Afterwood Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Afterwood 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	14% 10% 4 9% 6% 5% 5% 4 6% 7% 31%	149333
	Night	14% 10% 4 10% 6% 5% 5% 4 5% 6% 31%	141747
Transparency	Day	47% 9% 12% 20% 10% 1	152810
	Night	47% 10% 13% 19% 10% 1	146345
Seeing	Day	24% 21% 29% 17% 7% 2	17179
	Night	22% 20% 28% 22% 7% 1	26204

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	9% 13% 7% 21% 9% 6% 5% 3 4 5% 18%	4661
	11th to 20th	Day	10% 15% 5% 14% 7% 6% 5% 5% 5% 6% 22%	4650
	21st to end	Day	12% 12% 5% 11% 7% 6% 5% 4 6% 6% 27%	5405
February	1st to 10th	Day	13% 13% 5% 12% 8% 5% 6% 4 4 6% 23%	1890
	11th to 20th	Day	9% 11% 5% 15% 8% 5% 5% 4 6% 6% 26%	1996
	21st to end	Day	15% 14% 4 10% 6% 5% 4 3 5% 6% 27%	1730
March	1st to 10th	Day	18% 14% 5% 10% 6% 4 4 3 4 5% 28%	4576
	11th to 20th	Day	13% 12% 5% 10% 5% 4 4 3 4 6% 34%	4897
	21st to end	Day	17% 10% 4 10% 6% 5% 5% 3 4 5% 32%	6045
April	1st to 10th	Day	15% 13% 4 9% 5% 5% 4 3 5% 5% 31%	2520
	11th to 20th	Day	15% 11% 3 7% 5% 5% 4 3 5% 6% 34%	2494
	21st to end	Day	18% 11% 3 8% 5% 5% 4 3 5% 6% 32%	2583
May	1st to 10th	Day	26% 9% 3 8% 5% 4 4 4 5% 6% 25%	7871
	11th to 20th	Day	16% 10% 3 7% 5% 5% 5% 4 5% 7% 33%	8221
	21st to end	Day	20% 9% 3 8% 5% 5% 6% 5% 6% 7% 26%	9060
June	1st to 10th	Day	15% 8% 3 7% 5% 6% 5% 4 7% 8% 31%	2940
	11th to 20th	Day	15% 7% 2 7% 5% 6% 6% 5% 6% 9% 31%	2940
	21st to end	Day	14% 8% 3 8% 6% 5% 6% 4 6% 7% 33%	2940
July	1st to 10th	Day	12% 7% 3 8% 5% 6% 6% 5% 7% 9% 33%	8280
	11th to 20th	Day	7% 5% 2 6% 5% 6% 6% 5% 8% 12% 38%	8280
	21st to end	Day	14% 7% 2 7% 5% 5% 5% 4 5% 7% 38%	8993
August	1st to 10th	Day	15% 7% 3 10% 6% 6% 6% 4 6% 8% 29%	6367
	11th to 20th	Day	11% 8% 3 7% 5% 5% 6% 5% 8% 9% 33%	5520
	21st to end	Day	11% 7% 3 7% 5% 6% 6% 6% 8% 10% 32%	6080
September	1st to 10th	Day	13% 7% 2 7% 4 4 4 4 5% 8% 40%	2520
	11th to 20th	Day	12% 7% 2 6% 3 3 5% 4 5% 7% 47%	2366
	21st to end	Day	15% 7% 3 6% 4 5% 5% 5% 6% 8% 37%	2200
October	1st to 10th	Day	15% 10% 4 7% 5% 4 4 3 6% 6% 36%	2055
	11th to 20th	Day	14% 9% 3 8% 4 4 4 4 6% 8% 37%	2000
	21st to end	Day	19% 12% 3 8% 5% 4 4 3 5% 6% 30%	2017
November	1st to 10th	Day	13% 13% 3 5% 5% 4 4 3 5% 6% 40%	1851
	11th to 20th	Day	13% 11% 5% 9% 6% 5% 5% 3 4 6% 32%	1890
	21st to end	Day	15% 13% 5% 9% 5% 5% 5% 4 5% 6% 29%	1701
December	1st to 10th	Day	11% 15% 8% 15% 7% 6% 5% 4 5% 5% 20%	2680
	11th to 20th	Day	16% 12% 5% 11% 6% 5% 5% 4 5% 6% 27%	3066
	21st to end	Day	14% 13% 8% 12% 6% 4 5% 4 5% 6% 23%	4048
January	1st to 10th	Night	11% 13% 6% 20% 8% 5% 5% 3 5% 5% 18%	8234
	11th to 20th	Night	9% 15% 6% 18% 8% 6% 6% 4 5% 5% 18%	8235
	21st to end	Night	11% 10% 4 10% 8% 8% 6% 4 5% 7% 26%	8758
February	1st to 10th	Night	12% 13% 4 10% 8% 6% 6% 4 5% 7% 25%	2730
	11th to 20th	Night	10% 11% 6% 12% 8% 6% 5% 6% 6% 6% 23%	2510
