Challis Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Challis Astronomical Observatory clear sky chart from 2005-11-06 to 2026-09-29.

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	2 4 2 6% 5% 5% 6% 5% 7% 9% 49%	157209
	Night	2 4 2 6% 4 4 5% 5% 7% 9% 52%	151531
Transparency	Day	27% 7% 13% 23% 20% 10%	151943
	Night	26% 6% 17% 26% 17% 7%	147802
Seeing	Day	9% 15% 34% 28% 11% 3	17384
	Night	5% 8% 29% 38% 17% 2	26769

By Month

Percentage Cloud Cover SW (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 2 9% 7% 8% 10% 8% 12% 13% 27%	4722
	11th to 20th	Day	3 3 7% 5% 4 4 4 5% 9% 56%	4740
	21st to end	Day	4 3 9% 5% 5% 6% 6% 10% 10% 42%	5187
February	1st to 10th	Day	5% 3 11% 5% 7% 6% 6% 9% 11% 35%	1890
	11th to 20th	Day	4 3 10% 9% 7% 7% 6% 8% 10% 34%	1973
	21st to end	Day	5% 4 10% 8% 7% 8% 7% 10% 10% 31%	1730
March	1st to 10th	Day	5% 2 8% 6% 7% 8% 6% 9% 10% 39%	4618
	11th to 20th	Day	1 4 3 8% 5% 7% 7% 6% 9% 12% 39%	5100
	21st to end	Day	1 8% 4 11% 7% 8% 9% 7% 9% 9% 25%	5607
April	1st to 10th	Day	2 8% 4 11% 7% 7% 6% 5% 7% 9% 34%	2515
	11th to 20th	Day	2 7% 3 9% 7% 7% 6% 6% 7% 9% 37%	2645
	21st to end	Day	5% 10% 5% 11% 6% 5% 5% 4 7% 9% 34%	2730
May	1st to 10th	Day	3 7% 3 7% 4 4 5% 5% 6% 7% 48%	7626
	11th to 20th	Day	4 8% 3 8% 5% 5% 6% 5% 6% 8% 43%	7596
	21st to end	Day	5% 6% 3 7% 5% 5% 6% 5% 6% 8% 44%	9006
June	1st to 10th	Day	6% 6% 2 7% 6% 6% 6% 5% 8% 9% 38%	3150
	11th to 20th	Day	7% 5% 3 5% 5% 5% 5% 4 6% 7% 49%	3150
	21st to end	Day	2 4 2 5% 5% 4 4 4 5% 7% 60%	3141
July	1st to 10th	Day	1 2 1 3 3 4 5% 4 6% 6% 66%	8219
	11th to 20th	Day	1 2 1 3 3 3 4 4 6% 8% 65%	8220
	21st to end	Day	2 2 1 3 3 4 5% 4 7% 9% 59%	8922
August	1st to 10th	Day	2 2 1 3 3 4 5% 4 6% 7% 64%	7555
	11th to 20th	Day	1 1 1 3 3 4 5% 5% 6% 8% 63%	7565
	21st to end	Day	4 4 2 5% 4 4 4 4 6% 7% 56%	8213
September	1st to 10th	Day	4 3 1 4 3 4 4 3 5% 6% 64%	2457
	11th to 20th	Day	5% 5% 2 5% 5% 4 4 3 6% 7% 55%	2310
	21st to end	Day	5% 4 1 4 3 3 4 3 5% 8% 60%	2289
October	1st to 10th	Day	4 5% 2 4 4 3 4 3 4 4 64%	3574
	11th to 20th	Day	2 5% 1 5% 4 4 4 4 6% 8% 58%	3166
	21st to end	Day	1 4 2 6% 4 5% 6% 5% 6% 8% 53%	3200
November	1st to 10th	Day	2 5% 3 7% 7% 6% 6% 5% 8% 7% 44%	1854
	11th to 20th	Day	1 6% 3 6% 6% 6% 6% 5% 8% 11% 42%	1890
