Mountain Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mountain Sky Observatory clear sky chart from 2005-11-06 to 2024-11-12.

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 5% 4 5% 5% 4 7% 8% 52%	105167
	Night	3 5% 2 5% 3 3 4 4 6% 7% 58%	102736
Transparency	Day	24% 10% 13% 24% 22% 7%	104920
	Night	22% 9% 14% 26% 23% 7%	102573
Seeing	Day	9% 21% 30% 25% 12% 2	15439
	Night	6% 14% 32% 32% 14% 2	25502

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	4 2 5% 4 4 4 5% 6% 8% 58%	2538
	11th to 20th	Day	3 2 4 4 4 5% 4 7% 9% 56%	2857
	21st to end	Day	1 6% 2 8% 4 5% 4 4 6% 7% 53%	3146
February	1st to 10th	Day	5% 2 7% 3 3 4 3 6% 8% 58%	1710
	11th to 20th	Day	1 5% 3 5% 3 3 3 4 6% 9% 58%	1801
	21st to end	Day	1 5% 3 6% 4 3 4 4 5% 8% 57%	1570
March	1st to 10th	Day	1 5% 2 5% 4 4 4 4 5% 8% 57%	3226
	11th to 20th	Day	2 12% 4 7% 4 4 4 4 5% 7% 46%	3490
	21st to end	Day	1 8% 2 6% 5% 5% 6% 4 6% 8% 48%	3974
April	1st to 10th	Day	2 6% 2 4 4 4 4 4 6% 9% 55%	2280
	11th to 20th	Day	3 7% 2 6% 5% 4 5% 4 6% 8% 50%	2251
	21st to end	Day	4 7% 2 5% 5% 5% 5% 4 7% 9% 47%	2280
May	1st to 10th	Day	6% 9% 2 6% 5% 4 5% 4 6% 8% 46%	3814
	11th to 20th	Day	5% 7% 2 7% 5% 6% 6% 6% 7% 8% 41%	4378
	21st to end	Day	3 4 2 6% 5% 5% 5% 5% 6% 7% 51%	4825
June	1st to 10th	Day	3 4 1 5% 5% 4 6% 6% 9% 9% 48%	2660
	11th to 20th	Day	4 3 2 4 3 4 4 5% 7% 7% 58%	2660
	21st to end	Day	3 4 2 4 5% 5% 6% 5% 8% 9% 48%	2660
July	1st to 10th	Day	4 3 1 5% 5% 5% 6% 5% 7% 7% 53%	4430
	11th to 20th	Day	3 3 1 5% 6% 7% 9% 7% 9% 9% 40%	4403
	21st to end	Day	4 4 2 6% 6% 6% 6% 6% 8% 8% 43%	4454
August	1st to 10th	Day	6% 5% 2 8% 7% 8% 7% 6% 9% 9% 33%	4092
	11th to 20th	Day	3 4 2 6% 4 6% 7% 6% 8% 11% 44%	3742
	21st to end	Day	4 4 2 6% 6% 6% 7% 6% 10% 10% 37%	4180
September	1st to 10th	Day	5% 4 2 5% 5% 4 5% 5% 7% 8% 51%	2280
	11th to 20th	Day	5% 4 2 5% 4 4 5% 4 6% 7% 54%	2280
	21st to end	Day	6% 3 1 3 3 3 4 4 4 6% 63%	2058
October	1st to 10th	Day	3 3 1 3 3 2 2 2 4 5% 72%	3170
	11th to 20th	Day	6% 4 1 3 3 4 4 4 5% 8% 57%	3150
	21st to end	Day	2 3 1 3 2 3 2 2 5% 6% 71%	3438
November	1st to 10th	Day	1 3 1 3 3 3 3 2 5% 5% 71%	1796
	11th to 20th	Day	1 2 1 3 1 2 2 2 4 7% 76%	1719
	21st to end	Day	2 5% 2 5% 2 3 3 3 3 8% 65%	1615
December	1st to 10th	Day	1 5% 2 5% 3 3 4 3 3 8% 65%	1520
	11th to 20th	Day	1 4 2 4 2 3 3 4 6% 12% 57%	1926
	21st to end	Day	1 5% 3 5% 3 4 4 3 6% 8% 57%	2794
January	1st to 10th	Night	4 3 8% 5% 5% 5% 4 5% 6% 54%	4417
	11th to 20th	Night	1 3 1 9% 4 3 4 3 4 7% 61%	4142
	21st to end	Night	1 7% 3 5% 3 3 3 2 5% 7% 59%	4521
February	1st to 10th	Night	1 5% 2 6% 3 2 3 3 4 7% 64%	2470
	11th to 20th	Night	2 7% 2 4 3 2 2 2 4 6% 65%	2263
	21st to end	Night	1 7% 2 5% 2 3 3 2 5% 8% 61%	1884
March	1st to 10th	Night	1 8% 2 4 3 3 3 3 4 7% 62%	3624
