Computer Deconstruction Laboratory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Computer Deconstruction Laboratory clear sky chart from 2005-11-06 to 2025-09-19.

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% 7% 2 6% 4 4 4 4 5% 7% 40%	126267
	Night	16% 8% 2 6% 4 4 4 3 5% 6% 43%	122792
Transparency	Day	41% 6% 12% 22% 16% 3	129794
	Night	41% 5% 11% 23% 17% 4	127339
Seeing	Day	20% 11% 30% 24% 11% 4	16195
	Night	19% 9% 31% 29% 11% 1	25321

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	13% 7% 2 5% 3 3 3 3 4 5% 52%	3597
	11th to 20th	Day	14% 8% 3 6% 4 3 4 3 4 5% 46%	3570
	21st to end	Day	18% 12% 3 7% 3 3 3 2 3 5% 40%	4373
February	1st to 10th	Day	15% 9% 3 6% 4 4 4 3 4 6% 41%	1830
	11th to 20th	Day	16% 9% 3 6% 4 3 4 3 5% 6% 41%	1948
	21st to end	Day	17% 9% 3 7% 4 3 5% 3 4 5% 40%	1650
March	1st to 10th	Day	22% 6% 2 4 3 3 3 3 4 5% 45%	4476
	11th to 20th	Day	12% 9% 2 6% 4 4 4 4 5% 7% 43%	4785
	21st to end	Day	17% 7% 2 6% 4 4 4 3 4 6% 43%	5922
April	1st to 10th	Day	18% 8% 3 5% 5% 4 4 3 5% 7% 39%	2400
	11th to 20th	Day	15% 6% 2 5% 5% 4 4 4 6% 7% 41%	2374
	21st to end	Day	17% 6% 2 5% 4 4 5% 4 6% 7% 41%	2405
May	1st to 10th	Day	25% 8% 3 6% 4 4 4 4 6% 8% 28%	5886
	11th to 20th	Day	26% 8% 2 5% 3 3 3 3 4 6% 35%	6198
	21st to end	Day	21% 6% 2 5% 4 4 5% 4 6% 8% 35%	6949
June	1st to 10th	Day	15% 6% 2 4 5% 5% 5% 5% 7% 9% 38%	2800
	11th to 20th	Day	15% 6% 2 6% 5% 4 5% 4 6% 9% 39%	2800
	21st to end	Day	11% 4 3 6% 5% 5% 6% 5% 7% 9% 40%	2800
July	1st to 10th	Day	10% 5% 2 7% 5% 5% 5% 5% 7% 11% 37%	6360
	11th to 20th	Day	12% 7% 3 8% 5% 6% 7% 5% 7% 10% 30%	6360
	21st to end	Day	12% 7% 2 6% 4 5% 5% 4 6% 9% 39%	6926
August	1st to 10th	Day	18% 6% 2 7% 5% 5% 5% 4 7% 8% 32%	4215
	11th to 20th	Day	13% 6% 3 6% 4 5% 5% 4 6% 9% 40%	3850
	21st to end	Day	10% 4 1 5% 3 4 5% 4 7% 9% 47%	4312
September	1st to 10th	Day	16% 4 2 5% 3 4 4 4 5% 7% 45%	2400
	11th to 20th	Day	13% 6% 2 5% 3 4 4 4 5% 7% 48%	2288
	21st to end	Day	20% 7% 2 6% 4 3 4 3 5% 6% 39%	2090
October	1st to 10th	Day	21% 5% 1 5% 2 3 4 3 5% 6% 43%	1990
	11th to 20th	Day	15% 5% 2 5% 4 3 3 4 4 5% 51%	1900
	21st to end	Day	18% 7% 2 6% 2 3 3 3 5% 6% 46%	1952
November	1st to 10th	Day	13% 5% 2 6% 4 3 3 3 5% 6% 51%	1761
	11th to 20th	Day	16% 6% 2 5% 4 3 3 4 5% 5% 47%	1800
	21st to end	Day	16% 6% 1 4 4 4 4 3 5% 7% 46%	1754
December	1st to 10th	Day	13% 7% 2 6% 4 3 3 3 5% 7% 47%	2600
	11th to 20th	Day	23% 8% 2 4 3 2 2 2 3 4 46%	2986
	21st to end	Day	16% 6% 3 5% 4 4 5% 4 5% 7% 41%	3960
