BBC | 23 Degrees

Day 362: Global perspective

Wed, 28 Dec 2011 09:00:00 +0000

(Peter Gibbs is a BBC weather forecaster and appears as an expert meteorologist on "The Weather Show" for the BBC News channel. He started his first guest blog post for 23 degrees with 'What would happen if the Earth spun the other way' and provided much food for thought with his post on 'Abundance in fruits indicator to past British weather'. His post on 'What's in a name' cleared some misunderstandings that where flying around the web as remnants of hurricane Katia stirred it's way to the UK, and he provided us with a breakdown of the difference between cyclones, typhoons and hurricanes with his post on Cyclone Yasi. Keep up to date with Peter Gibbs - @peterg_weather)

Distance travelled ~ 929'745'600 km

It may seem surprising, but weather forecasters need to take a rather parochial view of the world. At an airport, the forecaster has to predict cloud base, visibility, wind speed and direction in great detail over a few hours for a very specific location. Even a forecaster with a national brief will tend to concentrate only on the weather systems moving across that country and give no more than a passing glance to the storm spiralling across neighbouring areas.

One of the advantages of working as a weather broadcaster on BBC World is that I get to see the whole picture and can begin to understand the interactions of the global weather system with its regular seasonal pulse. A group of thunderstorms produces newsworthy rainfall as it tracks westwards across equatorial Africa, grows into a hurricane over the tropical Atlantic to threaten east coast America, then gets caught up by the jetstream and races across the north Atlantic to bring rain and gales to northwest Europe, passing through several forecast jurisdictions en route.

Other rhythms overlay the annual one. Swings from El Nino to La Nina take place over periods of several years and enhance or diminish normal seasonal features, especially rainfall. 2011 has been mostly a La Nina year,

with unusually warm water washing into the western side of the Pacific. The extra atmospheric moisture this provided was the likely cause of January deluges in Sri Lanka and the Philippines, as well as the extraordinary flooding in Queensland where an area the size of France and Germany was underwater for a time.

Continental landmasses tend to produce the biggest temperature contrasts and hence the most violent weather, especially during the transitional periods of spring and autumn. April 2011 was a record month for tornadoes in the USA with an estimated 600, smashing the previous April record of 257 and even beating the all time monthly record of 542, set in May 2003. Arctic air pushed further south than usual, meeting air from the exceptionally warm waters of the Gulf of Mexico and combining with a jetstream pushed unusually far south by La Nina.

As the Atlantic warmed, an active hurricane season was expected and the predictions were spot-on with a total of 19 named storms, of which seven became hurricanes including three major hurricanes of category 3 or above. Surprising then, that we had to wait for a record eight tropical storms to come and go before our first hurricane. But once formed, hurricane Irene made the biggest impact, passing through the islands of the northern Caribbean before becoming the first landfalling hurricane in the USA since 2008.

La Nina was in the dock again as the likely culprit when weeks of heavy rain produced some of the worst floods on record in Thailand. The monsoon season started early and finished late, meaning there were even greater volumes of water than usual flowing from the mountainous north to the low-lying plains of the south.

Having a global perspective makes me even more appreciative of our UK climate. The British Isles are at the crossroads of European weather. Atlantic winds are a moderating influence, while the proximity of continental Europe can provide bigger swings from hot to cold. Last December found me gliding on Nordic skis across the snowfields of Berkshire, while this December the Christmas journeys to friends and family will be easier on roads kept clear of snow and ice by mild westerlies. There is the excitement of the occasional mid-latitude depression or summer thunderstorm, but without the devastation of hurricanes and monster tornadoes.

Meteorological variety without the jeopardy. If you have to be a parochial forecaster, the UK isn't such a bad place to be.

Blowing hot and cold

Mon, 10 Oct 2011 12:30:00 +0000

Distance travelled ~ 725'583'200 km

The first weekend of October saw UK beaches packed with day-trippers, while Londoners headed to the parks in an effort to stay cool during what turned out to be the hottest October weekend on record. Gravesend in Kent reached 29.9C, hotter than most days this summer.

Just a few days later the shorts were back in the drawer, the coats were on and happy hikers were tramping through fresh snow on the Scottish mountains. Elsewhere, late summer warmth was replaced by snow in the Alps and in California's Sierra Nevada, just 3 months after the last of the previous winter's snow had melted, 2 feet of fresh stuff arrived.

