Rocky Mountain Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rocky Mountain Sky Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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	5% 5% 2 5% 4 4 4 4 6% 8% 53%	124108
	Night	5% 6% 2 6% 4 4 4 3 5% 7% 54%	123907
Transparency	Day	28% 4 9% 25% 24% 10%	123851
	Night	29% 4 10% 26% 24% 8%	123754
Seeing	Day	9% 5% 19% 34% 25% 9%	16631
	Night	8% 3 20% 49% 18% 1	26624

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	1 8% 3 8% 4 3 3 3 5% 6% 55%	3638
	11th to 20th	Day	2 6% 3 8% 6% 5% 4 3 5% 7% 50%	3660
	21st to end	Day	1 6% 2 5% 3 4 4 3 5% 7% 59%	4510
February	1st to 10th	Day	4 7% 3 7% 4 4 5% 4 6% 8% 49%	1800
	11th to 20th	Day	1 7% 3 6% 4 4 4 4 6% 9% 52%	1921
	21st to end	Day	3 7% 3 6% 3 4 3 4 6% 7% 55%	1650
March	1st to 10th	Day	5% 7% 2 4 3 3 3 3 5% 7% 57%	4500
	11th to 20th	Day	5% 6% 2 6% 4 4 5% 4 6% 7% 50%	4898
	21st to end	Day	5% 7% 3 6% 4 4 4 5% 7% 10% 45%	5837
April	1st to 10th	Day	6% 7% 2 4 4 4 4 3 6% 8% 51%	2400
	11th to 20th	Day	11% 9% 2 5% 4 3 4 4 6% 9% 44%	2371
	21st to end	Day	10% 8% 2 5% 5% 5% 5% 4 6% 9% 43%	2400
May	1st to 10th	Day	10% 7% 2 7% 5% 4 5% 4 5% 6% 45%	5823
	11th to 20th	Day	8% 4 2 6% 4 5% 5% 5% 7% 9% 44%	6298
	21st to end	Day	9% 5% 2 5% 4 5% 5% 5% 6% 9% 44%	6925
June	1st to 10th	Day	5% 4 2 4 3 4 5% 5% 7% 11% 50%	2800
	11th to 20th	Day	6% 3 1 4 4 4 5% 5% 7% 9% 52%	2800
	21st to end	Day	4 3 1 4 3 4 5% 5% 8% 10% 52%	2800
July	1st to 10th	Day	5% 4 1 4 3 4 4 5% 6% 7% 56%	4570
	11th to 20th	Day	3 2 1 5% 5% 5% 6% 6% 9% 10% 48%	4570
	21st to end	Day	3 3 1 5% 3 4 5% 4 7% 9% 55%	4790
August	1st to 10th	Day	11% 4 1 5% 4 6% 6% 5% 9% 10% 39%	4092
	11th to 20th	Day	4 3 1 4 4 5% 6% 5% 8% 9% 50%	3733
	21st to end	Day	3 3 1 5% 3 4 5% 4 7% 9% 55%	4180
September	1st to 10th	Day	5% 4 1 3 4 3 3 4 6% 6% 61%	2280
	11th to 20th	Day	5% 2 1 4 3 4 4 4 6% 8% 59%	2252
	21st to end	Day	7% 4 1 3 2 2 3 3 3 6% 65%	2090
October	1st to 10th	Day	5% 4 1 2 2 2 3 2 3 5% 73%	3210
	11th to 20th	Day	5% 4 2 5% 3 3 3 3 5% 6% 61%	3150
	21st to end	Day	6% 5% 2 4 3 3 3 3 5% 6% 59%	3266
November	1st to 10th	Day	4 5% 1 4 4 3 3 3 6% 7% 60%	1763
	11th to 20th	Day	3 7% 2 4 3 3 4 3 5% 7% 60%	1800
	21st to end	Day	3 6% 2 5% 3 3 3 3 6% 8% 58%	1681
December	1st to 10th	Day	1 4 1 4 3 3 2 3 4 6% 67%	2620
	11th to 20th	Day	2 5% 2 3 3 3 3 2 4 7% 66%	3026
	21st to end	Day	4 7% 3 7% 4 4 5% 4 5% 7% 50%	4004
January	1st to 10th	Night	3 10% 3 7% 4 4 4 3 5% 9% 49%	6327
	11th to 20th	Night	1 6% 3 10% 5% 4 4 3 5% 6% 54%	6340
	21st to end	Night	3 7% 4 7% 4 3 3 3 3 5% 58%	6491
February	1st to 10th	Night	3 9% 4 7% 3 4 3 3 5% 7% 52%	2600
	11th to 20th	Night	3 8% 4 8% 5% 4 4 3 6% 7% 48%	2364
	21st to end	Night	3 9% 3 6% 4 3 3 2 4 6% 56%	1980
March	1st to 10th	Night	5% 10% 3 6% 4 3 3 3 4 5% 53%	5374
