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 2024-04-10.

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 4 4 3 4 5% 6% 8% 52%	83203
	Night	5% 6% 2 5% 4 3 4 4 5% 7% 54%	85671
Transparency	Day	29% 4 10% 27% 22% 8%	82908
	Night	29% 4 10% 27% 23% 7%	85489
Seeing	Day	9% 5% 18% 34% 25% 9%	14770
	Night	8% 3 20% 49% 18% 2	24967

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 5% 4 6% 5% 2 3 4 5% 7% 58%	2538
	11th to 20th	Day	3 7% 3 6% 6% 4 5% 4 6% 8% 48%	2546
	21st to end	Day	2 6% 2 5% 4 3 4 4 6% 8% 56%	3146
February	1st to 10th	Day	4 7% 4 6% 5% 3 5% 4 6% 7% 49%	1710
	11th to 20th	Day	2 6% 3 5% 4 3 4 5% 6% 9% 54%	1822
	21st to end	Day	3 7% 3 5% 5% 3 4 4 6% 7% 53%	1570
March	1st to 10th	Day	5% 8% 3 4 4 2 4 4 4 8% 56%	3130
	11th to 20th	Day	6% 6% 3 6% 5% 3 4 5% 5% 6% 50%	3416
	21st to end	Day	5% 7% 3 5% 5% 3 5% 6% 7% 10% 44%	4072
April	1st to 10th	Day	7% 6% 2 4 4 3 4 4 5% 8% 52%	2269
	11th to 20th	Day	11% 8% 2 5% 5% 3 4 4 5% 9% 44%	2131
	21st to end	Day	10% 8% 2 4 5% 3 5% 4 6% 9% 43%	2160
May	1st to 10th	Day	13% 6% 2 5% 5% 3 6% 5% 7% 8% 41%	2306
	11th to 20th	Day	15% 6% 2 4 5% 4 5% 5% 7% 8% 38%	2478
	21st to end	Day	8% 4 2 5% 6% 4 6% 6% 7% 10% 43%	2724
June	1st to 10th	Day	5% 4 2 3 4 2 6% 6% 7% 11% 50%	2520
	11th to 20th	Day	7% 3 2 3 4 3 5% 6% 7% 8% 52%	2520
	21st to end	Day	3 2 1 4 4 4 5% 6% 8% 9% 53%	2520
July	1st to 10th	Day	6% 3 2 4 4 3 5% 7% 8% 9% 49%	2520
	11th to 20th	Day	3 3 1 4 4 3 6% 6% 8% 9% 54%	2520
	21st to end	Day	5% 3 1 5% 5% 4 6% 6% 8% 12% 45%	2742
August	1st to 10th	Day	4 4 2 5% 5% 4 6% 7% 9% 11% 43%	2328
	11th to 20th	Day	4 3 1 3 4 3 5% 6% 7% 8% 56%	2130
	21st to end	Day	4 2 1 4 4 4 5% 5% 7% 9% 54%	2376
September	1st to 10th	Day	5% 4 1 2 4 2 3 5% 6% 7% 61%	2160
	11th to 20th	Day	6% 2 1 4 4 3 4 4 6% 8% 59%	2134
	21st to end	Day	7% 4 1 3 3 2 3 4 3 5% 65%	1980
October	1st to 10th	Day	8% 6% 2 3 3 2 3 3 4 7% 58%	1827
	11th to 20th	Day	5% 3 1 3 3 2 2 5% 4 6% 66%	1800
	21st to end	Day	7% 5% 2 3 4 2 2 3 5% 5% 61%	1888
November	1st to 10th	Day	3 5% 1 3 4 2 3 4 6% 8% 62%	1673
	11th to 20th	Day	3 7% 2 4 4 2 4 3 5% 7% 60%	1710
	21st to end	Day	3 6% 3 4 4 2 3 4 5% 8% 58%	1597
December	1st to 10th	Day	2 5% 3 5% 5% 3 3 5% 5% 9% 55%	1520
	11th to 20th	Day	3 6% 3 4 4 2 4 4 4 8% 57%	1926
	21st to end	Day	5% 8% 4 6% 5% 3 4 4 4 6% 50%	2794
January	1st to 10th	Night	2 8% 3 6% 5% 3 4 4 5% 8% 52%	4417
	11th to 20th	Night	1 6% 3 6% 5% 3 4 4 5% 7% 57%	4430
	21st to end	Night	3 7% 4 6% 6% 3 4 4 4 6% 53%	4544
February	1st to 10th	Night	3 9% 4 7% 4 3 3 4 4 7% 51%	2470
	11th to 20th	Night	4 7% 4 7% 5% 3 4 4 5% 7% 50%	2243
	21st to end	Night	3 9% 3 6% 5% 2 4 4 3 7% 55%	1884
March	1st to 10th	Night	4 10% 3 5% 4 2 3 4 5% 6% 54%	3734
	11th to 20th	Night	10% 9% 2 4 3 2 3 3 4 6% 51%	3557