Such wild swings in the weather are not unusual at this time of year as the seasons change. The exceptional UK heatwave was caused by persistent southerly winds bringing very warm air from North Africa and the Mediterranean, while the early Scottish, Alpine and Californian snowfalls were brought in by a rapid switch to northerly Arctic airstreams.

Having passed the equinox, the longer nights and shortening days in the northern hemisphere lead to a net loss of warmth during each 24-hour cycle. The Arctic ocean, surrounded by rapidly cooling land, begins to freeze over. Further south, the cooling is slower. The warm waters of the Atlantic and Mediterranean act as huge stores of summer heat. As executive producer Jonathan Renouf said recently in this blog (In a constant search for Equilibrium), weather is all about gradients. In this case it's the north/south temperature gradient that's tightening, which means a switch of wind direction can bring a big change in temperature.

So, wondering what to wear tomorrow? The answer, my friend, is blowing in the wind.

What's in a name?

Fri, 09 Sep 2011 13:00:00 +0000

Distance travelled ~ 647'166'400 km

(Peter Gibbs is a BBC weather forecaster and a regular blogger for 23 Degrees. With all the current Atlantic hurricane activity, here Peter tackles some of the questions flying around as hurricane Katia makes it's way to the UK.)

"Do we get hurricanes in the UK?"

No, it's not possible.

"But what about the 1987 storm, wasn't that a hurricane?"

Well, it did have hurricane force winds, which was why it knocked down 15 million trees, but it wasn't a hurricane.

"How does that work, then?"

I've lost count of the number of times that I've had that conversation during my career, but it's a reasonable question and one that's worth exploring during this very active Atlantic hurricane season.

Katia moving toward the northeast over the open atlantic

Weakening is indicated since the hurricane is already reaching cooler waters and Katia is forecast to become post-tropical in about 36 hours.

Hurricanes are creatures of the tropics, they need the warmth and humidity of tropical seas to develop and survive. The core of the storm consists entirely of warm air and it's the release of latent heat as this air rises and condenses into clouds which gives the hurricane its power. It's a bit like a pan of water coming to the boil as you apply heat from below.

Once formed, a hurricane moves through the surrounding atmosphere like a cork floating down a stream, becoming almost a separate entity. The strongest winds form in the lowest layers of the storm, close to the storm's centre just outside the eye.

Move into temperate latitudes and weather works differently. Extratropical storms (more commonly known as mid-latitude depressions) form over much colder waters and get their energy instead from the contrast between masses of warm and cold air. The bigger the contrast, the stronger the storm. Cold air makes up the core, digging under the warmer air mass and lifting it until it forms clouds and rain along a front.

The storm becomes an integral part of the atmospheric circulation, like an eddy in a river. Strongest winds are found high up, in the form of the jetstream, at around 30,000ft while the strongest surface winds tend to occur at some distance from the storm's centre and are spread out over an elongated area. All very different to a hurricane.

Where it gets messy is when a hurricane heads out of the tropics and into higher, temperate latitudes. It goes through an identity crisis as the supply of warmth from below is cut off and cold air is drawn into the circulation, eventually emerging as an extratropical storm after a fuzzy intermediate stage.

Fortunately, the long sea track ensures that any ex-hurricanes reaching the UK have gone through full transition before they arrive. The different amounts of available energy mean that even the most powerful of extratropical storms would barely make it onto the bottom of the hurricane scale.

So that's how it works

Hurricane Irene could still cross New York this weekend as category 3

Fri, 26 Aug 2011 17:00:00 +0000

Distance travelled ~ 613'398'400 km

Image courtesy of National Hurricane Center/NOAA

They're calling it the biggest storm in a generation. Hurricane Irene is set to bring flooding, power cuts and travel disruption to the east coast of America this weekend and looks like it may be the worst storm to hit the region since hurricane Bob in 1991.

Irene is vast. On Friday, the core of hurricane force winds extended up to 90 miles or 140km from the centre, while storm force winds spread out as far as 290 miles or 465km. If Irene was centred over London, there would be gales from Edinburgh to Paris.

Irene weakened slightly during Friday as slightly drier air was pulled into the storm along with stronger upper-level winds, but as it continues to track northwards over sea temperatures as high as 29C it is likely to be a category 3 hurricane as it makes landfall in North Carolina. (Check out NOAA for description of hurricane categories).

Over the weekend, the continued northerly track will take the hurricane right along the heavily populated east coast from North Carolina to New England. While interaction with the coast should weaken it, Irene could still cross New York as a full hurricane during Sunday.

