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 2024-03-06.

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% 3 7% 7% 4 6% 6% 7% 9% 43%	78234
	Night	2 5% 3 6% 6% 4 6% 6% 7% 10% 45%	81268
Transparency	Day	34% 7% 14% 22% 16% 7%	77938
	Night	32% 7% 16% 25% 14% 5%	81087
Seeing	Day	9% 14% 33% 29% 12% 3	13920
	Night	5% 7% 27% 39% 19% 3	23584

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 10% 11% 7% 11% 11% 11% 12% 23%	2539
	11th to 20th	Day	5% 3 9% 10% 5% 7% 7% 8% 10% 37%	2540
	21st to end	Day	1 6% 5% 11% 9% 6% 8% 10% 11% 11% 21%	2794
February	1st to 10th	Day	1 5% 4 10% 7% 5% 7% 7% 8% 11% 36%	1710
	11th to 20th	Day	1 4 4 9% 10% 5% 7% 7% 8% 11% 34%	1779
	21st to end	Day	4 5% 9% 10% 5% 8% 9% 11% 11% 28%	1570
March	1st to 10th	Day	1 5% 2 8% 8% 6% 9% 8% 9% 10% 32%	1851
	11th to 20th	Day	3 5% 4 8% 7% 5% 8% 8% 8% 12% 33%	1971
	21st to end	Day	3 9% 5% 10% 8% 6% 8% 7% 8% 9% 26%	2178
April	1st to 10th	Day	2 7% 5% 11% 8% 5% 7% 7% 7% 10% 31%	2155
	11th to 20th	Day	2 7% 4 8% 9% 4 7% 7% 7% 9% 37%	2261
	21st to end	Day	6% 10% 5% 10% 7% 3 5% 5% 6% 9% 35%	2340
May	1st to 10th	Day	5% 8% 4 9% 7% 4 5% 7% 6% 7% 38%	2340
	11th to 20th	Day	7% 9% 3 8% 8% 5% 6% 6% 7% 8% 34%	2301
	21st to end	Day	10% 11% 5% 9% 8% 5% 6% 7% 6% 8% 25%	2722
June	1st to 10th	Day	7% 6% 3 7% 7% 4 6% 7% 8% 10% 35%	2700
	11th to 20th	Day	8% 6% 3 5% 6% 4 5% 5% 6% 7% 45%	2700
	21st to end	Day	2 3 1 3 6% 3 5% 4 5% 7% 60%	2693
July	1st to 10th	Day	2 2 2 4 4 3 6% 6% 7% 8% 57%	2520
	11th to 20th	Day	3 3 1 2 3 2 4 4 5% 6% 67%	2520
	21st to end	Day	4 3 1 4 4 3 4 4 5% 6% 61%	2741
August	1st to 10th	Day	4 4 2 4 5% 4 5% 6% 7% 7% 51%	2334
	11th to 20th	Day	2 2 1 3 4 3 4 4 5% 7% 65%	2299
	21st to end	Day	3 3 2 4 4 3 4 5% 5% 7% 61%	2530
September	1st to 10th	Day	4 3 1 3 4 3 4 4 5% 6% 65%	2106
	11th to 20th	Day	5% 4 2 4 5% 3 4 4 6% 7% 56%	1980
	21st to end	Day	6% 4 1 4 3 2 4 4 5% 8% 58%	1980
October	1st to 10th	Day	7% 8% 3 6% 6% 3 5% 5% 5% 6% 46%	1969
	11th to 20th	Day	4 5% 2 4 6% 3 4 4 7% 9% 53%	1756
	21st to end	Day	2 6% 3 6% 6% 4 5% 5% 5% 9% 49%	1782
November	1st to 10th	Day	2 5% 3 6% 8% 4 7% 6% 8% 8% 44%	1674
	11th to 20th	Day	1 5% 3 6% 6% 4 6% 7% 8% 11% 43%	1710
	21st to end	Day	1 4 3 5% 4 4 6% 7% 10% 11% 44%	1544
December	1st to 10th	Day	1 5% 3 8% 8% 6% 7% 6% 8% 12% 37%	1379
	11th to 20th	Day	4 4 8% 9% 6% 9% 8% 9% 12% 31%	1685
