Sky Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sky Mountain Observatory clear sky chart from 2005-11-06 to 2025-03-27.

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 5% 4 4 5% 4 6% 8% 55%	112932
	Night	2 4 2 5% 3 3 4 4 6% 8% 61%	115249
Transparency	Day	25% 7% 12% 22% 24% 11%	112682
	Night	21% 6% 12% 25% 26% 10%	115089
Seeing	Day	9% 19% 32% 25% 12% 2	15882
	Night	5% 15% 36% 29% 13% 2	26097

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	3 2 8% 4 3 3 3 5% 8% 62%	3638
	11th to 20th	Day	2 1 5% 4 4 4 4 5% 7% 64%	3985
	21st to end	Day	5% 3 6% 3 3 3 3 5% 7% 61%	4510
February	1st to 10th	Day	5% 3 6% 4 3 4 3 6% 8% 59%	1804
	11th to 20th	Day	1 3 2 6% 3 3 5% 2 5% 8% 63%	1949
	21st to end	Day	1 5% 3 7% 4 3 4 3 6% 7% 58%	1650
March	1st to 10th	Day	4 3 7% 3 3 3 4 4 7% 61%	3248
	11th to 20th	Day	2 11% 6% 7% 6% 5% 4 5% 6% 6% 41%	3600
	21st to end	Day	1 5% 3 7% 4 4 5% 3 5% 9% 54%	4100
April	1st to 10th	Day	2 6% 2 5% 4 3 4 4 6% 9% 54%	2280
	11th to 20th	Day	3 8% 3 8% 5% 3 4 4 6% 7% 49%	2251
	21st to end	Day	4 7% 3 7% 5% 4 5% 5% 7% 8% 45%	2280
May	1st to 10th	Day	4 10% 3 7% 4 4 4 4 6% 6% 48%	3864
	11th to 20th	Day	4 6% 2 6% 5% 5% 5% 5% 7% 10% 44%	4288
	21st to end	Day	2 4 1 4 3 3 4 5% 8% 10% 56%	4825
June	1st to 10th	Day	2 4 2 5% 3 5% 6% 6% 9% 11% 48%	2660
	11th to 20th	Day	2 3 2 4 3 4 6% 5% 7% 8% 56%	2660
	21st to end	Day	2 4 2 6% 4 6% 7% 5% 9% 9% 47%	2660
July	1st to 10th	Day	3 3 1 4 4 5% 6% 5% 8% 9% 52%	4430
	11th to 20th	Day	2 3 2 6% 6% 8% 8% 6% 8% 9% 40%	4430
	21st to end	Day	4 4 2 7% 5% 6% 6% 6% 9% 9% 42%	4523
August	1st to 10th	Day	3 4 2 7% 6% 8% 9% 7% 9% 10% 35%	4037
	11th to 20th	Day	3 3 1 4 4 6% 7% 6% 8% 10% 48%	3726
	21st to end	Day	5% 5% 2 6% 5% 6% 7% 5% 9% 11% 39%	4180
September	1st to 10th	Day	4 5% 2 6% 4 4 6% 6% 7% 8% 49%	2280
	11th to 20th	Day	4 3 1 4 3 4 4 4 6% 7% 60%	2269
	21st to end	Day	4 3 1 5% 3 4 3 3 5% 5% 65%	2090
October	1st to 10th	Day	3 2 1 3 3 3 3 3 3 4 73%	3175
	11th to 20th	Day	2 2 2 4 3 3 4 4 6% 9% 62%	3150
	21st to end	Day	2 3 1 3 2 2 2 3 4 6% 73%	3417
November	1st to 10th	Day	1 3 2 4 2 2 3 2 6% 7% 68%	1763
	11th to 20th	Day	3 1 4 3 2 2 3 5% 7% 70%	1800
	21st to end	Day	1 4 2 5% 3 3 3 3 4 7% 66%	1760
December	1st to 10th	Day	2 1 4 2 2 2 2 4 5% 74%	2620
	11th to 20th	Day	2 1 3 2 3 3 3 6% 9% 67%	3026
	21st to end	Day	2 2 6% 4 4 4 4 6% 7% 61%	4004
January	1st to 10th	Night	2 2 9% 4 3 3 3 5% 8% 59%	6327
	11th to 20th	Night	1 1 3 3 3 4 4 5% 7% 69%	6036
	21st to end	Night	3 2 5% 3 3 4 4 5% 7% 64%	6468
February	1st to 10th	Night	4 3 6% 4 3 2 3 3 6% 65%	2595
	11th to 20th	Night	1 4 1 6% 4 3 4 3 5% 7% 62%	2338
	21st to end	Night	3 2 6% 3 3 3 3 5% 6% 66%	1980
March	1st to 10th	Night	1 6% 2 5% 2 3 3 3 5% 6% 65%	3832
	11th to 20th	Night	2 9% 3 6% 3 3 4 4 5% 7% 55%	3600
