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 2026-09-14.

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 10% 5% 5% 4 4 5% 6% 28%	149699
	Night	17% 13% 5% 11% 6% 5% 4 3 4 5% 27%	141380
Transparency	Day	55% 6% 11% 19% 9% 1	153218
	Night	58% 5% 10% 17% 9% 1	145950
Seeing	Day	27% 19% 29% 16% 7% 2	17058
	Night	25% 18% 27% 21% 7% 1	26185

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% 19% 10% 23% 10% 3 3 2 3 3 12%	4661
	11th to 20th	Day	10% 19% 11% 20% 8% 8% 4 3 3 3 10%	4650
	21st to end	Day	12% 16% 6% 15% 12% 9% 6% 3 4 4 13%	5693
February	1st to 10th	Day	13% 19% 7% 15% 10% 6% 6% 4 4 4 13%	1890
	11th to 20th	Day	13% 17% 8% 20% 10% 4 4 3 3 4 15%	1898
	21st to end	Day	17% 18% 6% 14% 9% 4 4 3 3 4 19%	1730
March	1st to 10th	Day	18% 16% 8% 14% 7% 5% 3 2 3 3 20%	4640
	11th to 20th	Day	11% 18% 7% 9% 4 3 3 2 4 6% 31%	5090
	21st to end	Day	17% 14% 6% 13% 5% 4 3 3 4 4 26%	5865
April	1st to 10th	Day	17% 18% 6% 10% 5% 3 3 3 4 5% 27%	2520
	11th to 20th	Day	17% 15% 4 7% 5% 4 4 3 5% 5% 32%	2494
	21st to end	Day	19% 16% 3 7% 4 3 4 3 4 5% 31%	2520
May	1st to 10th	Day	28% 11% 3 7% 4 4 4 3 5% 5% 26%	8168
	11th to 20th	Day	18% 10% 3 8% 5% 4 4 4 5% 8% 31%	8221
	21st to end	Day	22% 9% 3 6% 5% 5% 4 4 6% 8% 28%	9060
June	1st to 10th	Day	19% 8% 3 7% 5% 4 4 4 6% 8% 32%	2940
	11th to 20th	Day	18% 7% 2 6% 6% 5% 5% 5% 7% 8% 32%	2940
	21st to end	Day	16% 8% 2 7% 5% 5% 5% 4 6% 8% 32%	2940
July	1st to 10th	Day	14% 6% 2 6% 4 4 5% 5% 7% 10% 36%	8280
	11th to 20th	Day	10% 5% 2 6% 5% 5% 6% 5% 8% 11% 38%	8280
	21st to end	Day	16% 6% 2 6% 5% 4 5% 4 6% 7% 38%	8993
August	1st to 10th	Day	19% 7% 3 7% 5% 6% 5% 4 6% 7% 30%	6091
	11th to 20th	Day	17% 7% 2 6% 4 5% 5% 5% 7% 9% 32%	5745
	21st to end	Day	17% 7% 3 6% 5% 5% 5% 4 6% 8% 35%	6080
September	1st to 10th	Day	16% 6% 2 6% 4 5% 5% 4 6% 8% 38%	2520
	11th to 20th	Day	15% 7% 3 6% 4 4 4 3 5% 6% 44%	2436
	21st to end	Day	18% 8% 2 7% 5% 5% 5% 5% 6% 6% 33%	2139
October	1st to 10th	Day	19% 10% 3 7% 5% 5% 5% 3 5% 6% 33%	2000
	11th to 20th	Day	20% 11% 3 7% 4 4 5% 3 6% 6% 31%	2000
	21st to end	Day	23% 19% 3 7% 4 4 4 3 4 5% 24%	2100
November	1st to 10th	Day	13% 17% 3 8% 5% 4 5% 4 5% 6% 31%	1851
	11th to 20th	Day	15% 19% 6% 10% 4 5% 4 3 4 5% 26%	1790
	21st to end	Day	17% 21% 7% 11% 5% 5% 4 3 4 6% 18%	1680
December	1st to 10th	Day	16% 22% 11% 20% 5% 3 3 2 4 4 9%	2680
	11th to 20th	Day	18% 18% 10% 16% 5% 3 2 2 3 3 20%	3066
	21st to end	Day	16% 19% 8% 13% 7% 5% 4 3 4 5% 16%	4048
January	1st to 10th	Night	13% 17% 9% 27% 10% 4 3 2 3 3 9%	8234
	11th to 20th	Night	9% 19% 8% 23% 10% 5% 4 3 3 4 12%	8235
	21st to end	Night	11% 13% 6% 14% 11% 11% 6% 4 4 4 16%	8484
February	1st to 10th	Night	13% 18% 6% 17% 8% 5% 5% 3 4 4 17%	2730
	11th to 20th	Night	11% 15% 8% 18% 11% 6% 5% 3 4 3 16%	2606
