Angels Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Angels Observatory clear sky chart from 2005-11-06 to 2025-06-10.

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	18% 6% 2 6% 4 4 5% 4 6% 7% 38%	119632
	Night	18% 7% 2 6% 4 4 4 3 5% 6% 41%	118599
Transparency	Day	42% 5% 12% 23% 15% 2	123193
	Night	42% 5% 11% 23% 16% 3	123199
Seeing	Day	20% 8% 25% 26% 15% 6%	15631
	Night	20% 5% 26% 36% 12% 1	24942

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% 7% 2 5% 4 5% 5% 4 5% 6% 45%	3597
	11th to 20th	Day	13% 10% 4 8% 6% 5% 5% 4 5% 5% 34%	3593
	21st to end	Day	17% 8% 3 7% 4 5% 4 4 5% 5% 38%	4466
February	1st to 10th	Day	16% 7% 2 8% 5% 5% 5% 4 6% 6% 35%	1800
	11th to 20th	Day	15% 9% 3 6% 6% 4 4 4 5% 6% 38%	1884
	21st to end	Day	19% 8% 3 7% 5% 5% 4 3 6% 4 37%	1650
March	1st to 10th	Day	22% 5% 2 5% 4 5% 4 3 5% 6% 40%	4476
	11th to 20th	Day	12% 6% 3 7% 4 5% 4 4 6% 7% 42%	4975
	21st to end	Day	18% 7% 2 5% 3 3 4 3 6% 7% 42%	5800
April	1st to 10th	Day	18% 8% 3 7% 4 4 5% 4 6% 6% 36%	2400
	11th to 20th	Day	17% 7% 2 7% 5% 4 4 3 5% 7% 38%	2374
	21st to end	Day	17% 6% 2 6% 5% 4 4 4 6% 7% 39%	2400
May	1st to 10th	Day	28% 6% 2 5% 4 4 5% 3 5% 8% 29%	5957
	11th to 20th	Day	25% 8% 3 6% 4 4 4 3 4 7% 33%	6318
	21st to end	Day	21% 6% 2 5% 4 4 5% 4 7% 9% 34%	6949
June	1st to 10th	Day	15% 5% 2 7% 4 5% 5% 5% 6% 9% 37%	2786
	11th to 20th	Day	14% 6% 2 6% 5% 5% 5% 4 7% 9% 36%	2660
	21st to end	Day	13% 5% 2 6% 5% 4 5% 4 6% 9% 41%	2660
July	1st to 10th	Day	8% 6% 2 7% 5% 6% 6% 5% 7% 10% 38%	4440
	11th to 20th	Day	11% 5% 2 5% 5% 5% 5% 4 7% 10% 40%	4440
	21st to end	Day	15% 8% 3 7% 5% 5% 5% 4 7% 8% 33%	4793
August	1st to 10th	Day	23% 6% 2 6% 4 5% 5% 5% 6% 7% 31%	4104
	11th to 20th	Day	15% 5% 2 6% 4 5% 5% 3 6% 8% 42%	3765
	21st to end	Day	15% 5% 2 4 3 3 4 3 5% 8% 48%	4180
September	1st to 10th	Day	17% 5% 2 6% 4 5% 4 4 4 7% 42%	2280
	11th to 20th	Day	13% 5% 2 5% 3 3 4 3 5% 7% 51%	2252
	21st to end	Day	20% 6% 2 6% 4 4 5% 3 5% 5% 39%	2086
October	1st to 10th	Day	20% 6% 2 7% 3 3 3 3 6% 7% 40%	1900
	11th to 20th	Day	16% 5% 1 5% 3 4 4 3 5% 6% 46%	1900
	21st to end	Day	21% 5% 2 5% 4 3 5% 3 4 7% 40%	2005
November	1st to 10th	Day	14% 7% 2 4 3 2 3 2 4 5% 53%	1761
	11th to 20th	Day	18% 6% 1 4 3 4 5% 4 5% 6% 44%	1800
	21st to end	Day	18% 6% 2 4 4 3 4 4 5% 7% 41%	1635
December	1st to 10th	Day	18% 6% 2 6% 3 4 4 4 5% 6% 41%	2600
	11th to 20th	Day	23% 8% 2 4 3 3 4 4 5% 6% 38%	2986
	21st to end	Day	19% 7% 3 6% 5% 5% 5% 4 6% 7% 34%	3960
January	1st to 10th	Night	13% 6% 2 6% 4 4 4 4 5% 7% 45%	6349
	11th to 20th	Night	14% 8% 3 7% 4 4 4 4 6% 7% 40%	6287
	21st to end	Night	18% 9% 3 6% 4 3 4 3 5% 6% 41%	6490
February	1st to 10th	Night	16% 9% 3 6% 4 4 4 3 5% 7% 39%	2600
	11th to 20th	Night	17% 8% 3 7% 5% 4 5% 4 4 5% 39%	2401
	21st to end	Night	19% 7% 3 6% 4 4 4 4 4 5% 40%	1980
