Learning Outcomes
- Understand how temperature variations cause pressure differences.
- Recognize the key forces influencing the movement of air: Pressure gradient, Coriolis, and Frictional forces.
- Identify patterns in global atmospheric circulation.
- Learn how air masses form and interact, leading to various weather systems.
- Grasp the mechanisms of tropical and extra-tropical cyclones.
Atmospheric pressure plays a crucial role in setting the air in motion. As air expands when heated and contracts when cooled, pressure differences emerge across the Earth’s surface, causing air to move from areas of high pressure to low pressure. This chapter focuses on the forces influencing atmospheric circulation, wind patterns, and the weather phenomena that result from these processes.
Atmospheric pressure is the weight of a column of air from sea level to the top of the atmosphere and is measured in millibars (mb). At sea level, the average pressure is 1,013.2 mb. This pressure decreases with elevation and varies depending on the location. Instruments like mercury barometers and aneroid barometers measure atmospheric pressure. Wind is driven by variations in pressure, moving from high-pressure zones to low-pressure zones.
Pressure in the lower atmosphere decreases rapidly with height, with an approximate drop of 1 mb for every 10 meters increase in elevation. The decrease in pressure varies, as seen in Table 9.1:
| Level | Pressure (mb) | Temperature (°C) |
|---|---|---|
| Sea Level | 1,013.25 | 15.2 |
| 1 km | 898.76 | 8.7 |
| 5 km | 540.48 | -17.3 |
| 10 km | 265.00 | -49.7 |
Although the vertical pressure gradient is large, the gravitational force balances it, preventing strong upward winds.
Horizontal pressure variations influence wind direction and velocity. Isobars (lines connecting locations with equal pressure) on weather maps help visualize pressure systems. Low-pressure systems are enclosed by isobars with the lowest pressure at the center, while high-pressure systems have the highest pressure at the center. These systems shift with the sun’s movement, oscillating between hemispheres during different seasons.
Wind is a result of differences in atmospheric pressure. The combined effects of the pressure gradient force, Coriolis force, and frictional force determine wind movement.
In low-pressure areas, the wind circulates counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. This is reversed in high-pressure areas.
Geostrophic winds occur when the pressure gradient force and the Coriolis force balance each other, causing wind to flow parallel to isobars. Table 9.2 shows wind patterns in cyclones and anticyclones:
| System | Pressure Condition | Northern Hemisphere Direction | Southern Hemisphere Direction |
|---|---|---|---|
| Cyclone | Low pressure | Anticlockwise | Clockwise |
| Anticyclone | High pressure | Clockwise | Anticlockwise |
At the Earth’s surface, air converges over low-pressure areas and rises, while in high-pressure areas, air subsides and diverges.
Note: Coriolis force is zero at the equator, which prevents the formation of tropical cyclones near the equator.
Planetary wind patterns are driven by the following:
The general atmospheric circulation sets in motion the ocean’s currents, which significantly impact global climates.
Three primary wind cells determine the planet’s general circulation:
The interaction between these cells redistributes heat and moisture, maintaining Earth’s temperature balance.
Oceans play a critical role in atmospheric circulation by influencing the transfer of energy and moisture into the air. One significant phenomenon is the El Niño, which involves the movement of warm water across the Pacific Ocean, replacing the cooler Peruvian current. This disrupts typical weather patterns, causing extreme weather events across the globe.
Important Note: ENSO (El Niño-Southern Oscillation) is a combined phenomenon involving changes in Pacific Ocean temperatures and atmospheric pressures, leading to global weather variations.
Seasonal shifts in temperature and pressure modify wind patterns. The monsoon is the most well-known seasonal wind system, particularly in Southeast Asia. Monsoons are caused by the seasonal movement of pressure belts and maximum heating regions.
Local winds arise due to variations in the heating and cooling of Earth’s surfaces:
When air remains over a homogeneous region for a long period, it acquires that region’s characteristics, forming air masses. These air masses are classified based on their source regions:
When two air masses meet, they form a front. The interaction of warm and cold air masses at these fronts leads to weather changes, such as precipitation and storms.
Extra-tropical cyclones develop along the polar front, where cold polar air meets warmer air from the subtropics. These cyclones often have well-defined fronts and cause significant weather changes in mid and high latitudes. The interaction of warm fronts and cold fronts generates clouds and precipitation.
Note: Extra-tropical cyclones cover large areas and can form over both land and sea, unlike tropical cyclones, which form exclusively over oceans.
Tropical cyclones are violent storms that form over tropical oceans. These systems bring heavy rainfall, strong winds, and storm surges, causing widespread destruction. Conditions for tropical cyclone formation include:
Cyclones intensify through the release of energy from condensation in rising moist air. When a cyclone makes landfall, it loses strength due to the cut-off of moisture supply. Landfall is the point where the cyclone crosses the coast.
| Characteristics | Tropical Cyclones | Extra-Tropical Cyclones |
|---|---|---|
| Formation Area | Tropical Oceans | Mid and High Latitudes |
| Size | Smaller (600-1,200 km) | Larger |
| Wind Velocity | High (up to 250 km/h) | Moderate |
| Lifespan | Shorter (intensifies over the sea) | Longer |
**Important Note
**: The *eye* of the cyclone is a calm region, surrounded by spiraling winds in the eye wall, where wind speeds are the highest.
Thunderstorms are caused by intense convection on hot, moist days, forming cumulonimbus clouds that produce thunder, lightning, and sometimes hail. Tornadoes, which occur mainly in mid-latitudes, are violent, spiraling winds with extremely low pressure at their center. Tornadoes over the sea are called waterspouts.
Multiple Choice Question
If the surface air pressure is 1,000 mb, the air pressure at 1 km above the surface will be:
a) 700 mb
b) 1,100 mb
c) 900 mb (Correct Answer)
d) 1,300 mb