Kevin Myatt is The Roanoke Times' weather columnist.

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For the sake of our local climate, I'm glad we never switched to the metric system in temperature.

The Fahrenheit scale ideally situates 0 and 100 near the extremes of our temperature range. You know that when it gets to 0, it can't get much colder; when it gets to 100, it can't get much hotter.

We'll talk about below-zero temperatures when we get back to winter. In summer, 100 degrees is rarefied air for the Roanoke Valley.

How rare? Right now, we are in a stretch of seven years when Roanoke's official temperature, at Roanoke Regional Airport, has failed to hit 100 even once. Tuesday's high of 95 was 7 degrees above normal for the date, but it wasn't close to a record for July 18 (102 in 1977), and it fell 5 degrees short of ending the streak.

Since records began being kept at the former Woodrum Field in August 1948, Roanoke has had only one longer streak without a 100-degree temperature: 11 years, minus eight days, from July 14, 1966, to July 6, 1977. That was broken in a big way, with eight days of highs 100 or higher in 1977, the most on record.

In those 58 years of record-keeping at the airport, there have only been 38 triple-digit days. If you figure that there are about 90 days each year (mid-June to mid-September) when hitting 100 degrees is a legitimate possibility, then highs of 100 or more has happened on only 7/10 of 1 percent of possible days.

Roanoke has not exceeded 100 degrees since hitting 102 in August 1988. I had just graduated high school then, and many of the kids who just graduated high school in May were babies. Roanoke hit 100 three times in the 1990s, but never exceeded it. So far, we've been no higher than 97 in the 21st century.

For Blacksburg, hitting 100 degrees is even more rare than it is in Roanoke. In fact, it hasn't happened since record-keeping began there in 1954. Blacksburg has had a couple of 99-degree highs, the latest in August 1983, that brutally hot month when Roanoke hit its all-time record of 105 on Aug. 21, sandwiched between two days of 104.

So why is it so darn hard to hit 100 here?

The mountains play a major role. Elevation itself is important, as temperatures generally cool with upward changes in relief -- the major reason Blacksburg at 2,200 feet above sea level has failed to hit 100 even in the heat waves that have sent Roanoke, at 900 feet, above 100 degrees.

On a larger scale, the mountains form a natural barrier when hot, dry air moves at the surface from the Desert Southwest eastward across the Plains into the Ohio and Tennessee Valleys. It struggles to move over or around the mountains. The Appalachians also have a tendency to trap cooler air, moving south from Canada and New England, to the east.

Mountain ranges also tend to be a focus for shower and thunderstorm development with minimal moisture and instability. The resulting clouds, rain and thunderstorm downdrafts put a damper on rising temperatures.

Our placement with regard to the Gulf of Mexico and the Atlantic Ocean is also important. So often, even when we have hot weather, the prevailing winds also bring in more moisture from the seas. The humidity makes even marginally hot temperatures feel worse, but moist air also warms more slowly than dry air, so temperatures don't soar as quickly. Greater humidity also often means more clouds to block the sun and a greater likelihood for rain and thunderstorms.

So what does it take to reach 100 in our area?

It requires high pressure at the surface, drying the air out near the ground, topped by high pressure aloft, bringing in hot, dry air from the west high above the mountains and laying down a warm air "cap" that keeps thunderstorm clouds from being able to push upward through it.

This air mass usually needs to sit and simmer a few days, drying and heating even more, and allowing the ground and vegetation to dry out more so that additional solar energy isn't "wasted" on evaporation rather than heating the ground and air.

Each day it heats and dries under the deep layers of hot high pressure at the surface and aloft, the easier it becomes to pump temperatures high into the 90s, toward 100 degrees.

This was the kind of setup that began to develop Sunday and simmered through Tuesday. But it doesn't take much to disrupt a heat wave here. A southward-moving cold front triggering thunderstorms is likely to end any possibility of a rare 100-degree day the rest of this week.