As the Philadelphia region is in the middle of its fifth heatwave of the year, we thought it would be a great time to track the conditions and compare two hot days worth of data. We used a Kestrel 4500 with Bluetooth on the tripod and vane mount, with the built-in data logger recording every minute. We left our setup outside during the hottest part of the day- from around noon till 4pm.

When doing these observations, we put the Kestrel setup on an island of grass out in our small parking lot. Now, most meteorologists and weather buffs may scoff at even the idea of taking weather observations that don’t meed the World Meteorological Organization’s standards (eg. temperature measured at 2 meters, wind at 10 meters, away from buildings, etc.), but we wanted to make observations of what the weather really feels like. Not the weather taken at the airport next to the tarmac, but the weather you feel when you go outside. Makes sense, right? This is why the Kestrel meters offer a more true environmental snapshot of exactly where you are- it measures the conditions right where you are- not miles away at the airport or another weather station hub.

A quick note about the thermistor (temperature sensor): at all angles, the thermistor stayed in the shade of the Kestrel unit. So while we did not measure the temperature within a Stevenson screen or within a fan-aspirated radiation shield, the temperature is accurate to what you would feel while outside, at that exact minute. This is why the temperature has so much variation on the charts below.

When first investigated the data, the first thing we looked at was what the difference between the two days was. We knew the first day was warmer, but that hardly tells the whole story.

Recorded temperature during the afternoon on July 6 and 7, 2010 in Boothwyn, PA (just outside of Philadelphia).

In the graph, the darker line is the 6th and the brighter line is the 7th. At first glance, it looks like the temperature can really vary during the day. And in reality, it does. Most of the rises and dips in temperature can be explained by some passing clouds and winds (explained later). Even though the thermistor was not directly in the sun, when the sun went behind the clouds temperatures drifted down until the sun came back out. The high temperature recorded on the 6th was 105.5 F, while the high temperature recorded on the 7th was 104.5 F. Compare this to the local airport high’s of 102 and 103, respectively.

Now about that wind. I figured the wind had an influence on the temperature because to our west is a small woods area, and to the east is a large asphalt parking lot. I figured the temperature would become cooler with an easterly wind and warmer with a westerly wind. To investigate this, I took a day (the 7th) and plotted the temperature and wind direction on separate axes. The result is below, with wind direction in green and temperature in red.

Temperature vs. Wind Direction for a small period of time on the 7th.

Temperature vs. Wind Speed on July 7.

I noticed that there was a slight correlation between the direction and temperature, but it was not consistent. Also, I only took a small sample of the data where this effect looked better- in some segments of the data it was hardly noticeable. However, for times when the data did seem to align it was almost always the same. Temperatures rose with an Easterly wind, and temperatures decreased with a Westerly wind. After seeing this, I decided to compare the Temperature and Wind Speed during this same time period (below).

Now, the wind speed correlation seemed much more defined. As winds increased, the temperature reading dropped from the mixing going on in the air.

Looking closer at the data, I calculated the correlation factor for both variables with respect to temperature. (For those unfamiliar with correlation factors, -1 means a complete negative correlation, 1 means a complete positive correlation and 0 means no correlation at all.) Comparing the temperature to wind direction for the period graphed, the correlation factor was a measly -0.03. The wind speed had a much better correlation with temperature with a correlation factor of -0.69. For the entire day, temperature and wind had a correlation of -0.46 on the 6th and -0.50 on the 7th. Not an exact match, but a pretty strong observation that I wouldn’t be surprised to see during the summer.

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