Above the Clouds Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Above the Clouds Observatory clear sky chart from 2005-11-06 to 2025-05-11.

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	14% 9% 3 9% 6% 5% 5% 4 6% 7% 32%	113624
	Night	13% 9% 4 9% 6% 5% 5% 4 6% 7% 31%	115312
Transparency	Day	48% 7% 13% 20% 11% 1	117153
	Night	50% 7% 13% 19% 10% 1	119934
Seeing	Day	24% 16% 29% 19% 9% 3	15384
	Night	20% 14% 27% 29% 10% 1	24460

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	9% 10% 6% 22% 8% 5% 4 4 5% 5% 23%	3597
	11th to 20th	Day	10% 13% 5% 14% 8% 5% 5% 4 5% 5% 26%	3570
	21st to end	Day	10% 12% 6% 11% 6% 5% 5% 5% 5% 6% 29%	3960
February	1st to 10th	Day	11% 11% 4 11% 6% 7% 5% 4 6% 7% 27%	1764
	11th to 20th	Day	9% 10% 5% 12% 9% 7% 5% 4 6% 7% 27%	1948
	21st to end	Day	13% 13% 4 9% 6% 5% 5% 4 5% 6% 29%	1650
March	1st to 10th	Day	15% 14% 5% 13% 6% 5% 5% 3 4 4 26%	4476
	11th to 20th	Day	12% 10% 4 9% 6% 5% 5% 4 6% 6% 34%	4645
	21st to end	Day	18% 11% 4 10% 5% 5% 5% 4 4 5% 30%	5970
April	1st to 10th	Day	17% 11% 4 9% 6% 4 4 3 4 6% 33%	2400
	11th to 20th	Day	15% 11% 3 7% 5% 6% 5% 4 5% 7% 34%	2374
	21st to end	Day	20% 9% 3 9% 5% 5% 4 4 5% 6% 30%	2580
May	1st to 10th	Day	22% 11% 4 9% 6% 5% 4 3 5% 5% 26%	4259
	11th to 20th	Day	16% 8% 3 8% 6% 5% 6% 4 7% 7% 30%	4398
	21st to end	Day	12% 5% 2 6% 4 5% 5% 5% 8% 9% 40%	4837
June	1st to 10th	Day	18% 9% 3 7% 5% 5% 5% 5% 6% 7% 29%	2660
	11th to 20th	Day	16% 7% 2 6% 5% 5% 6% 4 6% 8% 35%	2660
	21st to end	Day	17% 8% 3 7% 6% 6% 5% 4 6% 8% 28%	2660
July	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 5% 8% 10% 37%	4440
	11th to 20th	Day	9% 5% 2 7% 5% 6% 7% 5% 9% 11% 34%	4440
	21st to end	Day	13% 7% 3 7% 5% 5% 5% 4 7% 7% 37%	4814
August	1st to 10th	Day	14% 6% 2 7% 6% 6% 6% 5% 8% 9% 32%	4361
	11th to 20th	Day	14% 8% 3 7% 5% 5% 6% 4 7% 8% 33%	3922
	21st to end	Day	10% 6% 2 6% 5% 5% 5% 5% 7% 11% 40%	4180
September	1st to 10th	Day	11% 5% 2 6% 4 3 5% 4 7% 9% 45%	2280
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 5% 6% 48%	2147
	21st to end	Day	15% 7% 2 6% 4 4 4 3 7% 8% 41%	2090
October	1st to 10th	Day	19% 7% 2 8% 6% 5% 5% 4 5% 6% 34%	2050
	11th to 20th	Day	13% 10% 3 8% 5% 5% 4 3 5% 7% 37%	1834
	21st to end	Day	22% 11% 3 7% 5% 5% 4 5% 4 6% 30%	1881
November	1st to 10th	Day	10% 9% 4 6% 5% 5% 5% 4 7% 6% 38%	1761
	11th to 20th	Day	14% 11% 3 7% 5% 4 5% 5% 5% 6% 34%	1800
	21st to end	Day	16% 11% 4 9% 5% 4 4 4 6% 7% 29%	1670
December	1st to 10th	Day	11% 14% 6% 16% 8% 6% 5% 4 6% 6% 18%	2600
	11th to 20th	Day	16% 11% 4 9% 6% 5% 6% 4 6% 6% 28%	2986
	21st to end	Day	16% 14% 4 10% 6% 5% 6% 4 5% 4 28%	3960
January	1st to 10th	Night	11% 8% 6% 19% 8% 5% 5% 4 5% 6% 24%	6349
	11th to 20th	Night	7% 12% 5% 16% 8% 7% 6% 4 6% 7% 21%	6315
	21st to end	Night	11% 11% 4 12% 7% 6% 5% 4 6% 7% 28%	6996
February	1st to 10th	Night	10% 12% 4 10% 6% 6% 5% 6% 7% 8% 26%	2632