	21st to end	Night	14% 12% 5% 11% 6% 6% 5% 3 6% 7% 26%	2076
March	1st to 10th	Night	16% 10% 3 11% 8% 5% 4 4 5% 6% 28%	5484
	11th to 20th	Night	13% 12% 5% 9% 5% 4 4 4 5% 6% 33%	5279
	21st to end	Night	17% 12% 5% 11% 5% 4 4 3 4 5% 29%	5128
April	1st to 10th	Night	16% 12% 4 10% 5% 5% 6% 4 5% 6% 27%	2100
	11th to 20th	Night	19% 11% 3 6% 4 3 4 3 4 7% 35%	2075
	21st to end	Night	19% 10% 4 6% 4 4 4 3 5% 5% 36%	2037
May	1st to 10th	Night	27% 11% 3 7% 4 4 4 3 5% 6% 26%	5083
	11th to 20th	Night	15% 11% 3 7% 4 4 5% 4 6% 8% 33%	4639
	21st to end	Night	19% 10% 3 9% 5% 5% 4 4 6% 7% 28%	5114
June	1st to 10th	Night	15% 6% 3 6% 5% 4 3 4 5% 7% 41%	1680
	11th to 20th	Night	14% 8% 2 6% 4 4 4 4 6% 8% 39%	1680
	21st to end	Night	11% 4 1 7% 5% 5% 5% 4 6% 10% 42%	1680
July	1st to 10th	Night	11% 7% 2 7% 5% 5% 5% 4 6% 9% 39%	4680
	11th to 20th	Night	9% 6% 3 7% 5% 5% 5% 5% 7% 9% 41%	4680
	21st to end	Night	13% 6% 2 6% 4 4 5% 3 5% 7% 44%	5084
August	1st to 10th	Night	17% 8% 3 7% 5% 4 5% 4 5% 8% 34%	3778
	11th to 20th	Night	13% 8% 3 7% 4 4 4 4 5% 7% 40%	4467
	21st to end	Night	10% 7% 3 7% 5% 5% 5% 4 7% 8% 40%	5045
September	1st to 10th	Night	13% 7% 2 5% 4 4 4 4 6% 7% 44%	2100
	11th to 20th	Night	13% 5% 2 5% 3 3 4 3 4 6% 53%	2101
	21st to end	Night	13% 6% 3 7% 5% 4 5% 3 5% 6% 43%	2200
October	1st to 10th	Night	18% 9% 3 7% 4 4 3 3 5% 7% 39%	2340
	11th to 20th	Night	16% 8% 3 6% 5% 4 4 4 5% 6% 40%	2400
	21st to end	Night	19% 12% 3 6% 5% 4 4 3 6% 6% 32%	2821
November	1st to 10th	Night	13% 11% 3 7% 4 3 4 4 5% 6% 39%	2667
	11th to 20th	Night	14% 12% 4 8% 5% 5% 5% 4 5% 6% 32%	2730
	21st to end	Night	13% 12% 5% 8% 6% 5% 4 4 6% 6% 32%	2914
December	1st to 10th	Night	12% 14% 6% 13% 6% 5% 5% 4 6% 7% 22%	4720
	11th to 20th	Night	19% 12% 5% 10% 5% 4 4 3 4 5% 28%	5348
	21st to end	Night	18% 13% 6% 13% 5% 4 4 3 4 5% 24%	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	54% 16% 11% 14% 4 1	4662
	11th to 20th	Day	55% 9% 10% 13% 11% 2	4680
	21st to end	Day	50% 8% 10% 21% 10% 1	5405
February	1st to 10th	Day	56% 9% 12% 14% 8% 1	1890
	11th to 20th	Day	54% 8% 12% 17% 8% 1	2005
	21st to end	Day	53% 8% 12% 17% 10%	1730
March	1st to 10th	Day	55% 7% 11% 13% 10% 4	4579
	11th to 20th	Day	47% 9% 15% 19% 6% 4	4897
	21st to end	Day	50% 8% 9% 13% 17% 3	6045
April	1st to 10th	Day	53% 7% 8% 20% 10% 2	2520
	11th to 20th	Day	48% 7% 12% 19% 13% 1	2494
	21st to end	Day	51% 7% 11% 16% 13% 3	2583
May	1st to 10th	Day	56% 8% 11% 15% 9% 1	7883
	11th to 20th	Day	48% 8% 12% 22% 9%	8225
	21st to end	Day	51% 10% 12% 18% 8%	9061
June	1st to 10th	Day	47% 9% 13% 19% 11% 1	2940
	11th to 20th	Day	45% 10% 15% 19% 11% 1	2940
	21st to end	Day	47% 8% 15% 19% 10% 1	2940
July	1st to 10th	Day	44% 9% 14% 21% 11%	8210
	11th to 20th	Day	34% 10% 16% 29% 11%	8205
	21st to end	Day	43% 7% 11% 25% 13%	8795
August	1st to 10th	Day	50% 10% 13% 15% 10% 1	6353
