Hotspot

Now that the wildfire season seems behind us, no concern is likely to be raised by a discussion of something called the hotspot in the forest.

The hotspot is an optical phenomenon in the form of a diffuse bright region around the antisolar point. This is the point on the celestial sphere directly opposite the sun. It is also the location of the head of an observer’s shadow, if it can be seen. In the scene, below, the hotspot appears in the geometric centre of the picture. However, the hotspot is not fixed to the ground. It moves with the observer and the Sun.

The explanation of the phenomenon, also known as the dry heiligenschein, is remarkably simple. On an irregular surface, such as a forest, the view is of a mixture of the trees (or other objects) and their shadows. The exception to this blend of light and dark occurs at the antisolar point. Here one is looking directly down the sunbeam and here each tree covers up its own shadow leaving that spot brighter than elsewhere.

There is a hotspot in the forest in the centre of this picture of a tree-covered mountainside.