Observing Atmospheric Optical Phenomena
Atmospheric optical phenomena depend on the position and conditions of the light source, which is usually the sun, and the atmospheric conditions such as is it cloudy, foggy, or raining. These conditions will determine what type of phenomena which can be observed.
Since the sun is usually the light source, positions in relation to the sun are important. Right after the actual location of the sun the anti solar point is the next most important location. The anti solar point is the position directly opposite the sun, which is usually best found by looking for the shadow of your head. If the sun is hidden by cloud then you just have to try and guess where the sun is and where the shadow of your head would be. Another key point is the zenith, which is directly above the observer.
Once key positions are located, angles need to be roughly determined to find the position of a phenomenon. The method I use all the time is the width of my out stretched hand at arms length. From the tip of your little finger to thumb is roughly 20 degrees and the width of you finger at arms length is about 1 degrees. The width of the sun and moon is about 0.5 degrees. Also remember that a circle, the angle all the way around the horizon is 360 degrees, the angle from horizon to zenith is 90 degrees, and therefore from horizon to horizon through the zenith is 180 degrees.
Time can be important, especially if you are waiting for a sunset. The earth rotates once every 24 hours, or 1440 minutes. From sunrise to sunrise the sun travels through 360 degrees in 24 hours. Thus the sun moves 1 degree across the sky every 1440/360 = 4 minutes. Thus if the sun is the width of you finger (1 degree) above the horizon it will about 4 minutes until it touches the horizon. Or if it is the width of you out stretched hand (20 degrees) it is about 80 minutes until sunset. Another way of stating this is that in 1 hour the sun moves across the sky 360/24 = 15 degrees.
The key ingredients for a rainbow are sunlight and rain. The primary rainbow is an arc 42 degrees around the anti solar point. This is an arc about 2 hand widths around the shadow of your head. To see a rainbow, sunlight must be illuminating water drops along this arc. If you were wondering where to stand to see a rainbow in a fountain or waterfall, it would be where ever puts the fountain or waterfall in this arc. The secondary arc is at about 51 degrees, a further half hand width out beyond the primary bow.
Another important factor for viewing rainbows is the hight of the sun. If the sun is too high the rainbow will be below the horizon. To see a rainbow reaching high into the sky the sun needs to be low. To tell quickly do the 2 hand trick - if two hand widths from the shadow of your head is below the horizon the sun isn't high enough to see a rainbow in the sky.
Halos are an ice crystal phenomena. The most likely place to see a halo is in high thin cirrus cloud (a type of cloud made up of ice crystals). The most common halo is the 22 degree halo, which is seen as an arc 22 degrees around the sun. If you put your thumb of your out stretched hand at arms length over the sun, being very careful not to look at the sun by closing one eye, and if high cirrus cloud is along an arc this distance from the sun there is likely to be a halo. 22 degree halos are often seen around the moon. So on days when there is high thin cirrus at sunset, and there is a full moon, go outside in the evening and look for a halo.
Parhelia, better known as sun dogs, are also ice crystal halos. They are seen about 22 degrees either side of the sun at the same hight as the sun above the horizon. They are often seen as bright spots either side of the sun on a 22 degree halo. Depending on the ice crystals they can also be seen without a 22 degree halo. They can sometimes be very colourful and sometimes very bright. They are easily found by looking the width of your out stretched hand at arms length either side of the sun. Again there needs to be ice crystals for parhelia, and this is most common with thin cirrus cloud.
Under perfect conditions ice crystals can settle onto a snow field and this layer of ice crystals can produce a halo, usually a 22 degree halo. The sun must be less than 22 degrees above the horizon for you to see this. What is extremely unusual about this is that the halo is being formed on a plane, the snow surface. A 22 degree halo is normally seen as light scattered and reflected from a cone of 22 degrees radius around the anti solar point. But in this case it is formed at the intersection of this cone, and the plane surface which is the snow surface. The intersection of a cone and plane surface is a parabola, and that is how this halo will be seen, as a parabola on the surface of the snow.
Every time you fly in an aircraft it is an opportunity to view a glory. A glory is an interference effect which produces a circle around the anti solar point of the observer on cloud or fog. The diameter of the circle depends on a number of things including the distance to the cloud. When ever you are in an aircraft try and determine where the position of the sun is, and thus where the shadow of the aircraft will be. If the shadow lands on cloud look for a slightly rainbow coloured circle around the shadow. This is a glory.