	21st to end	Day	5% 3 8% 8% 5% 9% 7% 9% 13% 33%	2581
January	1st to 10th	Night	1 5% 4 10% 9% 5% 7% 9% 12% 12% 27%	4406
	11th to 20th	Night	5% 4 12% 8% 6% 7% 6% 7% 9% 35%	4420
	21st to end	Night	1 8% 5% 11% 9% 5% 8% 8% 7% 11% 26%	4862
February	1st to 10th	Night	1 6% 4 9% 8% 5% 6% 7% 8% 10% 36%	2470
	11th to 20th	Night	1 5% 3 8% 6% 4 6% 6% 8% 11% 44%	2284
	21st to end	Night	1 4 3 7% 7% 3 7% 7% 8% 11% 42%	1884
March	1st to 10th	Night	4 2 6% 7% 5% 7% 7% 9% 12% 41%	2225
	11th to 20th	Night	1 5% 3 5% 5% 2 5% 6% 9% 13% 45%	1989
	21st to end	Night	3 7% 3 8% 7% 4 5% 7% 7% 11% 38%	2178
April	1st to 10th	Night	4 6% 4 5% 6% 4 5% 6% 6% 9% 44%	1803
	11th to 20th	Night	2 7% 3 7% 5% 4 6% 6% 8% 9% 43%	1646
	21st to end	Night	5% 7% 3 7% 5% 3 5% 5% 6% 11% 44%	1620
May	1st to 10th	Night	2 7% 3 5% 6% 3 5% 6% 7% 11% 46%	1620
	11th to 20th	Night	7% 6% 2 6% 7% 3 4 5% 8% 10% 42%	1594
	21st to end	Night	8% 9% 3 7% 7% 3 5% 7% 5% 10% 36%	1546
June	1st to 10th	Night	7% 5% 3 6% 5% 2 5% 5% 8% 8% 47%	1260
	11th to 20th	Night	7% 5% 2 4 5% 2 5% 5% 4 7% 52%	1260
	21st to end	Night	3 3 1 3 3 2 4 4 5% 8% 64%	1261
July	1st to 10th	Night	2 1 1 3 3 2 4 4 6% 8% 67%	1440
	11th to 20th	Night	1 1 1 2 1 3 4 5% 6% 75%	1440
	21st to end	Night	1 1 1 2 2 2 3 4 5% 6% 72%	1576
August	1st to 10th	Night	3 3 1 3 4 2 4 5% 6% 7% 61%	1602
	11th to 20th	Night	1 2 1 2 2 1 3 3 6% 7% 71%	1602
	21st to end	Night	4 2 1 4 3 2 3 4 5% 6% 65%	1824
September	1st to 10th	Night	3 3 2 3 2 1 3 4 4 6% 70%	1853
	11th to 20th	Night	4 3 1 3 3 2 3 4 6% 7% 62%	1980
	21st to end	Night	6% 4 2 3 2 2 3 3 4 6% 65%	1980
October	1st to 10th	Night	6% 7% 2 4 4 2 4 5% 6% 7% 53%	1989
	11th to 20th	Night	3 5% 2 3 3 3 4 5% 6% 8% 58%	2200
	21st to end	Night	2 4 2 5% 5% 4 5% 5% 7% 10% 51%	2574
November	1st to 10th	Night	1 6% 2 6% 5% 4 5% 6% 8% 10% 46%	2405
	11th to 20th	Night	1 5% 3 5% 5% 4 6% 7% 9% 12% 44%	2470
	21st to end	Night	1 3 3 5% 4 3 5% 6% 8% 11% 50%	2613
December	1st to 10th	Night	1 3 3 6% 7% 5% 7% 9% 9% 11% 41%	2797
	11th to 20th	Night	1 6% 4 9% 7% 4 8% 7% 9% 12% 33%	3506
	21st to end	Night	1 5% 3 8% 9% 6% 10% 10% 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	36% 12% 21% 21% 10%	1851
	11th to 20th	Day	37% 9% 22% 24% 8%	1971
	21st to end	Day	44% 12% 21% 17% 5% 1	2178
April	1st to 10th	Day	42% 9% 19% 21% 8% 2	2155