	11th to 20th	Night	9% 8% 4 11% 8% 6% 6% 6% 7% 6% 29%	2338
	21st to end	Night	10% 11% 3 10% 8% 5% 5% 4 5% 8% 31%	1980
March	1st to 10th	Night	17% 12% 3 9% 6% 5% 5% 5% 6% 6% 26%	5364
	11th to 20th	Night	11% 9% 4 9% 6% 6% 6% 4 5% 6% 33%	5296
	21st to end	Night	16% 13% 4 9% 5% 4 5% 3 5% 5% 31%	4962
April	1st to 10th	Night	16% 9% 3 9% 5% 5% 5% 4 5% 6% 33%	2000
	11th to 20th	Night	15% 10% 3 8% 5% 5% 5% 3 6% 6% 34%	1975
	21st to end	Night	18% 9% 4 7% 4 4 4 3 6% 6% 35%	1820
May	1st to 10th	Night	24% 12% 3 7% 5% 4 5% 3 6% 7% 25%	2924
	11th to 20th	Night	15% 10% 3 8% 7% 5% 5% 4 5% 6% 30%	2509
	21st to end	Night	12% 5% 2 6% 5% 5% 6% 5% 7% 9% 40%	2738
June	1st to 10th	Night	17% 6% 2 6% 4 5% 6% 4 6% 9% 34%	1520
	11th to 20th	Night	17% 7% 3 4 5% 3 3 5% 5% 8% 40%	1520
	21st to end	Night	16% 6% 1 7% 5% 5% 5% 4 6% 8% 36%	1520
July	1st to 10th	Night	10% 7% 2 6% 5% 5% 5% 5% 7% 9% 38%	2520
	11th to 20th	Night	13% 5% 2 7% 4 4 4 5% 7% 8% 42%	2520
	21st to end	Night	11% 7% 2 7% 5% 5% 5% 4 7% 7% 40%	2739
August	1st to 10th	Night	16% 9% 3 8% 4 5% 5% 3 5% 7% 35%	2536
	11th to 20th	Night	14% 6% 3 7% 5% 4 4 4 6% 8% 39%	2902
	21st to end	Night	10% 5% 1 5% 4 3 5% 4 8% 10% 46%	3476
September	1st to 10th	Night	11% 5% 2 5% 4 4 4 3 6% 8% 48%	1900
	11th to 20th	Night	12% 5% 2 6% 3 3 4 3 6% 5% 51%	2033
	21st to end	Night	12% 6% 2 5% 4 4 4 4 7% 7% 45%	2090
October	1st to 10th	Night	17% 7% 3 9% 5% 4 4 3 5% 8% 35%	2128
	11th to 20th	Night	15% 8% 3 5% 4 5% 5% 5% 6% 9% 35%	2346
	21st to end	Night	20% 8% 3 7% 5% 5% 5% 4 7% 7% 30%	2717
November	1st to 10th	Night	12% 9% 4 8% 5% 5% 5% 4 4 6% 37%	2537
	11th to 20th	Night	14% 7% 3 6% 5% 5% 5% 5% 6% 8% 35%	2600
	21st to end	Night	13% 10% 4 8% 6% 5% 5% 4 6% 7% 32%	2726
December	1st to 10th	Night	12% 12% 5% 16% 8% 7% 6% 4 5% 7% 18%	4580
	11th to 20th	Night	18% 11% 4 10% 6% 6% 5% 3 4 6% 28%	5208
	21st to end	Night	14% 15% 5% 12% 8% 6% 5% 3 4 5% 23%	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	61% 6% 9% 19% 4	3598
	11th to 20th	Day	58% 6% 10% 15% 8% 3	3600
	21st to end	Day	53% 5% 10% 18% 12% 2	3960
February	1st to 10th	Day	53% 6% 13% 16% 9% 2	1764
	11th to 20th	Day	55% 7% 11% 17% 8% 2	1957
	21st to end	Day	54% 6% 12% 15% 12% 2	1650
March	1st to 10th	Day	60% 6% 9% 11% 9% 6%	4479
	11th to 20th	Day	49% 6% 16% 18% 9% 3	4645
	21st to end	Day	55% 6% 6% 14% 17% 3	5970
April	1st to 10th	Day	52% 5% 9% 19% 11% 3	2400
	11th to 20th	Day	48% 6% 11% 19% 15% 1	2374
	21st to end	Day	54% 5% 10% 17% 12% 2	2580
May	1st to 10th	Day	59% 5% 10% 18% 7% 1	4273
	11th to 20th	Day	50% 7% 17% 16% 9% 1	4402
	21st to end	Day	37% 8% 15% 27% 12% 1	4837
June	1st to 10th	Day	51% 7% 15% 17% 9%	2660
	11th to 20th	Day	45% 7% 12% 20% 14% 2	2660
	21st to end	Day	52% 8% 12% 18% 9% 1	2660
July	1st to 10th	Day	39% 7% 16% 24% 13%	4370
	11th to 20th	Day	38% 9% 20% 25% 8%	4365
	21st to end	Day	43% 8% 13% 24% 12%	4661