	21st to end	Night	16% 17% 6% 15% 8% 5% 3 3 5% 5% 18%	2076
March	1st to 10th	Night	16% 13% 6% 14% 9% 5% 4 3 4 5% 22%	5419
	11th to 20th	Night	15% 16% 6% 10% 5% 3 3 3 4 6% 30%	5089
	21st to end	Night	20% 14% 5% 13% 4 4 4 3 3 5% 26%	5308
April	1st to 10th	Night	18% 16% 4 9% 5% 5% 4 3 4 6% 26%	2100
	11th to 20th	Night	18% 15% 3 8% 4 4 4 3 5% 5% 32%	2075
	21st to end	Night	20% 12% 3 6% 5% 3 4 3 4 5% 34%	2100
May	1st to 10th	Night	28% 13% 3 6% 4 4 3 3 4 5% 26%	4771
	11th to 20th	Night	17% 10% 3 7% 5% 4 4 3 5% 7% 34%	4639
	21st to end	Night	23% 10% 3 6% 4 4 4 3 6% 7% 32%	5114
June	1st to 10th	Night	18% 7% 2 6% 5% 4 4 4 6% 7% 37%	1680
	11th to 20th	Night	18% 8% 3 5% 4 4 4 4 5% 9% 35%	1680
	21st to end	Night	16% 5% 2 6% 4 5% 4 4 6% 7% 41%	1680
July	1st to 10th	Night	13% 7% 2 7% 5% 4 5% 3 6% 9% 38%	4680
	11th to 20th	Night	15% 5% 1 5% 4 4 4 3 6% 8% 44%	4680
	21st to end	Night	16% 7% 2 5% 4 3 4 4 5% 6% 44%	5084
August	1st to 10th	Night	22% 8% 2 6% 4 4 5% 3 5% 7% 35%	4041
	11th to 20th	Night	18% 9% 2 6% 4 4 4 3 4 6% 39%	4254
	21st to end	Night	16% 8% 2 6% 4 4 4 4 5% 7% 39%	5045
September	1st to 10th	Night	16% 7% 2 6% 4 4 4 3 5% 7% 43%	2100
	11th to 20th	Night	15% 8% 1 5% 3 3 3 3 4 6% 48%	2031
	21st to end	Night	18% 9% 2 6% 4 5% 4 3 5% 6% 36%	2257
October	1st to 10th	Night	19% 9% 3 7% 5% 4 4 3 5% 5% 35%	2400
	11th to 20th	Night	20% 11% 3 6% 5% 4 4 3 5% 6% 33%	2400
	21st to end	Night	24% 17% 3 7% 5% 3 4 3 4 5% 25%	2738
November	1st to 10th	Night	14% 17% 4 9% 4 4 4 3 4 5% 33%	2667
	11th to 20th	Night	18% 18% 5% 8% 4 3 4 3 4 5% 27%	2825
	21st to end	Night	15% 20% 8% 11% 6% 4 4 3 5% 6% 20%	2940
December	1st to 10th	Night	14% 20% 11% 17% 6% 5% 4 4 4 4 12%	4720
	11th to 20th	Night	20% 19% 8% 12% 4 3 3 2 3 4 20%	5348
	21st to end	Night	20% 20% 8% 13% 7% 4 3 2 3 4 17%	7150

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	78% 3 8% 9% 3	4662
	11th to 20th	Day	79% 4 6% 6% 4 1	4680
	21st to end	Day	72% 4 8% 11% 4	5693
February	1st to 10th	Day	72% 5% 7% 12% 3	1890
	11th to 20th	Day	72% 5% 6% 13% 4	1907
	21st to end	Day	68% 5% 8% 14% 5%	1730
March	1st to 10th	Day	69% 4 10% 11% 6% 1	4643
	11th to 20th	Day	54% 4 18% 13% 8% 2	5090
	21st to end	Day	61% 4 6% 14% 14% 1	5865
April	1st to 10th	Day	60% 3 8% 18% 8% 2	2520
	11th to 20th	Day	53% 5% 10% 19% 11% 1	2494
	21st to end	Day	55% 4 9% 18% 11% 2	2520
May	1st to 10th	Day	59% 5% 9% 17% 9% 1	8180
	11th to 20th	Day	52% 6% 10% 23% 8%	8225
	21st to end	Day	52% 7% 12% 20% 9%	9061
June	1st to 10th	Day	48% 7% 10% 21% 12% 1	2940
	11th to 20th	Day	48% 8% 12% 22% 10% 1	2940
	21st to end	Day	47% 8% 14% 21% 10% 1	2940
July	1st to 10th	Day	42% 7% 15% 24% 11%	8210
	11th to 20th	Day	38% 7% 16% 29% 10%	8205
	21st to end	Day	43% 6% 11% 28% 11%	8795
August	1st to 10th	Day	52% 7% 12% 17% 11% 1	6077
	11th to 20th	Day	45% 7% 20% 23% 5%	5784