	11th to 20th	Day	38% 7% 17% 25% 12% 1	2264
	21st to end	Day	43% 7% 15% 26% 9%	2340
May	1st to 10th	Day	38% 7% 15% 28% 10% 1	2340
	11th to 20th	Day	42% 8% 16% 25% 9% 1	2304
	21st to end	Day	51% 7% 15% 20% 6% 1	2723
June	1st to 10th	Day	39% 7% 16% 27% 10% 1	2700
	11th to 20th	Day	35% 6% 8% 22% 22% 8%	2700
	21st to end	Day	22% 5% 6% 22% 34% 11%	2693
July	1st to 10th	Day	21% 6% 6% 20% 27% 21%	2451
	11th to 20th	Day	17% 3 3 13% 27% 35%	2445
	21st to end	Day	22% 4 4 13% 30% 28%	2589
August	1st to 10th	Day	28% 6% 7% 19% 25% 15%	2318
	11th to 20th	Day	18% 4 5% 24% 33% 16%	2299
	21st to end	Day	21% 4 6% 20% 32% 17%	2530
September	1st to 10th	Day	20% 4 5% 20% 32% 20%	2106
	11th to 20th	Day	26% 5% 9% 25% 26% 10%	1980
	21st to end	Day	23% 4 8% 31% 28% 6%	1980
October	1st to 10th	Day	36% 5% 10% 28% 17% 3	1969
	11th to 20th	Day	27% 5% 18% 31% 17% 2	1756
	21st to end	Day	30% 7% 17% 30% 16% 1	1782
November	1st to 10th	Day	34% 7% 17% 24% 16% 2	1674
	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	31% 8% 23% 27% 11%	2225
	11th to 20th	Night	27% 8% 25% 30% 10%	1989
	21st to end	Night	37% 9% 23% 25% 6% 1	2178
April	1st to 10th	Night	35% 6% 21% 26% 11% 2	1803
	11th to 20th	Night	33% 8% 18% 27% 14% 1	1646
	21st to end	Night	34% 5% 21% 30% 10%	1620
May	1st to 10th	Night	29% 7% 17% 36% 10% 1	1620
	11th to 20th	Night	34% 6% 18% 30% 12% 1	1595
	21st to end	Night	42% 7% 20% 25% 6%	1546
June	1st to 10th	Night	32% 7% 19% 30% 11% 1	1260
	11th to 20th	Night	31% 6% 9% 26% 21% 8%	1260
	21st to end	Night	19% 4 9% 25% 32% 12%	1261
July	1st to 10th	Night	14% 5% 7% 24% 32% 18%	1400
	11th to 20th	Night	10% 3 5% 19% 28% 35%	1392
	21st to end	Night	12% 4 6% 15% 34% 30%	1488
August	1st to 10th	Night	21% 5% 9% 21% 25% 19%	1594
	11th to 20th	Night	12% 4 7% 25% 33% 18%	1602
	21st to end	Night	19% 3 6% 23% 30% 19%	1824
September	1st to 10th	Night	17% 3 6% 22% 33% 20%	1853
	11th to 20th	Night	20% 4 11% 24% 29% 12%	1980
	21st to end	Night	21% 3 8% 31% 30% 7%	1980
October	1st to 10th	Night	29% 4 11% 33% 19% 4	1989
	11th to 20th	Night	25% 3 14% 37% 19% 2	2200
	21st to end	Night	28% 6% 17% 32% 17% 1	2574
November	1st to 10th	Night	30% 7% 18% 27% 15% 2	2405
	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% 20% 44% 26% 5%	441
	11th to 20th	Day	5% 16% 36% 32% 11%	420
	21st to end	Day	10% 19% 42% 23% 6%	462
