and unusually slow performances are:  wet bulb temperature,  percent sky cover, and the presence or absence of a light precipitation. Record breaking performances are characterized by a wet bulb temperature of <7.8°c, abundant cloud cover, and a light drizzle. On the other hand, unusually slow marathons usually have a wet bulb temperature of>7.8°C, and a sky cover of 50% or less with no precipitation.
Wanna play meteorologist? A quick technique that many forecasters use to determine the wet-bulb temperature is called the “1/3 rule”. The technique is to first find the dewpoint depression (temperature minus dewpoint). Then take this number and divide by 3. Subtract this number from the temperature. You now have an approximation for the wet-bulb temperature.
Here is an example of tomorrow’s weather: suppose the temperature is 84 degrees Fahrenheit with a dewpoint of 52 degrees Fahrenheit.
The dewpoint depression is 84 – 52 = 32.
Now divide 32 by 3 = 10.5.
Now subtract 10.5 from the original temperature of 84.
84 – 10.5 = 73.5 estimate Wet Bulb temp…the lowest the temperature could fall if all moisture was evaporated from the air. It is the temperature one would feel on their skin when wet or exposed to moving air.
As dewpoint depression or temperature increase, the evaporational potential increases. This technique does not give the exact wet bulb temperature but it does give a pretty close approximation. Warmer air will cool at a greater rate than colder air since more water vapor can evaporate into warm air. Evaporation is a cooling process that absorbs latent heat, therefore the more evaporation the more cooling.
Otherwise a cold front pushes through Tuesday with a cooler NW winds to follow for the midweek. Lighter winds will begin to shift onshore and cool down the coast come Wednesday and Thursday. Conditions look dry and bright with sun and clouds. The brushfire potential continues with the dry breezy conditions. The pollen count will be off the charts this week with trees starting to bloom. We do have a shot of some beneficial rain heading into next weekend. I would like to see a few more model runs before I get my hopes up about this. Good Luck to all the runners tomorrow. Be safe and have a great run!

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