to [ash the state. hundreds of thousands remain without power. authorities are warning the flooding is likely to worsen before the waters subside. the king of spain has led a huge crowd of demonstrators through barcelona in defiance of the islamist militants who killed 15 people in catalonia this month. police say 500,000 people took part. the main banner bore the slogan "i am not afraid." swiss officials have called off a search for eight people missing since a huge landslide struck near the border with italy on wednesday. rescue services say they are likely to be buried under millions of tons of rock. now on bbc news, weather world. this time on weather world, we're at the highest point of the uk's biggest city, uncovering the secrets of urban weather.

and finding out why heat waves 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 driver's—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

been 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, you'll 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. it's 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. it's 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. there's also a lack of evaporation in cities due to less vegetation and fewer bodies of 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 world's population living in urban areas, this heat island effect has a huge impact on our weather and daily lives. we're swapping one high—rise for another as we cross the city from south to north, joining a team of scientists investigating urban weather. what they're discovering could one day narrow down weather forecasts from whole to parts of the city, and perhaps even individual 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 that's a thermal imaging infrared camera. this camera is looking down at the city and is taking measurements of temperature coming off different materials —

buildings, for instance, roads and vegetation too. so to tell us more about this camera and what is it's measuring, we're 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. let's have a look at how this works in cation, looking down at the city. —— let's have a look at how this works in action, looking down at the city. absolutely. so, will, you've 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 we're seeing —— talk us through what it looks like and what we're 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 there's 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. i'm 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 we'll have more from the rooftop later. when the going gets hot, you do what you can to stay cool. it's 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 here, 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 campsites here in france, injuly, spending the night on beaches near saint—tropez, as the smoke billowed above them. portugal injune, and a catastrophic forest fire kills more than 60 people — the country's 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 country's 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 mother's walking frame among the piles of debris and next to it, even her kitchen sink. it's happened. we can't put it back. you just have to get on and carry on and do what we can. we'll get back to normal. we're cornish! earlier in the summer, a heatwave provoked an unusual protest at a devon school. boys told they couldn't wear shorts in the heat because it wasn't official school uniform wore skirts instead. everyone was like, "oh, if everyone else does it, they can't 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. that's enough of that. but that's 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. that's 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, i'm feeling the air blowing right at me sideways now, but when you are talking about different surfaces heating the air above, it's rising

from the ground upwards, which you're also measuring with this. and you would expect that air to be rising differently, depending on where it's 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 what's going on just here, but you've also got another piece of kit here which is measuring over a bigger distance, isn't it? that's 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 likm away. so this really is the invisible world of what's going on in the air right above this cityscape and above us? that's 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? that's 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? that's 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 what's going on in the air around us. you're 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. we're 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 earth's 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. —— become a bbc weather watcher by signing up online. and coming up, how a blanket could help produce a phenomenon —— and coming up, how a blanket could help reduce a phenomenon known as glacial melt.

so far on weather world, we've been getting very high—tech at this urban weather experiment on this windy rooftop in london. but along with the high—tech, i'm glad ifound something that's a little bit more familiar, at least to me, which is a weather station. we've 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, we've got a thermometer and it's 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 we've also got, look at this, a cup anemometer. look at it moving in the wind today. and we've been to the birthplace of these anemometers on our travels in weather world, to armagh, in northern ireland. michael, i've got an amateur weather station in my back garden and it has

this on it. and to think it all started here, in armagh. that's right. a simple design, four cups that spin in the wind, and you can measure the wind speed and that's how we know wind speeds around the world today. well, this anemometer is getting a good workout today, but it's 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 driver's 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 russia is 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 trump's 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. i'm at the top of the stubai glacier in the austrian alps, at about 3,000m high.

it's an area that's very popular for skiing. and actually, there are about 80 separate glaciers in this area. but there's 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, and what can they do to stop it? dr andrea fischer is a world—renowned glaciologist, who's made it her life's work to halt the melt of this glacier. 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, it's very cost intensive and needs much labour. to save 1% of the glacier seems futile, but with the local economy relying on skiing and tourism, here,

officials say it's worth it. 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 sun's 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 around the world. and in the expansion of renewable energy, this is a landmark development: wind turbines floating off the sea of norway. they will form the world's first floating wind farm. so, you've look down towards the ground, and we've 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 can't see the laser. it measures outside the visible spectrum. so this looks 7000 kilometres into the sky. so this looks 7

