Munching Maggots, Noah's Flood & TV Heart Attacks: And Other Cataclysmic Science Moments

Munching Maggots, Noah's Flood & TV Heart Attacks: And Other Cataclysmic Science Moments

by Karl Kruszelnicki, Kruszelnicki


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Product Details

ISBN-13: 9780471378501
Publisher: Wiley
Publication date: 12/23/1999
Pages: 189
Product dimensions: 7.07(w) x 10.30(h) x 0.54(d)

About the Author

Karl Kruszelnicki is the Julius Sumner Miller Fellow at the University of Sydney and lectures at schools throughout Australia. He's also been a scientist, a medical doctor, an engineer, a carmechanic, a TV weatherman, and a roadie for Bo Diddley and Chuck Berry. He enjoys telling peopleabout the wonders and curiosities of science through radio, TV, the Internet, and his many books.You can check out his Web site at:

Read an Excerpt

Note: The Figures and/or Tables mentioned in this sample chapter do not appear on the Web.


In the movie Butch Cassidy and the Sundance Kid, there's a famous song called 'Raindrops Keep Falling on My Head'. Unfortunately, raindrops also fall on the front and back and sides of your body -- and you get wet. Usually, people prefer not to get wet in the rain, and run away from some raindrops that they would have otherwise caught. When most of us get caught in the rain without an umbrella, we make a mad instinctive dash for the nearest shelter.

But is our natural instinct to run, correct? Scientifically, what is the best tactic to use when you're caught in the rain -- run, or walk?

It's actually quite a tricky question. If you run, you'll spend less time in the rain. On the other hand, you'll run into some raindrops that would have otherwise missed you. Which effect is greater?

Run or Walk Debate - 1975

One of the first people to write about this was Jearl Walker back in 1975, in his famous book The Flying Circus of Physics. Basically, he favoured running instead of walking -- but he didn't really explain why.

Angle of Falling Rain

Jearl Walker mainly talked about the angle at which the rain is falling. If it's coming from directly above, or from in front of you, his advice is that you should run as quickly as possible to the nearest shelter. If the rain is falling on you from behind, you should try to run at the same horizontal speed as the rain. In other words, if you were running at the same horizontal speed as the falling rain, you would sweep up (or soak up) only one lot of raindrops -- and you should stay fairly dry after that.

The 'run or walk-in-the-rain' question had been asked, but had not been fully answered.

Run or Walk Debate - January 1995

Nobody did much about the 'run or walk-in- the-rain' controversy for the next 20 years. But in January 1995, this same question appeared again in the back pages of the New Scientist magazine -- along with three answers from three readers.

Wind Direction

Matthew Wright from the University of Southampton agreed with Jearl Walker about the importance of wind direction. His answer was a limerick:

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 fast as your legs can make track.

Horizontal Rain

Martin Whittle wrote about the weather in the Lake District in England, where horizontal rain happens quite frequently. He agreed with Jearl Walker when he said that 'by running to keep up with the rain it is theoretically possible to stay dry'.


If all the water vapour in the atmosphere were condensed into a single layer of water over the whole planet, that layer would be 25 mm (1 inch) thick. Water molecules have a cycle where they evaporate from the surface, travel through the atmosphere, and then fall as precipitation (liquid water, ice, snow, etc.). The average rainfall is about 1000 mm (about 40 inches). So the average annual time that an individual molecule of water spends in the atmosphere is about nine days.


Mike Stevenson talked about your clothes drying due to evaporation, and said that if you ran quickly, you'd have less time for this evaporative drying. He recommended that if the rain was quite light, you should probably walk.

The readers each had an answer -- but none of them had done the experiment.

Run or Walk Debate - November 1995

Now this kind of soft theorizing from the back pages of the New Scientist wasn't good enough for some meteorologists from the University of Reading. So in November 1995, Holden, Belcher, Horvath and Pytharoulis (hereafter called HBHP) released their ground-breaking paper, 'Raindrops Keep Falling on My Head'.

Like good scientists, they first searched the literature for all of the other scientific papers that had been written on this important topic of whether to run or walk in the rain. They then used this knowledge as a base to do more research -- by adding mathematics.

They realized that there are two ways in which a human can be made wet by a bunch of raindrops. First, the raindrops can fall on the top of your head. Second, as you move forward, you and your clothes can sweep up any raindrops that you happen to run into.


Making raindrops is a two-stage process.

In the first stage, as the water vapour rises from the oceans and the land, it cools. The water vapour will 'condense' on a tiny particle called a 'condensation nucleus' into tiny 'cloud droplets' of liquid. Condensation nuclei can include sea salt particles, combustion particles from fires, and clay-silicate particles lifted from the ground. These condensation nuclei are very small (0.002 mm) and need only very tiny upward wind currents (0.0000001 metres per second) to keep them floating. The cloud droplets (0.01 to 0.02 mm) are much bigger than the condensation nuclei, but are still small enough to be kept floating by a small upward wind current (0.01 m/ s).