	11th to 20th	Night	2 10% 2 6% 4 4 5% 4 7% 7% 51%	3490
	21st to end	Night	1 7% 3 6% 4 5% 5% 4 6% 7% 51%	3674
April	1st to 10th	Night	2 5% 2 5% 3 3 4 3 5% 7% 61%	1900
	11th to 20th	Night	4 7% 2 5% 4 4 5% 3 5% 6% 55%	1877
	21st to end	Night	5% 7% 2 5% 3 3 3 4 5% 6% 56%	1900
May	1st to 10th	Night	6% 6% 2 4 3 2 3 3 5% 7% 59%	3153
	11th to 20th	Night	7% 7% 2 6% 4 4 4 4 5% 7% 49%	2524
	21st to end	Night	5% 4 1 4 3 3 3 3 5% 5% 65%	2762
June	1st to 10th	Night	2 3 1 3 3 3 3 5% 7% 8% 61%	1520
	11th to 20th	Night	3 3 1 3 3 3 3 4 6% 7% 65%	1520
	21st to end	Night	3 2 2 4 5% 4 4 5% 6% 7% 57%	1520
July	1st to 10th	Night	5% 3 2 6% 4 4 4 3 6% 7% 56%	2540
	11th to 20th	Night	3 3 1 4 4 5% 5% 4 9% 12% 49%	2562
	21st to end	Night	3 3 2 4 4 5% 6% 5% 8% 11% 50%	3109
August	1st to 10th	Night	7% 6% 2 7% 5% 5% 6% 6% 9% 12% 35%	2847
	11th to 20th	Night	3 3 1 5% 5% 5% 6% 5% 9% 12% 44%	3084
	21st to end	Night	4 4 2 5% 5% 6% 7% 7% 9% 11% 40%	3487
September	1st to 10th	Night	6% 3 2 5% 4 4 4 5% 7% 8% 53%	1900
	11th to 20th	Night	6% 3 1 3 3 3 3 3 7% 8% 60%	1900
	21st to end	Night	6% 5% 1 4 2 2 2 2 4 7% 64%	2116
October	1st to 10th	Night	4 3 1 3 2 2 2 3 4 4 72%	3800
	11th to 20th	Night	4 4 1 4 3 3 4 3 5% 7% 60%	3778
	21st to end	Night	4 3 1 3 2 2 2 2 3 5% 73%	4120
November	1st to 10th	Night	2 4 1 3 2 3 2 3 4 8% 70%	2483
	11th to 20th	Night	1 2 1 2 1 2 2 2 4 6% 76%	2484
	21st to end	Night	3 6% 2 3 2 2 3 3 4 6% 65%	2561
December	1st to 10th	Night	1 3 2 3 3 3 3 2 5% 8% 67%	2660
	11th to 20th	Night	1 5% 2 4 3 3 3 4 6% 10% 60%	3271
	21st to end	Night	1 5% 2 6% 4 4 4 4 5% 6% 59%	4873

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% 8% 19% 31% 20% 1	2539
	11th to 20th	Day	17% 18% 21% 27% 16% 2	2857
	21st to end	Day	27% 10% 12% 31% 17% 2	3146
February	1st to 10th	Day	23% 9% 18% 26% 18% 5%	1710
	11th to 20th	Day	21% 9% 14% 28% 24% 4	1811
	21st to end	Day	25% 9% 10% 24% 26% 6%	1570
March	1st to 10th	Day	19% 10% 11% 26% 30% 6%	3226
	11th to 20th	Day	29% 11% 14% 30% 13% 2	3490
	21st to end	Day	24% 12% 19% 28% 16% 2	3974
April	1st to 10th	Day	23% 10% 13% 24% 23% 8%	2280
	11th to 20th	Day	26% 12% 11% 25% 21% 5%	2254
	21st to end	Day	26% 13% 12% 24% 22% 3	2280
May	1st to 10th	Day	28% 13% 11% 19% 20% 10%	3814
	11th to 20th	Day	30% 15% 14% 23% 14% 4	4382
	21st to end	Day	20% 15% 10% 19% 29% 7%	4825
June	1st to 10th	Day	24% 12% 14% 21% 20% 8%	2660
	11th to 20th	Day	20% 9% 10% 17% 24% 19%	2660
	21st to end	Day	24% 11% 12% 20% 21% 12%	2660
July	1st to 10th	Day	23% 10% 8% 17% 21% 21%	4361
	11th to 20th	Day	30% 11% 18% 25% 12% 3	4328
	21st to end	Day	29% 12% 15% 20% 14% 9%	4314
August	1st to 10th	Day	39% 12% 20% 14% 11% 4	4078
	11th to 20th	Day	27% 12% 16% 17% 21% 6%	3778
	21st to end	Day	31% 11% 17% 17% 19% 5%	4179
September	1st to 10th	Day	25% 10% 13% 19% 23% 10%	2280
