Were at the highest point of the uks biggest city, uncovering the secrets of urban weather. And finding out why heatwaves hit cities and those of us who live in them the hardest. We take to the rooftops of london to see just how man made environments can change the weather. What are you hoping to achieve in the future . When we do Weather Forecasts, we will give more detailed forecasts, so there will be differences between different parts of the city. Also on weather world out of nowhere. The drivers eye view of mudslides and storms, as we take a meteorological tour of the world. Scorching summer in Southern Europe and the deadly consequences of wildfires that rage out of control. Watch the birth of a flood as it surges down a dry riverbed, to the floods that sweep away everything in their path and the disaster they cause. Plus, cold comfort. The blanket response to melting glaciers in the austrian alps. Scientists have realised there has an unprecedented glacial melt, so the questions now are how serious is that melt and what can they do to stop it . Welcome to weather world and a view once seen youll never forget. The view from the shard, a skyscraper which stands above Everything Else in london. From up here you can see the haze that stretches over this urban landscape and tells you that the air is different here compared with in the countryside. And it is the unique properties of urban air which produces weather and climate differences which impact everyone who lives and works in a city. Its called the urban heat island effect. An urban heat island is a built up or man made area that is significantly warmer than the surrounding countryside. Its often most noticeable overnight when the temperature difference can be as much as 12 celsius between the inner city and the rural surroundings. There are several causes of the urban heat island effect. Firstly, dark surfaces and the type of materials used in cities, such as concrete and tarmac, tend to absorb more solar radiation during the day and heat up quicker than the countryside. This heat is then released into the city air, creating a warming effect. With higher levels of particles and pollutants in the air in cities, that air is more capable of holding onto the heat, whereas in the countryside where the air is often clearer, more of that heat can pass through the atmosphere. Theres also a lack of evaporation in cities due to less vegetation and fewer bodies water. In more rural areas, this evaporation from trees and water removes heat from the surrounding area. As well as the materials used in cities, the other major contributing factor is the topography of the structure of urban areas. Imagine, for instance, wind flowing in a relatively Straight Line across open countryside. When that wind blows across an urban area, the buildings create more friction to the flow, making it slower and more turbulent. This in turn reduces the winds capacity to disperse heat. So with more than half the worlds population living in urban areas, this heat island effect has a huge impact on our weather and daily lives. Were swapping one high rise for another, as we cross the city from south to north, joining a team of scientists investigating urban weather. What theyre discovering could one day narrow down Weather Forecasts from whole to parts of the city and perhaps even individuals streets. Behind me you can see the shard on the london skyline and as part of the Research Project here there are various pieces of meteorological equipment taking readings of different elements of the weather. One of those pieces of equipment is inside this silver box here and thats a thermal imaging infrared camera. This camera is looking down at the city and is taking measurements of temperature coming of different materials, buildings, for instance, roads and vegetation too. So to tell us more about this camera and what is its measuring were going to meet a phd student will morrison. Hi, will. Hi, sarah. Talk me through this camera. What exactly is it looking at and what is it showing us . This is a Long Wave Infrared camera, sensitive to Long Wave Infrared radiation, which can be used to determine surface temperatures. So if i was to point the camera at different surfaces, like my hand, surfaces of different temperature would appear a different colour . Yes, this is a thermal graph of the temperatures which you can see through the camera. Your hand is appearing cooler than the surroundings. The different colours represent different temperatures, in this case. Great. Lets have a look at how this works in action looking down at the city. Absolutely. So, will, youve got several of these infrared cameras positioned at different places around the building. This one here is looking down at some buildings, some roads and vegetation down below us. Talk as through what it looks like and what were seeing on the infrared image. This is a time lapse of the imagery. You can see the same principle as with your hand, different colours representing different temperatures. We can see theres a variety of colours, meaning the surface and temperatures are highly variable. Why are we interested in these measurements . Typically we use satellite based measurements of surface temperatures, which are looking straight down at the surface. That means they have a preferential view of the roof and ground surfaces, as opposed to all surfaces. Im doing a ground proving exercise here, with lots of measurements from different angles, to build up a complete picture. So these surfaces are really important for evaluation and input for forecasts. And well have more from the rooftop later. When the going gets hot, you do what you can to stay cool. Its been a summer of extreme heat in Southern Europe, with temperatures soaring well into the 40s celsius from spain, across to the balkans. In italy, the heat exacerbated drought. This is the bed of the river her, so the pope ordered the famous fountains of the vatican to be turned off, to show solidarity with the people of italy and rome, where temperatures hit 43 celsius. Hot, dry and now on fire. Wildfires led to thousands being evacuated from camp sites here