Why Can't Elephants Jump?: And 101 Other Tantalising Science Questions - New Scientist (2010)

Chapter 6. Weird weather

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 Rain imminent

During the Monaco Grand Prix, I was watching the drivers battle with the damp conditions when the commentator said that rain was expected at the track in 6 minutes. How can forecasting be so accurate? If such technology is available, why isn’t it offered to the public? Incidentally, the rain didn’t arrive on this occasion, but was predicted with such confidence that presumably the forecasting must be accurate most of the time.

Derek Bolon

Tetbury, Gloucestershire, UK

The reason we can achieve such accuracy in forecasting at each Grand Prix is because we have radar and weather stations on site provided by the the FIA, the governing body of world motor sport. In Monaco, these are operated by the French meteorological office, Météo France, which has an experienced forecaster on site to predict when inclement weather will arrive.

Each racing team is offered a subscription to the service, which is then fed to the team’s timing stand at the trackside. This displays radar images, temperatures and pressures in near real time. There is also a minute-by-minute rain update provided via the trackside TV feed system, which also updates on-track events such as the blue warning flags waved at cars about to be lapped.

Joseph Birkett

Trackside IT Engineer

Red Bull Racing

Milton Keynes, Buckinghamshire, UK

Many readers thought the answer was probably weather radar. The US, Australia, the Netherlands and Germany all appear to have publicly available services. Here’s a typical example – Ed.

I assume they use a weather radar. We have one sited 200 kilometres to the west of us, at West Takone, Tasmania, and by checking on the Australian Bureau of Meteorology website we can see a real-time picture of where the rain is falling and how heavy it is, or a loop showing how fast the rain is moving towards us. Great for getting the washing dry.

When our roof collapsed in the middle of winter and we were living under tarpaulins, it was wonderfully useful. The builders kept an eye on the weather radar, and when there looked to be at least 20 minutes clear they would whip off the tarps and work until I gave them a 5-minute warning to put them back again.

Jan Horton

West Launceston, Tasmania, Australia

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 Chill factor

Is it possible for it to be too cold to light a fire? In the same vein, is it possible for it to be so cold that a fire, with enough fuel to keep it going under normal circumstances, goes out?

Ian Tilly

Ashford, Kent, UK

A fire is a rapid exothermic (heat-producing) chemical reaction which occurs between a fuel and an oxidant, usually oxygen. The rate of any chemical reaction increases with temperature, as the molecules move faster and collide more often. Most fuels and oxidants can coexist at room temperature without spontaneously igniting. Although they may slowly react together, the rate of reaction is so slow that the heat produced is dissipated before the mixture can heat up – think of a slowly rusting iron nail.

To start a fire, you need to heat the mixture to its ignition point. This is the temperature at which the rate of reaction is high enough to produce heat more quickly than it is lost to the surroundings. Heat energy starts to accumulate, driving up the rate of the reaction, producing even more heat, and so on. This runaway reaction is what we call a fire.

So the answer to whether it can ever be too cold to start and sustain a fire depends on the difference between the ignition point and the temperature of the surroundings. Heat is lost to the surroundings by a combination of radiation and convection. The colder the surroundings, the greater the rate of heat loss via these processes. Therefore, for low-grade fuels, such as wood, that do not produce much heat when they combust, a fire would not be able to sustain itself if the surroundings were cold enough. Instead, to keep it burning you would need a continual supply of heat from an external source.

However, there is a limit to how cold you can make the surroundings, culminating at absolute zero. So fuels with a high enough heat of reaction will always be able to sustain a fire, no matter how cold the surroundings. Conversely, even fuels that are normally difficult to ignite can be made to burn if the surroundings are hot enough.

Simon Iveson

Department of Chemical Engineering

University of Newcastle

New South Wales, Australia

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 Surround sound

I live a kilometre north of a busy motorway. When the wind is coming from the south the noise of the motorway is noticeably greater than when the wind is coming from the north. Assuming a wind speed of a mere 30 kilometres per hour, how can the wind direction affect the level of traffic noise I hear when the speed of sound is more than 1,235 kilometres per hour?

