3 Angels Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the 3 Angels Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	10% 5% 2 6% 5% 5% 5% 4 7% 9% 41%	126631
	Night	11% 6% 2 6% 4 4 4 4 6% 8% 45%	125387
Transparency	Day	35% 9% 14% 19% 16% 7%	130457
	Night	34% 7% 14% 19% 18% 8%	129621
Seeing	Day	12% 8% 22% 26% 22% 11%	15904
	Night	12% 6% 20% 29% 25% 7%	25865

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	14% 5% 2 5% 4 3 4 3 5% 7% 50%	4616
	11th to 20th	Day	13% 5% 2 5% 4 3 3 3 5% 6% 51%	4620
	21st to end	Day	22% 7% 2 5% 4 4 5% 3 5% 6% 37%	5082
February	1st to 10th	Day	17% 8% 3 6% 4 4 5% 3 4 6% 41%	2000
	11th to 20th	Day	16% 7% 3 5% 4 3 4 3 4 7% 43%	1948
	21st to end	Day	16% 7% 2 6% 5% 5% 4 3 6% 7% 38%	1650
March	1st to 10th	Day	17% 7% 3 7% 4 4 4 4 5% 7% 38%	4480
	11th to 20th	Day	12% 5% 2 5% 4 3 4 3 6% 8% 49%	4761
	21st to end	Day	16% 6% 2 7% 4 4 4 3 4 5% 43%	5967
April	1st to 10th	Day	11% 6% 2 5% 5% 5% 4 5% 6% 8% 43%	2400
	11th to 20th	Day	13% 5% 2 6% 4 4 4 4 6% 9% 43%	2374
	21st to end	Day	8% 4 2 5% 4 4 4 3 7% 8% 51%	2400
May	1st to 10th	Day	13% 7% 3 6% 5% 5% 6% 5% 7% 9% 36%	5405
	11th to 20th	Day	10% 4 2 5% 4 5% 5% 4 7% 10% 44%	5467
	21st to end	Day	6% 4 2 6% 5% 5% 5% 4 8% 12% 45%	6433
June	1st to 10th	Day	8% 5% 2 7% 6% 6% 8% 6% 9% 11% 31%	2600
	11th to 20th	Day	5% 4 2 6% 7% 6% 8% 6% 10% 12% 35%	2600
	21st to end	Day	8% 7% 3 8% 6% 7% 6% 6% 10% 11% 28%	2600
July	1st to 10th	Day	4 4 2 7% 7% 7% 9% 8% 11% 14% 28%	5634
	11th to 20th	Day	3 4 2 7% 7% 7% 8% 7% 11% 14% 30%	5440
	21st to end	Day	4 4 2 8% 7% 8% 9% 8% 11% 14% 26%	5922
August	1st to 10th	Day	3 3 1 5% 5% 5% 8% 6% 10% 13% 41%	3905
	11th to 20th	Day	5% 4 2 7% 5% 6% 7% 6% 9% 11% 38%	3849
	21st to end	Day	6% 5% 3 9% 7% 7% 7% 6% 8% 10% 32%	4312
September	1st to 10th	Day	9% 6% 2 7% 6% 6% 7% 4 8% 9% 34%	2400
	11th to 20th	Day	9% 6% 3 7% 5% 4 5% 4 6% 9% 40%	2235
	21st to end	Day	7% 5% 2 5% 5% 5% 5% 5% 6% 7% 48%	2200
October	1st to 10th	Day	8% 4 2 5% 4 4 5% 4 5% 7% 53%	2200
	11th to 20th	Day	7% 5% 1 4 3 4 3 4 5% 8% 57%	2115
	21st to end	Day	10% 4 2 4 4 3 4 3 5% 8% 54%	2200
November	1st to 10th	Day	8% 4 2 5% 3 3 4 3 5% 6% 57%	1943
	11th to 20th	Day	12% 4 2 5% 3 2 4 3 4 5% 57%	1890
	21st to end	Day	11% 6% 2 6% 5% 4 4 4 5% 7% 46%	1890
December	1st to 10th	Day	16% 6% 2 6% 4 3 3 3 5% 6% 45%	3020
	11th to 20th	Day	10% 6% 2 6% 3 3 3 3 5% 6% 52%	3463
	21st to end	Day	17% 7% 3 6% 3 3 3 3 4 6% 44%	4610
January	1st to 10th	Night	15% 6% 2 5% 4 4 4 3 4 5% 48%	5549
	11th to 20th	Night	18% 8% 3 6% 3 3 2 2 3 5% 47%	5560
	21st to end	Night	21% 7% 3 6% 3 4 4 3 5% 6% 39%	6116
February	1st to 10th	Night	14% 7% 2 6% 4 4 3 3 5% 6% 44%	2401
	11th to 20th	Night	19% 7% 2 5% 4 4 3 3 5% 6% 42%	2338
	21st to end	Night	17% 7% 2 6% 5% 4 5% 4 5% 6% 39%	1980
March	1st to 10th	Night	20% 10% 3 6% 4 3 4 3 5% 6% 36%	5373
