Rolling Roof Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rolling Roof Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	4 3 1 3 2 3 3 3 4 6% 69%	139296
	Night	4 3 1 3 3 3 3 3 4 6% 67%	139051
Transparency	Day	12% 36% 4 11% 19% 19%	134383
	Night	13% 35% 4 11% 18% 19%	134937
Seeing	Day	6% 8% 26% 35% 19% 7%	17280
	Night	6% 3 14% 35% 35% 8%	27146

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	8% 3 1 3 3 3 3 2 4 5% 65%	4761
	11th to 20th	Day	4 2 1 3 2 2 2 2 4 7% 71%	5340
	21st to end	Day	7% 4 2 5% 3 3 4 3 6% 9% 55%	6197
February	1st to 10th	Day	9% 5% 2 4 2 2 3 3 5% 5% 60%	2100
	11th to 20th	Day	8% 3 1 3 3 2 3 3 5% 7% 60%	2001
	21st to end	Day	7% 4 2 5% 3 4 4 3 4 7% 58%	1650
March	1st to 10th	Day	10% 5% 2 5% 3 3 3 3 5% 6% 54%	4518
	11th to 20th	Day	7% 4 2 5% 3 3 3 3 4 7% 59%	4780
	21st to end	Day	7% 4 2 5% 4 5% 5% 4 6% 9% 50%	5942
April	1st to 10th	Day	6% 4 1 4 3 2 2 3 6% 8% 61%	2400
	11th to 20th	Day	5% 3 1 4 3 3 3 2 4 7% 64%	2369
	21st to end	Day	4 5% 1 3 2 2 2 2 4 7% 67%	2400
May	1st to 10th	Day	4 5% 2 4 2 2 3 3 4 7% 64%	5440
	11th to 20th	Day	2 4 2 4 3 3 4 3 6% 9% 61%	5841
	21st to end	Day	2 3 1 4 3 4 3 4 6% 8% 63%	6423
June	1st to 10th	Day	3 5% 1 5% 3 3 3 3 6% 9% 59%	2735
	11th to 20th	Day	2 3 1 3 2 3 3 3 4 9% 67%	2800
	21st to end	Day	1 3 1 2 2 2 2 2 4 6% 76%	2800
July	1st to 10th	Day	1 1 1 1 1 1 2 4 86%	6319
	11th to 20th	Day	1 1 1 1 2 2 2 2 3 5% 80%	6198
	21st to end	Day	1 1 2 1 2 2 2 3 5% 82%	6080
August	1st to 10th	Day	1 1 2 1 1 2 2 2 5% 83%	5346
	11th to 20th	Day	1 1 1 1 1 1 2 4 87%	5364
	21st to end	Day	1 1 1 1 2 1 3 4 86%	5984
September	1st to 10th	Day	1 2 1 2 1 2 2 2 3 6% 79%	2400
	11th to 20th	Day	2 2 1 2 2 2 3 1 3 5% 78%	2295
	21st to end	Day	2 2 1 2 2 2 2 2 3 5% 78%	2200
October	1st to 10th	Day	2 1 2 1 1 1 2 3 4 82%	3583
	11th to 20th	Day	2 3 1 4 3 3 2 2 4 5% 70%	3261
	21st to end	Day	2 2 1 3 2 3 4 3 5% 5% 70%	3558
November	1st to 10th	Day	3 2 1 3 3 2 3 2 4 6% 72%	1865
	11th to 20th	Day	6% 3 1 3 3 3 2 3 4 5% 68%	1890
	21st to end	Day	4 3 2 4 3 3 4 3 5% 7% 62%	1890
December	1st to 10th	Day	4 3 1 3 3 4 4 3 5% 6% 65%	3040
	11th to 20th	Day	9% 5% 2 4 3 3 4 4 5% 7% 53%	3434
	21st to end	Day	9% 5% 2 5% 5% 4 4 4 6% 9% 48%	4092
January	1st to 10th	Night	8% 4 2 4 3 3 3 2 3 5% 61%	8132
	11th to 20th	Night	4 3 1 3 2 2 3 2 4 6% 69%	7650
	21st to end	Night	8% 7% 2 5% 3 3 4 3 5% 6% 54%	8018
February	1st to 10th	Night	8% 6% 2 4 3 2 3 2 5% 5% 61%	2520
	11th to 20th	Night	9% 4 1 4 3 3 3 3 4 6% 60%	2398
	21st to end	Night	10% 5% 1 4 3 3 3 3 4 5% 59%	1980
March	1st to 10th	Night	7% 7% 2 5% 3 3 4 3 6% 7% 53%	5376
	11th to 20th	Night	5% 6% 2 5% 4 4 3 3 5% 5% 58%	5203
	21st to end	Night	9% 5% 2 5% 4 3 3 3 5% 6% 56%	5033
