Chapter Notes

Atmospheric Circulation and Weather Systems

The way the sun heats the Earth is uneven. This simple fact is the starting point for understanding all weather. When air is heated, it expands and becomes lighter, and when it's cooled, it gets compressed and becomes denser. This creates differences in atmospheric pressure. Just like water flowing downhill, air naturally moves from areas of high pressure to areas of low pressure. This movement is what we call wind.

Wind is incredibly important because it acts like the planet's circulatory system. It moves heat and moisture around the globe, helping to maintain a relatively constant temperature for the planet as a whole. When moist air rises, it cools, forming clouds and eventually leading to rain or snow (precipitation). This chapter explores why pressure differences exist, the forces that guide the wind, and how these processes create everything from gentle breezes to violent storms.

Atmospheric Pressure

You might not feel it, but your body is constantly under a great deal of pressure from the air around you. When you go up a mountain, the air gets thinner, and you might feel breathless. This is because there's less air above you pushing down.

Atmospheric pressure is defined as the weight of a column of air over a specific unit of area, measured from sea level to the top of the atmosphere.

• Unit of Measurement: It is expressed in milibar (mb).
• Average Sea Level Pressure: The standard pressure at sea level is 1,013.2 milibar.
• Measurement Tools: Pressure is measured using a mercury barometer or an aneroid barometer.

Because of gravity, air is densest at the surface, which is why pressure is highest there. This variation in pressure is the main driver of wind.

Vertical Variation of Pressure

As you move up in altitude, atmospheric pressure decreases rapidly.

• In the lower atmosphere, the pressure drops by about 1 mb for every 10 meters of elevation gain.
• This vertical pressure gradient is actually much stronger than the horizontal one. However, it is balanced by the force of gravity pulling air down. This balance is why we don't experience constant strong upward winds.

Horizontal Distribution of Pressure

Even small differences in pressure across the Earth's surface can have a huge impact on wind direction and speed. To study this, meteorologists create weather maps with isobars, which are lines that connect places with equal atmospheric pressure.

To make fair comparisons, pressure readings from different elevations are adjusted to what they would be at sea level. This removes the effect of altitude.

• Low-pressure system: An area enclosed by one or more isobars with the lowest pressure at the center.
• High-pressure system: An area enclosed by one or more isobars with the highest pressure at the center.

World Distribution of Sea Level Pressure

Globally, there are distinct belts of high and low pressure.

• Equatorial low: A low-pressure belt found near the equator.
• Subtropical highs: High-pressure areas located around 30° N and 30° S latitudes.
• Sub polar lows: Low-pressure belts found along 60° N and 60° S latitudes.
• Polar high: High-pressure areas near the poles.

Forces Affecting the Velocity and Direction of Wind

Wind doesn't just flow in a straight line from high to low pressure. Its path and speed are influenced by a combination of forces. For winds near the surface, three main forces are at play:

Pressure Gradient Force

This is the primary force that gets the air moving. The pressure gradient is the rate at which pressure changes over a distance.

• When isobars are close together, the pressure gradient is strong, and the wind is fast.
• When isobars are far apart, the pressure gradient is weak, and the wind is light.

Frictional Force

This force acts to slow the wind down. It is caused by the wind dragging against the Earth's surface.

• Friction is strongest at the surface and its effect can extend up to an elevation of 1-3 km.
• Over a smooth sea surface, friction is minimal, allowing winds to be stronger.

Coriolis Force

This is an effect caused by the Earth's rotation. Instead of being a true "force," it's an apparent deflection of moving objects (like wind) when viewed from a rotating frame of reference.

• It deflects wind to the right in the Northern Hemisphere.
• It deflects wind to the left in the Southern Hemisphere.
• The Coriolis force is strongest at the poles and is absent at the equator.
• The deflection increases as wind speed increases.

Pressure and Wind

The final direction and speed of wind is a result of these forces working together.

