On the record – observing a ‘heatwave’

7 07 2015

Last week on 1 July the UK saw its warmest July daily max temperature on record (records date back as far as 1853), with 36.7 °C at Heathrow. This has led to considerable interest in the wider context of the record temperatures. Here, Mark McCarthy, Manager of the Met Office National Climate Information Centre discusses records and how we record them.

Where were record temperatures observed on 1 July 2015?

Although Heathrow measured the highest temperature recorded by the Met Office observing network on a July day, record temperatures were reported across a wide stretch of the country, including from some of the Met Office’s very long running climate stations.

Temperatures exceeded 35 °C at a handful of locations in London and the south east, but also reached the low 30s across the Midlands, East Anglia and parts of north-west and north-east England. It is in these areas that July temperature records were broken.

Map showing stations recording new July temperature records, 1 July 2015

Map showing stations recording new July temperature records, 1 July 2015

The table below lists those stations with more than 50 years of observations for which 1 July 2015 was a record. These data show that record temperatures for July were not confined to London or other major urban centres. The records were, in fact, part of a larger scale pattern of high temperatures extending through Spain, Portugal and France.

SITE DATE OF PREV JULY RECORD PREV RECORD JULY MAX (°C) 1 JULY 2015 MAX (°C) YEARS OF DATA
Durham 31/7/1943, 10/7/1921 30.6 31 133
Sheffield 31/7/1943, 10/7/1921 31.7 33.3 130
Bradford 31/7/1943, 13/7/1935 30.6 30.9 106
Cranwell 22/7/1996 32.6 34.3 93
Sutton Bonnington 19/7/2006 32.9 33.6 84
Stonyhurst 3/7/1976 31.1 32.6 75
Manston 15/7/1983 31.4 33.6 74
Goudhurst 3/7/1976 32.8 33.3 74
Waddington 12/7/1949 32.2 33.1 67
Heathrow 19/7/2006 35.5 36.7 66
Nottingham (Watnall) 3/7/1976 32.3 33.9 64
Marham 3/7/1976, 5/7/1959 32.8 33.5 58
Wittering 5/7/1959 32.8 35.3 53
St James’s Park 5/7/1959 34.4 34.7 52

How does this compare to past heatwaves?

Temperatures over 36 °C reported at any station in the UK observing network are very rare, with only a handful of notable heatwaves seeing such extremes. The heatwaves of August 1990, August 2003, and July 2006 each saw a number of stations exceed 36 °C, whereas on 1 July 2015 Heathrow was the only station.

The Met Office maintains a list of climate extremes for the UK. It is standard practice to report the highest and lowest temperature every month as part of our routine monitoring of UK weather and climate. It is therefore always noteworthy when one of these records is broken.

While there is no doubt that some previous heatwaves have seen more extreme or more widespread high temperatures overall – particularly in the climatologically warmer period from late July into early August – 1 July 2015 has the honour of holding the highest recorded temperature for a July day with 36.7 °C at Heathrow.

How do you ensure the data are reliable?

To ensure consistency, Met Office weather records are only given for stations with standard instruments and exposure. This means that our records would not represent the extremes that may have occurred in places where we do not have standard instruments. This may have been the case on 1 July 2015, where the availability of additional data from amateur observers contributing to Met Office WOW show peak temperatures in the range 35 to 37 °C to the west London.

It is reasonable to ask whether Heathrow, as a major international airport, can provide a reliable climatological record. Are the observations biased by the presence of runways and air traffic?

The instrumentation and station enclosure are managed so that they meet the standards required by the Met Office and set out by the World Meteorological Organization. The site has been operating for 66 years and provides an excellent long observational series for west London.

The first thing we can do is compare the climatological temperatures with a nearby station at Kew Gardens. The average daytime maximum temperatures for the two sites are very close:

Site June July August
Heathrow 21.04 °C 23.54 °C 23.15 °C
Kew 21.02 °C 23.48 °C 23.15 °C

On 1 July the maximum temperature recorded at Heathrow (36.7 °C) was higher than Kew (35.7 °C). Modern instrumentation means we can look at the temperatures minute-by-minute at the two sites, as shown below. The two locations recorded very similar temperatures through most of the afternoon and the average temperature at the two sites between 12:00 and 18:00 GMT agree to within 0.02 °C. However, there was a peak in temperature at Heathrow between 14:00 and 14:30 GMT that was not seen at Kew Gardens. What could cause such a peak?

