The water cycle describes the continuous movement of water on, in, and above the Earth. Energy from the sun drives the water cycle by evaporating water from oceans, lakes, and vegetation into water vapor in the air through evaporation and transpiration. Water vapor condenses into clouds and precipitation falls to the ground as rain or snow through condensation. Water re-enters oceans and lakes through surface runoff and underground streams, where it will again evaporate and continue the cycle. The water cycle is essential for life on Earth and influences climate and weather patterns.

1. Water cycle

2. Definition:- The water cycle, also known as the hydrologic cycle or the hydrological cycle, describes the
continuous movement of water on, above and below the surface of the Earth.
During this process, water changes its state from one phase to another, but the total number of water particles
remains the same.
Water changes its state through a variety of processes from evaporation, melting and freezing, to sublimation,
condensation, and deposition. All these changes require the application of energy.
Stages of Water Cycle :-
1. Evaporation -
• The sun is the ultimate source of energy and it powers most of the evaporation that occurs on earth.
• Evaporation generally happens when water molecules at the surface of water bodies become excited and rise
into the air.
• These molecules with the highest kinetic energy accumulate into water vapour clouds.
• Evaporation usually takes place below the boiling point of water.
• Another process called evapotranspiration occurs when evaporation occurs through the leaves of plants. This
process contributes to a large percentage of water in the atmosphere.
2. Sublimation -
• Sublimation occurs when snow or ice changes directly into water vapour without becoming water. It usually
occurs as a result of dry winds and low humidity.
• Sublimation can be observed on mountain peaks, where the air pressure is quite low.
• The low air pressure helps to sublimate the snow into water vapour as less energy is utilised in the process. On
earth, the primary source of sublimation is from the ice sheets covering the poles of the earth.

3. 3. Condensation -
• The water vapour that accumulated in the atmosphere eventually cools down due to the low temperatures
found at high altitudes. These vapours become tiny droplets of water and ice, eventually coming together to
form clouds.
4. Precipitation -
• Above 0 degrees centigrade, the vapours will condense into water droplets. But it cannot condense without dust
or other impurities.
• Hence, water vapours attach itself on to the particle’s surface. When enough droplets merge, it falls out of the
clouds and on to the ground below. This process is called precipitation (or rainfall).
• In particularly cold weather or extremely low air pressure, the water droplets freeze and fall as snow or hail.
5. Infiltration -
• Rainwater gets absorbed into the ground through the process of infiltration. The level of absorption varies based
on the material the water has seeped into. For instance, rocks will retain comparatively less water than soil.
Groundwater can either follows streams or rivers. But sometimes, it might just sink deeper, forming aquifers.
6. Runoff -
• If the water from rainfall does not form aquifers, it follows gravity, often flowing down the sides of mountains
and hills; eventually forming rivers. This process is called runoff.
• In colder regions, icecaps form when the amount of snowfall is faster than the rate of evaporation or
sublimation.
• The biggest icecaps on earth are found at the poles.

4. Implications of Water Cycle :-
• The water cycle has a tremendous impact on the climate. For instance, the greenhouse effect will cause a rise in temperature.
Without the evaporative cooling effect of the water cycle, the temperature on earth would rise drastically.
• The water cycle is also an integral part of other biogeochemical cycles.
• Water cycle affects all life processes on earth.
• The water cycle is also known to clean the air. For instance, during the process of precipitation, water vapours have to attach
themselves on to particles of dust. In polluted cities, the raindrops, apart from picking up dust, also pick up water-soluble gas
and pollutants as they fall from the clouds. Raindrops are also known to pick up biological agents such as bacteria and
industrial soot particles and smoke.
• By transferring water from one reservoir to another, the water cycle purifies water, replenishes the land with freshwater, and
transports minerals to different parts of the globe. It is also involved in reshaping the geological features of the Earth, through
such processes as erosion and sedimentation.
• Finally, the water cycle figures significantly in the maintenance of life and ecosystems on Earth.
A short Description :- (for short note)
The Sun, which drives the water cycle, heats water in oceans and seas. Water evaporates as water vapor into the air. Ice and snow
can sublimate directly into water vapor. Evapotranspiration is water transpired from plants and evaporated from the soil. Rising
air currents take the vapor up into the atmosphere where cooler temperatures cause it to condense into clouds. Air currents
move water vapor around the globe, cloud particles collide, grow, and fall out of the upper atmospheric layers as precipitation.
Some precipitation falls as snow or hail, sleet, and can accumulate as ice caps and glaciers, which can store frozen water for
thousands of years. Most water falls back into the oceans or onto land as rain, where the water flows over the ground as surface
runoff. A portion of runoff enters rivers in valleys in the landscape, with streamflow moving water towards the oceans. Runoff and
groundwater are stored as freshwater in lakes. Not all runoff flows into rivers, much of it soaks into the ground as infiltration.
Some water infiltrates deep into the ground and replenishes aquifers, which store freshwater for long periods of time. Some
infiltration stays close to the land surface and can seep back into surface-water bodies (and the ocean) as groundwater discharge.
Some groundwater finds openings in the land surface and comes out as freshwater springs. Over time, the water returns to the
ocean, where our water cycle started.

5. Some definitions:-
• Preciptation - Condensed water vapor that falls to the Earth's surface . Most precipitation occurs as rain, but also includes
snow, hail, fog drip, graupel, and sleet.
• Runoff - The variety of ways by which water moves across the land. This includes both surface runoff and channel runoff.
• Infiltration - The flow of water from the ground surface into the ground.
• Evaporation - The transformation of water from liquid to gas phases as it moves from the ground or bodies of water into
the overlying atmosphere. The source of energy for evaporation is primarily solar radiation.
• Sublimation - The state change directly from solid water (snow or ice) to water vapor.
• Deposition - This refers to changing of water vapor directly to ice.
• Advection - The movement of water — in solid, liquid, or vapour states — through the atmosphere. Without advection,
water that evaporated over the oceans could not precipitate over land.
• Condensation - The transformation of water vapor to liquid water droplets in the air, creating clouds and fog.
• Transpiration - The release of water vapor from plants and soil into the air.
• Percolation - Water flows horizontally through the soil and rocks under the influence of gravity.