	21st to end	Day	1 5% 3 5% 5% 5% 5% 6% 10% 11% 45%	1706
December	1st to 10th	Day	1 3 2 7% 5% 6% 5% 4 7% 8% 53%	2439
	11th to 20th	Day	3 2 9% 8% 8% 9% 6% 10% 11% 33%	2715
	21st to end	Day	4 2 8% 6% 7% 9% 7% 11% 13% 33%	3789
January	1st to 10th	Night	4 2 9% 6% 7% 9% 8% 11% 13% 31%	8172
	11th to 20th	Night	3 2 9% 5% 5% 5% 4 5% 9% 53%	8220
	21st to end	Night	1 5% 2 7% 4 5% 6% 5% 7% 10% 48%	8981
February	1st to 10th	Night	1 6% 4 10% 6% 6% 6% 6% 9% 11% 35%	2730
	11th to 20th	Night	5% 3 8% 6% 6% 6% 4 8% 11% 42%	2530
	21st to end	Night	1 4 2 8% 6% 6% 6% 5% 8% 11% 42%	2076
March	1st to 10th	Night	5% 3 7% 5% 5% 5% 5% 7% 12% 46%	5496
	11th to 20th	Night	1 4 3 7% 5% 5% 6% 5% 9% 17% 38%	5119
	21st to end	Night	1 6% 3 9% 6% 6% 5% 5% 8% 12% 39%	5610
April	1st to 10th	Night	3 7% 3 6% 5% 5% 5% 4 7% 9% 45%	2103
	11th to 20th	Night	2 7% 3 7% 4 5% 6% 5% 8% 9% 44%	1922
	21st to end	Night	5% 7% 3 7% 4 4 5% 4 7% 10% 44%	1890
May	1st to 10th	Night	3 8% 3 7% 4 3 3 3 5% 7% 54%	5321
	11th to 20th	Night	6% 8% 2 5% 3 3 4 3 5% 7% 53%	5296
	21st to end	Night	3 6% 2 6% 4 3 4 4 5% 9% 54%	5133
June	1st to 10th	Night	6% 4 2 5% 4 3 5% 4 8% 8% 49%	1470
	11th to 20th	Night	6% 5% 2 5% 4 4 4 4 4 7% 54%	1470
	21st to end	Night	3 4 1 4 3 2 4 3 5% 8% 63%	1472
July	1st to 10th	Night	1 1 1 2 2 2 3 3 5% 7% 74%	4740
	11th to 20th	Night	1 1 1 2 2 2 3 3 5% 7% 73%	4740
	21st to end	Night	1 1 1 2 2 3 3 4 6% 9% 67%	5153
August	1st to 10th	Night	1 1 1 2 2 3 3 3 5% 7% 72%	5264
	11th to 20th	Night	1 1 1 2 2 4 3 6% 9% 71%	5291
	21st to end	Night	4 3 1 3 3 3 3 3 6% 8% 62%	5996
September	1st to 10th	Night	3 3 1 3 2 2 3 3 4 7% 69%	2162
	11th to 20th	Night	4 4 1 4 3 3 4 3 6% 8% 59%	2310
	21st to end	Night	6% 3 2 3 2 3 3 2 5% 6% 66%	2288
October	1st to 10th	Night	3 4 1 3 2 2 3 3 4 7% 68%	3623
	11th to 20th	Night	3 5% 2 4 3 4 4 4 5% 6% 60%	3977
	21st to end	Night	2 4 2 5% 4 4 5% 4 7% 10% 52%	4600
November	1st to 10th	Night	1 6% 2 6% 5% 5% 6% 5% 8% 10% 47%	2665
	11th to 20th	Night	1 5% 2 5% 5% 5% 6% 6% 9% 12% 43%	2730
	21st to end	Night	1 3 2 6% 4 4 5% 5% 8% 11% 50%	2889
December	1st to 10th	Night	2 2 5% 4 5% 5% 5% 6% 8% 57%	4958
	11th to 20th	Night	5% 3 8% 6% 6% 7% 6% 10% 12% 35%	5696
	21st to end	Night	1 5% 2 9% 8% 8% 10% 8% 10% 10% 30%	7438

