Britain faces its fourth heatwave of the year next week as a heat health alert is issued and the extremely dry weather continues with no rain for 36 consecutive days.The Met Office expects temperatures to reach at least 28C in south-east England between next Tuesday and Thursday – meeting the criteria for another heatwave.The third heatwave of the year ended last weekend in London after a fortnight, but has continued for 19 days so far at Exeter in Devon and Usk in Monmouthshire.Temperatures could reach 30C in some areas today and tomorrow, prompting the UK Health Security Agency (UKHSA) to issue six 48-hour yellow heat health alerts.These will cover regions across the South and the Midlands from 9am today until 9am on Sunday and warn of 'increased use of healthcare services by vulnerable people'.Officials said there is a 'greater risk to life of vulnerable people' while 'water‑related incidents may increase, including risks from cold‑water shock and drowning'.The Met Office defines an official heatwave as three days in a row of temperatures above a certain threshold, which varies by UK region but is 28C in London.The heatwave temperature threshold will be met for only two days today and tomorrow, before conditions turn much fresher across the country by Sunday. An aerial view of the drought sun-scorched fairways and rough which surround the watered 15th green at the Bridport and West Dorset Golf Club at West Bay in Dorset yesterday Scorched grass at Blackheath in south-east London yesterday as the dry weather continues Dried grass at Peckham Common in south London yesterday after a prolonged lack of rainfall Holiday traffic at the Port of Dover in Kent today as drivers make their way across the Channel The Met Office forecast for London shows the heatwave criteria of temperatures at least 28C for three consecutive days could be met between Tuesday and Thursday next weekRain is also possible in very dry areas – with Wisley in Surrey now having had 36 consecutive days with no rain. London has had no rain for 27 days since June 27.But while heavy rain could fall in the North of England, the areas most in need of it in the South and Wales are set to see just a few millimetres at most and possibly none.Temperatures will get to 27C on Monday, but the fourth heatwave is then set to start from Tuesday with 28C highs in London before 30C on Wednesday and Thursday.An official drought has now been declared in North Wales and the Upper Severn, after record dry conditions, heatwaves, wildfires and dwindling rivers. This is the first part of the UK to declare drought status amid a summer of record-breaking heatwaves.Many parts of England are in prolonged dry weather status as the country has received just 3 per cent of its average July rainfall, with hosepipe bans in place for 23 million customers and suppliers warning they may stay in place all summer.Wildfires have been raging in several places in England, while Scotland has also been hit by a major wildfire in the Cairngorms, and in the Republic of Ireland, a nationwide hosepipe ban is in place in response to the dry conditions.The South-East has seen the longest no-rainfall spell for more than 30 years. Three heatwaves have brought the UK's hottest ever days in May and June, while the latest one earlier this month saw 12 consecutive days of temperatures above 30C.Met Office forecaster Craig Snell said rain will move southwards tomorrow, spreading from Scotland and Northern Ireland into northern England and parts of Wales.