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 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	14% 10% 4 9% 6% 5% 5% 4 5% 7% 31%	113717
	Night	14% 10% 4 10% 6% 5% 5% 4 5% 6% 32%	115200
Transparency	Day	48% 9% 12% 19% 10% 2	117240
	Night	47% 10% 13% 18% 11% 1	119827
Seeing	Day	24% 21% 29% 17% 8% 2	15394
	Night	22% 20% 28% 21% 7% 1	24624

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% 11% 7% 22% 9% 5% 4 3 4 5% 21%	3597
	11th to 20th	Day	11% 16% 5% 14% 8% 5% 5% 4 5% 5% 22%	3570
	21st to end	Day	14% 13% 6% 11% 5% 5% 4 4 5% 6% 27%	4172
February	1st to 10th	Day	13% 13% 5% 12% 8% 5% 7% 4 4 6% 22%	1800
	11th to 20th	Day	9% 11% 5% 14% 8% 5% 6% 5% 5% 6% 27%	1898
	21st to end	Day	16% 14% 4 10% 6% 5% 5% 3 5% 6% 27%	1650
March	1st to 10th	Day	18% 14% 5% 11% 6% 4 4 3 4 5% 28%	4476
	11th to 20th	Day	12% 12% 5% 10% 5% 4 4 3 4 6% 35%	4791
	21st to end	Day	17% 10% 4 10% 6% 5% 5% 3 4 5% 33%	5914
April	1st to 10th	Day	15% 13% 4 9% 6% 5% 4 3 5% 5% 32%	2400
	11th to 20th	Day	16% 11% 3 8% 5% 5% 4 3 5% 6% 35%	2374
	21st to end	Day	18% 11% 3 8% 5% 5% 4 3 5% 6% 32%	2460
May	1st to 10th	Day	24% 10% 3 8% 5% 4 3 3 4 6% 28%	4362
	11th to 20th	Day	18% 9% 3 7% 5% 5% 5% 4 5% 8% 31%	4398
	21st to end	Day	11% 6% 2 7% 5% 6% 5% 5% 7% 8% 38%	4837
June	1st to 10th	Day	16% 8% 3 8% 5% 6% 6% 4 7% 8% 30%	2660
	11th to 20th	Day	16% 7% 2 7% 5% 6% 6% 4 6% 9% 31%	2660
	21st to end	Day	14% 8% 3 8% 6% 5% 6% 4 6% 7% 32%	2660
July	1st to 10th	Day	10% 7% 3 7% 5% 6% 6% 6% 7% 10% 33%	4440
	11th to 20th	Day	8% 5% 2 7% 6% 7% 7% 5% 8% 12% 32%	4440
	21st to end	Day	12% 8% 3 9% 6% 6% 6% 4 6% 7% 32%	4814
August	1st to 10th	Day	17% 7% 3 8% 6% 5% 5% 4 6% 8% 31%	4338
	11th to 20th	Day	11% 9% 3 7% 6% 6% 6% 5% 8% 9% 29%	3747
	21st to end	Day	13% 7% 3 7% 4 5% 6% 5% 7% 9% 34%	4180
September	1st to 10th	Day	13% 7% 2 7% 5% 4 4 4 5% 8% 40%	2280
	11th to 20th	Day	12% 7% 2 6% 3 3 5% 3 5% 7% 46%	2214
	21st to end	Day	14% 7% 3 5% 4 5% 5% 4 6% 8% 38%	2090
October	1st to 10th	Day	16% 10% 4 7% 5% 4 4 3 6% 6% 34%	1952
	11th to 20th	Day	14% 9% 3 8% 4 4 4 4 6% 7% 37%	1900
	21st to end	Day	19% 13% 3 8% 5% 4 4 3 6% 6% 30%	1916
November	1st to 10th	Day	13% 12% 3 5% 5% 4 4 3 5% 6% 41%	1761
	11th to 20th	Day	14% 10% 5% 8% 6% 5% 5% 3 5% 6% 33%	1800
	21st to end	Day	15% 12% 5% 9% 5% 5% 5% 4 5% 6% 30%	1620
December	1st to 10th	Day	11% 15% 9% 15% 7% 6% 5% 4 5% 5% 19%	2600
	11th to 20th	Day	16% 13% 5% 11% 5% 5% 5% 4 5% 6% 27%	2986
	21st to end	Day	14% 14% 8% 12% 6% 4 5% 4 5% 6% 23%	3960
January	1st to 10th	Night	12% 11% 5% 21% 7% 5% 4 3 5% 5% 21%	6349
	11th to 20th	Night	9% 14% 5% 19% 8% 6% 6% 4 5% 6% 19%	6315
	21st to end	Night	13% 11% 5% 12% 7% 6% 5% 3 5% 6% 26%	6766
February	1st to 10th	Night	13% 14% 4 10% 8% 5% 5% 4 5% 7% 25%	2600
	11th to 20th	Night	10% 11% 5% 12% 9% 6% 5% 6% 6% 6% 24%	2388
	21st to end	Night	14% 12% 5% 10% 6% 6% 5% 3 6% 7% 25%	1980
