Precipitation Class 9 Geography Notes Maharashtra State Board
70.8% part of the earth’s surface is full of water. The distribution of this water is very uneven. At some places, the water storage is limited while it is ample at other places. We see/experience the different forms of condensation as shown in the pictures above and the figure. These forms of condensation occur due to the water vapour in the atmosphere.
As the atmospheric conditions change, we see changes in the forms of condensation. On winter mornings, we find dew. In areas located at higher altitudes, snowfall occurs; while it rains in other places. Some places experience dense fog while some experience hailstones suddenly and face crop destruction.
Precipitation:
Waterfalls in a solid or liquid state from the clouds to the ground. Snow, hailstones, and rainfall are the major forms of precipitation. See the images in the figures.
Snow:
When the temperature in the atmosphere falls below the freezing point, water vapour directly turns into snowflakes. This is called sublimation. Here, the vapour in the form of gas transforms into solid snow. Precipitation in the form of solid particles of snow is known as snowfall. In high latitudinal and temperate regions, snowfall occurs at the mean sea level while in tropical areas, snowfall occurs at places located higher than the snowline altitude.
As snow is in the solid form, it does not flow like water. Layers of snow get deposited. When the snow accumulates on a large scale, then the transport and communication system of that area collapses. People living in those regions have to protect themselves from frostbite. When the snow melts, the region gets water.
Snow-capped Mountains and Frozen Lake
There is a difference between ice and snow. In areas located at higher altitudes and high latitudes, where the temperatures are below 0°C precipitation occurs in the form of snow. Snow is friable and opaque. This snow accumulates in the form of layers on top of each other. Because of the pressure from the upper layers, the lower layers of the snow become homogeneous, massive, and transparent. Massive transparent snow formed in such a way is called ice. When the temperature drops below freezing point, a layer of ice forms on the lake’s surface, and this layer floats on the surface of the reservoir. This ice is not related to snowfall directly.
Hail:
When there is a lot of heat on the earth’s surface, the upward air flow blows at a greater speed. Because of this upward flow, the temperature of the air reduces and the condensation of the water vapour takes place. Dark clouds are formed. Because of the upward movement of air, these water droplets go to a higher altitude. Here, solidification of these droplets occurs and hailstones are formed.
As hailstones are heavy, they fall towards the ground, but because of the frequent upward flow of air, they are repeatedly taken upwards. Here, a new layer of snow encapsulates the hail. This happens quite a few times. Hence, concentric layers are formed while the hail grows in size. These big heavy hailstones fall rapidly to the ground because of gravity. We call the precipitation of this type as hail showers. Because of hail, crops may get destroyed and loss of life and property may also occur. Hails occur in summer in India, Africa, and in some parts of South East Asia. Hails do not occur in equatorial areas because of the heat in the atmosphere. Hails do not occur in cold zones because of a lack of upward flow.
A vertical glass container with a metal lid, nail, hammer, hot water, ice cubes, and a handful of salt. Take the container with the metal lid. Remove the lid. Hit the lid on the upper side at several points with a nail and the hammer (take care that the lid does not get any holes because of this). Fill 1/3rd of the container with steamed water (Not boiled). Now put the lid on the container and close it tightly. Take care that the steam in the container should not go out. On the lid, put ice cubes, a handful of rock salt, and a little water. Observe the container. Experience rain! (Note: It may take 10-15 minutes for rain to occur)
As the vapour in the container is light, it travels upwards. As the lid is tightly closed, the vapour cannot escape. Because of the ice cubes above the lid, condensation of the vapour occurs. Consequently, water droplets formed from the vapor accumulate on the inner side of the lid. Because of the punches on the lid, these droplets gather together and fall as drops. During rainfall, a similar process happens in the atmosphere on a large scale.
Rainfall:
We generally get water in the form of rainfall. The temperature of the air with water vapour reduces when it goes higher. Condensation of the vapour occurs. Clouds form when the condensed water droplets and dust particles accumulate. The water droplets increase in size. When they cannot float in the air anymore because of their weight, they come down as rainfall.
Convectional Rainfall:
In equatorial areas, the surface gets heated because of the sun’s heat, and the air near it also gets heated. As it gets heated, it spreads becomes lighter, and moves upwards. The air cools down when it goes upward. The moisture-holding capacity of cold air is less. Consequently, condensation of the water vapor occurs and rainfall occurs. In equatorial areas, such a type of rainfall occurs almost daily in the afternoons. Rainfall is accompanied by lightning and thunder. The Congo basin of Africa and the Amazon basin in South America experience convectional rainfall. Such rainfall occurs in a very limited area in the world.
