Arnold Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Arnold Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	8% 4 1 3 2 2 2 2 4 5% 67%	139642
	Night	10% 5% 1 3 2 2 3 2 4 5% 62%	138949
Transparency	Day	22% 4 7% 17% 26% 24%	134624
	Night	26% 4 7% 18% 24% 21%	134948
Seeing	Day	13% 13% 33% 27% 11% 3	16836
	Night	12% 4 20% 36% 23% 5%	26330

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	19% 8% 2 3 3 2 2 2 4 6% 49%	4722
	11th to 20th	Day	11% 5% 1 4 3 3 2 2 3 5% 62%	5117
	21st to end	Day	12% 7% 2 6% 3 4 4 3 5% 7% 46%	5843
February	1st to 10th	Day	20% 9% 1 4 3 2 3 3 4 5% 47%	1891
	11th to 20th	Day	14% 12% 2 4 3 3 4 3 5% 6% 46%	2050
	21st to end	Day	17% 11% 3 4 3 3 3 2 4 5% 43%	1730
March	1st to 10th	Day	14% 15% 3 7% 4 4 4 4 6% 6% 33%	4556
	11th to 20th	Day	16% 10% 2 4 2 3 3 3 4 5% 48%	5000
	21st to end	Day	17% 10% 3 6% 4 4 4 3 5% 6% 38%	5784
April	1st to 10th	Day	13% 8% 2 3 3 3 4 3 5% 6% 48%	2400
	11th to 20th	Day	12% 6% 2 4 3 2 3 3 4 6% 54%	2369
	21st to end	Day	11% 5% 1 3 3 2 3 2 4 6% 59%	2400
May	1st to 10th	Day	6% 3 1 3 2 2 2 2 4 4 71%	5561
	11th to 20th	Day	4 2 1 2 2 2 2 2 4 4 74%	6145
	21st to end	Day	4 2 1 2 1 2 2 2 2 4 79%	6902
June	1st to 10th	Day	4 3 1 4 3 2 2 3 4 6% 67%	2800
	11th to 20th	Day	1 2 1 2 2 2 1 2 3 4 81%	2800
	21st to end	Day	2 2 1 1 1 2 2 2 2 3 83%	2800
July	1st to 10th	Day	1 1 1 1 1 1 2 94%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 87%	6320
	21st to end	Day	1 1 2 1 1 1 1 3 3 86%	6502
August	1st to 10th	Day	1 1 1 1 1 1 1 2 3 86%	5702
	11th to 20th	Day	1 1 1 1 1 2 2 91%	5364
	21st to end	Day	2 1 1 2 2 2 2 2 2 3 82%	5984
September	1st to 10th	Day	2 1 1 1 1 1 1 2 3 87%	2400
	11th to 20th	Day	3 2 1 3 2 2 2 2 3 5% 76%	2386
	21st to end	Day	4 2 3 1 1 2 2 3 4 79%	2200
October	1st to 10th	Day	3 3 1 3 1 2 2 3 4 5% 74%	3285
	11th to 20th	Day	5% 2 1 2 2 2 2 2 3 4 76%	3261
	21st to end	Day	6% 2 1 4 2 3 3 2 5% 6% 66%	3558
November	1st to 10th	Day	11% 5% 1 4 3 3 3 3 5% 6% 55%	1858
	11th to 20th	Day	12% 5% 2 3 3 3 3 3 4 6% 55%	1890
	21st to end	Day	12% 6% 1 4 3 3 3 2 5% 6% 55%	1843
December	1st to 10th	Day	8% 3 1 3 2 4 3 3 4 7% 60%	2700
	11th to 20th	Day	25% 8% 1 5% 3 3 3 3 5% 5% 38%	3108
	21st to end	Day	21% 8% 2 6% 3 4 4 4 6% 8% 33%	4092
January	1st to 10th	Night	17% 8% 2 4 2 3 3 3 4 7% 45%	8172
	11th to 20th	Night	12% 4 1 3 3 3 3 3 4 5% 60%	7847
	21st to end	Night	13% 7% 2 6% 4 4 4 3 4 6% 48%	8371
February	1st to 10th	Night	18% 8% 2 5% 3 2 2 2 4 6% 49%	2727
	11th to 20th	Night	16% 9% 2 4 3 3 3 3 5% 7% 46%	2458
	21st to end	Night	16% 9% 2 4 3 3 3 2 4 5% 50%	2076
March	1st to 10th	Night	16% 16% 3 4 3 3 3 3 5% 5% 37%	5367
	11th to 20th	Night	17% 11% 2 4 2 2 3 2 4 5% 48%	5000
	21st to end	Night	20% 11% 2 4 3 3 4 3 5% 6% 39%	5189