	11th to 20th	Night	14% 7% 2 5% 4 3 4 3 5% 7% 48%	5238
	21st to end	Night	15% 6% 2 5% 4 4 4 3 5% 6% 47%	4962
April	1st to 10th	Night	14% 7% 2 6% 5% 4 3 4 5% 7% 44%	2000
	11th to 20th	Night	12% 5% 2 5% 4 3 5% 4 6% 6% 49%	1975
	21st to end	Night	6% 4 1 5% 3 4 3 4 5% 7% 58%	2000
May	1st to 10th	Night	13% 7% 3 7% 5% 5% 5% 4 6% 8% 37%	4490
	11th to 20th	Night	12% 6% 2 6% 4 4 4 4 5% 8% 45%	4408
	21st to end	Night	6% 4 2 5% 5% 5% 5% 4 7% 12% 46%	4421
June	1st to 10th	Night	7% 5% 2 6% 4 5% 6% 5% 9% 12% 40%	1800
	11th to 20th	Night	3 4 1 5% 4 5% 5% 5% 9% 14% 46%	1800
	21st to end	Night	5% 6% 2 6% 6% 6% 6% 5% 9% 13% 37%	1800
July	1st to 10th	Night	3 4 2 6% 6% 6% 6% 6% 10% 13% 38%	4304
	11th to 20th	Night	2 3 1 5% 5% 5% 6% 6% 9% 11% 48%	4517
	21st to end	Night	2 4 2 7% 6% 7% 7% 6% 11% 15% 34%	4929
August	1st to 10th	Night	2 2 1 4 3 4 5% 5% 9% 15% 52%	3244
	11th to 20th	Night	3 3 1 4 4 4 6% 6% 9% 14% 46%	3200
	21st to end	Night	4 4 2 6% 5% 5% 6% 5% 8% 11% 42%	3586
September	1st to 10th	Night	6% 6% 2 7% 6% 4 5% 6% 8% 12% 39%	2000
	11th to 20th	Night	7% 6% 2 6% 4 5% 5% 4 6% 9% 46%	2144
	21st to end	Night	6% 5% 2 5% 4 5% 5% 4 5% 8% 52%	2200
October	1st to 10th	Night	8% 4 2 5% 3 4 4 3 4 9% 55%	2200
	11th to 20th	Night	7% 3 1 4 4 3 3 3 4 6% 63%	2280
	21st to end	Night	10% 4 2 5% 3 3 3 3 3 6% 57%	2640
November	1st to 10th	Night	9% 4 1 6% 4 3 3 3 4 6% 56%	2571
	11th to 20th	Night	11% 5% 2 5% 3 3 3 3 5% 6% 54%	2730
	21st to end	Night	10% 5% 2 6% 4 4 5% 3 5% 7% 50%	2730
December	1st to 10th	Night	13% 7% 2 6% 5% 3 4 4 5% 7% 44%	4380
	11th to 20th	Night	12% 5% 2 5% 4 4 4 3 5% 5% 51%	4952
	21st to end	Night	18% 7% 3 7% 4 3 3 3 4 4 44%	6569

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	34% 5% 6% 19% 26% 9%	4617
	11th to 20th	Day	35% 4 6% 17% 28% 10%	4620
	21st to end	Day	47% 5% 7% 14% 15% 11%	5082
February	1st to 10th	Day	44% 5% 9% 13% 19% 10%	2000
	11th to 20th	Day	40% 6% 9% 16% 20% 10%	1958
	21st to end	Day	42% 9% 9% 15% 16% 9%	1650
March	1st to 10th	Day	44% 6% 9% 18% 14% 9%	4480
	11th to 20th	Day	31% 7% 9% 16% 22% 15%	4761
	21st to end	Day	41% 6% 11% 19% 14% 9%	5969
April	1st to 10th	Day	35% 8% 13% 17% 17% 9%	2400
	11th to 20th	Day	35% 7% 9% 20% 20% 9%	2374
	21st to end	Day	28% 7% 12% 24% 21% 8%	2400
May	1st to 10th	Day	38% 11% 15% 22% 11% 3	5440
	11th to 20th	Day	30% 8% 19% 27% 14% 1	5471
	21st to end	Day	28% 9% 18% 32% 12% 2	6447
June	1st to 10th	Day	37% 13% 17% 22% 10% 1	2600
	11th to 20th	Day	32% 13% 20% 23% 10% 2	2600
	21st to end	Day	39% 14% 23% 16% 8% 1	2600
July	1st to 10th	Day	38% 13% 30% 17% 3	5574
	11th to 20th	Day	34% 17% 35% 12% 1	5377
	21st to end	Day	39% 15% 26% 19% 2	5752
August	1st to 10th	Day	27% 12% 19% 24% 17% 2	3893
	11th to 20th	Day	32% 12% 20% 24% 11%	3885
	21st to end	Day	42% 13% 16% 17% 11% 2	4311
September	1st to 10th	Day	37% 12% 19% 17% 11% 4	2400