Orographic Rainfall:
Winds coming from lakes or seas are moisture-laden. They are obstructed by the high mountain ranges coming in their way. They start going upwards along the slope of the mountains, The temperature of these winds drops condensation occurs and rainfall takes place. Thus because of the obstruction of the mountains, this type of rainfall occurs. The windward side of the mountains gets more rain; the amount of vapour in the air reduces after crossing the mountain and the moisture holding capacity of the air increases. The leeward side of the mountain gets lesser rainfall and hence this area is identified as a rain-shadow area. The effect of Monsoon rainfall is important when we think of the Indian subcontinent. We have studied this in the previous standard. The rainfall occurring in India because of these winds is orographic type rainfall.
Cyclonic Rainfall:
A cyclone is a specific air formation when the pressure in an area is less than the surrounding regions. This is called a cyclone. Air from the surrounding region comes toward the center of the cyclone and starts moving upwards. As it rises, the temperature of the air reduces, condensation occurs and rainfall takes place. It rains in areas over which the cyclone passes. Cyclonic rainfall occurs more in temperate zones and cyclone’s area is also quite extensive. Comparatively, cyclonic rainfall occurring in tropical regions is limited in extent and it is stormy.
Orographic rainfall occurs in most of parts the world. Convectional rainfall is regional. There is a certainty in the convectional rainfall occurring in the equatorial areas. Comparatively, the orographic and cyclonic rainfalls are less certain. Therefore, such areas are prone to very heavy rainfall, floods, or droughts frequently. The factors which affect the amount of rainfall in a region are the amount of water vapour in the atmosphere, air pressure, and temperature. The topography and latitudinal position of a place also affects its rainfall.
Rain Gauge:
The instrument that is used to measure rainfall is called a rain gauge. The funnel that is used in the rain gauge has a specific diameter. The rain falling in this funnel is collected in a bottle fitted with the gauge. The collected water is then measured with the help of a measuring jar. In areas of heavy rainfall, the reading of the rain is taken every three hours. The measuring jar reads in millimeters. The gauge is kept in open ground on a 30cm high flat-mount. Hence, the rainwater is collected without any obstruction.
How much water does 1 mm of rain mean?
If we do not let 1 mm of rainwater move as runoff, penetrate the ground, or evaporate, then how much water can be accumulated? Let’s understand this by an example. If 1mm rainfall occurs over 1 sq. km area then we get 10 lakh litres of water from it.
How is snowfall measured?
Snowfall can also be measured with the help of a rain gauge. For this, the container full of snow particles is heated carefully to melt the snow. Then the water obtained is measured. A layer of 120 mm of ice is equivalent to 10mm of rainfall.
Fog, Dew, and Frost:
When the condensation or solidification of the water vapour in the atmosphere occurs near the earth’s surface, then we can see fog, dew, or frost. The temperature of the layers of the air near the surface of the earth reduces. As temperature reduces, water vapour condenses. In this process, vapor turns into microscopic water particles and floats in the air. When the density of these droplets in the air increases, fog occurs.
When moisture-laden air near the earth’s surface comes into contact with very cold objects, condensation of the vapor takes place. They turn into very small water droplets. These water droplets stick to the surface of the cold objects. This is called dew. If the temperature of the air is less than 0°C, the water droplets stuck to the surfaces of cold objects freeze. This frozen water droplet is called frost. Dew and frost occur on a large scale in winter.
Effects of Precipitation:
The main source of potable water available on the earth is precipitation. As excessive rainfall is destructive so is the absence of rain. Floods may occur because of heavy rainfall causing loss of life and property. If precipitation does not take place, then conditions of drought arise. There is a shortfall of food and food may have to be imported. Farmers’ condition becomes grave. A country’s economy is affected.
The economy of an agrarian country like India is dependent on agriculture. Agriculture in India to a large extent is dependent on the Monsoons. Hence, the rainfall in India is important to the whole country. A good rainfall at the right time increases crop production while untimely rain can cause damage to the crops. The rainfall in India is quite erratic. Visibility is reduced because of highly dense fog. It affects the means of transportation like roads, railways, waterways, and the airways. Accidents may take place in such conditions. Trains, flights, and other transport services may have to be canceled. Frost is harmful to crops and causes accidents if spread on the road. Fog and dew damage some crops by spreading diseases while it may be beneficial to some crops.
Acid Rain:
Because of air pollution in industrial areas, various gases get mixed in the air. Different acids are created when the water vapour in the air reacts chemically with these gases. Such as nitric acid, sulphuric acid, etc. Precipitation of water with dissolved acids reaches the ground. Such rainfall is called ‘acid rain’. These rains are harmful to living organisms as well as non-living objects.
Detailed Maharashtra State Board Class 9 Geography Notes Precipitation are particularly useful for answering essay questions.