Here comes the sun | Weather

As many celebrated the arrival of spring with warm, sunny days, some may have also noticed that sun glare has become a bit of an issue for that early morning drive.

Sun glare refers to the contrast lowering effects of stray light on a visual scene.  This stray or extra light received on the human retina tends to wash out the contrast in the field of view.

Sun glare can impact pedestrians, drivers, boaters and pilots.  It can be intense and sudden.  And the results can be serious, even deadly.

Glare from the sun comes in two forms:  1:  From looking directly into the area of the sky in which the sun is located, and 2:  reflected off a surface such as water or snow (the level of reflectivity is known as the “albedo”).

Sun glare is most commonly an issue early and late in the day, during which time the sun is low in the sky. This increases the chance of receiving sun glare directly from the sun itself as well as indirectly off a secondary surface.

Also important in this discussion is local topography which, for example, can impact the angle of a car relative to the sun, as well as create rapid changes in the field of view.

Sometimes I, as a forensic meteorologist, am asked by attorneys to determine a future date or dates when sky and sun conditions will be comparable to those experienced in an ongoing case in order to replicate conditions that a driver faced.

In any sun glare investigation, as well as in the “replication” of a comparable sun glare pattern for future analysis, we look at three important scientific factors:

The first factor we address is the analysis of the sky cover at the time.  Were clouds present?  Were they opaque clouds?  Was there haze or fog?  

These and other factors can influence the intensity of sunlight reaching the earth’s surface.

If we determine that prevailing sky conditions cannot allow the ruling-out of sun glare, we continue our work and calculate the position of the sun in the sky.

We do this by positioning the sun via altitude and azimuth.

The altitude is simply how high the sun is above the horizon and is measured in degrees.

If the sun were directly overhead, its altitude would be 90 degrees.  Just below the horizon places its altitude at 0 degrees.

The second component, the azimuth, is the numbers of degrees east of due north, in a circle of 360 degrees.

Therefore, if the sun is due east it is at 90 degrees.  If it is due south it is at 180 degrees.  And, if it is due west, it is at 270 degrees.

Taking these three factors into consideration allows a forensic meteorologist to determine what date(s) in the future will best represent the condition that the driver in question faced at the time of a given incident.  

This in turn allows for the determination to be made as to whether sun glare was or was not a contributing factor.

Sometime sun glare is almost impossible to avoid.  But utilizing sun glasses and sun visors can make for a less challenging drive in such cases and are highly recommended.