Angel Light Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Angel Light 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	16% 13% 5% 10% 6% 5% 4 4 5% 6% 27%	113907
	Night	16% 14% 5% 12% 6% 4 4 3 4 5% 26%	115010
Transparency	Day	56% 6% 11% 18% 9% 1	117472
	Night	58% 5% 11% 16% 9% 1	119609
Seeing	Day	26% 19% 29% 17% 7% 2	15275
	Night	25% 19% 28% 21% 7% 1	24606

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	10% 18% 12% 24% 8% 4 3 2 3 4 13%	3597
	11th to 20th	Day	11% 20% 11% 21% 8% 6% 3 3 3 3 11%	3570
	21st to end	Day	14% 18% 7% 15% 9% 5% 4 3 4 4 16%	4393
February	1st to 10th	Day	13% 19% 7% 15% 10% 5% 5% 4 4 4 13%	1800
	11th to 20th	Day	12% 17% 7% 20% 10% 5% 4 3 3 4 15%	1805
	21st to end	Day	17% 17% 6% 14% 9% 4 4 3 3 4 19%	1650
March	1st to 10th	Day	18% 17% 8% 14% 7% 5% 3 2 3 3 20%	4539
	11th to 20th	Day	11% 18% 7% 9% 4 3 3 2 4 6% 31%	4980
	21st to end	Day	17% 14% 6% 13% 5% 4 3 2 4 4 27%	5738
April	1st to 10th	Day	17% 18% 6% 9% 5% 3 3 3 4 5% 27%	2400
	11th to 20th	Day	17% 15% 4 7% 5% 4 4 3 5% 5% 33%	2374
	21st to end	Day	19% 16% 3 7% 4 4 4 3 4 5% 31%	2400
May	1st to 10th	Day	24% 11% 3 7% 4 3 4 3 5% 6% 30%	4524
	11th to 20th	Day	17% 10% 4 9% 6% 5% 5% 4 6% 8% 27%	4398
	21st to end	Day	14% 7% 2 6% 4 4 4 4 6% 9% 38%	4837
June	1st to 10th	Day	20% 8% 3 7% 6% 4 4 4 6% 8% 30%	2660
	11th to 20th	Day	17% 7% 2 5% 6% 5% 5% 5% 7% 8% 33%	2660
	21st to end	Day	16% 9% 2 7% 5% 5% 5% 5% 6% 9% 31%	2660
July	1st to 10th	Day	13% 7% 2 6% 4 4 5% 5% 8% 11% 34%	4440
	11th to 20th	Day	12% 6% 2 7% 6% 6% 6% 5% 8% 10% 32%	4440
	21st to end	Day	14% 6% 2 7% 5% 5% 6% 5% 7% 8% 34%	4814
August	1st to 10th	Day	19% 6% 3 7% 5% 5% 5% 5% 7% 8% 31%	4151
	11th to 20th	Day	17% 8% 3 7% 4 5% 5% 5% 7% 8% 29%	3898
	21st to end	Day	17% 8% 3 6% 4 4 4 3 5% 6% 38%	4180
September	1st to 10th	Day	17% 7% 2 6% 4 5% 5% 4 6% 7% 37%	2280
	11th to 20th	Day	16% 8% 3 6% 4 4 4 3 5% 6% 42%	2280
	21st to end	Day	18% 8% 2 7% 5% 5% 5% 5% 6% 5% 34%	2032
October	1st to 10th	Day	20% 10% 3 7% 5% 5% 5% 3 5% 6% 30%	1900
	11th to 20th	Day	20% 11% 3 7% 4 4 5% 3 6% 6% 31%	1900
	21st to end	Day	23% 19% 3 7% 4 4 4 3 4 5% 24%	1995
November	1st to 10th	Day	13% 16% 3 8% 6% 4 5% 4 5% 6% 32%	1761
	11th to 20th	Day	15% 18% 5% 10% 4 5% 4 3 4 5% 26%	1705
	21st to end	Day	17% 20% 6% 11% 5% 5% 4 3 4 6% 18%	1600
December	1st to 10th	Day	16% 23% 11% 20% 5% 3 3 2 4 4 9%	2600
	11th to 20th	Day	18% 18% 10% 16% 5% 3 2 2 3 3 20%	2986
	21st to end	Day	16% 19% 8% 13% 7% 5% 4 3 4 5% 16%	3960
January	1st to 10th	Night	13% 15% 10% 29% 10% 4 3 2 3 3 9%	6349
	11th to 20th	Night	9% 18% 9% 24% 8% 5% 4 3 4 4 13%	6315
	21st to end	Night	14% 15% 6% 16% 9% 7% 5% 3 4 4 16%	6559
February	1st to 10th	Night	13% 19% 6% 17% 8% 5% 5% 3 4 4 17%	2600
	11th to 20th	Night	12% 15% 7% 18% 11% 6% 5% 3 4 3 16%	2479
