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 2025-06-10.

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% 9% 3 8% 5% 5% 5% 4 6% 8% 37%	123456
	Night	7% 10% 4 9% 6% 5% 5% 4 6% 8% 36%	123486
Transparency	Day	43% 6% 15% 24% 10% 2	123201
	Night	44% 6% 14% 23% 11% 2	123384
Seeing	Day	13% 9% 27% 31% 16% 5%	16084
	Night	11% 6% 31% 35% 14% 2	25223

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 10% 7% 13% 8% 6% 6% 4 6% 6% 32%	3179
	11th to 20th	Day	2 11% 4 10% 7% 5% 7% 6% 7% 7% 35%	3180
	21st to end	Day	3 8% 3 7% 4 4 5% 5% 7% 10% 43%	4004
February	1st to 10th	Day	6% 11% 5% 12% 8% 6% 5% 4 7% 9% 28%	1600
	11th to 20th	Day	6% 12% 4 11% 7% 7% 6% 5% 7% 8% 27%	1724
	21st to end	Day	3 11% 6% 11% 7% 6% 6% 5% 7% 7% 31%	1600
March	1st to 10th	Day	3 9% 5% 11% 7% 5% 5% 4 6% 10% 36%	4518
	11th to 20th	Day	5% 12% 5% 10% 5% 4 4 4 6% 8% 38%	4928
	21st to end	Day	9% 21% 6% 12% 5% 5% 6% 4 5% 5% 22%	5590
April	1st to 10th	Day	7% 16% 4 10% 7% 5% 6% 5% 7% 7% 27%	2457
	11th to 20th	Day	12% 14% 5% 9% 5% 4 5% 4 6% 7% 28%	2570
	21st to end	Day	11% 13% 4 9% 5% 4 5% 4 6% 8% 31%	2600
May	1st to 10th	Day	14% 10% 3 7% 5% 5% 4 4 5% 6% 38%	6296
	11th to 20th	Day	13% 11% 4 9% 6% 6% 6% 5% 7% 8% 24%	6734
	21st to end	Day	12% 8% 3 8% 6% 5% 5% 4 5% 6% 35%	7405
June	1st to 10th	Day	16% 8% 3 8% 6% 6% 5% 5% 8% 8% 29%	2988
	11th to 20th	Day	20% 9% 3 8% 7% 6% 6% 5% 7% 7% 23%	2850
	21st to end	Day	9% 6% 2 7% 5% 6% 6% 6% 7% 10% 36%	2850
July	1st to 10th	Day	8% 5% 2 5% 4 5% 5% 4 7% 8% 47%	4740
	11th to 20th	Day	9% 3 1 4 3 5% 5% 5% 8% 11% 46%	4740
	21st to end	Day	6% 4 2 5% 4 4 4 5% 7% 9% 51%	5055
August	1st to 10th	Day	10% 5% 2 7% 4 5% 6% 4 8% 8% 41%	4401
	11th to 20th	Day	8% 5% 2 6% 5% 5% 6% 4 8% 11% 41%	4099
	21st to end	Day	11% 4 2 5% 4 4 4 4 6% 8% 49%	4521
September	1st to 10th	Day	12% 6% 2 5% 4 3 4 4 5% 7% 47%	2367
	11th to 20th	Day	15% 8% 2 5% 4 3 4 3 5% 7% 44%	2090
	21st to end	Day	9% 8% 2 7% 4 4 5% 5% 6% 7% 43%	2090
October	1st to 10th	Day	7% 6% 3 7% 4 5% 6% 5% 8% 9% 41%	3344
	11th to 20th	Day	7% 7% 2 8% 5% 5% 6% 5% 8% 10% 39%	2887
	21st to end	Day	7% 8% 3 8% 6% 5% 5% 3 5% 7% 42%	3101
November	1st to 10th	Day	9% 16% 5% 9% 5% 4 4 3 6% 7% 31%	1623
	11th to 20th	Day	6% 14% 4 9% 5% 5% 4 4 6% 8% 35%	1495
	21st to end	Day	5% 14% 5% 8% 6% 5% 5% 5% 7% 9% 33%	1400
December	1st to 10th	Day	5% 10% 3 11% 7% 7% 7% 6% 7% 7% 30%	2290
	11th to 20th	Day	5% 10% 4 10% 7% 7% 7% 4 6% 8% 32%	2642
	21st to end	Day	3 9% 4 9% 5% 4 5% 4 6% 8% 40%	3498
January	1st to 10th	Night	3 13% 6% 14% 7% 6% 6% 5% 7% 7% 25%	6766
	11th to 20th	Night	2 10% 4 12% 6% 6% 6% 5% 6% 7% 34%	6739
	21st to end	Night	3 9% 4 8% 5% 5% 5% 4 6% 8% 40%	6975
February	1st to 10th	Night	5% 10% 5% 14% 8% 8% 5% 4 6% 6% 29%	2800
	11th to 20th	Night	4 12% 5% 12% 7% 6% 6% 4 7% 7% 29%	2562
	21st to end	Night	2 11% 6% 10% 6% 6% 5% 5% 6% 8% 34%	2028