March	1st to 10th	Night	19% 9% 2 6% 4 3 3 3 5% 5% 41%	5364
	11th to 20th	Night	13% 7% 3 5% 4 4 4 3 4 5% 49%	4983
	21st to end	Night	17% 8% 3 7% 4 4 4 3 4 4 42%	5130
April	1st to 10th	Night	18% 7% 2 7% 6% 4 4 4 4 6% 36%	2000
	11th to 20th	Night	18% 6% 3 6% 4 4 4 3 5% 8% 39%	1975
	21st to end	Night	20% 5% 2 5% 3 3 3 4 6% 6% 44%	2000
May	1st to 10th	Night	31% 7% 2 5% 3 4 4 3 5% 7% 28%	4023
	11th to 20th	Night	24% 9% 3 6% 4 4 4 3 5% 6% 33%	3575
	21st to end	Night	23% 6% 2 6% 3 4 4 4 5% 7% 37%	3926
June	1st to 10th	Night	17% 7% 2 4 4 3 4 4 6% 8% 42%	1598
	11th to 20th	Night	13% 6% 2 5% 4 4 4 5% 6% 9% 42%	1520
	21st to end	Night	13% 4 1 5% 4 4 4 4 7% 10% 44%	1520
July	1st to 10th	Night	11% 6% 2 7% 5% 6% 5% 5% 6% 9% 38%	2520
	11th to 20th	Night	12% 5% 1 6% 5% 5% 5% 4 7% 11% 38%	2520
	21st to end	Night	18% 6% 2 5% 4 4 5% 5% 6% 8% 38%	2756
August	1st to 10th	Night	22% 7% 2 7% 5% 3 4 4 5% 8% 33%	2825
	11th to 20th	Night	16% 7% 3 6% 4 4 4 4 5% 6% 41%	3031
	21st to end	Night	13% 4 1 4 3 4 4 3 5% 8% 49%	3476
September	1st to 10th	Night	15% 6% 2 5% 4 4 4 2 5% 6% 46%	1900
	11th to 20th	Night	13% 5% 2 4 4 3 3 3 4 7% 52%	1928
	21st to end	Night	19% 6% 2 6% 3 3 4 3 4 6% 44%	2090
October	1st to 10th	Night	23% 6% 3 5% 3 3 3 3 4 6% 43%	2280
	11th to 20th	Night	17% 6% 1 5% 3 3 3 3 4 6% 49%	2280
	21st to end	Night	21% 6% 2 4 3 3 3 3 5% 5% 45%	2591
November	1st to 10th	Night	13% 5% 2 5% 3 3 4 4 5% 5% 52%	2537
	11th to 20th	Night	18% 7% 2 5% 4 3 3 3 5% 5% 46%	2600
	21st to end	Night	15% 7% 2 5% 3 4 4 3 6% 7% 44%	2760
December	1st to 10th	Night	14% 8% 3 6% 4 4 4 3 4 6% 44%	4580
	11th to 20th	Night	25% 7% 2 4 3 3 4 3 5% 5% 40%	5208
	21st to end	Night	21% 10% 3 6% 4 3 4 3 4 5% 37%	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	38% 4 10% 28% 18% 2	3598
	11th to 20th	Day	49% 5% 10% 19% 13% 5%	3623
	21st to end	Day	47% 5% 11% 23% 13% 1	4466
February	1st to 10th	Day	46% 5% 13% 21% 13% 2	1800
	11th to 20th	Day	46% 5% 9% 24% 14% 2	1893
	21st to end	Day	49% 5% 10% 22% 12% 3	1650
March	1st to 10th	Day	44% 4 10% 23% 12% 7%	4479
	11th to 20th	Day	39% 6% 13% 27% 11% 4	4975
	21st to end	Day	41% 5% 11% 14% 23% 5%	5800
April	1st to 10th	Day	47% 5% 9% 21% 15% 5%	2400
	11th to 20th	Day	43% 5% 10% 22% 15% 4	2374
	21st to end	Day	42% 5% 10% 23% 16% 5%	2400
May	1st to 10th	Day	52% 7% 13% 17% 10% 1	5968
	11th to 20th	Day	51% 5% 9% 22% 12% 1	6322
	21st to end	Day	44% 6% 11% 24% 14% 1	6949
June	1st to 10th	Day	42% 7% 12% 24% 14% 1	2786
	11th to 20th	Day	42% 7% 14% 24% 11% 3	2660
	21st to end	Day	37% 8% 14% 27% 14% 1	2660
July	1st to 10th	Day	39% 9% 13% 22% 15% 2	4370
	11th to 20th	Day	37% 7% 17% 25% 13% 1	4365
	21st to end	Day	47% 7% 10% 19% 16% 1	4640
August	1st to 10th	Day	49% 8% 14% 20% 10%	4091
	11th to 20th	Day	39% 6% 12% 29% 12% 1	3802
	21st to end	Day	35% 5% 10% 32% 16% 2	4180
