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 2024-03-23.

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 5% 5% 3 5% 5% 5% 7% 40%	78936
	Night	14% 7% 3 5% 5% 3 4 4 5% 7% 43%	80980
Transparency	Day	42% 6% 11% 22% 16% 3	78647
	Night	41% 5% 10% 23% 18% 4	80790
Seeing	Day	16% 7% 21% 24% 21% 11%	14370
	Night	17% 6% 23% 32% 19% 3	24318

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% 8% 6% 4 4 4 4 5% 31%	2527
	11th to 20th	Day	19% 10% 3 7% 6% 3 4 4 4 4 35%	2530
	21st to end	Day	25% 11% 5% 5% 4 2 3 4 5% 6% 30%	3357
February	1st to 10th	Day	18% 10% 3 7% 4 3 4 4 4 7% 36%	1900
	11th to 20th	Day	21% 9% 4 6% 4 3 3 3 5% 6% 36%	1848
	21st to end	Day	18% 10% 4 7% 5% 3 4 3 4 5% 37%	1570
March	1st to 10th	Day	16% 10% 3 6% 5% 3 5% 4 4 5% 39%	1900
	11th to 20th	Day	18% 10% 4 6% 5% 3 4 3 4 5% 37%	1966
	21st to end	Day	17% 8% 4 8% 6% 3 5% 5% 5% 6% 32%	2370
April	1st to 10th	Day	13% 6% 2 5% 5% 3 4 5% 4 7% 46%	2160
	11th to 20th	Day	15% 7% 3 5% 6% 3 4 3 5% 6% 42%	2134
	21st to end	Day	15% 7% 3 6% 6% 3 4 4 5% 7% 40%	2170
May	1st to 10th	Day	16% 6% 3 5% 5% 3 4 5% 5% 8% 41%	2340
	11th to 20th	Day	14% 7% 3 7% 6% 4 4 6% 5% 6% 39%	2301
	21st to end	Day	9% 4 3 6% 7% 4 6% 6% 7% 10% 38%	2531
June	1st to 10th	Day	10% 5% 2 5% 4 3 5% 5% 6% 10% 45%	2459
	11th to 20th	Day	5% 3 2 4 5% 4 5% 7% 7% 10% 48%	2520
	21st to end	Day	5% 3 2 4 5% 4 6% 6% 9% 13% 44%	2520
July	1st to 10th	Day	8% 6% 2 6% 7% 5% 7% 7% 7% 11% 34%	2520
	11th to 20th	Day	8% 4 2 5% 6% 4 6% 6% 6% 10% 41%	2520
	21st to end	Day	8% 5% 2 5% 6% 4 7% 7% 7% 11% 38%	2594
August	1st to 10th	Day	6% 4 2 4 6% 4 7% 8% 8% 10% 40%	2186
	11th to 20th	Day	10% 5% 3 6% 6% 4 5% 6% 6% 9% 40%	2125
	21st to end	Day	8% 5% 2 5% 6% 4 6% 6% 7% 9% 42%	2376
September	1st to 10th	Day	11% 5% 3 4 6% 3 6% 5% 6% 7% 43%	2160
	11th to 20th	Day	12% 5% 2 5% 6% 3 4 5% 4 7% 46%	1985
	21st to end	Day	11% 5% 3 4 4 3 4 4 4 8% 51%	1980
October	1st to 10th	Day	11% 4 1 4 3 4 4 3 4 6% 55%	1980
	11th to 20th	Day	13% 5% 2 5% 5% 2 4 3 4 6% 52%	1888
	21st to end	Day	18% 6% 3 4 5% 2 4 4 4 5% 46%	1821
November	1st to 10th	Day	12% 5% 2 4 3 2 3 3 4 6% 55%	1672
	11th to 20th	Day	15% 5% 3 4 4 2 3 4 5% 6% 50%	1710
	21st to end	Day	16% 7% 3 5% 4 2 4 5% 4 5% 45%	1710
December	1st to 10th	Day	20% 9% 3 5% 4 3 3 5% 4 6% 38%	1710
	11th to 20th	Day	16% 10% 3 6% 5% 3 4 4 5% 6% 36%	2113
	21st to end	Day	16% 11% 4 7% 7% 3 5% 5% 5% 8% 30%	2783
January	1st to 10th	Night	18% 10% 4 5% 4 2 4 4 5% 7% 36%	4428
	11th to 20th	Night	18% 11% 4 9% 6% 3 4 4 4 4 35%	4440
	21st to end	Night	24% 11% 4 5% 4 3 3 3 3 6% 34%	4294
February	1st to 10th	Night	22% 10% 4 7% 4 3 4 3 5% 5% 33%	2280
	11th to 20th	Night	20% 8% 4 6% 4 3 4 3 4 4 38%	2218
	21st to end	Night	21% 8% 4 6% 6% 3 4 5% 4 6% 33%	1884
March	1st to 10th	Night	17% 8% 3 5% 5% 2 4 3 6% 7% 41%	2280
	11th to 20th	Night	17% 9% 4 6% 5% 3 3 3 4 6% 40%	2214
	21st to end	Night	15% 9% 4 7% 6% 3 5% 4 5% 6% 37%	2046
April	1st to 10th	Night	11% 5% 2 5% 4 3 4 5% 5% 8% 49%	1800
	11th to 20th	Night	16% 6% 2 5% 3 2 4 4 5% 7% 47%	1775