April	1st to 10th	Night	6% 5% 2 4 3 3 3 2 5% 6% 62%	2000
	11th to 20th	Night	4 4 2 3 2 3 3 3 4 6% 67%	1979
	21st to end	Night	3 5% 1 3 3 3 3 2 5% 8% 66%	2000
May	1st to 10th	Night	2 4 1 4 4 5% 4 3 5% 5% 63%	4544
	11th to 20th	Night	1 3 1 4 3 3 3 4 5% 7% 65%	4074
	21st to end	Night	1 2 3 2 3 3 3 4 6% 74%	4474
June	1st to 10th	Night	2 4 2 3 3 3 4 3 5% 8% 65%	1658
	11th to 20th	Night	1 2 1 2 2 2 2 3 4 7% 74%	1600
	21st to end	Night	1 3 1 2 2 2 1 2 4 5% 78%	1600
July	1st to 10th	Night	1 1 1 1 1 2 3 4 86%	3640
	11th to 20th	Night	1 1 2 2 2 2 4 5% 79%	3749
	21st to end	Night	1 1 2 2 2 2 2 3 5% 80%	4791
August	1st to 10th	Night	1 1 1 1 2 2 2 4 5% 81%	4486
	11th to 20th	Night	1 1 1 1 2 3 2 3 4 83%	4506
	21st to end	Night	1 2 1 1 3 2 3 3 84%	5015
September	1st to 10th	Night	2 3 1 3 2 2 2 2 3 5% 77%	2000
	11th to 20th	Night	3 2 1 3 2 2 2 1 3 4 78%	2105
	21st to end	Night	2 1 1 2 2 2 2 2 4 5% 79%	2200
October	1st to 10th	Night	2 1 1 2 2 1 2 2 2 3 83%	3600
	11th to 20th	Night	2 2 1 5% 2 3 4 3 5% 6% 66%	3897
	21st to end	Night	2 3 1 2 2 2 2 2 3 4 77%	4253
November	1st to 10th	Night	3 3 1 3 2 2 3 2 4 6% 71%	2651
	11th to 20th	Night	4 3 2 4 2 2 2 3 4 5% 69%	2730
	21st to end	Night	5% 3 1 4 4 3 3 3 5% 6% 62%	2730
December	1st to 10th	Night	4 3 1 3 2 3 3 3 4 5% 67%	4370
	11th to 20th	Night	11% 4 2 4 3 3 3 4 6% 8% 52%	4983
	21st to end	Night	9% 6% 2 6% 4 5% 6% 4 7% 9% 44%	7106

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	18% 20% 6% 14% 24% 18%	4762
	11th to 20th	Day	12% 21% 5% 11% 15% 36%	5340
	21st to end	Day	21% 22% 2 13% 23% 20%	6197
February	1st to 10th	Day	13% 59% 2 7% 10% 9%	2100
	11th to 20th	Day	11% 58% 2 6% 12% 10%	2011
	21st to end	Day	10% 62% 3 8% 9% 9%	1650
March	1st to 10th	Day	23% 29% 6% 13% 16% 14%	4518
	11th to 20th	Day	19% 29% 5% 24% 15% 8%	4780
	21st to end	Day	21% 30% 10% 23% 14% 3	5942
April	1st to 10th	Day	8% 63% 4 9% 12% 5%	2400
	11th to 20th	Day	8% 62% 2 7% 13% 7%	2372
	21st to end	Day	8% 62% 5% 10% 11% 4	2400
May	1st to 10th	Day	17% 28% 7% 13% 20% 14%	5440
	11th to 20th	Day	17% 28% 8% 17% 24% 6%	5844
	21st to end	Day	13% 29% 5% 24% 23% 6%	6424
June	1st to 10th	Day	9% 61% 4 10% 12% 5%	2735
	11th to 20th	Day	7% 61% 2 7% 13% 11%	2800
	21st to end	Day	5% 60% 2 4 13% 16%	2800
July	1st to 10th	Day	3 24% 1 4 13% 54%	6251
	11th to 20th	Day	6% 24% 1 6% 31% 32%	6123
	21st to end	Day	5% 24% 1 11% 34% 25%	5906
August	1st to 10th	Day	4 34% 1 6% 32% 23%	3890
	11th to 20th	Day	2 34% 1 4 22% 37%	3880
	21st to end	Day	2 34% 1 6% 22% 34%	4311
September	1st to 10th	Day	6% 56% 2 4 15% 17%	2400
	11th to 20th	Day	9% 56% 2 6% 16% 12%	2295
	21st to end	Day	5% 56% 3 7% 16% 13%	2200
October	1st to 10th	Day	3 34% 1 5% 11% 46%	3583
