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 2024-11-01.

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	17% 12% 4 9% 5% 5% 4 4 5% 6% 28%	101476
	Night	17% 14% 5% 10% 6% 4 4 3 4 5% 28%	97460
Transparency	Day	55% 6% 11% 19% 9% 1	101234
	Night	57% 6% 11% 18% 8% 1	97322
Seeing	Day	26% 19% 29% 17% 7% 2	14687
	Night	24% 18% 28% 21% 8% 1	23861

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	12% 22% 12% 15% 5% 5% 4 3 3 4 17%	2517
	11th to 20th	Day	12% 21% 10% 18% 6% 6% 3 3 3 3 13%	2520
	21st to end	Day	17% 21% 8% 14% 10% 5% 4 3 4 4 11%	3065
February	1st to 10th	Day	14% 20% 7% 14% 10% 5% 5% 4 4 4 13%	1710
	11th to 20th	Day	12% 17% 8% 20% 9% 5% 4 3 3 4 15%	1712
	21st to end	Day	18% 17% 6% 14% 8% 4 4 3 3 5% 19%	1570
March	1st to 10th	Day	21% 16% 6% 13% 6% 4 3 2 4 4 22%	3165
	11th to 20th	Day	10% 22% 8% 10% 5% 3 3 3 4 5% 26%	3480
	21st to end	Day	17% 13% 6% 12% 5% 4 3 2 5% 5% 28%	4004
April	1st to 10th	Day	17% 18% 5% 9% 5% 3 3 3 4 5% 28%	2280
	11th to 20th	Day	17% 14% 4 7% 5% 4 4 3 5% 5% 33%	2254
	21st to end	Day	20% 16% 3 7% 4 3 4 3 4 5% 31%	2280
May	1st to 10th	Day	23% 11% 3 7% 4 4 4 3 5% 6% 31%	4385
	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% 23%	1995
November	1st to 10th	Day	13% 16% 3 8% 6% 3 5% 4 5% 6% 31%	1671
	11th to 20th	Day	16% 19% 5% 11% 4 5% 4 3 4 5% 25%	1620
	21st to end	Day	17% 19% 6% 11% 5% 5% 4 2 4 7% 18%	1520
December	1st to 10th	Day	16% 24% 7% 14% 6% 3 3 3 5% 5% 13%	1520
	11th to 20th	Day	20% 18% 8% 14% 5% 3 3 3 4 4 20%	1906
	21st to end	Day	20% 23% 9% 12% 6% 5% 4 3 3 4 11%	2772
January	1st to 10th	Night	15% 17% 10% 20% 7% 5% 4 2 4 4 12%	4429
	11th to 20th	Night	11% 20% 9% 17% 9% 5% 4 2 3 4 15%	4440
	21st to end	Night	19% 18% 7% 17% 10% 7% 3 3 2 3 11%	4587
February	1st to 10th	Night	13% 19% 6% 16% 8% 5% 5% 3 4 4 17%	2470
	11th to 20th	Night	11% 15% 7% 18% 11% 6% 5% 3 4 3 17%	2352
	21st to end	Night	16% 15% 5% 15% 8% 5% 4 3 5% 6% 18%	1884
March	1st to 10th	Night	18% 14% 5% 12% 6% 5% 4 3 4 5% 24%	3674
	11th to 20th	Night	12% 20% 8% 11% 6% 3 3 3 4 5% 24%	3479
	21st to end	Night	18% 15% 5% 12% 4 4 4 3 3 4 29%	3627
April	1st to 10th	Night	17% 17% 4 9% 5% 5% 5% 2 4 6% 27%	1900
	11th to 20th	Night	17% 14% 3 8% 4 4 4 3 5% 5% 32%	1875
	21st to end	Night	20% 12% 3 7% 5% 3 4 4 4 5% 33%	1900
May	1st to 10th	Night	24% 12% 3 6% 4 4 4 3 5% 5% 30%	2569
	11th to 20th	Night	15% 11% 3 8% 5% 3 5% 4 5% 8% 33%	2495
	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% 33%	2414
	11th to 20th	Night	18% 17% 5% 9% 5% 3 4 3 4 5% 26%	2555
	21st to end	Night	14% 18% 8% 11% 6% 4 4 3 5% 6% 21%	2660
December	1st to 10th	Night	18% 21% 9% 13% 6% 4 4 4 3 4 15%	2660
	11th to 20th	Night	18% 17% 8% 14% 5% 4 4 3 4 5% 18%	3288
	21st to end	Night	23% 18% 9% 13% 5% 4 3 3 4 5% 13%	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	70% 4 10% 12% 4	2518
	11th to 20th	Day	75% 5% 7% 9% 4 1	2520
	21st to end	Day	77% 4 8% 8% 3 1	3065
February	1st to 10th	Day	72% 5% 8% 12% 3	1710
	11th to 20th	Day	71% 5% 6% 14% 4	1721
	21st to end	Day	68% 4 8% 14% 6%	1570
March	1st to 10th	Day	67% 4 10% 12% 7% 1	3168
	11th to 20th	Day	59% 5% 15% 11% 9% 1	3480
	21st to end	Day	58% 4 7% 17% 13% 1	4004
April	1st to 10th	Day	59% 3 7% 19% 9% 2	2280
	11th to 20th	Day	53% 5% 10% 20% 11% 1	2254
	21st to end	Day	56% 4 9% 18% 11% 3	2280
May	1st to 10th	Day	54% 5% 10% 20% 11% 1	4399