• Geostrophic Wind: In the upper atmosphere (2-3 km above the surface), the frictional force is negligible. Here, the pressure gradient force and the Coriolis force balance each other out. This causes the wind to blow parallel to the isobars. This balanced wind is known as the geostrophic wind.
• Cyclonic and Anti-cyclonic Circulation: Wind circulation around a low-pressure center is called cyclonic circulation. Around a high-pressure center, it's called anti-cyclonic circulation. The direction of rotation depends on the hemisphere:
  • Cyclone (Low Pressure): Anticlockwise in the Northern Hemisphere, Clockwise in the Southern Hemisphere.
  • Anticyclone (High Pressure): Clockwise in the Northern Hemisphere, Anticlockwise in the Southern Hemisphere.
• Convergence and Divergence: Over a low-pressure area, air converges at the surface and rises. Over a high-pressure area, air sinks from above and diverges (spreads out) at the surface. The rising of air is essential for forming clouds and precipitation.

General circulation of the atmosphere

The large-scale, planet-wide pattern of winds is called the general circulation of the atmosphere. It's driven by factors like uneven heating between latitudes, the formation of pressure belts, and the Earth's rotation. This circulation is described by a three-cell model in each hemisphere:

• Hadley Cell: This cell operates in the tropics.

  1. Intense solar heating at the Inter Tropical Convergence Zone (ITCZ) near the equator causes air to rise.
  2. This air travels towards the poles at high altitudes.
  3. Around 30° N and S, the air cools, sinks, and creates the subtropical high-pressure belts.
  4. At the surface, this air flows back towards the equator as the easterlies.

• Ferrel Cell: This cell is found in the middle latitudes. It is a circulation of sinking cold air from the poles and rising warm air from the subtropical highs. At the surface, this creates the prevailing westerlies.

• Polar Cell: At the poles, cold, dense air sinks and flows towards the middle latitudes as the polar easterlies.

General Atmospheric Circulation and its Effects on Oceans

The atmosphere and oceans are closely linked. Large-scale winds drive ocean currents, and oceans provide heat and moisture to the atmosphere.

A key example of this interaction is the El Nino Southern Oscillation (ENSO).

• El Nino: The appearance of unusually warm water in the central Pacific Ocean that drifts towards the coast of South America, replacing the normally cool Peruvian current.
• Southern Oscillation: A see-saw pattern of atmospheric pressure changes between the central Pacific and Australia.
• ENSO: The combined phenomenon of El Nino and the Southern Oscillation.

In strong ENSO years, weather patterns around the world are disrupted, causing events like heavy rainfall on the arid coast of South America, drought in Australia and India, and floods in China.

Seasonal Wind

The general circulation pattern is modified by the seasons because the zones of maximum heating shift with the sun. The most dramatic example of this seasonal shift is the monsoon, especially over southeast Asia.

Local Winds

Local differences in heating and cooling create smaller-scale, regional winds.

Land and Sea Breezes

This is a daily cycle common in coastal areas.

• Sea Breeze (Day): Land heats up faster than the sea, creating a low-pressure area over the land. The cooler, higher-pressure air from the sea blows inland.
• Land Breeze (Night): Land cools down faster than the sea. The pressure over the land becomes higher than over the sea, so the wind blows from the land out to the sea.

Mountain and Valley Winds

This is a similar daily cycle that occurs in mountainous regions.

• Valley Breeze (Day): Mountain slopes heat up, causing air to rise. Air from the valley flows upslope to fill the gap.
• Mountain Wind (Night): Slopes cool down quickly. The denser, cool air drains down into the valley. A katabatic wind is a powerful version of this, involving cold air from high plateaus or ice fields draining into a valley.

Air Masses

An airmass is a huge body of air with nearly uniform temperature and humidity. It gets these characteristics by remaining over a large, homogenous surface (like an ocean or a vast plain) for a long time. These surfaces are called source regions.

