﻿ Weird weather - Abraham Lincoln - James M. McPherson ﻿

## Why Don't Penguins' Feet Freeze?: And 114 Other Questions - Mick O'Hare (2009)

### Chapter 7. Weird weather

Forked frolics

Why does lightning fork and what is the diameter of a bolt of lightning?

Michael Lee

London, UK

Lightning usually brings the negative charge from a thunderstorm down to the ground. A negatively charged leader precedes the visible lightning, moving downwards below the clouds and through air containing pockets of positive charge. These are caused by point discharge ions released from the ground by the thunderstorm’s high electric field.

The leader branches in its attempt to find the path of least resistance. When one of these branches gets close to the ground, the negative charges attract positive ions from pointed objects, such as grass and trees, to form a conducting path between cloud and ground. The negative charges then drain to ground starting from the bottom of the leader channel. This is the visible ‘return stroke’ whose luminosity travels upwards as the charges move down. Those branches of the leader that were not successful in reaching the ground become brighter when their charges drain into the main channel.

Photographs of lightning often overestimate the channel width because the film can be overexposed. Damaged objects that have been struck by lightning show channel diameters of between 2 and 100 millimetres.

R. Saunders

Atmospheric Physics Group

Manchester University, UK

Wave power

What mechanism transforms gusting wind energy into the regular wave train of ocean swells and what determines their amplitude and frequency?

Frank Scahill

Eastonville, New South Wales, Australia

When the wind blows over a flat sea surface, small ripples form. These probably correspond to individual strong gusts, are disorganised and have no fixed direction or frequency.

However, as the wind continues to blow, two things happen. First, the waves interact with each other to produce longer waves which means lower frequency. Secondly, the wind pushes these larger waves and puts even more energy into them. As long as the storm lasts, the wind will make the waves larger and the wave dynamics will create longer and longer waves.

Some waves will become too steep and break but, in general, the total amount of energy will keep increasing. These locally generated waves are known as ‘wind-sea’. Their energy depends on how long the wind has been blowing (its duration) and over what distance (the fetch). The waves on the sea surface are not a simple wave train but a complicated random surface.

It is impossible to give a simple amplitude and frequency for a system as complex as this. Instead, significant wave height, the mean height of the highest third of the waves, is used to describe how large the waves are, and the peak period, the time between the dominant or most energetic waves, is used as a measure of frequency. On average, there will be a wave twice the significant wave height every three hours.

Eventually, the energy put into the sea by the wind will be balanced by the loss of energy, mainly through waves breaking. At this point, the waves will cease to grow and the sea is described as ‘fully developed’. In a wind of 20 metres per second (a Force 8 gale), a fully developed sea would have a significant wave height of 9 metres and a peak period of 15 seconds.

Waves can travel thousands of kilometres from the point of generation. Unlike light or sound waves, as sea waves become longer (and the frequency gets smaller), they also travel faster.

Waves that escape from the storm which generated them are known as ‘swell’. They have a much narrower range of periods and are almost regular wave trains. Because no more energy is put into them, none is dissipated by breaking, and they continue across the ocean until they hit the land.

Because different frequencies travel at different speeds, as swell travels across the ocean it separates into its individual components. So the significant wave height and peak period of the swell are set by the wind speed, duration and fetch from the storm that generated them.

Peter Challenor

Southampton Oceanography Centre

Hampshire, UK

Wind energy first gives rise to a wind-sea. Waves in a wind-sea are steeper and more chaotic than swell, and are accompanied by whitecaps, the breaking crests of waves. The longer the wind blows, the longer the wavelength of the predominant waves in the wind-sea.

When the wind ceases or the wind-sea waves move out of the generating region, whitecapping continues for a time and is accompanied by a lengthening of the waves, until they are no longer steep enough to sustain whitecaps. The wind-sea then becomes swell.

Surface waves on liquids are dispersive, which means that different wavelengths travel with different velocities. The longer wavelength swell travels faster and arrives at the observer first.