January	1st to 10th	Night	13% 6% 2 4 3 3 3 3 4 5% 53%	6349
	11th to 20th	Night	15% 8% 2 5% 4 3 3 3 4 5% 49%	6315
	21st to end	Night	18% 9% 2 6% 3 3 4 3 5% 6% 41%	6582
February	1st to 10th	Night	17% 9% 3 6% 4 3 4 3 5% 6% 41%	2569
	11th to 20th	Night	19% 8% 3 5% 4 3 4 4 4 6% 42%	2338
	21st to end	Night	18% 7% 2 6% 4 3 3 3 4 6% 43%	1980
March	1st to 10th	Night	19% 9% 2 5% 3 2 2 2 3 6% 46%	5364
	11th to 20th	Night	13% 12% 3 5% 4 3 3 3 4 5% 45%	5172
	21st to end	Night	18% 8% 3 7% 4 4 4 3 5% 5% 41%	5008
April	1st to 10th	Night	17% 9% 4 6% 4 4 4 3 4 5% 40%	2000
	11th to 20th	Night	17% 7% 3 7% 4 4 3 3 4 6% 42%	1975
	21st to end	Night	17% 7% 2 4 3 4 3 4 5% 7% 43%	1995
May	1st to 10th	Night	23% 10% 3 7% 5% 4 4 4 5% 9% 27%	4054
	11th to 20th	Night	24% 9% 3 6% 3 4 3 3 4 7% 34%	3723
	21st to end	Night	21% 8% 3 6% 4 4 4 3 5% 7% 36%	3926
June	1st to 10th	Night	16% 6% 2 7% 5% 4 4 3 5% 9% 40%	1600
	11th to 20th	Night	15% 7% 2 5% 5% 4 5% 3 6% 8% 41%	1600
	21st to end	Night	9% 5% 2 7% 5% 6% 6% 5% 8% 10% 39%	1600
July	1st to 10th	Night	9% 7% 3 7% 6% 6% 6% 4 6% 10% 36%	3600
	11th to 20th	Night	13% 9% 3 7% 4 5% 4 5% 8% 9% 33%	3600
	21st to end	Night	12% 7% 2 6% 4 4 4 4 6% 7% 42%	3926
August	1st to 10th	Night	22% 7% 2 6% 4 4 5% 3 6% 8% 32%	2898
	11th to 20th	Night	12% 6% 3 7% 4 4 5% 3 6% 7% 42%	3197
	21st to end	Night	8% 5% 2 5% 4 3 4 3 5% 8% 53%	3586
September	1st to 10th	Night	14% 6% 2 6% 4 4 5% 3 4 6% 46%	2000
	11th to 20th	Night	13% 5% 2 5% 3 4 4 3 5% 6% 50%	2068
	21st to end	Night	20% 8% 2 6% 4 3 3 3 5% 6% 41%	2090
October	1st to 10th	Night	19% 6% 2 5% 4 4 4 3 4 6% 43%	2185
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 5% 6% 51%	2280
	21st to end	Night	18% 8% 2 5% 3 3 3 3 4 5% 46%	2646
November	1st to 10th	Night	12% 5% 2 5% 4 4 3 2 4 6% 53%	2537
	11th to 20th	Night	17% 7% 2 5% 4 3 3 3 3 5% 48%	2600
	21st to end	Night	14% 5% 2 5% 4 4 4 4 5% 6% 47%	2645
December	1st to 10th	Night	14% 8% 2 5% 3 3 3 3 4 6% 49%	4580
	11th to 20th	Night	24% 7% 2 4 3 3 3 2 4 6% 43%	5208
	21st to end	Night	18% 9% 3 6% 3 3 3 3 5% 5% 42%	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	34% 4 8% 29% 24% 3	3598
	11th to 20th	Day	41% 4 8% 24% 17% 6%	3600
	21st to end	Day	48% 4 9% 23% 14% 2	4373
February	1st to 10th	Day	45% 3 13% 24% 14% 2	1830
	11th to 20th	Day	44% 4 9% 24% 16% 3	1957
	21st to end	Day	46% 4 10% 22% 14% 5%	1650
March	1st to 10th	Day	41% 4 10% 22% 16% 8%	4479
	11th to 20th	Day	39% 6% 9% 27% 14% 5%	4785
	21st to end	Day	42% 5% 9% 15% 23% 6%	5922
April	1st to 10th	Day	45% 5% 11% 18% 16% 5%	2400