Jim Turton

By email, no address supplied

Wind is the single most influential meteorological factor within approximately 150 metres of a noise source such as a highway.

The wind’s effects are mostly confined to noise paths close to the ground. The reason for this is what is known as the wind shear phenomenon: the wind speed is lower in the vicinity of the ground because of friction.

This velocity gradient tends to bend sound waves downward when they are travelling in the same direction as the wind and upward when in the opposite direction. This process, called refraction, creates a noise reduction upwind from the source of the sound and a noise increase downwind from the source.

Over distances greater than 150 metres, vertical air temperature gradients are more important. This is because under certain stable atmospheric conditions, temperature increases with height either from the ground up, or from some altitude above the ground. Such an inversion occurs when a layer of warm air is trapped between layers of cold air. This inversion increases the speed of sound with increasing altitude, causing sound waves to be refracted back towards the ground. This would lead to an increase in ambient noise levels for far-away listeners.

Victor Zeuzem

San Mateo, California, US

The wind does not appreciably speed up the sound and, even if it did, this would not explain why the sound should be louder. What happens is that the sound is refracted, or ‘bent’, in rather the same way as a ray of light is refracted as it passes from air into water.

This happens because wind speed is not constant with height. At 100 metres altitude, say, the air is moving at 30 kilometres an hour. Closer to the ground, however, trees and buildings get in the way, so the wind speed is lower. At ground level, in between the blades of grass, the wind speed is close to zero.

Sound moving horizontally through air when there is a velocity gradient like this will be bent upwards if it is moving against the wind, and downwards if moving with the wind.

The best way to visualise this is to imagine a row of joggers with their arms hooked together running in a straight line on a beach. If the sand is uniformly firm, they all run together at the same speed and the line moves straight ahead. Now imagine that the sand is moist but firm at the end of the line of joggers nearest the water (providing fast-going conditions), and dry and soft at the other end away from the water (providing very slow running conditions). In this case the line will curve around because the fast runners have to stay hooked to the slow runners.

So, by the same reasoning, if sound travels 30 kilometres an hour faster at 100 metres altitude than it does at ground level, the sound wave front, which can be thought of as a planar disturbance, bends downwards.

Hugh Hunt

Department of Engineering

University of Cambridge, UK

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 Hair-raising event

Walking along the breakwater at Berwick-upon-Tweed in northeast England, my granddaughter and her mother noticed their hair was standing on end. It started to rain soon afterwards, but there was no thunder or lightning that day. What was happening?

Richard Turner

Harrogate, North Yorkshire, UK

From one of my physics textbooks I recall a hair-raising picture of a woman standing on an exposed viewing platform at Sequoia National Park in California. She was in grave danger; lightning struck only minutes after she left, killing one person and injuring seven others (Fundamentals of Physics, 6th Edition, by David Halliday, Robert Resnick and Jearl Walker). It’s likely that similar conditions were abroad on this day.

Most lightning clouds carry a negative charge at their base. Anything close to the cloud would feel the effect of electrostatic forces: electrons in a person’s hair would be repelled downwards, leaving the ends of the hair positively charged. The positive hair tips then get attracted to the cloud – and repelled by each other – and stand on end. It’s rather like rubbing a balloon on someone’s hair to make the hair stand on end: the balloon becomes negatively charged and the hair is attracted to it.

Lightning victims often describe how they felt tingly and their hair stood on end before they got struck. Fortunately, air is a good electrical insulator and, in this instance, the charge in the clouds wasn’t high enough to jump down to earth, so there was no lightning. However, this was probably a lucky escape for your family. If your hair stands on end outdoors or your skin is tingling, lighting may be imminent and it’s best to run for suitable shelter.

Iain Longstaff

Linlithgow, West Lothian, UK

The phenomenon described above is known as luck – the two people were fortunate not to have been struck by lightning. Experienced hikers and climbers know that this hair-raising phenomenon can be a precursor to a lightning strike and are taught to flatten themselves or, if climbing, dive for lower ground.

There is a vertical voltage gradient in the atmosphere, typically in the order of 100 volts per metre on a clear, dry day. For an average adult male, then, there will be a 180 to 200-volt difference between the toes and the top of their head.