	21st to end	Night	13% 6% 3 5% 5% 3 5% 4 5% 9% 43%	1787
May	1st to 10th	Night	13% 5% 3 5% 5% 3 3 5% 4 7% 44%	1620
	11th to 20th	Night	10% 6% 3 6% 6% 3 5% 5% 6% 9% 41%	1592
	21st to end	Night	6% 4 1 4 6% 3 5% 7% 7% 12% 44%	1744
June	1st to 10th	Night	12% 6% 2 5% 4 3 4 4 6% 7% 48%	1495
	11th to 20th	Night	3 3 2 3 5% 3 5% 4 6% 10% 57%	1440
	21st to end	Night	3 2 1 4 4 3 6% 6% 7% 12% 51%	1440
July	1st to 10th	Night	6% 5% 3 4 4 4 5% 6% 7% 13% 43%	1440
	11th to 20th	Night	6% 5% 2 5% 5% 3 5% 5% 6% 10% 48%	1440
	21st to end	Night	6% 3 2 5% 5% 3 5% 5% 7% 9% 50%	1723
August	1st to 10th	Night	5% 5% 2 5% 5% 2 5% 6% 6% 9% 50%	1747
	11th to 20th	Night	6% 5% 2 4 6% 4 5% 5% 7% 10% 47%	1776
	21st to end	Night	7% 4 1 4 5% 3 4 6% 7% 10% 49%	1980
September	1st to 10th	Night	12% 4 2 5% 4 2 4 4 6% 8% 49%	1800
	11th to 20th	Night	10% 5% 2 4 4 2 3 4 6% 7% 52%	1957
	21st to end	Night	9% 4 2 4 4 2 4 4 4 7% 55%	1980
October	1st to 10th	Night	9% 4 2 2 3 2 3 4 4 5% 61%	1980
	11th to 20th	Night	16% 5% 2 5% 3 3 3 3 3 6% 51%	2069
	21st to end	Night	16% 7% 2 4 4 2 4 4 3 5% 48%	2533
November	1st to 10th	Night	13% 5% 2 4 3 2 3 3 4 6% 55%	2406
	11th to 20th	Night	12% 5% 2 4 4 2 4 3 5% 7% 50%	2470
	21st to end	Night	16% 8% 2 4 4 3 4 3 4 6% 47%	2470
December	1st to 10th	Night	20% 11% 3 5% 5% 3 3 4 5% 5% 36%	2470
	11th to 20th	Night	18% 8% 3 4 4 2 4 4 3 6% 43%	3078
	21st to end	Night	17% 12% 5% 8% 6% 4 5% 5% 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	48% 3 8% 19% 17% 5%	1900
	11th to 20th	Day	50% 4 9% 21% 14% 2	1966
	21st to end	Day	51% 5% 11% 20% 11% 2	2370
April	1st to 10th	Day	37% 6% 10% 24% 21% 3	2160
	11th to 20th	Day	42% 4 8% 21% 20% 4	2134
	21st to end	Day	44% 4 9% 25% 15% 2	2170
May	1st to 10th	Day	42% 6% 12% 25% 13% 1	2340
	11th to 20th	Day	44% 7% 10% 23% 15% 1	2305
	21st to end	Day	39% 10% 15% 24% 12% 1	2531
June	1st to 10th	Day	35% 8% 12% 23% 20% 3	2459
	11th to 20th	Day	29% 8% 15% 26% 18% 3	2520
	21st to end	Day	29% 8% 17% 25% 16% 5%	2520
July	1st to 10th	Day	40% 10% 14% 20% 12% 4	2451
	11th to 20th	Day	36% 8% 17% 22% 13% 3	2445
	21st to end	Day	36% 9% 16% 25% 11% 2	2450
August	1st to 10th	Day	35% 10% 17% 25% 13% 1	2173
	11th to 20th	Day	38% 8% 13% 23% 16% 2	2125
	21st to end	Day	36% 8% 13% 23% 16% 3	2376
September	1st to 10th	Day	39% 8% 10% 22% 18% 2	2160
	11th to 20th	Day	38% 6% 9% 23% 22% 3	1985
	21st to end	Day	33% 5% 10% 24% 22% 6%	1980
October	1st to 10th	Day	31% 4 9% 24% 27% 5%	1980
	11th to 20th	Day	34% 5% 6% 21% 26% 8%	1888
	21st to end	Day	41% 4 6% 21% 22% 5%	1821
November	1st to 10th	Day	31% 4 8% 23% 28% 5%	1672
	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	44% 5% 8% 16% 20% 7%	2280
	11th to 20th	Night	47% 4 9% 23% 14% 3	2214
	21st to end	Night	47% 5% 11% 22% 13% 2	2046
April	1st to 10th	Night	34% 5% 10% 26% 22% 3	1800
	11th to 20th	Night	38% 5% 8% 20% 23% 7%	1775
	21st to end	Night	39% 5% 11% 25% 18% 2	1787
May	1st to 10th	Night	40% 5% 10% 24% 17% 4	1620
	11th to 20th	Night	37% 9% 14% 23% 16% 1	1593
	21st to end	Night	30% 9% 18% 24% 16% 2	1745
