Overview
This lecture introduces the jet stream and its critical role in controlling mid-latitude weather patterns, air mass movements, and the formation of high and low-pressure systems.
Jet Stream Overview
- The jet stream is a fast-moving river or tube of air in the upper atmosphere found between major atmospheric circulation cells.
- It separates air masses, preventing mixing of cold and warm air unless the jet stream shifts.
- The jet stream influences weather events by controlling the movement and formation of weather systems, especially in mid-latitudes.
Atmospheric Circulation and Location
- Major circulation cells: Hadley, Ferrel, and Polar cells.
- Jet streams are found between these cells, where temperature gradients are strongest.
- Subtropical jet stream: between the Hadley and Ferrel cells (~30°N).
- Polar jet stream: between the Ferrel and Polar cells (~60°N).
Characteristics and Effects
- Jet streams have wind speeds over 50 knots and can exceed 200 knots.
- Their altitude is typically at pressures of 200–300 millibars (~40,000 feet).
- The jet stream moves seasonally as the sun’s position changes and is depicted as wavy lines on weather maps.
- They carry airborne particles, such as volcanic ash, over long distances.
Jet Stream Dynamics
- The jet stream dips (troughs) and rises (ridges) due to temperature and pressure differences.
- Troughs (cooler, denser air): lower altitude, promote rising air and inclement weather.
- Ridges (warmer, less dense air): higher altitude, promote sinking air and fair weather.
- Upward motion (rising jet stream): causes low pressure at the surface; downward motion: causes high pressure.
Wind, Pressure, and Weather Fronts
- Wind flows from high to low pressure, affected by the pressure gradient and Coriolis effect (deflects wind to the right in the Northern Hemisphere).
- Wind around high pressure: spirals outward clockwise; around low pressure: spirals inward counterclockwise.
- Geostrophic wind results when pressure gradient and Coriolis effect balance.
- Cold fronts: advancing cold air lifts warm air, causing rapid temperature drops and thunderstorms.
- Warm fronts: advancing warm air rises over retreating cold air, producing clouds and rain.
- Stationary fronts: no significant air movement, can later become warm or cold fronts.
Key Terms & Definitions
- Jet Stream — Fast, narrow band of wind in the upper atmosphere separating air masses.
- Hadley/Ferrel/Polar Cell — Major atmospheric circulation cells that distribute heat.
- Trough — Downward dip in the jet stream associated with cooler air and rising motion.
- Ridge — Upward bulge in the jet stream associated with warmer air and sinking motion.
- Pressure Gradient — Difference in atmospheric pressure that drives wind.
- Coriolis Effect — Deflection of moving air due to Earth's rotation.
- Cold Front — Boundary where advancing cold air undercuts warm air.
- Warm Front — Boundary where advancing warm air rises over cold air.
- Stationary Front — Boundary with little to no movement between differing air masses.
Action Items / Next Steps
- Review key atmospheric processes for understanding climate change and upcoming modules on inclement weather.