Arclight Observatory
Clear Sky Chart
History

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

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	13% 7% 3 7% 5% 4 4 4 5% 7% 41%	119710
	Night	14% 8% 3 7% 4 4 4 3 5% 7% 43%	119194
Transparency	Day	42% 5% 10% 23% 18% 3	123414
	Night	41% 4 9% 23% 19% 4	123617
Seeing	Day	16% 8% 22% 24% 20% 11%	16179
	Night	17% 6% 23% 32% 19% 3	25955

By Month

Percentage Cloud Cover SE (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	15% 11% 8% 10% 5% 4 4 3 4 5% 31%	3617
	11th to 20th	Day	15% 9% 3 10% 4 3 3 3 4 5% 42%	3620
	21st to end	Day	20% 9% 4 5% 3 3 3 3 5% 6% 39%	4814
February	1st to 10th	Day	18% 11% 3 7% 4 5% 3 3 5% 6% 36%	2000
	11th to 20th	Day	21% 10% 4 7% 4 4 3 3 5% 6% 34%	1948
	21st to end	Day	17% 10% 3 7% 4 4 3 3 4 5% 38%	1650
March	1st to 10th	Day	17% 13% 4 7% 4 4 4 3 4 4 37%	4480
	11th to 20th	Day	12% 9% 3 6% 4 3 3 3 5% 5% 48%	4694
	21st to end	Day	12% 7% 3 8% 5% 4 4 4 5% 6% 43%	5910
April	1st to 10th	Day	14% 7% 2 6% 4 4 4 4 4 7% 45%	2400
	11th to 20th	Day	14% 7% 2 7% 5% 4 4 3 6% 6% 42%	2374
	21st to end	Day	14% 7% 3 7% 6% 4 3 4 5% 7% 40%	2412
May	1st to 10th	Day	18% 10% 3 8% 5% 5% 5% 4 6% 8% 27%	5862
	11th to 20th	Day	15% 8% 3 9% 6% 5% 5% 4 6% 7% 32%	5861
	21st to end	Day	14% 6% 3 8% 5% 4 5% 4 5% 7% 39%	6433
June	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 6% 9% 44%	2733
	11th to 20th	Day	6% 4 2 6% 5% 5% 6% 5% 7% 10% 46%	2800
	21st to end	Day	4 4 2 4 5% 6% 5% 5% 9% 13% 44%	2800
July	1st to 10th	Day	7% 5% 2 6% 5% 5% 6% 5% 7% 11% 39%	4511
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 4 7% 10% 43%	4440
	21st to end	Day	15% 10% 3 9% 6% 6% 6% 5% 6% 8% 26%	4541
August	1st to 10th	Day	4 3 1 4 4 4 5% 4 6% 9% 55%	3851
	11th to 20th	Day	8% 5% 2 7% 5% 5% 6% 5% 7% 10% 40%	3729
	21st to end	Day	5% 4 1 4 4 4 5% 5% 6% 8% 55%	4180
September	1st to 10th	Day	11% 6% 2 5% 5% 5% 5% 4 6% 7% 44%	2280
	11th to 20th	Day	13% 5% 2 6% 5% 4 4 4 5% 7% 46%	2095
	21st to end	Day	12% 6% 2 5% 4 4 4 3 4 7% 48%	2090
October	1st to 10th	Day	10% 4 1 4 3 5% 3 2 4 6% 56%	2090
	11th to 20th	Day	12% 5% 2 5% 3 3 4 3 4 6% 53%	1992
	21st to end	Day	18% 6% 3 5% 4 3 4 2 4 5% 47%	1922
November	1st to 10th	Day	13% 6% 2 4 3 3 2 2 5% 6% 54%	1762
	11th to 20th	Day	14% 6% 3 5% 3 3 3 3 4 6% 49%	1800