	11th to 20th	Day	13% 34% 1 8% 25% 18%	3261
	21st to end	Day	12% 34% 1 4 27% 21%	3558
November	1st to 10th	Day	4 52% 2 6% 13% 22%	1865
	11th to 20th	Day	13% 54% 3 6% 8% 16%	1890
	21st to end	Day	12% 54% 3 7% 9% 14%	1890
December	1st to 10th	Day	13% 34% 1 7% 21% 25%	3040
	11th to 20th	Day	22% 29% 4 10% 18% 17%	3433
	21st to end	Day	25% 27% 7% 16% 19% 6%	4092
January	1st to 10th	Night	22% 20% 5% 10% 26% 18%	8132
	11th to 20th	Night	14% 21% 3 12% 13% 37%	7650
	21st to end	Night	25% 21% 2 9% 21% 21%	8018
February	1st to 10th	Night	13% 58% 3 6% 10% 9%	2520
	11th to 20th	Night	11% 58% 3 6% 9% 12%	2399
	21st to end	Night	10% 61% 2 6% 11% 9%	1980
March	1st to 10th	Night	20% 32% 6% 12% 21% 10%	5376
	11th to 20th	Night	19% 30% 7% 22% 12% 10%	5203
	21st to end	Night	21% 29% 8% 21% 18% 2	5033
April	1st to 10th	Night	9% 62% 3 10% 11% 6%	2000
	11th to 20th	Night	8% 61% 3 7% 14% 7%	1979
	21st to end	Night	6% 62% 5% 11% 12% 5%	2000
May	1st to 10th	Night	14% 29% 10% 13% 20% 13%	4541
	11th to 20th	Night	12% 29% 11% 19% 23% 7%	4075
	21st to end	Night	6% 31% 5% 25% 26% 7%	4474
June	1st to 10th	Night	7% 62% 6% 10% 12% 4	1658
	11th to 20th	Night	3 61% 4 7% 14% 12%	1600
	21st to end	Night	6% 59% 2 6% 12% 15%	1600
July	1st to 10th	Night	2 24% 1 5% 14% 54%	3600
	11th to 20th	Night	5% 24% 2 6% 32% 32%	3701
	21st to end	Night	6% 24% 2 12% 32% 24%	4653
August	1st to 10th	Night	5% 34% 1 10% 27% 23%	3256
	11th to 20th	Night	2 35% 1 5% 23% 34%	3256
	21st to end	Night	3 34% 1 5% 24% 32%	3608
September	1st to 10th	Night	6% 56% 3 4 15% 16%	2000
	11th to 20th	Night	7% 56% 3 5% 16% 13%	2105
	21st to end	Night	5% 56% 3 4 17% 15%	2200
October	1st to 10th	Night	3 34% 1 4 11% 46%	3600
	11th to 20th	Night	12% 35% 1 11% 20% 21%	3897
	21st to end	Night	8% 34% 1 5% 24% 28%	4253
November	1st to 10th	Night	4 52% 2 6% 13% 22%	2651
	11th to 20th	Night	11% 53% 4 6% 10% 16%	2730
	21st to end	Night	12% 54% 3 8% 10% 13%	2730
December	1st to 10th	Night	11% 33% 2 9% 17% 28%	4370
	11th to 20th	Night	22% 30% 5% 9% 14% 21%	4983
	21st to end	Night	28% 27% 7% 19% 15% 5%	7106

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% 11% 33% 30% 9%	233
	11th to 20th	Day	14% 14% 36% 27% 9%	430
	21st to end	Day	13% 15% 39% 24% 8% 2	537
February	1st to 10th	Day	13% 13% 37% 25% 10% 2	540
	11th to 20th	Day	11% 15% 36% 29% 7% 2	516
	21st to end	Day	11% 15% 40% 26% 7% 1	414
March	1st to 10th	Day	13% 19% 34% 24% 9% 1	500
	11th to 20th	Day	14% 14% 35% 26% 9% 2	500
	21st to end	Day	11% 16% 35% 28% 8% 2	550
April	1st to 10th	Day	9% 12% 33% 30% 13% 4	500
	11th to 20th	Day	7% 15% 33% 30% 11% 4	492
	21st to end	Day	5% 13% 31% 32% 12% 8%	500
May	1st to 10th	Day	9% 9% 30% 36% 12% 4	503
	11th to 20th	Day	5% 10% 27% 35% 19% 4	528