	21st to end	Night	1 6% 2 5% 3 3 4 4 6% 7% 60%	3684
April	1st to 10th	Night	1 4 1 4 3 3 4 2 6% 8% 63%	1900
	11th to 20th	Night	3 8% 2 6% 3 3 4 3 5% 8% 56%	1877
	21st to end	Night	3 7% 2 6% 4 3 4 4 6% 8% 54%	1900
May	1st to 10th	Night	4 6% 2 5% 4 4 3 3 5% 6% 58%	3090
	11th to 20th	Night	6% 8% 2 5% 4 3 4 4 6% 9% 49%	2613
	21st to end	Night	4 3 1 3 3 3 3 3 4 8% 64%	2762
June	1st to 10th	Night	3 4 1 4 2 4 4 4 6% 9% 57%	1520
	11th to 20th	Night	1 3 2 3 2 3 3 4 5% 8% 65%	1520
	21st to end	Night	2 3 2 5% 5% 6% 5% 5% 7% 11% 50%	1520
July	1st to 10th	Night	3 3 1 5% 5% 5% 6% 6% 7% 9% 51%	2540
	11th to 20th	Night	2 3 1 4 4 5% 7% 7% 10% 13% 44%	2540
	21st to end	Night	3 4 1 5% 5% 6% 6% 5% 8% 12% 46%	3036
August	1st to 10th	Night	4 4 2 8% 5% 6% 7% 6% 11% 12% 34%	2899
	11th to 20th	Night	2 3 1 4 4 4 6% 6% 11% 13% 46%	3113
	21st to end	Night	2 5% 2 7% 5% 5% 6% 6% 10% 11% 40%	3487
September	1st to 10th	Night	4 4 2 5% 4 5% 4 3 8% 9% 52%	1900
	11th to 20th	Night	4 4 1 3 3 3 4 3 5% 8% 62%	1909
	21st to end	Night	3 3 1 5% 3 3 4 3 4 6% 65%	2090
October	1st to 10th	Night	2 2 1 3 2 2 2 2 4 4 75%	3790
	11th to 20th	Night	2 2 2 4 3 3 4 4 6% 7% 64%	3778
	21st to end	Night	2 3 1 3 2 1 2 2 3 5% 75%	4120
November	1st to 10th	Night	1 2 2 3 2 2 2 3 5% 7% 71%	2525
	11th to 20th	Night	2 1 4 2 2 3 3 4 6% 74%	2600
	21st to end	Night	1 4 1 4 3 2 3 3 5% 7% 67%	2635
December	1st to 10th	Night	2 1 4 2 2 2 2 3 6% 76%	4570
	11th to 20th	Night	2 2 4 2 2 2 3 5% 7% 71%	5181
	21st to end	Night	3 2 6% 3 3 4 4 6% 8% 60%	6974

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	20% 4 12% 29% 26% 9%	3639
	11th to 20th	Day	18% 5% 11% 24% 25% 17%	3985
	21st to end	Day	22% 6% 8% 26% 25% 13%	4510
February	1st to 10th	Day	24% 4 13% 27% 21% 11%	1804
	11th to 20th	Day	20% 5% 11% 26% 29% 9%	1959
	21st to end	Day	24% 5% 9% 21% 26% 16%	1650
March	1st to 10th	Day	19% 6% 10% 17% 36% 12%	3248
	11th to 20th	Day	34% 10% 11% 25% 16% 4	3600
	21st to end	Day	22% 7% 13% 34% 21% 3	4100
April	1st to 10th	Day	27% 6% 11% 21% 23% 12%	2280
	11th to 20th	Day	32% 6% 9% 25% 19% 10%	2254
	21st to end	Day	34% 7% 12% 21% 20% 6%	2280
May	1st to 10th	Day	30% 8% 13% 16% 20% 12%	3864
	11th to 20th	Day	29% 12% 15% 21% 17% 6%	4292
	21st to end	Day	19% 8% 11% 19% 27% 16%	4825
June	1st to 10th	Day	24% 11% 14% 20% 20% 10%	2660
	11th to 20th	Day	21% 8% 11% 16% 24% 20%	2660
	21st to end	Day	29% 10% 12% 19% 19% 11%	2660
July	1st to 10th	Day	26% 8% 11% 15% 18% 21%	4361
	11th to 20th	Day	34% 10% 13% 25% 12% 5%	4355
	21st to end	Day	31% 12% 13% 19% 15% 10%	4380
August	1st to 10th	Day	40% 11% 20% 16% 9% 4	4023
	11th to 20th	Day	30% 8% 18% 16% 18% 10%	3762
	21st to end	Day	37% 9% 16% 16% 18% 4	4179
September	1st to 10th	Day	28% 8% 12% 21% 20% 10%	2280
	11th to 20th	Day	22% 7% 11% 18% 29% 14%	2269