There are five major types of air masses, classified by their source region:

• Maritime tropical (mT): Warm and moist, from tropical oceans.
• Continental tropical (cT): Warm and dry, from subtropical hot deserts.
• Maritime polar (mP): Cold and moist, from high-latitude oceans.
• Continental polar (cP): Cold and dry, from cold, high-latitude continents.
• Continental arctic (cA): Very cold and dry, from permanently ice-covered regions.

Fronts

When two different air masses meet, they don't mix easily. The boundary zone between them is called a front. The process of front formation is frontogenesis. Fronts are common in the middle latitudes and bring abrupt changes in weather, often causing clouds and precipitation.

There are four types of fronts:

• Stationary Front: The boundary remains stationary because neither air mass is advancing.
• Cold Front: Cold air actively advances and pushes warmer air up. This often leads to the development of cumulus clouds and intense, short-lived precipitation.
• Warm Front: Warm air glides up and over a colder air mass. This typically creates a sequence of widespread clouds and gentle, prolonged precipitation.
• Occluded Front: A cold front overtakes a warm front, lifting the entire warm air mass off the ground.

Extra Tropical Cyclones

These are the large, low-pressure systems that develop in the middle and high latitudes (beyond the tropics). They are also known as middle-latitude cyclones and are responsible for much of the day-to-day weather changes in these regions.

• Formation: They form along the polar front, where cold polar air meets warm subtropical air. A disturbance can cause the front to buckle, creating a cyclonic (anticlockwise in the Northern Hemisphere) circulation.
• Structure: A developed extra-tropical cyclone has a distinct warm front and cold front, with a "warm sector" of air wedged in between.
• Dissipation: The cold front moves faster than the warm front. When it catches up, it lifts the warm sector completely off the ground, creating an occluded front, and the cyclone dissipates.

Extra Tropical vs. Tropical Cyclones

Tropical Cyclones

Tropical cyclones are violent, destructive storms that form over warm tropical oceans. They are known by different names around the world:

• Cyclones in the Indian Ocean
• Hurricanes in the Atlantic
• Typhoons in the Western Pacific
• Willy-willies in Western Australia

Conditions for Formation

1. A large sea surface with a temperature higher than 27°C.
2. The presence of the Coriolis force (so they don't form at the equator).
3. Small variations in vertical wind speed.
4. A pre-existing weak low-pressure area.
5. Upper divergence (air spreading out) above the sea level system.

Structure and Characteristics

• Energy Source: The storm is fueled by the energy released during condensation in towering cumulonimbus clouds. It needs a continuous supply of moisture from the warm ocean to survive.
• Dissipation: When the cyclone moves over land (landfall), its moisture supply is cut off, and it quickly weakens.
• The Eye: At the center of a mature storm is the eye, a region of calm weather with sinking air.
• The Eye Wall: Surrounding the eye is the eye wall, the most dangerous part of the storm. Here, air spirals upward violently, wind speeds are at their maximum (up to 250 km/h), and rainfall is torrential.
• Storm Surges: The powerful winds can push a large mound of seawater towards the coast, causing severe flooding known as a storm surge.

Thunderstorms and Tornadoes

These are other types of severe, but more localized, storms.

• Thunderstorms: Caused by intense convection on hot, moist days. A thunderstorm is essentially a well-developed cumulonimbus cloud that produces thunder and lightning. If the cloud reaches very high, cold altitudes, it can produce hail. In dry conditions, it can generate dust storms.
• Tornadoes: A tornado is a violently rotating column of air that descends from a severe thunderstorm, looking like the trunk of an elephant. It has extremely low pressure at its center and causes massive destruction. Tornadoes mostly occur in the middle latitudes. A tornado that forms over the sea is called a water spout.

Ultimately, all these violent storms are manifestations of the atmosphere trying to balance out the uneven distribution of energy across the globe. They convert heat energy into the kinetic energy of wind, helping the atmosphere return to a more stable state.