Holmes Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Holmes Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	7% 4 1 4 3 3 3 3 5% 7% 60%	135879
	Night	8% 5% 2 5% 3 4 4 3 5% 6% 55%	133598
Transparency	Day	25% 4 9% 20% 25% 16%	130824
	Night	30% 5% 10% 18% 21% 15%	129639
Seeing	Day	9% 9% 22% 32% 22% 6%	17035
	Night	10% 5% 15% 33% 31% 7%	26509

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	13% 6% 2 5% 3 3 3 3 6% 7% 49%	4029
	11th to 20th	Day	12% 5% 2 6% 4 4 3 2 3 5% 54%	4190
	21st to end	Day	14% 8% 2 7% 5% 4 5% 4 5% 6% 41%	4609
February	1st to 10th	Day	21% 6% 2 4 3 3 3 3 5% 6% 43%	1801
	11th to 20th	Day	14% 6% 3 5% 5% 4 4 4 6% 7% 43%	1864
	21st to end	Day	14% 7% 3 6% 4 4 4 3 5% 5% 44%	1650
March	1st to 10th	Day	13% 10% 3 9% 7% 6% 6% 5% 5% 6% 31%	4868
	11th to 20th	Day	12% 7% 2 5% 4 4 4 3 5% 7% 46%	5000
	21st to end	Day	14% 8% 3 8% 6% 4 6% 4 6% 9% 33%	5569
April	1st to 10th	Day	13% 7% 2 5% 4 4 3 3 5% 7% 46%	2400
	11th to 20th	Day	9% 5% 2 5% 3 3 3 4 5% 8% 52%	2369
	21st to end	Day	6% 5% 2 5% 4 4 3 4 5% 7% 55%	2400
May	1st to 10th	Day	5% 3 1 2 2 2 2 2 4 6% 72%	5528
	11th to 20th	Day	2 2 1 4 2 2 3 2 5% 8% 68%	5917
	21st to end	Day	2 4 1 3 2 2 2 3 4 6% 71%	6902
June	1st to 10th	Day	4 4 2 4 3 3 3 4 5% 8% 60%	2800
	11th to 20th	Day	2 2 1 2 3 2 2 3 5% 7% 71%	2800
	21st to end	Day	2 2 1 2 3 2 2 3 3 7% 74%	2800
July	1st to 10th	Day	1 1 2 2 2 2 3 6% 81%	6275
	11th to 20th	Day	1 1 2 2 3 2 2 4 10% 73%	5880
	21st to end	Day	1 1 3 3 3 3 2 5% 9% 70%	6402
August	1st to 10th	Day	1 1 1 2 2 2 4 3 4 8% 73%	5777
	11th to 20th	Day	1 2 2 2 2 2 3 6% 81%	5457
	21st to end	Day	1 1 2 2 3 2 2 4 8% 75%	5984
September	1st to 10th	Day	2 2 1 2 2 2 2 2 3 6% 77%	2400
	11th to 20th	Day	3 3 1 5% 2 3 3 3 5% 8% 62%	2400
	21st to end	Day	4 2 1 2 2 3 2 2 4 5% 73%	2124
October	1st to 10th	Day	2 3 1 3 2 2 3 2 4 7% 71%	3280
	11th to 20th	Day	3 3 1 3 2 3 3 3 5% 7% 67%	3261
	21st to end	Day	4 4 1 3 3 3 4 4 5% 8% 61%	3558
November	1st to 10th	Day	8% 5% 2 4 3 3 4 3 5% 7% 56%	2036
	11th to 20th	Day	10% 7% 3 4 3 4 4 3 5% 8% 50%	1890
	21st to end	Day	12% 4 2 3 3 3 3 3 4 8% 55%	1759
December	1st to 10th	Day	8% 4 2 5% 4 3 3 3 6% 9% 53%	2700
	11th to 20th	Day	22% 8% 2 5% 4 4 4 3 5% 6% 36%	3108
	21st to end	Day	23% 11% 3 6% 4 4 4 3 5% 6% 31%	4092
January	1st to 10th	Night	10% 6% 2 5% 3 3 3 3 5% 8% 51%	6134
	11th to 20th	Night	12% 6% 2 5% 3 4 4 3 5% 5% 50%	6010
	21st to end	Night	14% 8% 2 7% 4 4 4 3 5% 6% 43%	6611
February	1st to 10th	Night	18% 7% 2 5% 3 3 3 3 4 5% 47%	2600
	11th to 20th	Night	13% 6% 2 5% 4 3 4 5% 6% 8% 44%	2422
	21st to end	Night	11% 7% 2 4 3 3 3 3 5% 7% 50%	1980
March	1st to 10th	Night	16% 10% 3 8% 6% 5% 6% 4 5% 5% 33%	5013
	11th to 20th	Night	14% 6% 2 5% 3 4 3 2 5% 6% 49%	5000
