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Distance travelled ~ 876'413'600 km

The depression is expected to bring gales and heavy rain from the west for Thursday.

Snow is likely for a time on Thursday morning across parts of central and northern Scotland, particularly affecting higher ground, but with some slushy deposits locally to lower levels, and all parts of the UK will have heavy rain for a time during the day.

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But the main feature of this depression is the gale to storm force winds it will bring. Gusts of 60 to 70 mph likely to become quite widespread across northern and central Britain, but 70 to 80 mph are expected across much of Scotland with 90mph gusts are possible in exposed places, particularly central and southern Scotland. Elsewhere across England and Wales the wind will gusts of 50-60 mph. ()

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As the storm passes through and moves away to the east of the UK on Friday north-westerly winds will bring in cold air which will see a drop in temperatures. This is expected to bring snow to northern and western parts of the UK, giving accumulations to low levels in the north. Accumulations will tend to be confined to higher ground across north Wales and much of northwest England. Untreated surfaces will also become icy at times. Other parts of the country will see a cold night on Friday with widespread frost.

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The forecast for the rest of the month is for the unsettled weather to continue, with spells of wet and windy weather interspersed with brighter, colder periods when we can expect to see frost and snow showers - the heaviest of any snowfalls are expected across higher ground from North Wales northwards, but we could occasionally see some snow cover at lower levels as well.

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Distance travelled ~ 819'651'200 km

Last week was quiet week weather wise for the UK. Most of the country saw mainly cloudy, drizzly conditions with some mist and fog.

Despite a few frosty mornings in the north west of Scotland, temperatures remained mild for the time of year. Double figure highs were reached every day and a maximum of temperature of 18.1 °C was recorded on Sunday at Otterbourne, near Winchester, Hampshire.

Elsewhere in the world, flooding continues to cause havoc in Bangkok, where the death toll has now risen to over 500. The weather has started to ease after months of monsoon rain, but the volume of flood water is continuing to cause problems.

Heavy rainfall has also affected Italy, where thousands were forced to evacuate around the River Po in Turin when water levels rose by 4 metres. Seven people are thought to have died as a result of the storms and torrential rain in the country.

Over in North America, severe winter storms have hit both Canada and Alaska. In British Columbia a snow storm caused severe disruption to travel networks and power supplies, with ferries to Vancouver Island forced to stop sailing. Meanwhile, Alaska saw winds of up to 100mph combined with high seas and blizzard conditions. During the storm the rate of ice accretion - the process of ice building up on solid objects - was more than 15.6 inches an hour.

The week ahead

The UK:

• The quiet weather is expected to continue with temperatures remaining generally around or slightly above normal throughout. However, there is the potential for some heavy rain and strong winds across north-western parts of the UK later in the week.

Across Europe:

• Low pressure over the eastern Mediterranean brings strong northeasterly winds through the Aegean Sea early in the week. Severe weather warnings have been in place for much of Greece, with storm force winds forecast in some areas. Although the strongest winds are likely to be on Monday, it will stay pretty breezy in the area for the next few days.

• Bitterly cold weather continues in eastern Turkey, Armenia, Georgia and Azerbaijan, although temperatures should start to rise from Tuesday.

Across the Americas:
• Following the recent storms affecting the west coast of Alaska, the state is set to see temperatures falling dramatically over the next few days to be 10-15 °C below average by Friday. Night time lows of -35 °C look possible in places, with daytime temperatures at times not far above.

• In South America there's heavy rain, strong winds and some relatively low temperatures for a time in southern Brazil at the start of this week, before the associated low pressure system moves east into the Atlantic around Tuesday.

Across Africa:
• The same eastern Mediterranean low pressure that brings strong winds to Greece is also causing lower than average temperatures over large parts of Egypt, mainly the north east, over the next few days, with above average winds and precipitation as well at first.

Across Asia:
• A spell of unsettled weather is forecast for the Philippines over the next few days. The system causing this will move into the South China Sea around mid-week and may later affect parts of southern China.

• Some unseasonably low temperatures are expected in the Himalayas - Tibet, Bhutan and Nepal - with daytime temperatures at Lhasa, Tibet potentially around ten degrees below normal.

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Distance travelled ~ 817'507'200

Air is funny stuff. The oxygen that we take from it is our most fundamental necessity, but air is invisible, odourless, colourless and easily ignored. Day to day, we have better things to think about. But air is doing some interesting things while we're busy ignoring it, and one of those things has the potential to cause huge damage in Southern California this month.

Let's have a think about what air is. What you're taking into your lungs at this very moment is a bustling crowd of billions of molecules, zooming about at speeds of about 1150mph, bouncing off each other and anything else they hit. It's busy down there in the microscopic world that we can't see.

To get to Californian weather, we need to know something about gases. Air temperature is just a way of measuring how fast the gas molecules are all zooming about. If they're travelling on average at 1100 mph, it's about zero degrees Celsius. If they're moving at 1300 mph, it's 100 degrees Celsius, and so on. So temperature represents the amount of energy that's carried by those air molecules.

