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 2026-10-29.

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	12% 7% 3 7% 4 4 4 4 6% 8% 41%	148986
	Night	13% 8% 3 7% 4 4 4 3 5% 7% 43%	143259
Transparency	Day	41% 4 11% 24% 16% 3	152648
	Night	41% 4 10% 24% 18% 4	147659
Seeing	Day	15% 8% 22% 24% 20% 10%	17999
	Night	17% 6% 23% 32% 19% 3	27670

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% 7% 10% 5% 4 4 3 4 5% 32%	4691
	11th to 20th	Day	13% 8% 3 9% 4 3 3 2 4 4 49%	4710
	21st to end	Day	21% 9% 4 6% 3 4 3 3 4 5% 37%	6239
February	1st to 10th	Day	18% 11% 3 7% 4 5% 3 3 5% 6% 36%	2100
	11th to 20th	Day	21% 10% 4 7% 4 4 3 3 4 6% 35%	2048
	21st to end	Day	17% 10% 3 7% 5% 4 3 3 4 5% 38%	1730
March	1st to 10th	Day	17% 13% 4 7% 4 4 4 3 4 4 37%	4580
	11th to 20th	Day	12% 9% 3 6% 4 3 3 3 5% 5% 48%	4797
	21st to end	Day	12% 7% 3 7% 5% 4 4 3 5% 6% 43%	6040
April	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 4 7% 45%	2520
	11th to 20th	Day	14% 7% 2 7% 5% 4 4 3 6% 7% 42%	2494
	21st to end	Day	14% 7% 3 6% 6% 4 4 4 5% 7% 40%	2533
May	1st to 10th	Day	16% 9% 3 7% 5% 5% 5% 4 6% 8% 32%	7616
	11th to 20th	Day	14% 7% 3 8% 5% 4 5% 4 6% 7% 37%	7626
	21st to end	Day	18% 8% 4 9% 5% 5% 5% 4 5% 7% 31%	8391
June	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 6% 9% 44%	2870
	11th to 20th	Day	6% 4 2 6% 5% 5% 6% 5% 7% 10% 46%	2940
	21st to end	Day	5% 4 2 4 5% 5% 5% 5% 9% 13% 44%	2940
July	1st to 10th	Day	8% 6% 2 6% 5% 5% 6% 5% 7% 12% 37%	8280
	11th to 20th	Day	8% 5% 2 7% 5% 6% 6% 5% 9% 13% 34%	8280
	21st to end	Day	10% 7% 2 7% 5% 5% 5% 5% 7% 12% 34%	8483
August	1st to 10th	Day	6% 4 2 5% 4 5% 5% 4 7% 9% 48%	5641
	11th to 20th	Day	6% 4 2 6% 4 4 5% 5% 7% 10% 47%	5486
	21st to end	Day	4 3 1 4 3 3 4 4 5% 8% 60%	6080
September	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 4 6% 7% 45%	2520
	11th to 20th	Day	12% 5% 2 5% 5% 4 4 4 5% 8% 48%	2315
	21st to end	Day	12% 6% 2 6% 4 4 4 3 5% 7% 47%	2310
October	1st to 10th	Day	11% 4 1 5% 3 4 3 2 4 6% 56%	2310
	11th to 20th	Day	12% 5% 2 5% 3 3 3 2 4 6% 55%	2202
	21st to end	Day	18% 6% 3 5% 4 3 4 3 4 5% 47%	2097
November	1st to 10th	Day	13% 6% 2 4 3 3 2 3 5% 6% 53%	1852
	11th to 20th	Day	14% 6% 3 5% 3 3 3 3 4 6% 49%	1890
	21st to end	Day	15% 8% 3 6% 4 3 4 3 4 5% 44%	1890
December	1st to 10th	Day	16% 9% 2 6% 3 4 3 4 4 6% 43%	3020
	11th to 20th	Day	18% 10% 3 8% 5% 4 4 3 5% 6% 34%	3395
	21st to end	Day	17% 11% 4 9% 6% 5% 5% 4 5% 7% 28%	4070
January	1st to 10th	Night	17% 10% 3 8% 3 4 4 3 5% 6% 38%	8232
	11th to 20th	Night	15% 9% 3 8% 4 3 3 3 3 4 44%	8280
	21st to end	Night	19% 11% 4 8% 4 4 3 2 3 5% 37%	7929
February	1st to 10th	Night	21% 11% 3 8% 4 3 4 3 5% 5% 33%	2520
	11th to 20th	Night	21% 9% 4 7% 4 4 3 3 4 4 36%	2458
	21st to end	Night	21% 8% 3 7% 5% 4 3 3 5% 5% 36%	2076