April	1st to 10th	Night	13% 6% 2 5% 3 3 4 3 4 6% 52%	2000
	11th to 20th	Night	11% 6% 1 4 3 3 3 2 4 6% 56%	1979
	21st to end	Night	9% 6% 1 3 3 2 3 3 5% 6% 60%	2000
May	1st to 10th	Night	3 4 1 2 1 2 2 2 3 5% 74%	4432
	11th to 20th	Night	3 2 1 2 1 1 2 2 4 5% 77%	3736
	21st to end	Night	2 2 1 2 1 1 2 2 2 4 81%	3972
June	1st to 10th	Night	5% 3 1 2 2 2 3 2 4 7% 69%	1600
	11th to 20th	Night	1 1 1 1 2 2 2 3 4 84%	1600
	21st to end	Night	1 1 1 2 2 2 1 1 2 3 85%	1600
July	1st to 10th	Night	1 1 1 1 95%	3640
	11th to 20th	Night	2 2 1 2 1 2 2 2 2 3 83%	3640
	21st to end	Night	2 1 1 2 2 2 2 1 3 4 81%	4349
August	1st to 10th	Night	2 2 1 2 1 1 1 1 2 3 85%	4114
	11th to 20th	Night	1 1 1 1 1 1 1 2 2 91%	4506
	21st to end	Night	3 1 1 1 1 2 1 2 2 3 82%	5015
September	1st to 10th	Night	1 1 1 1 1 2 1 2 3 88%	2000
	11th to 20th	Night	3 1 1 2 2 2 2 2 3 4 79%	2013
	21st to end	Night	2 2 1 2 1 2 2 2 2 4 80%	2200
October	1st to 10th	Night	3 2 1 2 2 2 2 2 3 3 79%	3910
	11th to 20th	Night	3 2 1 2 1 1 2 2 3 4 80%	3897
	21st to end	Night	7% 3 1 2 2 3 3 3 4 5% 68%	4253
November	1st to 10th	Night	11% 5% 1 3 2 2 2 2 4 6% 61%	2660
	11th to 20th	Night	11% 4 1 4 3 3 3 3 5% 7% 56%	2730
	21st to end	Night	12% 5% 1 3 3 3 3 3 4 6% 57%	2774
December	1st to 10th	Night	10% 4 1 3 2 3 3 3 4 6% 63%	4700
	11th to 20th	Night	24% 8% 2 4 3 3 3 4 6% 9% 35%	5316
	21st to end	Night	20% 9% 3 7% 5% 5% 4 4 5% 6% 33%	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	38% 4 12% 21% 18% 8%	4723
	11th to 20th	Day	28% 4 5% 11% 25% 28%	5117
	21st to end	Day	36% 5% 7% 16% 21% 14%	5843
February	1st to 10th	Day	40% 4 8% 20% 17% 12%	1891
	11th to 20th	Day	39% 4 9% 20% 22% 7%	2060
	21st to end	Day	43% 4 8% 20% 18% 7%	1730
March	1st to 10th	Day	48% 8% 9% 12% 17% 5%	4556
	11th to 20th	Day	38% 5% 14% 26% 10% 6%	5000
	21st to end	Day	43% 10% 13% 21% 11% 2	5784
April	1st to 10th	Day	36% 6% 13% 22% 18% 5%	2400
	11th to 20th	Day	31% 6% 13% 25% 18% 6%	2372
	21st to end	Day	26% 6% 13% 30% 22% 3	2400
May	1st to 10th	Day	17% 4 7% 29% 30% 12%	5561
	11th to 20th	Day	14% 4 7% 31% 37% 7%	6148
	21st to end	Day	12% 3 6% 27% 44% 7%	6904
June	1st to 10th	Day	19% 5% 10% 26% 29% 11%	2800
	11th to 20th	Day	10% 3 6% 21% 36% 24%	2800
	21st to end	Day	10% 2 3 11% 33% 41%	2800
July	1st to 10th	Day	3 1 1 5% 21% 70%	6251
	11th to 20th	Day	5% 2 3 6% 15% 70%	6245
	21st to end	Day	6% 2 3 13% 33% 45%	6314
August	1st to 10th	Day	9% 2 3 7% 26% 53%	4153
	11th to 20th	Day	5% 1 2 6% 35% 51%	3880
	21st to end	Day	7% 1 3 8% 40% 42%	4311
September	1st to 10th	Day	7% 1 3 10% 33% 46%	2400
	11th to 20th	Day	13% 3 6% 20% 39% 19%	2386
	21st to end	Day	12% 2 5% 18% 36% 27%	2200
October	1st to 10th	Day	14% 3 5% 11% 28% 41%	3285