March	1st to 10th	Night	16% 10% 3 11% 8% 5% 4 4 5% 6% 27%	5364
	11th to 20th	Night	13% 12% 5% 8% 5% 4 4 4 5% 7% 34%	5165
	21st to end	Night	17% 12% 5% 11% 5% 4 4 3 4 5% 29%	5017
April	1st to 10th	Night	15% 12% 4 10% 5% 5% 5% 4 6% 6% 27%	2000
	11th to 20th	Night	19% 11% 3 6% 5% 3 4 3 4 7% 35%	1975
	21st to end	Night	18% 10% 4 6% 4 4 4 3 5% 6% 36%	1940
May	1st to 10th	Night	24% 12% 3 7% 4 3 4 3 4 7% 29%	2828
	11th to 20th	Night	16% 11% 4 8% 5% 5% 5% 4 5% 7% 32%	2501
	21st to end	Night	11% 7% 3 9% 5% 5% 5% 4 6% 9% 37%	2738
June	1st to 10th	Night	15% 6% 3 6% 5% 5% 3 5% 5% 7% 40%	1520
	11th to 20th	Night	14% 7% 2 6% 3 4 4 4 6% 9% 40%	1520
	21st to end	Night	10% 4 1 7% 5% 5% 5% 4 6% 10% 42%	1520
July	1st to 10th	Night	8% 7% 2 7% 6% 6% 5% 5% 6% 8% 39%	2520
	11th to 20th	Night	11% 6% 3 7% 4 4 4 4 7% 9% 41%	2520
	21st to end	Night	12% 8% 2 6% 5% 5% 5% 4 5% 6% 41%	2739
August	1st to 10th	Night	19% 7% 3 7% 5% 4 5% 4 5% 7% 36%	2587
	11th to 20th	Night	14% 7% 3 8% 4 4 4 4 5% 7% 41%	3037
	21st to end	Night	11% 7% 2 6% 4 4 4 3 5% 8% 45%	3476
September	1st to 10th	Night	13% 7% 2 5% 5% 4 5% 4 6% 7% 44%	1900
	11th to 20th	Night	13% 6% 2 4 3 3 4 3 4 6% 53%	1966
	21st to end	Night	13% 6% 3 7% 5% 4 5% 3 5% 6% 43%	2090
October	1st to 10th	Night	18% 9% 3 7% 4 4 4 4 5% 6% 37%	2223
	11th to 20th	Night	15% 8% 3 6% 5% 4 4 4 5% 6% 40%	2280
	21st to end	Night	20% 13% 3 6% 5% 3 4 3 6% 6% 32%	2680
November	1st to 10th	Night	13% 10% 3 7% 4 3 4 4 5% 6% 39%	2537
	11th to 20th	Night	15% 11% 4 8% 5% 5% 5% 4 5% 6% 32%	2600
	21st to end	Night	13% 12% 5% 8% 6% 5% 4 4 5% 6% 32%	2775
December	1st to 10th	Night	13% 14% 6% 13% 6% 5% 5% 4 6% 7% 22%	4580
	11th to 20th	Night	19% 12% 5% 10% 5% 4 4 3 4 4 28%	5208
	21st to end	Night	18% 13% 6% 12% 5% 4 4 3 4 5% 24%	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	52% 16% 11% 15% 5% 1	3598
	11th to 20th	Day	57% 11% 9% 14% 8% 2	3600
	21st to end	Day	51% 8% 10% 19% 9% 2	4172
February	1st to 10th	Day	58% 9% 12% 13% 7% 1	1800
	11th to 20th	Day	53% 7% 12% 17% 9% 1	1907
	21st to end	Day	53% 8% 12% 16% 10%	1650
March	1st to 10th	Day	55% 7% 11% 13% 10% 4	4479
	11th to 20th	Day	46% 9% 15% 20% 6% 4	4791
	21st to end	Day	50% 8% 8% 13% 17% 4	5914
April	1st to 10th	Day	53% 7% 8% 20% 10% 2	2400
	11th to 20th	Day	49% 7% 11% 18% 13% 1	2374
	21st to end	Day	51% 7% 11% 16% 12% 3	2460
May	1st to 10th	Day	55% 7% 10% 16% 10% 1	4376
	11th to 20th	Day	49% 9% 14% 20% 8% 1	4402
	21st to end	Day	37% 12% 15% 24% 12% 1	4837
June	1st to 10th	Day	48% 10% 14% 18% 10% 1	2660
	11th to 20th	Day	46% 10% 15% 19% 10% 1	2660
	21st to end	Day	48% 9% 15% 19% 9% 1	2660
July	1st to 10th	Day	42% 10% 14% 22% 11% 1	4370
	11th to 20th	Day	39% 11% 18% 22% 10% 1	4365
	21st to end	Day	48% 8% 13% 19% 11%	4661
August	1st to 10th	Day	49% 10% 14% 18% 10%	4324
