All-Star Telescope Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the All-Star Telescope Observing Site clear sky chart from 2005-11-06 to 2026-10-14.

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	9% 8% 3 8% 5% 5% 5% 4 7% 8% 38%	154521
	Night	7% 9% 4 9% 6% 5% 5% 4 6% 8% 37%	145974
Transparency	Day	42% 6% 15% 25% 10% 2	154222
	Night	43% 6% 15% 23% 11% 2	145856
Seeing	Day	13% 9% 28% 30% 16% 4	17944
	Night	11% 7% 31% 35% 14% 2	26858

By Month

Percentage Cloud Cover NW (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	2 9% 6% 12% 8% 7% 7% 5% 6% 7% 30%	4125
	11th to 20th	Day	2 10% 3 9% 6% 5% 6% 5% 6% 7% 41%	4154
	21st to end	Day	3 7% 4 10% 5% 5% 5% 4 6% 9% 42%	5186
February	1st to 10th	Day	6% 11% 5% 12% 8% 5% 5% 4 7% 9% 28%	1680
	11th to 20th	Day	5% 12% 4 11% 7% 7% 6% 5% 7% 8% 28%	1813
	21st to end	Day	3 11% 6% 10% 7% 6% 6% 5% 7% 7% 31%	1678
March	1st to 10th	Day	3 9% 5% 11% 7% 5% 5% 4 6% 9% 36%	4618
	11th to 20th	Day	5% 12% 5% 10% 5% 4 5% 4 6% 8% 38%	5036
	21st to end	Day	9% 21% 6% 12% 5% 5% 6% 4 5% 5% 21%	5713
April	1st to 10th	Day	7% 16% 4 10% 7% 5% 6% 5% 7% 7% 27%	2580
	11th to 20th	Day	12% 14% 5% 9% 5% 4 5% 4 6% 7% 28%	2700
	21st to end	Day	11% 13% 4 9% 5% 4 5% 4 6% 8% 31%	2730
May	1st to 10th	Day	12% 10% 3 7% 5% 5% 5% 4 6% 7% 38%	8159
	11th to 20th	Day	13% 11% 4 9% 6% 6% 7% 5% 7% 8% 24%	8755
	21st to end	Day	13% 8% 3 8% 6% 5% 5% 4 6% 7% 33%	9649
June	1st to 10th	Day	16% 7% 3 8% 6% 6% 5% 5% 8% 8% 28%	3150
	11th to 20th	Day	19% 9% 3 8% 7% 6% 6% 5% 7% 7% 24%	3150
	21st to end	Day	10% 6% 3 7% 5% 6% 6% 6% 7% 9% 35%	3150
July	1st to 10th	Day	7% 5% 2 6% 4 5% 5% 4 7% 9% 46%	8820
	11th to 20th	Day	10% 4 1 5% 3 5% 5% 4 8% 10% 44%	8820
	21st to end	Day	8% 4 2 5% 4 4 4 4 7% 9% 50%	9416
August	1st to 10th	Day	9% 5% 2 6% 4 4 5% 4 7% 8% 45%	6441
	11th to 20th	Day	11% 5% 2 6% 5% 5% 6% 4 7% 10% 39%	6037
	21st to end	Day	8% 4 2 4 4 4 4 4 6% 8% 54%	6570
September	1st to 10th	Day	13% 6% 2 5% 4 3 4 4 5% 7% 46%	2616
	11th to 20th	Day	14% 8% 2 5% 4 3 4 3 5% 7% 46%	2310
	21st to end	Day	9% 8% 2 7% 4 4 5% 5% 6% 7% 44%	2310
October	1st to 10th	Day	7% 6% 3 7% 4 5% 6% 5% 8% 9% 41%	3556
	11th to 20th	Day	6% 7% 2 8% 5% 5% 6% 4 8% 10% 39%	3007
	21st to end	Day	7% 8% 3 8% 6% 5% 5% 4 5% 7% 43%	3200
November	1st to 10th	Day	9% 16% 5% 9% 5% 4 4 4 6% 7% 32%	1706
