Amateur Scientist

Tracking Heat Islands

Use data loggers and DIY sensors to map temperatures where you live.

Written by Forrest M. Mims III

Time Required: 1 Day Cost: $25–$350

Measure bubbles of warmth for accurate outdoor temps.

FORREST M. MIMS III
(forrestmims.org) an amateur scientist and Rolex Award winner, was named by Discover magazine as one of the “50 Best Brains in Science.” His books have sold more than 7 million copies.

ACCURATELY MONITORING THE TEMPERATURE OUTDOORS ISN’T EASY. That’s because air temperatures are influenced by virtually anything heated by sunlight, and by engines and other equipment that generate heat.

Bubbles of warmth are known as heat islands. They make major contributions to the temperature in and around towns and cities, which are usually warmer than the countryside. The National Weather Service has developed guidelines for placing weather instruments to avoid heat islands (see makezine.com/cwop). But with the passage of time, fewer weather stations meet the guidelines, often because of the arrival of new buildings, sidewalks, and paved roads.

Electronic Thermometers

Stand-alone temperature loggers are available from Onset Computer, Lascar, Extech, and others. Data are downloaded through USB or optical ports; software converts the data into graphs.

You can modify an Onset Pendant logger for temperature transects by boring an entry hole through its cap and an exit hole in its base, to allow air to flow over the thermistor, a tiny resistor whose resistance varies rapidly with temperature.

While temperature loggers work well, they require you to note times and locations during a transect so you can analyze the data. No notes are needed with a Vernier LabQuest 2 (vernier.com). This 12-bit, fully programmable logger has three analog inputs for external sensors, and a built-in GPS that records the location of every measurement. Best of all, it displays a realtime graph of temperature as it’s measured (Figure 1). After making a transect, you can send up to 1,000 measurements to Vernier, which will return a Google map with a color-coded line that indicates the temperature along the route. While the LabQuest 2 is expensive ($329), mine has become essential. Figure 3 shows my Excel plot of 942 nocturnal temperature measurements across San Antonio, logged with my LabQuest 2 and a thermistor installed on my pickup. Figure 4 shows the same data on the map generated by Vernier.

Suitable Temperature Sensors

Dedicated temperature loggers have a built-in sensor. The LabQuest 2 works with many external sensors, including a Vernier temperature sensor ($23).

If you already have a voltage logger, you can use it to measure temperature with the help of a thermistor.

» Make a DIY temperature sensor by connecting a 10kΩ thermistor (Jameco #207037 or similar) in series with a 10kΩ resistor to form a voltage divider (Figure 2). Connect the free end of the thermistor to the logger’s positive voltage and the free end of the resistor to ground. The voltage between the thermistor-resistor junction and ground will be directly proportional to the temperature. You can swap the resistor for a 20kΩ–50kΩ potentiometer to adjust the sensitivity of the probe.

Mounting Temperature Sensors

When mounting temperature loggers on a car, remember that the temperature sensor must be shielded from sunlight. If the car will need to make stops at traffic signs or lights, the sensor should be mounted away from the vehicle to avoid heat buildup from the engine or metal surfaces. Here are some methods I’ve used:

» Mount an Onset Pendant logger on a car roof with a pair of Adams suction cup clamps and a safety line (Figure 5).

» Install an external thermistor in a drinking straw or paper tube taped to the roof (Figure 6). Insert the sensor into the tube's forward opening, tape its cable to the door frame, and run the cable through the window.

» Use a car-window flag mount for an external sensor (Figure 7). Remove the cap and the flag. Extend the sensor just beyond the upper end of the flagpole and secure it by wrapping a few inches of insulated, solid wire around the sensor leads and the groove at the top of the flag mount. Use tape or binder clips to secure the cable to the flagpole.

» Use 2 feet of ½” PVC pipe with a tee fitting to make a sturdy sensor mount that’s shielded from sunlight. Bore a hole through a ¾”× ⁹/₁₆” rubber stopper and push the sensor through so that it protrudes ⁵/₁₆” from the large end. Insert the sensor cable into one end of the pipe and press the stopper in. Bend the free end of the cable back toward the sensor and tape it securely to the pipe. Finally, place a ½” tee over the end of the pipe so the sensor is visible through both ends (Figure 8).