What do you mean by water in the atmosphere? What are the different types of rainfall? Learn more about the topic.
Water in the atmosphere plays an important role in weather phenomena.
Water, the universal solvent, exists in the atmosphere in different forms. Each of these forms has different characteristics and behaves differently according to the change in the surroundings.
What do you mean by water in the atmosphere?
There are three different forms of water in the atmosphere: gaseous, liquid, and solid.
The gaseous form of water in the atmosphere is called water vapour.
- It is the source of all kinds of precipitation.
- The amount of water in the atmosphere as a vapour form, which affects meteorological phenomena, ranges from 0% to 4% of the volume of the atmosphere (with an average of 2%).
- Through evaporation from water bodies and transpiration from plants, the atmosphere gets its moisture. Thus, through the processes of evaporation, transpiration, condensation, and precipitation, there is a constant exchange of water between the atmosphere, the seas, and the continents.
- With height, the amount of water vapour drops. It likewise gets smaller as you get closer to the poles from the equator (or from low latitudes) (or towards the high latitudes).
- Its maximum amount of water in the atmosphere could be up to 4% which is found in the warm and wet regions.
- Water in the atmosphere (vapour) retains the heat that the planet radiates by absorbing some of the incoming solar radiation (insolation) from the sun. As a result, it works as a blanket, keeping the ground from getting too hot or too cold.
- Water vapour also contributes to stability and instability in the air.
To know about atmospheric circulation, click here.
Water vapour present in the air is known as Humidity.
- Absolute humidity
- Relative humidity
- Absolute humidity is the measure of the actual water vapour content of the air.
- It is measured in terms of grams per cubic meter and represents the weight of water vapour per volume of air.
- The absolute humidity differs from place to place on the surface of the earth.
- The temperature of the air determines simply whether or not it can hold water vapour (Warm air can hold more moisture than cold air).
- The relative humidity is the ratio of the amount of moisture in the air to its maximum capacity at a particular temperature.
- The ability to hold moisture increases or decreases as the air temperature changes and the relative humidity is also impacted.
- Over oceans, the relative humidity is higher, whereas, over continents, it is lower (absolute humidity is greater over oceans because of the greater availability of water for evaporation).
- The amount and rate of evaporation are determined by the relative humidity, making it a crucial climatic component.
- At a specific temperature, the air is considered to be saturated when it is fully saturated with moisture.
- It signifies that at that point, the air at the specified temperature is unable to hold any more moisture.
- The dew point is the temperature at which a given sample of air becomes saturated.
- Dew point occurs when Relative Humidity is 100%
Evaporation And Condensation
Water in the atmosphere consists of evaporation and condensation forms.
- Water changes from a liquid to a gas through the process of evaporation.
- Evaporation is mostly caused by heat. The latent heat of vapourization is the term used to describe the temperature at which water begins to evaporate.
- The given parcel of air can absorb and hold more water when the temperature rises. Similar to this, low moisture content air has the potential to absorb and hold moisture. The saturated layer is replaced by the unsaturated layer as a result of air movement. Therefore, evaporation increases in direct proportion to air movement.
- It is the process by which water vapour turns into actual water.
- Condensation is caused by the loss of heat.
- The degree of cooling and the relative humidity of the air affect condensation.
- The amount of air present, the temperature, the pressure, and the humidity all affect condensation.
- Both when the dew point is lower than the freezing point and when it is greater than the freezing point, condensation occurs.
- It takes place:
when the temperature of the air is dropped to dew point while maintaining its volume constant (adiabatically);
when both the volume and the temperature are reduced;
And when moisture is added to the air through evaporation.
After condensation, the water vapour or the moisture in the atmosphere takes one of the following forms — dew, frost, fog, and clouds.
- Dew is formed when moisture is deposited as water droplets on cooler surfaces of solid objects (rather than nuclei in the air on the surface) such as stones, grass blades, and plant leaves.
- The ideal conditions for its formation are a clear sky, calm air, high relative humidity, and cold and long nights.
- For the formation of dew, it is necessary that the dew point is above the freezing point.
- Frost forms on cold surfaces when condensation takes place below the freezing point (0° C), i.e. the dew point is at or below the freezing point.
- Instead of water droplets, the extra moisture is deposited as minute ice crystals.
- The optimum conditions for the creation of white frost are the same as those for the formation of dew, with the exception that the air temperature must be at or below freezing.
- When the temperature of an air mass containing a substantial amount of water vapour suddenly drops, condensation occurs on small dust particles inside the air mass.
- So, the fog is a cloud with its base at or very near the ground. Because of the fog and mist, the visibility becomes poor to zero.
Radiation smog often arises at night when surface heat is reflected into space. As the earth’s surface cools, the air becomes completely humid, transforming into a fog.
Advection fog is similar to radiation fog in appearance, but it is generated when warm moist air travels horizontally over a cold surface, generating condensation. Sea fog is a typical kind of advection smog.
Ice fog is formed from ice crystals. Ice fog forms when the air temperature is below freezing.
Freezing fog is made up of supercooled water droplets that turn from liquid to ice when they come into touch with a cold surface. Objects exposed to freezing fog frequently develop ice covering.