	11th to 20th	Day	6% 13% 4 9% 5% 5% 4 4 6% 8% 36%	1569
	21st to end	Day	5% 14% 5% 8% 6% 5% 5% 5% 6% 9% 32%	1470
December	1st to 10th	Day	5% 10% 4 11% 7% 7% 7% 6% 7% 7% 29%	2360
	11th to 20th	Day	5% 10% 4 10% 7% 7% 7% 4 6% 8% 32%	2712
	21st to end	Day	3 9% 4 9% 5% 4 5% 4 6% 8% 40%	3575
January	1st to 10th	Night	3 13% 6% 14% 7% 6% 6% 5% 6% 7% 26%	8768
	11th to 20th	Night	2 9% 4 11% 6% 5% 6% 5% 6% 7% 38%	8779
	21st to end	Night	2 9% 4 11% 6% 6% 6% 4 6% 8% 38%	9017
February	1st to 10th	Night	5% 10% 5% 13% 8% 8% 5% 4 6% 7% 29%	2940
	11th to 20th	Night	4 12% 5% 11% 7% 6% 6% 4 7% 8% 30%	2693
	21st to end	Night	2 12% 6% 10% 6% 6% 5% 5% 6% 8% 34%	2126
March	1st to 10th	Night	3 9% 4 12% 6% 6% 6% 5% 7% 9% 35%	5496
	11th to 20th	Night	5% 10% 4 10% 5% 5% 5% 5% 7% 9% 34%	5183
	21st to end	Night	8% 19% 6% 13% 7% 5% 5% 4 5% 7% 22%	5490
April	1st to 10th	Night	8% 14% 4 7% 7% 5% 5% 5% 7% 8% 31%	2039
	11th to 20th	Night	10% 14% 4 10% 7% 5% 6% 4 6% 7% 28%	1871
	21st to end	Night	11% 11% 4 9% 6% 5% 5% 4 7% 7% 30%	1890
May	1st to 10th	Night	15% 10% 3 8% 5% 4 4 3 5% 6% 37%	4768
	11th to 20th	Night	15% 9% 3 8% 5% 5% 5% 4 7% 10% 30%	4105
	21st to end	Night	10% 7% 3 8% 7% 6% 6% 5% 7% 8% 34%	4522
June	1st to 10th	Night	17% 7% 3 8% 4 5% 5% 4 7% 7% 32%	1470
	11th to 20th	Night	17% 10% 3 7% 7% 6% 6% 4 7% 7% 26%	1470
	21st to end	Night	10% 6% 3 7% 4 5% 6% 4 8% 7% 40%	1470
July	1st to 10th	Night	6% 4 2 6% 5% 5% 5% 6% 7% 9% 45%	4140
	11th to 20th	Night	8% 3 2 5% 4 4 5% 4 7% 9% 50%	4140
	21st to end	Night	9% 5% 2 5% 3 3 4 3 5% 11% 50%	4649
August	1st to 10th	Night	11% 7% 2 6% 4 4 4 3 5% 7% 46%	3709
	11th to 20th	Night	13% 6% 2 6% 5% 4 5% 5% 7% 10% 37%	4022
	21st to end	Night	7% 5% 2 5% 4 4 5% 4 6% 8% 49%	4578
September	1st to 10th	Night	10% 7% 2 5% 3 4 4 4 5% 9% 47%	2001
	11th to 20th	Night	14% 6% 2 5% 3 4 4 4 6% 8% 44%	2310
	21st to end	Night	10% 6% 2 6% 4 4 4 4 6% 8% 46%	2310
October	1st to 10th	Night	8% 6% 2 6% 5% 4 5% 4 6% 8% 46%	3859
	11th to 20th	Night	6% 8% 3 7% 4 5% 5% 4 7% 7% 45%	4206
	21st to end	Night	5% 8% 3 7% 5% 5% 5% 5% 7% 8% 42%	4600
November	1st to 10th	Night	7% 14% 4 9% 5% 5% 5% 4 5% 7% 34%	2790
	11th to 20th	Night	4 12% 4 9% 6% 5% 5% 5% 7% 9% 34%	3050
	21st to end	Night	4 11% 4 9% 6% 6% 6% 4 7% 9% 35%	3150
December	1st to 10th	Night	5% 11% 4 11% 7% 6% 6% 5% 7% 8% 30%	5040
