Q.In India, why is the day temperature maximum in May and why not after the summer solstice?

The maximum amount of solar radiation, or insolation, in the Northern Hemisphere occurs around the summer solstice, typically June 21st, when the sun's rays are most direct. However, the Earth's surface and atmosphere do not immediately reach their peak temperature at this exact moment.

Here's why the day temperature maximum in India is observed in May:

• Heat Accumulation: The Earth's surface and the atmosphere continuously absorb solar energy. It takes time for this absorbed energy to accumulate and for the land and air to heat up to their maximum extent.
• Thermal Lag: There is a thermal lag between the time of maximum insolation and the time of maximum temperature. Even as insolation begins to slightly decrease after the solstice, the net heat gain continues for some time because the outgoing radiation is still less than the incoming radiation. …

To understand why day temperatures in India reach their maximum in May, rather than after the summer solstice (which occurs around June 21st), we need to consider the interplay of incoming solar radiation, heat absorption, and the onset of the monsoon. This phenomenon is primarily explained by the concept of the Insolation Lag Effect.

The summer solstice marks the day when the Northern Hemisphere receives the maximum amount of direct solar radiation, or insolation, due to the Earth's tilt. One might intuitively expect the highest temperatures to occur precisely on or immediately after this day. However, the Earth's surface and atmosphere do not heat up instantaneously. They absorb solar energy over time, and this accumulated heat leads to a gradual rise in temperature.

In India, May is typically a month of intense solar heating. During this period, the sun's rays are high in the sky, daylight hours are long, and crucially, the skies over much of the subcontinent are largely clear and cloudless. This allows for maximum absorption of solar radiation by the land surface. With minimal cloud cover to reflect sunlight and very little rainfall to provide a cooling effect, the land heats up significantly, leading to a rapid and substantial increase in air temperatures. The dry conditions also mean that less energy is used for evaporation, leaving more available to raise the air temperature.

As we move into June, while the sun's position is still very high and insolation remains strong, a critical climatic event for India begins: the onset of the Southwest Monsoon. The arrival of the monsoon brings about several changes that drastically alter the thermal regime:

• Increased Cloud Cover: The monsoon winds carry vast amounts of moisture, leading to extensive cloud formation. These clouds act as a blanket, reflecting a significant portion of the incoming solar radiation back into space, thereby reducing the amount of energy reaching the Earth's surface. …