Thursday, December 03, 2009

Low Temperatures

After I planted my orchard during the early 90s, I decided to check out any microclimate effects of river proximity. Perhaps right next to the river would be warmer at night and cooler during the afternoon? So in 1995 I bought four Taylor high-low thermometers and set about testing my hypothesis.

First I cross-calibrated the thermometers, to compensate for any differences between their readings. I gave each thermometer a number so I could keep track of them. I set them outside at night, side by side, and read their temperatures at dawn. I also did this during the afternoon. I wanted to see how the temperatures compared at both the high and low end of the range. Not surprisingly, there was a little variation between the thermometers – a couple of degrees, as I recall. For a couple of the thermometers, I had to apply a correction of +1 degree or –1 degree so that all the thermometers, when corrected, would register the same temperature when they were next to each other. Then, when they were moved to their separate locations, I could apply the correction for each thermometer and be confident that the temperatures I ended up with were meaningful. This is known as “correcting for instrumental error.”

Then I built little housings for each thermometer. You don’t want to leave a thermometer exposed to the summer sun, for one thing. For another, just setting a thermometer outside at night will give an inaccurate reading. This is because of “radiative cooling,” when a surface exposed to the sky will become cooler than the air temperature. This is why a windshield can get frost on it when the air temperature remains above freezing. The same thing happens with a thermometer – it will cool off below air temperature, and will give an inaccurate reading. So I built little foam housings to protect the thermometers from exposure to the sun, and also to protect them from the nighttime sky, while leaving the front and bottom open so that plenty of air could get in, since it was the air temperature I wanted to measure.

(The temperatures reported in my “Signs of the Seasons” column are inaccurate, because the thermometers were subject to radiative cooling, and gave inaccurately low readings.)

So... with cross-calibrated thermometers with their foam housings, I set up one in the pasture in front of the house, and two more in the orchard -- one of them right next to the river, and the other one about 50 feet away from the river. I’ll have to hunt up the notebook with the original observations, but as I recall, the river thermometer was cooler during the afternoon, but not warmer at night. I was disappointed, because I was hoping that somehow I could figure out a way to utilize the warmth of the river to keep my orchard from freezing at night. But evidently the river is too small to have a significant temperature-modification impact on the surrounding land.

I took the orchard thermometers down after a couple of years, but the thermometer in front of the house has remained in the same spot since 1995 – same thermometer, same spot, same housing. So I have confidence that the readings are meaningful.

Whenever the weather gets unusually cold during the winter, I check the thermometer and write the temperature down on the calendar. Last week for the first time, I went through my pile of old calendars and scanned through them until I found the lowest temperature for each winter. The results are interesting:

95-96    12 degrees

96-97    11

97-98    12

98-99      8

99-00    10

00-01    10

01-02      7

02-03    14

03-04      4

04-05      9

05-06      5

06-07    11

07-08      6

08-09    10

Not surprisingly, the lows for each year tend to occur during what I call the “pit of winter” – the dark time near the solstice when nights are long and the sun seems feeble and far away. This means that most winter lows will occur during December or January. But not always – in 2002 we had 7 degrees on March 4. So a lot depends on the jet stream and how far south the coldest of the Arctic air can penetrate.

It also looks like winters have been getting colder, in terms of the lowest temperature. (This is distinguished from the average temperature for the winter. We could have a relatively warm winter and still have the occasional extreme cold front bringing very low temperatures with it.) The average of the first six winters is 10.5 degrees; the average for the last six winters is 7.5 degrees. This seems like a significant difference. I don’t know why this would be. Obviously, global warming is a hoax! Snark, snark.

My farmlet is located on the very edge of the pomegranate zone. If the lowest winter temperature is 10 degrees or above, the pomegranates are untouched. But the lower the temperature gets below 10 deg., the worse the damage. One variety gets frozen to the ground at the slightest provocation. Others might lose branches, or the tips of branches. When temperatures reach 7 degrees or less, I can expect to see severe damage the following spring.

Another interesting effect of low temperatures is ice on the river. Once during the early 90s the ice was so thick near the bank of the river, Neil and his friends were playing on the ice with no fear of breaking through. Soon I’ll run another “Signs of the Seasons” column that talks more about the “ice on the river” phenomenon. We live in a different climate zone here, only 20 miles from Las Cruces.