	11th to 20th	Day	45% 10% 17% 25% 4	3783
	21st to end	Day	38% 10% 14% 28% 8% 1	4180
September	1st to 10th	Day	42% 8% 12% 24% 14% 1	2280
	11th to 20th	Day	36% 7% 12% 24% 19% 1	2214
	21st to end	Day	41% 8% 13% 23% 13% 2	2090
October	1st to 10th	Day	44% 8% 9% 21% 17% 1	3238
	11th to 20th	Day	38% 7% 11% 23% 17% 5%	3140
	21st to end	Day	37% 8% 15% 26% 11% 3	3128
November	1st to 10th	Day	42% 7% 13% 22% 13% 2	1761
	11th to 20th	Day	49% 9% 11% 17% 13% 2	1800
	21st to end	Day	52% 8% 15% 17% 7% 1	1620
December	1st to 10th	Day	60% 10% 11% 15% 4	2600
	11th to 20th	Day	53% 9% 13% 16% 8% 1	2986
	21st to end	Day	56% 10% 9% 13% 11% 2	3960
January	1st to 10th	Night	52% 14% 13% 13% 7% 1	6349
	11th to 20th	Night	55% 14% 13% 11% 6% 1	6360
	21st to end	Night	53% 8% 12% 12% 13% 3	6766
February	1st to 10th	Night	53% 9% 14% 15% 7% 1	2600
	11th to 20th	Night	53% 10% 14% 14% 9% 1	2390
	21st to end	Night	52% 10% 13% 16% 8% 2	1980
March	1st to 10th	Night	50% 10% 13% 16% 10% 1	5374
	11th to 20th	Night	46% 9% 19% 17% 7% 2	5166
	21st to end	Night	50% 9% 8% 15% 16% 1	5017
April	1st to 10th	Night	54% 9% 12% 16% 8% 1	2000
	11th to 20th	Night	48% 8% 13% 18% 14% 1	1975
	21st to end	Night	48% 7% 12% 17% 13% 3	1940
May	1st to 10th	Night	55% 8% 10% 15% 10% 2	2834
	11th to 20th	Night	49% 9% 13% 20% 8% 1	2502
	21st to end	Night	39% 13% 15% 19% 13% 1	2739
June	1st to 10th	Night	42% 8% 12% 24% 12% 2	1520
	11th to 20th	Night	38% 10% 16% 25% 10%	1520
	21st to end	Night	35% 11% 17% 24% 12%	1520
July	1st to 10th	Night	37% 11% 17% 20% 14% 1	2480
	11th to 20th	Night	36% 9% 18% 25% 12%	2472
	21st to end	Night	42% 9% 13% 24% 12% 1	2651
August	1st to 10th	Night	45% 11% 14% 22% 8%	2577
	11th to 20th	Night	41% 9% 17% 25% 8%	3066
	21st to end	Night	32% 10% 13% 33% 10% 1	3476
September	1st to 10th	Night	37% 9% 13% 27% 13% 1	1900
	11th to 20th	Night	32% 8% 12% 25% 22% 1	1966
	21st to end	Night	40% 9% 11% 23% 15% 1	2090
October	1st to 10th	Night	45% 9% 12% 22% 11% 1	3704
	11th to 20th	Night	34% 9% 14% 21% 18% 4	3779
	21st to end	Night	39% 8% 16% 26% 10% 1	4418
November	1st to 10th	Night	42% 8% 13% 19% 15% 2	2537
	11th to 20th	Night	47% 9% 14% 17% 11% 3	2600
	21st to end	Night	48% 9% 17% 19% 6% 1	2775
December	1st to 10th	Night	52% 13% 14% 18% 3	4580
	11th to 20th	Night	54% 8% 12% 14% 12%	5208
	21st to end	Night	59% 8% 9% 11% 11% 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% 29% 29% 9% 3 1	200
	11th to 20th	Day	36% 31% 23% 9% 2 1	200
	21st to end	Day	29% 31% 27% 9% 3	267
February	1st to 10th	Day	36% 34% 25% 4 1	300
	11th to 20th	Day	36% 34% 22% 8% 1	446
	21st to end	Day	31% 29% 30% 9% 1	414
March	1st to 10th	Day	32% 27% 29% 11% 2	500
	11th to 20th	Day	34% 25% 27% 11% 2	500
	21st to end	Day	35% 28% 23% 9% 4 1	550
April	1st to 10th	Day	35% 26% 21% 11% 5% 1	500
	11th to 20th	Day	30% 31% 22% 11% 4 2	492
	21st to end	Day	32% 22% 28% 12% 4 2	500
