Orb with a tail reproduced with a falling water droplet (yellow/orange tinge is not significant)
How fast is rain?
How fast does rain fall? Is it really fast enough to show up as trail in a flash photo?
When an object falls, it accelerates due to gravity. However, it does not keep going faster and faster because air friction limits the speed. Every object has a terminal velocity where gravity equals air friction. For rain, the terminal velocity depends on the diameter of the raindrop. Essentially, small raindrops (drizzle) go much slower than heavy rain.
Raindrop terminal velocity varies roughly between 1 m/s (for drops of one third of a mm in diameter) to 9 m/s (6mm drops). Typical middle-sized drops fall at around 5 m/s. That's around 0.5 cm in one thousandth of a second.
The raindrop (or insect - see right) might be around 5-15 cm from the lens (a typical distance for the orb zone). At that distance, an orb would easily get a tail, particularly if the flash lasts 2 or 3 times longer than usual (see top right).
Orbs with tails
Sometimes people take photos of orbs with tails, resembling tiny comets. They certainly look impressively mysterious. Are they orbs on the move or something even stranger?
Many apparently moving orbs are, in fact, multiple orbs superimposed on each other. Tailed orbs are completely different. There are not made up of overlapping circles (even when examined closely) but instead show a clear, linear fading trail going away from a main bright orb (see photo left).
So what clues are there to the true nature of these unusual and rare orbs? Firstly, they are, almost without exception, in photos taken outdoors. Secondly, the background is often very dark because the camera flash cannot reach the 'subject' of the photo which is too far away. Thirdly, the tail almost always goes downwards, either straight down or at a diagonal - it seldom, if ever, stretches upwards.
All these clues add up to one thing - these tailed orbs are caused by raindrops. This is not as obvious as it sounds as there are technical problems that require explanation.
So, if tailed orbs are raindrops, why do the tails usually go down instead of up? And why isn't the camera flash, which typically only takes a thousandth of a second, fast enough to freeze the movement of falling raindrops?
Rain typically falls at 0.5 cm in one thousandth of a second (though it varies a lot - see left). This is ample to show up as a trail when the drop is inside the orb zone, just a few cm from the camera lens. Though the tails go downwards, it is usually at a diagonal because of the wind which easily deflects rain.
So why do the tails fade away downwards? This is because the photo uses a full flash discharge. When a subject is too far away to illuminate, the flash goes on for a longer time than usual and fades away gradually. It also takes longer, allowing the raindrop to fall further while illuminated and show producing a tail (see right).
The photo above (left) was produced deliberately by dripping water droplets in front of a camera, just like rain on a dark night. The yellow-orange tinge is not significant - tailed orbs can be any colour (most often white or grey). For comparison the orb (right) is a 'natural' one caused by rain.
Insects as well
Some tailed orbs are reported indoors. The obvious answer here is insects, as dust would move too slowly. So how fast do insects fly? Different species can move at anything from 0.5 m/s up to 7 m/s! So an 'average' insect might do 3 m/s or around 0.3 cm in one thousandth of a second. That's easily fast enough to show up as a tailed orb in a flash photo.
If you take a photo in the dark of a subject that is far away, your camera flash will carry on for as long as it can, to try to illuminate the scene properly. The flash intensity does not stay constant, however. Instead,. it falls away gradually, like the curve below, because it is draining a capacitor. The brilliant initial light, followed by the gradual fading, explains the way 'tails' diminish in intensity as the drop falls.
Light intensity falls gradually with a full flash discharge
A full flash discharge takes longer than a normal one (2 to 3 milliseconds instead of 1). This means the raindrop will move further giving a greater likelihood of a tail.
If you take a photo in the dark where the subject is close enough for the flash to illuminate it adequately, the flash duration is cut short by the camera (see graph below). In this situation, any falling raindrops will NOT have a tail but will appear as single orbs.
Light intensity is cut off by the camera when the subject is illuminated adequately.
The camera can work out how long to let the flash go on for by using a very short 'pre-flash' of known intensity and measuring the light reflected back.
In 3 ms, a typical raindrop might move 1.5 cm and an insect 0.9 cm (see left). At a distance of around 5-15 cm from the lens, this would easily show up as a tail.
© Maurice Townsend 2007