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 2024-11-03.

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	13% 7% 3 6% 4 4 4 4 5% 7% 41%	100998
	Night	13% 7% 3 6% 4 4 4 3 5% 7% 44%	98011
Transparency	Day	41% 5% 10% 24% 17% 3	100795
	Night	40% 5% 10% 24% 18% 4	97885
Seeing	Day	15% 7% 21% 24% 21% 11%	15300
	Night	17% 6% 23% 32% 20% 3	25046

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	13% 13% 6% 10% 5% 5% 4 3 4 5% 31%	2527
	11th to 20th	Day	19% 10% 3 10% 4 3 4 3 4 4 35%	2530
	21st to end	Day	25% 11% 5% 6% 3 3 3 3 5% 6% 30%	3357
February	1st to 10th	Day	18% 11% 3 7% 3 5% 3 3 5% 6% 36%	1900
	11th to 20th	Day	21% 10% 3 7% 4 4 3 3 5% 6% 35%	1848
	21st to end	Day	18% 11% 3 7% 5% 4 3 3 4 5% 37%	1570
March	1st to 10th	Day	22% 14% 3 7% 4 4 4 3 4 4 33%	3120
	11th to 20th	Day	13% 8% 2 5% 4 3 3 3 5% 5% 48%	3278
	21st to end	Day	12% 7% 3 8% 5% 4 4 4 5% 6% 42%	4122
April	1st to 10th	Day	13% 7% 2 6% 4 5% 4 4 4 7% 45%	2280
	11th to 20th	Day	15% 7% 2 6% 5% 4 4 3 5% 6% 42%	2254
	21st to end	Day	14% 7% 3 7% 6% 4 3 4 5% 7% 40%	2291
May	1st to 10th	Day	16% 8% 3 7% 4 5% 5% 4 6% 8% 34%	4082
	11th to 20th	Day	16% 8% 3 7% 5% 4 4 4 5% 7% 37%	4081
	21st to end	Day	10% 6% 3 7% 6% 5% 5% 4 6% 8% 40%	4476
June	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 6% 9% 44%	2596
	11th to 20th	Day	5% 4 1 5% 5% 5% 6% 5% 7% 10% 47%	2660
	21st to end	Day	4 4 2 4 5% 6% 5% 5% 9% 13% 44%	2660
July	1st to 10th	Day	8% 5% 2 6% 5% 6% 6% 5% 7% 11% 39%	4440
	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 2 4 2 4 5% 47%	1922
November	1st to 10th	Day	12% 6% 1 4 3 3 2 2 5% 6% 56%	1690
	11th to 20th	Day	15% 5% 3 5% 4 3 3 3 5% 6% 49%	1710
	21st to end	Day	16% 8% 2 6% 4 3 4 4 5% 5% 45%	1710
December	1st to 10th	Day	20% 10% 2 6% 4 3 3 4 4 6% 37%	1710
	11th to 20th	Day	16% 11% 3 7% 5% 4 4 3 5% 6% 36%	2113
	21st to end	Day	15% 11% 4 8% 5% 4 5% 4 5% 8% 29%	2783
January	1st to 10th	Night	18% 10% 4 6% 3 3 4 3 5% 7% 36%	4428
	11th to 20th	Night	17% 11% 4 10% 5% 4 4 3 4 4 35%	4440
	21st to end	Night	24% 12% 4 6% 3 3 3 2 3 6% 33%	4294
February	1st to 10th	Night	22% 11% 3 8% 4 3 3 3 5% 5% 33%	2280
	11th to 20th	Night	20% 9% 3 7% 4 4 3 3 4 4 38%	2218
	21st to end	Night	21% 9% 3 7% 5% 4 4 4 5% 5% 33%	1884
March	1st to 10th	Night	24% 9% 3 7% 4 3 3 3 5% 5% 34%	3733
	11th to 20th	Night	13% 8% 2 5% 4 3 4 3 4 5% 49%	3682
	21st to end	Night	13% 8% 3 7% 5% 4 3 3 5% 6% 42%	3538
April	1st to 10th	Night	11% 6% 2 6% 3 4 4 3 6% 7% 48%	1900
	11th to 20th	Night	16% 6% 2 5% 3 3 4 3 4 7% 47%	1875
	21st to end	Night	13% 7% 2 6% 5% 5% 4 3 6% 8% 43%	1886
May	1st to 10th	Night	16% 8% 3 7% 4 4 4 4 5% 6% 39%	2813
	11th to 20th	Night	12% 8% 2 7% 4 4 4 4 6% 8% 41%	2812
	21st to end	Night	8% 6% 2 7% 5% 4 5% 4 7% 10% 43%	3079
June	1st to 10th	Night	12% 6% 2 6% 3 4 3 4 6% 8% 47%	1578
	11th to 20th	Night	3 4 2 3 5% 3 5% 3 6% 10% 56%	1520
	21st to end	Night	3 2 1 5% 4 5% 5% 5% 7% 12% 51%	1520
July	1st to 10th	Night	6% 5% 2 6% 4 5% 5% 4 8% 12% 42%	2530
	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	13% 5% 2 4 3 3 3 2 4 6% 55%	2433
	11th to 20th	Night	12% 6% 2 5% 4 3 4 3 5% 7% 50%	2470
	21st to end	Night	16% 8% 2 4 4 4 3 2 5% 5% 47%	2470
December	1st to 10th	Night	20% 12% 3 6% 5% 4 3 3 5% 5% 36%	2470
	11th to 20th	Night	18% 9% 3 5% 3 3 4 3 4 5% 43%	3078
	21st to end	Night	16% 12% 5% 9% 5% 5% 5% 3 5% 6% 29%	4884

