Dark Arts Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dark Arts Observatory clear sky chart from 2005-11-06 to 2024-01-12.

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	15% 10% 4 8% 6% 3 5% 5% 5% 7% 31%	74687
	Night	17% 11% 5% 8% 7% 3 5% 5% 5% 7% 28%	75030
Transparency	Day	52% 7% 12% 18% 10% 1	74397
	Night	54% 7% 12% 17% 9% 1	74854
Seeing	Day	20% 15% 30% 23% 10% 3	13419
	Night	21% 15% 29% 24% 10% 1	22853

By Month

Percentage Cloud Cover NE (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% 17% 10% 10% 9% 6% 6% 5% 4 6% 13%	1517
	11th to 20th	Day	15% 20% 9% 17% 9% 5% 5% 4 3 3 11%	1514
	21st to end	Day	14% 16% 7% 15% 12% 6% 6% 4 5% 5% 10%	1782
February	1st to 10th	Day	16% 15% 7% 13% 9% 3 6% 5% 4 6% 15%	1620
	11th to 20th	Day	14% 15% 8% 15% 10% 4 6% 4 4 5% 16%	1573
	21st to end	Day	20% 14% 6% 14% 9% 4 5% 4 4 5% 15%	1426
March	1st to 10th	Day	15% 12% 7% 12% 8% 4 5% 4 4 6% 21%	1927
	11th to 20th	Day	14% 14% 4 9% 7% 3 5% 5% 4 5% 30%	1980
	21st to end	Day	18% 12% 5% 8% 6% 3 4 4 4 6% 31%	2205
April	1st to 10th	Day	16% 12% 5% 7% 6% 3 4 4 4 6% 33%	2160
	11th to 20th	Day	19% 8% 3 6% 5% 2 4 4 4 6% 38%	2134
	21st to end	Day	20% 9% 3 7% 6% 2 4 5% 5% 6% 33%	2298
May	1st to 10th	Day	14% 9% 3 6% 7% 4 5% 4 6% 7% 35%	2402
	11th to 20th	Day	14% 7% 3 5% 5% 3 4 5% 6% 7% 40%	2478
	21st to end	Day	11% 5% 3 5% 5% 4 5% 5% 5% 9% 43%	2725
June	1st to 10th	Day	14% 6% 3 5% 5% 3 5% 5% 6% 9% 39%	2520
	11th to 20th	Day	12% 5% 2 4 4 4 5% 5% 6% 11% 42%	2520
	21st to end	Day	12% 5% 3 5% 5% 3 4 4 5% 8% 43%	2520
July	1st to 10th	Day	8% 4 2 4 5% 3 4 5% 6% 9% 49%	2520
	11th to 20th	Day	11% 5% 2 5% 5% 3 5% 6% 7% 10% 40%	2520
	21st to end	Day	10% 5% 2 6% 6% 3 6% 6% 6% 9% 40%	2664
August	1st to 10th	Day	9% 5% 2 5% 5% 3 6% 6% 7% 9% 43%	2327
	11th to 20th	Day	9% 6% 2 6% 6% 4 6% 6% 6% 10% 38%	2303
	21st to end	Day	12% 6% 2 5% 5% 3 4 5% 6% 9% 43%	2399
September	1st to 10th	Day	14% 6% 2 5% 4 4 5% 5% 6% 7% 42%	2160
	11th to 20th	Day	14% 5% 2 5% 5% 3 5% 4 5% 8% 45%	2156
	21st to end	Day	15% 6% 3 7% 6% 4 6% 5% 5% 8% 36%	1881
October	1st to 10th	Day	21% 8% 3 5% 6% 4 4 4 5% 7% 32%	1800
	11th to 20th	Day	18% 10% 4 8% 7% 4 5% 5% 6% 7% 27%	1800
	21st to end	Day	28% 11% 3 7% 7% 3 6% 4 4 4 23%	1980
November	1st to 10th	Day	14% 11% 5% 7% 7% 4 5% 5% 5% 6% 31%	1638
	11th to 20th	Day	14% 15% 6% 9% 8% 5% 4 4 5% 6% 24%	1520
	21st to end	Day	20% 17% 5% 9% 7% 3 4 4 5% 7% 18%	1520
December	1st to 10th	Day	19% 20% 9% 11% 7% 3 4 4 4 5% 13%	1520
	11th to 20th	Day	18% 19% 8% 15% 7% 3 4 4 4 5% 13%	1906
	21st to end	Day	26% 22% 7% 13% 6% 3 4 3 3 3 10%	2772
January	1st to 10th	Night	17% 17% 8% 11% 10% 4 5% 4 4 5% 15%	2649
	11th to 20th	Night	17% 18% 7% 15% 11% 4 5% 3 3 5% 12%	2483
	21st to end	Night	13% 15% 7% 13% 12% 7% 5% 4 5% 5% 13%	2574
February	1st to 10th	Night	17% 16% 7% 12% 8% 4 5% 4 5% 5% 17%	2340
