A heat wave is a prolonged period of excessively hot weather. In England and Wales, heatwave conditions are reached when the maximum daytime temperature and minimum night-time temperature rise above the threshold for a particular region. (The length of time spent above the threshold determines the level. Level 1: Normal summer conditions. Level 2: 60% or higher risk that the temperature will be above the threshold levels for two days and the intervening night. Level 3: Temperature above the threshold for the preceding day and night, with a 90% or higher chance that it will stay above the threshold in the following day. Level 4: Anything above the other three levels. Each of the first three levels is associated with a particular state of readiness and response by the social and health services, and Level 4 is associated with more widespread response. The UK is currently set at Level 3.) http://www.metoffice.gov.uk/public/weather/heat-health/