	21st to end	Night	16% 17% 5% 15% 8% 5% 4 3 5% 5% 17%	1980
March	1st to 10th	Night	16% 13% 6% 15% 9% 5% 4 3 4 5% 21%	5300
	11th to 20th	Night	15% 16% 6% 10% 5% 3 3 3 4 6% 30%	4979
	21st to end	Night	20% 14% 5% 13% 4 4 4 3 3 5% 26%	5193
April	1st to 10th	Night	17% 16% 4 9% 5% 5% 5% 3 4 6% 26%	2000
	11th to 20th	Night	17% 15% 3 8% 4 4 4 3 5% 5% 32%	1975
	21st to end	Night	19% 13% 3 6% 5% 3 4 4 4 5% 34%	2000
May	1st to 10th	Night	25% 12% 3 6% 4 4 3 3 5% 5% 29%	2651
	11th to 20th	Night	15% 11% 3 8% 5% 3 5% 4 5% 8% 34%	2501
	21st to end	Night	14% 7% 3 6% 4 4 4 4 7% 8% 38%	2738
June	1st to 10th	Night	17% 7% 2 6% 5% 4 4 4 6% 7% 37%	1520
	11th to 20th	Night	17% 7% 3 5% 4 5% 4 4 5% 10% 36%	1520
	21st to end	Night	15% 5% 2 6% 4 5% 4 4 6% 7% 41%	1520
July	1st to 10th	Night	10% 7% 3 8% 6% 5% 5% 4 6% 8% 39%	2520
	11th to 20th	Night	14% 6% 2 6% 5% 4 4 4 6% 7% 43%	2520
	21st to end	Night	15% 7% 2 5% 4 3 4 4 6% 7% 42%	2739
August	1st to 10th	Night	23% 6% 2 5% 4 3 5% 3 5% 7% 38%	2761
	11th to 20th	Night	17% 8% 2 8% 4 4 4 3 5% 6% 39%	2899
	21st to end	Night	14% 9% 2 5% 4 3 4 4 5% 7% 43%	3476
September	1st to 10th	Night	15% 7% 2 6% 4 4 4 3 5% 7% 43%	1900
	11th to 20th	Night	15% 8% 1 5% 3 3 2 3 4 6% 48%	1900
	21st to end	Night	18% 8% 2 7% 4 5% 4 3 5% 6% 36%	2144
October	1st to 10th	Night	20% 10% 3 7% 5% 4 4 3 5% 5% 33%	2280
	11th to 20th	Night	20% 11% 3 6% 5% 4 4 3 4 6% 33%	2280
	21st to end	Night	25% 17% 3 7% 5% 3 4 3 4 5% 25%	2601
November	1st to 10th	Night	13% 17% 4 9% 4 4 4 3 4 5% 34%	2537
	11th to 20th	Night	18% 17% 5% 8% 4 3 4 3 4 5% 27%	2690
	21st to end	Night	15% 19% 8% 11% 6% 4 4 3 5% 6% 20%	2800
December	1st to 10th	Night	14% 20% 10% 17% 6% 5% 4 4 4 4 12%	4580
	11th to 20th	Night	20% 20% 8% 11% 4 3 3 2 3 4 21%	5208
	21st to end	Night	20% 20% 8% 12% 7% 4 3 2 3 4 17%	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	76% 3 8% 11% 3	3598
	11th to 20th	Day	78% 4 6% 7% 4 1	3600
	21st to end	Day	70% 4 8% 12% 6%	4393
February	1st to 10th	Day	73% 5% 7% 12% 3	1800
	11th to 20th	Day	72% 5% 6% 13% 4	1814
	21st to end	Day	68% 4 8% 13% 6%	1650
March	1st to 10th	Day	70% 3 10% 10% 6% 1	4542
	11th to 20th	Day	54% 4 18% 13% 8% 2	4980
	21st to end	Day	61% 4 6% 14% 14% 1	5738
April	1st to 10th	Day	60% 3 7% 19% 8% 2	2400
	11th to 20th	Day	53% 5% 10% 20% 11% 1	2374
	21st to end	Day	55% 4 9% 18% 11% 2	2400
May	1st to 10th	Day	54% 5% 9% 19% 11% 1	4538
	11th to 20th	Day	53% 8% 11% 21% 6% 1	4402
	21st to end	Day	40% 9% 14% 24% 13% 1	4837
June	1st to 10th	Day	50% 8% 10% 20% 12% 1	2660
	11th to 20th	Day	47% 7% 13% 22% 10% 1	2660
	21st to end	Day	47% 8% 14% 20% 10%	2660
July	1st to 10th	Day	41% 9% 15% 24% 11% 1	4370
	11th to 20th	Day	42% 8% 15% 25% 9%	4365
	21st to end	Day	46% 8% 13% 23% 9%	4661
August	1st to 10th	Day	49% 7% 13% 20% 11% 1	4137