	11th to 20th	Day	14% 3 4 18% 41% 21%	3261
	21st to end	Day	20% 4 5% 18% 37% 16%	3558
November	1st to 10th	Day	30% 3 9% 21% 22% 15%	1858
	11th to 20th	Day	31% 4 10% 18% 25% 12%	1890
	21st to end	Day	31% 4 8% 17% 27% 14%	1843
December	1st to 10th	Day	25% 4 6% 23% 26% 16%	2700
	11th to 20th	Day	49% 4 8% 12% 18% 9%	3108
	21st to end	Day	45% 8% 13% 16% 12% 5%	4092
January	1st to 10th	Night	38% 5% 13% 20% 17% 8%	8172
	11th to 20th	Night	27% 4 5% 11% 25% 27%	7847
	21st to end	Night	36% 5% 7% 15% 19% 18%	8371
February	1st to 10th	Night	39% 3 7% 17% 20% 13%	2727
	11th to 20th	Night	38% 4 9% 22% 19% 8%	2459
	21st to end	Night	38% 4 10% 22% 19% 7%	2076
March	1st to 10th	Night	47% 6% 8% 16% 15% 8%	5367
	11th to 20th	Night	39% 4 10% 25% 14% 7%	5000
	21st to end	Night	44% 8% 13% 20% 12% 3	5189
April	1st to 10th	Night	34% 7% 13% 21% 21% 5%	2000
	11th to 20th	Night	29% 6% 13% 26% 18% 8%	1979
	21st to end	Night	24% 6% 12% 35% 19% 5%	2000
May	1st to 10th	Night	15% 4 9% 32% 29% 12%	4429
	11th to 20th	Night	11% 4 9% 31% 35% 10%	3736
	21st to end	Night	10% 3 6% 34% 40% 7%	3972
June	1st to 10th	Night	16% 6% 12% 25% 28% 12%	1600
	11th to 20th	Night	6% 4 7% 24% 35% 25%	1600
	21st to end	Night	8% 2 2 13% 34% 41%	1600
July	1st to 10th	Night	2 1 1 4 22% 71%	3600
	11th to 20th	Night	9% 2 3 5% 18% 63%	3592
	21st to end	Night	9% 2 3 13% 27% 45%	4225
August	1st to 10th	Night	11% 2 2 8% 26% 51%	2984
	11th to 20th	Night	5% 2 3 8% 31% 53%	3256
	21st to end	Night	8% 1 2 10% 38% 42%	3608
September	1st to 10th	Night	5% 1 3 10% 35% 46%	2000
	11th to 20th	Night	11% 3 4 19% 41% 21%	2013
	21st to end	Night	11% 2 5% 20% 36% 27%	2200
October	1st to 10th	Night	12% 3 3 11% 27% 43%	3910
	11th to 20th	Night	11% 2 4 21% 39% 22%	3897
	21st to end	Night	19% 4 5% 18% 36% 19%	4253
November	1st to 10th	Night	25% 4 9% 22% 23% 16%	2660
	11th to 20th	Night	28% 4 8% 21% 26% 13%	2730
	21st to end	Night	29% 4 6% 19% 26% 15%	2774
December	1st to 10th	Night	24% 4 6% 22% 25% 19%	4700
	11th to 20th	Night	46% 5% 8% 13% 21% 8%	5316
	21st to end	Night	49% 7% 10% 17% 12% 6%	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	32% 14% 25% 20% 9%	220
	11th to 20th	Day	20% 7% 26% 32% 13% 2	350
	21st to end	Day	20% 10% 29% 25% 13% 3	473
February	1st to 10th	Day	28% 14% 23% 24% 9% 2	431
	11th to 20th	Day	23% 19% 31% 18% 8% 1	535
	21st to end	Day	21% 18% 34% 20% 6%	446
March	1st to 10th	Day	28% 21% 29% 16% 6%	509
	11th to 20th	Day	27% 20% 25% 18% 7% 2	500
	21st to end	Day	24% 19% 32% 20% 5%	550
April	1st to 10th	Day	21% 19% 33% 20% 6% 1	500
	11th to 20th	Day	20% 22% 31% 20% 6% 1	492
	21st to end	Day	12% 21% 31% 23% 10% 3	500
May	1st to 10th	Day	14% 22% 33% 22% 7% 2	503
	11th to 20th	Day	14% 18% 31% 22% 11% 3	528
	21st to end	Day	11% 15% 30% 28% 12% 4	573