	11th to 20th	Night	8% 8% 2 5% 3 4 4 3 4 6% 53%	5093
	21st to end	Night	8% 9% 3 7% 5% 4 4 3 5% 7% 45%	5108
April	1st to 10th	Night	6% 8% 2 6% 4 3 4 4 5% 6% 53%	2000
	11th to 20th	Night	13% 7% 3 5% 4 4 3 3 5% 7% 46%	1977
	21st to end	Night	14% 8% 2 5% 5% 3 3 4 4 8% 44%	2000
May	1st to 10th	Night	12% 7% 3 6% 4 4 4 4 6% 7% 43%	4141
	11th to 20th	Night	9% 6% 2 7% 4 4 5% 4 7% 9% 43%	3617
	21st to end	Night	12% 7% 2 5% 3 4 4 3 5% 7% 48%	3972
June	1st to 10th	Night	7% 3 2 4 3 3 3 4 6% 11% 54%	1600
	11th to 20th	Night	5% 3 1 4 3 3 4 3 6% 8% 59%	1600
	21st to end	Night	5% 3 1 5% 3 3 5% 4 7% 9% 56%	1600
July	1st to 10th	Night	5% 3 1 4 3 5% 4 4 6% 8% 57%	2620
	11th to 20th	Night	3 2 1 4 4 4 4 5% 9% 11% 53%	2620
	21st to end	Night	3 3 1 5% 4 4 5% 5% 7% 9% 53%	2761
August	1st to 10th	Night	11% 3 2 6% 4 5% 6% 5% 8% 9% 40%	2844
	11th to 20th	Night	3 4 2 5% 4 4 5% 4 8% 10% 49%	3104
	21st to end	Night	2 3 2 4 4 5% 5% 5% 8% 10% 52%	3487
September	1st to 10th	Night	5% 3 1 5% 4 3 4 3 5% 8% 58%	1900
	11th to 20th	Night	6% 2 1 4 3 3 4 3 5% 7% 62%	1928
	21st to end	Night	7% 3 1 4 3 3 3 3 5% 5% 64%	2090
October	1st to 10th	Night	6% 3 1 4 2 2 2 2 4 5% 69%	3757
	11th to 20th	Night	7% 5% 2 5% 3 3 3 3 5% 7% 58%	3778
	21st to end	Night	7% 5% 1 4 2 3 3 2 4 7% 61%	4259
November	1st to 10th	Night	4 5% 2 4 3 3 3 4 4 7% 62%	2535
	11th to 20th	Night	4 5% 2 4 3 3 4 3 4 7% 62%	2600
	21st to end	Night	4 7% 2 4 3 3 3 3 5% 6% 59%	2715
December	1st to 10th	Night	2 5% 1 4 3 3 3 2 4 5% 69%	4570
	11th to 20th	Night	2 5% 2 4 3 3 3 3 6% 9% 61%	5181
	21st to end	Night	5% 10% 3 7% 4 4 4 3 4 7% 50%	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	30% 2 7% 35% 19% 6%	3639
	11th to 20th	Day	33% 3 9% 22% 25% 8%	3660
	21st to end	Day	25% 2 10% 27% 26% 10%	4510
February	1st to 10th	Day	32% 4 10% 29% 20% 5%	1800
	11th to 20th	Day	28% 4 8% 30% 25% 5%	1931
	21st to end	Day	27% 4 7% 26% 25% 11%	1650
March	1st to 10th	Day	27% 2 5% 21% 34% 11%	4500
	11th to 20th	Day	32% 3 11% 23% 25% 5%	4898
	21st to end	Day	32% 5% 11% 36% 14% 2	5837
April	1st to 10th	Day	30% 5% 10% 27% 21% 8%	2400
	11th to 20th	Day	37% 4 8% 24% 20% 8%	2374
	21st to end	Day	38% 4 8% 27% 21% 2	2400
May	1st to 10th	Day	37% 6% 9% 27% 17% 4	5823
	11th to 20th	Day	34% 4 11% 23% 20% 9%	6302
	21st to end	Day	33% 6% 13% 26% 19% 4	6926
June	1st to 10th	Day	26% 5% 18% 25% 18% 7%	2800
	11th to 20th	Day	27% 5% 11% 20% 24% 14%	2800
	21st to end	Day	24% 5% 9% 24% 25% 13%	2800
July	1st to 10th	Day	24% 4 8% 18% 30% 16%	4501
	11th to 20th	Day	26% 6% 10% 23% 21% 14%	4495
	21st to end	Day	24% 4 9% 22% 21% 21%	4639
August	1st to 10th	Day	36% 6% 13% 21% 15% 9%	4078
	11th to 20th	Day	26% 6% 12% 20% 23% 13%	3769
	21st to end	Day	24% 4 8% 21% 26% 16%	4179
September	1st to 10th	Day	22% 3 5% 23% 27% 19%	2280