	21st to end	Night	6% 8% 3 6% 6% 3 4 4 6% 8% 47%	3559
April	1st to 10th	Night	6% 7% 2 5% 5% 2 4 4 5% 6% 53%	1900
	11th to 20th	Night	12% 7% 3 5% 4 3 4 3 6% 7% 47%	1777
	21st to end	Night	14% 8% 3 4 5% 2 4 4 4 8% 44%	1800
May	1st to 10th	Night	14% 6% 2 5% 5% 3 5% 5% 7% 6% 43%	1651
	11th to 20th	Night	17% 6% 2 5% 5% 3 3 4 6% 7% 44%	1417
	21st to end	Night	8% 5% 1 4 4 3 6% 5% 6% 9% 49%	1552
June	1st to 10th	Night	6% 3 2 3 4 2 4 4 7% 11% 55%	1440
	11th to 20th	Night	6% 3 2 4 4 2 4 4 6% 8% 58%	1440
	21st to end	Night	5% 3 1 4 3 2 4 5% 7% 9% 57%	1440
July	1st to 10th	Night	8% 3 1 3 4 3 5% 5% 7% 10% 51%	1440
	11th to 20th	Night	2 2 1 3 4 3 4 5% 8% 9% 57%	1440
	21st to end	Night	3 4 2 5% 5% 4 5% 6% 7% 10% 49%	1576
August	1st to 10th	Night	5% 3 2 6% 5% 4 6% 6% 7% 10% 46%	1608
	11th to 20th	Night	4 3 1 4 3 2 4 4 8% 9% 56%	1769
	21st to end	Night	3 4 2 4 5% 3 5% 5% 5% 9% 56%	1980
September	1st to 10th	Night	5% 3 2 4 5% 3 4 4 5% 8% 57%	1800
	11th to 20th	Night	7% 2 1 3 3 2 4 4 5% 8% 62%	1826
	21st to end	Night	7% 2 1 3 3 2 3 4 5% 6% 64%	1980
October	1st to 10th	Night	9% 4 2 4 2 2 3 3 5% 8% 57%	2131
	11th to 20th	Night	5% 4 2 3 3 2 3 3 4 6% 65%	2160
	21st to end	Night	8% 7% 2 4 3 2 3 3 3 6% 60%	2448
November	1st to 10th	Night	4 4 2 3 3 2 3 4 4 7% 64%	2405
	11th to 20th	Night	5% 4 2 4 3 2 4 4 4 7% 62%	2470
	21st to end	Night	5% 7% 3 4 4 3 2 4 5% 6% 59%	2579
December	1st to 10th	Night	3 7% 3 4 5% 2 3 4 5% 7% 58%	2660
	11th to 20th	Night	3 6% 2 4 4 2 4 4 6% 10% 55%	3271
	21st to end	Night	5% 11% 3 6% 5% 3 4 4 4 7% 49%	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	27% 2 8% 35% 21% 7%	2539
	11th to 20th	Day	33% 4 12% 27% 18% 6%	2546
	21st to end	Day	27% 2 12% 33% 22% 4	3146
February	1st to 10th	Day	33% 4 10% 29% 18% 5%	1710
	11th to 20th	Day	27% 4 8% 31% 25% 5%	1832
	21st to end	Day	29% 4 7% 27% 24% 9%	1570
March	1st to 10th	Day	28% 3 7% 23% 32% 8%	3130
	11th to 20th	Day	34% 3 14% 31% 14% 3	3416
	21st to end	Day	33% 6% 11% 36% 12% 2	4072
April	1st to 10th	Day	29% 5% 10% 27% 20% 9%	2269
	11th to 20th	Day	37% 4 9% 24% 19% 8%	2134
	21st to end	Day	38% 4 8% 27% 21% 2	2160
May	1st to 10th	Day	40% 5% 13% 26% 15% 3	2306
	11th to 20th	Day	42% 5% 13% 25% 13% 2	2482
	21st to end	Day	34% 5% 17% 24% 14% 5%	2724
June	1st to 10th	Day	26% 5% 18% 26% 18% 8%	2520
	11th to 20th	Day	28% 5% 11% 19% 24% 14%	2520
	21st to end	Day	24% 4 8% 25% 25% 13%	2520
July	1st to 10th	Day	27% 5% 10% 24% 21% 12%	2451
	11th to 20th	Day	23% 6% 8% 26% 24% 13%	2445
	21st to end	Day	29% 5% 12% 28% 17% 8%	2590
August	1st to 10th	Day	31% 5% 15% 24% 16% 9%	2313
	11th to 20th	Day	23% 5% 10% 24% 25% 14%	2130
	21st to end	Day	25% 5% 7% 24% 27% 14%	2376
September	1st to 10th	Day	23% 3 5% 23% 26% 20%	2160
	11th to 20th	Day	23% 4 6% 21% 29% 18%	2134