March	1st to 10th	Night	19% 11% 3 6% 4 3 4 3 5% 6% 36%	5493
	11th to 20th	Night	12% 7% 2 5% 4 4 4 3 5% 6% 49%	5383
	21st to end	Night	13% 8% 3 7% 5% 4 4 3 5% 7% 41%	5174
April	1st to 10th	Night	12% 6% 2 6% 3 4 4 4 6% 7% 47%	2100
	11th to 20th	Night	15% 6% 2 5% 3 3 4 3 5% 7% 47%	2075
	21st to end	Night	13% 7% 2 6% 5% 5% 4 3 6% 8% 42%	2084
May	1st to 10th	Night	16% 8% 3 8% 5% 4 4 4 5% 7% 35%	5237
	11th to 20th	Night	12% 8% 3 8% 5% 5% 5% 4 6% 8% 37%	5234
	21st to end	Night	18% 9% 3 8% 5% 4 5% 4 6% 8% 31%	5761
June	1st to 10th	Night	11% 6% 2 6% 3 4 4 4 6% 8% 47%	1744
	11th to 20th	Night	4 4 1 4 5% 3 5% 3 6% 9% 54%	1680
	21st to end	Night	3 2 1 5% 5% 5% 5% 5% 7% 12% 50%	1680
July	1st to 10th	Night	6% 5% 2 6% 4 5% 5% 5% 7% 11% 43%	4708
	11th to 20th	Night	6% 5% 2 6% 4 5% 6% 6% 9% 13% 39%	4707
	21st to end	Night	8% 6% 2 7% 5% 5% 5% 4 8% 11% 37%	5586
August	1st to 10th	Night	5% 4 2 5% 4 4 5% 4 6% 8% 53%	4503
	11th to 20th	Night	4 3 1 4 4 4 4 4 6% 10% 56%	4560
	21st to end	Night	4 3 1 4 3 3 4 3 6% 8% 61%	5046
September	1st to 10th	Night	10% 5% 2 5% 3 3 4 3 6% 8% 51%	2100
	11th to 20th	Night	9% 5% 2 5% 4 3 4 3 6% 7% 52%	2284
	21st to end	Night	10% 5% 2 5% 4 3 4 4 5% 6% 52%	2310
October	1st to 10th	Night	9% 4 2 3 2 3 3 3 4 5% 62%	2310
	11th to 20th	Night	14% 5% 2 5% 3 3 3 2 3 6% 54%	2414
	21st to end	Night	16% 7% 2 5% 3 3 4 3 3 5% 50%	2918
November	1st to 10th	Night	14% 5% 2 5% 3 3 3 2 4 6% 53%	2666
	11th to 20th	Night	12% 6% 2 5% 4 3 4 3 5% 7% 49%	2730
	21st to end	Night	16% 8% 2 4 4 4 3 2 5% 5% 46%	2730
December	1st to 10th	Night	18% 10% 2 7% 4 3 3 3 4 4 42%	4380
	11th to 20th	Night	21% 8% 3 6% 3 3 3 3 4 5% 41%	5017
	21st to end	Night	18% 12% 5% 10% 5% 5% 5% 3 4 5% 27%	7150

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 11% 22% 9% 1	4692
	11th to 20th	Day	40% 2 5% 28% 23% 1	4710
	21st to end	Day	50% 3 7% 21% 14% 5%	6239
February	1st to 10th	Day	50% 3 9% 23% 13% 2	2100
	11th to 20th	Day	53% 3 8% 20% 13% 4	2058
	21st to end	Day	49% 3 8% 19% 15% 5%	1730
March	1st to 10th	Day	53% 2 4 17% 18% 7%	4580
	11th to 20th	Day	40% 3 8% 24% 20% 6%	4797
	21st to end	Day	42% 4 10% 27% 17% 1	6042
April	1st to 10th	Day	38% 5% 11% 23% 20% 2	2520
	11th to 20th	Day	41% 4 9% 22% 20% 4	2494
	21st to end	Day	44% 4 10% 25% 16% 2	2533
May	1st to 10th	Day	49% 5% 13% 22% 10% 1	7651
	11th to 20th	Day	46% 5% 13% 21% 14% 2	7630
	21st to end	Day	53% 5% 9% 17% 13% 2	8405
June	1st to 10th	Day	36% 7% 11% 23% 19% 3	2870
	11th to 20th	Day	32% 7% 15% 26% 18% 3	2940
	21st to end	Day	30% 7% 17% 26% 16% 5%	2940
July	1st to 10th	Day	40% 5% 16% 28% 10% 1	8211
	11th to 20th	Day	42% 5% 20% 26% 6% 1	8205
	21st to end	Day	42% 3 22% 24% 7% 1	8297
August	1st to 10th	Day	31% 5% 14% 30% 19% 1	5628
	11th to 20th	Day	33% 4 15% 31% 16% 1	5522
	21st to end	Day	23% 4 10% 37% 25% 2	6079