	21st to end	Day	45% 7% 13% 28% 6% 1	6080
September	1st to 10th	Day	43% 6% 12% 27% 11% 1	2520
	11th to 20th	Day	41% 5% 10% 26% 18% 1	2436
	21st to end	Day	50% 6% 11% 21% 11% 1	2139
October	1st to 10th	Day	46% 6% 11% 26% 11% 1	3260
	11th to 20th	Day	47% 4 14% 19% 15% 1	3240
	21st to end	Day	48% 5% 15% 24% 8% 1	3377
November	1st to 10th	Day	51% 6% 11% 19% 11% 1	1851
	11th to 20th	Day	61% 5% 9% 15% 8% 2	1790
	21st to end	Day	68% 5% 12% 11% 4	1680
December	1st to 10th	Day	80% 4 8% 7% 2	2680
	11th to 20th	Day	71% 3 9% 10% 7%	3066
	21st to end	Day	71% 4 8% 10% 6% 1	4048
January	1st to 10th	Night	80% 4 7% 6% 3	8234
	11th to 20th	Night	75% 4 7% 7% 6% 1	8280
	21st to end	Night	69% 4 8% 10% 8% 1	8484
February	1st to 10th	Night	69% 4 8% 13% 6%	2730
	11th to 20th	Night	70% 5% 8% 12% 4	2608
	21st to end	Night	66% 5% 10% 12% 7% 1	2076
March	1st to 10th	Night	65% 5% 11% 12% 7%	5429
	11th to 20th	Night	55% 5% 16% 15% 5% 3	5090
	21st to end	Night	60% 4 5% 17% 11% 1	5308
April	1st to 10th	Night	59% 6% 10% 16% 7% 2	2100
	11th to 20th	Night	53% 5% 10% 18% 13%	2075
	21st to end	Night	52% 4 10% 19% 12% 3	2100
May	1st to 10th	Night	61% 4 8% 14% 10% 3	4777
	11th to 20th	Night	49% 7% 12% 22% 10% 1	4640
	21st to end	Night	52% 5% 12% 19% 10% 1	5115
June	1st to 10th	Night	45% 6% 11% 22% 15% 1	1680
	11th to 20th	Night	44% 7% 14% 21% 13% 1	1680
	21st to end	Night	40% 7% 13% 27% 12% 1	1680
July	1st to 10th	Night	42% 7% 15% 22% 13% 1	4640
	11th to 20th	Night	39% 6% 14% 27% 13% 1	4632
	21st to end	Night	41% 5% 12% 25% 17% 1	4967
August	1st to 10th	Night	50% 6% 13% 20% 10% 2	4031
	11th to 20th	Night	46% 5% 15% 27% 7%	4281
	21st to end	Night	43% 6% 13% 27% 10%	5045
September	1st to 10th	Night	41% 5% 13% 26% 13% 1	2100
	11th to 20th	Night	36% 5% 10% 30% 17% 1	2031
	21st to end	Night	48% 5% 11% 22% 12% 1	2257
October	1st to 10th	Night	53% 5% 10% 22% 11% 1	3920
	11th to 20th	Night	48% 4 13% 16% 12% 6%	3899
	21st to end	Night	48% 5% 13% 26% 7%	4411
November	1st to 10th	Night	52% 5% 10% 19% 12% 2	2667
	11th to 20th	Night	58% 5% 10% 17% 7% 2	2825
	21st to end	Night	64% 4 14% 14% 4	2940
December	1st to 10th	Night	74% 5% 9% 10% 1	4720
	11th to 20th	Night	68% 5% 8% 11% 9%	5348
	21st to end	Night	73% 4 6% 8% 7% 2	7150

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	33% 30% 27% 8% 2	220
	11th to 20th	Day	43% 28% 24% 5% 1	220
	21st to end	Day	33% 31% 28% 7% 1	346
February	1st to 10th	Day	37% 31% 26% 5%	330
	11th to 20th	Day	39% 31% 26% 3	385
	21st to end	Day	38% 27% 27% 7% 2	446
March	1st to 10th	Day	33% 25% 29% 11% 2	540
	11th to 20th	Day	38% 24% 25% 12% 2	540
	21st to end	Day	38% 24% 27% 8% 3 1	594
April	1st to 10th	Day	38% 22% 29% 8% 3	540
	11th to 20th	Day	35% 25% 24% 9% 5% 2	532
	21st to end	Day	33% 21% 27% 14% 4 1	540