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	55% 3 7% 22% 11% 2	2528
	11th to 20th	Day	51% 4 9% 21% 14% 1	2530
	21st to end	Day	54% 4 9% 22% 10% 1	3357
February	1st to 10th	Day	50% 3 9% 23% 13% 2	1900
	11th to 20th	Day	51% 3 7% 20% 13% 4	1858
	21st to end	Day	51% 3 9% 17% 14% 6%	1570
March	1st to 10th	Day	56% 3 5% 16% 13% 7%	3120
	11th to 20th	Day	37% 5% 9% 27% 18% 5%	3278
	21st to end	Day	42% 5% 10% 25% 17% 1	4122
April	1st to 10th	Day	38% 6% 10% 23% 20% 3	2280
	11th to 20th	Day	42% 5% 8% 21% 20% 4	2254
	21st to end	Day	44% 4 9% 25% 15% 2	2291
May	1st to 10th	Day	45% 8% 12% 22% 12% 1	4120
	11th to 20th	Day	45% 8% 10% 25% 13% 1	4085
	21st to end	Day	39% 8% 11% 20% 21% 1	4489
June	1st to 10th	Day	36% 8% 12% 23% 19% 3	2596
	11th to 20th	Day	30% 8% 15% 25% 19% 3	2660
	21st to end	Day	29% 8% 17% 24% 16% 5%	2660
July	1st to 10th	Day	38% 6% 18% 28% 9% 2	4371
	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	30% 4 9% 23% 29% 6%	2090
	11th to 20th	Day	33% 4 6% 22% 26% 8%	1992
	21st to end	Day	40% 4 6% 22% 23% 5%	1922
November	1st to 10th	Day	31% 4 8% 24% 29% 5%	1690
	11th to 20th	Day	36% 3 8% 23% 24% 6%	1710
	21st to end	Day	42% 4 7% 24% 19% 4	1710
December	1st to 10th	Day	49% 4 8% 23% 15% 2	1710
	11th to 20th	Day	47% 4 10% 20% 16% 2	2110
	21st to end	Day	52% 6% 12% 19% 11% 1	2783
January	1st to 10th	Night	47% 4 7% 26% 14% 1	4428
	11th to 20th	Night	53% 5% 6% 20% 14% 3	4440
	21st to end	Night	54% 3 8% 22% 12% 1	4294
February	1st to 10th	Night	53% 4 9% 18% 14% 1	2280
	11th to 20th	Night	51% 3 7% 20% 15% 4	2219
	21st to end	Night	53% 5% 7% 15% 16% 4	1884
March	1st to 10th	Night	52% 4 7% 13% 18% 5%	3733
	11th to 20th	Night	37% 5% 10% 24% 14% 10%	3682
	21st to end	Night	43% 4 9% 24% 18% 1	3538
April	1st to 10th	Night	35% 5% 10% 25% 21% 3	1900
	11th to 20th	Night	38% 5% 8% 20% 23% 7%	1875
	21st to end	Night	40% 5% 10% 25% 18% 2	1886
May	1st to 10th	Night	44% 7% 13% 22% 12% 2	2840
	11th to 20th	Night	40% 8% 13% 23% 15% 1	2813
	21st to end	Night	33% 9% 15% 23% 18% 1	3087
June	1st to 10th	Night	35% 6% 13% 24% 19% 3	1578
	11th to 20th	Night	23% 6% 17% 27% 23% 4	1520
	21st to end	Night	24% 8% 18% 28% 16% 6%	1520
July	1st to 10th	Night	33% 6% 16% 32% 11% 2	2490
	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	25% 3 8% 24% 31% 9%	2090
	11th to 20th	Night	35% 4 7% 21% 27% 7%	2184
	21st to end	Night	38% 3 7% 23% 24% 6%	2674
November	1st to 10th	Night	32% 3 7% 25% 25% 7%	2433
	11th to 20th	Night	34% 4 7% 23% 24% 8%	2470
	21st to end	Night	40% 3 7% 26% 20% 4	2470
December	1st to 10th	Night	50% 4 8% 21% 15% 2	2470
	11th to 20th	Night	43% 4 8% 22% 20% 3	3080
	21st to end	Night	56% 5% 9% 17% 11% 2	4884