	21st to end	Day	16% 8% 2 6% 4 3 4 3 5% 5% 44%	1800
December	1st to 10th	Day	16% 8% 2 6% 3 4 3 4 4 6% 44%	2930
	11th to 20th	Day	19% 11% 3 8% 5% 4 4 3 5% 6% 33%	3307
	21st to end	Day	17% 11% 4 9% 6% 5% 5% 4 5% 7% 28%	3982
January	1st to 10th	Night	16% 11% 4 10% 3 3 4 3 5% 7% 34%	6348
	11th to 20th	Night	17% 10% 3 9% 4 3 3 3 3 3 41%	6360
	21st to end	Night	20% 10% 4 6% 4 3 3 2 3 5% 39%	6126
February	1st to 10th	Night	22% 11% 3 8% 4 3 4 3 5% 5% 33%	2400
	11th to 20th	Night	21% 9% 4 7% 4 4 3 3 4 4 36%	2338
	21st to end	Night	20% 9% 3 7% 5% 4 4 4 5% 5% 35%	1980
March	1st to 10th	Night	19% 11% 3 6% 4 3 4 3 5% 6% 36%	5373
	11th to 20th	Night	12% 6% 2 5% 4 3 4 3 5% 6% 49%	5266
	21st to end	Night	13% 8% 3 8% 5% 4 4 3 5% 7% 41%	5062
April	1st to 10th	Night	12% 6% 2 6% 3 4 4 3 6% 7% 47%	2000
	11th to 20th	Night	15% 6% 2 5% 3 3 4 3 5% 7% 47%	1975
	21st to end	Night	13% 7% 2 6% 5% 5% 4 3 6% 8% 42%	1985
May	1st to 10th	Night	16% 9% 3 9% 5% 5% 5% 4 6% 7% 32%	4033
	11th to 20th	Night	14% 9% 3 9% 5% 5% 5% 4 6% 8% 32%	4032
	21st to end	Night	14% 7% 2 7% 5% 4 4 4 6% 9% 38%	4421
June	1st to 10th	Night	11% 6% 2 6% 3 4 3 4 6% 8% 47%	1661
	11th to 20th	Night	3 4 2 4 5% 4 5% 3 6% 10% 55%	1600
	21st to end	Night	3 2 1 5% 5% 5% 5% 5% 7% 12% 51%	1600
July	1st to 10th	Night	6% 5% 2 6% 4 5% 5% 4 8% 12% 43%	2570
	11th to 20th	Night	5% 4 2 5% 4 4 4 4 6% 10% 52%	2530
	21st to end	Night	10% 9% 3 9% 6% 6% 5% 4 7% 8% 32%	3013
August	1st to 10th	Night	4 4 1 4 3 3 4 3 5% 7% 61%	3073
	11th to 20th	Night	4 4 1 5% 5% 5% 6% 4 7% 11% 49%	3100
	21st to end	Night	4 3 1 4 4 4 4 4 6% 8% 58%	3476
September	1st to 10th	Night	11% 5% 2 5% 3 3 4 3 6% 8% 50%	1900
	11th to 20th	Night	10% 6% 2 5% 4 3 4 3 6% 7% 51%	2066
	21st to end	Night	10% 5% 2 5% 4 3 4 4 5% 6% 54%	2090
October	1st to 10th	Night	9% 4 2 3 2 3 3 3 4 5% 62%	2090
	11th to 20th	Night	15% 5% 1 5% 3 3 3 3 3 6% 53%	2184
	21st to end	Night	16% 7% 2 4 3 3 4 3 4 5% 49%	2674
November	1st to 10th	Night	14% 5% 2 4 3 3 3 2 4 6% 53%	2536
	11th to 20th	Night	12% 6% 2 5% 4 3 4 3 5% 7% 49%	2600
	21st to end	Night	16% 8% 2 4 4 4 3 2 5% 5% 47%	2600
December	1st to 10th	Night	18% 10% 2 7% 4 3 3 3 4 4 42%	4250
	11th to 20th	Night	21% 9% 3 6% 3 3 3 3 3 5% 41%	4886
	21st to end	Night	18% 13% 5% 10% 5% 5% 5% 3 4 5% 27%	6996