March	1st to 10th	Night	3 9% 4 12% 6% 6% 6% 5% 7% 9% 34%	5376
	11th to 20th	Night	5% 10% 4 10% 5% 6% 5% 5% 7% 9% 34%	5071
	21st to end	Night	8% 19% 5% 13% 7% 5% 5% 4 5% 7% 22%	5371
April	1st to 10th	Night	7% 14% 4 7% 7% 5% 5% 5% 7% 8% 31%	1942
	11th to 20th	Night	9% 14% 4 10% 7% 5% 5% 4 6% 7% 27%	1781
	21st to end	Night	11% 10% 4 9% 6% 6% 5% 4 8% 7% 30%	1800
May	1st to 10th	Night	16% 10% 3 7% 5% 4 3 3 5% 6% 37%	3668
	11th to 20th	Night	15% 9% 4 8% 5% 5% 5% 4 7% 9% 29%	3159
	21st to end	Night	11% 9% 3 9% 7% 6% 5% 5% 6% 6% 34%	3470
June	1st to 10th	Night	17% 7% 3 8% 5% 5% 5% 4 7% 7% 33%	1396
	11th to 20th	Night	19% 10% 3 7% 6% 6% 6% 4 7% 7% 26%	1330
	21st to end	Night	9% 6% 3 7% 5% 5% 5% 4 8% 7% 41%	1330
July	1st to 10th	Night	8% 5% 2 6% 5% 6% 5% 6% 7% 8% 42%	2220
	11th to 20th	Night	6% 3 1 4 5% 5% 5% 5% 8% 10% 48%	2220
	21st to end	Night	5% 5% 2 4 3 3 5% 4 6% 10% 52%	2494
August	1st to 10th	Night	11% 6% 2 6% 5% 4 4 4 6% 8% 44%	2529
	11th to 20th	Night	8% 6% 2 7% 5% 4 5% 4 7% 11% 41%	2737
	21st to end	Night	10% 6% 2 5% 4 4 4 4 5% 7% 49%	3146
September	1st to 10th	Night	10% 7% 2 5% 3 4 4 4 5% 9% 47%	1810
	11th to 20th	Night	15% 7% 2 5% 4 4 4 4 6% 8% 43%	2090
	21st to end	Night	10% 7% 2 6% 4 4 4 4 7% 8% 45%	2090
October	1st to 10th	Night	9% 6% 2 6% 5% 4 4 4 6% 8% 46%	3631
	11th to 20th	Night	6% 7% 3 8% 4 5% 5% 4 7% 8% 44%	4041
	21st to end	Night	5% 8% 4 7% 5% 5% 5% 5% 7% 8% 42%	4457
November	1st to 10th	Night	8% 14% 5% 9% 5% 5% 5% 4 5% 7% 34%	2654
	11th to 20th	Night	4 12% 4 9% 6% 5% 5% 5% 7% 9% 33%	2905
	21st to end	Night	4 11% 4 9% 6% 6% 6% 4 7% 9% 34%	3000
December	1st to 10th	Night	5% 11% 4 10% 7% 6% 6% 5% 7% 8% 30%	4890
	11th to 20th	Night	4 12% 5% 12% 9% 7% 6% 5% 6% 8% 27%	5550
	21st to end	Night	3 10% 4 9% 5% 5% 5% 4 6% 8% 41%	7458

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% 22% 11% 2	3179
	11th to 20th	Day	45% 7% 11% 15% 19% 3	3201
	21st to end	Day	36% 6% 13% 33% 11% 1	4004
February	1st to 10th	Day	53% 6% 13% 17% 9% 2	1600
	11th to 20th	Day	52% 6% 14% 18% 9% 2	1733
	21st to end	Day	48% 7% 13% 20% 10% 2	1600
March	1st to 10th	Day	42% 6% 14% 23% 15%	4518
	11th to 20th	Day	42% 5% 13% 24% 15% 1	4929
	21st to end	Day	62% 6% 11% 14% 6% 1	5590
April	1st to 10th	Day	51% 7% 15% 19% 8%	2457
	11th to 20th	Day	53% 6% 10% 21% 9% 1	2570
	21st to end	Day	49% 6% 15% 22% 7% 1	2600
May	1st to 10th	Day	46% 4 10% 22% 15% 3	6296
	11th to 20th	Day	52% 8% 13% 20% 7%	6738
	21st to end	Day	46% 6% 14% 23% 10%	7406
June	1st to 10th	Day	48% 7% 16% 19% 9% 1	2988
	11th to 20th	Day	56% 7% 14% 16% 5% 1	2850
	21st to end	Day	38% 8% 21% 24% 7% 1	2850
July	1st to 10th	Day	31% 7% 16% 28% 14% 3	4666
	11th to 20th	Day	29% 7% 20% 35% 9% 1	4659
	21st to end	Day	26% 6% 16% 28% 19% 4	4895
August	1st to 10th	Day	38% 7% 18% 31% 6% 1	4384
	11th to 20th	Day	34% 7% 25% 27% 6% 1	4139
