Temperature is the most important climatic factor affecting the life of agricultural crops. Temperature plays a fundamental role in the process of photosynthesis and in the rate of crop growth and maturity. Each agricultural crop has three thermal thresholds for growth: minimum, optimum, and maximum, for each stage of its growth, starting from the germination stage Germination, passing through the seedling emergence stage to the soil surface Growth, and ending with the maturity stage Maturity. In other words, each crop has a minimum growth temperature Minimum Growth Temperature, known as the zero temperature of growth Zero Temperature of Growth, and a maximum growth temperature Maximum Growth Temperature. If the temperature drops below the minimum or exceeds the maximum, that crop will stop growing. Conversely, each agricultural crop has an optimum growth temperature Optimum Growth Temperature that represents the best thermal conditions for increasing plant growth speed, where crop growth rate gradually decreases as the temperature drops or rises from that optimal level. These three thermal thresholds are called the cardinal growth temperatures for agricultural crops Cardinal Growth Temperature, and these thresholds differ among crops, being lower for winter crops such as wheat and barley and higher for summer crops such as corn and rice.

The table below represents the cardinal growth temperatures for winter and summer agricultural crops in general

Cardinal Growth Temperatures for Agricultural Crops (Cardinal growth temperature)
Crop Type	Minimum Growth Temperature              (Zero temperature  of growth)	Optimum Growth Temperature	Maximum                         Growth temperature
Winter Crops	C°  6 – 0 32 – 42  F	25 – 31  C° 77 – 87.8  F	31 -37  C° 87.8 – 98.6  F
Summer Crops	15 – 18  C° 59 – 64.4   F	31 – 37  C° 87.8 – 98.6  F	40 – 44  C° 111.2 – 121 F

Every plant requires, in order to complete its life cycle and reach maturity, a specific number of thermal units that must accumulate during its life above the "zero temperature of growth". These units are called Accumulated Temperature Accumulated Temperature

On this basis, accumulated temperature can be defined as the sum of thermal units or degrees that accumulate above the minimum daily average temperature at which plants can generally grow. According to most researchers, this is 6°C. Accumulated temperature can be calculated for a single day, a week, a month, or any other period. As for the accumulated temperature for the growing season, it is the sum of temperatures that accumulate during all the months included in that season. On this basis, every plant needs, in order to complete its life, a specific number of days and also a specific number of daily thermal units. Knowing the sum of daily thermal units G.D.D Growing Degree Day  is essential for determining growth periods, maturity dates, and harvest times for cultivated crops. It can also help farmers determine the timing of insecticide application and weed control. Furthermore, estimating the accumulated temperature for the growing season is of great importance in determining the type of crops that can be grown in a particular climate.

On this page, we display the monthly accumulated temperature values for network stations distributed across different regions of the country, using two values for the zero temperature of growth Zero Temperature of Growth: the first is (0°C) and the second is (6°C). The monthly accumulated temperature represents the sum of daily accumulated temperature values for that month, calculated as follows:

Monthly Accumulated Temperature = Sum of Daily Accumulated Temperatures for All Days of the Month

Daily Accumulated Temperature = Daily Average Temperature – Zero Growth Temperature (0 or 6)

Daily Average Temperature = (Daily Maximum Temperature + Daily Minimum Temperature) / 2

* If the daily average temperature is below the zero growth temperature, the accumulated temperature for that day equals zero.