I went to the store and looked for the cheapest thermometers I could find, which turned out to be the ACURITE brand thermometer and hydrometer for around $1 each. I bought all they had, and a pack of 12 landscape stakes. While the kids were napping I attached all the thermometers to the stakes using wood screws and rubber bands so that they would be approximately six inches above the ground. I then tried to strategically place the thermometers throughout the yard:
#1 - Open area of the yard (control)
#2 - By 1,000 gallon rain tank
#3 - In side-yard by blueberries
#4 - By covered back porch
#5 - Southeast corner of yard by fence
#6 - Back fence by Cara Cara
#7 - Right outside kitchen window
#8 - By 300 gallon rain tank
#9 - Against north side of greenhouse
#10 - Under pine tree by Wilma avocado
#11 - Under pine tree in front
#12 - Out front by pomegranates
#13 - On play south side of play fort
#14 - On back fence by Republic of Texas
So when the alarm went off at 5 am on Tuesday morning, I put on my coat, turned on my head lamp, grabbed the clipboard, and headed out to take measurements. I'm sure if any of my neighbors saw me they thought I was even crazier than they had previously thought. The Weather Channel app on my phone said that the outdoor temperature was 28° F, so I didn't expect to see anything too drastic. Most of the measurements were consistently within one degree of 32°F, and I didn't notice any significant effects from thermometers being adjacent to the rain tank, the house, or the greenhouse. Dr. Wife wisely pointed out that I didn't place two thermometers side by side to test their precision, and therefore I don't know the potential margin of error. However, I had compared a few of them when I first brought them home, so I'm confident that they are precise enough for this exercise.
I noticed that measurements from thermometers #13 and #14 seemed significantly higher than all the others, but I'm pretty sure it was because I had attached them directly to 4x4 posts of the play fort and fence respectively after I had run out of stakes. By attaching them this way, the entire back of the thermometer was insulated, and therefore these measurements aren't really useful. The coldest measurement of 28° F was recorded at thermometer #5, located in the southeast corner of the yard. This was also the only location where there was no underlying grass cover, and I believe it was the release of the latent heat of the water on and in the grass as it froze that kept the temperature at most of the locations at or near 32°F for the few hours that were below freezing.
I'm going to leave the thermometers out just in case we get a hard freeze, and I really wouldn't expect to see an appreciable difference unless temperatures dropped significantly below freezing. I think I also need to set out a tall stake with thermometers positioned at different heights so that I can measure the vertical temperature gradient. By understanding how individual areas of my yard respond to extreme cold events, I can better plan, plant, and protect my trees, and avoid the damage caused by the heartbreaking freezes of 2010 and 2011. Since then I've learned a great deal about protecting citrus from freezing temperatures, and with the unstable Polar Vortex constantly threatening to collapse, I've got the C7 lights, sheets, mulch, and gallon jugs ready to go at a moment's notice!