Kevin Myatt is The Roanoke Times' weather columnist.

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So, finally, I can quit writing this phrase: "The temperature has not reached 100 in Roanoke since July 6, 1999."

The Roanoke Regional Airport recorded daily record highs of 100 on Wednesday and 101 on Thursday, ending a span of eight years, a month and two days when temperatures in Roanoke failed to reach 100. It almost happened again Friday, when the high reached 99.

Also, and even more remarkably, Thursday's low dropped only to 79, which was the warmest low temperature recorded in 59 years of record keeping at the airport.

So just why did it get so hot this time?

Related

Why it hit 100 degrees

• Large high-pressure system over much of the eastern two-thirds of the nation
• Dry ground and vegetation from regional drought
• Westerly winds blowing downslope into Roanoke Valley and Piedmont

Graphic

• Enlarge the graphic above

Let's take a look at three factors, moving from the large scale to the small scale.

At the largest scale is the high-altitude continental weather pattern. I've been writing off and on for a few weeks about how the large high-pressure system over the West was showing signs of moving, albeit slowly, eastward.

It got delayed once by that weird, whirligig upper-level low that got stuck over us. Remember those "frigid" days in July when the highs were in the lower 70s?

Eventually, the high did build over us. But a similar weather pattern has set up for weeks in previous summers without our getting to 100 degrees. So there must be more to it.

Moving to the regional scale, much of the Southeast and mid-Atlantic are experiencing drought.

While most of the Roanoke and New River valleys are considered just "abnormally dry," or only at the edge of drought, moderate to severe drought covers most of Virginia, according to the latest map from the National Drought Mitigation Center in Lincoln, Neb.

Drought and heat waves feed each other. The drier the ground and vegetation are, the faster those surfaces can heat, so the air also heats more. The hotter it gets, the faster the ground and vegetation dry out.

The dry ground under our feet and for hundreds of miles around us undoubtedly played a major role in our extreme temperatures.

At the local level, there was a subtle but significant factor at work on Wednesday, Thursday and Friday to drive our temperatures up a few more degrees.

Each day featured a west wind blowing down the slopes of the taller mountains west of Interstate 81 down into the Roanoke Valley, the Shenandoah Valley and eventually over the Blue Ridge and down into the Piedmont.

As wind blows downslope, air particles compress, causing heating. Downslope winds also dry out, because a large part of what moisture they may carry has been lifted and condensed by upslope flow on the other side of the elevation rise.

Roanoke's dew points were steamy early on Wednesday, Thursday and Friday, hovering near 70. But each day as westerly winds blew into the valley, the dew point began to drop by late morning, reaching the mid-60s. The dew point fell as low as the upper 50s at Martinsville on Thursday.

Dropping dew points are a trade-off. The heat index, or apparent temperature to the human body considering humidity as a factor, will be fewer degrees above the actual air temperature than it would with higher dew points. But drier air also heats faster, so the temperature may climb a few more degrees than it would if it were more humid.

These westerly breezes, which felt like someone had opened an oven door, may have made the difference in a humid 96-degree day and a somewhat less humid 100-degree day for us.

The large-scale pattern is shifting slightly the next few days, with the high-pressure dome building over the central U.S. rather than over us. So the extreme heat will take a break, but hot, sticky weather appears likely to linger through much of the rest of August.