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 2025-05-26.

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 5% 2 7% 5% 6% 6% 5% 7% 8% 47%	117138
	Night	2 5% 2 7% 5% 5% 5% 5% 7% 9% 49%	120910
Transparency	Day	29% 7% 14% 23% 18% 9%	116882
	Night	29% 7% 17% 26% 16% 6%	120757
Seeing	Day	9% 15% 33% 29% 12% 3	15547
	Night	5% 8% 28% 39% 18% 2	25144

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 10% 8% 8% 10% 8% 12% 12% 27%	3639
	11th to 20th	Day	4 4 9% 7% 6% 5% 5% 7% 9% 45%	3640
	21st to end	Day	1 5% 3 10% 5% 5% 6% 6% 9% 9% 40%	4004
February	1st to 10th	Day	1 5% 3 12% 6% 7% 6% 6% 9% 11% 34%	1800
	11th to 20th	Day	4 3 9% 9% 7% 7% 6% 8% 10% 34%	1876
	21st to end	Day	5% 4 10% 8% 7% 8% 7% 10% 11% 29%	1650
March	1st to 10th	Day	5% 2 7% 6% 7% 8% 6% 9% 10% 39%	4518
	11th to 20th	Day	1 4 3 8% 5% 7% 7% 6% 9% 12% 38%	4991
	21st to end	Day	1 8% 4 12% 8% 9% 9% 7% 9% 9% 24%	5486
April	1st to 10th	Day	2 8% 4 12% 8% 7% 6% 5% 7% 9% 32%	2395
	11th to 20th	Day	2 7% 3 9% 7% 7% 5% 6% 7% 9% 38%	2517
	21st to end	Day	5% 10% 5% 11% 6% 5% 5% 4 6% 9% 34%	2600
May	1st to 10th	Day	5% 10% 4 11% 7% 6% 6% 5% 7% 6% 33%	4240
	11th to 20th	Day	4 7% 3 7% 5% 5% 5% 4 6% 7% 47%	4201
	21st to end	Day	6% 9% 3 9% 6% 6% 6% 5% 6% 7% 36%	4899
June	1st to 10th	Day	7% 6% 3 8% 6% 6% 6% 6% 8% 10% 35%	2850
	11th to 20th	Day	7% 6% 3 6% 5% 5% 5% 4 6% 7% 47%	2850
	21st to end	Day	2 3 1 4 5% 5% 4 3 5% 7% 61%	2842
July	1st to 10th	Day	1 1 1 3 3 4 4 3 5% 6% 69%	4420
	11th to 20th	Day	2 2 1 3 3 3 4 4 5% 7% 65%	4420
	21st to end	Day	3 2 1 3 3 3 4 4 5% 7% 64%	4790
August	1st to 10th	Day	3 3 1 4 4 6% 6% 5% 8% 8% 52%	4098
	11th to 20th	Day	1 1 1 3 3 4 4 4 6% 8% 64%	4028
	21st to end	Day	2 3 2 4 3 3 3 3 4 6% 67%	4438
September	1st to 10th	Day	4 3 1 4 3 4 3 3 5% 6% 65%	2223
	11th to 20th	Day	5% 5% 2 5% 5% 4 4 3 6% 7% 55%	2090
	21st to end	Day	5% 4 1 4 2 3 4 3 5% 8% 60%	2090
October	1st to 10th	Day	4 5% 1 4 3 3 4 3 4 4 64%	3465
	11th to 20th	Day	2 5% 1 5% 3 4 4 4 6% 8% 58%	3069
	21st to end	Day	1 4 2 6% 4 5% 6% 5% 6% 9% 53%	3101
November	1st to 10th	Day	2 5% 3 7% 7% 6% 6% 5% 8% 7% 43%	1764
	11th to 20th	Day	1 6% 3 7% 5% 6% 6% 5% 8% 11% 43%	1800
	21st to end	Day	1 5% 3 5% 5% 5% 5% 6% 10% 11% 44%	1625
December	1st to 10th	Day	1 3 2 7% 5% 6% 5% 4 7% 8% 53%	2367
	11th to 20th	Day	3 3 9% 8% 8% 10% 6% 10% 11% 32%	2645
	21st to end	Day	4 2 8% 6% 7% 9% 7% 11% 13% 32%	3707
January	1st to 10th	Night	5% 3 10% 7% 7% 9% 8% 12% 12% 29%	6306
	11th to 20th	Night	4 3 12% 7% 6% 6% 5% 7% 9% 42%	6320
	21st to end	Night	1 6% 3 9% 5% 5% 6% 5% 6% 9% 44%	6939
February	1st to 10th	Night	1 6% 4 10% 6% 7% 6% 6% 9% 11% 34%	2600
	11th to 20th	Night	5% 3 8% 6% 6% 5% 4 8% 10% 44%	2407
	21st to end	Night	1 4 2 8% 6% 5% 6% 6% 8% 11% 42%	1980
March	1st to 10th	Night	5% 3 7% 5% 5% 5% 5% 7% 12% 46%	5376