	21st to end	Day	6% 8% 29% 34% 18% 5%	573
June	1st to 10th	Day	5% 5% 22% 35% 26% 7%	530
	11th to 20th	Day	3 4 19% 40% 26% 8%	530
	21st to end	Day	3 2 16% 39% 30% 11%	530
July	1st to 10th	Day	1 9% 46% 33% 11%	526
	11th to 20th	Day	2 1 6% 38% 34% 20%	524
	21st to end	Day	1 7% 42% 31% 19%	575
August	1st to 10th	Day	2 1 10% 46% 28% 14%	527
	11th to 20th	Day	2 1 7% 44% 31% 16%	517
	21st to end	Day	1 1 13% 45% 27% 13%	583
September	1st to 10th	Day	2 2 18% 43% 25% 11%	530
	11th to 20th	Day	3 4 24% 42% 20% 6%	530
	21st to end	Day	2 5% 22% 40% 23% 8%	530
October	1st to 10th	Day	2 6% 28% 38% 21% 5%	530
	11th to 20th	Day	4 5% 30% 40% 18% 3	530
	21st to end	Day	4 7% 33% 32% 19% 6%	583
November	1st to 10th	Day	5% 5% 23% 35% 23% 8%	341
	11th to 20th	Day	8% 11% 29% 35% 14% 3	330
	21st to end	Day	9% 10% 33% 30% 16% 2	330
December	1st to 10th	Day	10% 8% 32% 32% 15% 2	330
	11th to 20th	Day	5% 10% 29% 34% 20% 2	316
	21st to end	Day	15% 14% 30% 29% 10% 1	242
January	1st to 10th	Night	11% 5% 24% 40% 16% 4	840
	11th to 20th	Night	8% 6% 28% 40% 16% 2	840
	21st to end	Night	13% 6% 26% 37% 15% 2	924
February	1st to 10th	Night	11% 6% 21% 38% 20% 4	840
	11th to 20th	Night	9% 4 21% 42% 19% 5%	804
	21st to end	Night	12% 6% 23% 38% 19% 2	660
March	1st to 10th	Night	10% 5% 23% 43% 18% 2	800
	11th to 20th	Night	8% 4 16% 41% 25% 5%	800
	21st to end	Night	8% 5% 17% 44% 23% 3	880
April	1st to 10th	Night	6% 4 15% 43% 29% 4	800
	11th to 20th	Night	4 4 17% 40% 31% 4	791
	21st to end	Night	5% 4 11% 41% 30% 9%	800
May	1st to 10th	Night	6% 2 10% 41% 35% 5%	797
	11th to 20th	Night	3 2 11% 38% 41% 5%	591
	21st to end	Night	2 2 12% 35% 42% 7%	650
June	1st to 10th	Night	2 1 5% 33% 48% 10%	458
	11th to 20th	Night	1 1 1 30% 57% 10%	400
	21st to end	Night	2 2 26% 60% 11%	400
July	1st to 10th	Night	2 1 11% 71% 16%	390
	11th to 20th	Night	1 1 7% 63% 27%	432
	21st to end	Night	2 1 1 11% 62% 24%	797
August	1st to 10th	Night	2 1 1 19% 61% 17%	796
	11th to 20th	Night	1 1 1 14% 62% 22%	792
	21st to end	Night	1 1 18% 59% 20%	880
September	1st to 10th	Night	2 1 3 22% 55% 16%	800
	11th to 20th	Night	2 1 3 29% 50% 14%	800
	21st to end	Night	2 4 34% 48% 12%	800
October	1st to 10th	Night	4 7% 43% 42% 5%	800
	11th to 20th	Night	4 2 11% 39% 37% 7%	800
	21st to end	Night	4 2 15% 39% 30% 10%	880
November	1st to 10th	Night	4 3 14% 42% 30% 7%	820
	11th to 20th	Night	6% 2 18% 42% 26% 5%	840
	21st to end	Night	6% 4 25% 38% 23% 4	840
December	1st to 10th	Night	8% 5% 24% 38% 19% 5%	840
	11th to 20th	Night	12% 5% 22% 40% 19% 2	840
	21st to end	Night	11% 7% 23% 39% 18% 3	924

Page generated at 2026-02-12 22:42:12. Version ac07

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