	21st to end	Day	22% 5% 7% 23% 33% 10%	2090
October	1st to 10th	Day	16% 5% 6% 15% 39% 18%	3175
	11th to 20th	Day	22% 5% 8% 29% 27% 9%	3150
	21st to end	Day	15% 4 5% 21% 42% 12%	3417
November	1st to 10th	Day	15% 4 10% 21% 36% 13%	1763
	11th to 20th	Day	14% 5% 9% 30% 32% 10%	1800
	21st to end	Day	20% 4 8% 22% 31% 15%	1760
December	1st to 10th	Day	14% 4 9% 29% 35% 10%	2620
	11th to 20th	Day	15% 5% 10% 29% 32% 8%	3024
	21st to end	Day	20% 6% 11% 26% 25% 12%	4004
January	1st to 10th	Night	21% 5% 10% 27% 29% 8%	6327
	11th to 20th	Night	15% 4 9% 28% 28% 15%	6036
	21st to end	Night	18% 7% 11% 25% 25% 14%	6468
February	1st to 10th	Night	22% 4 11% 29% 23% 11%	2595
	11th to 20th	Night	23% 4 11% 28% 26% 7%	2338
	21st to end	Night	18% 5% 9% 24% 31% 14%	1980
March	1st to 10th	Night	18% 5% 9% 21% 37% 10%	3832
	11th to 20th	Night	27% 8% 12% 28% 21% 4	3600
	21st to end	Night	20% 7% 17% 33% 21% 2	3684
April	1st to 10th	Night	19% 6% 14% 26% 26% 9%	1900
	11th to 20th	Night	26% 6% 11% 25% 24% 7%	1877
	21st to end	Night	28% 7% 12% 28% 20% 5%	1900
May	1st to 10th	Night	24% 8% 13% 23% 23% 11%	3090
	11th to 20th	Night	28% 9% 15% 26% 15% 7%	2614
	21st to end	Night	17% 8% 13% 22% 30% 11%	2762
June	1st to 10th	Night	22% 8% 18% 21% 18% 13%	1520
	11th to 20th	Night	16% 8% 11% 22% 24% 20%	1520
	21st to end	Night	26% 9% 13% 19% 20% 12%	1520
July	1st to 10th	Night	26% 8% 12% 19% 19% 16%	2500
	11th to 20th	Night	27% 8% 22% 28% 10% 4	2492
	21st to end	Night	28% 11% 17% 23% 16% 5%	2941
August	1st to 10th	Night	40% 13% 20% 15% 8% 4	2890
	11th to 20th	Night	29% 9% 24% 16% 17% 5%	3143
	21st to end	Night	36% 9% 17% 22% 14% 3	3487
September	1st to 10th	Night	27% 7% 15% 23% 21% 7%	1900
	11th to 20th	Night	20% 6% 12% 21% 26% 14%	1909
	21st to end	Night	20% 6% 9% 23% 34% 8%	2090
October	1st to 10th	Night	14% 4 9% 23% 38% 12%	3790
	11th to 20th	Night	21% 5% 12% 25% 28% 9%	3778
	21st to end	Night	15% 3 5% 25% 38% 13%	4120
November	1st to 10th	Night	14% 4 10% 26% 32% 14%	2525
	11th to 20th	Night	13% 5% 8% 31% 32% 12%	2600
	21st to end	Night	17% 5% 10% 22% 33% 13%	2635
December	1st to 10th	Night	12% 4 9% 29% 36% 9%	4570
	11th to 20th	Night	13% 5% 9% 28% 37% 7%	5182
	21st to end	Night	22% 6% 13% 20% 29% 10%	6974

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	16% 22% 33% 19% 8% 1	300
	11th to 20th	Day	16% 23% 40% 18% 3	300
	21st to end	Day	5% 25% 41% 21% 7% 1	330
February	1st to 10th	Day	11% 37% 39% 11% 1 1	300
	11th to 20th	Day	11% 39% 36% 10% 3	300
	21st to end	Day	19% 30% 35% 14% 1	249
March	1st to 10th	Day	14% 29% 33% 19% 5%	305
	11th to 20th	Day	12% 32% 34% 17% 5% 1	500
	21st to end	Day	16% 37% 35% 9% 3	531
April	1st to 10th	Day	15% 32% 35% 14% 2 1	460
	11th to 20th	Day	17% 35% 33% 13% 2	452
	21st to end	Day	15% 34% 34% 13% 3	460