kilometres into the sky. what is it measuring? it is measuring reflection from particles in the atmosphere. this could be cloud droplets or air pollution. with that, you can see at what height the clouds are. so you can get a picture of where the clouds and have a picture of how polluted the air is at any time. yes. ok, so we've had a look at this cellometer. we will go around and talk to sue, who will give us an indication of what the output from the cellometer looks like and how we can use that data. now, sue is an urban climatologist with the university of reading, and she's heading up the research here. so, sue, hi, we havejust been talking to natalie about the cellometer, and how that measures cloud height and particles in the atmosphere. so how is that information useful to you? it's helping us with our weather forecast. we can see where clouds form. what we are looking at now is the green is the rural area, and the greater city. you can see the clouds are forming, what happens to them as they form, and over cities, that is different.

we can compare the difference in the real city with what we can see in forecasts like this. so with all the research going on here, then, how does that tie together, and what are you hoping to achieve in the future? what we are hoping to do, and as we we think about forecasting into the future, what we are expecting is that models will allow us to look at things in much more detail, so we can look at, for example, the differences between the northern part of the city and the southern part of the city. hopefully this will make it better for citizens within the city and living in different neighbours. wonderful. so we're looking at more accurate urban forecasting the future. they you forjoining us, sue, and for all your team, too. sometimes you need to look of your shoulder to see what the weather's doing. this photo of a man unwilling to abandon mowing lawn in canada injune took certain media by storm. apparently, the tornado was further away than it looked. we often show pictures of the aurora borealis,

or northern lights, but these are the southern hemisphere equivalent — the aurora australis, putting on a spectacular show in the skies above australia and new zealand in may. and finally, the weather can make us all a little hot and bothered at times. and apparently the same goes for gorillas. zola creating a splash at dallas zoo. and that is it for this time on weather world. we will be back later in the year. until then, keep checking the forecast. hello. if you approved of saturday's weather, then sunday shouldn't disappoint. let's take a look at a couple of images from saturday. it was often cloudy, but there were some warm, sunny spells coming through. most places stayed dry. there were just one or two isolated showers around,

particularly across northern parts of the united kingdom. this finger of high pressure that has moved in across the uk will keep most places dry for part two of the weekend, during sunday. again, it won't be clear blue sky. in fact, from the cloud to begin with, there could just be a few spots of drizzle around north wales, north western england, and a lot of cloud feeding into scotland, northern ireland, and north—west england. but elsewhere, that cloud will break, there will be some sunny spells coming through. again, while most places will be dry, there is a weather system close to north—west scotland — the thickest cloud here, more breeze. some patchy outbreaks of rain, especially into south wales. and some spots into the mid—20s. we're looking fine at the test match at headingley. as that continues, there'll be some sunny spells coming through that cloud. a bit more breeze, though, by the time we get to monday. for the notting hill carnival, glorious weather. temperatures head up even further by the time we get to the bank holiday. and the winds remain very light. here is how it is looking through sunday evening, so if you're planning a barbecue,

you're in good shape. but as we go through sunday night, you'll notice the rain starting to gather a bit more towards the north—west of the uk into northern scotland. some outbreaks of rain moving through. 0n towards the northern isles as well. the wind is picking up. these are overnight temperatures. and monday's big picture does look a little bit different. high pressure being squeezed away, weather fronts coming in. and they're poised to move their way southwards during monday into tuesday. now, what that means for monday is that we'll see for scotland and northern ireland, more cloud, stronger winds, and outbreaks of rain moving south. going into monday evening, some will be moving into northern england and north wales. whereas, to the south of that, it will continue notjust dry, but with plenty of sunshine. a range of weather on monday. a range of temperatures, as well, from the breeze and the rain to where we see a break from the breeze in the rain to where we see the best of the sunshine, with some places reaching into the upper 20s, to nearly 30 celsius. as we look further ahead to tuesday and wednesday, we will take that cool and fresher air, and gradually filter that further southwards across the uk.

that's how it is looking here for the next two days. there isn't a great deal of change in texas. even though harvey has been downgraded to a tropical storm, that's a wind classification, it's still producing prolific amounts of flooding rain, and will continue to do so for several more days to come, as an extremely dangerous situation continues in texas. hello this is breakfast, with rogerjohnson and rachel burden. it was billed as one of the greatest sporting shows on earth and it didn't disappoint. good morning it's sunday 27th august. also ahead, a big shift on brexit by labour — the party says it wants the britain to remain in the single market after leaving eu. texas braces itself for further "catastrophic" flooding as tropical storm harvey sweeps across the state. strong winds are hampering rescue efforts and thousands