	11th to 20th	Day	23% 8% 13% 19% 27% 9%	2280
	21st to end	Day	20% 7% 8% 24% 35% 7%	2058
October	1st to 10th	Day	15% 5% 5% 19% 39% 16%	3170
	11th to 20th	Day	22% 5% 8% 33% 25% 6%	3150
	21st to end	Day	14% 5% 7% 37% 30% 7%	3438
November	1st to 10th	Day	15% 6% 11% 26% 37% 5%	1796
	11th to 20th	Day	10% 7% 11% 38% 30% 5%	1719
	21st to end	Day	21% 5% 10% 30% 25% 9%	1615
December	1st to 10th	Day	18% 9% 20% 29% 18% 6%	1520
	11th to 20th	Day	20% 9% 16% 30% 23% 2	1924
	21st to end	Day	22% 10% 15% 21% 23% 9%	2794
January	1st to 10th	Night	24% 11% 15% 28% 20% 2	4417
	11th to 20th	Night	15% 14% 20% 30% 17% 4	4142
	21st to end	Night	23% 11% 14% 31% 18% 3	4521
February	1st to 10th	Night	22% 7% 19% 27% 20% 6%	2470
	11th to 20th	Night	21% 8% 14% 29% 25% 4	2263
	21st to end	Night	21% 7% 10% 26% 31% 6%	1884
March	1st to 10th	Night	16% 12% 9% 25% 34% 4	3624
	11th to 20th	Night	26% 12% 14% 30% 16% 2	3490
	21st to end	Night	25% 11% 17% 28% 17% 2	3674
April	1st to 10th	Night	22% 8% 11% 28% 24% 6%	1900
	11th to 20th	Night	26% 9% 13% 25% 22% 5%	1877
	21st to end	Night	25% 9% 13% 27% 23% 3	1900
May	1st to 10th	Night	22% 10% 12% 20% 31% 6%	3153
	11th to 20th	Night	28% 11% 16% 25% 14% 6%	2525
	21st to end	Night	17% 8% 13% 23% 31% 8%	2762
June	1st to 10th	Night	18% 10% 15% 25% 20% 13%	1520
	11th to 20th	Night	16% 7% 11% 22% 23% 22%	1520
	21st to end	Night	21% 10% 12% 20% 23% 13%	1520
July	1st to 10th	Night	26% 8% 9% 18% 16% 23%	2500
	11th to 20th	Night	23% 8% 26% 31% 8% 5%	2514
	21st to end	Night	24% 12% 22% 24% 12% 7%	3011
August	1st to 10th	Night	38% 12% 25% 12% 8% 6%	2838
	11th to 20th	Night	27% 14% 22% 16% 16% 5%	3114
	21st to end	Night	31% 13% 17% 20% 15% 3	3487
September	1st to 10th	Night	25% 10% 14% 20% 20% 10%	1900
	11th to 20th	Night	20% 9% 13% 20% 27% 12%	1900
	21st to end	Night	22% 6% 9% 24% 32% 8%	2116
October	1st to 10th	Night	15% 6% 6% 24% 32% 17%	3800
	11th to 20th	Night	21% 5% 10% 29% 28% 7%	3778
	21st to end	Night	16% 4 5% 35% 32% 8%	4120
November	1st to 10th	Night	13% 7% 11% 29% 33% 7%	2483
	11th to 20th	Night	12% 5% 11% 37% 30% 6%	2484
	21st to end	Night	20% 6% 8% 28% 30% 8%	2561
December	1st to 10th	Night	17% 7% 19% 30% 22% 5%	2660
	11th to 20th	Night	19% 9% 14% 30% 26% 3	3272
	21st to end	Night	23% 9% 14% 22% 24% 8%	4873

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	11% 23% 35% 25% 4 1	270
	11th to 20th	Day	13% 31% 36% 17% 3	270
	21st to end	Day	4 25% 43% 23% 5% 1	297
February	1st to 10th	Day	5% 41% 40% 12% 1 1	270
	11th to 20th	Day	13% 36% 34% 13% 3	270
	21st to end	Day	17% 38% 33% 10% 2	225
March	1st to 10th	Day	10% 37% 30% 21% 2	327
	11th to 20th	Day	11% 34% 31% 18% 5%	460
	21st to end	Day	14% 43% 31% 11% 2	506
April	1st to 10th	Day	15% 39% 31% 12% 3	460
	11th to 20th	Day	19% 39% 28% 10% 4	452
	21st to end	Day	13% 38% 31% 13% 3 1	460