August	1st to 10th	Day	45% 9% 16% 20% 10%	4347
	11th to 20th	Day	46% 8% 17% 24% 5%	3961
	21st to end	Day	35% 9% 17% 25% 13% 1	4180
September	1st to 10th	Day	34% 6% 14% 31% 13% 1	2280
	11th to 20th	Day	37% 4 11% 29% 17% 1	2147
	21st to end	Day	39% 6% 15% 26% 12% 2	2090
October	1st to 10th	Day	43% 9% 12% 21% 14% 1	3401
	11th to 20th	Day	43% 4 13% 25% 13%	3024
	21st to end	Day	40% 7% 12% 29% 12% 1	3081
November	1st to 10th	Day	43% 7% 14% 23% 13%	1761
	11th to 20th	Day	48% 7% 11% 17% 15% 2	1800
	21st to end	Day	53% 6% 18% 17% 6%	1670
December	1st to 10th	Day	63% 9% 13% 12% 3	2600
	11th to 20th	Day	53% 9% 10% 17% 10% 1	2986
	21st to end	Day	59% 5% 8% 13% 13% 2	3960
January	1st to 10th	Night	58% 8% 15% 13% 6%	6349
	11th to 20th	Night	61% 8% 12% 12% 7% 1	6360
	21st to end	Night	54% 7% 10% 13% 13% 3	6996
February	1st to 10th	Night	52% 6% 14% 17% 9% 1	2632
	11th to 20th	Night	51% 7% 12% 19% 10% 1	2340
	21st to end	Night	50% 6% 14% 19% 8% 3	1980
March	1st to 10th	Night	53% 6% 11% 14% 12% 2	5374
	11th to 20th	Night	50% 6% 17% 15% 10% 2	5297
	21st to end	Night	53% 5% 7% 15% 16% 2	4962
April	1st to 10th	Night	51% 4 10% 20% 12% 2	2000
	11th to 20th	Night	49% 5% 11% 22% 12% 1	1975
	21st to end	Night	49% 5% 10% 20% 13% 3	1820
May	1st to 10th	Night	57% 5% 11% 17% 8% 1	2930
	11th to 20th	Night	54% 6% 14% 17% 8% 1	2510
	21st to end	Night	39% 8% 19% 20% 13% 2	2739
June	1st to 10th	Night	45% 8% 16% 19% 11%	1520
	11th to 20th	Night	43% 7% 16% 20% 12% 2	1520
	21st to end	Night	45% 8% 14% 23% 10% 1	1520
July	1st to 10th	Night	41% 7% 19% 20% 13%	2480
	11th to 20th	Night	37% 10% 17% 26% 11%	2472
	21st to end	Night	40% 8% 12% 29% 10% 1	2651
August	1st to 10th	Night	48% 7% 14% 21% 9%	2526
	11th to 20th	Night	41% 8% 19% 22% 9%	2929
	21st to end	Night	29% 9% 22% 27% 12% 1	3476
September	1st to 10th	Night	33% 7% 17% 29% 13%	1900
	11th to 20th	Night	33% 6% 14% 30% 16% 1	2033
	21st to end	Night	37% 6% 15% 25% 15% 2	2090
October	1st to 10th	Night	47% 9% 13% 22% 8% 1	3544
	11th to 20th	Night	41% 6% 13% 23% 14% 2	3895
	21st to end	Night	41% 6% 13% 29% 9% 1	4467
November	1st to 10th	Night	47% 6% 12% 22% 11% 2	2537
	11th to 20th	Night	45% 7% 12% 21% 12% 2	2600
	21st to end	Night	50% 7% 16% 20% 7%	2726
December	1st to 10th	Night	62% 9% 13% 14% 2	4580
	11th to 20th	Night	55% 7% 9% 14% 14%	5208
	21st to end	Night	61% 6% 9% 14% 7% 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% 24% 28% 17% 6% 1	200
	11th to 20th	Day	31% 22% 29% 13% 6% 1	200
	21st to end	Day	21% 27% 30% 15% 6% 1	220
February	1st to 10th	Day	33% 28% 28% 8% 2	289
	11th to 20th	Day	28% 24% 30% 15% 2	496
	21st to end	Day	27% 22% 30% 17% 4	414
March	1st to 10th	Day	29% 18% 32% 18% 3	500
	11th to 20th	Day	32% 22% 28% 13% 5%	500
	21st to end	Day	31% 23% 25% 12% 5% 3	550
April	1st to 10th	Day	36% 17% 26% 15% 5% 2	500
	11th to 20th	Day	38% 19% 24% 12% 7% 1	492