Devastating winds of over 100mph would seem to be the most obvious threat, but in fact most hurricane deaths are due to flooding. The storm surge ahead of Irene is expected to lift sea levels by 2 or 3 metres, with dangerous waves on top of that. Rainfall totals of 10-15 inches (25 to 37cm) will be falling in areas that have already seen record rainfall during August.

It's been a long wait for the first hurricane of the 2011 season, but Irene seems to be making up for lost time.

Looks like an interesting weekend!

Keep updated with Hurricane Irene's development over the weekend via BBC Weather.

Turbulent weekend across USA leaves a trail of destruction

Tue, 16 Aug 2011 11:30:00 +0000

Distance travelled ~ 585'258'400 km

A turbulent weekend across the USA left a trail of destruction from the Midwest to the east coast.

Forecasters began warning of severe thunderstorms during Saturday, as a developing area of low pressure began to interact with warm, humid air across the Great Plains. Towards evening, weather radars in Indiana showed storms lining up and advancing towards Indianapolis. Tragedy followed, when a freak gust of 60-70mph toppled a stadium at the Indiana State Fair as the storm front approached.

Image from Accuweather.com

Severe thunderstorms produce vicious downdraughts as heavy rain and hail falling through the cloud cools the surrounding air, making it heavier and denser. As this hits the ground, it spreads outwards as a gust front which can arrive several minutes ahead of the main storm. These "downbursts" are a major hazard for aircraft as they can change the airspeed over the wings in a matter of seconds, causing it to stall. They are also very difficult to forecast precisely, as the strongest gusts can be very localised with all the energy focused into a narrow zone. This seems to be what may have happened at the Indiana State Fair.

The developing area of low pressure created different problems as it pushed further east. Slowing as it reached the east coast on Sunday, the swirling system began to replenish its store of warm, moist air from the Atlantic waters. This was converted into torrential rain which fell for hour after hour across New Jersey and New York City. JFK airport smashed their all-time daily rainfall record with a massive total of 198mm, nearly twice the amount they would normally see in the whole of the month.

Some of that rain would have been very welcome further south. In Waco, Texas, they've had 62 days this year with a temperature of 100F (38.4C) or above. One more and they'll equal the record set in 1980.

Abundance in UK summer fruits an indicator to past British weather?

Thu, 14 Jul 2011 12:00:00 +0000

Distance travelled ~ 500'409'600 km: day 195 in Earth's orbit

There's been a strange groaning sound emanating from gardens across the country this summer. It's not coming from the creaking limbs of aged gardeners, but from the strained boughs of fruit trees laden down with a bumper crop of steadily swelling apples and pears.

Lots of people remarked on the stunning display of blossom this spring and this has translated into one of the most prolific fruit crops in years. Not just tree fruit either, blackcurrants and other berries seem to have done equally well.

This can all be put down to the combination of a cold winter followed by a warm spring. Most plants that we grow in the UK go into a period of hibernation during the winter, before bursting back into growth as temperatures rise in spring. If they don't get a cold enough spell of weather during the winter they never go into full dormancy, which means they have less energy left over for spring growth.

Once the blossom appears, it's vulnerable to frost damage if there's a late cold snap. Cold weather also means that pollinating insects stay at home. This all means that the fruit doesn't set and you end up with a poor crop.

So why is this year so good? Well, the coldest December in a century was just the thing to push fruit trees and berry shrubs into a good dormant spell, followed by the extravagant display of blossom as temperatures soared through March and a record-breaking April. Late frosts held off until May, by which time the fruit had set.

So all this suggests that an abundance of berries in the hedgerow is more reliable as an indicator of past weather, rather than a prediction of a hard winter to come as weather lore would have it.

But then again, it was a meandering jetstream and the resulting blocked weather pattern that brought us two cold winters in succession. There is some evidence that North Atlantic weather may occasionally get stuck in a blocked pattern for a few years at a time.

So maybe the berries are trying to tell us something. As ever with weather lore, there could be a grain of truth - but don't bet your holidays on it.

Cyclone Yasi - The most powerful to hit Australia

Fri, 04 Feb 2011 16:52:00 +0000

d ~ 90'048'000 km: day 35 of Earth's orbit

1000km (620 miles) wide and packing winds of 300km per hour (186mph), the recent cyclone Yasi was possibly the biggest and most powerful to hit the Australian coast in recorded history.

