Introduction

1. 1. Types of clouds:

1. 1. • High clouds:

1. 1. 1. • Cirrus: indicate fair weather and a good sunset. Look fibrous or wispy.

1. 1. 1. • Cirrocumulus: Appear like a white globular mass.

1. 1. 1. • Cirrostratus: resemble a thin white sheet or veil.

1. 1. • Medium clouds:

1. 1. 1. • Altocumulus: indicate fair weather.

1. 1. 1. • Altostratus; dense, grayish clouds

1. 1. • Low clouds:

1. 1. 1. • Stratocumulus: rough, bumpy cloud

1. 1. 1. • Stratus: Brings dull weather with light drizzle.

1. 1. 1. • Nimbostratus: Rain clouds. It also brings snow or sleet.

1. 1. • Clouds with great vertical height:

1. 1. 1. • Cumulus: Typically seen in tropical, humid regions.

1. 1. 1. • Cumulonimbus: Thunder cloud brings rain with thunder and lightning.

The phenomenon caused by High Humidity

Haze: It is caused by smoke or dust in industrial areas or when there is unequal refraction of light in the air of different densities in the lower atmosphere. This is seen in regions of low humidity [Relative humidity < 75%].

Mist: It occurs in areas of high humidity where the relative humidity is 75% +. Condensation of water vapor in the air causes small water droplets to float about in the lower atmosphere forming clouds.

Fog: These are formed when water vapors condense on smoke or dust particles. A dense ground cloud is formed reducing visibility. Although fogs are seen in tropics and temperate areas, they are denser in high and middle latitudes than in the tropics. Fogs are more common overseas than on land and prevalent in coastal areas.

Haze and Mist occur more commonly in dry interiors.

Clouds and their types

Low altitude clouds:

1. Nimbostratus – Rain-bearing
2. Stratocumulus and cumulus are fair-weather clouds.

Middle altitude clouds:

1. Altocumulus
2. Altostratus

High altitude clouds:

1. Cirrostratus, cirrocumulus and cirrus.

Clouds with vertical development are cumulonimbus which is stormy clouds and an indicator of cyclonic heavy rainfall.

Rainfall and its type

The term “rainfall” is used to describe precipitation in the form of water drops of sizes larger than 0.5 mm. Other forms are snow, drizzle, glaze, sleet, and hail.

Types of rainfall:

1. Frontal and cyclonic – Seen in temperate regions and is the reason for rainfall during winters there. Warm air rises over cool air and is cooled due to it. this leads to condensation and rain.
2. Convectional – Hot air formed due to the sun’s heating effect on oceans rises upwards and condenses. This rain is seen in equatorial and tropics.
3. Orographic – This is seen where moist air is forced to climb a mountain barrier. The windward sides of mountains see this rainfall. It is also called “Relief rain”. The leeward side of the mountain, however, doesn’t receive much rainfall and is called the “Rainshadow regions”. E.g: Western ghats

Pressure systems of the world:

The entire earth is divided into four pressure belts. Belts aren’t continuous but pockets of low or high pressure. Low pressure is created by heat and the rising movement of air. High pressure is created by low temperature and descending air.

The belts are:

1. Equatorial low-pressure belt.
2. Subtropical high-pressure belt
3. Subpolar low-pressure belt
4. Polar high-pressure belt.

Equatorial low-pressure belt OR intertropical convergence zone:

The direct insolation leads to low pressure as the air is heated. The heated air rises upwards and condenses to bring evening rain.: The region along the equator and within 5 degrees North and South are the equatorial low-pressure belt is called Doldrums.

This convectional rain is common throughout the year in equatorial areas.

Calm belt with slow winds.

Subtropical high-pressure belt:

The air above the equator moves towards the poles but due to Coriolis force it is deflected and its path increases. The air loses energy and cools down; it descends and creates a high-pressure belt.

Subpolar low-pressure belt:

The cold air from the poles moves to the equator but the warm air of the sub-tropic high-pressure zone collides with it. The warm air rises here and creates a low-pressure zone. About 30 degrees North and South is the area of descending air currents or wind divergence or anticyclones. These are the horse latitudes. 60 degrees North and South are the temperate low-pressure belts and zones of cyclonic activity. Then at 90 degrees north and south are the polar high-pressure belts.

Polar high-pressure belt:

The air from the low-pressure sub-polar belt descends here creating high pressure. Also, the low temperature cools the air.

Planetary winds:

These winds blow in the same direction throughout the year. But due to Coriolis’s force, their direction is deflected. Winds tend to blow from high-pressure belts to low pressure but due to Earth’s rotation are deflected to right in the northern hemisphere and left in the southern hemisphere – “Ferrel’s law”. Coriolis force is absent at the equator but increases towards the poles. Due to the large expanse of oceans in the Southern hemisphere compared to the North, planetary winds blow harder.

Trade winds or Easterlies:

They blow from subtropical high to the equatorial low. They are deflected from east to west due to Coriolis’s force.

Desert winds:

The trade winds blow from east to west. But they lose moisture as they pass over the continents. The western margins of the continents don’t receive rain. Hence deserts are formed called trade wind deserts.

The second reason for deserts is they are in contact with cold currents that reach the western margins of the desert. These cold currents create a desiccating effect on the trade winds.

Westerlies:

They blow from the sub-tropic high-pressure zone to the sub-polar low-pressure zone. Their direction is from west to east due to Coriolis’s force. Due to the absence of landmass in the southern hemisphere, they have high velocities.

Polar easterlies:

Blow from Polar Regions to sub-Polar Regions. The direction is east to west.

Seasonal winds:

These are monsoon winds in the Indian subcontinent. The easterlies blow from north to south in winter and south to north in summer over some areas only.

The reason is the earth’s tilted axis which causes apparent movement of the sun in the north during June – July. This causes the shifting of the low-pressure belt or ITCZ to the north.

This brings rainfall to India.

Land and sea breezes:

During the morning the sea gets heated slower than land. The land remains under low pressure and the sea is under high pressure. Thus we get the sea breeze. At the night the land cools faster than the sea due to lower specific heat. This creates low pressure over the sea and high pressure over land. Hence we get a land breeze.

The fishermen use this system to move from land to sea and back.

Upper troposphere winds:

The troposphere has a different situation than land. Here there is high pressure over the equator and low over the poles. They blow from west to east “Westerlies”. In these winds, there is a strong, narrow band of winds called Jet Streams [speed – 300 kmph].

Cyclones

Tropical cyclones:

8-20° latitude. Movement from east to west. They are created due to thermal conditions. Their path is difficult to predict. They have winds of high speed but quickly dissipate on land. They mostly affect coastal areas.

Location: the Bay of Bengal, western pacific i.e. Philippines, etc, and the Gulf of Mexico when formed over these areas they make landfall hence the coastal parts of these areas are largely affected.

The other regions where they are formed but can’t make landfall are the Arabian sea, the east coast of Africa, the west coast of Mexico, and the southwest USA.

Formation:

1. The increase in the sea surface temperature causes air to warm and rise. This air also has moisture. The low-pressure zone at sea level is where air converges.
2.  As air moves upwards the Coriolis force causes spiral movement. After reaching the top it dissipates.
3. The continuous process above causes a cyclone where a high-pressure eye is surrounded by a low-pressure region. But as this cyclone is fueled by moisture when it makes landfall it dissipates quickly. This is due to being cut off from moisture.

Temperate cyclones:

30-40° latitude. Movement from west to east. They are formed due to the movement of air [rising]. They have winds at 40-50 mph. They don’t dissipate quickly and cause destruction.

They are located on the western coast of the USA and Europe. They are influenced by the Westerlies.