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	35% 13% 24% 22% 6%	4723
	11th to 20th	Day	25% 5% 25% 31% 11% 3	4740
	21st to end	Day	31% 9% 19% 26% 15%	5187
February	1st to 10th	Day	36% 12% 22% 21% 8% 1	1890
	11th to 20th	Day	36% 11% 20% 24% 10% 1	1983
	21st to end	Day	36% 12% 16% 22% 11% 2	1730
March	1st to 10th	Day	30% 9% 17% 27% 15% 2	4618
	11th to 20th	Day	28% 9% 23% 31% 9%	5100
	21st to end	Day	41% 13% 21% 20% 5%	5607
April	1st to 10th	Day	40% 9% 20% 23% 7% 2	2515
	11th to 20th	Day	37% 7% 18% 25% 12% 1	2648
	21st to end	Day	42% 7% 15% 26% 10%	2730
May	1st to 10th	Day	28% 7% 12% 30% 19% 4	7626
	11th to 20th	Day	33% 6% 11% 24% 19% 7%	7599
	21st to end	Day	31% 8% 11% 29% 18% 3	9008
June	1st to 10th	Day	37% 7% 15% 27% 14% 2	3150
	11th to 20th	Day	32% 5% 8% 21% 25% 9%	3150
	21st to end	Day	22% 4 6% 22% 34% 12%	3141
July	1st to 10th	Day	16% 4 5% 15% 27% 32%	8151
	11th to 20th	Day	16% 3 6% 14% 28% 33%	8145
	21st to end	Day	19% 5% 6% 18% 29% 24%	8723
August	1st to 10th	Day	18% 4 5% 12% 28% 33%	5983
	11th to 20th	Day	19% 5% 11% 20% 33% 11%	5964
	21st to end	Day	23% 4 7% 19% 32% 15%	6444
September	1st to 10th	Day	19% 4 6% 21% 32% 17%	2457
	11th to 20th	Day	26% 5% 9% 26% 25% 9%	2310
	21st to end	Day	22% 4 9% 31% 28% 7%	2289
October	1st to 10th	Day	24% 4 7% 23% 35% 8%	3574
	11th to 20th	Day	24% 4 16% 31% 23% 2	3166
	21st to end	Day	25% 6% 14% 31% 22% 1	3200
November	1st to 10th	Day	33% 7% 18% 25% 15% 2	1854
	11th to 20th	Day	32% 8% 22% 27% 9% 2	1890
	21st to end	Day	27% 8% 23% 27% 13% 2	1706
December	1st to 10th	Day	27% 6% 28% 29% 9% 1	2439
	11th to 20th	Day	38% 10% 27% 20% 4	2714
	21st to end	Day	36% 11% 19% 20% 11% 4	3789
January	1st to 10th	Night	35% 13% 22% 25% 5%	8172
	11th to 20th	Night	29% 5% 25% 29% 9% 3	8220
	21st to end	Night	30% 7% 18% 30% 14% 1	8981
February	1st to 10th	Night	39% 9% 22% 21% 9% 1	2730
	11th to 20th	Night	32% 8% 23% 25% 11%	2531
	21st to end	Night	32% 9% 19% 27% 12% 2	2076
March	1st to 10th	Night	27% 6% 18% 25% 22% 1	5496
	11th to 20th	Night	28% 9% 24% 31% 8%	5119
	21st to end	Night	33% 9% 27% 26% 5%	5610
April	1st to 10th	Night	32% 7% 21% 28% 10% 1	2103
	11th to 20th	Night	31% 8% 19% 28% 14% 1	1922
	21st to end	Night	34% 5% 19% 31% 10%	1890
May	1st to 10th	Night	28% 5% 11% 37% 16% 3	5319
	11th to 20th	Night	29% 5% 11% 27% 24% 5%	5297
	21st to end	Night	26% 5% 14% 34% 19% 1	5134
June	1st to 10th	Night	30% 6% 17% 31% 15% 1	1470
	11th to 20th	Night	29% 6% 9% 25% 24% 9%	1470
	21st to end	Night	20% 3 9% 24% 33% 12%	1472
July	1st to 10th	Night	10% 3 7% 24% 23% 33%	4700
	11th to 20th	Night	12% 2 5% 18% 28% 34%	4692
	21st to end	Night	14% 3 9% 23% 27% 24%	5040
August	1st to 10th	Night	13% 3 5% 15% 29% 35%	4160
	11th to 20th	Night	12% 5% 13% 25% 32% 13%	4168
	21st to end	Night	18% 4 12% 21% 29% 15%	4693
September	1st to 10th	Night	17% 3 7% 25% 32% 17%	2162
	11th to 20th	Night	22% 4 11% 26% 27% 10%	2310
	21st to end	Night	21% 3 9% 29% 31% 7%	2288
October	1st to 10th	Night	17% 3 6% 28% 35% 11%	3623
	11th to 20th	Night	25% 3 14% 31% 24% 2	3977
	21st to end	Night	25% 6% 17% 32% 19% 1	4600
November	1st to 10th	Night	28% 7% 20% 27% 15% 2	2665
	11th to 20th	Night	30% 9% 23% 28% 8% 1	2730
	21st to end	Night	26% 8% 20% 31% 14% 2	2889
December	1st to 10th	Night	22% 8% 33% 27% 9% 1	4958
	11th to 20th	Night	36% 10% 26% 23% 4	5697
	21st to end	Night	43% 10% 18% 16% 11% 2	7438