	11th to 20th	Night	4 12% 5% 12% 9% 7% 6% 5% 6% 8% 26%	5700
	21st to end	Night	3 10% 4 9% 5% 5% 5% 4 6% 8% 41%	7623

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	51% 5% 9% 21% 12% 2	4125
	11th to 20th	Day	40% 6% 14% 18% 18% 3	4175
	21st to end	Day	38% 5% 13% 31% 11%	5187
February	1st to 10th	Day	51% 6% 13% 17% 11% 2	1680
	11th to 20th	Day	50% 6% 13% 19% 10% 2	1822
	21st to end	Day	48% 6% 14% 21% 10% 1	1678
March	1st to 10th	Day	42% 6% 14% 23% 14%	4618
	11th to 20th	Day	42% 5% 13% 24% 14% 1	5037
	21st to end	Day	62% 6% 11% 14% 6% 1	5713
April	1st to 10th	Day	51% 7% 15% 19% 7%	2580
	11th to 20th	Day	52% 6% 11% 21% 9% 1	2700
	21st to end	Day	49% 6% 14% 22% 8% 1	2730
May	1st to 10th	Day	43% 5% 10% 26% 13% 2	8159
	11th to 20th	Day	52% 8% 11% 22% 7% 1	8759
	21st to end	Day	46% 6% 15% 24% 9%	9650
June	1st to 10th	Day	49% 7% 15% 19% 8% 1	3150
	11th to 20th	Day	55% 7% 14% 17% 5% 1	3150
	21st to end	Day	39% 8% 20% 25% 7% 1	3150
July	1st to 10th	Day	31% 7% 17% 31% 12% 2	8746
	11th to 20th	Day	32% 7% 20% 28% 10% 3	8739
	21st to end	Day	28% 6% 18% 29% 16% 3	9211
August	1st to 10th	Day	34% 6% 18% 32% 8% 2	6424
	11th to 20th	Day	37% 7% 24% 25% 6% 1	6077
	21st to end	Day	28% 5% 18% 38% 11% 1	6569
September	1st to 10th	Day	36% 5% 19% 25% 11% 4	2616
	11th to 20th	Day	38% 5% 15% 29% 13% 1	2310
	21st to end	Day	37% 6% 16% 25% 13% 3	2310
October	1st to 10th	Day	36% 6% 21% 30% 6% 1	3556
	11th to 20th	Day	39% 6% 19% 22% 12% 2	3007
	21st to end	Day	40% 4 10% 34% 11% 1	3200
November	1st to 10th	Day	51% 6% 12% 20% 10% 2	1706
	11th to 20th	Day	46% 4 16% 23% 10% 1	1569
	21st to end	Day	47% 6% 15% 21% 9% 2	1470
December	1st to 10th	Day	52% 6% 13% 21% 7% 1	2360
	11th to 20th	Day	49% 6% 19% 19% 6% 1	2714
	21st to end	Day	39% 5% 11% 23% 18% 4	3575
January	1st to 10th	Night	55% 6% 10% 18% 9% 2	8769
	11th to 20th	Night	42% 6% 15% 20% 13% 4	8820
	21st to end	Night	42% 5% 17% 24% 11% 1	9017
February	1st to 10th	Night	53% 4 11% 18% 12% 2	2940
	11th to 20th	Night	49% 6% 11% 19% 13% 2	2694
	21st to end	Night	46% 6% 10% 22% 14% 2	2126
March	1st to 10th	Night	43% 7% 16% 22% 11% 1	5496
	11th to 20th	Night	43% 8% 13% 22% 12% 1	5183
	21st to end	Night	59% 7% 9% 15% 8% 1	5490
April	1st to 10th	Night	49% 6% 13% 20% 11% 1	2039
	11th to 20th	Night	53% 5% 12% 20% 9% 1	1871
