Upper Level Winds

Wind velocity in the upper atmosphere is inversely proportional to air density.
Wind speeds increase with height up to the tropopause due to decreasing density.
Wind velocity is directly proportional to the Pressure Gradient Force (PGF).
At a given altitude, lower pressure is observed in cold air masses compared to warm air masses.
Stronger temperature differences create a stronger Pressure Gradient Force.
High wind speeds are expected in regions with significant temperature contrasts due to stronger PGF.

Contour Charts are maps showing lines of equal pressure or geo-potential height (Isohypses).
Closely spaced isohypses indicate strong temperature and pressure gradients, resulting in higher wind speeds.
A 300 mb chart shows contours of equal height where pressure equals 300 mb.
In cold air, the 300 mb level occurs at lower heights, indicating lower geo-potential height.
In warm air, the 300 mb level occurs at higher heights, indicating higher geo-potential height.

Extended Buys Ballot’s Law describes wind direction in the upper atmosphere.
In the Northern Hemisphere, with your back to the upper-level wind, cold air is to your left.
In the Southern Hemisphere, with your back to the upper-level wind, cold air is to your right.

Polar regions are colder than equatorial regions, creating strong temperature gradients.
As a result, westerly winds dominate in both hemispheres.
In the Northern Hemisphere, cold air lies to the left of the flow.
In the Southern Hemisphere, cold air lies to the right of the flow.
Typical wind speeds:
- ~20 knots at 20° latitude
- ~40 knots at 60° latitude

The thermal equator shifts northward during Northern Hemisphere summer.
At times, 23.5°N becomes warmer than the equator due to land heating.
Easterly winds are observed in both hemispheres in equatorial regions during summer conditions.
These occur due to the reversed temperature gradient near the equator.

Jet Streams are narrow bands of strong winds in the upper troposphere.
Wind speeds exceed 60 knots.
They are caused by strong horizontal temperature gradients and result in wind shear.
Typical dimensions:
- Length: up to 1500 km
- Width: ~200 km
- Vertical depth: ~1200 ft
The axis is the central line of maximum wind speed.
The core is the region surrounding the axis with the strongest winds (110–180 knots or more).

Jet streams are associated with frontal zones due to strong temperature contrasts.
They are identified on upper air charts where wind speeds exceed 60 knots.
Jet streams typically occur just below the tropopause.
They are prominent near the fringes of weather systems.
They are also intensified on the leeward side of mountain ranges.
Mountain waves and jet streams together can cause severe Clear Air Turbulence (CAT), often indicated by lenticular clouds.

The Subtropical Jet Stream is a strong westerly jet found near 30° latitude.
It occurs at about 9–12 km altitude (~200 hPa level).
It forms at the boundary between the Hadley and Ferrel cells.
Winter:
- Strong (100–200 knots)
- Located near ~27°N
- Lower altitude due to lower tropopause
Summer:
- Weaker
- Shifted toward ~35°N
Can intensify up to 400 knots south of the Tibetan Plateau due to strong thermal contrast between land and ocean.

Influences India mainly from October to March, strongest around December–January.
Mean position: ~27°N at ~12 km altitude.
Average wind speed: ~100 knots; peaks up to 200 knots.
Between October–May: typically 60–70 knots.
January peak: 100–120 knots.
In February, it shifts south to ~22°N with ~100 knots speed.
It may split near foothills and rejoin over China.
Strong vertical and horizontal wind shear occurs around the jet core.
Western disturbances can intensify it to 130–150 knots.

Arctic jets occur over the Arctic front during winter.
They form at the boundary between very cold Arctic air and slightly warmer polar air.
Altitude is lower (~7–8 km) due to lower tropopause height.

The Tropical Easterly Jet is a strong easterly wind in the upper troposphere during summer.
It forms due to northward shift of the thermal equator and heating of the Tibetan Plateau.
Occurs mainly south of ~25° latitude at ~100 hPa (~16 km / 50,000 ft).
Found over Asia and Africa, not typically over oceans.
Wind speeds range from 80–100 knots, sometimes reaching 150 knots.
Most prominent over peninsular India (June–August).

Low-Level Jets: Occur at 1–3 km altitude, especially in subtropical regions.
Over India, they strengthen the Southwest Monsoon, particularly along the Somali coast.
Stratospheric Jets: Occur around 20 km in polar regions.
They are westerly in winter and easterly in summer.

In the Northern Hemisphere, cold air lies to the left when facing the jet stream.
In the Southern Hemisphere, cold air lies to the right.
Cloud patterns:
- Roll-type clouds form on the warm side.
- Cirrus clouds form on the cold side near the tropopause.
A thin haze layer is often present on the warm side.
Clear Air Turbulence (CAT) occurs near jet boundaries due to strong wind shear.
CAT is strongest below the jet axis on the cold side.
Mountain wave interactions can intensify turbulence significantly.