1. Global Atmospheric Heating and Circulation & Precipitation Patterns

Global Heating Patterns:

The sun’s radiation heats the Earth’s surface unevenly. The equator receives the most direct sunlight, leading to warmer temperatures. Poles receive sunlight at an angle, resulting in cooler temperatures.

Atmospheric Circulation:

This uneven heating drives global atmospheric circulation patterns:

• Hadley Cells (Tropical): Warm air rises near the equator, creating a low-pressure zone. This air cools and sinks at around 30° N and 30° S latitude, forming high-pressure zones (subtropical highs). The sinking air creates dry conditions. At the surface, winds blow from the highs towards the equator (trade winds). As the warm air rises again near the equator, it cools and condenses, leading to high precipitation.
• Ferrell Cells (Mid-Latitudes): Rising air in the subtropics creates a zone of high pressure. Prevailing winds blow westerly, towards the poles. These winds meet cooler polar air masses, creating areas of low pressure and frontal boundaries. Upward air movement along these fronts leads to condensation and precipitation in temperate latitudes.
• Polar Cells (Polar Regions): Cold, dense air at the poles sinks, creating high-pressure zones. Prevailing winds blow outwards towards the mid-latitudes. Due to the cold temperatures and minimal evaporation, precipitation is low in these regions.

Mechanisms for Precipitation:

• Tropical High Precipitation: As warm, moist air rises in the tropics (Hadley Cells), it cools and condenses, forming clouds and heavy precipitation.
• Temperate High Precipitation: In temperate regions, the meeting of warm and cold air masses (frontal boundaries) creates instability that leads to condensation and precipitation.
• Tropical Low Precipitation: Sinking air in the subtropics (Hadley Cells) warms and dries, inhibiting cloud formation and resulting in low precipitation (deserts).

2. Seasonality in Tropical Dry Forests and Savannas

Seasonal Sun Angle and Circulation:

The Earth’s tilt on its axis causes the sun’s direct rays to move north and south throughout the year. This significantly impacts atmospheric circulation patterns in tropical regions.

Tropical Dry Forests and Savannas:

These biomes experience a distinct wet and dry season due to the interplay of atmospheric circulation and the sun’s angle.

• Wet Season: When the sun is directly overhead, increased solar radiation intensifies heating and evaporation. Rising air creates low pressure, attracting moist winds and leading to heavy precipitation.
• Dry Season: As the sun’s angle shifts away, the region falls under the influence of the subtropical high-pressure zone. Sinking air suppresses precipitation, leading to a dry period.

Example:

• In the African Savanna, the wet season coincides with the sun being directly overhead (roughly May to September). This brings heavy rainfall.
• During the dry season (roughly November to March), the sun’s angle shifts, and the region experiences dry conditions due to the influence of the subtropical high pressure.

Summary: The seasonal migration of the sun’s direct rays and its impact on atmospheric circulation patterns create the characteristic wet and dry seasons in tropical dry forests and savannas.