The Chemistry of Weather: Condensation and Evaporation

Water Constantly Changes Its "State" When Travelling Through the Atmosphere

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Condensation and evaporation are two terms that appear early on and often when learning about weather processes. They are essential to understanding how water -- which is always present (in some form) in the atmosphere -- behaves.

Condensation Definition

Condensation is the process by which water residing in the air changes from water vapor (a gas) to liquid water. This happens when the water vapor is cooled to the dew point temperature, which leads to saturation.

Anytime you have warm air rising up into the atmosphere, you can expect condensation to eventually occur. There are also many examples of condensation in our daily lives, such as the formation of water droplets on the outside of a cold drink. (When the cold drink is left sitting on a table, the moisture (water vapor) in the room's air comes in contact with the cold bottle or glass, cools, and condenses on the drink's outside.)

Condensation: A Warming Process

You'll often hear condensation called a "warming process," which can be confusing since condensation has to do with cooling. While condensation does cool the air inside of the air parcel, in order for that cooling to occur, that parcel must release heat into the surrounding environment. Thus, when speaking about the effect of condensation on the overall atmosphere, it warms it. Here's how it works:

Remember from chemistry class that molecules in a gas are energetic and move very fast, while those in a liquid move slower.

In order for condensation to happen, the water vapor molecules must release energy so that they can slow their movement. (This energy is hidden and is therefore called latent heat.)

Thank Condensation for this Weather...

A number of well-known weather phenomenon are caused by condensation, including:

Evaporation Definition

The opposite of condensation is evaporation. Evaporation is the process of changing liquid water into water vapor (a gas). It transports water from the Earth's surface to the atmosphere.

(It should be noted that solids, like ice, can also evaporate or be transformed directly into a gas without first becoming a liquid. In meteorology, this is called sublimation.)

Evaporation: A Cooling Process

For water molecules to go from a liquid to an energized gaseous state, they must first absorb heat energy. They do this by colliding with other water molecules.

Evaporation is called a "cooling process" because it removes heat from the surrounding air. Evaporation in the atmosphere is a crucial step in the water cycle. Water on Earth's surface will evaporate into the atmosphere as energy is absorbed by liquid water. Water molecules that exist in the liquid phase are free-flowing and in no particular fixed position. Once energy is added to water by heat from the sun, the bonds between the water molecules gain kinetic energy or energy in motion. They then escape the surface of the liquid and become a gas (water vapor), which then rises into the atmosphere.

This process of water evaporating from the surface of the Earth happens continually and continually transports water vapor into the air.

 The rate of evaporation depends on air temperature, wind speed, cloudiness.

Thank Evaporation for this Weather...

Evaporation is responsible for several weather phenomena, including:

  • Humidity
  • Clouds