	11th to 20th	Day	23% 3 6% 21% 29% 18%	2252
	21st to end	Day	22% 2 4 23% 36% 12%	2090
October	1st to 10th	Day	17% 2 4 18% 34% 25%	3210
	11th to 20th	Day	24% 3 9% 27% 28% 9%	3150
	21st to end	Day	25% 2 6% 26% 32% 9%	3266
November	1st to 10th	Day	23% 3 8% 28% 26% 11%	1763
	11th to 20th	Day	23% 4 5% 28% 32% 8%	1800
	21st to end	Day	25% 2 8% 25% 30% 9%	1681
December	1st to 10th	Day	19% 3 7% 34% 28% 9%	2620
	11th to 20th	Day	20% 3 6% 34% 31% 5%	3024
	21st to end	Day	33% 4 9% 26% 21% 8%	4004
January	1st to 10th	Night	33% 3 11% 29% 18% 6%	6327
	11th to 20th	Night	31% 4 10% 23% 24% 8%	6340
	21st to end	Night	30% 2 9% 25% 26% 7%	6491
February	1st to 10th	Night	32% 3 10% 28% 22% 5%	2600
	11th to 20th	Night	34% 4 10% 27% 21% 5%	2364
	21st to end	Night	30% 2 5% 26% 26% 11%	1980
March	1st to 10th	Night	33% 3 5% 18% 34% 7%	5374
	11th to 20th	Night	33% 2 10% 18% 28% 9%	5093
	21st to end	Night	38% 4 12% 31% 14% 2	5108
April	1st to 10th	Night	31% 4 8% 27% 21% 8%	2000
	11th to 20th	Night	38% 3 10% 24% 19% 6%	1977
	21st to end	Night	40% 3 8% 28% 18% 3	2000
May	1st to 10th	Night	39% 5% 10% 27% 13% 5%	4139
	11th to 20th	Night	37% 3 15% 22% 16% 7%	3618
	21st to end	Night	36% 4 16% 27% 13% 4	3972
June	1st to 10th	Night	24% 4 21% 31% 13% 7%	1600
	11th to 20th	Night	24% 4 12% 23% 23% 14%	1600
	21st to end	Night	23% 5% 10% 26% 28% 8%	1600
July	1st to 10th	Night	24% 5% 11% 23% 27% 10%	2580
	11th to 20th	Night	22% 6% 16% 26% 24% 8%	2572
	21st to end	Night	24% 5% 13% 25% 20% 13%	2674
August	1st to 10th	Night	36% 7% 15% 22% 15% 4	2836
	11th to 20th	Night	27% 7% 16% 23% 17% 10%	3134
	21st to end	Night	25% 5% 13% 23% 25% 9%	3487
September	1st to 10th	Night	25% 4 9% 22% 22% 18%	1900
	11th to 20th	Night	20% 4 8% 26% 25% 17%	1928
	21st to end	Night	22% 3 7% 22% 33% 12%	2090
October	1st to 10th	Night	19% 2 7% 18% 31% 23%	3757
	11th to 20th	Night	27% 3 9% 26% 27% 8%	3778
	21st to end	Night	25% 2 5% 26% 30% 11%	4259
November	1st to 10th	Night	22% 4 8% 27% 27% 12%	2535
	11th to 20th	Night	23% 3 5% 31% 30% 8%	2600
	21st to end	Night	26% 3 8% 26% 30% 8%	2715
December	1st to 10th	Night	20% 2 8% 37% 27% 6%	4570
	11th to 20th	Night	21% 3 5% 38% 29% 4	5182
	21st to end	Night	35% 3 7% 25% 25% 4	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	8% 3 11% 44% 28% 6%	300
	11th to 20th	Day	6% 8% 15% 47% 21% 3	300
	21st to end	Day	6% 3 21% 48% 18% 4	330
February	1st to 10th	Day	6% 7% 29% 42% 13% 3	300
	11th to 20th	Day	3 3 33% 46% 11% 3	300
	21st to end	Day	6% 6% 26% 45% 13% 3	249
March	1st to 10th	Day	10% 6% 20% 39% 19% 6%	300
	11th to 20th	Day	14% 9% 25% 32% 16% 5%	450
	21st to end	Day	12% 10% 29% 32% 15% 3	550
April	1st to 10th	Day	12% 10% 26% 31% 18% 2	500
	11th to 20th	Day	17% 14% 27% 27% 11% 3	492
	21st to end	Day	18% 9% 25% 29% 15% 4	500