	11th to 20th	Day	35% 11% 14% 18% 16% 6%	2235
	21st to end	Day	31% 8% 12% 24% 17% 8%	2200
October	1st to 10th	Day	28% 5% 8% 13% 18% 28%	3590
	11th to 20th	Day	15% 4 6% 19% 25% 31%	3420
	21st to end	Day	20% 4 6% 12% 39% 19%	3538
November	1st to 10th	Day	27% 4 9% 21% 25% 13%	1943
	11th to 20th	Day	29% 4 7% 18% 29% 13%	1890
	21st to end	Day	36% 7% 9% 17% 20% 12%	1890
December	1st to 10th	Day	40% 4 9% 16% 23% 7%	3020
	11th to 20th	Day	32% 5% 10% 23% 20% 11%	3460
	21st to end	Day	41% 5% 7% 15% 27% 5%	4610
January	1st to 10th	Night	37% 5% 7% 16% 26% 10%	5549
	11th to 20th	Night	41% 4 4 14% 29% 8%	5560
	21st to end	Night	46% 4 7% 13% 19% 10%	6116
February	1st to 10th	Night	40% 5% 7% 17% 20% 11%	2401
	11th to 20th	Night	43% 5% 6% 14% 20% 12%	2339
	21st to end	Night	43% 9% 9% 14% 13% 12%	1980
March	1st to 10th	Night	47% 6% 8% 13% 18% 8%	5373
	11th to 20th	Night	35% 7% 9% 14% 20% 15%	5238
	21st to end	Night	36% 6% 13% 20% 15% 10%	4962
April	1st to 10th	Night	38% 7% 11% 15% 18% 11%	2000
	11th to 20th	Night	33% 7% 10% 19% 22% 9%	1975
	21st to end	Night	24% 6% 13% 26% 24% 8%	2000
May	1st to 10th	Night	41% 9% 17% 18% 13% 3	4520
	11th to 20th	Night	35% 7% 18% 26% 13% 1	4409
	21st to end	Night	27% 8% 21% 31% 12% 1	4429
June	1st to 10th	Night	32% 11% 22% 25% 10% 1	1800
	11th to 20th	Night	26% 12% 26% 25% 11% 1	1800
	21st to end	Night	34% 13% 25% 19% 9% 1	1800
July	1st to 10th	Night	33% 13% 33% 19% 3	4256
	11th to 20th	Night	26% 14% 44% 14% 1	4460
	21st to end	Night	34% 17% 24% 23% 1	4785
August	1st to 10th	Night	18% 11% 24% 30% 16% 1	3232
	11th to 20th	Night	24% 11% 27% 29% 8%	3230
	21st to end	Night	34% 15% 21% 18% 10% 3	3586
September	1st to 10th	Night	35% 11% 21% 18% 12% 5%	2000
	11th to 20th	Night	33% 10% 17% 18% 15% 7%	2144
	21st to end	Night	28% 9% 13% 24% 17% 9%	2200
October	1st to 10th	Night	33% 5% 8% 13% 20% 21%	3590
	11th to 20th	Night	15% 3 6% 18% 27% 30%	3701
	21st to end	Night	20% 3 6% 10% 43% 19%	4252
November	1st to 10th	Night	30% 4 8% 18% 26% 15%	2571
	11th to 20th	Night	31% 4 6% 17% 27% 14%	2730
	21st to end	Night	33% 5% 8% 20% 21% 12%	2730
December	1st to 10th	Night	39% 4 8% 16% 23% 10%	4380
	11th to 20th	Night	33% 5% 8% 23% 20% 11%	4954
	21st to end	Night	43% 5% 6% 17% 23% 6%	6569

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	18% 17% 38% 21% 6%	330
	11th to 20th	Day	15% 18% 35% 23% 8% 1	330
	21st to end	Day	22% 20% 36% 18% 3	363
February	1st to 10th	Day	21% 19% 35% 18% 5% 1	300
	11th to 20th	Day	25% 19% 35% 15% 5%	300
	21st to end	Day	14% 22% 45% 15% 4	249
March	1st to 10th	Day	15% 19% 41% 22% 3	300
	11th to 20th	Day	17% 22% 38% 17% 6%	303
	21st to end	Day	14% 13% 35% 27% 10% 1	550
April	1st to 10th	Day	18% 13% 33% 25% 10% 1	500
	11th to 20th	Day	15% 14% 31% 25% 13% 2	492
	21st to end	Day	8% 7% 31% 31% 18% 4	500