	21st to end	Day	23% 3 4 23% 36% 11%	1980
October	1st to 10th	Day	27% 3 7% 23% 29% 11%	1827
	11th to 20th	Day	20% 3 8% 26% 30% 13%	1800
	21st to end	Day	25% 2 7% 28% 25% 13%	1888
November	1st to 10th	Day	21% 3 9% 29% 27% 12%	1673
	11th to 20th	Day	24% 4 5% 29% 32% 7%	1710
	21st to end	Day	25% 2 8% 24% 30% 10%	1597
December	1st to 10th	Day	26% 4 10% 32% 21% 7%	1520
	11th to 20th	Day	26% 4 8% 27% 31% 3	1924
	21st to end	Day	36% 3 7% 24% 24% 5%	2794
January	1st to 10th	Night	31% 3 10% 30% 21% 4	4417
	11th to 20th	Night	27% 5% 13% 25% 24% 7%	4430
	21st to end	Night	33% 3 12% 32% 17% 4	4544
February	1st to 10th	Night	33% 3 11% 28% 21% 4	2470
	11th to 20th	Night	34% 3 9% 28% 21% 5%	2243
	21st to end	Night	31% 2 5% 26% 25% 10%	1884
March	1st to 10th	Night	31% 4 6% 21% 33% 6%	3734
	11th to 20th	Night	35% 2 13% 24% 23% 3	3557
	21st to end	Night	35% 4 14% 31% 13% 3	3559
April	1st to 10th	Night	31% 4 8% 26% 21% 8%	1900
	11th to 20th	Night	37% 3 11% 25% 18% 6%	1777
	21st to end	Night	39% 3 8% 27% 19% 3	1800
May	1st to 10th	Night	40% 5% 13% 24% 16% 2	1651
	11th to 20th	Night	40% 3 18% 23% 14% 2	1418
	21st to end	Night	32% 6% 20% 25% 10% 7%	1552
June	1st to 10th	Night	23% 4 22% 30% 14% 7%	1440
	11th to 20th	Night	25% 3 13% 22% 23% 14%	1440
	21st to end	Night	22% 5% 10% 28% 28% 7%	1440
July	1st to 10th	Night	28% 6% 14% 25% 21% 6%	1400
	11th to 20th	Night	20% 6% 11% 30% 22% 12%	1392
	21st to end	Night	28% 6% 17% 29% 16% 5%	1488
August	1st to 10th	Night	30% 7% 18% 25% 13% 6%	1600
	11th to 20th	Night	22% 4 10% 29% 22% 12%	1769
	21st to end	Night	25% 4 9% 28% 23% 11%	1980
September	1st to 10th	Night	25% 4 9% 22% 22% 18%	1800
	11th to 20th	Night	20% 4 8% 26% 24% 17%	1826
	21st to end	Night	22% 3 7% 23% 34% 11%	1980
October	1st to 10th	Night	26% 4 11% 20% 28% 11%	2131
	11th to 20th	Night	21% 3 8% 24% 29% 14%	2160
	21st to end	Night	28% 2 7% 24% 27% 11%	2448
November	1st to 10th	Night	21% 4 9% 27% 27% 12%	2405
	11th to 20th	Night	23% 3 6% 31% 30% 7%	2470
	21st to end	Night	26% 3 8% 25% 30% 8%	2579
December	1st to 10th	Night	27% 3 11% 29% 20% 8%	2660
	11th to 20th	Night	25% 4 7% 32% 29% 3	3272
	21st to end	Night	36% 3 8% 26% 23% 4	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	7% 4 11% 43% 28% 6%	270
	11th to 20th	Day	7% 7% 14% 47% 21% 3	270
	21st to end	Day	6% 3 20% 49% 18% 5%	297
February	1st to 10th	Day	7% 6% 27% 43% 14% 3	270
	11th to 20th	Day	3 3 34% 44% 12% 4	270
	21st to end	Day	7% 6% 27% 45% 12% 3	225
March	1st to 10th	Day	9% 5% 20% 40% 19% 6%	270
	11th to 20th	Day	14% 7% 25% 33% 16% 5%	412
	21st to end	Day	12% 11% 29% 31% 15% 2	506
April	1st to 10th	Day	12% 10% 27% 31% 18% 3	457
	11th to 20th	Day	17% 13% 29% 26% 10% 4	412
	21st to end	Day	18% 10% 26% 27% 15% 5%	420
May	1st to 10th	Day	15% 5% 24% 35% 18% 3	567