in france, injuly, spending the night on beaches near saint tropez, as the smoke billowed above them. Portugal, in june, and a catastrophic forest fire kills more than 60 people. The countrys worst disaster for more than 25 years. The majority of people were killed on this road, rapidly engulfed by fire as families already fleeing from their homes were trying to drive to safety. More fire, but a different continent. This is south africa, near cape town, injune, as severe drought combines with strong winds to spread wildfire sparked by lightning. They followed a major storm that inundated the coastline here, with high seas and big waves. The south African Weather Service said it was the worst winter storm here in 30 years. Big storms have hit europe too. This is moscow, in may. A severe thunderstorm, the worst in a century, kills 16 people, toppling trees, with winds of up to 70 mph. And astonishing scenes from poland, in august, as storms bring down tens of thousands of trees, killing six people. The countrys chief forecaster called it the worst disaster in the history of polish and perhaps european forestry. Storms brought devastating downpours too. Turkey and istanbul, injuly. Roads turn to rivers. Rail tracks turned to rivers. There have been a number of flooding storms in the city this summer. In the uk, storms followed a hot start to the summer. This is cornwall, injuly. And in coverack, the torrential rain and hail lasted over an hour, causing a flash flood to sweep through the village, taking everything it could carry towards the sea. This woman found her elderly mothers walking frame among the piles of debris and next to it even her kitchen sink. Its happened. We cant put it back. You just have to get on and carry on and do what we can. Well get back to normal. Were cornish earlier in the summer, a heatwave provoked an unusual protest at a devon school. Boys told they couldnt wear shorts in the heat, because it wasnt official school uniform, wore skirts instead. Everyone was like, oh, if everyone else does it, they cant stop anyone else doing it, so they might bring shorts back for the summer. Well, girls are allowed to wear skirts all year round. They get cold legs and we have to sit there sweating the school says as hot weather becomes more common, it may consider a change. Protest is real we are back at our urban experiment on a rooftop in london. Earlier, sarah and will were using thermal imaging to look at how different surfaces, grass, concrete, humans, heat and release the heat at different levels. Thats enough of that. But thats really important because those different surfaces are also heating the air above them at different levels, different rising of that air. That goes on to affect the air around us in the city, but also the weather too. Ben crawford is monitoring just how much the air is moving around us with this piece of kit here. Tell us about it, ben. Thats right. This is a 3d sonic anemometer. So it measures wind speeds in three dimensions. With this piece of equipment we can measure the cumulative effect of all the different surfaces and their heating of the air. So, importantly, im feeling the air blowing right at me sideways, now, but when you are talking about different surfaces heating the air above, its rising from the ground upwards, which youre also measuring with this. And you would expect that air to be rising differently, depending on where its come from grass, concrete, tarmac . Yeah, exactly right. If you can imagine a natural environment, like a field or forest, and you replace that with a city, with glass and concrete and brick, you will change the thermal properties of that and change how the air flows and moves. This is measuring whats going on just here, but youve also got another piece of kit here, which is measuring over a bigger distance, isnt it . Thats right. This is called a scintillometer. So it measures disturbances from the air, from the heat rising, similar to shimmers coming off a hot roof or a hot piece of concrete. So this is pointed across the city towards suffolk towers, about four kilometres away. So this really is the invisible world of whats going on in the air right above this cityscape and above us . Thats right. And with the data from this instrument we can make the invisible visible. So here we have a series of days in the summertime here in london and we can see during the midday, the hottest part of the day, we have the greatest heat emissions and we can see this rise in the graph. 0vernight, when the surface has cooled down, we can see less heat emissions coming off the surface. But what does it matter . How does knowing about all of this turbulent air possibly improve a Weather Forecast for a city . Thats a good question. Heat emissions and turbulence affects a lot of things. It affects air temperature, wind speed, it affects the dispersal of wind pollutants and can even impact things like clouds. So we can narrow down the forecast to a smaller part of the city . Thats right. We can give a specific forecast for a city, versus a rural area, and maybe in the future even different neighbourhoods within a city. Thanks for revealing the hidden world of whats going on in the air around us. Youre very welcome. Oh my god clear turbulence and updraughts when airborne are what hot air balloons try to avoid. But getting airborne is the problem here. Were trying to get air up, so we can get up. Two balloons collide injune. Swirling, shifting winds blow them off course. Luckily, what could have ended in tragedy only left one man with minor injuries. Lightning sparks across the south wales sky in may. The earths surface is struck about 100 times every second and whenever lightning strikes you can be sure bbc weather watchers will try to succeed where many of us fail to get a good photo of it. These were all taken when storms swept through the uk injuly. Become a bbc weather watchers by signing up online. And coming up, how a blanket could help produce a phenomenon known as glacial melt. So far on weather world, weve been getting very high tech at this urban weather experiment on this windy rooftop in london. But along with the