	11th to 20th	Day	40% 6% 13% 21% 16% 3	2374
	21st to end	Day	41% 6% 11% 20% 19% 4	2405
May	1st to 10th	Day	53% 7% 14% 15% 9% 1	5897
	11th to 20th	Day	51% 4 10% 22% 12% 1	6202
	21st to end	Day	45% 6% 11% 22% 14% 1	6949
June	1st to 10th	Day	40% 8% 14% 22% 15% 2	2800
	11th to 20th	Day	41% 8% 13% 24% 11% 2	2800
	21st to end	Day	37% 8% 15% 23% 16% 1	2800
July	1st to 10th	Day	40% 10% 14% 21% 13% 1	6290
	11th to 20th	Day	45% 10% 18% 17% 9% 1	6285
	21st to end	Day	39% 8% 11% 22% 20% 1	6728
August	1st to 10th	Day	46% 8% 15% 21% 9%	4201
	11th to 20th	Day	39% 7% 14% 27% 11% 1	3886
	21st to end	Day	31% 7% 14% 28% 18% 2	4312
September	1st to 10th	Day	37% 5% 14% 22% 20% 2	2400
	11th to 20th	Day	35% 5% 10% 24% 23% 2	2288
	21st to end	Day	45% 5% 10% 18% 20% 3	2090
October	1st to 10th	Day	35% 5% 12% 20% 24% 5%	3302
	11th to 20th	Day	25% 3 9% 23% 34% 6%	3140
	21st to end	Day	27% 3 14% 25% 27% 4	3190
November	1st to 10th	Day	34% 4 12% 27% 21% 2	1761
	11th to 20th	Day	38% 4 9% 22% 24% 3	1800
	21st to end	Day	38% 4 12% 30% 13% 2	1754
December	1st to 10th	Day	38% 3 15% 26% 16% 2	2600
	11th to 20th	Day	43% 3 9% 30% 13% 3	2986
	21st to end	Day	40% 7% 10% 23% 18% 2	3960
January	1st to 10th	Night	34% 3 8% 25% 23% 7%	6349
	11th to 20th	Night	39% 3 10% 24% 17% 7%	6360
	21st to end	Night	44% 4 9% 22% 18% 3	6582
February	1st to 10th	Night	44% 4 15% 23% 13% 2	2569
	11th to 20th	Night	44% 3 9% 20% 19% 5%	2340
	21st to end	Night	42% 3 9% 22% 19% 5%	1980
March	1st to 10th	Night	42% 3 8% 23% 13% 10%	5374
	11th to 20th	Night	41% 5% 12% 23% 14% 5%	5173
	21st to end	Night	45% 5% 6% 15% 24% 5%	5008
April	1st to 10th	Night	46% 4 9% 19% 18% 5%	2000
	11th to 20th	Night	43% 4 10% 19% 21% 3	1975
	21st to end	Night	40% 5% 9% 20% 21% 5%	1995
May	1st to 10th	Night	54% 7% 12% 16% 9% 2	4060
	11th to 20th	Night	51% 5% 11% 17% 14% 2	3724
	21st to end	Night	47% 6% 11% 21% 13% 2	3927
June	1st to 10th	Night	43% 6% 10% 21% 20% 1	1600
	11th to 20th	Night	41% 7% 13% 25% 13% 1	1600
	21st to end	Night	36% 9% 15% 23% 15% 2	1600
July	1st to 10th	Night	43% 9% 13% 21% 13% 1	3560
	11th to 20th	Night	45% 9% 16% 22% 8% 1	3552
	21st to end	Night	38% 7% 15% 22% 17% 1	3809
August	1st to 10th	Night	49% 7% 18% 16% 9% 1	2888
	11th to 20th	Night	38% 6% 13% 31% 11% 1	3227
	21st to end	Night	29% 6% 11% 31% 21% 2	3586
September	1st to 10th	Night	40% 6% 11% 22% 19% 2	2000
	11th to 20th	Night	34% 4 11% 22% 24% 4	2068
	21st to end	Night	44% 5% 7% 22% 20% 2	2090
October	1st to 10th	Night	38% 5% 10% 23% 22% 2	3640
