Upper-Level Troughs and Ridges

• We plot upper-level maps with contours, which are curves that connect places with the same heights (isoheights / isohypse).

• Ridges = areas on upper-level maps with greater heights, representing warmer columns of air.

• Troughs = areas on upper-level maps with lower heights, representing cooler, shorter columns of air.

• Generally, colder, shorter columns are found in the polar regions and the warmer, taller columns are found in the tropical and subtropical regions.

• On upper-level maps, the air is accelerated from the region of greater heights toward the region of lower heights (from high to low pressure).

• Warm air tends to rise ahead of the trough axis, while cold air tends to sink behind the trough axis.

• The rising, warmer air may generate clouds and precipitation ahead of the trough axis, while the sinking, colder air may generate clearing skies behind the trough axis.

• The upper-atmosphere dictates what happens at the surface!

• Wind speed is inversely proportional to the spacing between the height contours (isobars).
• Close height contours = faster wind speeds.
• Far apart height contours = slower wind speeds.

• The upper-level flow is dominated by the location of very large amplitude, synoptic-scale disturbances (troughs and ridges), which generally change in intensity and position from day-to-day.

• Generally, ridges and troughs cancel each other out when averaged for a month, season, or year, providing a more zonal appearance!

http://mp1.met.psu.edu/~fxg1/HEMI500/5dayloop.html

Longwaves

(aka Rossby / Planetary Waves)

• Longwaves have wavelengths of thousands of kilometers and represent the large scale, global flow.

• Generally, move slowly from west to east, but may become stationary or retrogress slowly from east to west.


• In the northern hemisphere, we see 3-7 long waves with wavelengths of 50-120°, with the wave number changing over days or weeks, and through the long waves move the faster short waves.

• Influence the locations of large regions of warm versus cold temperatures, wet versus dry conditions, the position of the jet streams and storm tracks.

• Longwaves are barotropic (in which pressure depends only on density).

Advection

• Advection = horizontal movement of air.

• Warm Advection occurs when the wind blows across the gradient of temperature from higher toward lower temperature / thickness.

• Cold Advection occurs when wind blows across the gradient of temperature from lower to higher temperature / thickness.

Shortwaves

• Shortwaves tend to have wavelengths of less than 3,000 km.

• They move rapidly from west to east, around or through longwaves.

• Represent smaller pools of cold and warm air aloft.
• Shortwaves are baroclinic

• Pools of cold air aloft may lead to instability, outbreaks or rain/snow showers, or thunderstorms (in extreme cases).
• Significant "weather-makers"

• Dry channels ahead, moist channel behind.