	21st to end	Day	32% 5% 21% 32% 9% 1	4520
September	1st to 10th	Day	35% 5% 20% 24% 12% 4	2367
	11th to 20th	Day	40% 5% 15% 28% 11% 1	2090
	21st to end	Day	37% 6% 16% 24% 13% 3	2090
October	1st to 10th	Day	37% 6% 21% 30% 5% 1	3344
	11th to 20th	Day	39% 6% 20% 23% 11% 2	2887
	21st to end	Day	40% 4 10% 35% 10% 1	3101
November	1st to 10th	Day	52% 6% 12% 19% 9% 2	1623
	11th to 20th	Day	46% 4 15% 22% 10% 1	1495
	21st to end	Day	46% 6% 16% 21% 9% 2	1400
December	1st to 10th	Day	51% 6% 13% 21% 7% 1	2290
	11th to 20th	Day	49% 6% 20% 19% 5% 1	2644
	21st to end	Day	39% 5% 11% 23% 18% 4	3498
January	1st to 10th	Night	56% 6% 11% 19% 6% 2	6766
	11th to 20th	Night	47% 6% 12% 17% 14% 4	6780
	21st to end	Night	40% 5% 16% 27% 12% 1	6975
February	1st to 10th	Night	54% 4 11% 18% 11% 2	2800
	11th to 20th	Night	50% 6% 11% 19% 12% 2	2563
	21st to end	Night	46% 5% 10% 22% 14% 2	2028
March	1st to 10th	Night	43% 7% 16% 21% 11% 1	5376
	11th to 20th	Night	42% 8% 14% 22% 12% 1	5071
	21st to end	Night	59% 8% 9% 15% 8% 1	5371
April	1st to 10th	Night	48% 6% 13% 20% 11% 1	1942
	11th to 20th	Night	54% 5% 12% 20% 9% 1	1781
	21st to end	Night	50% 6% 14% 22% 8% 2	1800
May	1st to 10th	Night	47% 5% 9% 21% 14% 4	3665
	11th to 20th	Night	48% 6% 16% 21% 8%	3159
	21st to end	Night	46% 8% 14% 24% 7% 1	3470
June	1st to 10th	Night	47% 7% 16% 20% 9% 1	1396
	11th to 20th	Night	55% 6% 15% 15% 6% 2	1330
	21st to end	Night	39% 7% 21% 25% 7% 2	1330
July	1st to 10th	Night	36% 8% 14% 23% 16% 2	2185
	11th to 20th	Night	27% 7% 22% 33% 7% 4	2178
	21st to end	Night	27% 6% 14% 33% 18% 2	2415
August	1st to 10th	Night	36% 6% 17% 32% 9%	2521
	11th to 20th	Night	34% 8% 26% 26% 6%	2763
	21st to end	Night	33% 6% 21% 29% 9% 1	3146
September	1st to 10th	Night	34% 4 17% 26% 14% 5%	1810
	11th to 20th	Night	38% 6% 16% 26% 11% 4	2090
	21st to end	Night	35% 5% 17% 26% 13% 3	2090
October	1st to 10th	Night	35% 5% 17% 27% 14% 1	3631
	11th to 20th	Night	36% 5% 19% 26% 13% 1	4041
	21st to end	Night	38% 5% 12% 31% 12% 1	4457
November	1st to 10th	Night	48% 7% 12% 21% 11% 2	2654
	11th to 20th	Night	45% 6% 16% 20% 11% 1	2905
	21st to end	Night	45% 7% 16% 20% 9% 3	3000
December	1st to 10th	Night	50% 6% 17% 19% 7% 2	4890
	11th to 20th	Night	55% 5% 16% 17% 6% 1	5547
	21st to end	Night	39% 4 10% 25% 17% 4	7458

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	6% 8% 30% 35% 16% 7%	200
	11th to 20th	Day	9% 9% 35% 29% 17% 3	200
	21st to end	Day	8% 10% 31% 30% 18% 4	220
February	1st to 10th	Day	11% 11% 34% 27% 12% 6%	200
	11th to 20th	Day	10% 7% 35% 33% 12% 3	285
	21st to end	Day	6% 10% 29% 34% 18% 4	364
March	1st to 10th	Day	8% 9% 34% 32% 13% 4	500
	11th to 20th	Day	8% 9% 33% 35% 11% 5%	500
	21st to end	Day	15% 10% 33% 25% 11% 6%	550
April	1st to 10th	Day	12% 13% 33% 27% 13% 3	500
	11th to 20th	Day	16% 13% 28% 27% 14% 2	492