NASA Earth Observatory

It was the strongest storm globally since Super Typhoon Megi, which hit the Philippines in October 2010 and comparable to Hurricane Ike which raged across the Caribbean and Gulf of Mexico in 2008.

So what's the difference between cyclones, typhoons and hurricanes? Simply geography. They are all the same phenomenon, but given different names around the world. Cyclones in the southwest Pacific and Indian Ocean, typhoons in the northwest Pacific and hurricanes in the tropical Atlantic and eastern Pacific. To make things easier, I'll just refer to tropical cyclones from now on!

The habitat of these heat-fuelled beasts is confined to a zone between 5 and 30 degrees of latitude north and south of the equator. Any closer to the equator and our old friend the Coriolis force is too weak to induce any spin to the developing storm, while outside of the 30th parallel there just isn't enough heat to provide the necessary energy.

To make a tropical cyclone, you need very warm sea water with a temperature of at least 27C. This is why the storms occur seasonally, once summer sunshine has pushed sea temperatures beyond the threshold. They work like a huge heat engine, sucking up very warm, moist air from the sea surface which is then lifted and condenses into clouds. This releases even more heat which keeps the air buoyant and moving upwards before it finally cools and flows outwards at higher levels.

Cyclones play a very important role in transferring heat from the tropics to higher latitudes, acting as a natural safety valve. They also provide a large percentage of the annual rainfall in places such as the Philippines.

So despite the destructive power of cyclones, without their sustaining rain survival would be much more difficult in many parts of the world.

More on Tropical Cyclones:
Mechanism of a Cyclone
Anatomy of a cyclone
Hurricane FAQS

What would happen if the Earth spun the other way?

Mon, 24 Jan 2011 19:08:31 +0000

d ~ 61'747'200 km: day 24 of Earth's orbit

"Drivetime" seemed a misnomer for Simon Mayo's early evening show on Radio 2 as my car inched out of BBC TV Centre and into the homeward crawl.

Twenty minutes and 200 yards later, Simon read out a question from a listener whose five year-old daughter had asked "What would be different if the Earth spun the other way?" Easy, I thought - everything would travel the other way across the sky and we would greet the sun from the west every morning.

My smugness lasted until Chiswick, when I realised I'd forgotten the little matter of Coriolis.

The Coriolis effect transfers the spin of the earth into the circular motion of winds around a weather system. Storms spin anti-clockwise in the northern hemisphere and clockwise in the south. Reverse the rotation of the Earth and you put the storms into reverse too. Interesting, but apart from confused weather forecasters I couldn't imagine a huge impact.

Reaching the M4, the pace of traffic is picking up to a gentle jog and this seemingly simple question is quickening my thought processes too. What about the jet stream?

This river of high altitude, fast-moving air steers the mid-latitude depressions across the planet from west to east. Swirling masses of cloud and rain are pushed from Japan to the Pacific coast of America, and from Newfoundland to Cornwall. Reverse the flow and climate changes dramatically. The British Isles loses the moderating effect of weather from the Atlantic. A harsher continental climate becomes more likely, with a predominantly easterly flow bringing bitter Siberian winds in winter and hot, dry weather in summer. Goodbye green and pleasant land.

Finally on to the M4. Now that we're really moving, the constant stream of traffic reminds me of the trade winds, another crucial part of our planet's circulation system.

The sun heats the atmosphere more at the equator than it does at the poles. On a stationary Earth the warm air would rise at the equator, moving to the poles where it would sink and flow back to the equator along the surface. Nice and simple.

Rotation complicates things. The flow breaks up into three separate cells known as the Hadley cell, the Ferrell cell and the Polar cell. Northward and southward-moving surface winds generated by the cells are then deflected to right or left by our old friend the Coriolis effect and we end up with the trade winds.

These constant easterly winds in the tropical regions were the motorways of the seas for sailing ships. A captain heading out of southern Spain could depend on picking up the northeast trades for a free ride to the Caribbean. Again, reverse the Earth's spin and the whole thing switches. Patterns of human discovery, subsequent empire-building and the resulting political geography would all be different.

The trade winds also affect the distribution of rainfall across large parts of the planet, influencing the position of some deserts and rainforests and interacting with periodic events like El Nino. It's reasonable to assume that a reversal would alter the pattern of habitable land.

Conclusion - change something as simple as direction of rotation and you change the planet we know.

And beware the innocent questions of five year-olds!

Peter Gibbs is a BBC weather forecaster and Met Office meteorologist

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