The reason is the high specific heat of water : 4.181 J/g/°K or °C.
Water has a very high specific
heat capacity. It stores a lot of heat energy in it. Water absorbs a lot of
heat energy before the temperature goes up by 1 °C. Same way water
gives out a lot of heat energy when it cools by 1°C.
the atmosphere has a lower heat capacity and it gets hotter quicker and also it
gives out less heat when it cools down.
summer during the day time the air around us on land gets heated very quickly
to a high temperature. But a water lake remains cool because it takes
a lot of heat to get hotter. So the air close to the lake is cooler. When air
blows, we feel the coolness.
winters the water absorbs heat during the day and it takes a long time to cool
down. So water lake radiates the heat well in to the night. Thus the
atmosphere, which cools down quickly after sunset, gets heated up due to heat
from the lake.