September	1st to 10th	Day	37% 7% 9% 23% 19% 4	2520
	11th to 20th	Day	36% 5% 9% 26% 22% 2	2315
	21st to end	Day	37% 5% 9% 22% 22% 6%	2310
October	1st to 10th	Day	22% 2 5% 17% 44% 10%	3700
	11th to 20th	Day	24% 3 6% 24% 31% 12%	3494
	21st to end	Day	31% 2 5% 25% 29% 8%	3322
November	1st to 10th	Day	34% 4 8% 23% 27% 4	1852
	11th to 20th	Day	37% 3 8% 23% 23% 6%	1890
	21st to end	Day	43% 4 7% 24% 19% 4	1890
December	1st to 10th	Day	44% 2 6% 28% 19% 1	3020
	11th to 20th	Day	52% 3 8% 20% 15% 3	3392
	21st to end	Day	54% 4 11% 22% 8%	4070
January	1st to 10th	Night	49% 2 13% 24% 11% 1	8233
	11th to 20th	Night	46% 3 3 22% 23% 3	8280
	21st to end	Night	53% 2 7% 20% 14% 4	7929
February	1st to 10th	Night	54% 4 9% 18% 14% 1	2520
	11th to 20th	Night	53% 3 6% 20% 15% 3	2459
	21st to end	Night	51% 4 7% 16% 18% 4	2076
March	1st to 10th	Night	50% 3 6% 15% 20% 6%	5493
	11th to 20th	Night	37% 4 8% 22% 19% 11%	5383
	21st to end	Night	44% 3 9% 26% 16% 2	5174
April	1st to 10th	Night	37% 4 11% 25% 20% 3	2100
	11th to 20th	Night	37% 4 9% 21% 22% 6%	2075
	21st to end	Night	40% 4 10% 26% 18% 2	2084
May	1st to 10th	Night	49% 4 12% 23% 12% 1	5264
	11th to 20th	Night	46% 5% 13% 19% 16% 1	5235
	21st to end	Night	51% 6% 13% 18% 11% 1	5771
June	1st to 10th	Night	35% 6% 12% 24% 19% 3	1744
	11th to 20th	Night	25% 6% 16% 27% 22% 3	1680
	21st to end	Night	25% 7% 18% 29% 16% 5%	1680
July	1st to 10th	Night	35% 6% 17% 28% 12% 1	4670
	11th to 20th	Night	37% 7% 21% 26% 7% 1	4662
	21st to end	Night	40% 4 23% 25% 8% 1	5459
August	1st to 10th	Night	29% 4 13% 32% 21% 1	4492
	11th to 20th	Night	23% 4 18% 37% 17% 1	4590
	21st to end	Night	23% 3 9% 37% 27% 2	5046
September	1st to 10th	Night	30% 6% 11% 26% 21% 6%	2100
	11th to 20th	Night	32% 4 8% 27% 25% 4	2284
	21st to end	Night	33% 4 8% 22% 26% 6%	2310
October	1st to 10th	Night	20% 2 4 17% 43% 12%	3700
	11th to 20th	Night	21% 2 4 22% 37% 13%	3848
	21st to end	Night	29% 2 6% 24% 28% 11%	4643
November	1st to 10th	Night	34% 3 7% 25% 24% 6%	2666
	11th to 20th	Night	34% 4 7% 24% 24% 8%	2730
	21st to end	Night	41% 3 8% 26% 18% 4	2730
December	1st to 10th	Night	47% 2 6% 21% 21% 1	4380
	11th to 20th	Night	47% 2 7% 23% 19% 2	5019
	21st to end	Night	59% 3 8% 19% 9% 2	7150

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	23% 16% 34% 20% 7%	330
	11th to 20th	Day	27% 17% 38% 15% 3	330
	21st to end	Day	26% 13% 41% 16% 4	363
February	1st to 10th	Day	25% 17% 33% 20% 4 2	330
	11th to 20th	Day	31% 19% 34% 12% 3 1	330
	21st to end	Day	29% 14% 37% 14% 6%	273
March	1st to 10th	Day	25% 13% 39% 19% 5% 1	330
	11th to 20th	Day	21% 18% 36% 19% 5% 1	403
	21st to end	Day	22% 14% 36% 20% 7% 1	594
April	1st to 10th	Day	19% 13% 30% 26% 11% 2	540
	11th to 20th	Day	17% 12% 28% 28% 13% 2	532
	21st to end	Day	17% 9% 35% 24% 10% 5%	540