	11th to 20th	Night	1 4 3 7% 5% 5% 7% 5% 10% 17% 37%	5009
	21st to end	Night	1 6% 3 9% 6% 6% 6% 5% 8% 12% 39%	5489
April	1st to 10th	Night	3 7% 3 6% 5% 5% 5% 4 7% 9% 45%	2003
	11th to 20th	Night	2 7% 3 7% 4 5% 5% 5% 8% 9% 44%	1830
	21st to end	Night	5% 7% 3 7% 4 4 5% 4 7% 10% 44%	1800
May	1st to 10th	Night	4 11% 4 10% 5% 3 4 4 6% 9% 40%	2950
	11th to 20th	Night	4 6% 1 5% 4 3 4 4 7% 7% 54%	2924
	21st to end	Night	4 8% 2 7% 5% 4 5% 5% 5% 9% 46%	2785
June	1st to 10th	Night	7% 5% 2 5% 5% 3 5% 4 9% 9% 47%	1330
	11th to 20th	Night	7% 6% 2 5% 4 4 4 4 5% 7% 52%	1330
	21st to end	Night	2 3 1 4 3 2 4 3 5% 7% 64%	1332
July	1st to 10th	Night	1 1 1 2 2 2 2 2 4 7% 76%	2540
	11th to 20th	Night	1 1 1 2 2 2 2 3 6% 8% 73%	2540
	21st to end	Night	1 1 1 2 2 3 3 3 5% 6% 72%	2761
August	1st to 10th	Night	2 2 1 4 3 4 4 4 6% 9% 61%	2838
	11th to 20th	Night	1 1 2 2 2 4 2 5% 7% 74%	2811
	21st to end	Night	2 2 1 3 3 3 3 3 5% 6% 70%	3230
September	1st to 10th	Night	3 3 1 3 2 2 2 3 4 6% 70%	1956
	11th to 20th	Night	4 4 1 4 3 3 4 3 6% 7% 60%	2090
	21st to end	Night	6% 4 1 3 2 3 3 2 4 6% 66%	2090
October	1st to 10th	Night	3 4 1 3 2 2 3 3 5% 7% 68%	3512
	11th to 20th	Night	3 5% 2 4 3 4 4 4 5% 6% 61%	3855
	21st to end	Night	2 4 2 5% 4 4 5% 4 7% 10% 52%	4457
November	1st to 10th	Night	1 7% 2 6% 5% 5% 5% 5% 8% 10% 46%	2535
	11th to 20th	Night	1 6% 2 5% 5% 5% 6% 6% 9% 11% 44%	2600
	21st to end	Night	1 4 2 6% 4 4 5% 5% 8% 11% 50%	2751
December	1st to 10th	Night	2 2 5% 4 5% 5% 5% 6% 8% 58%	4811
	11th to 20th	Night	5% 3 9% 6% 7% 7% 6% 10% 12% 34%	5546
	21st to end	Night	5% 3 9% 8% 8% 10% 8% 10% 10% 29%	7277

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	36% 14% 21% 23% 5%	3639
	11th to 20th	Day	33% 6% 23% 27% 9% 3	3640
	21st to end	Day	35% 7% 17% 25% 15%	4004
February	1st to 10th	Day	37% 12% 21% 20% 9% 1	1800
	11th to 20th	Day	36% 11% 20% 23% 10% 1	1886
	21st to end	Day	37% 13% 16% 22% 11% 2	1650
March	1st to 10th	Day	30% 9% 17% 26% 15% 2	4518
	11th to 20th	Day	28% 9% 23% 31% 9%	4991
	21st to end	Day	42% 13% 21% 20% 4	5486
April	1st to 10th	Day	41% 10% 19% 21% 7% 2	2395
	11th to 20th	Day	37% 7% 18% 25% 12% 1	2520
	21st to end	Day	43% 7% 15% 26% 9%	2600
May	1st to 10th	Day	40% 10% 12% 23% 13% 2	4240
	11th to 20th	Day	30% 8% 11% 31% 12% 8%	4204
	21st to end	Day	39% 10% 12% 30% 8% 2	4900
June	1st to 10th	Day	38% 7% 15% 27% 11% 1	2850
	11th to 20th	Day	34% 5% 8% 21% 23% 9%	2850
	21st to end	Day	21% 5% 6% 22% 34% 12%	2842
July	1st to 10th	Day	14% 4 4 12% 25% 42%	4351
	11th to 20th	Day	16% 3 5% 10% 22% 45%	4345
	21st to end	Day	17% 4 5% 13% 30% 32%	4639
August	1st to 10th	Day	25% 5% 6% 16% 30% 17%	4083
	11th to 20th	Day	16% 3 6% 23% 38% 14%	4067
	21st to end	Day	19% 3 5% 18% 37% 19%	4437