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	56% 2 10% 19% 11% 2	3618
	11th to 20th	Day	46% 3 6% 24% 20% 1	3620
	21st to end	Day	47% 3 7% 21% 17% 6%	4814
February	1st to 10th	Day	50% 3 9% 23% 13% 2	2000
	11th to 20th	Day	53% 3 8% 19% 13% 4	1958
	21st to end	Day	49% 3 9% 19% 15% 6%	1650
March	1st to 10th	Day	52% 2 4 17% 18% 7%	4480
	11th to 20th	Day	39% 3 7% 24% 20% 6%	4694
	21st to end	Day	42% 4 9% 27% 17% 1	5912
April	1st to 10th	Day	39% 6% 10% 23% 21% 3	2400
	11th to 20th	Day	42% 4 8% 22% 20% 4	2374
	21st to end	Day	44% 4 9% 25% 15% 2	2412
May	1st to 10th	Day	53% 6% 13% 19% 9% 1	5897
	11th to 20th	Day	50% 6% 13% 20% 10%	5865
	21st to end	Day	43% 6% 9% 22% 17% 3	6447
June	1st to 10th	Day	36% 8% 11% 23% 19% 3	2733
	11th to 20th	Day	32% 8% 14% 25% 18% 3	2800
	21st to end	Day	30% 8% 17% 25% 16% 5%	2800
July	1st to 10th	Day	37% 6% 17% 28% 10% 2	4442
	11th to 20th	Day	36% 5% 13% 34% 11% 2	4365
	21st to end	Day	55% 6% 11% 18% 8% 1	4397
August	1st to 10th	Day	25% 6% 13% 32% 23% 1	3838
	11th to 20th	Day	38% 5% 11% 26% 18% 1	3765
	21st to end	Day	27% 6% 9% 33% 23% 2	4179
September	1st to 10th	Day	39% 8% 10% 21% 19% 4	2280
	11th to 20th	Day	38% 6% 9% 23% 21% 3	2095
	21st to end	Day	36% 5% 10% 23% 21% 6%	2090
October	1st to 10th	Day	21% 2 5% 17% 45% 10%	3480
	11th to 20th	Day	24% 3 7% 25% 31% 11%	3284
	21st to end	Day	31% 2 5% 25% 30% 7%	3147
November	1st to 10th	Day	33% 4 8% 23% 28% 5%	1762
	11th to 20th	Day	37% 3 8% 22% 24% 6%	1800
	21st to end	Day	42% 4 7% 24% 19% 4	1800
December	1st to 10th	Day	44% 2 5% 28% 19% 1	2930
	11th to 20th	Day	52% 3 8% 20% 14% 3	3304
	21st to end	Day	54% 4 10% 22% 8%	3982
January	1st to 10th	Night	51% 3 9% 23% 13% 1	6348
	11th to 20th	Night	49% 3 4 21% 19% 3	6360
	21st to end	Night	50% 2 6% 22% 16% 5%	6126
February	1st to 10th	Night	54% 4 9% 18% 14% 1	2400
	11th to 20th	Night	53% 3 7% 19% 15% 4	2339
	21st to end	Night	51% 5% 7% 16% 17% 4	1980
March	1st to 10th	Night	50% 3 5% 15% 20% 6%	5373
	11th to 20th	Night	36% 4 8% 21% 19% 12%	5266
	21st to end	Night	45% 3 9% 25% 16% 2	5062
April	1st to 10th	Night	37% 5% 10% 25% 20% 3	2000
	11th to 20th	Night	38% 4 8% 22% 22% 6%	1975
	21st to end	Night	40% 5% 11% 25% 18% 2	1985
May	1st to 10th	Night	51% 6% 12% 21% 10% 1	4060
	11th to 20th	Night	50% 7% 14% 19% 11% 1	4033
	21st to end	Night	42% 6% 13% 23% 14% 1	4429
June	1st to 10th	Night	34% 6% 12% 25% 19% 3	1661
	11th to 20th	Night	25% 6% 17% 27% 22% 4	1600
	21st to end	Night	24% 8% 18% 28% 16% 6%	1600
July	1st to 10th	Night	32% 6% 16% 32% 12% 2	2530
	11th to 20th	Night	28% 6% 11% 39% 13% 2	2482
	21st to end	Night	49% 7% 14% 20% 9% 1	2917
August	1st to 10th	Night	22% 4 10% 39% 23% 1	3062
	11th to 20th	Night	29% 5% 18% 30% 17% 1	3130
	21st to end	Night	24% 4 10% 33% 26% 3	3476
September	1st to 10th	Night	31% 7% 11% 25% 20% 6%	1900
	11th to 20th	Night	32% 4 9% 25% 25% 4	2066
	21st to end	Night	31% 4 9% 23% 26% 6%	2090
October	1st to 10th	Night	19% 2 5% 17% 45% 12%	3480
	11th to 20th	Night	21% 2 5% 22% 37% 13%	3618
	21st to end	Night	28% 2 6% 24% 29% 11%	4399
November	1st to 10th	Night	34% 3 7% 24% 25% 7%	2536
	11th to 20th	Night	35% 4 7% 23% 24% 8%	2600
	21st to end	Night	40% 3 7% 26% 19% 4	2600
December	1st to 10th	Night	47% 3 6% 21% 22% 1	4250
	11th to 20th	Night	47% 2 7% 23% 18% 2	4888
	21st to end	Night	59% 4 8% 19% 9% 2	6996

