# Photo Thermic Indexing

Plants require a certain amount of heat to develop from one point in their life cycles to another. This measure of accumulated heat is known as physiological time. Physiological time is often expressed and approximated in units called degree days (D). All plants tend to respond to the seasonal and daily variation in the duration of night and day time periods. Plant responses to this variation were attributed to the variation in the day time duration(Photoperiod) rather than to its complementary part of dark time duration (Nyctoperiod). Rice has been classified as a quantitative short day plant. In other words, it is a long night requiring plant. Hence nyctoperiods are also considered.

### Heat Units/Degree Days( ℃ d)

The growth and development of both plants and insects is strongly dependent on temperature. Below the base temperature (Td) and above the maximum temperature (Tm) the rate of development is zero. Three cardinal temperatures base temperature, optimum temperature(To) and maximum temperatures are identified to compute the heat units. The development rate over time is expressed in daily heat units/degree days(℃ d).

Daily heat units were calculated using the following formula

where h is time of the day. Hourly increments in Heat Units (HUH) are calculated
if Td <= Tbase and Td >= Thigh then HUH=0
if Tbase < Td and Td <= Topt then HUH=(Td - Tbase)/24
if Topt < Td and Td < Thigh then
HUH=[Topt - (Td-Topt) X (Topt-Tbase) / (Thigh-Topt)] / 24

### Daylengh (Photoperiod in hours)

These calculations involve some empirical relationships that calculate the day length and integral of the sine of the solar angle from the day number and latitude. Nyctoperiod is calculated by subtracting the photoperiod from 24.(total no. of hours /day)

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