May	1st to 10th	Day	14% 29% 38% 15% 4 1	501
	11th to 20th	Day	9% 17% 35% 26% 11% 2	675
	21st to end	Day	7% 20% 31% 29% 12% 2	734
June	1st to 10th	Day	4 13% 32% 31% 17% 3	680
	11th to 20th	Day	5% 14% 36% 28% 14% 3	680
	21st to end	Day	4 11% 35% 29% 19% 4	680
July	1st to 10th	Day	6% 5% 25% 37% 21% 5%	671
	11th to 20th	Day	4 2 18% 40% 29% 7%	668
	21st to end	Day	5% 3 18% 40% 26% 7%	574
August	1st to 10th	Day	7% 4 23% 39% 22% 5%	487
	11th to 20th	Day	5% 4 31% 40% 17% 3	477
	21st to end	Day	8% 6% 26% 39% 18% 3	539
September	1st to 10th	Day	6% 8% 28% 36% 17% 5%	490
	11th to 20th	Day	5% 12% 39% 28% 15% 2	479
	21st to end	Day	6% 11% 28% 30% 22% 4	300
October	1st to 10th	Day	8% 16% 27% 30% 17% 3	300
	11th to 20th	Day	8% 24% 28% 26% 10% 3	300
	21st to end	Day	11% 21% 35% 22% 10% 1	330
November	1st to 10th	Day	8% 28% 32% 22% 9% 1	300
	11th to 20th	Day	7% 25% 39% 19% 9% 2	300
	21st to end	Day	7% 20% 42% 25% 5% 1	300
December	1st to 10th	Day	13% 25% 34% 20% 8% 1	300
	11th to 20th	Day	7% 33% 38% 16% 6%	300
	21st to end	Day	12% 34% 37% 13% 4	330
January	1st to 10th	Night	4 24% 37% 25% 9% 2	1000
	11th to 20th	Night	5% 24% 41% 23% 6% 1	865
	21st to end	Night	2 17% 48% 25% 7% 1	880
February	1st to 10th	Night	3 21% 56% 16% 4	800
	11th to 20th	Night	5% 23% 50% 17% 5%	784
	21st to end	Night	4 22% 53% 20% 1	660
March	1st to 10th	Night	5% 25% 44% 21% 4 1	800
	11th to 20th	Night	6% 15% 43% 27% 9% 1	800
	21st to end	Night	6% 21% 44% 24% 4	737
April	1st to 10th	Night	8% 20% 45% 24% 4	570
	11th to 20th	Night	9% 19% 40% 26% 5%	563
	21st to end	Night	7% 19% 37% 28% 8%	570
May	1st to 10th	Night	8% 14% 40% 30% 8% 1	570
	11th to 20th	Night	10% 10% 34% 33% 13% 1	561
	21st to end	Night	8% 15% 34% 29% 13% 1	617
June	1st to 10th	Night	4 9% 26% 38% 21% 1	570
	11th to 20th	Night	2 11% 33% 41% 13% 1	570
	21st to end	Night	4 5% 27% 39% 24% 2	570
July	1st to 10th	Night	7% 2 16% 41% 32% 3	555
	11th to 20th	Night	5% 1 11% 38% 39% 6%	552
	21st to end	Night	4 1 10% 37% 42% 6%	624
August	1st to 10th	Night	5% 3 13% 39% 35% 5%	567
	11th to 20th	Night	4 4 13% 45% 31% 4	564
	21st to end	Night	5% 2 16% 41% 30% 5%	627
September	1st to 10th	Night	4 4 18% 41% 27% 6%	570
	11th to 20th	Night	5% 7% 26% 36% 21% 4	570
	21st to end	Night	4 8% 28% 41% 16% 3	570
October	1st to 10th	Night	5% 10% 32% 36% 14% 2	750
	11th to 20th	Night	2 13% 36% 32% 14% 2	760
	21st to end	Night	6% 14% 36% 28% 14% 2	836
November	1st to 10th	Night	3 16% 40% 29% 11% 1	965
	11th to 20th	Night	4 19% 41% 25% 9% 2	1000
	21st to end	Night	6% 16% 41% 28% 8% 1	1000
December	1st to 10th	Night	5% 20% 41% 26% 7%	1000
	11th to 20th	Night	4 22% 46% 21% 7% 1	1000
	21st to end	Night	5% 26% 43% 21% 4 1	1100

Page generated at 2025-03-24 07:04:31. Version ac07

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