Atmospheric Pressure

Atmospheric pressure refers to the weight of the air above a particular point on the Earth’s surface. It is the force exerted by the atmosphere on everything below it. Air pressure is highest at sea level because the entire atmosphere is pressing down on it and decreases as you go higher up into the atmosphere.

Vertical Variation of Pressure

• Air pressure decreases with height. At higher altitudes, such as on mountains, there is less air above, so the pressure is lower.
• The pressure drop is roughly 1 millibar (mb) for every 10 meters of elevation gain.
• The higher you go, the thinner the air becomes, which is why breathing can be difficult at high altitudes, like on Mount Everest.

Horizontal Distribution of Pressure

• Pressure also varies horizontally across the Earth’s surface, creating areas of high pressure and low pressure.
• High-pressure systems (where air is heavier) usually bring clear, calm weather, while low-pressure systems (where air is lighter) are associated with clouds, rain, and storms.
• Isobars are lines on weather maps that connect places with the same pressure. If the isobars are close together, it means the pressure is changing rapidly, leading to stronger winds.

World Distribution of Sea Level Pressure

• Equatorial Low-Pressure Belt: At the equator, warm air rises, creating a zone of low pressure.
• Subtropical High-Pressure Belt: At around 30° N and S of the equator, the air that rose at the equator cools and sinks, creating zones of high pressure (e.g., Sahara Desert).
• Polar High-Pressure Areas: Near the poles, the cold air sinks, forming zones of high pressure. This is why the Arctic and Antarctica are cold and dry.

Forces Affecting Wind

Wind is simply the movement of air from areas of high pressure to areas of low pressure. However, several forces influence the direction and speed of wind:

Pressure Gradient Force

• This is the most basic force that drives wind. It causes air to move from areas of high pressure to areas of low pressure.
• The steeper the pressure gradient (the closer the isobars are on a weather map), the stronger the wind.

Frictional Force

• Friction affects the movement of wind, especially near the Earth’s surface. It slows down wind and alters its direction.
• Friction is greater over land (because of obstacles like mountains, forests, and buildings) and lower over oceans.

Coriolis Force

• Due to the rotation of the Earth, winds are deflected:
  • Right in the Northern Hemisphere.
  • Left in the Southern Hemisphere.
• The Coriolis force is stronger at the poles and weaker at the equator, and it affects the direction of global wind patterns.

Wind Systems and Circulation

Winds are not random; they follow specific patterns based on the distribution of pressure and the Earth’s rotation.

General Circulation of the Atmosphere

The Earth’s atmosphere has a global pattern of air circulation that redistributes heat from the equator to the poles. The general circulation is divided into three major wind belts or cells:

1. Hadley Cell:
   • Air rises at the equator (due to intense heat), moves towards higher latitudes, and sinks around 30° N and S.
   • This sinking air forms high-pressure zones and creates the Trade Winds that blow from east to west in tropical regions.
2. Ferrel Cell:
   • Found between 30° and 60° in both hemispheres.
   • Here, air moves poleward, rises around 60° N and S, and flows back toward the equator, forming the westerlies (winds that blow from west to east in the mid-latitudes).
3. Polar Cell:
   • Cold, dense air sinks at the poles, creating high-pressure areas.
   • Air flows away from the poles toward the mid-latitudes (forming polar easterlies).

These large-scale circulation patterns help distribute heat around the Earth, preventing the tropics from becoming too hot and the poles from becoming too cold.

Local Winds

In addition to the global wind patterns, there are smaller, more localized wind systems:

• Land and Sea Breezes: During the day, land heats up faster than the sea, causing a sea breeze (wind blowing from the sea to the land). At night, the land cools down faster, creating a land breeze (wind blowing from the land to the sea).
• Mountain and Valley Winds: During the day, the air in valleys heats up and rises, creating a valley breeze. At night, cool air flows down from the mountains into the valleys, forming a mountain breeze.

Air Masses and Fronts

An air mass is a large body of air with consistent temperature and humidity throughout. Air masses form over large areas like oceans or continents and bring specific weather patterns when they move.

Types of Air Masses

• Maritime Tropical (mT): Warm and moist air, typically found over tropical oceans.
• Continental Tropical (cT): Hot and dry air, often formed over deserts.
• Maritime Polar (mP): Cool and moist air, found over high-latitude oceans.
• Continental Polar (cP): Cold and dry air, found over snow-covered regions.
• Continental Arctic (cA): Extremely cold, dry air from the Arctic region.

Fronts

When air masses with different temperatures and humidity levels meet, they form fronts. A front is a boundary between two different air masses:

• Cold Front: Cold air pushes into warm air, forcing the warm air to rise. This usually leads to thunderstorms and a drop in temperature.
• Warm Front: Warm air moves over cold air, bringing gradual, steady rain and an increase in temperature.
• Stationary Front: Neither air mass moves much, leading to prolonged periods of rain or cloudiness.
• Occluded Front: Cold air overtakes warm air, lifting it off the ground and bringing complex weather, including thunderstorms and sometimes snow.

