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 2024-11-02.

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	3 5% 2 7% 5% 6% 6% 5% 7% 8% 46%	101050
	Night	2 5% 2 7% 5% 5% 5% 5% 7% 10% 47%	98057
Transparency	Day	31% 7% 13% 22% 17% 10%	100794
	Night	30% 7% 15% 25% 16% 7%	97904
Seeing	Day	9% 14% 33% 29% 12% 3	14890
	Night	5% 7% 27% 39% 18% 2	24336

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 3 12% 9% 9% 11% 8% 12% 12% 23%	2539
	11th to 20th	Day	5% 3 11% 8% 7% 6% 5% 8% 9% 37%	2540
	21st to end	Day	1 7% 5% 14% 7% 8% 8% 8% 11% 11% 21%	2794
February	1st to 10th	Day	1 5% 3 12% 5% 7% 6% 6% 9% 11% 35%	1710
	11th to 20th	Day	1 5% 3 10% 9% 7% 7% 6% 8% 10% 34%	1779
	21st to end	Day	5% 4 10% 9% 7% 8% 7% 10% 11% 28%	1570
March	1st to 10th	Day	3 2 9% 7% 9% 10% 7% 10% 11% 34%	3138
	11th to 20th	Day	1 4 3 8% 5% 6% 6% 5% 7% 13% 43%	3481
	21st to end	Day	1 10% 5% 13% 8% 10% 9% 7% 9% 8% 20%	3825
April	1st to 10th	Day	2 8% 4 12% 7% 7% 6% 5% 7% 9% 31%	2275
	11th to 20th	Day	2 7% 3 9% 8% 7% 5% 6% 7% 9% 37%	2389
	21st to end	Day	5% 10% 4 11% 6% 5% 5% 4 6% 9% 34%	2470
May	1st to 10th	Day	5% 11% 4 11% 7% 6% 6% 5% 7% 7% 32%	4110
	11th to 20th	Day	4 7% 3 7% 5% 5% 5% 3 6% 7% 48%	4071
	21st to end	Day	7% 9% 4 9% 6% 6% 6% 5% 7% 7% 35%	4826
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	7% 8% 2 7% 5% 4 5% 4 5% 6% 48%	2078
	11th to 20th	Day	3 6% 1 5% 4 4 4 4 6% 8% 54%	1853
	21st to end	Day	2 6% 3 7% 5% 5% 5% 4 6% 10% 49%	1881
November	1st to 10th	Day	2 5% 3 7% 7% 6% 6% 6% 8% 7% 43%	1683
	11th to 20th	Day	1 6% 3 7% 5% 6% 6% 5% 8% 11% 43%	1710
	21st to end	Day	1 5% 3 5% 5% 6% 5% 6% 10% 11% 44%	1544
December	1st to 10th	Day	1 5% 3 9% 7% 8% 5% 5% 8% 12% 37%	1379
	11th to 20th	Day	4 3 10% 8% 8% 9% 6% 9% 12% 31%	1685
	21st to end	Day	6% 3 10% 6% 7% 8% 6% 9% 12% 33%	2581
January	1st to 10th	Night	6% 3 12% 7% 7% 8% 7% 12% 12% 27%	4406
	11th to 20th	Night	6% 4 14% 6% 7% 6% 5% 7% 9% 35%	4420
	21st to end	Night	1 9% 4 13% 7% 7% 8% 6% 8% 11% 26%	4862
February	1st to 10th	Night	1 6% 4 11% 6% 6% 6% 6% 8% 10% 35%	2470
	11th to 20th	Night	1 6% 3 8% 6% 5% 5% 4 8% 10% 44%	2284
	21st to end	Night	1 5% 3 8% 6% 5% 6% 6% 8% 11% 41%	1884
March	1st to 10th	Night	3 2 7% 6% 6% 6% 6% 9% 15% 41%	3736
	11th to 20th	Night	1 4 3 6% 4 4 5% 4 8% 18% 44%	3499
	21st to end	Night	2 8% 4 10% 7% 6% 6% 6% 8% 12% 33%	3828
April	1st to 10th	Night	3 8% 3 6% 5% 5% 5% 5% 7% 9% 44%	1903
	11th to 20th	Night	2 7% 3 8% 4 5% 5% 5% 9% 9% 42%	1738
	21st to end	Night	5% 7% 3 7% 4 4 5% 4 7% 10% 44%	1710
May	1st to 10th	Night	4 11% 4 10% 5% 4 4 4 6% 9% 40%	2860
	11th to 20th	Night	4 6% 1 5% 4 3 4 4 7% 7% 55%	2834
	21st to end	Night	4 8% 2 7% 5% 5% 5% 5% 5% 9% 45%	2742
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	5% 7% 2 4 3 3 4 3 6% 7% 55%	2100
	11th to 20th	Night	3 5% 2 4 3 3 4 4 6% 8% 58%	2323
	21st to end	Night	2 4 2 6% 4 5% 5% 4 7% 10% 51%	2717
November	1st to 10th	Night	1 7% 2 6% 5% 5% 5% 5% 8% 10% 46%	2418
	11th to 20th	Night	1 5% 2 5% 4 5% 6% 6% 9% 11% 44%	2470
	21st to end	Night	1 4 2 6% 4 4 5% 5% 8% 11% 50%	2613
December	1st to 10th	Night	1 3 3 7% 6% 7% 7% 7% 9% 11% 40%	2797
	11th to 20th	Night	1 6% 4 10% 6% 6% 7% 5% 10% 12% 33%	3506
	21st to end	Night	1 6% 3 10% 8% 8% 10% 8% 9% 9% 30%	5089