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% 15% 33% 29% 15% 2	220
	11th to 20th	Day	6% 11% 41% 25% 11% 6%	220
	21st to end	Day	4 6% 48% 28% 12% 2	242
February	1st to 10th	Day	6% 23% 43% 20% 5% 2	330
	11th to 20th	Day	4 19% 41% 25% 7% 2	459
	21st to end	Day	7% 19% 41% 25% 7%	446
March	1st to 10th	Day	6% 20% 41% 27% 6%	540
	11th to 20th	Day	9% 18% 34% 27% 11% 1	540
	21st to end	Day	12% 21% 38% 23% 6%	594
April	1st to 10th	Day	11% 18% 34% 27% 8% 2	540
	11th to 20th	Day	8% 26% 37% 22% 5% 2	532
	21st to end	Day	15% 18% 33% 24% 8% 2	540
May	1st to 10th	Day	11% 17% 34% 27% 11% 1	543
	11th to 20th	Day	23% 15% 27% 23% 10% 2	568
	21st to end	Day	17% 13% 29% 24% 14% 3	627
June	1st to 10th	Day	12% 16% 26% 28% 15% 3	780
	11th to 20th	Day	15% 19% 32% 22% 12% 1	780
	21st to end	Day	9% 12% 28% 34% 16% 2	771
July	1st to 10th	Day	4 14% 36% 34% 10% 1	566
	11th to 20th	Day	4 14% 39% 32% 10% 1	564
	21st to end	Day	6% 10% 35% 31% 13% 4	619
August	1st to 10th	Day	6% 11% 32% 31% 15% 4	567
	11th to 20th	Day	4 10% 32% 34% 15% 5%	557
	21st to end	Day	9% 14% 30% 28% 15% 5%	627
September	1st to 10th	Day	6% 12% 27% 33% 15% 6%	570
	11th to 20th	Day	11% 13% 28% 30% 13% 5%	570
	21st to end	Day	9% 13% 31% 27% 17% 4	563
October	1st to 10th	Day	13% 9% 29% 29% 17% 3	517
	11th to 20th	Day	9% 16% 25% 29% 15% 6%	330
	21st to end	Day	10% 16% 35% 29% 8% 2	363
November	1st to 10th	Day	13% 13% 36% 25% 12% 1	330
	11th to 20th	Day	7% 17% 35% 30% 9% 2	330
	21st to end	Day	7% 9% 42% 30% 11% 1	330
December	1st to 10th	Day	7% 12% 34% 26% 16% 5%	247
	11th to 20th	Day	10% 17% 37% 28% 7% 2	220
	21st to end	Day	9% 16% 37% 26% 11% 2	242
January	1st to 10th	Night	4 12% 34% 35% 14% 1	840
	11th to 20th	Night	4 7% 35% 38% 13% 4	840
	21st to end	Night	2 6% 35% 40% 14% 2	924
February	1st to 10th	Night	4 15% 36% 32% 11% 2	840
	11th to 20th	Night	3 9% 40% 34% 13% 2	824
	21st to end	Night	5% 10% 36% 34% 13% 2	692
March	1st to 10th	Night	3 9% 38% 37% 11% 1	840
	11th to 20th	Night	4 13% 34% 32% 14% 2	840
	21st to end	Night	7% 11% 32% 37% 11% 1	924
April	1st to 10th	Night	6% 8% 32% 38% 14% 2	840
	11th to 20th	Night	5% 10% 39% 33% 12% 1	674
	21st to end	Night	9% 8% 30% 35% 15% 2	630
May	1st to 10th	Night	6% 9% 24% 38% 21% 2	630
	11th to 20th	Night	13% 7% 24% 30% 23% 2	621
	21st to end	Night	13% 4 23% 31% 26% 3	462
June	1st to 10th	Night	8% 7% 21% 40% 21% 2	420
	11th to 20th	Night	10% 7% 28% 41% 13% 2	420
	21st to end	Night	5% 3 19% 46% 25% 2	420
July	1st to 10th	Night	2 2 15% 46% 32% 4	410
	11th to 20th	Night	2 1 19% 56% 22%	408
	21st to end	Night	2 1 16% 47% 32% 2	460
August	1st to 10th	Night	5% 3 17% 47% 26% 2	618
	11th to 20th	Night	2 3 17% 47% 28% 2	624
	21st to end	Night	5% 5% 20% 39% 28% 2	743
September	1st to 10th	Night	5% 2 17% 44% 27% 4	840
	11th to 20th	Night	6% 4 17% 41% 26% 6%	840
	21st to end	Night	7% 5% 21% 37% 26% 5%	832
October	1st to 10th	Night	9% 5% 20% 43% 21% 2	800
	11th to 20th	Night	6% 10% 23% 38% 18% 4	800
	21st to end	Night	5% 12% 29% 38% 13% 3	880
November	1st to 10th	Night	6% 10% 30% 38% 15% 1	820
	11th to 20th	Night	5% 15% 34% 36% 9% 1	840
	21st to end	Night	4 10% 33% 35% 15% 3	999
December	1st to 10th	Night	5% 10% 33% 39% 12% 2	1050
	11th to 20th	Night	6% 11% 33% 36% 13% 1	1050
	21st to end	Night	5% 15% 35% 30% 12% 2	1074

Page generated at 2026-09-26 02:18:42. Version ac07

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