Cyclones

A cyclone is a large, spiraling storm system with a center of low pressure. Cyclones can be categorized into two main types: tropical cyclones and extra-tropical cyclones.

Tropical Cyclones

• These are intense storms that form over warm tropical oceans. They are known for producing heavy rainfall, strong winds, and storm surges that can cause widespread destruction.
• Conditions needed for tropical cyclones:
  • Warm ocean waters above 27°C.
  • Coriolis force to initiate rotation.
  • A pre-existing low-pressure system.
• Tropical cyclones go by different names depending on the region:
  • Hurricanes in the Atlantic Ocean.
  • Cyclones in the Indian Ocean.
  • Typhoons in the Western Pacific.

Extra-Tropical Cyclones

• These cyclones form in mid-latitudes, beyond the tropics, and are associated with the meeting of cold and warm air masses along the polar front.
• Extra-tropical cyclones are usually less intense than tropical cyclones but can cover larger areas. They bring rain, snow, and strong winds and usually move from west to east.

Thunderstorms and Tornadoes

Thunderstorms

• Thunderstorms are caused by the rapid rise of warm, moist air, which forms cumulonimbus clouds. They are often accompanied by lightning, thunder, heavy rain, and strong winds.
• Thunderstorms are more common in tropical regions and during hot, humid afternoons.

Tornadoes

• A tornado is a violent, rotating column of air that extends from a thunderstorm to the ground. Tornadoes can be extremely destructive, with winds reaching speeds of over 300 km/h.
• Tornadoes are most common in North America, particularly in the central United States, known as Tornado Alley.
• Tornadoes over water are called waterspouts.

The Influence of Oceans

Oceans play a crucial role in regulating the Earth’s climate and influencing weather systems. One of the most well-known ocean-related phenomena is El Niño:

• El Niño is a warming of the surface waters in the Pacific Ocean, which disrupts normal weather patterns.
• It is part of the El Niño-Southern Oscillation (ENSO) cycle, which can cause extreme weather events like droughts, floods, and heatwaves.
• La Niña is the opposite phase, characterized by cooler-than-normal ocean temperatures in the Pacific, which can also affect global weather patterns.

El Niño and La Niña (Simple Explanation)

El Niño and La Niña are weather patterns that happen because of changes in water temperatures in the Pacific Ocean. They cause unusual weather around the world, affecting rainfall, temperatures, and even storms.

What is El Niño?

El Niño happens when the Pacific Ocean near the equator becomes warmer than usual.

• Normally, trade winds blow warm water from South America toward Asia (from east to west).
• During El Niño, these trade winds weaken, and the warm water stays near the west coast of South America.
• This warming changes the usual weather patterns.

Effects of El Niño:

• More Rain: Countries along the west coast of South America, like Peru, get heavier rainfall and sometimes floods.
• Droughts: Places like Australia, Indonesia, and India may get less rain, leading to droughts.
• Warmer Winters: Parts of North America can have warmer winters during El Niño.
• Fewer Hurricanes: El Niño can also reduce the number of hurricanes in the Atlantic Ocean.

What is La Niña?

La Niña is the opposite of El Niño. It happens when the Pacific Ocean near the equator becomes cooler than usual.

• Trade winds become stronger, pushing warm water even further toward Asia.
• This causes the eastern Pacific (near South America) to be cooler than normal.

Effects of La Niña:

• More Rain in Asia: Countries like Australia, Indonesia, and parts of Southeast Asia get more rainfall and sometimes flooding.
• Drier in South America: Countries like Peru and Ecuador often get drier weather.
• Cooler Winters: La Niña can lead to colder winters in places like North America.
• More Hurricanes: La Niña can lead to more hurricanes in the Atlantic Ocean.

Key Differences:

• El Niño: Warmer water in the Pacific, causes heavier rains in South America and drier weather in Asia and Australia.
• La Niña: Cooler water in the Pacific, causes more rain in Asia and drier weather in South America.

Summary:

• El Niño = Warm Pacific Ocean, leading to unusual weather like floods in South America and droughts in Asia.
• La Niña = Cool Pacific Ocean, leading to heavy rains in Asia and dry weather in South America.

These changes in the Pacific Ocean can affect weather patterns worldwide, causing floods, droughts, and changing storm activity.

Conclusion

The atmosphere is constantly in motion, driven by differences in pressure, temperature, and the Earth’s rotation. These movements create wind systems, weather patterns, and storms like cyclones and tornadoes. Understanding how air masses interact and how pressure systems form is key to predicting the weather and preparing for extreme events like hurricanes and thunderstorms.