May	1st to 10th	Day	10% 8% 32% 26% 20% 5%	503
	11th to 20th	Day	11% 3 22% 32% 23% 9%	527
	21st to end	Day	8% 2 16% 35% 26% 12%	573
June	1st to 10th	Day	9% 2 13% 33% 27% 16%	530
	11th to 20th	Day	9% 2 12% 31% 28% 18%	530
	21st to end	Day	10% 2 12% 25% 30% 21%	530
July	1st to 10th	Day	8% 1 8% 21% 34% 28%	525
	11th to 20th	Day	9% 1 6% 19% 35% 31%	524
	21st to end	Day	7% 1 6% 23% 40% 24%	576
August	1st to 10th	Day	4 1 4 21% 38% 32%	526
	11th to 20th	Day	8% 1 6% 17% 36% 33%	519
	21st to end	Day	9% 1 10% 21% 35% 24%	583
September	1st to 10th	Day	9% 1 8% 25% 35% 22%	530
	11th to 20th	Day	11% 2 10% 26% 37% 14%	530
	21st to end	Day	8% 3 12% 31% 33% 13%	530
October	1st to 10th	Day	9% 4 20% 36% 25% 6%	530
	11th to 20th	Day	4 5% 21% 33% 30% 6%	445
	21st to end	Day	15% 7% 22% 36% 15% 4	363
November	1st to 10th	Day	14% 9% 25% 35% 16% 2	330
	11th to 20th	Day	15% 11% 28% 29% 15% 2	330
	21st to end	Day	11% 13% 37% 28% 10% 1	330
December	1st to 10th	Day	21% 12% 31% 26% 9% 2	330
	11th to 20th	Day	19% 14% 37% 23% 7% 1	330
	21st to end	Day	19% 15% 30% 24% 9% 2	363
January	1st to 10th	Night	18% 10% 32% 33% 6%	840
	11th to 20th	Night	15% 13% 28% 35% 9%	840
	21st to end	Night	21% 11% 32% 31% 5%	924
February	1st to 10th	Night	18% 10% 31% 32% 8%	801
	11th to 20th	Night	23% 10% 33% 25% 8% 1	783
	21st to end	Night	17% 14% 34% 30% 5%	660
March	1st to 10th	Night	16% 11% 32% 32% 9%	800
	11th to 20th	Night	15% 9% 32% 29% 14% 1	720
	21st to end	Night	13% 6% 31% 34% 16%	660
April	1st to 10th	Night	17% 9% 26% 32% 15% 1	600
	11th to 20th	Night	12% 7% 31% 30% 19% 2	593
	21st to end	Night	8% 5% 23% 36% 27% 2	600
May	1st to 10th	Night	12% 3 18% 38% 26% 3	600
	11th to 20th	Night	8% 2 14% 41% 30% 5%	591
	21st to end	Night	7% 1 10% 34% 39% 8%	650
June	1st to 10th	Night	8% 1 8% 29% 44% 10%	600
	11th to 20th	Night	4 2 6% 25% 50% 14%	600
	21st to end	Night	8% 1 6% 21% 44% 20%	600
July	1st to 10th	Night	7% 4 15% 50% 24%	585
	11th to 20th	Night	3 1 3 10% 45% 38%	582
	21st to end	Night	4 1 3 14% 49% 28%	657
August	1st to 10th	Night	3 2 10% 51% 34%	597
	11th to 20th	Night	7% 1 2 11% 47% 33%	594
	21st to end	Night	8% 2 5% 17% 49% 19%	660
September	1st to 10th	Night	8% 2 5% 23% 40% 23%	600
	11th to 20th	Night	9% 1 8% 27% 44% 10%	744
	21st to end	Night	8% 2 10% 32% 42% 7%	800
October	1st to 10th	Night	11% 2 17% 37% 29% 5%	800
	11th to 20th	Night	8% 4 18% 34% 31% 6%	800
	21st to end	Night	11% 5% 20% 36% 25% 3	880
November	1st to 10th	Night	11% 5% 18% 41% 21% 3	820
	11th to 20th	Night	12% 8% 25% 36% 18% 2	840
	21st to end	Night	12% 7% 30% 35% 15% 2	840
December	1st to 10th	Night	16% 8% 28% 32% 15% 1	840
	11th to 20th	Night	18% 7% 28% 32% 13% 1	840
	21st to end	Night	22% 9% 31% 28% 10% 1	924

Page generated at 2026-01-29 00:21:39. Version ac07

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