Mid-Latitude Weather Hazards and Drought

While tropical storms dominate media coverage, mid-latitude weather systems and drought are significant hazards affecting millions of people. This lesson examines depressions, anticyclones, jet stream behaviour, drought classification, wildfire, and the Sahel as a case study of long-term climate variability.

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Mid-Latitude Depressions

Formation

Mid-latitude depressions (also called extra-tropical cyclones) form along the polar front — the boundary between cold polar air and warm tropical air. They are driven by the interaction between contrasting air masses:

1. Warm, moist tropical air meets cold, dense polar air along the polar front
2. A wave develops along the front, with warm air rising over the cold air
3. The developing low-pressure system deepens as the warm front (leading edge of warm air) and cold front (leading edge of cold air) extend from the centre
4. The cold front moves faster, eventually overtaking the warm front to form an occluded front
5. The system eventually fills and decays

Associated Hazards

• Heavy rainfall — especially at fronts and in orographic uplift areas; can cause river flooding (e.g., Storm Desmond, December 2015 — 341 mm of rainfall in 24 hours at Honister Pass, Cumbria; widespread flooding across northern England)
• High winds — the tighter the pressure gradient, the stronger the winds; the Great Storm of October 1987 brought gusts of 160 km/h to south-east England, killing 18 people
• Storm surges — deep low-pressure centres can raise sea levels by 30–60 cm; combined with high tides and onshore winds, this can cause significant coastal flooding (e.g., the 1953 North Sea storm surge killed over 300 people in England and 1,800 in the Netherlands)

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Anticyclones and Extreme Weather

Blocking Anticyclones

• Large, slow-moving high-pressure systems that can persist for days or weeks
• They block the normal west-to-east progression of depressions, diverting them north or south
• In summer: clear skies, high temperatures, drought conditions, increased wildfire risk, heat-related mortality
• In winter: clear skies, very cold temperatures (especially at night due to radiation cooling), frost, fog, poor air quality (temperature inversions trap pollutants)

The European Heatwave (2003)

• A persistent blocking anticyclone brought record temperatures across Western Europe
• France recorded 44.1 °C; temperatures exceeded 40 °C for over a week
• An estimated 70,000 excess deaths across Europe (15,000 in France alone), predominantly among the elderly
• Highlighted the health risks of extreme heat in countries without widespread air conditioning or adequate heat action plans

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The Jet Stream

The polar jet stream is a narrow ribbon of fast-flowing air (150–300 km/h) in the upper troposphere (~10 km altitude) that broadly follows the polar front. Its position and behaviour strongly influence mid-latitude weather:

• A strong, zonal (west-to-east) jet stream keeps weather patterns moving and limits prolonged extremes
• A weak, meridional (wavy) jet stream creates large meanders (Rossby waves) that can become stationary, locking weather patterns in place for extended periods
• Blocking patterns are associated with persistent ridges (high pressure) and troughs (low pressure) in the jet stream

Climate Change and the Jet Stream