	21st to end	Night	50% 5% 13% 22% 8% 2	1890
May	1st to 10th	Night	47% 5% 9% 23% 13% 3	4765
	11th to 20th	Night	47% 6% 15% 22% 9%	4105
	21st to end	Night	44% 7% 16% 26% 6% 1	4526
June	1st to 10th	Night	48% 7% 15% 20% 9% 1	1470
	11th to 20th	Night	55% 6% 15% 16% 6% 2	1470
	21st to end	Night	40% 7% 20% 25% 7% 1	1470
July	1st to 10th	Night	32% 8% 18% 29% 11% 1	4105
	11th to 20th	Night	29% 5% 22% 29% 11% 4	4098
	21st to end	Night	29% 6% 21% 28% 15% 2	4549
August	1st to 10th	Night	37% 5% 16% 31% 10% 1	3701
	11th to 20th	Night	39% 8% 26% 23% 4	4048
	21st to end	Night	31% 6% 21% 33% 8% 1	4578
September	1st to 10th	Night	34% 5% 17% 27% 13% 4	2001
	11th to 20th	Night	36% 6% 15% 28% 12% 4	2310
	21st to end	Night	34% 5% 16% 27% 14% 3	2310
October	1st to 10th	Night	34% 5% 18% 27% 14% 1	3859
	11th to 20th	Night	36% 5% 18% 26% 14% 2	4206
	21st to end	Night	38% 5% 12% 32% 13% 1	4600
November	1st to 10th	Night	48% 6% 12% 21% 11% 2	2790
	11th to 20th	Night	44% 6% 17% 20% 11% 1	3050
	21st to end	Night	44% 6% 16% 21% 9% 3	3150
December	1st to 10th	Night	51% 6% 16% 19% 7% 2	5040
	11th to 20th	Night	55% 5% 16% 17% 6% 1	5697
	21st to end	Night	40% 4 10% 25% 17% 4	7623

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	5% 8% 31% 34% 15% 6%	220
	11th to 20th	Day	10% 10% 35% 27% 15% 2	220
	21st to end	Day	7% 10% 31% 31% 17% 3	242
February	1st to 10th	Day	10% 10% 37% 26% 11% 6%	220
	11th to 20th	Day	10% 7% 33% 34% 13% 3	314
	21st to end	Day	6% 11% 30% 32% 17% 4	394
March	1st to 10th	Day	7% 11% 34% 31% 12% 4	540
	11th to 20th	Day	8% 10% 33% 34% 10% 4	540
	21st to end	Day	15% 12% 33% 25% 10% 5%	594
April	1st to 10th	Day	12% 13% 33% 26% 13% 3	540
	11th to 20th	Day	16% 14% 28% 27% 14% 2	532
	21st to end	Day	17% 12% 27% 30% 11% 3	540
May	1st to 10th	Day	16% 13% 26% 28% 14% 3	606
	11th to 20th	Day	16% 8% 25% 31% 17% 4	775
	21st to end	Day	15% 7% 21% 30% 21% 6%	844
June	1st to 10th	Day	17% 11% 27% 30% 12% 4	780
	11th to 20th	Day	20% 11% 24% 29% 13% 2	780
	21st to end	Day	12% 6% 23% 32% 21% 6%	780
July	1st to 10th	Day	12% 5% 22% 35% 21% 5%	771
	11th to 20th	Day	14% 5% 24% 32% 21% 4	768
	21st to end	Day	8% 4 24% 36% 21% 8%	797
August	1st to 10th	Day	10% 5% 27% 30% 21% 7%	567
	11th to 20th	Day	10% 6% 22% 38% 19% 5%	557
	21st to end	Day	10% 7% 23% 32% 21% 7%	627
