Basics of Weather :
Seasons & Circulations
Understanding Earth's Tilt and Its Impact on Weather
Introduction: The Dance of the Earth and the Sun
Have you ever wondered why we experience different seasons, or why weather patterns shift throughout the year? The answers lie in the complex interplay between Earth’s axial tilt and atmospheric circulations. These two factors together dictate the rhythms of our planet’s weather, influencing everything from the warmth of summer days to the chill of winter nights.
In this guide, we’ll explore the science behind seasons and circulations, demystifying how Earth’s tilt creates seasons and how large-scale atmospheric circulations shape our weather in the troposphere. This knowledge not only enhances our appreciation of the natural world but also deepens our understanding of the forces that drive our planet’s climate.
1. The Mechanics of Seasons: Earth's Tilt and Orbit
1.1 Earth's Tilt: The Reason for the Seasons
1.1.1 What is Earth's Axial Tilt?
Earth’s axis is tilted at an angle of approximately 23.5 degrees relative to its orbit around the Sun. This tilt, known as axial tilt or obliquity, is the primary reason we experience seasons. If Earth’s axis were not tilted, we would not have the distinct seasons we know today; instead, the equator would consistently receive the most direct sunlight year-round, leading to minimal seasonal variation.
1.1.2 How Tilt Creates Seasons
As Earth orbits the Sun over the course of a year, different parts of the planet receive varying amounts of sunlight. During summer in the Northern Hemisphere, the North Pole is tilted toward the Sun, resulting in longer days and more direct sunlight. Conversely, during winter, the North Pole is tilted away from the Sun, leading to shorter days and less direct sunlight. The opposite occurs in the Southern Hemisphere, where the seasons are reversed.
1.2 Earth's Orbit: The Eccentricity Effect
1.2.1 The Shape of Earth's Orbit
While Earth’s tilt is the primary driver of seasons, the shape of Earth’s orbit also plays a role. Earth’s orbit around the Sun is not a perfect circle; it is slightly elliptical, meaning that the distance between Earth and the Sun varies throughout the year. However, this variation, known as orbital eccentricity, has a relatively minor effect on seasonal temperature changes compared to axial tilt.
1.2.2 Perihelion and Aphelion
When asked about when Earth is closest to the Sun, many children may say “in the summer”, because logically, it makes sense to them. However, the reality is more complicated.
At perihelion (around early January – winter for the Northern Hemisphere), Earth is closest to the Sun, while at aphelion (around early July), it is farthest from the Sun. Despite these differences in distance, the variation in solar energy received by Earth is minimal, so it does not significantly influence the seasons. However, it can slightly modulate the intensity of seasons, making Northern Hemisphere winters slightly milder and summers slightly cooler compared to the Southern Hemisphere.
2. The Four Seasons: A Closer Look
2.1 Spring: The Season of Rebirth
2.1.1 Vernal Equinox: Equal Day and Night
Spring begins with the vernal equinox, around March 21st, when day and night are nearly equal in length. During this time, the Sun is directly over the equator, and both hemispheres receive roughly equal amounts of sunlight. This balance of sunlight marks the transition from winter to spring, leading to warmer temperatures and the awakening of plant life in the Northern Hemisphere.
2.1.2 Weather Patterns in Spring
Increasing temperatures, longer days, and the melting of winter snow characterize spring. It is also a season of volatile weather, as the warming land interacts with lingering cold air masses, often leading to thunderstorms, especially in temperate regions. The increased solar energy also drives stronger atmospheric circulations, contributing to the development of spring storms and tornadoes in some areas.
2.2 Summer: The Season of Sunlight
2.2.1 Summer Solstice: The Longest Day
The summer solstice, occurring around June 21st, marks the longest day of the year in the Northern Hemisphere. During this time, the North Pole is tilted closest to the Sun, and the Sun’s rays strike the Earth at their most direct angle. This results in the warmest temperatures of the year, as the Northern Hemisphere experiences its peak solar energy input.
2.2.2 Weather Patterns in Summer
Summer is synonymous with hot temperatures, clear skies, and, in many regions, dry conditions. However, it can also be a time of extreme weather, such as heatwaves, droughts, and severe thunderstorms. The increased solar heating during summer fuels the development of strong convection currents, leading to powerful thunderstorms and, in some cases, tropical cyclones (hurricanes) in coastal regions.