	11th to 20th	Night	14% 12% 7% 12% 11% 4 6% 5% 5% 5% 18%	2273
	21st to end	Night	18% 12% 6% 14% 7% 4 4 4 5% 7% 19%	1829
March	1st to 10th	Night	15% 14% 5% 12% 7% 4 5% 5% 5% 5% 24%	2017
	11th to 20th	Night	16% 11% 4 8% 5% 3 5% 4 5% 7% 30%	1979
	21st to end	Night	17% 10% 5% 8% 6% 3 4 4 5% 5% 33%	2150
April	1st to 10th	Night	19% 10% 5% 6% 6% 3 5% 5% 5% 6% 30%	1800
	11th to 20th	Night	16% 9% 3 7% 6% 3 5% 5% 5% 7% 35%	1775
	21st to end	Night	19% 8% 3 7% 4 3 4 5% 6% 7% 34%	1677
May	1st to 10th	Night	17% 8% 2 5% 6% 3 5% 6% 5% 7% 36%	1558
	11th to 20th	Night	12% 5% 3 6% 4 3 4 5% 6% 8% 42%	1415
	21st to end	Night	11% 4 2 5% 5% 3 4 5% 5% 8% 48%	1550
June	1st to 10th	Night	13% 5% 3 7% 5% 4 5% 6% 8% 9% 36%	1440
	11th to 20th	Night	12% 5% 2 5% 5% 4 6% 6% 6% 8% 41%	1440
	21st to end	Night	11% 5% 2 7% 6% 4 5% 6% 6% 8% 41%	1440
July	1st to 10th	Night	6% 5% 2 5% 5% 3 4 4 6% 10% 49%	1440
	11th to 20th	Night	11% 5% 2 5% 4 3 4 5% 7% 8% 47%	1440
	21st to end	Night	9% 5% 3 5% 4 2 5% 5% 6% 10% 46%	1650
August	1st to 10th	Night	9% 5% 2 4 4 4 5% 5% 6% 10% 46%	1611
	11th to 20th	Night	8% 5% 3 6% 6% 4 4 6% 6% 10% 42%	1598
	21st to end	Night	8% 6% 2 6% 5% 3 5% 5% 7% 8% 46%	1938
September	1st to 10th	Night	14% 5% 2 5% 5% 3 4 3 5% 8% 45%	1800
	11th to 20th	Night	15% 6% 2 5% 5% 3 4 4 4 8% 45%	1800
	21st to end	Night	15% 6% 3 6% 5% 3 5% 5% 6% 8% 38%	2078
October	1st to 10th	Night	20% 7% 3 6% 6% 3 4 5% 5% 6% 34%	2160
	11th to 20th	Night	23% 8% 3 6% 6% 3 5% 4 5% 7% 29%	2160
	21st to end	Night	29% 10% 3 6% 6% 3 4 5% 5% 5% 23%	2376
November	1st to 10th	Night	16% 10% 4 6% 5% 3 5% 6% 5% 7% 33%	2438
	11th to 20th	Night	15% 11% 5% 8% 8% 4 6% 5% 6% 7% 25%	2660
	21st to end	Night	20% 13% 6% 8% 7% 3 5% 5% 6% 7% 20%	2660
December	1st to 10th	Night	20% 18% 8% 11% 7% 4 5% 4 5% 6% 14%	2660
	11th to 20th	Night	18% 18% 8% 11% 7% 3 5% 4 5% 6% 14%	3288
	21st to end	Night	30% 18% 7% 11% 6% 3 4 4 5% 4 9%	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	72% 8% 9% 9% 2	1518
	11th to 20th	Day	78% 6% 9% 5% 1 1	1514
	21st to end	Day	76% 7% 10% 6% 1	1782
February	1st to 10th	Day	72% 7% 9% 10% 2	1620
	11th to 20th	Day	72% 5% 8% 12% 3	1582
	21st to end	Day	71% 5% 10% 9% 4	1426
March	1st to 10th	Day	64% 6% 12% 13% 4 1	1930
	11th to 20th	Day	57% 6% 10% 16% 11% 1	1980
	21st to end	Day	56% 5% 8% 18% 10% 2	2205
April	1st to 10th	Day	54% 6% 11% 17% 10% 2	2160
	11th to 20th	Day	48% 6% 10% 21% 14% 2	2134
	21st to end	Day	52% 8% 8% 17% 13% 2	2298
May	1st to 10th	Day	49% 7% 10% 20% 12% 2	2402
	11th to 20th	Day	43% 7% 12% 22% 13% 3	2482
	21st to end	Day	38% 8% 14% 24% 14% 1	2725
June	1st to 10th	Day	43% 8% 11% 19% 18% 1	2520
	11th to 20th	Day	38% 8% 14% 24% 15% 1	2520
	21st to end	Day	39% 8% 14% 23% 16% 1	2520
July	1st to 10th	Day	32% 8% 13% 27% 17% 2	2450
	11th to 20th	Day	36% 10% 17% 25% 12%	2445