May	1st to 10th	Day	32% 20% 29% 11% 6% 2	543
	11th to 20th	Day	23% 16% 37% 16% 8% 1	567
	21st to end	Day	21% 16% 32% 21% 8% 2	617
June	1st to 10th	Day	25% 15% 27% 24% 8% 1	570
	11th to 20th	Day	22% 15% 31% 22% 7% 2	570
	21st to end	Day	19% 17% 36% 20% 7% 1	570
July	1st to 10th	Day	16% 9% 32% 30% 12% 2	565
	11th to 20th	Day	13% 13% 31% 29% 12% 4	564
	21st to end	Day	14% 9% 39% 26% 9% 3	620
August	1st to 10th	Day	16% 8% 28% 28% 14% 5%	566
	11th to 20th	Day	18% 10% 28% 26% 11% 6%	559
	21st to end	Day	15% 15% 30% 22% 12% 5%	627
September	1st to 10th	Day	20% 12% 27% 23% 12% 6%	570
	11th to 20th	Day	14% 11% 26% 21% 18% 9%	542
	21st to end	Day	22% 15% 28% 20% 11% 4	530
October	1st to 10th	Day	24% 16% 27% 16% 14% 3	530
	11th to 20th	Day	30% 20% 27% 16% 6% 1	530
	21st to end	Day	35% 19% 28% 15% 3	528
November	1st to 10th	Day	31% 22% 24% 16% 7%	425
	11th to 20th	Day	31% 26% 25% 14% 4	330
	21st to end	Day	39% 27% 30% 5%	220
December	1st to 10th	Day	41% 23% 27% 6% 2	220
	11th to 20th	Day	40% 27% 26% 6% 1	220
	21st to end	Day	34% 26% 33% 5% 1	242
January	1st to 10th	Night	27% 31% 30% 11% 1	840
	11th to 20th	Night	29% 27% 28% 13% 3	840
	21st to end	Night	24% 27% 32% 15% 2	924
February	1st to 10th	Night	28% 25% 32% 15% 1	840
	11th to 20th	Night	28% 27% 31% 13% 1	823
	21st to end	Night	31% 26% 27% 13% 2	692
March	1st to 10th	Night	27% 25% 30% 16% 2	840
	11th to 20th	Night	28% 25% 29% 16% 2	840
	21st to end	Night	30% 22% 27% 15% 5% 1	924
April	1st to 10th	Night	30% 20% 31% 17% 3	840
	11th to 20th	Night	28% 22% 25% 18% 7%	831
	21st to end	Night	28% 16% 29% 22% 5%	840
May	1st to 10th	Night	26% 15% 26% 26% 7%	466
	11th to 20th	Night	19% 10% 34% 29% 7% 1	414
	21st to end	Night	18% 8% 33% 31% 10%	455
June	1st to 10th	Night	20% 8% 28% 33% 11% 1	420
	11th to 20th	Night	19% 11% 24% 34% 12%	420
	21st to end	Night	15% 6% 28% 41% 10%	420
July	1st to 10th	Night	10% 4 28% 42% 14% 2	410
	11th to 20th	Night	14% 4 23% 40% 18% 1	408
	21st to end	Night	16% 5% 23% 42% 13% 1	460
August	1st to 10th	Night	15% 4 27% 34% 19% 1	585
	11th to 20th	Night	16% 4 21% 34% 21% 3	692
	21st to end	Night	17% 7% 23% 33% 20% 1	924
September	1st to 10th	Night	19% 8% 25% 31% 15% 2	840
	11th to 20th	Night	16% 8% 21% 30% 21% 4	813
	21st to end	Night	21% 12% 24% 25% 17% 2	800
October	1st to 10th	Night	24% 14% 24% 21% 15% 2	800
	11th to 20th	Night	27% 15% 30% 22% 5% 1	800
	21st to end	Night	33% 18% 27% 18% 3	880
November	1st to 10th	Night	26% 18% 31% 18% 6%	820
	11th to 20th	Night	31% 21% 28% 14% 5% 1	840
	21st to end	Night	28% 25% 28% 15% 3	840
December	1st to 10th	Night	31% 28% 26% 13% 3	840
	11th to 20th	Night	31% 26% 28% 13% 2	840
	21st to end	Night	31% 29% 27% 11% 2	924

Page generated at 2026-09-11 04:11:58. Version ac07

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