2.3 Autumn: The Season of Transition
2.3.1 Autumnal Equinox: The Balance of Light
Autumn begins with the autumnal equinox, around September 21st, when day and night are again nearly equal. The Sun is once more directly over the equator, signaling the gradual return of shorter days and cooler temperatures. This period of transition is marked by the changing color of leaves and the preparation of flora and fauna for the coming winter.
2.3.2 Weather Patterns in Autumn
Autumn is a time of cooling temperatures and declining solar energy. The reduction in sunlight leads to a gradual cooling of the land and atmosphere, which can cause fog, frost, and the first snowfalls in some regions. Autumn also brings increased storm activity, particularly in temperate regions, as warm and cold air masses clash.
2.4 Winter: The Season of Cold
2.4.1 Winter Solstice: The Shortest Day
The winter solstice, occurring around December 21st, marks the shortest day of the year in the Northern Hemisphere. During this time, the North Pole is tilted furthest from the Sun, resulting in the least direct sunlight and the coldest temperatures of the year.
2.4.2 Weather Patterns in Winter
Winter is best known for cold temperatures, snow, and ice in many regions. The lack of solar energy leads to prolonged periods of cold, particularly in high-latitude areas. Winter storms, such as blizzards and ice storms, are common during this season, driven by the interaction of cold polar air with warmer, humid air masses.
3. Global Circulations: The Engine of Earth's Weather
To understand how atmospheric circulations affect weather, it’s essential to understand the three-cell model of atmospheric circulation, which divides each hemisphere into three main circulation cells: the Hadley cell, the Ferrel cell, and the Polar cell.
3.1 The Hadley Cell: Tropical Heat Engine
3.1.1 What is the Hadley Cell?
The Hadley Cell is a large-scale atmospheric circulation pattern that occurs between the equator and about 30 degrees latitude in both hemispheres. It is driven by the intense solar heating at the equator, which causes warm air to rise, creating a region of low pressure known as the Intertropical Convergence Zone (ITCZ) — known for heavy rainfall and thunderstorms. This warm rising air moves poleward at high altitudes, cools, and descends around 30° latitude, forming high-pressure zones that are associated with the world’s deserts.
3.1.2 How the Hadley Cell Affects Weather
As warm air rises at the equator, it cools and spreads out toward the poles. Around 30 degrees latitude, the air descends, creating regions of high pressure known as the subtropical highs. This circulation pattern is responsible for the trade winds, tropical rain belts, and the arid conditions found in many subtropical deserts.
3.2 The Ferrel Cell: Mid-Latitude Circulation
3.1.1 What is the Ferrel Cell?
The Ferrel Cell is located between 30 and 60 degrees latitude in both hemispheres. It acts as a bridge between the tropical Hadley Cell and the polar regions. Unlike the Hadley and Polar Cells, the Ferrel Cell is not driven directly by solar heating but by the interactions between the nearby cells.
3.1.2 How the Ferrel Cell Affects Weather
The Ferrel Cell is responsible for the westerly winds that dominate the mid-latitudes. These winds drive weather systems from west to east, influencing the weather patterns experienced in regions such as North America and Europe. The Ferrel Cell also plays a role in the formation of mid-latitude cyclones, which are a major driver of weather variability in these regions.
3.3 The Polar Cell: The Arctic and Antarctic Circulation
3.3.1 What is the Polar Cell?
The Polar Cell is the smallest and weakest of the three atmospheric circulation cells. It extends from about 60 degrees latitude to the poles. In this cell, cold air descends at the poles, creating high-pressure areas, and then flows equatorward at the surface.
3.3.2 How the Polar Cell Affects Weather
Cold (and often dry) air flows equatorward at the surface, where it meets the warmer air from the Ferrel cell, creating a boundary known as the polar front. This front is a zone of frequent storm activity, particularly in the winter months. The interaction between the Polar and Ferrel Cells is crucial in driving the weather patterns experienced at higher latitudes.
4. The Jet Streams: Rivers of Wind
4.1 What are Jet Streams?
Jet streams are fast-flowing, narrow air currents, often moving from west to east. Winds in a jet stream can reach speeds of up to 200 miles per hour and play a significant role in shaping weather patterns by steering storm systems and influencing temperature distributions. Formed by the temperature contrasts between different air masses, jet streams are found near the boundaries of the Hadley, Ferrel, and Polar cells.
4.2 How Jet Streams Influence Weather
4.2.1 The Polar Jet Stream: A Winter Powerhouse
The polar jet stream, found at the boundary between the Ferrel and Polar cells, is particularly strong in winter when the temperature contrast between the cold polar air and the warmer mid-latitude air is greatest. This jet stream can bring cold Arctic air down into the mid-latitudes, leading to significant temperature drops and winter storms.