May	1st to 10th	Day	33% 22% 27% 11% 5% 2	503
	11th to 20th	Day	19% 21% 33% 18% 8% 1	487
	21st to end	Day	19% 12% 34% 22% 11% 3	529
June	1st to 10th	Day	18% 19% 33% 22% 8% 1	490
	11th to 20th	Day	16% 17% 32% 24% 10% 2	490
	21st to end	Day	18% 18% 32% 22% 9% 1	490
July	1st to 10th	Day	11% 12% 37% 25% 13% 3	485
	11th to 20th	Day	9% 10% 36% 31% 11% 2	484
	21st to end	Day	13% 11% 40% 24% 10% 3	532
August	1st to 10th	Day	11% 10% 34% 26% 15% 4	486
	11th to 20th	Day	13% 12% 29% 29% 11% 6%	479
	21st to end	Day	13% 15% 28% 25% 14% 5%	539
September	1st to 10th	Day	17% 11% 29% 22% 15% 6%	490
	11th to 20th	Day	14% 13% 26% 21% 18% 9%	490
	21st to end	Day	19% 12% 30% 22% 12% 5%	490
October	1st to 10th	Day	22% 17% 26% 17% 14% 3	490
	11th to 20th	Day	23% 21% 26% 23% 6% 1	490
	21st to end	Day	25% 23% 29% 18% 4	446
November	1st to 10th	Day	25% 23% 25% 17% 10% 1	395
	11th to 20th	Day	30% 25% 26% 15% 4 1	400
	21st to end	Day	31% 25% 29% 14% 1	220
December	1st to 10th	Day	29% 34% 24% 9% 5%	200
	11th to 20th	Day	37% 26% 27% 9% 2	200
	21st to end	Day	28% 31% 29% 9% 3	220
January	1st to 10th	Night	24% 31% 32% 12% 2	800
	11th to 20th	Night	26% 24% 32% 15% 2 1	800
	21st to end	Night	21% 30% 33% 16% 1	880
February	1st to 10th	Night	25% 26% 36% 12% 1	800
	11th to 20th	Night	26% 30% 30% 12% 1	783
	21st to end	Night	29% 26% 29% 14% 2	660
March	1st to 10th	Night	28% 27% 30% 13% 2	800
	11th to 20th	Night	24% 29% 28% 16% 3	800
	21st to end	Night	28% 23% 30% 15% 4	880
April	1st to 10th	Night	25% 27% 29% 16% 3	800
	11th to 20th	Night	25% 23% 29% 16% 6%	791
	21st to end	Night	26% 21% 27% 19% 6%	740
May	1st to 10th	Night	29% 17% 28% 19% 5% 1	543
	11th to 20th	Night	21% 16% 30% 28% 5% 1	376
	21st to end	Night	15% 9% 31% 32% 11% 1	411
June	1st to 10th	Night	16% 12% 31% 28% 12% 1	380
	11th to 20th	Night	15% 14% 26% 33% 13%	380
	21st to end	Night	11% 8% 34% 34% 12% 1	380
July	1st to 10th	Night	11% 7% 26% 40% 15% 1	370
	11th to 20th	Night	13% 8% 25% 37% 17% 1	368
	21st to end	Night	12% 5% 26% 43% 13% 1	416
August	1st to 10th	Night	9% 6% 31% 34% 19% 1	420
	11th to 20th	Night	14% 7% 23% 36% 17% 3	714
	21st to end	Night	14% 8% 24% 32% 20% 1	836
September	1st to 10th	Night	14% 10% 28% 31% 16% 1	760
	11th to 20th	Night	16% 10% 24% 26% 19% 5%	760
	21st to end	Night	17% 11% 26% 31% 13% 3	760
October	1st to 10th	Night	22% 15% 24% 23% 14% 1	760
	11th to 20th	Night	20% 18% 29% 25% 8%	760
	21st to end	Night	25% 21% 29% 21% 4	836
November	1st to 10th	Night	21% 22% 26% 25% 6%	780
	11th to 20th	Night	24% 25% 26% 18% 6% 1	800
	21st to end	Night	24% 26% 26% 19% 4 1	800
December	1st to 10th	Night	23% 30% 29% 14% 4	800
	11th to 20th	Night	27% 25% 30% 16% 3	800
	21st to end	Night	27% 29% 27% 13% 2	880

Page generated at 2025-05-08 09:33:55. Version ac07

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