Deep Dive 15/09/2026 – Wavey jet keeping us things mixed – Met Office weekly weather forecast UK

Name: Deep Dive 15/09/2026 – Wavey jet keeping us things mixed – Met Office weekly weather forecast UK
Uploaded: 2026-09-15T14:14:18+00:00
Duration: 26 min 59 s
Channel: Met Office

00:00Aliens, spacecraft and UFOs and cows udders. There's always something interesting in the

00:05skies to look at. We're going to be talking clouds today, but what about the forecast?

00:10Well, that's kind of trough, ridge, trough, ridge. No, that's not a new kind of dance.

00:15That is the weather pattern that we've kind of got at the moment, which is fairly typical

00:20for this time of year. All will be revealed in this week's Met Office Deep Dive. Welcome

00:26along. We do the deep dive every Tuesday on YouTube. So make sure you hit subscribe and

00:32then you'll never miss one of these 20, 25 minutes talking about the weather, the meteorological

00:38mayhem that is going on over the UK and beyond. So yes, do make sure you subscribe, hit that

00:45thumbs up button and share the love. Let everyone know that the Met Office does the deep dive

00:50every Tuesday. Any other Met heads that you know who might be interested in this? And

00:55if you've got a question or a suggestion, something that you'd like us to cover in another deep

01:00dive, then do put those in the comments and we'll answer some of the questions during the

01:05live that we do every Friday on YouTube. So yet another reason to subscribe. Okay. What

01:11are we talking about today? Clouds mostly. These are the current clouds. This is the satellite

01:17picture that has been going on. Satellite imagery showing the clouds over the past five days or

01:23so. But what I want to talk about is the different types of clouds. Last week, the Cloud Appreciation

01:29Society had their Cloud Appreciation Day. So I thought we'd look in a bit more detail about

01:34the different types of clouds and two clouds in particular. But how do we define clouds?

01:41Well, the WMO, the World Meteorological Organization, has a fantastic cloud atlas. I do suggest you go

01:48and check that out. We might even put a link in the description. The Cloud Atlas describes all the

01:54different types of clouds that are known around the globe. And there are 10 basic types, 10 genera as

02:01they're known. And they're shown here with a couple of bonus ones. So the 10 basic types can be split

02:08into three layers, low level cloud, medium level cloud, and upper level cloud. The three low level

02:14ones, Stratus on that gloomy, often dank and drizzly day when the cloud base is really low. Then you've

02:21got Strata Cumulus, perhaps the most common of all the clouds around the UK. Cumulus, which is the light

02:28fluffy cotton wool ones that get a bit bigger at times. They're the low level clouds. Then we're going up

02:33to the medium level clouds. That's where Alto comes in. Alto Stratus, so that's a layer of cloud,

02:39but it's a little higher up and you can make the sun out through it. Alto Cumulus, basically Cumulus

02:45that's just a little bit higher up. And then Nimbo Stratus, Nimbo meaning rain, so you know you're going

02:50to get wet if you've got Nimbo Stratus above your head. And then higher up than that, where the

02:54airplanes fly at the top of the troposphere, the top of where the weather happens. Cirrus,

02:58Cirro Cumulus and Cirro Stratus. And then the 10th member is Cumulo Nimbus. And that's because it goes

03:05all the way, it starts at the bottom, but it goes all the way up to the top of the

03:08atmosphere. So

03:09that's the one that extends through all the main layers. So they're the 10 basic, as I say, genera,

03:15as the WMO define them. But there's also within that different varieties of cloud. And this one,

03:23Lenticular. That's one of the varieties of cloud. Most common form is Altacumulus Lenticularis.

03:30And we'll talk a little bit about that in a moment. And I also want to talk a little bit

03:33about

03:34Mammatus clouds. They're not actually a defined variety. They're a supplementary type of cloud,

03:41usually associated with these, with thunderstorms. And again, we'll have more on that in a moment.

