March 16, 2022 Program Meeting

Speaker: Allan Rahill

Topic: Weather Forecasts

The March 16 program meeting speaker is Allan Rahill, a meteorologist who will discuss the weather forecasts behind many of the forecast tools we amateurs use.

In 2000, Allan Rahill, a meteorologist at the Canadian Meteorological Centre (CMC) and amateur astronomer, created a forecast processing step that took data from CMC’s Global Element Multi-scale (GEM) forecast model and created a new forecast of cloud cover. Rahill specially designed his cloud forecast to consider the formation of cirrus clouds. The cirrus cloud modeling distinguishes Rahill’s model from other cloud forecast models, as sufficient cirrus clouds to make a night unusable for astronomers is still called “clear” by civil weather forecasts.

In later years, Rahill added a forecast of astronomical transparency, which is a measure of how much starlight traverses the Earth’s atmosphere when otherwise free of clouds. Rahill also added a forecast of astronomical seeing which uses forecast data of turbulence and temperature gradients in the atmosphere to forecast its optical steadiness.

In 2001, Attilla Danko, computer programmer and amateur astronomer, began to summarize Rahill’s hundreds of forecast maps by displaying only one pixel, from each map, laid out in rows.[3] The resulting meteogram, called a clear sky chart, showed all of Rahill’s forecast data, but for only one location. This is the origin of the Clear Sky Clock. Allan’s data also forms the backbone for Astropheric.

Due to the COVID-19 pandemic, program meetings are currently being held via Zoom. You can register in advance for the next meeting at the following link. After registering, you will receive a confirmation email containing information about joining the meeting. You may be required to log in with a Zoom login and password in order to attend the meeting.

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