4.2.2 The Subtropical Jet Stream: A Weather Divider
The subtropical jet stream, located at the boundary of the Hadley and Ferrel cells, is generally weaker but still influential. It often acts as a boundary between tropical and temperate weather systems, influencing the paths of storms and the distribution of precipitation.
4.3 How Atmospheric Circulations Affect Weather
4.3.1 Tropical Rain Belts and Deserts
The Hadley cell’s circulation is responsible for the positioning of tropical rain belts and deserts. The ITCZ, where the air rises and cools, is a zone of heavy rainfall and is responsible for the tropical rainforests near the equator. In contrast, the descending air at around 30° latitude creates arid conditions, leading to the formation of deserts like the Sahara and the Arabian Desert.
4.3.2 Mid-Latitude Storms
In the mid-latitudes, the interaction between the Ferrel and Polar cells creates a region of frequent storms, particularly in winter. These storms are often driven by the polar jet stream, which can bring cold Arctic air into contact with warmer air from the tropics, leading to the development of powerful storm systems.
4.3.3 Polar Vortex and Extreme Cold Events
Americans are becoming more familiar with this last event type, as tv meteorologists / broadcasters mention the term “Polar Vortex” each winter. The Polar cell’s influence extends to the polar vortex, a large area of low pressure and cold air surrounding the poles. Occasionally, the polar vortex can weaken and allow frigid air to spill southward into the mid-latitudes, leading to extreme cold events and severe winter weather.
5. The Interaction Between Seasons and Circulations
5.1 Seasonal Shifts in Circulation Patterns
As the Earth orbits the Sun and the angle of sunlight changes, the positions of the ITCZ, jet streams, and other circulation features shift accordingly. These shifts lead to changes in weather patterns, such as the onset of monsoons, the migration of storm tracks, and the development of seasonal droughts and floods.
5.2 Monsoons: Seasonal Winds and Rain
Monsoons are a prime example of how seasonal shifts in circulation patterns can have dramatic effects on weather. Driven by the differential heating of land and sea, monsoons bring seasonal winds and heavy rains to regions like South Asia and West Africa. These rains are crucial for agriculture but can also lead to devastating floods.
5.3 El Niño and La Niña: The Pacific's Influence on Global Weather
El Niño and La Niña (ENSO – El Nino Southern Oscillation) are phenomena associated with variations in sea surface temperatures in the central and eastern Pacific Ocean. These events can disrupt normal circulation patterns, leading to significant weather changes across the globe, especially in the winter months.
El Niño typically brings warmer and wetter conditions to the eastern Pacific, while La Niña is associated with cooler and drier conditions. Of course, this is highly simplified – in the United States alone there are many small geographical areas that may experience generally higher or lower temperatures and precipitation depending on the ENSO oscillation’s current configuration (strong El Nino, weak El Nino, neutral, weak La Nina, strong La Nina).
There are other global circulations at play on the Earth, but El Nino remains the most discussed by non-meteorologists and the most studied by climatologists and weather scientists. For example, the North Pacific Oscillation (NPO) affects the winter temperatures over most of North America, but we rarely see this term used by the media. For more information about other oscillations, please check out Woods Hole Oceanographic Institution’s excellent summary.
5.4 How Seasons and Circulations Together Influence Weather
The interplay between Earth’s tilt and atmospheric circulations creates the seasonal weather patterns we experience. For example, during the winter solstice, the polar jet stream tends to dip southward, bringing cold air and snow to regions that are typically mild during the summer months. Conversely, during the summer, the ITCZ shifts northward, bringing rain and thunderstorms to regions that were dry during the winter.
About Climate Change & Its Impact on Seasons & Atmospheric Circulations
As the climate changes, so too do the patterns of atmospheric circulation. Warming temperatures are expected to shift the positions of the jet streams, ITCZ, and other key circulation features, potentially leading to more extreme weather events, such as stronger storms, more intense heatwaves, and changes in precipitation patterns.
Climate change is also expected to alter the length and intensity of seasons. Winters may become shorter and milder, while summers may become longer and hotter. These changes will have significant impacts on agriculture, water resources, and ecosystems around the world.
Understanding how seasons and circulations interact is crucial for adapting to the impacts of climate change. Improving our knowledge of circulation patterns can help us better predict and mitigate the effects of these changes on our weather and climate systems. By studying the mechanisms behind seasons and circulations, we can develop more accurate climate models and prepare for the shifts in weather patterns that are already beginning to occur.