03:48Why are we talking about those two clouds in particular? Well, because we've been sent

03:52fantastic examples of those clouds on our email. So this is a beautiful example of Lenticularis

04:00clouds taken last month, Linda Joseph, in Cumbria. Cumbria is crucial here. These kind of clouds

04:07are generally generated by hills and mountains. So they're quite common in Cumbria, or of course,

04:11Wales, or to the east of the Pennines, and of course, across parts of Scotland. So often generated by

04:17mountains. Beautiful example here of Lenticularis. You can see those layers within the cloud. And

04:22that's why I talked about alien spaceships, because they're often described as UFO clouds and have

04:27quite frequently been confused with UFOs and people phoning the police saying they can spot

04:33an alien spacecraft in the skies when really it's just an Altacumulus Lenticularis. And the other one

04:40I want to talk about in detail today, Mamatus. So another fantastic example here. Alec Longhurst

04:45sent this one in from Huntington in North Worcestershire. Again, taken last month. Mamatus,

04:52they're clouds that hang down from the bottom of cumulonimbus clouds. They're the opposite of what

04:58you might imagine. Clouds are normally flat-bottomed, but these are bulbous blobs that kind of hang down.

05:04Mamatus coming from the Latin for udders or breasts, and that's where they get their name from. Mamatus

05:11clouds. A beautiful example of that there. And Mamatus clouds, as I say, generally come from

05:17cumulonimbus clouds. And here's another example. Just wanted to show this, taken in Monmouth in

05:23South Wales again last month. Some cracking examples here of how cumulus grow through the atmosphere,

05:28turning into cumulonimbus clouds from that light fluffy cotton wool ball into a powerful agitated

05:35bit of cauliflower, if you like, before it forms into a fully blown cumulonimbus. And just some

05:41beautiful shading on these pictures here. You can see that just the two different types of cumulus,

05:46one in front of the other. They're all the same, but one just looks that much darker because the light

05:51isn't getting through. So it's just all about the way that the light gets to it or doesn't get to

05:56it,

05:56as depending on the shade of the cloud. That's why cumulonimbus clouds, the bottom of them,

06:01is so dark because the light can't get through because the clouds going all the way to the top

06:06of the troposphere. So there's no light can get to the bottom. And that is why the bottom of cumulonimbus

06:10clouds are so foreboding and are so dark because the light can't get through. So I just thought I'd show

06:14that because that's another absolutely cracking example of some of the beautiful clouds that we see

06:20in our skies. And if you see or take any cracking images of clouds, something unusual you want us

06:27to look at, then there's the email address to send your weather picks into. Weatherpicks

06:32at metoffice.gov.uk. And we'll either discuss them maybe on a deep dive or in our live on YouTube

06:38on Friday. Again, another reason to subscribe. Okay, let's look a little bit more in detail then about

06:46these clouds, lenticularis and mammatus in particular. And when we're talking about clouds,

06:53and when we're talking about weather in general, as meteorologists, we're often looking at the stability

06:59of the atmosphere. We often talk about how stable the air is. The air around us is always moving,

07:06it's always shifting, it's always in different layers. But the ability for the air to rise is

07:12dictated by how unstable the air is. So the air is always moving around, we know that the winds

07:17shift and change in height as well. But the air can also be moving up and down. And when the

07:23air is

07:23moving up rapidly, we talk about unstable weather conditions. So when the air is unstable, you've

07:32generally got an area of warmer air down close to the ground. And then you've got colder air

07:43above. And Mother Nature wants to balance things out. So warm air rises. So what we get is that warm

07:50air rising rapidly from the ground up into the colder air, trying to balance everything out. So

07:56when you've got this set up, it's often the case during a summer's day, for example, it starts off

08:02clear blue skies, the sun gets higher in the sky, warms the very bottom part of the atmosphere,

08:09warms the ground, it warms this layer here, and that air then rushes up, starts to rise. Initially,

08:15just forming little pretty cumulus clouds. But eventually, if it's unstable enough, if the air is

08:21colder up here, or if it warms up enough at the surface, then that air will rush up further,

08:27generating towering cumulus clouds, and eventually potentially then into cumulonimbus clouds. So when

08:33the air is very unstable, you get the air rising rapidly. And that's crucial for the formation of

08:40things like thunderstorms. But what can happen is, well, you haven't got a lot of instability,

08:46it's often colder higher up than it is lower down, but it's not to such an extent that you're going

08:51to get the air rapidly rising. That's when the air is fairly stable. But you can also find another

08:56situation where the air is very stable indeed. And that's often the case when you've got the warmer

09:02air, you've got to... I knew that was going to happen. You get a warmer layer above a cooler layer.