	21st to end	Night	17% 10% 2 6% 4 4 4 3 5% 7% 39%	5404
April	1st to 10th	Night	11% 6% 1 4 4 4 5% 4 5% 7% 48%	2000
	11th to 20th	Night	10% 6% 2 5% 4 4 4 3 5% 7% 51%	1979
	21st to end	Night	5% 4 1 4 3 3 4 3 6% 10% 56%	2000
May	1st to 10th	Night	2 3 1 5% 3 3 3 2 4 5% 68%	4450
	11th to 20th	Night	1 2 2 8% 4 5% 5% 4 6% 7% 54%	3987
	21st to end	Night	1 2 1 3 3 3 4 4 5% 6% 68%	3972
June	1st to 10th	Night	3 2 1 4 3 3 6% 4 7% 8% 58%	1600
	11th to 20th	Night	1 2 1 3 3 3 5% 4 6% 6% 65%	1600
	21st to end	Night	2 3 1 2 3 4 6% 4 4 5% 68%	1600
July	1st to 10th	Night	3 3 3 4 3 3 5% 76%	3680
	11th to 20th	Night	1 1 4 4 4 5% 4 5% 7% 63%	4100
	21st to end	Night	2 2 7% 4 5% 7% 5% 6% 8% 54%	4472
August	1st to 10th	Night	1 2 1 3 4 4 5% 3 6% 7% 65%	4035
	11th to 20th	Night	1 2 2 2 4 4 4 6% 74%	4405
	21st to end	Night	1 1 1 3 3 3 4 3 4 6% 72%	5015
September	1st to 10th	Night	1 1 2 2 3 3 3 5% 7% 72%	2000
	11th to 20th	Night	3 2 1 6% 3 4 4 3 5% 8% 61%	2000
	21st to end	Night	3 4 1 3 2 3 2 2 4 6% 71%	2270
October	1st to 10th	Night	2 2 1 4 3 3 2 2 4 5% 72%	3920
	11th to 20th	Night	3 4 1 4 3 3 4 3 5% 6% 65%	3897
	21st to end	Night	5% 4 1 4 2 2 3 3 4 6% 65%	4253
November	1st to 10th	Night	7% 5% 2 4 3 3 3 3 5% 7% 59%	2481
	11th to 20th	Night	10% 5% 2 4 3 3 4 4 6% 7% 51%	2730
	21st to end	Night	9% 5% 2 4 3 3 3 3 4 6% 56%	2856
December	1st to 10th	Night	9% 4 1 4 3 3 4 4 5% 6% 58%	4700
	11th to 20th	Night	21% 8% 2 6% 4 4 4 4 7% 8% 32%	5316
	21st to end	Night	25% 10% 3 6% 4 4 4 3 5% 5% 31%	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	31% 4 19% 25% 17% 5%	4029
	11th to 20th	Day	33% 4 8% 15% 18% 21%	4190
	21st to end	Day	42% 4 7% 14% 18% 14%	4609
February	1st to 10th	Day	41% 3 7% 20% 18% 11%	1801
	11th to 20th	Day	38% 5% 11% 20% 19% 8%	1873
	21st to end	Day	38% 5% 8% 23% 18% 8%	1650
March	1st to 10th	Day	44% 7% 8% 19% 19% 2	4868
	11th to 20th	Day	33% 4 16% 29% 10% 8%	5000
	21st to end	Day	42% 8% 16% 20% 11% 2	5569
April	1st to 10th	Day	34% 5% 15% 21% 21% 4	2400
	11th to 20th	Day	29% 6% 14% 25% 20% 6%	2372
	21st to end	Day	28% 5% 13% 28% 21% 4	2400
May	1st to 10th	Day	16% 4 8% 29% 29% 14%	5528
	11th to 20th	Day	20% 4 10% 33% 28% 6%	5920
	21st to end	Day	18% 4 9% 31% 36% 3	6904
June	1st to 10th	Day	25% 6% 12% 26% 25% 6%	2800
	11th to 20th	Day	18% 4 10% 23% 32% 14%	2800
	21st to end	Day	18% 3 4 17% 32% 25%	2800
July	1st to 10th	Day	11% 3 6% 8% 23% 50%	6208
	11th to 20th	Day	15% 3 9% 6% 19% 48%	5811
	21st to end	Day	15% 4 5% 20% 31% 26%	6217
August	1st to 10th	Day	18% 3 6% 14% 29% 29%	4206
	11th to 20th	Day	11% 3 3 12% 46% 25%	3950
	21st to end	Day	16% 3 4 13% 35% 28%	4311
September	1st to 10th	Day	15% 3 3 15% 28% 36%	2400
	11th to 20th	Day	23% 3 8% 23% 30% 14%	2400
	21st to end	Day	16% 2 6% 21% 36% 18%	2124
October	1st to 10th	Day	15% 2 6% 13% 30% 34%	3280