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	25% 15% 34% 19% 7%	300
	11th to 20th	Day	28% 15% 38% 16% 3	300
	21st to end	Day	25% 13% 42% 17% 4	330
February	1st to 10th	Day	25% 17% 33% 19% 4 2	300
	11th to 20th	Day	32% 19% 34% 11% 3 1	300
	21st to end	Day	29% 13% 38% 15% 5%	249
March	1st to 10th	Day	25% 14% 36% 20% 5% 1	300
	11th to 20th	Day	21% 17% 36% 20% 6% 1	370
	21st to end	Day	23% 14% 34% 20% 7% 1	550
April	1st to 10th	Day	20% 13% 30% 24% 11% 2	500
	11th to 20th	Day	18% 12% 26% 28% 13% 2	492
	21st to end	Day	18% 9% 34% 24% 11% 5%	500
May	1st to 10th	Day	19% 12% 27% 25% 13% 4	503
	11th to 20th	Day	16% 7% 23% 25% 20% 9%	527
	21st to end	Day	12% 4 19% 27% 22% 16%	573
June	1st to 10th	Day	11% 4 15% 29% 23% 18%	663
	11th to 20th	Day	5% 3 15% 26% 33% 17%	730
	21st to end	Day	5% 3 15% 32% 28% 17%	730
July	1st to 10th	Day	11% 2 11% 24% 28% 24%	697
	11th to 20th	Day	9% 3 10% 23% 31% 23%	668
	21st to end	Day	10% 3 10% 26% 28% 23%	581
August	1st to 10th	Day	9% 2 7% 28% 35% 20%	486
	11th to 20th	Day	11% 3 9% 26% 36% 13%	479
	21st to end	Day	11% 2 5% 20% 36% 26%	539
September	1st to 10th	Day	10% 4 8% 25% 33% 21%	490
	11th to 20th	Day	12% 3 8% 22% 37% 19%	490
	21st to end	Day	17% 6% 14% 24% 27% 12%	490
October	1st to 10th	Day	11% 2 16% 31% 28% 12%	490
	11th to 20th	Day	12% 8% 30% 28% 17% 5%	392
	21st to end	Day	23% 10% 28% 23% 15% 1	330
November	1st to 10th	Day	18% 9% 28% 25% 16% 4	300
	11th to 20th	Day	15% 15% 31% 27% 12%	300
	21st to end	Day	22% 10% 30% 23% 14% 2	300
December	1st to 10th	Day	24% 13% 33% 20% 10%	300
	11th to 20th	Day	22% 16% 29% 23% 10% 1	300
	21st to end	Day	23% 17% 33% 22% 6%	330
January	1st to 10th	Night	22% 12% 34% 28% 5%	1000
	11th to 20th	Night	26% 11% 34% 25% 5%	1000
	21st to end	Night	24% 9% 36% 27% 3	939
February	1st to 10th	Night	26% 9% 36% 26% 4	800
	11th to 20th	Night	26% 13% 35% 24% 2	783
	21st to end	Night	25% 12% 37% 25% 2	660
March	1st to 10th	Night	20% 12% 31% 31% 5%	800
	11th to 20th	Night	22% 9% 33% 29% 7%	800
	21st to end	Night	19% 10% 31% 32% 8%	708
April	1st to 10th	Night	15% 8% 33% 36% 8% 1	600
	11th to 20th	Night	19% 5% 30% 33% 12%	593
	21st to end	Night	16% 5% 31% 36% 12% 1	600
May	1st to 10th	Night	18% 3 28% 33% 16% 2	600
	11th to 20th	Night	12% 5% 20% 37% 23% 4	591
	21st to end	Night	8% 3 13% 41% 30% 5%	650
June	1st to 10th	Night	12% 4 10% 40% 30% 4	600
	11th to 20th	Night	5% 2 9% 40% 41% 4	600
	21st to end	Night	3 2 10% 41% 37% 8%	600
July	1st to 10th	Night	7% 2 6% 28% 47% 9%	570
	11th to 20th	Night	8% 3 6% 28% 48% 7%	552
	21st to end	Night	8% 2 7% 32% 43% 8%	624
August	1st to 10th	Night	7% 1 6% 31% 47% 8%	567
	11th to 20th	Night	7% 2 6% 40% 41% 4	564
	21st to end	Night	7% 1 4 32% 47% 9%	627
September	1st to 10th	Night	12% 2 8% 30% 42% 6%	570
	11th to 20th	Night	13% 1 10% 29% 40% 6%	736
	21st to end	Night	11% 3 16% 30% 35% 5%	760
October	1st to 10th	Night	11% 3 16% 35% 29% 6%	760
	11th to 20th	Night	18% 4 22% 34% 20% 2	760
	21st to end	Night	19% 4 27% 33% 16% 2	836
November	1st to 10th	Night	16% 5% 22% 39% 16% 2	780
	11th to 20th	Night	15% 7% 34% 32% 12% 1	800
	21st to end	Night	19% 7% 29% 33% 11% 1	800
December	1st to 10th	Night	26% 9% 27% 31% 7%	800
	11th to 20th	Night	23% 8% 30% 32% 7%	825
	21st to end	Night	26% 11% 30% 30% 5%	1100

Page generated at 2025-07-03 00:30:14. Version ac07

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