	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	52% 6% 10% 18% 12% 1	1900
	11th to 20th	Day	52% 6% 13% 21% 8% 1	1900
	21st to end	Day	62% 4 12% 14% 7% 1	1995
November	1st to 10th	Day	51% 7% 12% 18% 11% 1	1671
	11th to 20th	Day	61% 5% 10% 14% 7% 2	1620
	21st to end	Day	66% 5% 12% 12% 4	1520
December	1st to 10th	Day	74% 4 10% 9% 3	1520
	11th to 20th	Day	70% 4 13% 9% 3	1906
	21st to end	Day	77% 4 6% 9% 3	2772
January	1st to 10th	Night	73% 6% 10% 7% 3	4429
	11th to 20th	Night	72% 5% 7% 10% 6%	4440
	21st to end	Night	78% 4 7% 9% 3 1	4587
February	1st to 10th	Night	69% 4 9% 13% 5%	2470
	11th to 20th	Night	69% 6% 7% 13% 4 1	2354
	21st to end	Night	64% 5% 10% 13% 7% 1	1884
March	1st to 10th	Night	62% 5% 9% 15% 7%	3684
	11th to 20th	Night	60% 6% 11% 18% 5%	3480
	21st to end	Night	58% 5% 6% 19% 11% 2	3627
April	1st to 10th	Night	58% 6% 10% 17% 7% 2	1900
	11th to 20th	Night	52% 5% 10% 18% 14%	1875
	21st to end	Night	52% 4 10% 19% 12% 3	1900
May	1st to 10th	Night	55% 6% 10% 17% 9% 3	2575
	11th to 20th	Night	49% 9% 14% 20% 7% 1	2496
	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	51% 5% 12% 20% 11% 1	2280
	11th to 20th	Night	51% 6% 14% 17% 9% 2	2280
	21st to end	Night	62% 4 11% 15% 7% 1	2601
November	1st to 10th	Night	52% 5% 11% 18% 12% 2	2414
	11th to 20th	Night	58% 5% 11% 16% 7% 3	2555
	21st to end	Night	62% 5% 15% 15% 4	2660
December	1st to 10th	Night	72% 5% 11% 10% 2	2660
	11th to 20th	Night	68% 6% 9% 12% 4	3288
	21st to end	Night	73% 5% 8% 10% 3	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	30% 27% 30% 10% 3	180
	11th to 20th	Day	40% 28% 26% 5% 1	180
	21st to end	Day	27% 33% 30% 8% 2	282
February	1st to 10th	Day	36% 33% 27% 5%	270
	11th to 20th	Day	36% 33% 27% 3	319
	21st to end	Day	38% 25% 27% 8% 2	382
March	1st to 10th	Day	32% 26% 28% 11% 3	460
	11th to 20th	Day	35% 25% 26% 12% 2	460
	21st to end	Day	36% 23% 28% 9% 3 1	506
April	1st to 10th	Day	36% 21% 30% 9% 4	460
	11th to 20th	Day	33% 26% 22% 10% 6% 2	452
	21st to end	Day	33% 20% 26% 15% 4 1	460
May	1st to 10th	Day	29% 21% 30% 13% 7% 2	463
	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% 18% 28% 15% 3	489
November	1st to 10th	Day	27% 23% 24% 17% 8%	365
	11th to 20th	Day	31% 27% 24% 13% 4	280
	21st to end	Day	36% 25% 34% 6%	180
December	1st to 10th	Day	41% 21% 28% 7% 3 1	180
	11th to 20th	Day	37% 28% 27% 7% 1	180
	21st to end	Day	32% 29% 33% 5% 1	198
January	1st to 10th	Night	26% 30% 31% 12% 1	760
	11th to 20th	Night	27% 26% 29% 14% 3	760
	21st to end	Night	22% 28% 33% 15% 2	836
February	1st to 10th	Night	28% 25% 32% 14% 1	760
	11th to 20th	Night	27% 29% 31% 12% 1	743
	21st to end	Night	30% 27% 26% 14% 3	628
March	1st to 10th	Night	26% 26% 30% 17% 2	760
	11th to 20th	Night	27% 26% 29% 16% 2	760
	21st to end	Night	28% 22% 27% 16% 5% 1	836
April	1st to 10th	Night	28% 20% 32% 18% 3	760
	11th to 20th	Night	27% 22% 26% 17% 7% 1	751
	21st to end	Night	28% 16% 29% 22% 5%	760
May	1st to 10th	Night	24% 15% 26% 27% 8%	422
	11th to 20th	Night	18% 10% 33% 29% 8% 1	374
	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%	743
	11th to 20th	Night	31% 21% 28% 14% 5% 1	760
	21st to end	Night	26% 25% 29% 16% 4	760
December	1st to 10th	Night	32% 28% 25% 13% 3	760
	11th to 20th	Night	30% 26% 29% 13% 2	760
	21st to end	Night	30% 31% 28% 10% 2	836

Page generated at 2024-10-29 11:20:45. Version ac07

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