When electrically charged would-be storm clouds scud overhead, an induced ground charge follows the clouds, markedly increasing the voltage gradient. If the potential difference is sufficient to overcome the resistance of the air – around 3 million volts per metre – then lightning leaps across the gap. In practice, lightning strikes are possible at substantially lower voltage differences. The fact that the reader saw no lightning and heard no thunder merely suggests that, luckily, the voltage never rose high enough for a lightning strike.

Larry Constantine

Department of Mathematics and Engineering

University of Madeira

Funchal, Portugal

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 Apple melt

The snow at the base of our small apple trees melts before snow elsewhere has melted. We’ve seen this under other trees too. Why?

Robert Campbell

Uetendorf, Switzerland

Snow, like everything else, including apple trees, emits and absorbs radiation. While ultraviolet and visible radiation are strongly reflected (not absorbed) by snow, it is however a strong absorber of infrared radiation. The battle between the absorption and emission of radiation determines whether there is net warming or cooling of the snow – or neither.

So why would snow under a tree melt faster? At night, snow in the open absorbs infrared radiation from the ground and from the sky – which can be below -30 °C when it is clear.

Snow underneath a tree absorbs radiation emitted by the ground and by the tree, which is likely to be significantly warmer than the sky, so it emits more infrared energy.

This difference is sufficient to explain why snow underneath a tree might melt faster than snow of the same depth that is out in the open, and also explains why frost often does not form around trees.

It is also possible that shelter provided by the tree when the snow was falling led to a thinner layer of snow there than in the rest of the immediate vicinity!

Thomas Smith

Environmental Monitoring and Modelling Group

Department of Geography

King’s College London, UK

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 Happy not sad

Some years ago, between rain showers, I noticed an upside-down rainbow (u-shaped rather than n-shaped). The colours were also reversed. It appeared around 40 degrees above the horizon and was smaller than an upright rainbow. It persisted in a semicircle for about a minute before slowly fading from one side, the remaining arm lasting for another minute. Can anyone explain this?

R. Dufton

Banbury, Oxfordshire, UK

Rainbows are indeed circular but intersect with the ground before their full circle can be achieved, giving an arc shape. As this rainbow was high in the sky (40 degrees above the horizon), we can assume it’s possible other factors were involved, including reflection.

Michael Edie

Petersfield, Hampshire, UK

This is possible if the viewer has a reflecting surface, such as a sheet of very calm water, behind them. The reflection of the Sun from this surface can produce a rainbow in front of the viewer. Because the Sun is reflecting from water behind the viewer, if the viewer were to turn around and observe the Sun’s reflection in the water it would appear as though it was beneath the surface of the water. The centre of the Sun would, therefore, appear to be below the horizon. A full rainbow circle could be produced under these circumstances appearing in front of the viewer as the Sun shines up into the sky rather than the usual sunlight which shines down. What was seen in this case was a part circle comprising the lower half of the circular rainbow.

Malcolm Brooks

The Met Office Press Office, London, UK

This radially challenged and disoriented rainbow sounds like a portion of a solar halo. This effect is caused by refraction of sunlight through a thin cloud of ice crystals such as that found in a cirro-stratus cloud veil. Various circles and arcs can be produced. This particular sighting would depend on the movement and extent of the veil and the rain clouds below it, relative to the Sun. Similar, but less colourful, haloes can be seen in moonlight.

Even smaller rainbows can be produced that are concentric with the Sun and Moon, by reflection and diffraction of light by water droplets in lower clouds. This effect is called a corona, which is not the same as the Sun’s corona in an eclipse. The fact that lunar haloes and coronas appear to lack certain colours is, I suspect, linked to the human eye’s reduced capability to discriminate colours in low light conditions.

Steve Mason

Eastleigh, Hampshire, UK

U-shaped rainbows are quite simply ones originating deep in the southern hemisphere. They only occasionally migrate north on exceptional weather systems. For one to persist as far north as Oxfordshire is probably a record.