In the second stage, the cloud droplet has to grow 100 times bigger. One way to do this is by a process called 'collision and coalescence'. Slightly bigger droplets will fall through the other droplets. As they fall, they can sweep up other droplets, and so grow bigger. This process is very effective in clouds in the tropics, which have turbulent wind currents inside the cloud to bring droplets into contact with other droplets.

First Theoretical Model

They assumed that a person was basically a 3-D rectangle, with different areas on the top, front and sides. They then set up a few equations to model the movement of this 3-D rectangle through a space filled with air and raindrops, and then solved these equations with a high-powered mathematics called calculus. They also assumed that an average walking pace is about 2 to 3 metres per second (m/ s), and looked at three different rainfall rates (5, 10 and 15 mm/ hour). Finally, after plugging in these numbers and then drawing some graphs, they had their answer. They concluded that if you ran as quickly as you could, you would soak up 10 per cent less water than if you walked -- hardly worth it! But they were wrong!

To Run or Not to Run?

They were a little surprised by this result. After all, in real life, people run in the rain, but their theoretical calculations did not show any big benefit in running.

But once again, the experiment had not been done.


You'll often see a falling raindrop drawn by a cartoonist as being round at the bottom, with a pointed tip at the top. It isn't real! Raindrops are quite close to round in shape, up to a size of about 2 mm. Above that size, as they get bigger, they get more flattened at the bottom. Once raindrops get to about 6 mm in diameter, they become unstable and can break apart due to the airflow.

Run or Walk Debate - 1997

In March 1997, two meteorologists, Thomas C. Peterson and Trevor W. R. Wallis, from the National Climatic Data Center in North Carolina in the USA, took the 'run/ walk' debate a little further. They published a paper called 'Running in the Rain'.

These meteorologists wrote their paper because they had been soaked by rain while jogging through the southern Appalachian forest near their office. They weren't surprised by the rain because, as meteorologists, they had watched the weather forecast. But as the rain gradually became heavier, and they slowed down as they ran up a steep hill, they began to speculate about what was the best speed at which to travel.

Once they were back in the office, they read the 'Raindrops Keep Falling on My Head' paper by HBHP. They found a few minor mistakes, and corrected them.

MISTAKE NO. 1: Peterson and Wallis realised that HBHP had overestimated the walking speed and running speed of humans.

HBHP had written that the average walking speed was about 2 to 3 m/ s. HBHP probably got slightly confused by using scientific units (m/ s), rather than commonplace units (such as kilometres or miles per hour). Two point five m/ s works out to 9 kph -- definitely faster than a walking pace. A typical walking pace is about 1.5 m/ s (5.4 kph or 3.4 mph). An Olympic runner could just reach 8 m/ s (28.8 kph, or 18 mph), so an average fit citizen in street clothing could probably reach 4 m/ s (14.4 kph, or 9 mph) on a wet and slippery surface.

MISTAKE NO. 2: Secondly, Peterson and Wallis found a mistake in the calculations of HBHP.


According to The Guinness Book of Records, the most intense rain-burst recorded in modern times was 38 mm (1.5 inches) in one minute at Barst in Guadeloupe on 26 November 1970.

Again according to The Guinness Book of Records, the location with more rainy days than anywhere else in the world is Mt Wai'ale'ale, on the island of Kauai in Hawaii -- up to 350 rainy days per year!

Going by average yearly rainfall, the wettest place in the world is in India -- Mawsynram, in the State of Meghalaya. It has 11.875 metres (467.5 inches) each year. In a single calendar month, the greatest rainfall ever recorded was 9.296 metres (366 inches), also in the State of Meghalaya in India, in July 1861, at Cherrapunji. Cherrapunji also holds the record for the greatest yearly rainfall -- 26.461 metres (1041.8 inches) from August 1860 to July 1861.

And the greatest rainfall ever in a 24-hour period was recorded on 15 and 16 March 1952, in Cilaos at La Réunion in the Indian Ocean. It was 1.87 metres (73.6 inches), which works out to 18,700 tonnes of water per hectare (about 8000 tons of water per acre).

For example, look at the case of a person with a top surface area of 0.1 square metres and a cross-sectional area of 0.6 square metres. Suppose that this person has to travel 100 metres at 1 m/ s (3.6 kph or 2.24 mph) through rain falling at 10 mm/ hour. HBHP's calculations showed that this person would encounter 0.18 kg (180 grams, or 6.35 ounces, or oz) of water -- but Peterson and Wallis used HBHP's own equations to come up with 0.06 kg (60 grams, or 2.13 oz). HBHP had made a simple mathematical mistake.