May	1st to 10th	Day	15% 6% 24% 34% 18% 4	663
	11th to 20th	Day	19% 4 15% 30% 26% 6%	725
	21st to end	Day	11% 6% 19% 31% 25% 8%	789
June	1st to 10th	Day	6% 5% 13% 32% 30% 14%	730
	11th to 20th	Day	7% 3 20% 34% 26% 8%	730
	21st to end	Day	6% 4 18% 35% 28% 10%	730
July	1st to 10th	Day	7% 2 13% 27% 35% 15%	721
	11th to 20th	Day	4 1 11% 33% 35% 16%	718
	21st to end	Day	7% 2 12% 24% 40% 16%	739
August	1st to 10th	Day	7% 2 15% 32% 29% 15%	487
	11th to 20th	Day	3 2 16% 35% 30% 15%	477
	21st to end	Day	5% 2 17% 29% 32% 14%	539
September	1st to 10th	Day	7% 3 15% 28% 32% 15%	490
	11th to 20th	Day	5% 3 16% 29% 33% 13%	462
	21st to end	Day	9% 1 17% 25% 30% 19%	300
October	1st to 10th	Day	13% 4 11% 24% 30% 19%	300
	11th to 20th	Day	3 6% 17% 36% 25% 12%	300
	21st to end	Day	11% 5% 20% 33% 23% 8%	330
November	1st to 10th	Day	9% 3 18% 39% 21% 10%	300
	11th to 20th	Day	9% 4 15% 44% 23% 6%	300
	21st to end	Day	5% 5% 16% 46% 23% 5%	300
December	1st to 10th	Day	4 6% 17% 44% 21% 8%	300
	11th to 20th	Day	5% 6% 17% 52% 18% 2	300
	21st to end	Day	8% 5% 25% 43% 15% 4	330
January	1st to 10th	Night	6% 3 23% 52% 13% 3	1000
	11th to 20th	Night	4 5% 28% 50% 11% 1	1000
	21st to end	Night	7% 2 24% 56% 11% 1	890
February	1st to 10th	Night	7% 3 33% 47% 9% 1	800
	11th to 20th	Night	7% 4 30% 50% 9%	784
	21st to end	Night	8% 3 33% 50% 5%	660
March	1st to 10th	Night	7% 4 28% 51% 10%	800
	11th to 20th	Night	11% 3 23% 49% 14% 1	800
	21st to end	Night	11% 4 24% 50% 10%	719
April	1st to 10th	Night	9% 7% 29% 48% 7%	600
	11th to 20th	Night	16% 5% 27% 41% 10% 1	593
	21st to end	Night	19% 5% 21% 40% 15%	600
May	1st to 10th	Night	15% 6% 20% 42% 17% 1	600
	11th to 20th	Night	19% 2 13% 43% 20% 2	591
	21st to end	Night	10% 3 21% 43% 22% 1	650
June	1st to 10th	Night	7% 3 15% 47% 27% 2	600
	11th to 20th	Night	7% 3 19% 51% 20%	600
	21st to end	Night	6% 2 14% 51% 27% 1	600
July	1st to 10th	Night	7% 2 10% 42% 36% 3	585
	11th to 20th	Night	3 1 7% 49% 36% 2	582
	21st to end	Night	5% 1 7% 40% 45% 3	624
August	1st to 10th	Night	8% 1 8% 41% 40% 2	567
	11th to 20th	Night	5% 1 8% 49% 35% 2	564
	21st to end	Night	4 2 9% 46% 36% 3	627
September	1st to 10th	Night	7% 1 10% 50% 26% 6%	570
	11th to 20th	Night	8% 1 10% 49% 28% 4	570
	21st to end	Night	7% 2 15% 49% 25% 2	570
October	1st to 10th	Night	11% 1 15% 47% 24% 2	730
	11th to 20th	Night	7% 2 19% 50% 21% 1	760
	21st to end	Night	11% 3 19% 49% 16% 2	913
November	1st to 10th	Night	6% 3 20% 54% 16% 1	975
	11th to 20th	Night	7% 5% 24% 54% 10% 1	1000
	21st to end	Night	8% 4 21% 51% 15% 1	1000
December	1st to 10th	Night	7% 3 23% 54% 12% 1	1000
	11th to 20th	Night	6% 4 25% 53% 10% 1	1000
	21st to end	Night	8% 4 26% 52% 10% 1	1100

Page generated at 2025-07-17 23:10:04. Version ac07

Copyright © 2025 Attilla Danko.