May	1st to 10th	Day	17% 34% 30% 14% 4	463
	11th to 20th	Day	11% 20% 29% 27% 11% 2	665
	21st to end	Day	10% 21% 28% 28% 11% 2	734
June	1st to 10th	Day	3 15% 27% 34% 18% 3	680
	11th to 20th	Day	7% 14% 35% 28% 13% 3	680
	21st to end	Day	4 14% 30% 29% 18% 4	680
July	1st to 10th	Day	7% 6% 22% 38% 23% 5%	671
	11th to 20th	Day	7% 3 17% 39% 28% 6%	654
	21st to end	Day	9% 3 20% 37% 27% 4	531
August	1st to 10th	Day	9% 5% 26% 38% 19% 3	487
	11th to 20th	Day	6% 8% 34% 32% 16% 3	477
	21st to end	Day	8% 9% 30% 32% 17% 4	539
September	1st to 10th	Day	8% 9% 26% 33% 19% 5%	490
	11th to 20th	Day	8% 15% 34% 27% 14% 3	490
	21st to end	Day	7% 17% 24% 31% 17% 5%	321
October	1st to 10th	Day	9% 19% 27% 23% 18% 4	300
	11th to 20th	Day	9% 23% 32% 20% 11% 4	300
	21st to end	Day	13% 26% 28% 24% 7% 1	330
November	1st to 10th	Day	8% 28% 35% 21% 7%	300
	11th to 20th	Day	7% 24% 40% 22% 7%	273
	21st to end	Day	6% 26% 40% 23% 4	270
December	1st to 10th	Day	11% 24% 39% 19% 6% 1	270
	11th to 20th	Day	5% 33% 37% 17% 7% 1	270
	21st to end	Day	14% 28% 36% 18% 4	297
January	1st to 10th	Night	4 21% 37% 28% 8% 2	950
	11th to 20th	Night	5% 20% 39% 27% 7% 2	779
	21st to end	Night	3 17% 38% 33% 8% 1	836
February	1st to 10th	Night	3 23% 45% 25% 4 1	760
	11th to 20th	Night	5% 23% 42% 24% 6%	744
	21st to end	Night	6% 21% 47% 24% 2	628
March	1st to 10th	Night	3 19% 44% 28% 6% 1	760
	11th to 20th	Night	5% 14% 39% 32% 10% 1	760
	21st to end	Night	3 22% 43% 27% 5%	746
April	1st to 10th	Night	8% 22% 41% 25% 5%	570
	11th to 20th	Night	9% 22% 36% 24% 9% 1	563
	21st to end	Night	9% 19% 40% 25% 6% 1	570
May	1st to 10th	Night	9% 15% 32% 32% 10% 1	570
	11th to 20th	Night	11% 8% 27% 39% 14% 2	561
	21st to end	Night	9% 12% 29% 37% 13% 1	617
June	1st to 10th	Night	3 8% 22% 43% 22% 3	570
	11th to 20th	Night	4 8% 29% 43% 15% 1	570
	21st to end	Night	3 4 21% 39% 29% 4	570
July	1st to 10th	Night	8% 3 13% 40% 33% 3	555
	11th to 20th	Night	6% 2 9% 38% 39% 7%	552
	21st to end	Night	7% 1 9% 31% 46% 6%	624
August	1st to 10th	Night	7% 3 12% 36% 37% 5%	567
	11th to 20th	Night	6% 3 11% 45% 31% 5%	564
	21st to end	Night	6% 3 13% 40% 34% 4	627
September	1st to 10th	Night	7% 3 15% 42% 27% 6%	570
	11th to 20th	Night	7% 3 19% 47% 20% 4	570
	21st to end	Night	8% 6% 24% 39% 19% 4	619
October	1st to 10th	Night	7% 8% 31% 37% 14% 2	760
	11th to 20th	Night	3 12% 35% 35% 13% 3	760
	21st to end	Night	7% 11% 38% 29% 13% 2	836
November	1st to 10th	Night	4 12% 38% 33% 11% 1	923
	11th to 20th	Night	3 18% 38% 30% 8% 2	956
	21st to end	Night	7% 16% 32% 33% 11%	950
December	1st to 10th	Night	4 21% 40% 28% 7%	950
	11th to 20th	Night	6% 19% 40% 25% 9%	950
	21st to end	Night	6% 21% 41% 25% 6% 1	1045

Page generated at 2024-11-08 20:45:38. Version ac07

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