Lecture 2:Heatwaves

• When a pattern of extreme weather persists for some time, such as a season, it may be classed as an extreme climate event, especially if it yields an average or total that is itself extreme (e.g., drought or heavy rainfall over a season).
• Heatwave


  A period of abnormally hot weather with respect to some threshold value, but there is no universally adopted scientific definition and a 'heatwave' in one country (or region) may be a less remarkable event in an other.
• Heat index


  A measure of the 'how it feels' or the 'apparent' temperature that factors in relative humidity.
• UK heatwave definition


  A UK heatwave occurs when a location records a period of 3 or more consecutive days with daily maximum temperatures equal to or exceeding the heatwave temperature threshold. Thresholds vary with region and change over time (~every 10 years) to reflect UK's changing climate.
• Atmospheric pressure


  • Air pressure (force per unit area) arises from the weight of the atmosphere. Pressure is larger closer to the Earth's surface and reduces with increasing height. Horizontal pressure gradients are small relative to vertical pressure gradients but are important as they drive winds.
• High pressure systems (anticyclones)


  Involve air descending forming high pressure area at the surface. Associated with settled conditions (sunny and light winds in summer) and clear skies.
• Low pressure systems (depressions)


  Involve air ascending forming an area of low pressure at the surface. Generally associated with unsettled weather (cloudy and wet).
• Heat dome


  Descending air in a persistent high pressure system, with stagnant conditions and light winds, may trap heat close to the surface.
• Urban Heat Island effect


  Local factors & micro-climate of a region are important. Tarmac and stone absorbs and stores heat during the daytime then emits heat at night, influencing heatwave occurrence and exacerbating heatwave effects.
• Heatwave characteristics of interest

  • Intensity (e.g. max T, 'tropical nights' etc.)
  • Duration
  • Frequency
  • Areal extent
• Heat-related illness and death

  • A short-term increase in mortality is generally observed during heatwaves. Extreme heat impairs human ability to regulate temperature ('silent killer'). Adverse affects on heart, lung and kidney function. Strain on health services.
• The 2003 European heatwave caused around 14,000 deaths in France (~60% of which over 75).
• Impacts on critical infrastructure


  • Conductors sagging and transformers overheating led to close to 8000 properties in Yorkshire, Lincolnshire and the North East without power in July 2022. Hot steel expands and this increases the risk of buckled tracks and derailment. Emergency speed restrictions and train cancellations were in place in July 2022.
• Why do we care about heatwaves?

  • Heat-related illness and death
  • Pressure on services
  • Heat-related economic impacts
  • Water/energy demand
  • Critical infrastructure impacts
  • Ecological impacts (including droughts, forest fires)
  • Co-occurrence of heatwaves and air pollution episodes
  • Vulnerable groups most at risk
• High risk groups


  Very old and very young, Chronic and severe illness, Inability to adapt behaviour to keep cool, Environmental factors and overexposure
• Heat-health alert system (UK)


  Provided by UKHSA & operates from June to September in the UK (in place since 2004). Forewarns of upcoming hot weather based on Met Office forecasts and data. There are 5 heatwave plans alert levels reflecting degrees of preparedness and action needed.
• Heatwave mitigation


  • Countries need a heatwave response plan
  • Coordinated action to educate & alert public + health providers
  • Identify & protect most at risk groups (including those 'hard to reach')
  • Healthcare capacity (not just winter!)
  • Protect critical infrastructure (airports, roads, rail, power)
  • Address urban heat islands
  • Address problematic buildings (e.g. those poorly ventilated, including hospitals, care and nursing homes)
  • Promote water efficiency and a water-saving culture
  • Optimise forecasting capability
• The frequency and intensity of hot extremes (including heatwaves) have increased, and those of cold extremes have decreased on the global scale since 1950 (virtually certain).
• Human-induced greenhouse gas forcing is the main driver of the observed changes in hot and cold extremes on the global scale (virtually certain).
• What is a heatwave?

  A period of abnormally hot weather