On a sunny day, rocks and bare soil store much more heat than does vegetation. These environmental hot spots are known as heat islands.

Many things built by people are also heat islands, especially asphalt roads, parking lots and building roofs.

You can measure the temperature of heat islands with various thermometers. During a recent record cool evening, I ventured out on a quest for heat islands by driving from Castroville to Seguin while monitoring the air temperature just ahead of my pickup's windshield with a sensitive electronic thermometer.

The temperature measurements were recorded by a Vernier LabQuest 2 (www. vernier.com), a miniature computer with a display that graphed the temperature along the way. After the drive, the data saved in the LabQuest 2 were sent to Vernier.

Within seconds, the data were transformed into a color-coded line on a Google map (maps.google.com) that traced the highways I drove. The coolest temperatures were indicated by violet and the warmest by red. Blue, green, yellow and orange indicated the temperatures in between.

Heat islands often are more pronounced after dark when the sky is clear, because vegetation and trees cool much more rapidly than roads and buildings. This was obvious on the color-coded temperature map.

On leaving Castroville on U.S. 90, the temperature fell around 4 degrees. Later, a brief 2-degree increase occurred adjacent to exposed soil or rock in a large pit.

As I neared San Antonio, the temperature began to rise, and there was a sharp increase of 4 degrees where U.S. 90 crossed Loop 410. The temperature fell 2.5 degrees at Leon Creek.

It then rose to 64 degrees, around 8 degrees warmer than west of Loop 1604, and stayed there while the San Antonio skyline was visible out my left (north) window. The temperature then dropped a surprising 11 degrees when the Salado Creek greenway was crossed before returning to 64 degrees.

The temperature fell back to the 50s after Interstate 10 crossed Loop 1604 on the way to Seguin. The one exception was a 7-degree increase downwind from a major industrial facility just before a brief dip to 48 degrees at the Guadalupe River. Downtown Seguin was 5 degrees warmer than the countryside.

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Country Scientist: Exploring heat islands