Heating and cooling of the atmosphere

A dark bare surface absorbs more energy and heats air rapidly. In exams link low albedo and low moisture with stronger heating.

Forests increase transpiration and shade while concrete increases sensible heat and storage. In exams treat land cover as a temperature factor.

When moisture is available much energy goes into latent heat. In exams link irrigated surfaces with evaporative cooling.

Slopes heat by day causing upslope flow and cool by night causing downslope dense air flow. In exams link slope winds with thermal cycle.

Cold dense air pools on the valley floor and calm night prevents mixing. In exams link valley inversion with cold air pooling.

The surface absorbs solar energy and emits it as longwave radiation which gases absorb. In exams remember atmospheric heating from below.

Water vapour is a strong absorber but its amount changes greatly by place and time. In exams treat it as a variable greenhouse gas.

Cloud effects differ with time because they affect incoming and outgoing energy differently. In exams remember different day-night cloud effects.

Positive net radiation means incoming energy exceeds outgoing energy. In exams link positive balance with heating.

In a negative balance the surface loses energy and temperature falls. In exams link negative radiation with night cooling.

Vegetation converts some energy into latent heat and reduces direct heating. In exams treat vegetation as a temperature moderator.

A warm surface lifts air and moisture can condense into clouds and rain. In exams link convective rain with heating and moisture.

Surface cooling brings air near saturation and water vapour condenses. In exams link fog with surface cooling and dew point.

A clear night cools the surface and calm air keeps moisture near it. In exams remember radiation fog by calm moist night.

Strong mixing prevents a saturated cold layer from forming near the surface. In exams consider calm air important for fog.

Warm moist air can cool over a cold surface and become saturated. In exams link advection fog with moist air and cold surface.

Conduction changes the thin surface air layer and then convection and mixing spread heat. In exams treat it as an initial contact process.

Horizontal arrival of an air mass is advection and changes temperature. In exams call horizontal heat transfer advection.

When energy is stored in the ground less remains for sensible heat. In exams link energy partitioning with temperature.

Moist soil uses energy in evaporation reducing direct temperature rise. In exams treat soil moisture as a cooling factor.

The direct answer is A. A sun-heated field warms the air touching its surface. The warm air expands; for the same amount of air, expansion lowers its density. Surrounding cooler air is denser, so the warm, lighter air experiences buoyancy and rises. If the surface heating is strong or uneven, pockets of warm air can rise rapidly, making vertical motion suddenly intensify and producing convection. Option A is correct because it states the density decrease and increased buoyancy. Option B is wrong because gravity does not disappear; gravity is essential to buoyancy. Option C is wrong because water vapour remains water vapour; it does not change into nitrogen through ordinary heating. Option D is wrong because solar radiation does not become lunar radiation; the two are different sources of radiation. The correct chain is heating, expansion, lower density, buoyancy and upward motion. Memory cue: warm air becomes lighter and rises.

Afternoon heating lifts moist air which cools and condenses aloft. In exams remember this sequence.

Convection allows warm air to rise and cooler air to descend causing vertical heat exchange. In exams link vertical redistribution with convection.

High water vapour can reduce evaporative cooling from the body so perceived heat rises. In exams distinguish actual and felt temperature.

Strong land heating expands and lifts air causing lower surface pressure. In exams treat this as the basis of sea breeze.