September	1st to 10th	Day	41% 5% 10% 22% 19% 2	2280
	11th to 20th	Day	33% 5% 9% 24% 26% 3	2252
	21st to end	Day	46% 4 8% 20% 20% 2	2086
October	1st to 10th	Day	39% 4 8% 21% 24% 5%	3160
	11th to 20th	Day	29% 3 10% 28% 25% 4	3140
	21st to end	Day	29% 3 14% 25% 25% 4	3282
November	1st to 10th	Day	35% 3 13% 24% 21% 3	1761
	11th to 20th	Day	39% 5% 10% 26% 18% 2	1800
	21st to end	Day	40% 5% 13% 29% 10% 2	1635
December	1st to 10th	Day	43% 4 15% 25% 10% 2	2600
	11th to 20th	Day	47% 4 16% 22% 11% 1	2986
	21st to end	Day	47% 6% 12% 20% 14% 1	3960
January	1st to 10th	Night	36% 4 10% 25% 20% 4	6349
	11th to 20th	Night	42% 5% 13% 21% 14% 6%	6332
	21st to end	Night	44% 4 10% 21% 17% 4	6490
February	1st to 10th	Night	46% 5% 13% 24% 11% 2	2600
	11th to 20th	Night	46% 5% 9% 20% 16% 4	2403
	21st to end	Night	45% 4 7% 23% 17% 3	1980
March	1st to 10th	Night	44% 4 10% 22% 11% 8%	5374
	11th to 20th	Night	36% 5% 15% 25% 14% 5%	4984
	21st to end	Night	45% 4 6% 16% 23% 5%	5130
April	1st to 10th	Night	47% 5% 9% 21% 14% 4	2000
	11th to 20th	Night	43% 6% 10% 19% 20% 2	1975
	21st to end	Night	39% 5% 10% 23% 19% 5%	2000
May	1st to 10th	Night	55% 6% 11% 17% 9% 2	4029
	11th to 20th	Night	52% 6% 11% 15% 13% 2	3576
	21st to end	Night	45% 7% 11% 24% 12% 2	3927
June	1st to 10th	Night	40% 5% 10% 24% 19% 1	1598
	11th to 20th	Night	36% 8% 15% 26% 13% 1	1520
	21st to end	Night	34% 7% 14% 29% 14% 1	1520
July	1st to 10th	Night	42% 9% 14% 17% 15% 2	2480
	11th to 20th	Night	39% 8% 14% 25% 13% 1	2472
	21st to end	Night	42% 8% 13% 17% 18% 1	2668
August	1st to 10th	Night	49% 7% 16% 18% 9% 1	2814
	11th to 20th	Night	43% 5% 12% 24% 15% 1	3060
	21st to end	Night	34% 5% 10% 32% 18% 1	3476
September	1st to 10th	Night	39% 4 10% 24% 21% 2	1900
	11th to 20th	Night	33% 5% 9% 24% 26% 3	1928
	21st to end	Night	42% 4 8% 22% 22% 2	2090
October	1st to 10th	Night	42% 4 10% 22% 21% 2	3800
	11th to 20th	Night	28% 4 7% 30% 26% 4	3779
	21st to end	Night	29% 4 13% 31% 22% 2	4264
November	1st to 10th	Night	33% 5% 12% 24% 22% 4	2537
	11th to 20th	Night	41% 4 9% 24% 19% 3	2600
	21st to end	Night	40% 4 13% 28% 14% 2	2760
December	1st to 10th	Night	42% 3 13% 31% 10% 2	4580
	11th to 20th	Night	47% 3 11% 22% 15% 2	5208
	21st to end	Night	49% 5% 10% 20% 15% 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% 16% 31% 21% 7% 2	200
	11th to 20th	Day	21% 21% 35% 17% 4 1	205
	21st to end	Day	19% 12% 36% 24% 7% 2	330
February	1st to 10th	Day	22% 16% 35% 22% 4 1	300
	11th to 20th	Day	24% 12% 35% 25% 3	432
	21st to end	Day	26% 9% 30% 31% 4	414
March	1st to 10th	Day	20% 17% 28% 27% 7% 1	500
	11th to 20th	Day	19% 14% 33% 26% 6% 2	500
	21st to end	Day	24% 15% 27% 23% 8% 3	550
April	1st to 10th	Day	24% 12% 31% 24% 8% 1	500
	11th to 20th	Day	21% 14% 29% 22% 10% 4	492
	21st to end	Day	23% 10% 30% 21% 13% 2	500
