User:Baudette/sandbox

proposed addition to the atmospheric optics page

Statistics of Halos
A study at Penn State University published in 1942 looked into the statistical significance of an often-cited piece of weather lore; if a ring is around the sun or moon, rain is expected soon. Weather events 48 hours following the sighting of a halo from the Fall of 1934 to the Fall of 1940 were analyzed. Generally, there was an increased chance of precipitation after the sighting of a halo. Specifically, there was a seven in one chance that precipitation occurred 48 hours after a halo in the winter months. On average, the time interval between a halo and precipitation over central Pennsylvania was 18.5 hours in the summer and 11.1 hours in the winter.

Certain atmospheric conditions are needed to create the ice crystals required for a halo. In the upper troposphere, particles for ice nucleation are sparse, therefore homogeneous nucleation is required. For the formation of ice crystals without aerosols to freeze onto, the temperature must be at least -38 ℃ (about -36 ℉). But characteristics of the ice crystals play an important role; as seen in simulations, aggregation and rimming of ice crystals have the ability to prevent halos. For a simulated 22° halo, more than 10% of columns or more than 40% of plates must be pristine (having an idealized hexagonal shape). The criteria for a simulated 46° halo is much higher, more than 50% of plates or more than 70% of columns must be pristine. This causes the formation of a 46° halo to be rarer than a 22° halo. To create more pristine crystals, slower large-scale rising motion in the atmosphere is favored. Lastly, the size of the ice particle must be large enough to cause refraction. A hypothesized minimum diameter for a 22° halo or a 46° halo is 10 micrometers.

In 2003, a study was published on the frequency of cirrus cloud optical phenomena over Salt Lake City, Utah, from the University of Utah Facility for Atmospheric Remote Sensing. Optical phenomena were recorded from March 1991 to November 2001, totaling 1561 hours of daylight observations. Of those hours, 52.9% of the 1-hour periods had some type of optical event. Of the total time period broken into 1-hour periods, 37.3% contained 22° halos, 8.6% contained upper tangent arcs, and 8.5% contained sundogs. The least active months were the coldest and warmest months of the year (e.g., July, August, and December).