	11th to 20th	Day	41% 10% 19% 26% 3	5556
	21st to end	Day	38% 12% 15% 26% 8%	6080
September	1st to 10th	Day	41% 8% 12% 25% 14% 1	2520
	11th to 20th	Day	36% 7% 12% 25% 19% 1	2366
	21st to end	Day	41% 8% 13% 23% 13% 2	2200
October	1st to 10th	Day	43% 8% 9% 21% 18% 1	3341
	11th to 20th	Day	38% 7% 11% 23% 17% 5%	3240
	21st to end	Day	37% 8% 14% 25% 12% 3	3229
November	1st to 10th	Day	43% 7% 12% 22% 13% 2	1851
	11th to 20th	Day	49% 9% 11% 17% 12% 2	1890
	21st to end	Day	53% 9% 14% 17% 6% 1	1701
December	1st to 10th	Day	59% 10% 11% 16% 4	2680
	11th to 20th	Day	53% 9% 13% 16% 8% 1	3066
	21st to end	Day	56% 10% 9% 13% 11% 2	4048
January	1st to 10th	Night	56% 13% 12% 12% 6% 1	8234
	11th to 20th	Night	56% 13% 11% 11% 7% 2	8280
	21st to end	Night	50% 9% 12% 14% 12% 2	8758
February	1st to 10th	Night	52% 9% 13% 16% 8% 1	2730
	11th to 20th	Night	53% 10% 13% 15% 8% 1	2512
	21st to end	Night	52% 9% 13% 16% 8% 2	2076
March	1st to 10th	Night	50% 10% 13% 16% 10% 1	5494
	11th to 20th	Night	46% 10% 18% 17% 7% 2	5280
	21st to end	Night	50% 9% 8% 15% 16% 1	5128
April	1st to 10th	Night	54% 9% 12% 16% 9% 1	2100
	11th to 20th	Night	48% 8% 12% 18% 14% 1	2075
	21st to end	Night	48% 7% 11% 17% 14% 3	2037
May	1st to 10th	Night	55% 8% 11% 14% 11% 1	5089
	11th to 20th	Night	46% 9% 14% 23% 8% 1	4640
	21st to end	Night	50% 10% 12% 18% 8%	5115
June	1st to 10th	Night	41% 8% 11% 25% 14% 1	1680
	11th to 20th	Night	39% 10% 15% 25% 10%	1680
	21st to end	Night	35% 11% 17% 24% 11% 1	1680
July	1st to 10th	Night	39% 10% 16% 22% 13% 1	4640
	11th to 20th	Night	35% 10% 16% 25% 13% 1	4632
	21st to end	Night	36% 9% 14% 28% 13% 1	4967
August	1st to 10th	Night	45% 11% 13% 20% 9% 1	3768
	11th to 20th	Night	39% 10% 17% 28% 6%	4496
	21st to end	Night	33% 13% 15% 28% 10% 1	5045
September	1st to 10th	Night	38% 9% 13% 27% 13% 1	2100
	11th to 20th	Night	32% 7% 12% 26% 22% 1	2101
	21st to end	Night	40% 9% 11% 23% 16% 1	2200
October	1st to 10th	Night	45% 9% 12% 22% 12% 1	3821
	11th to 20th	Night	35% 9% 14% 20% 18% 4	3899
	21st to end	Night	39% 8% 16% 26% 10% 1	4559
November	1st to 10th	Night	43% 8% 13% 20% 15% 2	2667
	11th to 20th	Night	47% 9% 14% 17% 11% 2	2730
	21st to end	Night	48% 10% 16% 19% 6% 1	2914
December	1st to 10th	Night	52% 13% 14% 18% 3	4720
	11th to 20th	Night	54% 8% 12% 14% 12%	5348
	21st to end	Night	59% 8% 9% 11% 11% 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	30% 30% 29% 8% 2	220
	11th to 20th	Day	38% 31% 21% 8% 1	220
	21st to end	Day	33% 30% 26% 8% 3	294
February	1st to 10th	Day	34% 32% 28% 5% 1	330
	11th to 20th	Day	37% 32% 23% 7% 1	484
	21st to end	Day	30% 31% 29% 9% 1	446
March	1st to 10th	Day	32% 27% 29% 10% 2	540
	11th to 20th	Day	36% 24% 27% 10% 2	540
	21st to end	Day	35% 29% 23% 8% 3 1	594
April	1st to 10th	Day	36% 26% 21% 11% 5% 1	540
	11th to 20th	Day	29% 31% 23% 11% 4 2	532