Fog is generally associated with hazardous driving conditions. Because drivers cannot see very far in front of them (oftentimes, their depth perception becomes skewed), foggy weather can cause many dangerous accidents.
- Smoke generates a large number of nuclei that aid in the development of fog and mist in urban and industrial areas. Smog is a phenomenon that occurs when fog and smoke mingle.
Comparison between fog and smog
Water droplets condensed around a dust particle.
Water droplets condensed around a particle of pollutant like SO2.
It reduces visibility and damages the crops.
Causes health issues.
- A cloud is a mass of minute water droplets or microscopic ice crystals generated by the condensation of water vapour in free air at high altitudes.
- As the clouds are formed at some height over the surface of the earth, they take various shapes.
- According to their height, expanse, density, and transparency or opaqueness, clouds are grouped under four types :
- Cirrus clouds are formed at high altitudes (8,000 – 12,000m)
- They are thin and detached clouds having a feathery appearance.
- They are always white.
- Cumulus clouds look like cotton wool.
- These are generally formed at a height of 4,000 – 7,000 m.
- They exist in patches and can be seen scattered here and there.
- These clouds have a flat base.
- These are layered clouds covering large portions of the sky.
- These clouds are generally formed either due to loss of heat or the mixing of air masses with different temperatures.
- Nimbus clouds are black or dark grey.
- They form at middle levels or very near to the surface of the earth.
- These are extremely dense and opaque to the rays of the sun.
- Nimbus clouds are shapeless masses of thick vapour.
A combination of these four basic types can give rise to the following types of clouds:
- High clouds – cirrus, cirrostratus, cirrocumulus;
- Middle clouds – altostratus and altocumulus;
- Low clouds – stratocumulus and nimbostratus (long duration rainfall cloud) and
- Clouds with extensive vertical development – cumulus and cumulonimbus (thunderstorm cloud)
After condensation, the release of moisture is known as precipitation.
It can occur in the following forms:
Rain: Precipitation in the form of water is called rain.
Drizzle: It sprays like rainfall which is very slow with water droplets having 0.5mm diameter. They mostly happen via stratus clouds.
Snow: When the temperature falls below 0° C, precipitation falls in the form of thin flakes of snow and is referred to as snowfall. Hexagonal crystals are formed as a result of the discharge of moisture. These crystals combine to make snowflakes.
Sleet: It’s made of frozen raindrops and refrozen melted snow. Sleet forms when a layer of air with a temperature above freezing overlies a layer of air with a temperature below freezing near the ground.
Types Of Rainfall
Based on origin, rainfall may be classified into three main types – convectional, orographic or relief, and cyclonic or frontal.
- Convectional rainfall happens when the sun’s energy heats the Earth’s surface, forcing water to evaporate and produce water vapour.
- When the land warms, the air above it warms as well. As a result, the air expands and rises. The rising air cools and condenses. This condensation mechanism creates clouds high in the atmosphere.
- This type of rainfall is very common in tropical areas.
- Because convective clouds are more vertical than horizontal, their strength changes rapidly and across a smaller area.
- In the tropics, convective rainfall results in hail storms whereas in mid-latitudes it results in cold fronts, squalls, and warm fronts.
- When warm wet air is blocked by a mountain, it climbs to a high altitude along the mountain slope.
- It condenses at high altitudes, resulting in severe rains. This is referred to as orographic rainfall.
- Cyclonic Rainfall is convectional rainfall on a large scale.
- Precipitation in a tropical cyclone is convectional, whereas precipitation in a temperate cyclone is due to frontal activity.
World Distribution of Rainfall
- Different places on the earth’s surface receive different amounts of rainfall in a year.
- Rainfall steadily decreases as we move from the equator to the pole.
- The equatorial zone and the monsoon region of Southeast Asia have the most rainfall. Precipitation is modest in the middle latitudes, but it is scarce in the desert regions of the subtropics and around the poles.
- Rain is greater on the eastern shores between latitudes 35° and 40° N and S of the equator and gradually decreases towards the west. However, due to the westerlies, rainfall is first obtained on the western borders of the continents and then decreases towards the east between 45° and 65° N and S of the equator.
- Major precipitation regimes of the world are characterized based on total yearly precipitation as follows: o The trade winds from both hemispheres converge in the equatorial belt, causing a general upward motion of air that becomes intensified locally in tropical storms that produce very heavy rains in the Caribbean, the Indian and southwest Pacific oceans, and the China Sea, as well as thunderstorms that are particularly frequent and active over land areas.
- Rainfall in the interior continental areas is moderate, ranging from 100 to 200 cm per year. The continents’ coastal parts receive a moderate quantity of rainfall.
- Rainfall in the central tropical area and the eastern and inner temperate lands ranges between 50 and 100 cm per year.
- Low precipitation at high latitudes, particularly in the polar regions, is caused partly by the subsidence of air in high-pressure belts and partly by cold temperatures. Snow and rainfall from time to time, but evaporation from the freezing sea and land surfaces is slow, and the frigid air has limited moisture capacity.
To learn more about the Composition and Structure of the Earth’s Atmosphere click here
Article Written by: Remya