Understanding the Differences Between Polar and Subtropical Jetstreams

Jetstreams are powerful, narrow bands of fast-moving air currents located high in the atmosphere. These streams circle the Earth from west to east, influencing weather patterns and climatic conditions across the globe. Among the various jetstreams, two principal types dominate: the Polar Jetstream and the Subtropical Jetstream. Understanding their differences is crucial for meteorologists, aviators, and anyone interested in weather dynamics. This article explores the characteristics, formation, and impacts of these two jetstreams.

What Are Jetstreams?

Jetstreams are strong air currents found near the tropopause—the boundary between the troposphere and the stratosphere—typically at altitudes of 9 to 16 kilometers (30,000 to 52,000 feet). They can reach speeds exceeding 400 kilometers per hour (250 miles per hour), although typical wind speeds are around 160-320 km/h (100-200 mph). These high-altitude winds flow predominantly from west to east due to the Earth’s rotation and temperature gradients between different atmospheric layers.

Jetstreams form as a result of atmospheric temperature contrasts—especially between cold polar air masses and warmer tropical air. The greater the temperature difference, the stronger the jetstream tends to be.

The Two Main Jetstreams: Polar vs. Subtropical

While there are several jetstreams around the world, two major ones stand out due to their strength and influence on weather:

1. Polar Jetstream
2. Subtropical Jetstream

Each has distinct features in terms of location, origin, wind speed, altitude, and impact.

Polar Jetstream

Location and Altitude

The polar jetstream forms near the polar front—the boundary between cold polar air and warmer mid-latitude air. It typically flows between latitudes 50°N and 60°N in the Northern Hemisphere and similarly near 50°S to 60°S in the Southern Hemisphere. Its altitude ranges from about 7 to 12 kilometers (23,000 to 39,000 feet), generally lower than the subtropical jetstream.

Formation Mechanism

The polar jetstream arises from sharp temperature contrasts between cold arctic or antarctic air masses and warmer temperate air masses. When these contrasting air masses collide at mid-latitudes, it creates a steep pressure gradient. Air moves swiftly in response to this gradient, leading to fast winds that form the jetstream.

The Coriolis effect—caused by Earth’s rotation—forces this airflow into a west-to-east direction.

Characteristics

• Stronger Winds: The polar jet is often faster due to more significant temperature gradients in winter.
• Variable Path: This jet is highly dynamic, exhibiting large meanders known as Rossby waves that cause dips (troughs) and rises (ridges).
• Weather Influence: The polar jet controls much of the day-to-day weather across mid-latitudes. It guides storm systems like cyclones and anticyclones.
• Seasonal Variation: It is stronger during winter when temperature differences are greatest and weaker during summer.

Impact on Weather

The polar jet plays a key role in bringing cold arctic air southward during troughs, which can cause severe winter storms or cold snaps in temperate zones. Its position can determine whether regions experience warm or cold weather periods.

For example:

• When the polar jet dips southward over North America or Europe, it can bring frigid polar air down into these regions.
• Conversely, when it shifts northward, warmer conditions prevail.

This oscillation helps drive many weather phenomena such as blizzards, heavy rains, and thunderstorms depending on local conditions.

Subtropical Jetstream

Location and Altitude

The subtropical jetstream forms closer to the equator than its polar counterpart. It typically resides between latitudes 25°N and 35°N in both hemispheres but can shift seasonally. Its altitude is higher than the polar jet’s, usually found around 10 to 16 kilometers (33,000 to 52,000 feet).

Formation Mechanism

The subtropical jet arises mainly from strong temperature gradients near the boundary between tropical and subtropical air masses. It is associated with the Hadley cell circulation—where warm air rises near the equator and sinks around 30° latitude.

The descending dry air in subtropics creates a zone of high pressure aloft called the subtropical ridge. Strong upper-level winds form along this ridge where pressure gradients exist between this zone and surrounding areas.

Additionally, conservation of angular momentum causes winds flowing poleward from the equator to accelerate eastward due to Earth’s spin.

Characteristics

• Generally Weaker Than Polar Jet: Although fast-moving at times, subtropical jets tend to have lower wind speeds on average.
• More Stable Path: The subtropical jet is less variable than the polar jet but still experiences seasonal shifts.
• Higher Altitude: Its position high in the atmosphere affects upper-level weather phenomena.
• Seasonal Strength: Often strongest during winter months when pressure differences increase.

Impact on Weather

The subtropical jet influences weather mostly in subtropical regions but can affect mid-latitude weather indirectly through interaction with other systems. It often steers storms developing over oceans or deserts.

Examples include:

• Guiding hurricane tracks or tropical cyclone development by providing upper-level wind shear.
• Affecting monsoon patterns by modulating moisture transport.
• Influencing dry or stable conditions under its ridge zone, contributing to desert climates.

Key Differences Between Polar and Subtropical Jetstreams

| Feature | Polar Jetstream | Subtropical Jetstream |
|————————–|—————————————|—————————————-|
| Typical Latitude | ~50°–60° (mid-high latitudes) | ~25°–35° (subtropics) |
| Altitude | ~7–12 km (lower troposphere) | ~10–16 km (upper troposphere) |
| Wind Speed | Generally stronger (up to 400+ km/h) | Generally weaker than polar |
| Temperature Gradient | Sharp gradient between cold polar & warm temperate air | Gradients between warm tropical and cooler subtropics |
| Seasonal Variation | Stronger in winter; highly variable | Also stronger in winter; more stable |
| Weather Influence | Directly affects mid-latitude weather; storm tracks | Affects subtropical climate; guides tropical systems |
| Variability | Highly meandering (Rossby waves) | Relatively stable flow |

Interaction Between Polar and Subtropical Jets

While both jets operate independently most of the time, they can interact under certain circumstances:

• In some cases, their paths may converge or overlap temporarily.
• This interaction can amplify storm systems or produce unusual weather patterns such as severe storms or blocking highs.
• For example, splitting or merging jets can create zones of enhanced upper-level divergence that aid storm development.

Importance of Studying Jetstreams

Understanding how these two main jets function provides insights into global climate behavior:

• Weather Forecasting: Meteorologists use knowledge of jetstreams to predict storm tracks and intensity.
• Aviation: Pilots rely on jetstream patterns for efficient routing—using tailwinds for faster travel or avoiding dangerous turbulence zones.
• Climate Research: Long-term changes in jetstream behavior can indicate shifts related to climate change such as Arctic warming altering polar jet strength.
• Disaster Preparedness: Recognizing jetstream anomalies helps prepare for extreme weather like floods or droughts.

Conclusion

The Polar and Subtropical Jetstreams are fundamental components of Earth’s atmospheric circulation system. Despite both being fast-moving westerly winds high above Earth’s surface, they differ significantly in latitude range, altitude, formation mechanisms, wind speed characteristics, and influence on weather patterns. The polar jet dominates mid-latitude weather with its powerful winds shaping storms that affect large populations. Meanwhile, the subtropical jet plays a vital role closer to the equator by governing upper-level wind patterns that influence tropical climates and monsoons.

By understanding these differences—as well as how these jets interact—we gain better tools for meteorological prediction and climate study. Continued research into their behavior will improve our ability to forecast extreme events and adapt to changing global climates.

References:

• Wallace, J.M., & Hobbs, P.V., Atmospheric Science: An Introductory Survey, Academic Press
• National Oceanic and Atmospheric Administration (NOAA)
• American Meteorological Society Glossary
• Holton, J.R., An Introduction to Dynamic Meteorology, Academic Press