	11th to 20th	Day	49% 7% 17% 21% 6%	3937
	21st to end	Day	45% 6% 11% 30% 7% 1	4180
September	1st to 10th	Day	45% 6% 12% 26% 11% 1	2280
	11th to 20th	Day	42% 5% 11% 25% 17% 1	2280
	21st to end	Day	50% 5% 11% 21% 11% 1	2032
October	1st to 10th	Day	47% 6% 11% 25% 11% 1	3160
	11th to 20th	Day	47% 4 14% 19% 15% 1	3140
	21st to end	Day	48% 5% 15% 24% 8% 1	3272
November	1st to 10th	Day	51% 6% 12% 19% 11% 1	1761
	11th to 20th	Day	60% 5% 10% 15% 8% 2	1705
	21st to end	Day	67% 5% 13% 12% 4	1600
December	1st to 10th	Day	80% 4 8% 6% 2	2600
	11th to 20th	Day	71% 3 9% 10% 7%	2986
	21st to end	Day	71% 4 8% 10% 6% 1	3960
January	1st to 10th	Night	80% 4 7% 6% 2	6349
	11th to 20th	Night	74% 4 8% 8% 5% 1	6360
	21st to end	Night	69% 3 8% 8% 10% 1	6559
February	1st to 10th	Night	69% 4 9% 13% 5%	2600
	11th to 20th	Night	70% 5% 8% 12% 4	2481
	21st to end	Night	66% 5% 10% 12% 7% 1	1980
March	1st to 10th	Night	65% 5% 11% 12% 7%	5310
	11th to 20th	Night	54% 5% 17% 15% 5% 4	4980
	21st to end	Night	60% 4 5% 18% 11% 1	5193
April	1st to 10th	Night	59% 6% 10% 17% 7% 2	2000
	11th to 20th	Night	53% 5% 10% 18% 14%	1975
	21st to end	Night	52% 4 10% 19% 12% 3	2000
May	1st to 10th	Night	56% 6% 10% 17% 9% 3	2657
	11th to 20th	Night	49% 9% 14% 21% 7% 1	2502
	21st to end	Night	42% 7% 14% 19% 16% 2	2739
June	1st to 10th	Night	44% 6% 12% 22% 14% 1	1520
	11th to 20th	Night	42% 7% 15% 21% 13% 1	1520
	21st to end	Night	40% 7% 13% 27% 12% 1	1520
July	1st to 10th	Night	41% 7% 14% 23% 14% 1	2480
	11th to 20th	Night	39% 7% 14% 27% 12% 1	2472
	21st to end	Night	41% 6% 11% 26% 15% 1	2651
August	1st to 10th	Night	47% 6% 15% 23% 9%	2751
	11th to 20th	Night	45% 5% 15% 26% 9%	2926
	21st to end	Night	41% 5% 12% 31% 10% 1	3476
September	1st to 10th	Night	40% 5% 14% 25% 14% 1	1900
	11th to 20th	Night	37% 5% 11% 29% 17% 1	1900
	21st to end	Night	47% 5% 12% 23% 12% 1	2144
October	1st to 10th	Night	54% 5% 10% 21% 10%	3800
	11th to 20th	Night	48% 4 13% 16% 12% 6%	3779
	21st to end	Night	48% 5% 13% 27% 6%	4274
November	1st to 10th	Night	51% 5% 10% 19% 12% 2	2537
	11th to 20th	Night	57% 5% 11% 17% 8% 3	2690
	21st to end	Night	63% 5% 14% 14% 4	2800
December	1st to 10th	Night	74% 5% 10% 10% 1	4580
	11th to 20th	Night	67% 5% 8% 11% 9%	5208
	21st to end	Night	73% 4 6% 8% 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	31% 31% 27% 9% 3	200
	11th to 20th	Day	40% 30% 25% 5% 1	200
	21st to end	Day	30% 32% 29% 7% 2	314
February	1st to 10th	Day	38% 32% 25% 4	300
	11th to 20th	Day	39% 33% 25% 3	352
	21st to end	Day	38% 25% 28% 7% 2	414
March	1st to 10th	Day	33% 26% 28% 11% 2	500
	11th to 20th	Day	35% 25% 26% 12% 2	500
	21st to end	Day	37% 23% 28% 8% 3 1	550
April	1st to 10th	Day	37% 22% 29% 8% 3	500
	11th to 20th	Day	35% 25% 23% 9% 6% 2	492
	21st to end	Day	33% 21% 26% 15% 4 1	500