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	42% 13% 19% 21% 5%	2539
	11th to 20th	Day	39% 8% 20% 22% 9% 3	2540
	21st to end	Day	49% 9% 17% 19% 5% 1	2794
February	1st to 10th	Day	37% 11% 22% 21% 8% 1	1710
	11th to 20th	Day	36% 10% 20% 23% 10%	1789
	21st to end	Day	38% 13% 16% 22% 10% 2	1570
March	1st to 10th	Day	32% 11% 16% 26% 15%	3138
	11th to 20th	Day	28% 8% 25% 32% 6%	3481
	21st to end	Day	46% 14% 19% 17% 4	3825
April	1st to 10th	Day	41% 10% 20% 20% 7% 2	2275
	11th to 20th	Day	38% 7% 18% 25% 11% 1	2392
	21st to end	Day	43% 7% 15% 26% 9%	2470
May	1st to 10th	Day	41% 10% 12% 22% 12% 2	4110
	11th to 20th	Day	29% 8% 11% 31% 12% 8%	4074
	21st to end	Day	39% 10% 12% 30% 8% 2	4827
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	35% 5% 10% 27% 19% 4	2078
	11th to 20th	Day	26% 5% 18% 30% 18% 2	1853
	21st to end	Day	30% 7% 16% 30% 17% 1	1881
November	1st to 10th	Day	34% 8% 17% 24% 16% 2	1683
	11th to 20th	Day	32% 8% 21% 27% 9% 2	1710
	21st to end	Day	28% 8% 21% 27% 14% 2	1544
December	1st to 10th	Day	38% 8% 21% 21% 10% 2	1379
	11th to 20th	Day	40% 9% 24% 23% 4	1684
	21st to end	Day	39% 10% 15% 19% 12% 5%	2581
January	1st to 10th	Night	42% 12% 17% 23% 6%	4406
	11th to 20th	Night	43% 8% 16% 21% 8% 4	4420
	21st to end	Night	50% 8% 16% 18% 6% 1	4862
February	1st to 10th	Night	40% 8% 21% 22% 8% 1	2470
	11th to 20th	Night	32% 8% 24% 25% 11%	2285
	21st to end	Night	33% 9% 19% 27% 10% 2	1884
March	1st to 10th	Night	27% 8% 17% 26% 22% 1	3736
	11th to 20th	Night	25% 6% 28% 32% 7%	3499
	21st to end	Night	38% 10% 24% 24% 3	3828
April	1st to 10th	Night	34% 7% 21% 26% 10% 1	1903
	11th to 20th	Night	32% 8% 19% 27% 13% 1	1738
	21st to end	Night	33% 5% 20% 31% 10%	1710
May	1st to 10th	Night	39% 6% 13% 29% 10% 2	2860
	11th to 20th	Night	26% 5% 13% 33% 19% 5%	2835
	21st to end	Night	34% 6% 14% 35% 11%	2742
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	27% 4 10% 32% 21% 5%	2100
	11th to 20th	Night	25% 3 14% 36% 19% 2	2323
	21st to end	Night	27% 6% 17% 32% 17% 1	2717
November	1st to 10th	Night	30% 7% 18% 27% 16% 2	2418
	11th to 20th	Night	30% 9% 23% 29% 9% 1	2470
	21st to end	Night	26% 7% 19% 30% 15% 2	2613
December	1st to 10th	Night	33% 10% 23% 21% 10% 2	2797
	11th to 20th	Night	39% 8% 22% 26% 5%	3507
	21st to end	Night	46% 9% 16% 15% 11% 3	5089

