Atlanta
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Atlanta clear sky chart from 2005-11-05 to 2008-11-27.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	11% 5% 5% 6% 8% 43%	12250
	Night	11% 5% 5% 8% 44%	12184
Transparency	Day	9% 16% 24% 23% 5%	9294
	Night	8% 14% 24% 25% 8%	9311
Seeing	Day	16% 21% 27% 22%	659
	Night	7% 26% 32% 24%	3708

By Month

Percentage Cloud Cover
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month		Occurrence of each Forecast	Data Points
January	Day	17% 5% 5% 5% 7% 41%	846
	Night	20% 6% 5% 6% 38%	1193
February	Day	16% 5% 6% 5% 5% 6% 38%	809
	Night	17% 6% 7% 42%	970
March	Day	11% 5% 6% 9% 45%	999
	Night	8% 5% 5% 8% 50%	1043
April	Day	12% 5% 6% 5% 7% 45%	1079
	Night	7% 5% 6% 5% 7% 9% 48%	900
May	Day	5% 5% 5% 5% 8% 52%	1208
	Night	5% 5% 5% 5% 7% 8% 47%	837
June	Day	5% 5% 7% 8% 13% 43%	1170
	Night	6% 5% 8% 5% 8% 10% 43%	810
July	Day	10% 5% 7% 9% 8% 7% 12% 24%	1209
	Night	5% 5% 7% 7% 7% 8% 7% 15% 27%	834
August	Day	9% 5% 6% 5% 6% 7% 7% 11% 32%	1135
	Night	9% 5% 5% 6% 7% 7% 8% 8% 33%	896
September	Day	9% 5% 5% 6% 6% 5% 6% 7% 41%	989
	Night	9% 5% 5% 5% 5% 5% 9% 42%	985
October	Day	14% 5% 56%	967
	Night	12% 5% 54%	1067
November	Day	10% 5% 5% 6% 52%	1002
	Night	11% 7% 55%	1441
December	Day	16% 5% 8% 44%	837
	Night	16% 5% 6% 44%	1208

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	7% 15% 25% 28% 7%	595
	Night	5% 11% 27% 24% 12%	791
February	Day	7% 16% 25% 25%	563
	Night	7% 13% 25% 29% 7%	700
March	Day	7% 20% 29% 22%	753
	Night	8% 18% 28% 26% 7%	802
April	Day	8% 15% 26% 28% 6%	808
	Night	6% 16% 28% 22% 11%	727
May	Day	8% 19% 27% 25%	989
	Night	9% 14% 29% 25% 6%	668
June	Day	14% 24% 30% 17%	894
	Night	13% 24% 24% 26%	574
July	Day	20% 19% 12% 16%	634
	Night	13% 23% 20% 13% 6%	444
August	Day	14% 20% 23% 10%	918
	Night	14% 16% 23% 10%	718
September	Day	9% 12% 18% 25%	831
	Night	10% 14% 17% 20% 8%	873
October	Day	8% 18% 32% 11%	876
	Night	9% 13% 32% 14%	952
November	Day	5% 10% 26% 34% 8%	794
	Night	10% 26% 34% 10%	1174
December	Day	6% 13% 28% 18% 10%	639
	Night	5% 12% 27% 26% 9%	888

Seeing (when cloud cover <= 80%)
Color Meanings: Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month		Occurrence of each Forecast	Data Points
January	Night	17% 36% 32% 5%	317
February	Night	12% 47% 29%	286
March	Day	6% 28% 35% 15%	64
	Night	11% 34% 38% 12%	355
April	Day	33% 28% 22%	84
	Night	10% 35% 38% 10%	286
May	Day	23% 26% 21% 21%	88
	Night	5% 28% 30% 28% 5%	275
June	Day	19% 22% 50%	81
	Night	12% 30% 47% 7%	249
July	Day	7% 7% 34% 43%	78
	Night	7% 28% 46% 13%	240
August	Day	5% 14% 29% 37%	88
	Night	5% 18% 49% 16%	262
September	Day	9% 24% 41% 9%	87
	Night	11% 36% 41%	336
October	Day	21% 19% 31% 8%	87
	Night	25% 32% 24%	354
November	Day	50% 50%	2
	Night	9% 32% 32% 18%	418
December	Night	9% 32% 34% 15%	330

Page generated at 2008-11-27 13:47:35. Version ab11