May	1st to 10th	Day	31% 19% 30% 12% 6% 2	503
	11th to 20th	Day	22% 14% 37% 17% 8% 2	487
	21st to end	Day	17% 16% 32% 24% 9% 2	529
June	1st to 10th	Day	24% 14% 29% 23% 8% 1	490
	11th to 20th	Day	18% 14% 32% 25% 9% 3	490
	21st to end	Day	19% 18% 35% 19% 7% 1	490
July	1st to 10th	Day	15% 9% 31% 30% 13% 3	485
	11th to 20th	Day	13% 12% 30% 29% 12% 4	484
	21st to end	Day	14% 9% 40% 25% 9% 3	532
August	1st to 10th	Day	16% 8% 29% 27% 14% 6%	486
	11th to 20th	Day	18% 10% 28% 26% 12% 6%	479
	21st to end	Day	15% 15% 29% 23% 13% 6%	539
September	1st to 10th	Day	19% 12% 27% 23% 13% 6%	490
	11th to 20th	Day	15% 11% 25% 22% 17% 9%	490
	21st to end	Day	23% 16% 27% 20% 11% 4	490
October	1st to 10th	Day	24% 17% 27% 15% 14% 3	490
	11th to 20th	Day	28% 19% 28% 17% 7% 1	490
	21st to end	Day	35% 19% 28% 15% 3	489
November	1st to 10th	Day	28% 23% 25% 16% 8%	395
	11th to 20th	Day	30% 26% 25% 14% 4	305
	21st to end	Day	37% 26% 32% 6%	200
December	1st to 10th	Day	40% 22% 29% 7% 3 1	200
	11th to 20th	Day	39% 28% 27% 7% 1	200
	21st to end	Day	31% 27% 35% 6% 1	220
January	1st to 10th	Night	26% 31% 30% 11% 1	800
	11th to 20th	Night	27% 27% 29% 14% 3	800
	21st to end	Night	23% 28% 33% 15% 2	880
February	1st to 10th	Night	29% 25% 31% 14% 1	800
	11th to 20th	Night	28% 28% 31% 12% 1	783
	21st to end	Night	31% 27% 27% 13% 3	660
March	1st to 10th	Night	27% 26% 29% 16% 2	800
	11th to 20th	Night	27% 25% 30% 16% 2	800
	21st to end	Night	29% 22% 28% 16% 5% 1	880
April	1st to 10th	Night	29% 20% 31% 18% 3	800
	11th to 20th	Night	28% 22% 26% 17% 7% 1	791
	21st to end	Night	28% 16% 29% 22% 5%	800
May	1st to 10th	Night	26% 14% 25% 27% 7%	444
	11th to 20th	Night	18% 10% 33% 29% 8% 1	376
	21st to end	Night	15% 9% 34% 31% 11%	411
June	1st to 10th	Night	19% 9% 28% 33% 11% 1	380
	11th to 20th	Night	17% 11% 23% 35% 14%	380
	21st to end	Night	14% 7% 30% 40% 9%	380
July	1st to 10th	Night	9% 4 26% 44% 14% 2	370
	11th to 20th	Night	13% 4 23% 40% 19% 1	368
	21st to end	Night	16% 5% 24% 41% 13% 1	416
August	1st to 10th	Night	15% 5% 28% 32% 19% 1	529
	11th to 20th	Night	15% 4 22% 33% 23% 3	626
	21st to end	Night	16% 7% 23% 33% 20% 1	836
September	1st to 10th	Night	18% 9% 26% 30% 14% 2	760
	11th to 20th	Night	17% 8% 21% 29% 20% 4	760
	21st to end	Night	21% 12% 25% 25% 16% 1	760
October	1st to 10th	Night	25% 15% 24% 20% 15% 2	760
	11th to 20th	Night	27% 15% 30% 22% 5% 1	760
	21st to end	Night	33% 18% 28% 17% 3	836
November	1st to 10th	Night	24% 18% 32% 18% 7%	780
	11th to 20th	Night	31% 21% 29% 14% 5% 1	800
	21st to end	Night	28% 25% 29% 15% 4	800
December	1st to 10th	Night	31% 28% 26% 13% 3	800
	11th to 20th	Night	31% 25% 28% 13% 2	800
	21st to end	Night	30% 30% 28% 11% 2	880

Page generated at 2025-05-08 09:33:56. Version ac07

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