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% 12% 31% 32% 17% 3	180
	11th to 20th	Day	7% 12% 36% 25% 12% 8%	180
	21st to end	Day	5% 6% 45% 28% 14% 2	198
February	1st to 10th	Day	4 22% 45% 22% 5% 2	270
	11th to 20th	Day	3 19% 41% 26% 8% 3	385
	21st to end	Day	5% 19% 41% 27% 8%	382
March	1st to 10th	Day	5% 19% 42% 28% 5%	460
	11th to 20th	Day	5% 15% 36% 31% 12% 2	460
	21st to end	Day	10% 19% 41% 24% 6%	506
April	1st to 10th	Day	10% 20% 34% 27% 7% 2	460
	11th to 20th	Day	8% 25% 37% 23% 6% 1	452
	21st to end	Day	16% 16% 34% 25% 7% 2	460
May	1st to 10th	Day	11% 18% 32% 27% 11% 1	463
	11th to 20th	Day	23% 11% 28% 25% 11% 2	488
	21st to end	Day	18% 12% 26% 25% 15% 4	538
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% 30% 8% 2	330
November	1st to 10th	Day	12% 12% 35% 28% 12% 1	273
	11th to 20th	Day	7% 14% 33% 33% 11% 2	270
	21st to end	Day	6% 11% 38% 32% 12% 1	270
December	1st to 10th	Day	4 11% 35% 28% 17% 5%	203
	11th to 20th	Day	7% 14% 38% 31% 8% 2	180
	21st to end	Day	7% 12% 38% 29% 11% 3	198
January	1st to 10th	Night	4 11% 32% 37% 15% 1	760
	11th to 20th	Night	4 8% 30% 40% 14% 4	760
	21st to end	Night	3 6% 31% 42% 15% 3	836
February	1st to 10th	Night	4 12% 36% 34% 12% 2	760
	11th to 20th	Night	3 9% 38% 35% 13% 2	744
	21st to end	Night	4 9% 36% 35% 14% 2	628
March	1st to 10th	Night	3 9% 36% 39% 12% 1	760
	11th to 20th	Night	3 11% 33% 36% 16% 2	760
	21st to end	Night	6% 10% 31% 40% 12% 1	836
April	1st to 10th	Night	6% 7% 31% 38% 15% 2	760
	11th to 20th	Night	5% 9% 37% 35% 14% 1	610
	21st to end	Night	9% 7% 30% 36% 16% 2	570
May	1st to 10th	Night	6% 9% 23% 39% 22% 2	570
	11th to 20th	Night	12% 6% 22% 32% 25% 2	561
	21st to end	Night	14% 4 20% 31% 28% 3	418
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% 40% 15% 1	744
	11th to 20th	Night	5% 13% 33% 38% 10% 1	760
	21st to end	Night	4 9% 31% 37% 16% 3	903
December	1st to 10th	Night	5% 8% 33% 40% 11% 2	950
	11th to 20th	Night	5% 8% 33% 39% 14% 1	950
	21st to end	Night	5% 12% 36% 32% 12% 3	972

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