Is temp taken in the shade?

The air temperature around your plants can mean the difference between a plant having frost damage or not. Temperature also plays a role in many plant growth factors such as blooming, breaking dormancy and germination. Knowing how much of a difference placing your plants in the shade makes given the air temperature can help you be a better gardener, with plants that live longer and that are more productive.

Table of Contents Show

Air Temperature Measurements
Solar Radiation
Importance of Temperature for Plants
Influencing Factors
Why do we take air temperature in the shade?
Why the sun makes a difference with testing air temperature
Why does this matter?

Air Temperature Measurements

Measuring the air temperature is more than just placing a thermometer anywhere and taking a reading from it. Air temperature is always read in the shade because the sun's radiation does not affect the temperature reading. Placing the thermometer near sidewalks, cars or other warm, reflective surfaces could create an inaccurate reading because heat bounces off these objects and magnifies before being read by the thermometer. Thermometers are not affected by wind blowing on them, even if the wind changes how people perceive the temperature.

Solar Radiation

Solar radiation can dramatically change the temperature of an object that it hits, heating it far above the actual air temperature. Jack Williams, former weather editor for "USA Today," stood outside in air that measured in the 20s Fahrenheit in the shade, but a thermometer in the sun behind him registered 80 degrees Fahrenheit. This is an extreme example of how solar radiation can cause areas in the sun to be warmer than those in the shade. Usually, the difference from the sun to shade will only be between 10 and 15 degrees Fahrenheit, says retired meteorologist Jim Lushine, in an interview with Florida's "Sun Sentinal." Moving potted plants into direct sunlight when a light freeze is forecast and covering up the plants at night helps the plants to take advantage of solar radiation to get through cold days, and the cover insulates that warmth to help the plant survive the chilly night.

Importance of Temperature for Plants

Air temperatures, whether in the sun or shade, trigger many events in the growth cycle of plants such as dormancy, dormancy breaking, germination and flowering. Photosynthesis also increases to a point with rising temperatures. While many plants grow best in direct sunlight, where temperatures may be warmer, too much heat and sunlight is never good for plants. The temperature of a plant's foliage may be higher than the outside temperature if the plant does not have enough moisture to evaporate and cool off the leaves. Signs that a plant is getting too much heat and not enough water include leaves that change position or curl. Saving sensitive plants from the cold should include planning a growing spot that is on a side of a building that is protected from cold winds, and that is in full sun to keep the plant warmer during chilly winter days. Do watch out for wind's dehydrating effects on your plants, but wind chill will not be an issue.

Influencing Factors

The time of the year and time of the day will both affect the angle of the sun's light and the intensity of the solar radiation and the temperature. Wind is only a factor in the perceived temperature difference in the sun and shade for humans, but not for plants.

Renew Economy ran a thought provoking article recently, referencing an article in Sydney’s Daily Telegraph. The article belittled the extreme temperatures parts of Australia has seen this summer. It suggested that we shouldn’t be concerned about climate change and this summer’s extreme heat wave because Australia has seen temperatures of 50°c+ before.

“Take March 18, 1832, which The Sydney Gazette reported as “insufferably warm”,” Devine wrote. She then went on to quote that newspaper: “At 1pm, the thermometer was 54°C in the sun. The cattle suffered much. Working bullocks dropped dead.”

Why do we take air temperature in the shade?

However, R.E. flagged up an interesting point; the report measured the air temperature in the sun. Giles Parkinson who wrote the article on R.E. commented that even a 6 year old will know you measure temperature in the shade. However, I didn’t know this and I thought some of our readers and customers might not know it either. So, for those of us that didn’t know here’s why you take temperature in the shade.

Why the sun makes a difference with testing air temperature

All temperatures taken for records of air temperature are taken in the shade. Any temperature gauges that are recording temperatures in the sun will be affected by the sun’s radiation and the temperature during that period will appear significantly hotter.

This article actually provides a complete breakdown on how to take the most efficient temperature reading.

Why does this matter?

The historic temperature reading provides reference for current temperatures as does whether it was recorded in the sun or shade. For example; in 1837 if the reading for the temperature in the sun was 54°c the shade temperature may have been 40°c or lower. Therefore recent temperatures in NSW of 50°c+ are extreme and do suggest that climate change is having an impact on Australian temperatures.

Belittling climate change affects by saying the country has experienced temperatures like this before is reaching. Especially if your reference point is a redundant (and technically inaccurate) record that was taken in the sun.