September	1st to 10th	Day	20% 4 5% 21% 32% 19%	2223
	11th to 20th	Day	27% 5% 9% 26% 24% 9%	2090
	21st to end	Day	22% 4 8% 30% 28% 7%	2090
October	1st to 10th	Day	23% 4 7% 23% 35% 8%	3465
	11th to 20th	Day	23% 4 16% 31% 23% 2	3069
	21st to end	Day	26% 6% 14% 31% 23% 1	3101
November	1st to 10th	Day	34% 7% 17% 24% 15% 2	1764
	11th to 20th	Day	32% 8% 21% 27% 10% 2	1800
	21st to end	Day	28% 7% 22% 28% 13% 2	1625
December	1st to 10th	Day	27% 6% 27% 30% 9% 1	2367
	11th to 20th	Day	39% 10% 27% 20% 4	2644
	21st to end	Day	36% 11% 18% 20% 11% 3	3707
January	1st to 10th	Night	39% 11% 19% 26% 5%	6306
	11th to 20th	Night	37% 7% 18% 25% 9% 4	6320
	21st to end	Night	36% 6% 14% 28% 15% 1	6939
February	1st to 10th	Night	40% 9% 21% 21% 9% 1	2600
	11th to 20th	Night	32% 8% 24% 25% 11%	2408
	21st to end	Night	32% 9% 20% 27% 11% 2	1980
March	1st to 10th	Night	27% 6% 18% 25% 22% 1	5376
	11th to 20th	Night	28% 9% 23% 31% 8%	5009
	21st to end	Night	33% 9% 27% 26% 4	5489
April	1st to 10th	Night	33% 7% 21% 27% 11% 1	2003
	11th to 20th	Night	31% 8% 19% 28% 13% 1	1830
	21st to end	Night	34% 5% 20% 31% 10%	1800
May	1st to 10th	Night	39% 6% 13% 30% 11% 2	2950
	11th to 20th	Night	26% 5% 13% 32% 19% 4	2925
	21st to end	Night	34% 6% 14% 35% 11%	2785
June	1st to 10th	Night	32% 7% 19% 31% 11% 1	1330
	11th to 20th	Night	30% 6% 8% 25% 22% 8%	1330
	21st to end	Night	18% 4 8% 24% 32% 13%	1332
July	1st to 10th	Night	8% 3 5% 18% 25% 41%	2500
	11th to 20th	Night	10% 2 4 13% 24% 47%	2492
	21st to end	Night	13% 3 4 18% 32% 30%	2674
August	1st to 10th	Night	20% 4 7% 20% 32% 18%	2830
	11th to 20th	Night	10% 3 6% 31% 36% 14%	2838
	21st to end	Night	15% 3 5% 23% 35% 19%	3230
September	1st to 10th	Night	16% 3 6% 24% 32% 19%	1956
	11th to 20th	Night	22% 4 12% 24% 27% 11%	2090
	21st to end	Night	20% 3 8% 31% 30% 8%	2090
October	1st to 10th	Night	17% 3 7% 27% 35% 11%	3512
	11th to 20th	Night	25% 3 14% 31% 25% 2	3855
	21st to end	Night	26% 6% 17% 31% 19% 1	4457
November	1st to 10th	Night	29% 7% 19% 27% 16% 2	2535
	11th to 20th	Night	30% 9% 23% 28% 9% 1	2600
	21st to end	Night	26% 7% 20% 31% 14% 2	2751
December	1st to 10th	Night	22% 7% 33% 27% 9% 1	4811
	11th to 20th	Night	37% 10% 26% 23% 4	5547
	21st to end	Night	43% 10% 17% 16% 11% 2	7277

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% 31% 15% 3	200
	11th to 20th	Day	7% 12% 40% 24% 12% 7%	200
	21st to end	Day	5% 6% 45% 28% 14% 2	220
February	1st to 10th	Day	6% 24% 43% 20% 4 2	300
	11th to 20th	Day	3 19% 41% 26% 7% 2	422
	21st to end	Day	6% 19% 41% 27% 8%	414
March	1st to 10th	Day	5% 19% 42% 28% 6%	500
	11th to 20th	Day	6% 16% 36% 29% 12% 1	500
	21st to end	Day	10% 20% 40% 24% 6%	550
April	1st to 10th	Day	11% 19% 33% 27% 8% 2	500
	11th to 20th	Day	8% 26% 36% 23% 5% 1	492
	21st to end	Day	15% 18% 34% 24% 7% 2	500