So when Peterson and Wallis used their more accurate assumptions (about walking and running speeds) and more accurate calculations, they found that a walking person would encounter 0.052 kg (52 grams, 1.83 oz) of water, but that a running person would soak up 0.040 kg (40 grams, 1.41 oz) -- a 23 per cent improvement. Twenty-three per cent is a lot better than the 10 per cent improvement of HBHP -- and suddenly, running in the rain made more sense.

Second Theoretical Model

But then Peterson and Wallis went a little further. HBHP were one of the first to explore this topic, and quite clearly stated that their model was simple. Peterson and Wallis had the advantage of reading HBHP's work, and so they came up with a more sophisticated model that was closer to the truth.

They took into account another three factors -- the velocity of the falling rain, the effect of leaning forward as you run, and rain that is driven by the wind.

Their improved model showed that in a light rain with no wind at all, running will give you only a 16 per cent reduction in wetness, as compared to walking. But if you're running rapidly and leaning forward in a heavy rain that is driven by the wind, you will end up 44 per cent less wet than if you had walked.


In a white fluffy cloud, the diameter of the 'cloud droplets' is about 0.01 to 0.02 mm.' Drizzle drops' are defined to be between 0.2 and 0.5 mm. A typical raindrop is about 1 to 2 mm in diameter, while large raindrops from a thunderstorm can be between 5 and 8 mm in diameter.

There are two main forces acting on raindrops -- the 'suck' of gravity downwards, and the 'up force' of the updraught of the wind.

Cloud droplets are so small that the wind speed needed to keep them aloft is very small -- 0.01 m/ s. But this wind speed needed increases as the size of the droplet increases -- 0.7 m/ s for a drizzle drop 0.2 mm in size, 4 m/ s for a raindrop 1.0 mm in size, and 9 m/ s for a large raindrop 5 mm in diameter.

The Experiment

At this stage, Peterson and Wallis showed that they were real scientists, and decided to do the experiment.

They didn't need an $80 million satellite or complex lab equipment -- which was just as well, seeing that they were paying for this out of their own pockets, and were doing the experiment on their own time. (They had done all their work because of 'intellectual curiosity'.)

Luckily they were roughly the same build, so they bought two identical sets of sweat shirts, pants and hats. They also bought two large plastic bags to wear underneath these clothes, so that any rain which ended up on their clothes would not get soaked into their underclothes. They then measured out a 100-metre track behind their United States National Climatic Data Center office and waited for some rain. Soon, some heavy rain came along -- falling at around 18 mm (or 3/ 4 of an inch) per hour. They made sure that they weighed the clothes both before and after the rain.

Dr Wallis ran the hundred metres at around 4 m/ s (about 14.4 kph, or 9 mph), and his clothes absorbed 130 grams (about 4.5 oz) of water. Dr Peterson walked his hundred metres at a much more leisurely 1.4 m/ s (about 5 kph, or 3.1 mph), but his clothes soaked up 217 grams (about 7.7 oz) of water. Running instead of walking meant that you got 40 per cent less wet, which was pretty darn close to their predicted 44 per cent.

Run! Run! Run!

So if you run in heavy rain (as compared with walking), you'll stay somewhere between 30 per cent and 50 per cent drier. The greatest benefit is achieved by running in heavy windy rainy conditions, and by leaning forward. There is less improvement in light rain, with no wind, and when you stay nearly vertical.

Of course, you could take an umbrella with you -- but that would give you lousy aerodynamics. But it's probably safer than running on slippery ground. Then again . . . if you're riding a bicycle . . . if you're in love . . . if it's raining . . . you may just dare to get wet.


The Guinness Book of Records, CD-ROM, Guinness Publishing, 1993.

J. J. Holden, S. E. Belcher, A. Horvath and I. Pytharoulis,' Raindrops Keep Falling on My Head', Weather, Vol. 50, November 1995, pp 367- 370.

Thomas C. Peterson and Trevor W. R. Wallis, 'Running in the Rain', Weather, Vol. 52, March 1997, p 93.

Jearl Walker, The Flying Circus of Physics, John Wiley & Sons, 1975, pp 23, 231- 232.

Martin Whittle, New Scientist, No. 1960, 14 January 1995, p 57.

Matthew Wright, New Scientist, No. 1960, 14 January 1995, p 57.

Mike Stevenson,' Drip Dry', New Scientist, No. 1960, 14 January 1995, p 57.

Table of Contents

Running in the Rain.

Noah's Flood.

Can't Get to the Stars in a Lifetime.

Can Get to the Stars in a Lifetime.

Coffee, Caffeine and Circles.

Maggots Give Life.

Maggots Date Death.

Is Mr. Smith Heavier Than Mr. Tailor?

Super Broccoli!

Life on Titan.

Life on Europa.

Flattened Fauna.

Melatonin -
Magic or Madness?

Name Your Own Element.

Planets Line Up.

TV, Heart Attacks and CPR.

Your Name Is Your Job.

Cold Baths, Olympic Games and Hot Bodies.

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