high tech, im glad ifound something thats a little bit more familiar, at least to me, which is a weather station. Weve seen a lot of these on our travels on weather world. Remember, in northumberland, we saw a fully working met Office Weather station there. Inside here weve got a thermometer and its measuring humidity with this gauge, all inside a very clever box which protects it from the direct sunshine and excessive wind. Of course that weather station had a stevenson screen behind which you had a thermometer. This, although not the same as a stevenson screen, acts like that, protecting that thermometer, so it can measure the temperature of the air, rather than the sun, getting to that thermometer. And weve also got, look at this, a cup anemometer. Look at it moving in the wind today. And weve been to the birthplace of these anemometers on our travels in weather world, to armagh, in Northern Ireland. Michael, ive got an amateur weather station in my back garden and it has this on it. And to think it all started here, in armagh. Thats right. A simple design, four cups that spin in the wind, and you can measure the wind speed and thats how we know wind speeds around the world today. Well, this anemometer is getting a good workout today, but its really good to know that wherever we go on weather world we can find a weather station. But we will get a little bit more high tech again later in the programme when we come back here. In car Dash Cam Technology can now give us a drivers eye view of what it is like to negotiate a mud covered, flooded road. But things are about to get much worse. China injuly, and a dramatic mudslide, after days of heavy rain, leaves drivers buried and many needing rescue. Here, only injuries, but in bangladesh, injune, a desperate search for survivors as heavy rain triggers a landslide that kills more than 100 people. From sri lanka north to nepal, the monsoon rains so vital for crops bring floods that affect millions. In nepal, the floods worsen in august, and elephants are used to rescue people trapped in water. Injune, torrential rain in taiwan. More than 600 millimetres in less than twelve hours. Similar scenes in japan in july. Described as unprecedented, the rain causes more than half a Million People out of their homes. When a flash flood rages through a major city, disaster strikes. Freetown, the capital of sierra leone, in august, and a massive downpour causes muddy water to certain industries. Then a mountainside collapses, burying whole communities and families, as they slept. Hundreds are under this mud, and the death toll may never be known. It is the rainy season here, but this year, it has come with unusual and deadly ferocity. And just how quickly can rain transform a dried out, rusty river bed, into a raging torrent . This quickly. The usa, and arizona, in august, as a flood of water, mud, and debris, suddenly surges out of nowhere, catching campers by surprise, as they scramble to rescue their belongings. A happier surprise to some in the Southern Hemisphere winter, as a rare snowfall his the chilean capital, santiago, in july. The snow caused widespread power outages, and treacherous driving conditions not that these motorists seem to care. But unusual cold stats look like local anomalies against a tide of record warned. Alaska. Parts of this us state had its warmestjuly on record. In may, the Artic Council meeting here signed an agreement accepting the need to tackle climate change. But less than a month later, this. In order to fulfil my solemn duty to protect america and its citizens, the United States will withdraw from the paris climate accord. President trumps announcement brought Widespread International condemnation. The World Meteorological association said that in the worst case, the us pull out could add 0. 3 degrees celsius by the end of the century. To the Global Political battles, there are smaller scale vote on the way to try and stop local environments from changing, perhaps irreversibly, as Sara Thornton has been finding out in austria. Im at the top of the stubai glacier in the austrian alps, at about 3,000m high. Its an area thats very popular for skiing. And actually, there are about 80 separate glaciers in this area. But theres a problem, because in the last few years, scientists have realised there has been unprecedented glacial melt. The question is, now, how serious is that melt, whn made it her lifss. Aggre she has hit upon a surprising answer, a blanket. On a very small, very local scale, we could prevent some very tiny glacial areas but covering them with sheer textiles in summer. But only about i of glacier area can be preserved with this method, and of course, its very cost intensive and needs much labour. There are 5000 alpine glaciers in the world. And some scientists predict that at the current rate of melting, in 20 years, half will be gone, and those that are left will be much smaller. But it is clear this inexpensive local solution can help on a global scale. Here on the vast Greenland Ice sheet, the problem is super sized. There are fears that the melting is happening faster than expected, in part due to algae growing on the surface of the ice. Darker than the ice, it absorbs more of the suns warmth. Nobody as saying that is going to melt in the next decade, the next 100, or even 1000 years. But it does not need to be completely melted for people to be in danger. Only a small portion is required to raise sea levels, and the threaten millions of people coastal communities the sea of norway. They will form the worlds first Floating Wind farm. So, youve look down towards the ground, and weve seen equipment measuring the weather at this level. But now we look into the skies. To do that, i am joined by natalie. Hi, natalie. Talk us through this piece of equipment, here. Ok, so this is the cellometer, a laser that takes a vertical snapshot of the atmosphere. This is the equipment that holds the laser. You cant see the laser. It measures outside the visible spectrum. So this looks 7000 kilometres into the sky. What is it measuring . It is measuring reflection from particles in the atmosphere. This could be cloud dr