Typical weather situations in the mid-latitudes

050-08-03Pablo Asensio Martínez2026-04-012 min

In the mid-latitudes (between 30° and 60°), the climate is dominated by the movement of pressure systems and fronts, influenced by the jet stream and the westerlies.

Cold Air Drop (Cold Pool)

Cut-off Low CIRA Satellite animation of a Cut-off low (Cold Drop) drifting.

Also known as "Gota Fría" in Spanish contexts.

  • Definition: It is a depression (low-pressure system) that exists only in the upper troposphere. It consists of a pocket of air much colder than the surroundings at high levels, which has detached from the main polar air mass.
  • Formation: Occurs when cold air aloft separates from the polar region and moves towards lower latitudes (over warmer air).
  • Detection:
    • Visible on upper charts (300 hPa, 500 hPa) as an area of low contours (low pressure).
    • NOT visible on surface charts: At the surface, there may be high pressure or a flat pressure gradient, making detection difficult without analyzing upper layers.
  • Associated Weather:
    • Generates deep instability (very cold air over warm surface air = large vertical temperature gradient).
    • Causes strong convective activity: Cumulonimbus, severe thunderstorms, and heavy rains.
    • Activity is maximum on summer afternoons over land (diurnal heating + cold air aloft).
  • Prediction: Difficult to forecast. Its movement is erratic, and numerical models sometimes fail to predict its exact track.
  • Duration: Typically about 3 days, although they can persist for weeks if associated with static anticyclones (like the Siberian High).

Westerly Waves

These are undulations in the general westerly flow corresponding to the movement of polar front depressions and their associated fronts.

  • Movement: From West to East.
  • Location: Mid-latitudes (40° - 60°).
  • Weather: Very variable ("changeable") weather. Alternation between the passage of fronts (rain, wind, clouds) and high-pressure ridges (dry and sunny weather).

Polar Front Depressions

Low-pressure systems that form at the discontinuity between polar air and tropical air.

  • Location: Generally between 40°N and 65°N.
  • Seasonal Displacement:
    • Winter: They shift South (towards the equator), pushed by the expansion of polar air. They are more intense due to the greater temperature contrast.
    • Summer: They shift North (towards the pole). They are less intense.

Westerlies

Prevailing winds blowing from the subtropical high-pressure belts towards the subpolar lows.

  • Northern Hemisphere: Stronger and more constant in winter (frequent gales in the North Atlantic). In summer, they are weaker.
  • Southern Hemisphere: Known as the "Roaring Forties" between 40°S and 50°S. They are very strong and constant throughout the year due to the absence of large landmasses to slow the flow.