	21st to end	Day	32% 22% 29% 12% 4 2	540
May	1st to 10th	Day	33% 24% 26% 10% 5% 2	543
	11th to 20th	Day	21% 23% 32% 16% 7% 1	567
	21st to end	Day	22% 12% 34% 19% 10% 2	617
June	1st to 10th	Day	18% 19% 32% 23% 8% 1	570
	11th to 20th	Day	17% 19% 32% 21% 9% 2	570
	21st to end	Day	19% 18% 32% 21% 9% 1	570
July	1st to 10th	Day	11% 12% 36% 26% 12% 3	565
	11th to 20th	Day	10% 12% 36% 30% 11% 2	564
	21st to end	Day	13% 11% 39% 24% 10% 3	620
August	1st to 10th	Day	10% 9% 34% 27% 15% 5%	566
	11th to 20th	Day	13% 13% 28% 28% 11% 6%	559
	21st to end	Day	12% 16% 29% 25% 13% 5%	627
September	1st to 10th	Day	18% 12% 30% 21% 14% 5%	570
	11th to 20th	Day	13% 12% 27% 22% 18% 9%	542
	21st to end	Day	19% 12% 29% 22% 12% 5%	530
October	1st to 10th	Day	22% 16% 26% 18% 15% 4	530
	11th to 20th	Day	24% 22% 25% 22% 6% 1	530
	21st to end	Day	25% 23% 30% 17% 4	481
November	1st to 10th	Day	26% 24% 24% 16% 9%	425
	11th to 20th	Day	30% 25% 26% 15% 4 1	430
	21st to end	Day	34% 25% 28% 12% 1	241
December	1st to 10th	Day	31% 31% 25% 9% 4	220
	11th to 20th	Day	38% 26% 26% 8% 1	220
	21st to end	Day	31% 29% 29% 9% 3	242
January	1st to 10th	Night	24% 30% 32% 12% 2	840
	11th to 20th	Night	28% 24% 31% 15% 2	840
	21st to end	Night	21% 30% 32% 15% 2	924
February	1st to 10th	Night	25% 25% 36% 12% 1	840
	11th to 20th	Night	26% 30% 31% 12% 1	823
	21st to end	Night	28% 26% 29% 14% 2	692
March	1st to 10th	Night	28% 27% 30% 13% 2	840
	11th to 20th	Night	26% 28% 27% 16% 2	840
	21st to end	Night	28% 24% 29% 14% 4	924
April	1st to 10th	Night	26% 26% 29% 16% 3	840
	11th to 20th	Night	25% 24% 30% 16% 5%	831
	21st to end	Night	26% 21% 28% 19% 6%	777
May	1st to 10th	Night	29% 18% 29% 18% 5% 1	570
	11th to 20th	Night	21% 17% 29% 27% 4 1	414
	21st to end	Night	16% 11% 31% 30% 10% 1	455
June	1st to 10th	Night	16% 11% 30% 31% 11% 1	420
	11th to 20th	Night	17% 14% 26% 32% 11%	420
	21st to end	Night	12% 8% 34% 35% 11% 1	420
July	1st to 10th	Night	11% 6% 27% 41% 14% 1	410
	11th to 20th	Night	12% 9% 25% 37% 16% 1	408
	21st to end	Night	12% 6% 25% 43% 13% 1	460
August	1st to 10th	Night	9% 6% 29% 35% 20% 1	465
	11th to 20th	Night	13% 8% 24% 36% 17% 3	790
	21st to end	Night	14% 9% 25% 32% 19% 1	924
September	1st to 10th	Night	15% 10% 27% 30% 16% 1	840
	11th to 20th	Night	15% 10% 23% 27% 20% 5%	813
	21st to end	Night	17% 11% 25% 31% 13% 2	800
October	1st to 10th	Night	22% 15% 23% 24% 15% 2	800
	11th to 20th	Night	22% 18% 28% 25% 7%	800
	21st to end	Night	25% 20% 29% 22% 4	880
November	1st to 10th	Night	22% 21% 26% 24% 6%	820
	11th to 20th	Night	25% 25% 26% 17% 6% 1	840
	21st to end	Night	25% 25% 26% 19% 4 1	840
December	1st to 10th	Night	23% 30% 29% 14% 4	840
	11th to 20th	Night	28% 24% 30% 16% 3	840
	21st to end	Night	29% 29% 27% 13% 2	924

Page generated at 2026-09-11 04:11:54. Version ac07

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