June	1st to 10th	Day	8% 12% 36% 26% 12% 5%	530
	11th to 20th	Day	5% 17% 40% 26% 7% 5%	530
	21st to end	Day	4 10% 39% 28% 12% 6%	530
July	1st to 10th	Day	3 10% 43% 28% 11% 5%	526
	11th to 20th	Day	1 4 40% 35% 14% 6%	524
	21st to end	Day	2 4 40% 35% 11% 8%	575
August	1st to 10th	Day	3 9% 42% 28% 14% 4	527
	11th to 20th	Day	2 8% 40% 33% 12% 5%	517
	21st to end	Day	2 5% 40% 34% 15% 3	583
September	1st to 10th	Day	3 8% 43% 31% 13% 1	530
	11th to 20th	Day	6% 13% 37% 32% 11% 2	530
	21st to end	Day	5% 13% 34% 37% 10% 2	530
October	1st to 10th	Day	6% 13% 34% 32% 12% 3	530
	11th to 20th	Day	8% 12% 30% 33% 14% 3	530
	21st to end	Day	11% 11% 29% 32% 14% 3	583
November	1st to 10th	Day	14% 12% 22% 31% 15% 7%	334
	11th to 20th	Day	17% 10% 24% 30% 14% 5%	330
	21st to end	Day	16% 12% 24% 30% 15% 3	305
December	1st to 10th	Day	16% 7% 26% 27% 17% 7%	220
	11th to 20th	Day	18% 11% 28% 24% 15% 4	220
	21st to end	Day	26% 12% 29% 18% 12% 3	242
January	1st to 10th	Night	23% 6% 25% 24% 16% 5%	840
	11th to 20th	Night	16% 7% 25% 35% 12% 4	840
	21st to end	Night	20% 7% 26% 28% 16% 4	924
February	1st to 10th	Night	21% 6% 27% 27% 16% 3	840
	11th to 20th	Night	20% 8% 23% 29% 15% 5%	824
	21st to end	Night	21% 7% 27% 31% 12% 2	692
March	1st to 10th	Night	25% 8% 22% 29% 14% 3	808
	11th to 20th	Night	21% 8% 19% 28% 20% 4	800
	21st to end	Night	21% 6% 23% 32% 16% 2	880
April	1st to 10th	Night	16% 5% 21% 38% 18% 2	800
	11th to 20th	Night	14% 7% 24% 34% 16% 4	791
	21st to end	Night	10% 4 23% 37% 21% 5%	800
May	1st to 10th	Night	9% 5% 24% 37% 21% 4	750
	11th to 20th	Night	7% 3 20% 40% 28% 2	449
	21st to end	Night	6% 3 16% 39% 32% 4	433
June	1st to 10th	Night	6% 2 12% 46% 30% 5%	400
	11th to 20th	Night	1 1 14% 48% 33% 3	400
	21st to end	Night	2 1 14% 45% 36% 3	400
July	1st to 10th	Night	1 8% 45% 43% 4	390
	11th to 20th	Night	1 1 7% 43% 43% 5%	388
	21st to end	Night	3 1 9% 33% 44% 10%	599
August	1st to 10th	Night	3 1 10% 41% 36% 8%	626
	11th to 20th	Night	1 1 10% 46% 35% 7%	792
	21st to end	Night	2 1 7% 46% 40% 4	880
September	1st to 10th	Night	2 1 9% 46% 36% 6%	800
	11th to 20th	Night	4 1 12% 42% 34% 8%	800
	21st to end	Night	2 3 16% 43% 28% 7%	800
October	1st to 10th	Night	5% 3 18% 45% 24% 5%	800
	11th to 20th	Night	7% 3 18% 39% 28% 5%	800
	21st to end	Night	7% 5% 20% 35% 24% 10%	880
November	1st to 10th	Night	14% 4 17% 35% 24% 7%	820
	11th to 20th	Night	13% 5% 25% 33% 18% 5%	840
	21st to end	Night	14% 5% 28% 32% 16% 5%	840
December	1st to 10th	Night	18% 5% 25% 33% 13% 5%	840
	11th to 20th	Night	25% 4 24% 29% 15% 3	840
	21st to end	Night	26% 8% 25% 26% 12% 2	924

Page generated at 2026-02-27 22:40:23. Version ac07

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