June	1st to 10th	Night	34% 6% 13% 25% 20% 3	1495
	11th to 20th	Night	23% 7% 18% 28% 21% 4	1440
	21st to end	Night	24% 8% 18% 29% 15% 6%	1440
July	1st to 10th	Night	30% 10% 17% 26% 14% 3	1400
	11th to 20th	Night	31% 9% 16% 24% 16% 4	1392
	21st to end	Night	27% 9% 17% 31% 14% 2	1627
August	1st to 10th	Night	29% 8% 15% 28% 19% 1	1736
	11th to 20th	Night	29% 9% 18% 24% 17% 2	1776
	21st to end	Night	29% 7% 14% 27% 17% 5%	1980
September	1st to 10th	Night	31% 7% 11% 26% 19% 5%	1800
	11th to 20th	Night	31% 5% 9% 26% 25% 5%	1957
	21st to end	Night	29% 4 8% 24% 28% 7%	1980
October	1st to 10th	Night	26% 4 8% 25% 29% 8%	1980
	11th to 20th	Night	37% 4 7% 20% 25% 7%	2069
	21st to end	Night	39% 3 6% 22% 24% 5%	2533
November	1st to 10th	Night	32% 3 7% 25% 25% 7%	2406
	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% 31% 21% 8% 1	470
April	1st to 10th	Day	17% 13% 30% 25% 13% 2	420
	11th to 20th	Day	19% 11% 25% 29% 14% 2	412
	21st to end	Day	19% 8% 33% 24% 11% 5%	420
May	1st to 10th	Day	18% 10% 28% 26% 13% 4	423
	11th to 20th	Day	17% 5% 22% 27% 19% 10%	447
	21st to end	Day	11% 2 15% 28% 25% 19%	485
June	1st to 10th	Day	11% 3 14% 27% 25% 20%	569
	11th to 20th	Day	4 3 15% 27% 33% 18%	630
	21st to end	Day	5% 3 15% 33% 28% 16%	630
July	1st to 10th	Day	11% 2 10% 24% 27% 25%	621
	11th to 20th	Day	10% 3 9% 22% 32% 24%	618
	21st to end	Day	9% 2 9% 25% 29% 25%	535
August	1st to 10th	Day	9% 2 6% 27% 36% 20%	446
	11th to 20th	Day	12% 3 8% 26% 37% 14%	439
	21st to end	Day	12% 2 5% 20% 37% 24%	495
September	1st to 10th	Day	10% 4 8% 23% 34% 22%	450
	11th to 20th	Day	11% 3 8% 21% 38% 20%	450
	21st to end	Day	16% 5% 14% 23% 29% 13%	450
October	1st to 10th	Day	11% 2 17% 31% 28% 11%	450
	11th to 20th	Day	13% 9% 30% 29% 15% 4	358
	21st to end	Day	25% 10% 26% 23% 14% 1	297
November	1st to 10th	Day	17% 7% 28% 26% 17% 4	270
	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	20% 9% 29% 33% 9%	646
April	1st to 10th	Night	13% 7% 34% 38% 8% 1	540
	11th to 20th	Night	20% 4 29% 33% 14%	533
	21st to end	Night	16% 4 31% 35% 13% 1	540
May	1st to 10th	Night	17% 3 29% 34% 16% 2	540
	11th to 20th	Night	11% 5% 20% 37% 23% 4	531
	21st to end	Night	7% 2 11% 42% 33% 5%	584
June	1st to 10th	Night	13% 3 10% 39% 31% 5%	540
	11th to 20th	Night	4 1 9% 41% 40% 4	540
	21st to end	Night	3 1 10% 42% 37% 7%	540
July	1st to 10th	Night	7% 2 6% 28% 47% 10%	525
	11th to 20th	Night	8% 2 6% 28% 48% 7%	522
	21st to end	Night	7% 2 7% 31% 45% 8%	591
August	1st to 10th	Night	7% 1 6% 30% 48% 8%	537
	11th to 20th	Night	7% 2 5% 39% 42% 4	534
	21st to end	Night	7% 1 4 32% 47% 9%	594
September	1st to 10th	Night	12% 2 7% 29% 43% 6%	540
	11th to 20th	Night	13% 2 10% 29% 41% 6%	697
	21st to end	Night	10% 3 16% 30% 36% 5%	720
October	1st to 10th	Night	11% 3 16% 34% 29% 6%	720
	11th to 20th	Night	19% 4 22% 34% 18% 2	720
	21st to end	Night	20% 4 27% 32% 15% 2	792
November	1st to 10th	Night	15% 4 22% 40% 17% 2	740
	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 2024-03-19 22:31:06. Version ac07

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