	21st to end	Day	39% 11% 15% 20% 14% 1	2515
August	1st to 10th	Day	36% 10% 17% 23% 13% 1	2314
	11th to 20th	Day	41% 10% 15% 24% 10%	2303
	21st to end	Day	38% 9% 14% 26% 11% 2	2399
September	1st to 10th	Day	40% 8% 13% 26% 12% 1	2160
	11th to 20th	Day	39% 7% 12% 26% 15%	2156
	21st to end	Day	45% 8% 13% 23% 10% 1	1881
October	1st to 10th	Day	51% 7% 13% 18% 11%	1800
	11th to 20th	Day	55% 8% 14% 17% 6% 1	1800
	21st to end	Day	64% 6% 10% 13% 5% 1	1980
November	1st to 10th	Day	54% 7% 11% 17% 9% 1	1638
	11th to 20th	Day	59% 7% 16% 12% 6% 1	1520
	21st to end	Day	66% 6% 14% 12% 2	1520
December	1st to 10th	Day	73% 6% 11% 9% 1	1520
	11th to 20th	Day	74% 7% 10% 6% 2	1906
	21st to end	Day	79% 6% 7% 6% 1	2772
January	1st to 10th	Night	72% 6% 10% 10% 2	2649
	11th to 20th	Night	77% 6% 9% 6% 2	2483
	21st to end	Night	72% 6% 10% 9% 3	2574
February	1st to 10th	Night	68% 7% 9% 12% 3	2340
	11th to 20th	Night	68% 6% 10% 12% 4	2275
	21st to end	Night	64% 8% 10% 13% 5%	1829
March	1st to 10th	Night	61% 6% 12% 16% 5%	2027
	11th to 20th	Night	55% 6% 11% 19% 9% 1	1980
	21st to end	Night	54% 6% 8% 18% 12% 2	2150
April	1st to 10th	Night	56% 6% 9% 20% 9% 1	1800
	11th to 20th	Night	48% 7% 12% 18% 14% 2	1775
	21st to end	Night	48% 7% 10% 20% 13% 2	1677
May	1st to 10th	Night	47% 8% 9% 19% 15% 2	1558
	11th to 20th	Night	39% 8% 13% 23% 15% 3	1416
	21st to end	Night	34% 10% 14% 21% 19% 2	1551
June	1st to 10th	Night	41% 9% 12% 23% 14% 2	1440
	11th to 20th	Night	38% 8% 11% 22% 19% 2	1440
	21st to end	Night	39% 9% 12% 23% 16% 1	1440
July	1st to 10th	Night	31% 9% 12% 26% 19% 3	1400
	11th to 20th	Night	33% 7% 13% 30% 16% 1	1392
	21st to end	Night	31% 9% 16% 28% 15% 2	1558
August	1st to 10th	Night	31% 9% 16% 29% 14% 1	1600
	11th to 20th	Night	36% 8% 18% 27% 11%	1598
	21st to end	Night	33% 9% 15% 29% 13% 2	1938
September	1st to 10th	Night	38% 7% 13% 29% 12% 2	1800
	11th to 20th	Night	37% 6% 14% 26% 17% 1	1800
	21st to end	Night	41% 9% 13% 24% 12% 1	2078
October	1st to 10th	Night	49% 8% 14% 17% 12% 1	2160
	11th to 20th	Night	53% 8% 15% 17% 8%	2160
	21st to end	Night	60% 7% 12% 12% 7% 1	2376
November	1st to 10th	Night	48% 8% 13% 19% 10% 2	2438
	11th to 20th	Night	55% 8% 13% 15% 7% 1	2660
	21st to end	Night	61% 8% 16% 12% 3	2660
December	1st to 10th	Night	69% 8% 14% 8% 2	2660
	11th to 20th	Night	69% 8% 11% 10% 2	3288
	21st to end	Night	77% 7% 9% 6% 1	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	25% 28% 32% 9% 6%	180
	11th to 20th	Day	30% 28% 30% 10% 2 1	192
	21st to end	Day	21% 28% 32% 15% 4	264
February	1st to 10th	Day	28% 30% 35% 7%	240
	11th to 20th	Day	24% 33% 36% 7%	240
	21st to end	Day	37% 21% 32% 10%	292
March	1st to 10th	Day	20% 27% 32% 18% 3	420
	11th to 20th	Day	25% 22% 33% 15% 4 1	420
	21st to end	Day	30% 18% 29% 16% 6% 1	462
April	1st to 10th	Day	23% 21% 32% 19% 5%	420