With the passage of time, the swell wavelength becomes shorter as shorter, slower wavelengths arrive. Swell from a storm that formed thousands of kilometres away may persist for several days, steadily getting shorter because of its dispersion.

Dispersion acts as a filter, so only swell within a narrow bandwidth is present in one region of ocean at any time. This is why swell looks so uniform when viewed from an aircraft.

Generally, swell decreases in amplitude as it travels away from the source region because its energy is spread over an ever larger region of ocean.

However, this is not the whole story. A following wind will generate a wind-sea that can transfer some of its energy to the swell and increase the amplitude of the swell without changing its wavelength. Likewise, an opposing wind-sea can diminish a swell.

John Reid

Formerly of Hobart Laboratories of the Division of Marine Research

Tasmania, Australia

Clouding the issue

Why do clouds darken to a very deep grey just before it is about to rain or prior to a heavy thunderstorm?

Matt Bourke

Graceville, Queensland, Australia

Clouds darken from a pleasant fluffy white just before rain begins to fall because they absorb more light.

Clouds normally appear white when the light which strikes them is scattered by the small ice or water particles from which they are composed. However, as the size of these ice and water particles increases – as it does just before clouds begin to deposit rain – this scattering of light is increasingly replaced by absorption.

As a result, much less light reaches the observer on the ground below and the clouds look darker.

Keith Appleyard

Dundee, Tayside, UK

Tainted tint

I have a photochromic coating on my glasses. Under a blazing Caribbean sun they were only moderately tinted. However, under a weak midwinter sun in the UK they go almost black. Why?

Jeff Lander

Whitwick, Leicestershire, UK

We have two types of explanation here: one physical, one chemical. It seems likely that chemistry is responsible for the greater effect – Ed.

I can only assume the questioner was walking around in the Caribbean, rather than lying on his back getting a tan. If so, the following may explain his experience.

The sun would be fairly low in the British winter sky, its rays shining almost directly on, and perpendicular to, the vertical plane of his lenses. In the tropics, the sun could be almost directly overhead, and if he was walking around, the sun’s rays would strike his glasses edge-on. A sliver of radiant energy would be all that each lens would receive, thus reducing their shading reaction.

Charles Kluepfel

Bloomfield, New Jersey, US

One of the little details opticians fail to mention about photo-chromic glasses is that they do not work as well when hot. Particles of silver halide trapped inside the glass are normally transparent, but when struck by ultraviolet light, they disassociate into halogen and metallic silver, which darkens the lenses.

As both components are trapped inside the glass, they will recombine when UV light is removed – when you go indoors – becoming transparent again. The recombination reaction, like many others, speeds up as the temperature rises. As the darkness of the glasses at any moment is a balance between UV light-induced disassociation and the temperature-sensitive re-association, it takes much more UV to reach a given level of darkening in a warm climate.

Alec Cawley

Newbury, Berkshire, UK

Photochromic materials are sensitive to temperature and darken more when they are colder. My sunglasses turn really dark on an overcast day but change little in the midday sun of Florida. This is fine for skiers but not much use to sun-lovers.

I also found, to my cost, that many photochromic lenses react almost entirely to UV radiation rather than to visible light, so they don’t darken properly inside a car.

William Darlington

Bell College of Technology

Hamilton, Strathclyde, UK

The response of photochromic lenses to light is affected by temperature. Lower temperatures change the kinetics of the photochemical reaction so the reverse reaction – lens lightening – is delayed.

Photochromic lenses become much darker at lower temperatures. Living in the American Midwest provides me with perfect experimental conditions to test the temperature effects. With summer temperatures around 30 °C my photo-chromic lenses respond with a bluish-grey tint, whereas in deep winter, at around -10 °C, they quickly become very dark.

The darker lens tint on sunny winter days is especially beneficial against strong snow-dazzle. However, this heavy darkening is disconcerting when going indoors on a sunny day because it takes about 10 minutes for the lenses to return to normal.

Barry Timms

University of South Dakota

Vermillion, US

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