09:10So you can actually get layers within the atmosphere where you actually find cooler layer below the warmer

09:16layer. So here, the air would be described as stable because it's not going to be rising,

09:21it's not going to be changing in its motion because the cold air isn't going to rise.

09:25So you have this, what's called a very stable atmosphere. And it's a stable atmosphere that you

09:32need for lenticularis clouds to form. So if you go to the lenticularis clouds,

09:39you can see them forming here in these kind of waves almost, and they get different layers

09:44within those lenticularis clouds. And what's going on here is you've got stable air.

09:51You also need some moisture. That's crucial as well. So if it's moist, stable air, then let's imagine

10:01the winds are coming in this direction. So the winds are blowing like this.

10:08Now, if the earth was flat, completely flat, then, you know, not a lot would happen. But as we know,

10:13the earth isn't flat, and that air is going to meet certain obstacles. In this example, a mountain.

10:19Now, the air can't go through the mountain, so what's it going to do? It's going to bounce over it.

10:23It is going to rise up over the mountain,

10:26but it can't keep going. It can't go higher because the air is stable. So what actually happens is it

10:31bounces back down, and then it actually sets up this wave-like pattern, and actually get the air coming in,

10:39hitting a mountain, and it sets up this wave. And we see this pretty frequently, actually. Wherever you've

10:46got mountains, the air will form and generate these waves. So downstream, the wind's coming this way.

10:53So if you're on the lee side of the mountain, you will, well, you probably won't see it unless you've

10:59got these clouds. But the air will be doing this. It will be bouncing up and down. It can't rise

11:04further

11:04because the air is stable. So it stays in this kind of pattern, and it sets up a wave. Now,

11:09if you've got

11:10moist air, what happens is it will hit the mountain, and you will find a cloud will form. Because it's

11:20rising here,

11:21it's cooling enough to get the moisture to condense, to form water droplets, and you get a little cloud

11:27here. And what can then happen is you get a slightly smaller one on top, and you get that capping

11:31effect

11:31of the mountain. So over the mountain, you can get lenticularis forming. What's more common is actually

11:37to see it as the air comes down and bounces, you will then get another pocket of lenticularis as it

11:45bounces. And then potentially another one down as well, and another, and another, until that wave

11:50is kind of evened itself out. So that is how lenticularis clouds form. You need the moisture,

11:56because if it's dry, you don't get the clouds. One thing you can tell then if you've got this mountain

12:01wave setting up, is actually the winds will be quite gusty to the lee of the mountains as well. So

12:07you get the waves, and as the winds come down here, you get some strong gusts, because they can't go

12:12into the ground. So they bounce and they spread out, and you get strong gusts of wind in these little

12:17troughs. So even if you don't get the clouds, you often will get that, and that's called lee wave,

12:23lee gustiness, lee wave gusts, that we see quite often over the Pennines if the wind's in the right

12:29direction. But those lenticularis clouds are formed in these locations, and one tent telltale sign about

12:36lenticularis cloud, is they don't move. They form in these peaks, but they don't shift. So the winds

12:42are blowing, but unlike a cumulus cloud that drifts with the wind, they will stay in the same location,

12:49and there's a good example of that in this video here, of how you can see the winds now coming

12:54this

12:54way, and forming that lenticularis cloud that, that's the lens shape, that's why they're called

13:01lenticularis, lentil shape. You can see the winds coming in here, and tailing off there. So the winds

13:06are actually blowing that way, but the clouds are pretty much stationary. They stay in that same

13:12location, because that's where the wind's peaked, and that's where you get. And you're going to get

13:15those set up one after another, where you get clear skies, lenticularis, clean skies, lenticularis,

13:21and you get those little waves and little ripples. And often you can see that on a satellite image from

13:25above, and you can actually see the little waves forming, particularly across parts of Scotland

13:29in particular. So you need stable air, you need enough moisture for the clouds to form, but not too much

13:35moisture that the whole thing's clouded out. And you need those bouncing, those waves. You need the

13:40way, the winds to be in the right direction, so that they're coming over the mountain and forming

13:45those waves as they go. So that is lenticularis. As I said, there are different types of lenticularis,

13:50but mostly you're going to get them in those middle layers. They are alta cumulus lenticularis,

13:56but you can get strata cumulus lenticularis as well. So that's one particular type of cloud,

14:02a cloud variety I wanted to talk about. Now let's talk about these, because these are the

14:08Martyrs clouds. As I said, not classified as a variety, but as a supplementary type,

14:14to a supplementary feature of a type of cloud, usually associated with a cumulonimbus cloud.

