A diurnal cycle is any pattern that recurs every 24 hours as a result of one full rotation of the Earth.
In climatology, the diurnal cycle is one of the most basic forms of climate patterns. The most familiar is the diurnal temperature variation. Diurnal temperature variation is a meteorological term that relates to the variation in temperature that occurs from the highs of the day to the cool of nights.
The diurnal temperature range is the changes in temperature within a 24 hour period at a paticular site. The diurnal temperature pattern is the daily temperature ranges recorded at a particular location, graphed. This diurnal range of air temperatures at a site is an important physiological feature which affects the growth of plants.
Some continental and coastal sites have small differences between day and night while others such as most deserts have a very wide daily diurnal range. The diurnal range will also vary with altitude.
Regardless of how favourable light and moisture conditions may be, plant growth ceases when the air and leaf temperature drops below a certain minimum or exceeds a certain maximum value. Between these limits, there is an optimum temperature at which growth proceeds with greatest rapidity. These three temperature points are the cardinal temperatures for a given plant; the cardinal temperatures are known for most plant species, at least approximately.
From the plants point of view, only temperatures between 10ºC and 30ºC are useful for plant growth. Below 10ºC, many plants automatically stop growing as a survival mechanism. Similarly, above 30ºC plants close their stomata (little openings on the undersides of leaves) to avoid overload of their natural moisture pumping mechanisms. Some rainforest plants such as gevuina nuts, developed in humid, even temperature environments, do not have a closing mechanism on their stomata openings and on very hot days, can literally pump themselves to death through stomatal osmosis.
This diagram shows the useful part of accumulated heat for plant growth.
Note that for most plants, additional heat received above or below the dashed lines is not useable by plants and in fact may be harmful, such as frosts. The other interesting thing to observe in this graph is the variability of temperature patterns between years (seen in both maximum and minimum temperature lines).
When you analyse temperatures for a horticultural farming enterprise it is important to take note of the diurnal temperature patterns, especially in reference to your proposed crops’ heat and chill requirements. Refer to the articles What are Microclimates? and Do you know your Chill Hours?