G. W. Storr

Bournemouth, Dorset, UK

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 Drip dry

If I alight from the bus and it is raining, I tend to run for my door, in the belief that I will arrive home less wet than if I walk. However, I have heard that the same number of raindrops will strike me whether I run or walk. Is this really the case?

Mark Haines

London, UK

The volume of space swept by the body between bus and door is identical. Therefore, assuming a constant rate of deluge, the number of falling rain drops (below the crown of the head) swept is the same. However, the number of raindrops falling directly on to the top of your head is proportional to the time spent exposed to the rain, so running reduces this component.

But running will deposit all the swept component in a shorter time. This will produce greater apparent wetting since normal evaporative drying has less time to work. So for light showers, with small swept and falling components, walking is probably preferable. We make this complex decision completely unconsciously, while also taking into account the likelihood of the rain becoming harder or lighter, the distance we have to travel, and our ability to run.

It would be interesting to confirm this theory by filming pedestrians, recording the rate of rainfall, and relating the latter to the point at which the former begins to run.

Mike Stevenson

Millom, Cumbria, UK

If the walker is in the Lake District, where horizontal rain is common (and always opposes the direction of travel), then it is recommended that they move as quickly as possible because the volume of drops swept out through the rain is now determined by the relative velocity of rain and walker multiplied by the time taken on the journey.

Indeed, if the rain is moving over the ground at speed vr (opposing travel) then the walker, moving at speed vp, will be 1 + vr/vp wetter than in vertical rain by the time they reach shelter. By running to keep up with the rain (defined as vp = -vr) it is theoretically possible to stay dry.

Martin Whittle

Sheffield, South Yorkshire, UK

The following (from memory) is attributed to one D. Brown of York:

When caught in the rain without mac,

Walk as fast as the wind at your back,

But when the wind’s in your face

The optimal pace

Is as fast as your legs can make track.

Matthew Wright

University of Southampton, UK

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 Getting the drift

Why is it that snowstorms can last a long time and precipitate a deep layer of snow, but hailstorms are brief and do not result in a deep layer of fallen hail?

Mathias Brust and Robert Wilson

University of Liverpool, UK

Most snow falls from extensive banks of stable nimbus clouds. The genesis of a snowflake requires fairly slow and continuous crystallisation with little turbulence. If the wind continues to carry clouds towards high ground, or if a warm air mass slides smoothly over a cold one, snow can persist for as long, and over as large a continuous area, as ordinary rainfall.

Hailstones are formed in conditions of violent convection inside tall, isolated, unstable cumulonimbus clouds, where there is very strong internal circulation. Most hailstones have a layered structure, showing that they have been cycled several times between freezing and melting levels. Eventually the cloud grows to the point where its shadow prevents the sun from heating the ground which normally supplies the rising thermals necessary to keep the stones airborne, and so they fall in a brief, localised burst.

Deep layers of hail do occur but, as the largest cumulonimbus clouds are formed on clear sunny days over hot ground, and as the same cloud may also precipitate warm rain, hailstones tend to melt quickly. As hail is macroscopically denser than fresh snow, the same mass of water will produce a much deeper layer of snow over a given area.

Alan Calverd

Bishop’s Stortford, Hertfordshire, UK

Very severe hailstorms, such as those that cause crop damage in the summer in North America, can produce a deep layer of fallen hail. I have seen such a storm in Nebraska deposit about 10 centimetres of 5-millimetre diameter hailstones over a wide area in less than 20 minutes. At first glance, the appearance on the ground was of heavy snowfall. Of course, the storm moved on, the sun came out and the hailstones melted. In the UK our thunderstorms are not so severe and we only rarely and briefly see a coating of hail on the ground.

Clive Saunders

Manchester, UK

It is not necessarily true that hailstorms cannot produce a deep layer of fallen hail. Last summer, on a hot, sultry day, we had a thunderstorm that lasted for several hours. In the course of it, there was a fall of enormous hailstones (the largest I have ever seen) and they piled up in impressive drifts. Obviously, it was too warm for them to last for a long time on the ground, and they melted, producing a flash flood. Our garage was flooded to a depth of about 5 centimetres as a result.

Diana Brown

Etagnières, Switzerland