Temperature (°C) graph for Heathrow and Kew Gardens 1 July 2015

There were scattered clouds in the area that afternoon. Both Heathrow and Kew Gardens have instruments measuring solar radiation, shown in the graph below. Both sites recorded a general dip in solar radiation due to clouds from approximately 13:30 to 15:00 GMT which corresponds to a slight cooling at both sites. Heathrow saw a short gap in the clouds shortly after 14:00 GMT which resulted in a similarly short lived peak in temperature, while Kew Gardens remained cloudy. In turn Kew Gardens then saw a brief spell being sunnier than Heathrow just before 15:00 GMT and became warmer than Heathrow for about an hour.

Solar radiation (W/m2) graph for Heathrow and Kew Gardens 1 July 2015





Hotter weather for the start of July

25 06 2015

The weather is showing signs of heating up next week for the start of the Wimbledon fortnight.

In contrast to June so far, which has seen temperatures often near or just below normal, next week could see a real change in the way it feels – with hot days and humid nights, especially across the south.

It looks like heat will start to build across Iberia later this weekend and spread northwards across France early next week as a tropical continental airmass begins to dominate the weather. Temperatures across Iberia and southern France could reach the low 40’s Celsius by midweek with northern France seeing temperatures into the mid to high 30s Celsius.

Data: European Centre for Medium Range Weather Forecasts

Data: European Centre for Medium Range Weather Forecasts

The UK is likely to be near the boundary between this tropical continental airmass and a tropical maritime airmass over the Atlantic, but we do expect to see temperatures rise across the whole of the UK for the start of July.

Scotland could see highs in the low to mid 20’s (although it may be cloudy here at times), and highs across southern Britain are likely to reach the low 30’s Celsius with a small chance of values in the mid 30’s here.

It’s worth saying that there is some uncertainty about how much of the hot weather from the continent will reach us, and it may only last a couple of days before temperatures drop a little. As is traditional with hot weather in the UK in the summer it may end with thunderstorms.





No heat wave just sunshine and showers this weekend

21 08 2013

You may have seen more headlines in the newspapers today talking about a heat wave heading our way for the weekend. Once again these stories seem a little over-hyped.

The Met Office forecast for the rest of the week does, indeed, show rising temperatures, peaking in the high 20’s Celsius in south-east England on Friday. Temperatures are then expected to return to around average during the course of the weekend, so not a heat wave.

The weather is expected to turn more unsettled from the west during Friday and Saturday, with the risk of some heavy, showery rain around. The rest of the weekend should see some good dry and sunny spells of weather at times, but the risk of occasional showers remains and there is some uncertainty about which areas will see the showers and when.

The uncertainty this weekend comes from complex weather patterns developing over the Atlantic and these will ultimately determine where the showers will be from day to day. For those who like technical speak, it’s called a trough disruption and forecasting this phenomenon continues to be a major challenge to both computer models and humans alike.

So the best thing to do is keep up to date with the forecast to get the latest on how things are expected to develop over this weekend.





Sorry, no heatwave in sight

8 08 2013

An article on the front page of today’s Daily Express suggests that there is a new UK heat wave in the offing for next week and temperatures could soar into the 90s °F (32-37 °C).

This is not a forecast from the Met Office and, sadly for those who enjoy the heat, there’s no sign that we will see a return to the prolonged hot and sunny weather we saw in July.

In the Express, the article talks about high temperatures on the continent – up to 104 °F (40 °C). This may be the case in parts of continental Europe, but that doesn’t mean we’ll see temperatures like that in the UK or even a heat wave of any description. For that continental air to impact us, you’d need a very specific weather pattern and – looking at several of the world’s leading weather prediction models – there is no sign of that at the moment.

However, we do expect to see periods of decent, sunny weather over the next ten days or so (including today and tomorrow for some parts of the country). These will be mixed in with periods of more unsettled, wetter and windier weather. In fact, it looks like fairly typical mixed weather for the UK at this time of year.