	21st to end	Day	17% 12% 26% 30% 11% 3	500
May	1st to 10th	Day	16% 13% 26% 28% 15% 3	563
	11th to 20th	Day	15% 8% 24% 31% 17% 4	725
	21st to end	Day	15% 7% 21% 30% 21% 6%	789
June	1st to 10th	Day	16% 11% 27% 30% 12% 4	726
	11th to 20th	Day	21% 10% 24% 30% 13% 3	680
	21st to end	Day	10% 6% 24% 34% 20% 6%	680
July	1st to 10th	Day	12% 5% 20% 36% 21% 6%	671
	11th to 20th	Day	15% 4 21% 32% 23% 4	668
	21st to end	Day	7% 4 24% 36% 21% 8%	692
August	1st to 10th	Day	10% 5% 27% 30% 21% 7%	487
	11th to 20th	Day	9% 5% 22% 39% 20% 5%	477
	21st to end	Day	11% 7% 24% 31% 20% 7%	539
September	1st to 10th	Day	14% 8% 29% 27% 16% 6%	490
	11th to 20th	Day	19% 6% 24% 29% 16% 5%	490
	21st to end	Day	15% 12% 32% 27% 11% 3	490
October	1st to 10th	Day	15% 9% 27% 30% 13% 6%	409
	11th to 20th	Day	12% 12% 30% 28% 14% 3	300
	21st to end	Day	15% 12% 34% 28% 9% 3	330
November	1st to 10th	Day	18% 7% 30% 30% 11% 4	300
	11th to 20th	Day	15% 13% 32% 27% 11% 2	247
	21st to end	Day	10% 11% 33% 24% 16% 7%	200
December	1st to 10th	Day	10% 7% 34% 30% 16% 4	200
	11th to 20th	Day	9% 10% 38% 28% 13% 3	200
	21st to end	Day	7% 5% 30% 30% 24% 4	220
January	1st to 10th	Night	8% 8% 37% 33% 12% 1	1000
	11th to 20th	Night	9% 8% 38% 34% 10% 1	1000
	21st to end	Night	12% 9% 35% 33% 10% 2	890
February	1st to 10th	Night	9% 9% 29% 34% 16% 3	800
	11th to 20th	Night	8% 7% 35% 36% 13% 1	784
	21st to end	Night	5% 7% 31% 32% 19% 5%	660
March	1st to 10th	Night	8% 5% 33% 37% 14% 3	800
	11th to 20th	Night	9% 6% 33% 37% 13% 2	800
	21st to end	Night	13% 5% 29% 35% 14% 3	880
April	1st to 10th	Night	11% 8% 30% 33% 15% 2	742
	11th to 20th	Night	16% 8% 32% 34% 10%	593
	21st to end	Night	16% 9% 27% 36% 11% 1	600
May	1st to 10th	Night	17% 8% 29% 32% 13%	475
	11th to 20th	Night	12% 5% 28% 39% 15% 1	394
	21st to end	Night	19% 5% 22% 36% 16% 1	433
June	1st to 10th	Night	17% 9% 23% 36% 14% 1	399
	11th to 20th	Night	19% 4 29% 33% 14% 1	380
	21st to end	Night	10% 4 22% 41% 22% 2	380
July	1st to 10th	Night	11% 4 18% 40% 26% 1	370
	11th to 20th	Night	12% 2 22% 43% 20% 1	368
	21st to end	Night	8% 3 18% 41% 28% 2	416
August	1st to 10th	Night	10% 3 14% 41% 29% 2	378
	11th to 20th	Night	10% 4 17% 43% 25% 1	514
	21st to end	Night	10% 3 22% 42% 22% 1	627
September	1st to 10th	Night	13% 3 24% 39% 18% 3	665
	11th to 20th	Night	17% 4 21% 36% 20% 2	760
	21st to end	Night	14% 5% 31% 35% 13% 2	760
October	1st to 10th	Night	16% 5% 28% 34% 14% 3	760
	11th to 20th	Night	11% 5% 33% 36% 12% 2	760
	21st to end	Night	11% 9% 34% 34% 12%	836
November	1st to 10th	Night	15% 7% 36% 29% 11% 2	899
	11th to 20th	Night	9% 10% 41% 29% 9% 2	1000
	21st to end	Night	7% 7% 39% 36% 10% 2	1000
December	1st to 10th	Night	11% 8% 33% 32% 13% 3	1000
	11th to 20th	Night	7% 7% 39% 36% 10% 1	1000
	21st to end	Night	11% 4 35% 36% 12% 2	1100

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