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	24% 14% 34% 20% 8%	270
	11th to 20th	Day	30% 12% 39% 15% 4	270
	21st to end	Day	26% 11% 40% 18% 4	297
February	1st to 10th	Day	26% 16% 32% 19% 5% 2	270
	11th to 20th	Day	32% 17% 35% 12% 3 1	270
	21st to end	Day	32% 13% 35% 13% 6%	225
March	1st to 10th	Day	26% 12% 37% 20% 5% 1	270
	11th to 20th	Day	20% 17% 37% 19% 6% 1	336
	21st to end	Day	24% 14% 33% 21% 8% 1	506
April	1st to 10th	Day	18% 13% 30% 24% 12% 2	460
	11th to 20th	Day	19% 12% 26% 29% 13% 2	452
	21st to end	Day	18% 8% 35% 23% 11% 5%	460
May	1st to 10th	Day	17% 11% 28% 25% 14% 5%	463
	11th to 20th	Day	17% 6% 22% 26% 20% 10%	487
	21st to end	Day	11% 3 18% 28% 23% 17%	529
June	1st to 10th	Day	12% 4 15% 27% 24% 19%	616
	11th to 20th	Day	4 3 15% 27% 33% 18%	680
	21st to end	Day	5% 3 16% 33% 27% 16%	680
July	1st to 10th	Day	11% 2 11% 25% 27% 24%	671
	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	17% 7% 27% 26% 18% 4	276
	11th to 20th	Day	13% 14% 31% 27% 13%	270
	21st to end	Day	21% 9% 31% 23% 14% 2	270
December	1st to 10th	Day	23% 13% 33% 20% 10%	270
	11th to 20th	Day	19% 15% 31% 23% 11% 1	270
	21st to end	Day	23% 18% 33% 21% 6%	297
January	1st to 10th	Night	22% 12% 34% 27% 5%	950
	11th to 20th	Night	26% 11% 34% 24% 5%	950
	21st to end	Night	24% 9% 36% 27% 3	893
February	1st to 10th	Night	26% 9% 35% 26% 4	760
	11th to 20th	Night	25% 13% 35% 25% 3	743
	21st to end	Night	25% 12% 38% 23% 2	628
March	1st to 10th	Night	20% 12% 31% 32% 5%	760
	11th to 20th	Night	22% 9% 33% 29% 7%	760
	21st to end	Night	19% 10% 30% 32% 8%	673
April	1st to 10th	Night	14% 8% 33% 37% 8% 1	570
	11th to 20th	Night	20% 5% 30% 32% 13%	563
	21st to end	Night	15% 5% 32% 35% 12% 1	570
May	1st to 10th	Night	18% 3 28% 33% 16% 2	570
	11th to 20th	Night	12% 4 19% 37% 23% 4	561
	21st to end	Night	7% 2 12% 42% 32% 5%	617
June	1st to 10th	Night	13% 3 10% 39% 30% 5%	570
	11th to 20th	Night	4 1 9% 40% 41% 4	570
	21st to end	Night	3 2 10% 42% 36% 7%	570
July	1st to 10th	Night	8% 2 6% 28% 47% 10%	555
	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	15% 5% 22% 40% 17% 2	749
	11th to 20th	Night	14% 6% 34% 32% 12% 1	760
	21st to end	Night	20% 7% 28% 33% 12% 1	760
December	1st to 10th	Night	27% 8% 27% 31% 7%	760
	11th to 20th	Night	22% 7% 30% 33% 8%	783
	21st to end	Night	26% 11% 30% 29% 5%	1045

Page generated at 2024-10-31 02:39:24. Version ac07

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