14:22And for these, you need the opposite. So this is when the air is very unstable. So you've got stable

14:28air

14:29to form the lenticularis. For these, you've got the opposite. Still needs to be moist, but you need

14:36unstable air. So you need the air to be rising rapidly. So what's going on with these? Well,

14:41I'll try and get my pen out again and show you if I can. Now we've got unstable air. So

14:49classically,

14:50in a summer scenario, perhaps, you've got that warm air being heated by the power of the sun.

14:56So the air at the lower part of the atmosphere is getting hotter and hotter. And then higher up,

15:02you've got the colder air. So the air is rising. The air is rising because it's unstable. Unstable air

15:08rising and rising rapidly. So then, start off with a nice fluffy little cumulus cloud, perhaps,

15:16on a nice summer's day. But if it's unstable enough, well, that cumulus cloud will start to grow

15:26and get bigger, become a towering cumulus. And eventually, it could hit the top of the troposphere.

15:36So up where the cirrus clouds are, 30,000 feet, where the airplanes generally fly,

15:40that's the top of the troposphere. The troposphere is the lowest layer of our atmosphere. It's where

15:44pretty much all of our weather happens. But above the troposphere, you've got the stratosphere. Now,

15:50what happens in the stratosphere is that temperatures start to rise as you go into the stratosphere. So,

15:56at the top of the troposphere, you've basically got a stop point. The air is not going to rise any

16:01further,

16:01because it's getting warmer, higher up. So the air is rising up to that point, up to the top of

16:05the troposphere.

16:07And then it gets stopped. So, we've got our rising cumulus into a towering cumulus that eventually forms

16:15into a cumulonimbus cloud. There's the base of it, and it's rising and rising and rising. It hits the top

16:22here.

16:22It can't go up any further. So this, what's it going to do? Well, it can't go any higher, so

16:29it has to be

16:29pushed out. And the winds, so the winds are going this way up here. The cloud will get pushed out

16:34up

16:34here, up to the top. And you've now got that classic cumulonimbus anvil shape. So this is your classic

16:42thunderstorm. It's raining hard here. You've got hailstones coming in as well. And at the top of the

16:46cloud, you've got the anvil stretching out above. And you can often see them from the distance.

16:51All a lot of ice packed in here. So you've got a really cold, very dense air in the anvil

16:58up here.

16:59Now, let's zoom in a little bit to that, to that anvil. I'll draw that again, just a little bit

17:06more

17:07zoomed in so we can see what's going on. So this is now just the anvil. You've got it coming

17:11in here.

17:11You've got a lot of cold, dense air in here. And as it's extending out from the top of the

17:19cloud,

17:20it's cold, it's dense air. And below it, you've got very dry air. So the air in here wants to

17:26drop,

17:27wants to fall, wants to sink. If I can get the arrows working, which I can't. It wants to sink

17:33down.

17:33The air wants to sink in there, because it's cold and it's dense. And you've got less dense drier air

17:40beneath it. So it starts to sink its way down. And then everything should kind of come into balance

17:44again. But because it's so cold at the top of the anvil, it's so cold and so dense. Actually,

17:50a lot of the energy is used in melting the ice crystals or turning the water back into water,

17:56droplets back into water vapour. So there's cold ice crystals, very cold ice crystals. They're

18:02generally cold. Lots of dense ice crystals in this area. The air starts to sink. A lot of energy is

18:08used

18:09in melting the ice. So as it sinks, it doesn't quite get in balance in time. And what happens is

18:15it stays

18:16colder than its surroundings because you're using the energy to change the form of the droplets,

18:23the ice into water that takes energy. So instead of warming up as quickly as it should do, it stays

18:29colder than it does. And so instead of just forming the end of the cloud, it actually,

18:33we get these overshoots. So it will overshoot the bottom of the cloud as it falls. It falls further

18:41than it would do normally if it wasn't so dense. And then the cloud actually tends to form into these

18:50udders underneath it. That's where you get those bulbs as that air descends through the bottom of the

18:55cloud and forms those little balls, those little udders that you see underneath the bottom of the cloud.