	11th to 20th	Day	28% 18% 27% 17% 8% 3	412
	21st to end	Day	24% 20% 31% 18% 6% 1	420
May	1st to 10th	Day	25% 13% 31% 20% 8% 3	423
	11th to 20th	Day	17% 10% 33% 30% 9% 2	447
	21st to end	Day	13% 11% 32% 26% 15% 4	485
June	1st to 10th	Day	16% 11% 32% 30% 9% 1	450
	11th to 20th	Day	16% 11% 26% 30% 14% 2	450
	21st to end	Day	13% 8% 35% 30% 12% 1	450
July	1st to 10th	Day	9% 6% 27% 35% 20% 4	445
	11th to 20th	Day	13% 6% 28% 31% 18% 4	444
	21st to end	Day	10% 7% 33% 35% 12% 2	488
August	1st to 10th	Day	9% 7% 24% 35% 20% 6%	446
	11th to 20th	Day	12% 7% 23% 34% 19% 6%	439
	21st to end	Day	12% 8% 26% 32% 16% 6%	495
September	1st to 10th	Day	14% 9% 25% 30% 15% 7%	450
	11th to 20th	Day	15% 7% 22% 27% 20% 9%	450
	21st to end	Day	16% 14% 26% 23% 14% 6%	450
October	1st to 10th	Day	23% 11% 26% 20% 15% 5%	450
	11th to 20th	Day	26% 18% 31% 17% 6% 2	450
	21st to end	Day	33% 16% 30% 16% 5%	495
November	1st to 10th	Day	21% 17% 34% 17% 8% 2	332
	11th to 20th	Day	23% 27% 31% 16% 2 2	180
	21st to end	Day	29% 21% 35% 13% 2	180
December	1st to 10th	Day	33% 22% 28% 13% 3 1	180
	11th to 20th	Day	30% 28% 27% 14% 2	180
	21st to end	Day	29% 26% 33% 10% 1 1	198
January	1st to 10th	Night	26% 26% 32% 13% 4	760
	11th to 20th	Night	26% 24% 33% 15% 2	728
	21st to end	Night	19% 26% 35% 17% 4	792
February	1st to 10th	Night	27% 27% 32% 14% 1	720
	11th to 20th	Night	23% 25% 35% 15% 2	703
	21st to end	Night	26% 22% 33% 16% 2	592
March	1st to 10th	Night	25% 18% 34% 19% 4	720
	11th to 20th	Night	24% 21% 31% 20% 4	720
	21st to end	Night	22% 14% 33% 23% 7% 1	792
April	1st to 10th	Night	24% 15% 36% 21% 4	720
	11th to 20th	Night	21% 16% 30% 25% 6% 1	711
	21st to end	Night	24% 10% 29% 28% 9%	597
May	1st to 10th	Night	22% 7% 27% 29% 14% 1	478
	11th to 20th	Night	15% 5% 31% 39% 10%	354
	21st to end	Night	12% 6% 30% 31% 20% 1	389
June	1st to 10th	Night	15% 6% 30% 32% 16% 1	360
	11th to 20th	Night	14% 5% 28% 34% 18%	360
	21st to end	Night	10% 6% 29% 42% 12%	360
July	1st to 10th	Night	7% 4 21% 42% 23% 3	350
	11th to 20th	Night	12% 2 29% 33% 23% 1	348
	21st to end	Night	11% 3 25% 41% 18% 2	470
August	1st to 10th	Night	10% 2 23% 39% 23% 3	537
	11th to 20th	Night	10% 5% 21% 36% 25% 3	534
	21st to end	Night	10% 5% 23% 35% 24% 2	750
September	1st to 10th	Night	16% 6% 22% 28% 24% 3	720
	11th to 20th	Night	17% 6% 19% 29% 24% 6%	720
	21st to end	Night	16% 10% 22% 29% 18% 5%	720
October	1st to 10th	Night	23% 10% 27% 23% 16% 2	720
	11th to 20th	Night	24% 16% 30% 22% 7% 1	720
	21st to end	Night	33% 12% 30% 21% 3	792
November	1st to 10th	Night	23% 14% 31% 24% 7% 1	740
	11th to 20th	Night	21% 23% 31% 17% 7% 1	760
	21st to end	Night	27% 21% 30% 17% 5%	760
December	1st to 10th	Night	32% 23% 29% 13% 3	760
	11th to 20th	Night	24% 27% 30% 16% 3	760
	21st to end	Night	33% 22% 30% 12% 2	836

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