	11th to 20th	Day	17% 4 8% 25% 31% 15%	3261
	21st to end	Day	21% 2 7% 24% 30% 16%	3558
November	1st to 10th	Day	28% 4 9% 25% 25% 10%	2036
	11th to 20th	Day	32% 4 11% 23% 22% 8%	1890
	21st to end	Day	29% 2 10% 19% 28% 11%	1759
December	1st to 10th	Day	28% 3 7% 23% 32% 8%	2700
	11th to 20th	Day	46% 4 11% 17% 14% 8%	3108
	21st to end	Day	51% 4 11% 20% 11% 3	4092
January	1st to 10th	Night	29% 4 20% 26% 16% 5%	6134
	11th to 20th	Night	33% 5% 10% 15% 22% 15%	6010
	21st to end	Night	40% 4 9% 13% 17% 17%	6611
February	1st to 10th	Night	39% 3 8% 17% 19% 14%	2600
	11th to 20th	Night	36% 5% 11% 20% 21% 7%	2424
	21st to end	Night	31% 4 12% 28% 17% 7%	1980
March	1st to 10th	Night	46% 5% 11% 18% 17% 3	5013
	11th to 20th	Night	34% 6% 11% 24% 16% 8%	5000
	21st to end	Night	42% 7% 13% 23% 13% 2	5404
April	1st to 10th	Night	33% 5% 15% 24% 18% 6%	2000
	11th to 20th	Night	32% 7% 11% 27% 17% 7%	1979
	21st to end	Night	25% 8% 12% 29% 20% 6%	2000
May	1st to 10th	Night	20% 4 11% 22% 28% 14%	4447
	11th to 20th	Night	32% 10% 8% 23% 21% 7%	3988
	21st to end	Night	17% 11% 10% 19% 40% 3	3972
June	1st to 10th	Night	25% 10% 10% 24% 23% 8%	1600
	11th to 20th	Night	22% 5% 8% 15% 30% 20%	1600
	21st to end	Night	24% 6% 4 13% 27% 27%	1600
July	1st to 10th	Night	16% 4 11% 6% 17% 45%	3640
	11th to 20th	Night	23% 5% 14% 6% 14% 38%	4046
	21st to end	Night	26% 8% 8% 15% 22% 21%	4345
August	1st to 10th	Night	26% 6% 10% 12% 24% 23%	2929
	11th to 20th	Night	16% 5% 8% 8% 35% 29%	3180
	21st to end	Night	20% 6% 8% 10% 35% 23%	3608
September	1st to 10th	Night	17% 5% 4 12% 27% 35%	2000
	11th to 20th	Night	25% 5% 5% 21% 29% 15%	2000
	21st to end	Night	19% 3 4 18% 34% 21%	2270
October	1st to 10th	Night	17% 3 6% 11% 21% 41%	3920
	11th to 20th	Night	20% 7% 7% 25% 23% 19%	3897
	21st to end	Night	22% 3 6% 22% 26% 22%	4253
November	1st to 10th	Night	26% 3 9% 26% 23% 13%	2481
	11th to 20th	Night	31% 4 11% 22% 23% 9%	2730
	21st to end	Night	28% 4 8% 21% 23% 15%	2856
December	1st to 10th	Night	27% 2 7% 24% 25% 15%	4700
	11th to 20th	Night	45% 5% 11% 16% 17% 7%	5316
	21st to end	Night	53% 4 11% 18% 10% 4	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	28% 12% 22% 21% 14% 2	361
	11th to 20th	Day	16% 10% 22% 29% 20% 4	430
	21st to end	Day	14% 9% 25% 30% 18% 4	473
February	1st to 10th	Day	23% 14% 20% 22% 16% 5%	401
	11th to 20th	Day	18% 16% 26% 26% 12% 2	450
	21st to end	Day	11% 14% 29% 30% 14% 2	414
March	1st to 10th	Day	22% 16% 28% 23% 9% 1	500
	11th to 20th	Day	22% 13% 24% 23% 13% 4	500
	21st to end	Day	16% 15% 31% 25% 10% 2	550
April	1st to 10th	Day	17% 14% 26% 29% 12% 3	500
	11th to 20th	Day	10% 16% 29% 27% 14% 4	492
	21st to end	Day	5% 16% 29% 31% 14% 6%	500
May	1st to 10th	Day	9% 12% 30% 30% 14% 5%	503
