Eye on nature

Light in the atmosphere

i.e., the interaction of light (generally from the sun or moon) with stuff in the air: molecules, water: ice and drops; crud; smoke...

General issues

Write for the fairly sophisticated reader - naturalist. Give boxes to explain simple principles to the less advanced reader. Put anything technical/mathematical in appendices with references. Concentrate on drawing analogies and pointing out the nature of the scientific method.

Introduction

• Day
• Light and molecules: Rayleigh scattering; molecular absorption; polarization; twilight phenomena
• Refraction: mirages; sunsets
• Aerosols and dust: sunbeams; aerial perspective
• Clouds and fog: shadows, fogbows, coronae, glories. nb. explain why a given concentration of water in the atmosphere can be transparent or opaque - depending on the size of the 'particles'
• Raindrops: rainbows
• Ice crystals: halos, sun pillars: the whiteness and brightness of snow
• Night
• The airglow
• Aurorae
• Zodiacal light?
• Lightning

Halos

Issues

• How to illustrate the general process? Everyone sees their own halo!
• Standard format for comparison of photo with simulation
• Drawings and photographs of crystals

Contents

• Introduction
• What to look for: common halos; coloured and white structures; symmetry and asymmetry
• Ice crystals: snowflakes; single crystals; plates, rods, pyramids hexagons, octagons?
• families of halos: reflections; refractions; 60deg and 90deg prisms (the limit of sqrt(2) on the refractive index)
• How crystals float in the air; the specific orientations mean that some halos change shape and position as the sun changes altitude
• Simulations: how they are done and what we can learn from them
• Rare halos: why are they rare?
• What to do when you see a halo: photography; angles, times, masking the sun
• History: ancient records in paintings/drawings; famous displays. Who understood what was going on: Bravais; Tape; Greenler; TrЉnkle & Pattloch; Schroeder & Cowley
• Forefront research
• Example of changing parhelion as a plane flies through turbulence

Introduction

The weather has been clear and sunny but a few whisps of feathery cirrus cloud are appearing. In a few hours, the blue sky has been replaced by a milky-white shroud telling us that rain is on the way. In temperate latitudes, this is a good time to see halos formed by clouds of ice crystals high in the atmosphere where the temperature is always well down below -30 deg. C or so.

What do halos look like and how do we recognise them? Most people feel comfortable looking at the sky away from the sun and know that this is the place to look for a rainbow when the conditions feel right. The most prominent halos, however, appear quite close to the sun where it can be uncomfortably bright to look. The most common is probably the 22 degree halo: a whitish circle around the sun with an angular radius of 22 degrees (give way of estimating size). The inner edge is quite sharp and has a distinctly reddish tinge. The brightest halo phenomena are the closely-related parhelia, known commonly as 'sun dogs' or 'mock suns'. When the sun is low in the sky, these appear as bright regions on the 22 degree halo on either side of the sun on the horizontal line passing through it. They are more distinctly red on their inner edges and can appear bluish on their outer sides. Sometimes there is a white band passing through them and extending on around the line parallel to the horizon. This is known as the parhelic circle.

These particular phenomena are in fact quite common and can be seen, in temperate latitudes, at least once a week by a keen observer. There are, however, a rich variety of other ice crystal halos and arcs that can be seen more rarely and which provide a rich source of information about the nature of ice crystals in the atmosphere.

In a sense, halos are like rainbows, glories and coronae in that they require the presence of a bright source of light, the Sun or the Moon, and water floating in the air. They differ, however, in the sense that the water has to be in the form of ice: not just the snowflakes that we are familiar with in the winter, but simple, regular crystals in the form of hexagonal plates and pencils (photograph). These simple crystals act as prisms that can reflect and refract light in very particular ways to produce the regularities and symmetries that we see in halo phenomena. The real scientific interest in halos - and the reason for their sometimes especiallly spectacular beauty and complexity - is that these crystals tend to float in the air in particular orientations. That is, each crystal knows about its local vertical and horizontal directions. The practical consequence of this is that many halo forms depend on the altitude of the sun, ie, tha angle between the vertical and the direction of illumination. In what follows, we'll try to explain how this happens.