April	1st to 10th	Day	10% 20% 34% 27% 7% 2	420
	11th to 20th	Day	8% 26% 38% 21% 6% 1	412
	21st to end	Day	17% 16% 33% 24% 8% 2	420
May	1st to 10th	Day	12% 17% 32% 28% 11% 1	423
	11th to 20th	Day	23% 11% 27% 25% 11% 2	448
	21st to end	Day	19% 12% 25% 24% 15% 4	493
June	1st to 10th	Day	12% 15% 24% 27% 17% 4	630
	11th to 20th	Day	16% 17% 28% 23% 14% 2	630
	21st to end	Day	7% 10% 25% 37% 20% 2	623
July	1st to 10th	Day	4 13% 34% 35% 11% 2	446
	11th to 20th	Day	4 14% 39% 32% 10% 1	444
	21st to end	Day	6% 9% 33% 33% 13% 5%	487
August	1st to 10th	Day	6% 9% 31% 34% 15% 5%	447
	11th to 20th	Day	4 8% 29% 35% 18% 6%	437
	21st to end	Day	8% 12% 30% 30% 15% 5%	495
September	1st to 10th	Day	6% 11% 28% 33% 16% 6%	450
	11th to 20th	Day	12% 12% 26% 30% 15% 4	450
	21st to end	Day	10% 12% 30% 28% 16% 3	450
October	1st to 10th	Day	14% 9% 27% 30% 17% 3	439
	11th to 20th	Day	9% 16% 24% 31% 16% 6%	270
	21st to end	Day	9% 15% 33% 31% 9% 3	297
November	1st to 10th	Day	12% 11% 36% 28% 12% 1	270
	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% 40% 12% 1	742
	11th to 20th	Night	3 11% 33% 36% 15% 2	720
	21st to end	Night	6% 10% 30% 40% 12% 1	792
April	1st to 10th	Night	7% 7% 30% 38% 16% 3	720
	11th to 20th	Night	5% 9% 37% 34% 14% 1	578
	21st to end	Night	9% 7% 30% 36% 16% 2	540
May	1st to 10th	Night	6% 9% 21% 39% 23% 2	540
	11th to 20th	Night	13% 5% 21% 32% 27% 2	531
	21st to end	Night	15% 4 19% 30% 29% 4	396
June	1st to 10th	Night	9% 6% 17% 41% 24% 3	360
	11th to 20th	Night	11% 7% 21% 44% 15% 2	360
	21st to end	Night	5% 1 15% 48% 29% 2	360
July	1st to 10th	Night	2 2 11% 45% 36% 4	350
	11th to 20th	Night	2 1 14% 58% 24% 1	348
	21st to end	Night	2 1 13% 47% 36% 3	394
August	1st to 10th	Night	5% 2 14% 47% 29% 2	528
	11th to 20th	Night	2 2 12% 48% 32% 3	534
	21st to end	Night	5% 4 17% 41% 31% 2	636
September	1st to 10th	Night	6% 2 16% 43% 29% 5%	720
	11th to 20th	Night	6% 3 15% 41% 29% 7%	720
	21st to end	Night	8% 5% 18% 37% 28% 6%	720
October	1st to 10th	Night	9% 4 18% 44% 23% 3	720
	11th to 20th	Night	6% 9% 22% 39% 19% 5%	720
	21st to end	Night	5% 10% 28% 40% 14% 4	792
November	1st to 10th	Night	6% 9% 29% 40% 16% 1	740
	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

Page generated at 2024-03-03 04:28:58. Version ac07

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