Kevin Myatt

Latest entries from the Weather Journal blog

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About Kevin

Kevin Myatt grew up in Arkansas to the tune of tornado sirens and the rhythm of hailstones, aspiring to be a meteorologist before his studies and career were turned to journalism instead. Though he often chases storms, he prefers living in the cooler, more tranquil weather of the Blue Ridge. He moved to Roanoke in 1999 to take a job on the copy desk of The Roanoke Times; writing headlines and editing copy is his principal work for the newspaper today.

Each May, Kevin assists Pulaski County High School / Virginia Tech meteorology instructor Dave Carroll in leading college and high school students to the Plains to observe severe weather firsthand. The accounts of many of his storm chases can be found here on the storm chasing page of his weather blog on roanoke.com.

Kevin was an editor for "Hurricanes and the Middle Atlantic States," a book written by D.C.-area weather enthusiast Rick Schwartz and published by Blue Diamond Books that documents hurricanes striking the mid-Atlantic states since colonial times.

The Weather Journal column began in 2003 and appears on Friday's Virginia section front in The Roanoke Times. The Weather Journal blog began in 2006 and follows weather day-by-day between the larger columns.

Even as the days get shorter, summer days can sizzle

Reader question: July and August are normally the hottest months of the year. Since the sun is at its highest point in June, why is June not the hottest month?

First, let's establish statistically that July and August are the hottest.

Based on current National Climatic Data Center norms for Roanoke, which use data from 1971 to 2000, the average daily high temperature is 87.5 degrees for July, 86 degrees for August and 83.3 degrees for June. For the remaining nine months, the average high is below 80 degrees.

From 1971 to 2000, Roanoke averaged a high of 88 each date from July 13 to Aug. 5, with average lows of 65 each of those dates except 66 on July 22 to July 24. Highs and lows are cooler before and after these dates.

Southeast Regional Climate Center data, averaging all the years back to 1947, is similar, with the hottest average day being July 20 with a high of 89 and low of 67.

So, yes, by multiple measures, the hottest days of the year, on average, occur some weeks after the summer solstice, the day with the year's longest period of sunlight and highest sun angle, which this year occurred on June 21.

A principle called the "heat lag" is involved.

Think about a fireplace in a cold room suddenly roaring with a hot fire, then slowly subsiding to a steady flame. The room will not be the warmest immediately when the fire is the biggest, but rather sometime later when the flame has settled.

The initial heat from the big fire takes some time to warm the air in the cold room. The steady fire will continue to add to that heat, warming more of the room to an even higher temperature, until the fire dwindles enough that less heat is being added than what is escaping the room.

The length of days and height of sun angle on summer days do not provide as dramatic an effect as a roaring fireplace in a cold room, but the principle is the same.

As the days only slowly shorten and the sun angle very slightly lessens in July and August, heat from the sun continues to build faster than the short nights can let it escape. This heat is added to what has gradually built through the spring as the days have lengthened and the sun angle has increased, peaking on the summer solstice.

At some point about mid-August, the sun angle becomes short enough and the nights become long enough so that, generally, more heat is escaping than is being added. Temperatures, on average, begin to decrease.

This continues through the fall and winter into mid-January, when the year reaches its historic temperature low point. This occurs about a month after the winter solstice, so the heat lag works on the other end too, as the slowly lengthening days of early January are still not long enough nor the sun angle steep enough to overcome the heat lost on those long nights.

It is very important to note that the quirks of weather and climate patterns shifting day to day, week to week and even year to year can alter when the hottest and coldest periods occur at a given location in any particular year. (Based on the weather pattern that appears to be developing, this summer may well be one of those times when the end of July isn't the hottest period of the year.)

But based on long-term historical averages in most Northern Hemisphere continental temperate climates like ours, the hottest days are typically near the end of July and the coldest ones in mid- to late January, about a month after the longest and shortest days of the year.

Kevin Myatt's column runs Monday, Wednesday and Friday.