	11th to 20th	Day	20% 4 14% 28% 26% 7%	625
	21st to end	Day	10% 6% 19% 30% 26% 9%	679
June	1st to 10th	Day	6% 5% 13% 31% 30% 15%	630
	11th to 20th	Day	8% 2 21% 36% 25% 8%	630
	21st to end	Day	5% 4 16% 35% 30% 10%	630
July	1st to 10th	Day	8% 1 12% 26% 37% 15%	621
	11th to 20th	Day	4 1 10% 32% 36% 16%	618
	21st to end	Day	7% 2 11% 23% 39% 17%	684
August	1st to 10th	Day	6% 2 15% 32% 29% 15%	447
	11th to 20th	Day	3 2 16% 33% 30% 15%	437
	21st to end	Day	5% 2 18% 28% 33% 15%	495
September	1st to 10th	Day	7% 3 16% 29% 32% 13%	450
	11th to 20th	Day	5% 3 15% 30% 34% 13%	424
	21st to end	Day	9% 1 17% 26% 29% 19%	270
October	1st to 10th	Day	14% 4 12% 24% 27% 19%	270
	11th to 20th	Day	3 7% 17% 37% 24% 12%	270
	21st to end	Day	11% 5% 21% 32% 23% 8%	297
November	1st to 10th	Day	7% 3 17% 40% 22% 10%	270
	11th to 20th	Day	10% 5% 15% 43% 22% 6%	270
	21st to end	Day	6% 4 16% 47% 22% 5%	270
December	1st to 10th	Day	4 6% 17% 46% 19% 7%	270
	11th to 20th	Day	5% 5% 17% 51% 19% 3	270
	21st to end	Day	7% 5% 26% 41% 16% 4	297
January	1st to 10th	Night	6% 3 23% 52% 13% 3	950
	11th to 20th	Night	5% 5% 28% 50% 12% 1	950
	21st to end	Night	7% 2 23% 55% 12% 1	846
February	1st to 10th	Night	8% 3 31% 49% 9% 1	760
	11th to 20th	Night	6% 4 30% 50% 10%	744
	21st to end	Night	8% 3 33% 50% 5%	628
March	1st to 10th	Night	7% 4 28% 51% 10%	760
	11th to 20th	Night	11% 3 22% 50% 14% 1	760
	21st to end	Night	10% 4 25% 50% 11%	683
April	1st to 10th	Night	9% 7% 28% 48% 7%	570
	11th to 20th	Night	15% 5% 28% 41% 10% 1	533
	21st to end	Night	19% 5% 20% 40% 17%	540
May	1st to 10th	Night	16% 6% 18% 43% 17% 1	540
	11th to 20th	Night	20% 2 13% 41% 21% 2	531
	21st to end	Night	10% 3 20% 42% 24% 1	584
June	1st to 10th	Night	7% 2 14% 46% 28% 2	540
	11th to 20th	Night	7% 3 18% 51% 21%	540
	21st to end	Night	7% 1 12% 50% 29% 1	540
July	1st to 10th	Night	8% 1 9% 41% 38% 3	525
	11th to 20th	Night	3 1 7% 48% 38% 3	522
	21st to end	Night	5% 1 6% 39% 46% 3	591
August	1st to 10th	Night	7% 1 8% 41% 41% 2	537
	11th to 20th	Night	5% 1 7% 49% 36% 2	534
	21st to end	Night	4 2 8% 45% 38% 4	594
September	1st to 10th	Night	7% 1 10% 50% 26% 6%	540
	11th to 20th	Night	9% 1 9% 48% 29% 4	540
	21st to end	Night	6% 2 15% 50% 25% 2	540
October	1st to 10th	Night	11% 1 15% 46% 24% 2	691
	11th to 20th	Night	7% 2 18% 50% 20% 1	720
	21st to end	Night	12% 3 18% 49% 16% 2	864
November	1st to 10th	Night	5% 2 20% 55% 16% 1	925
	11th to 20th	Night	7% 5% 23% 54% 10% 1	950
	21st to end	Night	8% 4 20% 51% 15% 1	950
December	1st to 10th	Night	7% 3 24% 54% 11% 1	950
	11th to 20th	Night	7% 4 23% 54% 11% 1	950
	21st to end	Night	8% 4 26% 51% 10% 1	1045

Page generated at 2024-04-07 07:28:09. Version ac07

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