	11th to 20th	Day	9% 9% 30% 30% 16% 5%	528
	21st to end	Day	6% 12% 25% 32% 19% 6%	573
June	1st to 10th	Day	5% 8% 24% 35% 23% 5%	530
	11th to 20th	Day	1 9% 28% 37% 21% 4	530
	21st to end	Day	2 3 19% 44% 25% 7%	530
July	1st to 10th	Day	2 1 17% 41% 31% 7%	526
	11th to 20th	Day	1 10% 47% 32% 10%	524
	21st to end	Day	1 11% 42% 35% 11%	575
August	1st to 10th	Day	2 2 18% 43% 28% 7%	527
	11th to 20th	Day	1 1 15% 42% 30% 11%	517
	21st to end	Day	2 11% 43% 33% 11%	583
September	1st to 10th	Day	2 4 11% 44% 29% 10%	530
	11th to 20th	Day	3 4 24% 37% 24% 8%	530
	21st to end	Day	5% 5% 19% 33% 30% 8%	530
October	1st to 10th	Day	4 6% 22% 29% 29% 10%	530
	11th to 20th	Day	5% 7% 19% 29% 30% 10%	530
	21st to end	Day	7% 7% 21% 30% 28% 7%	583
November	1st to 10th	Day	8% 11% 18% 25% 27% 10%	512
	11th to 20th	Day	11% 11% 20% 25% 24% 8%	330
	21st to end	Day	11% 6% 22% 30% 25% 6%	261
December	1st to 10th	Day	14% 10% 16% 29% 23% 8%	220
	11th to 20th	Day	15% 14% 20% 20% 19% 11%	220
	21st to end	Day	21% 19% 24% 19% 12% 5%	242
January	1st to 10th	Night	19% 7% 17% 30% 22% 5%	840
	11th to 20th	Night	16% 5% 14% 34% 24% 7%	840
	21st to end	Night	17% 5% 13% 35% 25% 5%	924
February	1st to 10th	Night	20% 9% 14% 30% 23% 5%	800
	11th to 20th	Night	15% 8% 19% 33% 19% 5%	784
	21st to end	Night	13% 8% 22% 34% 20% 3	660
March	1st to 10th	Night	20% 9% 19% 28% 21% 3	800
	11th to 20th	Night	17% 9% 16% 28% 25% 5%	800
	21st to end	Night	15% 8% 21% 35% 19% 3	880
April	1st to 10th	Night	13% 7% 21% 34% 22% 3	800
	11th to 20th	Night	11% 10% 20% 32% 22% 5%	791
	21st to end	Night	6% 7% 26% 31% 23% 7%	800
May	1st to 10th	Night	6% 5% 23% 35% 26% 5%	755
	11th to 20th	Night	4 3 15% 36% 35% 6%	557
	21st to end	Night	3 7% 18% 38% 29% 5%	433
June	1st to 10th	Night	3 3 16% 40% 34% 5%	400
	11th to 20th	Night	1 3 15% 47% 33% 2	400
	21st to end	Night	2 2 8% 39% 43% 6%	400
July	1st to 10th	Night	5% 36% 53% 6%	404
	11th to 20th	Night	1 3 29% 50% 17%	582
	21st to end	Night	1 4 26% 56% 12%	657
August	1st to 10th	Night	3 8% 34% 47% 8%	597
	11th to 20th	Night	1 1 9% 34% 46% 9%	741
	21st to end	Night	2 8% 30% 49% 11%	880
September	1st to 10th	Night	1 1 8% 32% 46% 12%	800
	11th to 20th	Night	4 1 10% 34% 42% 9%	800
	21st to end	Night	4 2 11% 30% 41% 12%	800
October	1st to 10th	Night	4 3 11% 34% 37% 11%	800
	11th to 20th	Night	6% 3 12% 28% 40% 11%	800
	21st to end	Night	6% 5% 10% 33% 30% 15%	880
November	1st to 10th	Night	7% 6% 14% 27% 36% 10%	820
	11th to 20th	Night	12% 4 14% 32% 31% 6%	840
	21st to end	Night	10% 4 15% 39% 26% 7%	840
December	1st to 10th	Night	16% 4 17% 33% 25% 6%	840
	11th to 20th	Night	20% 6% 15% 35% 21% 3	840
	21st to end	Night	20% 10% 21% 28% 18% 3	924

Page generated at 2026-01-28 22:21:43. Version ac07

Copyright © 2026 Attilla Danko.