Evaporation and Condensation

The Engine of Atmospheric Moisture

The atmosphere constantly gains and loses water vapour through two opposing processes. Evaporation adds water vapour to the air, while condensation removes it. These two transformations drive every form of precipitation and cloud formation on Earth.

Evaporation: Liquid to Gas

Evaporation is the process by which water changes from a liquid state to a gaseous state. Heat is the main cause. When water absorbs enough heat energy, its molecules gain the energy to escape into the air as vapour.

The heat required for this change is called latent heat of vaporisation — the amount of heat energy needed to convert a unit mass of a liquid into vapour without any change in temperature. This heat is absorbed during evaporation and released later during condensation.

Three factors control the rate of evaporation:

• Temperature — An increase in temperature raises the water absorption and retention capacity of a given parcel of air. Warmer air can hold more moisture.
• Moisture content — If the moisture content of the air is low, the air has a greater potential to absorb and retain moisture. Dry air evaporates water faster than humid air.
• Movement of air — Moving air replaces the saturated layer near the water surface with an unsaturated layer. The greater the movement of air, the greater the evaporation. Wind continually brings fresh, dry air into contact with the water.

Condensation: Gas to Liquid

Condensation is the transformation of water vapour back into water. It is caused by the loss of heat. When moist air is cooled, it may reach a level where its capacity to hold water vapour ceases. Beyond this point, the excess water vapour condenses into liquid form.

In free air, condensation results from cooling around very small particles called hygroscopic condensation nuclei. Particles of dust, smoke, and salt from the ocean are particularly good nuclei because they absorb water. Condensation can also take place when moist air comes into contact with a colder object, and it may occur when the temperature is close to the dew point.

Condensation therefore depends on two things: the amount of cooling and the relative humidity of the air. It is also influenced by the volume of air, temperature, pressure, and humidity.

Condensation takes place under three conditions:

1. When the temperature of the air is reduced to the dew point with its volume remaining constant.
2. When both the volume and the temperature are reduced.
3. When moisture is added to the air through evaporation.

The most favourable condition for condensation is a decrease in air temperature.

After condensation, the moisture in the atmosphere takes one of four forms — dew, frost, fog, and clouds. These forms can be classified on the basis of temperature and location. Condensation takes place whether the dew point is lower than the freezing point or higher than it.

Dew

When moisture is deposited as water droplets on the cooler surfaces of solid objects — such as stones, grass blades, and plant leaves — rather than on nuclei in the air above the surface, it is known as dew.

The ideal conditions for dew formation are:

• Clear sky
• Calm air
• High relative humidity
• Cold and long nights

For dew to form, the dew point must be above the freezing point.

Frost

Frost forms on cold surfaces when condensation takes place below the freezing point (0°C), meaning the dew point is at or below the freezing point. The excess moisture is deposited as minute ice crystals instead of water droplets.

The ideal conditions for white frost are the same as those for dew, except that the air temperature must be at or below the freezing point.

Fog and Mist

When the temperature of an air mass containing a large quantity of water vapour falls suddenly, condensation takes place within the air mass itself on fine dust particles. This makes fog a cloud with its base at or very near to the ground.

Fog and mist differ in their effect on visibility:

In urban and industrial centres, smoke provides plenty of nuclei that help fog and mist form. When fog mixes with smoke, the condition is called smog.

The only difference between mist and fog is that mist contains more moisture than fog. In mist, each nucleus contains a thicker layer of moisture. Mists are frequent over mountains because rising warm air up the slopes meets a cold surface. Fogs are drier than mist and are prevalent where warm currents of air come into contact with cold currents. Fogs are mini clouds in which condensation takes place around nuclei provided by dust, smoke, and salt particles.

Clouds

A cloud is a mass of minute water droplets or tiny crystals of ice formed by the condensation of water vapour in free air at considerable elevations. Because clouds form at some height above the Earth's surface, they take various shapes.

According to their height, expanse, density, and transparency or opaqueness, clouds are grouped into four types: …