19:01And that is what we think is how Mamatous cloud forms. I say we think because, you know, the science

19:08isn't actually completely settled on exactly how Mamatous cloud formed. That is the most common theory.

19:15It's that cooling, but it doesn't cool rapidly enough because it takes some of the energy to melt the ice

19:20and change the form of the cloud in there to form that Mamatous cloud. So it is quite rare,

19:27but it is always beautiful when you see Mamatous clouds forming. Absolutely gorgeous, that instance

19:33from last month. And here's just a nice little video of them forming. Each of the individual bulbs can

19:41only last about 10 minutes, generally speaking, but you can keep Mamatous clouds within a cumulonimbus

19:48cloud for pretty much the duration of it. Once the anvils formed, you can keep those coming. So they can

19:54last 15 minutes to an hour, depending on how long that thunderstorm is lasting for. But each of those

19:58individual globulets, globulets, is that a word, will last about 10 minutes or so. But absolutely

20:05beautiful, beautiful clouds. Some fantastic examples. Again, go and check out the WMO Cloud Atlas for

20:11for more examples of those clouds. But really, really interesting stuff. Well, I found it interesting

20:17anyway. Hope you did too. If you did, let us know in the comments. Or if you didn't, let us

20:20know in the comments

20:21as well. Got any questions about clouds? Do pop those in the chat as well, and we'll try and answer

20:25some of them in the live on Friday. Right, what's the weather doing? I talked at the beginning about

20:31how our weather's kind of stuck in a rut, but it's it's chopping and changing every day. Nothing too

20:36extreme, nothing too severe. Although there were some lively thunderstorms last night across Scotland.

20:43But equally, yeah, nothing extreme, too much in terms of

20:46of rainfall amounts. It's going to get windier, particularly as we head into the weekends. But

20:52again, for September, nothing exceptional. It doesn't look like at this stage. So what's going

20:57on? We're seeing these weather systems moving in from the Atlantic. That is where our winds often

21:01come from. Didn't do much during the summer, admittedly, but they often do at this time of year.

21:06Let's put the bigger picture on. I talked at the start about trough ridge, trough ridge. What on earth am

21:11I

21:11talking about? Well, it's when you get this waving pattern, not dissimilar to mountain waves, but now

21:17we're in the jet stream, that fast moving ribbon of air, high up at the top of the troposphere. And

21:22we've got a classic example of it here. You can see it as Canada, it's dipping, it's waving, it's

21:28dipping, it's waving, it's dipping, it's waving. These dips are called troughs, and where it goes like

21:33that, and the end shape, that is a ridge. And we're seeing a trough ridge, ridge, trough ridge,

21:39and you're seeing that pattern all the way pretty much around from North America, right across the

21:45Atlantic to the UK. And when you've got that set up, and the jet stream's reasonably active,

21:50they all just get shunted across the UK. When you've got a trough, that dip, well generally the

21:57air is rising, so we know that means clouds, often showers, and some rain. And when the air is ridged,

22:04when you've got that end shape, you've got the ridge, then the air is generally sinking,

22:06and you have drier and calmer weather. You can see it there in the jet stream, but you can also

22:12see

22:12if you take the jet stream off, in the isobar. So we've got a trough here, that dip, then there's

22:19a nice little ridge of high pressure out in the Atlantic, that's going to build in as we go through

22:23the rest of today and into tomorrow. Then there's the next trough, and associated with the trough,

22:28often low pressure systems and weather fronts, that will come in as we go through Thursday. Then

22:34there's that ridge across the north of Canada, Newfoundland, and then another ridge in there

22:41across the Great Lakes. And they'll all be heading across one after another, like a train really,

22:45because they're all being pushed along by the jet stream, if you like. So our weather is going to

22:49continue to chop and change day by day. We can see that in our weather. If we take a look