May	1st to 10th	Day	18% 13% 29% 25% 12% 4	543
	11th to 20th	Day	16% 7% 24% 26% 19% 8%	567
	21st to end	Day	14% 5% 19% 27% 21% 15%	617
June	1st to 10th	Day	12% 4 15% 28% 23% 18%	710
	11th to 20th	Day	6% 4 16% 27% 32% 16%	780
	21st to end	Day	5% 3 15% 32% 27% 17%	780
July	1st to 10th	Day	12% 2 11% 23% 28% 24%	771
	11th to 20th	Day	9% 4 9% 24% 33% 22%	768
	21st to end	Day	10% 3 11% 25% 29% 23%	674
August	1st to 10th	Day	9% 2 7% 28% 36% 19%	566
	11th to 20th	Day	11% 3 9% 27% 36% 14%	559
	21st to end	Day	10% 2 7% 22% 36% 25%	627
September	1st to 10th	Day	9% 4 8% 26% 32% 21%	570
	11th to 20th	Day	11% 3 7% 21% 38% 20%	570
	21st to end	Day	16% 5% 14% 26% 27% 11%	570
October	1st to 10th	Day	12% 3 15% 32% 28% 11%	570
	11th to 20th	Day	12% 7% 28% 29% 19% 5%	462
	21st to end	Day	22% 10% 28% 23% 15% 2	387
November	1st to 10th	Day	19% 10% 28% 25% 15% 4	330
	11th to 20th	Day	14% 16% 32% 26% 12%	330
	21st to end	Day	21% 12% 31% 22% 13% 2	330
December	1st to 10th	Day	23% 14% 35% 20% 9%	330
	11th to 20th	Day	21% 15% 30% 23% 9% 1	330
	21st to end	Day	23% 17% 35% 20% 5%	363
January	1st to 10th	Night	22% 12% 34% 28% 5%	1050
	11th to 20th	Night	25% 11% 35% 25% 4	1050
	21st to end	Night	23% 10% 36% 27% 3	986
February	1st to 10th	Night	25% 9% 36% 26% 4	840
	11th to 20th	Night	27% 13% 34% 23% 2	823
	21st to end	Night	24% 12% 37% 25% 2	692
March	1st to 10th	Night	20% 12% 32% 31% 5%	840
	11th to 20th	Night	22% 9% 33% 29% 7%	840
	21st to end	Night	18% 10% 32% 32% 8%	743
April	1st to 10th	Night	15% 8% 33% 36% 8%	630
	11th to 20th	Night	18% 5% 31% 34% 12%	623
	21st to end	Night	16% 5% 30% 37% 12% 1	630
May	1st to 10th	Night	18% 3 29% 34% 15% 2	630
	11th to 20th	Night	12% 5% 20% 37% 22% 4	621
	21st to end	Night	9% 3 13% 41% 30% 4	683
June	1st to 10th	Night	12% 3 10% 41% 29% 4	630
	11th to 20th	Night	5% 2 9% 40% 39% 4	630
	21st to end	Night	3 2 10% 41% 36% 7%	630
July	1st to 10th	Night	8% 2 6% 27% 48% 9%	615
	11th to 20th	Night	8% 2 5% 27% 50% 7%	612
	21st to end	Night	7% 2 8% 31% 43% 8%	690
August	1st to 10th	Night	7% 1 7% 30% 47% 8%	627
	11th to 20th	Night	6% 2 6% 39% 42% 4	624
	21st to end	Night	7% 1 5% 33% 46% 8%	693
September	1st to 10th	Night	11% 2 8% 31% 42% 6%	630
	11th to 20th	Night	12% 2 9% 28% 41% 7%	814
	21st to end	Night	12% 3 16% 31% 34% 4	840
October	1st to 10th	Night	11% 3 15% 37% 28% 6%	840
	11th to 20th	Night	18% 4 21% 36% 20% 1	840
	21st to end	Night	19% 4 26% 33% 16% 2	913
November	1st to 10th	Night	15% 5% 22% 40% 16% 2	820
	11th to 20th	Night	15% 7% 34% 32% 11% 1	840
	21st to end	Night	20% 7% 28% 33% 11% 1	840
December	1st to 10th	Night	26% 9% 27% 31% 7%	840
	11th to 20th	Night	22% 8% 31% 32% 7%	866
	21st to end	Night	26% 11% 29% 29% 4	1155

Page generated at 2026-10-26 03:37:45. Version ac07

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