Now here's the really interesting bit. It happens every time you open a pressurized fizzy drink. As you unscrew the bottle, high pressure air rushes out and when it meets the lower pressure outside, it expands. But for the gas to expand, those molecules have to move further apart from each other and that takes energy. So they use some of their movement energy. As they move apart, the molecules slow down, and that means that the temperature goes down. Put your hand over the bottle opening, immediately after you open it, and you'll feel the cold. So when air expands, it cools, and when it's compressed, it heats up - that's why your bicycle pump gets hot as you pump air into a tyre. Physicists call this adiabatic heating.

Why should all this matter for California? It's because an atmospheric version of the bicycle pump happens there on a huge scale at this time of year. To the east of California there are vast deserts at an altitude of 2 km. Weather systems over those deserts push air westwards, so it flows down the slope towards the ocean. Air pressure gets higher as you go downwards, because there's a greater weight of air above you. So the air flowing down the slope is compressed as it goes and it heats up. Then there isn't just a wind, there's a really hot wind, about 20 degrees Celsius higher than temperature in the desert. These winds are called the Santa Ana winds, and as the air flows down towards the ocean they get funnelled down canyons, increasing the wind speed even more.

The result is that after the hot dry summer, San Diego, LA and everything in between get dried out even more by a massive atmospheric hair dryer blowing down from the high deserts. And if a spark starts a fire, there isn't much to stop it. This is why wildfires are such a hazard at this time of year, and why California's fire service is now on high alert. There have been huge destructive fires in the past few years, and Californians just have to prepare for them and do everything they can to prevent a blaze starting. It all happens because of a fundamental rule of physics - that air gets hot when you squash it. At this time of year, it's a rule that many Californians probably think they could do without.

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Distance travelled ~ 809'788'800 km

High winds are a common occurrence on Earth but they don't often reach more than 150km/h. The record is held by the Tropical Cyclone Olivia as it moved across Australia in April 1996 which battered the land with gusts of 408 km/h. This is nothing compared to the rather more serene and beautiful looking planet Saturn. High in the atmosphere of this gas giant the wind speeds have been measured at a staggering 1800km/h.

The concept of what causes wind, which is effectively the flow of gas from one place to another, is pretty simple to understand. Take the Earth for example; warmth from the Sun heats the surface which then in turn heats the atmosphere in contact with it. As the air warms, it becomes less dense than the surrounding air causing it to rise which results in an area of low pressure as 'less air' is present. Other surface air will then rush in to effectively fill the void left from the rising air and we experience that as wind.

The storms we see on Earth are just extreme versions of this with areas of particularly low pressure at the centre. Typically they form over the oceans which are a vast reserve of energy. Water is very good at storing and retaining incoming solar energy and its this along with the moisture that gives storms their awesome power.

On Saturn the extreme storms that drive the winds are very similar in structure to those on Earth with low pressure systems but it's the source of the energy which sets them apart. Instead of vast bodies of water, the heat driving the storms on Saturn comes from deep within the planets core. When it formed around 5 billion years ago heat was generated when the pieces from the proto-planetary disk crashed together and its the slow but steady release of this energy which has driven Saturn's super-storms.

The Cassini spacecraft witnessed first hand one of Saturn's ferocious storms whilst it was orbiting the planet in December 2010. It was quite lucky given that Saturn is usually relatively storm free, unlike Jupiter however the lucky break gave planetary scientists a unique insight into the local weather system. The images show the storm covering nearly 4 billion square km and analysis of the lightning strikes showed a ten times more flashes than in other storms studied since 2004.

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Distance travelled ~ 647'488'000 km

Hurricane Katia, currently in the western Atlantic is set to steam due east towards the UK and is expected to . With it will come the risk of severe gales and heavy rain to parts of the UK. The strength and depth of this September storm is quite unusual, but similar storms that originated as hurricanes have affected the UK in the last 20 years several times.

Hurricane Bill - 2009

You only have to look back as far as 2009 to find a storm that crossed the Atlantic. Hurricane Bill formed on August 15th and reached the UK as a post tropical storm on August 25th, bringing severe gales and heavy rain two days after being downgraded from a hurricane to a tropical storm.

Hurricane Alberto, Gordon and Helene - 2006

In 2006, three post tropical storms reached the UK. Alberto, Gordon and Helene all brought wet and windy weather to the UK. Alberto combined with a cold front to the west of the UK whilst Gordon brought record warm temperatures as tropical air pushed north across the UK, but also strong winds that brought down power lines in Northern Ireland.

Hurricane Isaac and Leslie - 2000

These two hurricanes both affected the British Isles in the year 2000.

Hurricane Karl - 1998

Hurricane Karl made its way to southern Britain in 1998.

Hurricane Lili - 1996

Perhaps the most similar storm to Katia was in 1996 when the remains of hurricane Lili pushed across the UK just one day after being downgraded from a hurricane. The post tropical storm ran across Britain on 28th and 29th October. The storm brought gusts in excess of 90 mph, bringing widespread impacts across the UK and causing significant disruption.

Hurricane Katia - 2011

Katia is currently a category one hurricane off the east coast of the US and will run across the Atlantic through the weekend bringing the risk of severe gales and storm force winds in places later on Sunday and through Monday.

Although it is expected to be windy everywhere, it is uncertain as to exactly which parts of the country will see the very strongest winds and therefore you should stay up to date with .

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