Longest day (June 21st ) is not the hottest day of the year

On March 21st and September 23rd Sun rises at east and sets at west after tracing a semicircle on the sky. This great circle is the celestial equator of your place of observation. Each day sun travels in a circular path which is parallel to your equator. Day and night are equal on March 21st and September 23^rd. But on June 21st (rising about 23^o 27^' north of east) Sun takes the longest apparent route. This is the longest day for the region (shortest day for south torrid/temperate zone). It appears that the longest day (June 21st ) is the hottest day of the year. But this is not the case.

The seasonal changes in the temperature depend on many factors such as the duration of sun’s stay above horizon, sun’s meridian altitude (less heat and low temperature in morning), the distance of the sun, the intensity of radiation of sun beams, the absorption of heat by atmosphere and the amount of heat gained and lost by the earth. Due to the constant inclination of earth’s axis to its orbit and the revolution of the earth around the sun, for the same place on the surface of earth, the climatic conditions are different at different periods.

After June 21st, sun takes the lesser apparent path and day time decreases. Though the amount of heat received decreases, there is already some accumulated heat and for some days, more the heat lost is less than the heat gained. This continues till on some day in the first week of August when the heat gained and that lost are equal. This day is the hottest day of the year. This explains why the mid summer is hotter than mid spring even though the sun’s daily paths are identical in the two seasons. For similar reasons, mid winter is colder than mid autumn and December 22nd, the shortest day, is not the coldest day.  The coldest day occurs in the first week of February.  

These seasonal changes are not very conspicuous for places in the torrid zone like South India. Since the upper transit of the sun takes place very near the zenith throughout the year, the duration of day and night are not much different and therefore such places are having the days hot throughout the year.