Emory University Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Emory University Observatory 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%	12183
Transparency	Day	6% 17% 26% 24% 5%	9318
	Night	5% 15% 25% 26% 8%	9249
Seeing	Day	14% 22% 29% 20%	661
	Night	7% 25% 33% 24%	3676

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% 7% 41%	845
	Night	19% 7% 5% 6% 38%	1194
February	Day	16% 6% 5% 5% 6% 39%	809
	Night	16% 5% 5% 5% 7% 42%	970
March	Day	11% 6% 5% 9% 44%	1000
	Night	8% 5% 6% 8% 49%	1042
April	Day	12% 5% 5% 7% 46%	1077
	Night	8% 5% 6% 6% 9% 48%	900
May	Day	5% 5% 5% 5% 8% 53%	1208
	Night	6% 5% 5% 6% 9% 47%	837
June	Day	5% 5% 5% 6% 8% 13% 43%	1170
	Night	6% 8% 5% 7% 11% 43%	810
July	Day	11% 5% 6% 8% 8% 7% 12% 26%	1209
	Night	5% 7% 7% 6% 7% 10% 13% 28%	834
August	Day	9% 5% 5% 7% 6% 8% 10% 33%	1137
	Night	10% 5% 5% 6% 7% 7% 9% 33%	895
September	Day	9% 5% 6% 6% 6% 8% 42%	989
	Night	9% 5% 5% 7% 5% 5% 10% 41%	985
October	Day	14% 5% 57%	966
	Night	12% 5% 5% 54%	1068
November	Day	10% 5% 5% 6% 52%	1003
	Night	11% 8% 54%	1440
December	Day	17% 7% 44%	837
	Night	17% 5% 7% 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	17% 27% 28% 7%	589
	Night	10% 30% 25% 12%	789
February	Day	17% 27% 27%	559
	Night	12% 27% 31% 7%	675
March	Day	6% 21% 30% 22%	767
	Night	18% 30% 27% 8%	791
April	Day	15% 29% 28% 6%	794
	Night	16% 28% 23% 11%	727
May	Day	6% 21% 29% 26%	1001
	Night	5% 15% 31% 25% 5%	661
June	Day	10% 28% 31% 18%	910
	Night	9% 27% 25% 25%	583
July	Day	12% 24% 12% 17% 5%	634
	Night	9% 25% 23% 13% 6%	440
August	Day	10% 21% 24% 11%	940
	Night	8% 19% 25% 11%	712
September	Day	6% 11% 21% 26%	829
	Night	7% 15% 19% 20% 8%	863
October	Day	7% 18% 33% 11%	877
	Night	8% 14% 33% 14%	954
November	Day	5% 9% 27% 34% 9%	790
	Night	9% 27% 34% 10%	1175
December	Day	14% 30% 18% 10%	628
	Night	12% 28% 28% 8%	879

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	18% 35% 33% 6%	315
February	Night	12% 47% 29% 5%	285
March	Day	10% 23% 36% 16%	65
	Night	14% 30% 40% 11%	351
April	Day	8% 30% 27% 25%	84
	Night	6% 34% 42% 10%	280
May	Day	22% 27% 24% 20%	87
	Night	6% 26% 30% 27% 5%	274
June	Day	6% 19% 22% 48%	83
	Night	8% 32% 47% 8%	243
July	Day	12% 35% 44%	77
	Night	29% 51% 10%	238
August	Day	6% 12% 34% 32%	88
	Night	19% 49% 17%	261
September	Day	8% 26% 42% 8%	87
	Night	9% 37% 40%	335
October	Day	5% 19% 19% 32% 6%	87
	Night	23% 34% 23%	353
November	Day	33% 33% 33%	3
	Night	9% 30% 35% 17%	413
December	Night	7% 34% 34% 16%	328

Page generated at 2008-11-27 13:47:44. Version ab11