September	1st to 10th	Day	15% 8% 28% 28% 17% 6%	570
	11th to 20th	Day	17% 5% 23% 31% 17% 7%	570
	21st to end	Day	14% 12% 34% 27% 11% 3	570
October	1st to 10th	Day	14% 9% 28% 30% 12% 6%	481
	11th to 20th	Day	11% 12% 32% 27% 14% 3	339
	21st to end	Day	14% 13% 35% 27% 9% 2	363
November	1st to 10th	Day	18% 8% 31% 30% 10% 4	330
	11th to 20th	Day	14% 13% 32% 28% 11% 2	271
	21st to end	Day	11% 11% 32% 23% 16% 6%	220
December	1st to 10th	Day	9% 8% 35% 29% 16% 4	220
	11th to 20th	Day	10% 11% 40% 25% 11% 3	220
	21st to end	Day	7% 7% 31% 30% 21% 3	242
January	1st to 10th	Night	8% 8% 38% 33% 12% 1	1050
	11th to 20th	Night	9% 9% 39% 33% 10% 1	1050
	21st to end	Night	12% 9% 35% 33% 9% 2	934
February	1st to 10th	Night	9% 10% 30% 34% 15% 3	840
	11th to 20th	Night	8% 7% 36% 36% 13% 1	824
	21st to end	Night	6% 7% 32% 32% 19% 4	692
March	1st to 10th	Night	7% 6% 33% 37% 14% 3	840
	11th to 20th	Night	9% 7% 35% 35% 12% 2	840
	21st to end	Night	13% 6% 30% 35% 13% 3	924
April	1st to 10th	Night	12% 8% 29% 33% 15% 2	779
	11th to 20th	Night	15% 9% 31% 35% 10%	623
	21st to end	Night	16% 9% 27% 36% 11% 1	630
May	1st to 10th	Night	18% 8% 29% 32% 13%	499
	11th to 20th	Night	13% 5% 28% 38% 15%	414
	21st to end	Night	18% 5% 23% 36% 17% 1	455
June	1st to 10th	Night	18% 8% 23% 36% 14% 1	420
	11th to 20th	Night	18% 6% 28% 34% 13% 1	420
	21st to end	Night	11% 5% 20% 41% 21% 1	420
July	1st to 10th	Night	10% 4 18% 42% 25% 1	410
	11th to 20th	Night	12% 2 23% 44% 18% 1	408
	21st to end	Night	9% 3 17% 42% 27% 2	460
August	1st to 10th	Night	10% 4 15% 40% 28% 2	418
	11th to 20th	Night	11% 4 16% 42% 25% 1	568
	21st to end	Night	10% 3 22% 44% 20% 1	693
September	1st to 10th	Night	13% 4 24% 39% 18% 3	735
	11th to 20th	Night	16% 4 20% 37% 22% 2	840
	21st to end	Night	13% 5% 31% 36% 13% 2	840
October	1st to 10th	Night	15% 5% 29% 34% 14% 2	840
	11th to 20th	Night	12% 5% 33% 36% 12% 2	812
	21st to end	Night	11% 10% 34% 33% 12%	880
November	1st to 10th	Night	15% 8% 36% 28% 11% 1	945
	11th to 20th	Night	9% 10% 41% 30% 9% 1	1050
	21st to end	Night	7% 7% 39% 35% 10% 2	1050
December	1st to 10th	Night	11% 8% 34% 32% 12% 2	1050
	11th to 20th	Night	7% 8% 39% 35% 10% 1	1050
	21st to end	Night	11% 5% 36% 35% 12% 2	1155

Page generated at 2026-10-11 03:06:30. Version ac07

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