22:55at what's

22:55happening over the next few days, then you can see. This is today, we've got the trough, which is

23:01bringing that weather front that we've seen. It's been pretty feeble across the southeast,

23:04not much useful rain clearing through. And then by the time we get to Wednesday, if we fast forward

23:09to Wednesday, we've got a ridge building in. So although there's a few showers, the trough hasn't

23:14fully cleared. There are a few showers around. It's a mixture of sunshine and showers tomorrow,

23:18but I think the emphasis more on the dry weather tomorrow. By the time we get to Wednesday night,

23:23however, that next trough is coming in, bringing another spell of rain across the country. And then we're

23:28back to a brief ridge on Thursday before the next trough and the next weather system coming in.

23:34There we go on Friday. Yeah, Friday afternoon, another weather system, bringing another spell

23:40of rain. So it's going to be chopping and changing the weather through most of this week,

23:44till we get to the weekend, when it gets a little more complicated. Because by now,

23:48notice the jet stream there in the far, put it on the biggest picture. Actually, we can see it a

23:52bit better.

23:52Rather than that jet being wiggly wobbly and in wave pattern like that, it's kind of straighter

24:00across closer to our part of the world. So more typically taking the low pressures further north

24:04with ridging trying to build across the south, but where that weather front sits. So instead of

24:08chopping and changing, the weather patterns kind of stabilize and stop switching around quite so much

24:14as we head into the weekend. A bit of a question mark about where that weather front lies.

24:17A lot of troughing and ridging to get through before we get there. So that's something to watch

24:22as we go into the weekend. But certainly for the rest of this working week, the weather's going to

24:26be chopping and changing a fair bit. But not much in terms of temperatures, because our temperatures

24:31are really going to kind of stabilize now. Yes, it's still a bit warm across the southeast today.

24:37Even though there's a cold front moving through, there's still some warmth and a bit of welly in

24:43the sun, obviously lifting the temperatures into the low 20s. But that is cooler than it was yesterday.

24:47It got to 27 yesterday here in Exeter, warmest part of the UK. So that was quite humid still in

24:53the

24:53south yesterday. But look at this, if we move forward and show Wednesday, Thursday, Friday, Saturday,

24:58not a great deal of change. Temperatures are going to be high teens, maybe getting up to 20 Celsius,

25:03pretty much bang on average for the time of year. That's the daytime temperatures, nighttime temperatures as well.

25:08Well, if we put those on for the next few days, again, a bit warm in the south still this

25:15evening,

25:15but certainly compared to last night, much fresher, much cooler, much less humid air. And again,

25:22for the next few nights, rural spots will dip down into single figures. Most towns and cities staying

25:27in 10, 11, 12 degrees, that kind of ballpark. So nothing remarkable about the temperatures.

25:32There are some signs, potentially, as we go into the second half of the weekend,

25:40fast forward to Sunday, the jet itself is starting to move a little further north, taking low pressures,

25:46not dissimilar to most of the setup we had through the summer, pushing the lows up towards Iceland and

25:50Greenland, with high pressure building across the south. And tucked in here, there's actually some

25:56quite warm air, potentially. So there are signs that early next week we could see a bit of warmth

26:01of returning to parts of the south. But I won't spoil Aidan's 10-day trends too much. I'll let him

26:08talk a little bit more about that in the 10-day trend, which of course we do every Wednesday. So

26:14you

26:14want to subscribe to make sure you watch that. That will be with Aidan tomorrow. I think that's pretty

26:19much it from me for this week's Met Office Deep Dive. As I say, please do let us know in

26:24the comments

26:25what you like, if you've got any ideas for future deep dives, or if you've got any questions about

26:29weather, climate, about anything to do with the Met Office, pop those in the chat as well. And we will

26:35answer some of them in Friday's live broadcast, which we do at 12.15 every Friday. Aidan's got

26:42the 10-day trend tomorrow. But that's it from me for this week's Deep Dive. So please hit the like

26:48bump. Do us a favour and spread the love. Let everyone know that Met Office is doing this

26:52every Tuesday on YouTube. But thanks for watching. I'll see you again soon.

Deep Dive 15/09/2026 – Wavey jet keeping us things mixed – Met Office weekly weather forecast UK

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