In terms of temperatures, currently it looks like our highs will continue to be in the mid 20s °C – with the warmest weather being in south eastern parts of the country. Elsewhere temperatures are likely to peak in the high teens to low 20s °C.

You can see the outlook to the end of August and in to September on our website.





What is a heat wave?

4 07 2013

This weekend and into next week temperatures are expected to reach the high twenties Celsius in southern England. This is certainly warmer than we would expect at this time of year – the average maximum temperature for July in England is 20.9 °C – but does it constitute a heat wave?

How hot is a heat wave?

There’s actually no official definition of a heat wave in the UK. In America, where high temperatures are more likely, the official classification is based on the Heat Index. The Heat Index temperature is a ‘feels-like’ temperature calculated by combining the temperature and relative humidity.

Depending on the local climate, an excessive heat warning is issued when the Heat Index is expected to exceed 105 °- 110 °F (40 °C – 43 °C) for at least two consecutive days.

Australia also has variable definitions depending on the state. In Adelaide, a heat wave is defined as five consecutive days at or above 35 °C, or three consecutive days at or over 40 °C.

Heat health watch

Working in association with the Department of Health, the Met Office provides a heat health warning system for England.

The Heat-health watch system comprises four levels of response based upon threshold maximum daytime and minimum night-time temperatures. These thresholds vary by region, but an average threshold temperature is 30 °C by day and 15 °C overnight for at least two consecutive days.

When was the last time we had a prolonged spell of hot weather in the UK?

The last time we saw a long spell of warm weather was in July 2006, where temperatures were above 28 °C in many areas for a fortnight. We have had shorter spells of warm weather since, however, such as the 23 – 26 July last year, when temperatures peaked at 30.7 °C.

How hot will it get this weekend?

You can see the expected maximum temperature range for your area on our website. Temperatures are not currently expected to exceed the heat health watch threshold, but keep up to date with warnings on our website.

temprangelondon

Visit our summer pages for activity ideas for hot and sunny weather.

http://www.youtube.com/watch?v=GrBuD9eAmsk





Is there a UK ‘heatwave’ on the way?

25 03 2013

There have been some headlines today suggesting the Met Office has forecast a ‘heatwave’ by the end of spring.

The articles reference a line from our March to May three-month outlook for contingency planners which refers to how our Spring weather can change depending on where large areas of ‘blocking’ high pressure systems lie in relation to the UK – something we recently wrote about when contrasting the weather this March to that of last year.

This is not, however, a forecast of what the weather is expected to be like at the end of the spring or whether a ‘heatwave’ is likely or not, but is an indication of how average temperatures may differ from normal throughout the whole three month period of March, April and May.

Clearly this is the time of year when temperatures rise in response to the sun getting higher in the northern hemisphere sky, the days get longer and continental Europe warms up. So we will undoubtedly get some warmer spells of weather as the months go on and these will be picked up in our accurate five-day forecasts, as well as our forecasts out to 30 days ahead, which gives a more general view of the weather ahead over a longer-timescale. It is our accurate five-day forecasts and weather warnings provide the best possible advice and detail on what weather to expect in the UK.

This week is set to remain very cold with further wintry flurries in places.





What has brought the warm autumn weather to the UK?

27 09 2011

Over the next few days we are expecting a spell of very warm weather for this time of year across much of England and Wales and even parts of Scotland too. The reason why we are seeing this unseasonable warm spell is due to an area of high pressure which has developed across much of central Europe, centred on Germany and Poland.

This draws up very warm air from a long way south, from parts of France and Spain. That comes across a dry continent removing most of the moisture out of the air. As a result we see very little in the way of cloud with blue skies and plenty of sunshine. As a result the sunshine warms the ground and the ground warms the air so we see high temperatures for this time of the year.

In the video below Paul Gundersen, Met Office Chief Forecaster provides more details about this warm spell, how long it will last and whether this really is an ‘Indian Summer’.





What is an ‘Indian summer’?

26 09 2011

After a cooler than average summer, a spell of settled weather is expected later this week, with temperatures up to 27 °C possible in some areas from mid week.