	21st to end	Day	30% 21% 29% 11% 7% 1	500
May	1st to 10th	Day	35% 19% 25% 13% 7% 2	503
	11th to 20th	Day	19% 19% 32% 21% 6% 2	487
	21st to end	Day	18% 17% 34% 20% 9% 2	529
June	1st to 10th	Day	23% 14% 38% 17% 7% 1	490
	11th to 20th	Day	19% 16% 31% 21% 11% 2	490
	21st to end	Day	22% 14% 34% 20% 8% 2	490
July	1st to 10th	Day	16% 11% 31% 26% 11% 4	485
	11th to 20th	Day	13% 10% 35% 29% 10% 4	484
	21st to end	Day	14% 8% 38% 25% 11% 3	532
August	1st to 10th	Day	12% 11% 33% 22% 18% 5%	486
	11th to 20th	Day	18% 10% 25% 25% 14% 8%	479
	21st to end	Day	13% 12% 27% 27% 14% 7%	539
September	1st to 10th	Day	15% 12% 27% 23% 14% 9%	490
	11th to 20th	Day	15% 12% 25% 20% 17% 12%	490
	21st to end	Day	19% 11% 28% 22% 13% 7%	490
October	1st to 10th	Day	23% 13% 25% 19% 14% 6%	490
	11th to 20th	Day	22% 16% 25% 23% 12% 2	455
	21st to end	Day	34% 15% 23% 20% 8% 1	429
November	1st to 10th	Day	24% 16% 24% 21% 13% 2	395
	11th to 20th	Day	28% 19% 25% 20% 9% 1	400
	21st to end	Day	34% 16% 26% 15% 7% 1	270
December	1st to 10th	Day	27% 20% 31% 16% 7% 1	200
	11th to 20th	Day	32% 23% 28% 16% 2	200
	21st to end	Day	31% 16% 27% 20% 5%	220
January	1st to 10th	Night	20% 19% 35% 23% 4	800
	11th to 20th	Night	23% 18% 30% 26% 4	800
	21st to end	Night	16% 17% 34% 29% 4	880
February	1st to 10th	Night	18% 19% 35% 26% 3	800
	11th to 20th	Night	20% 19% 35% 22% 3	783
	21st to end	Night	21% 17% 32% 26% 3	660
March	1st to 10th	Night	23% 15% 33% 26% 3	800
	11th to 20th	Night	23% 19% 28% 25% 5%	800
	21st to end	Night	26% 16% 26% 24% 7%	880
April	1st to 10th	Night	24% 19% 28% 24% 5%	800
	11th to 20th	Night	23% 20% 28% 21% 7%	791
	21st to end	Night	27% 16% 27% 23% 7%	620
May	1st to 10th	Night	28% 12% 28% 24% 8%	600
	11th to 20th	Night	16% 13% 28% 35% 6% 1	384
	21st to end	Night	16% 9% 27% 33% 13%	411
June	1st to 10th	Night	19% 9% 27% 33% 12% 1	380
	11th to 20th	Night	20% 7% 23% 36% 14%	380
	21st to end	Night	17% 6% 24% 38% 13% 1	380
July	1st to 10th	Night	16% 3 20% 43% 16% 2	370
	11th to 20th	Night	13% 2 25% 40% 18% 2	368
	21st to end	Night	12% 5% 21% 45% 17% 1	416
August	1st to 10th	Night	14% 4 17% 40% 25%	395
	11th to 20th	Night	15% 7% 20% 35% 20% 3	630
	21st to end	Night	14% 6% 17% 37% 24% 2	836
September	1st to 10th	Night	14% 6% 25% 33% 20% 2	760
	11th to 20th	Night	15% 7% 19% 30% 24% 4	760
	21st to end	Night	15% 8% 26% 32% 18% 2	760
October	1st to 10th	Night	23% 12% 17% 30% 16% 2	760
	11th to 20th	Night	21% 12% 22% 31% 13% 1	760
	21st to end	Night	26% 14% 26% 26% 8%	836
November	1st to 10th	Night	20% 16% 26% 30% 9%	780
	11th to 20th	Night	19% 17% 27% 27% 9% 1	800
	21st to end	Night	21% 17% 27% 28% 7%	800
December	1st to 10th	Night	24% 18% 32% 22% 5%	800
	11th to 20th	Night	25% 17% 26% 27% 4	800
	21st to end	Night	27% 16% 30% 24% 4	880

Page generated at 2025-05-08 09:33:58. Version ac07

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