Why are coastal areas cooler in summer and warmer in winter than interiors at the same latitude?

At the same latitude, a coastal station such as Mumbai has a far smaller annual temperature range than an interior station such as Nagpur. The cause is the contrasting thermal behaviour of land and water, expressed geographically as continentality.

Why water moderates temperature

• High specific heat: Water needs roughly four to five times more heat than an equal mass of rock or soil for the same rise in temperature, so it warms and cools slowly.
• Transparency and depth of heating: Sunlight penetrates several metres into water but only a few centimetres into soil, spreading the same energy through a much larger mass.
• Mixing: Waves, currents and convection distribute heat vertically, whereas land conducts heat poorly and concentrates it at the surface.
• Evaporative cooling: A large share of incoming radiation over the sea is used as latent heat rather than raising temperature.

How that reaches the coast

• Onshore winds and sea breezes: Air moving inland from the sea carries the ocean's moderated temperature with it.
• Higher humidity and cloud: Coastal air suppresses daytime heating and reduces night-time radiative loss.
• Ocean currents modify the effect: Warm currents such as the North Atlantic Drift keep north-west European ports ice-free far north of their latitude, while cold currents such as the Benguela cool the Namibian coast.

Consequences

• Maritime climates have small diurnal and annual ranges; continental climates have large ones, as in Siberia's extreme range.
• The contrast underpins the classification of maritime and continental climate types and drives the monsoon through differential seasonal heating.

Latitude sets the amount of solar energy received. The land-sea distribution decides how much of it becomes temperature swing, which is why two places on the same parallel can feel like different climates.