Many media reports are calling this settled spell an ‘Indian summer’, however according to the Met Office’s Meteorological Glossary, it’s a little too early in the year. An Indian summer is defined as a warm, calm spell of weather occurring in autumn, especially in October and November.

William R Deedler, Weather Historian at the United States National Weather Service, describes it as “any spell of warm, quiet, hazy weather that may occur in October or even early November”.

The origins of the term Indian summer are uncertain, but several writers suggest it may be have been based on the warm, hazy conditions in autumn when native American Indians chose to hunt. The earliest record of the use of the term is in America at the end of the 18th century. Although William R Deedler also refers to a reference by a French man, John de Crevecoeur, in 1778:

“Sometimes the rain is followed by an interval of calm and warmth which is called the Indian Summer; its characteristics are a tranquil atmosphere and general smokiness. Up to this epoch the approaches of winter are doubtful; it arrives about the middle of November, although snows and brief freezes often occur long before that date.”

The term was first used in the British Isles at the beginning of the 19th century, but there is no statistical evidence to show that such a warm spell tends to recur each year. The warmest recorded temperatures in the UK in October and November are 29.4°C on 1 October 1985, in Cambridgeshire, and 21.1°C on 2 November 1938, in Essex and Suffolk.

For the latest weather forecast go to www.metoffice.gov.uk/weather





Dangerous Heat Wave Continues Across Much of Central and Eastern US

22 07 2011

The United States National Weather Service has said that the dangerous heat wave that has lead to a number of deaths is expected to continue across much of the central and eastern United States, with excessive heat and humidity expanding into the Ohio Valley and East Coast states for the remainder of the week. The highest heat index values are excepted across parts of the Midwest, Ohio Valley and Mid-Atlantic, where they are forecast to be between 105 and 115 degrees through Friday, with locally higher values possible. Excessive Heat Watches, Warnings and Heat Advisories are in effect over much of the central U.S., Ohio Valley and from the Carolinas northward into New England.

NOAA have produced guidance on a range of issues of coping with heat. 

In the UK, the Met Office works with the Department of Health and the Welsh Assembly to provide a  Heat-Health Watch system across England and Wales from 1 June to 15 September each year.

Heatwaves can be dangerous, especially for the very young, the very old or those with a chronic disease. Prolonged exposure to very high temperatures can mean the body is unable to reduce its own temperature, causing dehydration and heat stroke, which can be fatal. In particular, hot temperatures overnight make it difficult for the body to cool.

The Met Office forecasts day-time and night-time maximum temperatures. These are monitored regionally. When certain heat thresholds are passed, a warning is sent to relevant health professionals and people working in social care, so they can take action to minimise the impact of the heat on the health of people in their area.





‘Flaming June’ forecasts not from the Met Office

2 06 2011

Several media outlets are reporting that we are likely to see a heatwave in June and in some cases throughout the summer.  Some of the headlines attribute the June forecast to the Met Office.

These forecasts were based on comments from other forecasting organisations, not the Met Office.  At no time has the Met Office said that the whole of June will be hot nor have we issued forecasts for Wimbledon and Glastonbury or a summer forecast.

This weekend (3-5 June) will be warm in many places at first, with top temps in the mid to high 20s. But we do expect it to cool down later and into next week.

The longer term 6-15 day forecast issued on the Met Office website on 1 June for the period up to the middle of June said:

The UK entering the new week rather unsettled with a cooler feel when compared to the first few days of June. Some sunny spells are expected, but also rain or showers, with rainfall most persistent and heaviest towards the north and west, giving slightly below average temperatures here. Meanwhile the southeast may become locally warm, with a risk of heavy or thundery showers, whilst the best of any dry weather more likely in the west. Later in the week, conditions look to remain unsettled, with showers perhaps turning heavy at times with a risk of thunder. It should remain rather cool for many, although again, perhaps warmer towards the southeast. Towards the end of the period, conditions continue rather unsettled with rain or showers, separated by drier and brighter interludes.

Met Office weather forecasts for the next five days, 6 to 15 days ahead and 16 to 30 days ahead are available on the Met Office website.








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