May	1st to 10th	Day	12% 17% 34% 27% 11% 1	503
	11th to 20th	Day	23% 14% 27% 24% 11% 2	528
	21st to end	Day	18% 12% 27% 25% 15% 4	559
June	1st to 10th	Day	12% 15% 25% 28% 16% 4	680
	11th to 20th	Day	16% 18% 29% 23% 13% 2	680
	21st to end	Day	8% 11% 26% 35% 18% 2	672
July	1st to 10th	Day	4 14% 34% 34% 12% 2	486
	11th to 20th	Day	3 14% 38% 33% 11% 1	484
	21st to end	Day	6% 9% 35% 32% 13% 5%	531
August	1st to 10th	Day	7% 9% 31% 33% 15% 5%	487
	11th to 20th	Day	4 9% 30% 35% 17% 5%	477
	21st to end	Day	9% 12% 30% 29% 15% 5%	539
September	1st to 10th	Day	6% 11% 28% 34% 16% 6%	490
	11th to 20th	Day	12% 13% 28% 30% 14% 4	490
	21st to end	Day	10% 12% 31% 28% 16% 3	490
October	1st to 10th	Day	13% 9% 28% 30% 17% 3	478
	11th to 20th	Day	8% 15% 24% 31% 16% 6%	300
	21st to end	Day	10% 15% 35% 29% 8% 2	330
November	1st to 10th	Day	12% 12% 36% 26% 13% 1	300
	11th to 20th	Day	7% 18% 34% 30% 10% 2	300
	21st to end	Day	7% 10% 40% 31% 11% 1	300
December	1st to 10th	Day	4 11% 35% 28% 17% 5%	225
	11th to 20th	Day	8% 15% 38% 30% 8% 2	200
	21st to end	Day	7% 15% 39% 27% 10% 2	220
January	1st to 10th	Night	3 11% 33% 36% 14% 1	800
	11th to 20th	Night	4 8% 32% 39% 14% 4	800
	21st to end	Night	3 7% 33% 41% 15% 3	880
February	1st to 10th	Night	4 14% 36% 32% 11% 2	800
	11th to 20th	Night	3 9% 39% 34% 13% 2	784
	21st to end	Night	5% 10% 36% 35% 13% 2	660
March	1st to 10th	Night	2 9% 38% 38% 12% 1	800
	11th to 20th	Night	3 12% 34% 34% 15% 2	800
	21st to end	Night	6% 11% 31% 39% 12% 1	880
April	1st to 10th	Night	6% 8% 31% 38% 15% 2	800
	11th to 20th	Night	5% 9% 38% 34% 13% 1	642
	21st to end	Night	9% 7% 31% 36% 15% 2	600
May	1st to 10th	Night	7% 8% 23% 39% 22% 2	600
	11th to 20th	Night	14% 7% 23% 31% 24% 2	591
	21st to end	Night	14% 4 20% 31% 27% 3	429
June	1st to 10th	Night	9% 7% 19% 41% 23% 3	380
	11th to 20th	Night	11% 7% 23% 43% 14% 2	380
	21st to end	Night	5% 2 17% 47% 27% 2	380
July	1st to 10th	Night	2 2 13% 44% 35% 4	370
	11th to 20th	Night	2 1 15% 58% 23% 1	368
	21st to end	Night	2 1 14% 47% 34% 2	416
August	1st to 10th	Night	5% 3 15% 47% 28% 2	558
	11th to 20th	Night	2 2 14% 48% 31% 2	564
	21st to end	Night	5% 4 19% 40% 30% 2	672
September	1st to 10th	Night	6% 1 16% 44% 29% 4	760
	11th to 20th	Night	6% 3 16% 40% 28% 6%	760
	21st to end	Night	8% 5% 19% 36% 27% 5%	760
October	1st to 10th	Night	9% 4 19% 44% 22% 2	760
	11th to 20th	Night	6% 9% 23% 39% 19% 4	760
	21st to end	Night	5% 11% 28% 39% 13% 4	836
November	1st to 10th	Night	6% 9% 29% 39% 15% 1	780
	11th to 20th	Night	5% 14% 34% 36% 10% 1	800
	21st to end	Night	4 10% 33% 35% 16% 3	951
December	1st to 10th	Night	5% 8% 33% 40% 12% 2	1000
	11th to 20th	Night	5% 9% 33% 38% 14% 1	1000
	21st to end	Night	5% 14% 36% 31% 12% 2	1023

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