May	1st to 10th	Day	31% 10% 25% 18% 11% 4	503
	11th to 20th	Day	22% 8% 28% 24% 13% 5%	527
	21st to end	Day	20% 5% 20% 26% 21% 8%	573
June	1st to 10th	Day	16% 9% 22% 26% 19% 7%	526
	11th to 20th	Day	17% 6% 21% 27% 22% 7%	490
	21st to end	Day	17% 5% 21% 30% 21% 6%	490
July	1st to 10th	Day	14% 2 15% 29% 26% 14%	485
	11th to 20th	Day	13% 2 14% 31% 28% 12%	484
	21st to end	Day	17% 2 20% 30% 23% 8%	532
August	1st to 10th	Day	15% 3 16% 29% 26% 11%	486
	11th to 20th	Day	15% 3 13% 33% 23% 12%	479
	21st to end	Day	16% 2 17% 29% 23% 14%	539
September	1st to 10th	Day	19% 5% 18% 23% 23% 12%	490
	11th to 20th	Day	13% 3 17% 27% 22% 18%	490
	21st to end	Day	24% 6% 19% 27% 18% 7%	490
October	1st to 10th	Day	24% 6% 19% 23% 20% 8%	490
	11th to 20th	Day	16% 8% 30% 26% 15% 4	490
	21st to end	Day	23% 6% 31% 26% 12% 2	494
November	1st to 10th	Day	16% 8% 27% 30% 15% 4	395
	11th to 20th	Day	20% 13% 27% 30% 9% 1	400
	21st to end	Day	22% 11% 34% 24% 8% 1	235
December	1st to 10th	Day	24% 17% 32% 18% 8% 2	200
	11th to 20th	Day	26% 15% 29% 24% 7% 1	200
	21st to end	Day	25% 9% 30% 24% 9% 2	220
January	1st to 10th	Night	19% 8% 38% 30% 5%	800
	11th to 20th	Night	23% 8% 31% 33% 5%	800
	21st to end	Night	18% 5% 40% 34% 3	880
February	1st to 10th	Night	20% 6% 35% 36% 3	800
	11th to 20th	Night	21% 9% 42% 26% 2	783
	21st to end	Night	22% 8% 35% 32% 3	660
March	1st to 10th	Night	20% 7% 37% 32% 5%	800
	11th to 20th	Night	20% 7% 36% 31% 6%	800
	21st to end	Night	20% 6% 34% 32% 7% 1	880
April	1st to 10th	Night	21% 9% 37% 28% 5%	800
	11th to 20th	Night	20% 8% 32% 31% 9%	791
	21st to end	Night	22% 6% 27% 36% 9%	800
May	1st to 10th	Night	26% 6% 24% 34% 9% 1	595
	11th to 20th	Night	21% 3 26% 40% 10%	394
	21st to end	Night	18% 2 14% 46% 19% 1	433
June	1st to 10th	Night	18% 3 16% 42% 21% 1	400
	11th to 20th	Night	14% 2 17% 49% 17% 1	380
	21st to end	Night	14% 1 12% 54% 19%	380
July	1st to 10th	Night	15% 1 12% 44% 25% 2	370
	11th to 20th	Night	13% 2 11% 45% 28% 1	368
	21st to end	Night	13% 2 12% 47% 24% 1	421
August	1st to 10th	Night	14% 2 7% 46% 30% 2	567
	11th to 20th	Night	16% 2 9% 39% 31% 3	708
	21st to end	Night	17% 2 8% 41% 30% 2	836
September	1st to 10th	Night	19% 2 14% 38% 24% 3	760
	11th to 20th	Night	15% 2 10% 40% 28% 5%	760
	21st to end	Night	23% 3 14% 38% 21% 2	760
October	1st to 10th	Night	23% 4 19% 31% 20% 3	760
	11th to 20th	Night	19% 3 24% 41% 14% 1	760
	21st to end	Night	24% 3 26% 36% 11%	836
November	1st to 10th	Night	16% 4 27% 39% 13% 1	780
	11th to 20th	Night	20% 6% 31% 32% 10% 1	800
	21st to end	Night	17% 7% 32% 36% 8%	800
December	1st to 10th	Night	24% 5% 31% 33% 6% 1	800
	11th to 20th	Night	23% 5% 31% 35% 7%	800
	21st to end	Night	23% 6% 32% 36% 4	880

Page generated at 2025-06-07 10:28:19. Version ac07

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