This page made possible by the sponsorship of: .

JAT Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the JAT 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	15% 5% 6% 5% 7% 40%	12256
	Night	17% 5% 5% 5% 7% 39%	12134
Cloud Cover	Day	14% 5% 5% 6% 7% 39%	12272
	Night	17% 5% 5% 5% 7% 38%	12160
Transparency	Day	5% 17% 28% 19%	8956
	Night	5% 18% 27% 20%	8897
Seeing	Day	56% 9% 12% 8%	852
	Night	59% 12% 13% 5%	3827

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	16% 6% 5% 5% 5% 5% 5% 6% 37%	774
	Night	22% 5% 5% 9% 34%	1265
February	Day	15% 6% 6% 43%	779
	Night	15% 7% 5% 5% 6% 6% 40%	1012
March	Day	15% 5% 5% 6% 5% 7% 41%	990
	Night	16% 6% 5% 8% 39%	1007
April	Day	16% 5% 5% 5% 5% 6% 7% 37%	1078
	Night	21% 5% 5% 5% 5% 6% 37%	899
May	Day	11% 8% 5% 6% 8% 40%	1265
	Night	13% 5% 5% 6% 6% 42%	782
June	Day	14% 5% 6% 5% 7% 11% 32%	1251
	Night	15% 5% 6% 9% 38%	718
July	Day	12% 5% 5% 7% 5% 7% 9% 36%	1300
	Night	10% 5% 7% 8% 7% 10% 35%	740
August	Day	13% 5% 6% 6% 7% 41%	1135
	Night	17% 5% 6% 8% 40%	897
September	Day	16% 6% 47%	1041
	Night	12% 6% 52%	936
October	Day	14% 5% 48%	914
	Night	17% 5% 48%	1120
November	Day	22% 7% 5% 40%	985
	Night	23% 6% 5% 5% 5% 6% 32%	1461
December	Day	14% 6% 5% 7% 7% 38%	744
	Night	18% 5% 6% 7% 37%	1297

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	16% 6% 5% 5% 5% 5% 6% 37%	773
	Night	22% 6% 5% 9% 34%	1266
February	Day	15% 5% 5% 5% 6% 43%	776
	Night	14% 7% 5% 6% 7% 39%	1004
March	Day	15% 5% 5% 6% 5% 7% 40%	1011
	Night	16% 6% 5% 5% 9% 37%	1031
April	Day	16% 5% 5% 5% 6% 7% 36%	1077
	Night	20% 5% 5% 6% 7% 36%	899
May	Day	10% 5% 7% 6% 6% 8% 39%	1264
	Night	13% 6% 5% 7% 41%	782
June	Day	14% 5% 6% 5% 7% 10% 33%	1260
	Night	14% 5% 5% 5% 6% 10% 38%	720
July	Day	12% 5% 5% 5% 6% 6% 7% 9% 35%	1300
	Night	9% 5% 7% 7% 8% 10% 35%	741
August	Day	13% 5% 6% 5% 7% 6% 41%	1130
	Night	16% 5% 5% 6% 9% 39%	899
September	Day	15% 5% 7% 46%	1040
	Night	12% 6% 51%	937
October	Day	13% 5% 5% 6% 47%	914
	Night	17% 6% 48%	1120
November	Day	21% 8% 5% 40%	983
	Night	22% 6% 5% 5% 6% 32%	1460
December	Day	13% 6% 5% 6% 5% 7% 38%	744
	Night	18% 5% 5% 6% 7% 36%	1301

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	27% 28% 13%	519
	Night	5% 22% 35% 13%	835
February	Day	5% 23% 31% 14%	578
	Night	5% 23% 26% 19%	746
March	Day	18% 31% 22%	713
	Night	5% 21% 29% 19%	713
April	Day	5% 16% 21% 25% 6%	762
	Night	20% 17% 24% 5%	635
May	Day	6% 18% 30% 21%	921
	Night	15% 28% 26%	565
June	Day	11% 27% 34% 9%	852
	Night	10% 24% 34% 15%	471
July	Day	6% 15% 32% 17% 5%	693
	Night	14% 18% 23% 18%	412
August	Day	5% 10% 25% 21% 5%	958
	Night	12% 26% 22%	753
September	Day	9% 25% 30%	861
	Night	8% 30% 33%	783
October	Day	12% 29% 21%	805
	Night	12% 29% 24%	976
November	Day	18% 26% 16%	764
	Night	18% 25% 14%	1070
December	Day	6% 27% 24% 14%	530
	Night	29% 26% 13%	938

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	71% 11% 8%	371
February	Day	70% 14% 6%	48
	Night	72% 9% 8%	318
March	Day	75% 8% 9%	93
	Night	73% 9% 5%	371
April	Day	70% 7% 5%	90
	Night	70% 9% 8%	359
May	Day	69% 6% 12% 5%	93
	Night	70% 13% 9%	223
June	Day	64% 11% 8% 7%	84
	Night	65% 9% 15%	169
July	Day	61% 12% 14%	81
	Night	61% 9% 16% 9%	162
August	Day	38% 8% 16% 11% 13%	90
	Night	37% 9% 26% 14%	326
September	Day	34% 13% 12% 23% 5%	88
	Night	36% 11% 24% 18%	350
October	Day	38% 13% 21% 10%	91
	Night	37% 18% 22% 8%	367
November	Day	52% 13% 15%	94
	Night	54% 18% 12%	442
December	Night	70% 10% 6%	369

Page generated at 2008-11-27 13:47:42. Version ab11