	11th to 20th	Night	24% 3 9% 27% 31% 5%	3779
	21st to end	Night	27% 4 10% 30% 25% 4	4358
November	1st to 10th	Night	34% 4 11% 25% 22% 4	2537
	11th to 20th	Night	40% 4 7% 23% 21% 5%	2600
	21st to end	Night	36% 4 13% 27% 18% 2	2645
December	1st to 10th	Night	38% 3 12% 30% 15% 3	4580
	11th to 20th	Night	44% 3 10% 24% 14% 4	5208
	21st to end	Night	44% 4 11% 20% 18% 4	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	24% 17% 41% 14% 4 1	200
	11th to 20th	Day	30% 27% 28% 13% 4	200
	21st to end	Day	24% 19% 37% 16% 3 1	310
February	1st to 10th	Day	24% 21% 37% 15% 2 1	330
	11th to 20th	Day	28% 19% 38% 14% 1	496
	21st to end	Day	27% 20% 33% 17% 3	414
March	1st to 10th	Day	25% 17% 35% 17% 5%	500
	11th to 20th	Day	19% 20% 39% 16% 5% 1	500
	21st to end	Day	26% 21% 32% 17% 4 1	550
April	1st to 10th	Day	25% 19% 34% 15% 6% 1	500
	11th to 20th	Day	22% 16% 34% 18% 8% 2	492
	21st to end	Day	24% 14% 36% 19% 6% 2	500
May	1st to 10th	Day	28% 12% 32% 18% 7% 2	503
	11th to 20th	Day	23% 10% 36% 22% 7% 2	527
	21st to end	Day	22% 7% 25% 26% 15% 5%	573
June	1st to 10th	Day	18% 7% 28% 28% 15% 4	530
	11th to 20th	Day	15% 7% 33% 29% 12% 3	530
	21st to end	Day	13% 5% 30% 32% 17% 3	530
July	1st to 10th	Day	12% 4 24% 33% 19% 8%	525
	11th to 20th	Day	9% 4 21% 36% 22% 7%	524
	21st to end	Day	12% 5% 27% 34% 17% 5%	576
August	1st to 10th	Day	12% 4 24% 35% 18% 6%	526
	11th to 20th	Day	13% 5% 19% 33% 22% 8%	519
	21st to end	Day	11% 4 26% 31% 18% 9%	583
September	1st to 10th	Day	17% 7% 28% 24% 16% 8%	530
	11th to 20th	Day	14% 4 25% 32% 15% 10%	522
	21st to end	Day	25% 7% 26% 25% 13% 4	490
October	1st to 10th	Day	24% 8% 22% 24% 15% 7%	490
	11th to 20th	Day	16% 12% 30% 26% 13% 3	490
	21st to end	Day	20% 10% 34% 22% 11% 3	466
November	1st to 10th	Day	14% 12% 34% 25% 13% 2	395
	11th to 20th	Day	19% 16% 34% 22% 10%	400
	21st to end	Day	21% 11% 35% 23% 8% 1	354
December	1st to 10th	Day	25% 21% 35% 12% 7% 1	200
	11th to 20th	Day	30% 20% 30% 14% 7%	200
	21st to end	Day	25% 15% 32% 23% 5% 1	220
January	1st to 10th	Night	21% 14% 41% 20% 4	800
	11th to 20th	Night	25% 13% 34% 24% 4	800
	21st to end	Night	19% 14% 42% 22% 3	880
February	1st to 10th	Night	22% 12% 39% 24% 3	800
	11th to 20th	Night	25% 16% 41% 17% 1	783
	21st to end	Night	23% 17% 36% 20% 3	660
March	1st to 10th	Night	21% 13% 39% 22% 4	800
	11th to 20th	Night	22% 12% 39% 22% 5% 1	800
	21st to end	Night	22% 10% 40% 23% 5%	880
April	1st to 10th	Night	23% 12% 37% 23% 5%	800
	11th to 20th	Night	21% 10% 34% 29% 6%	791
	21st to end	Night	20% 8% 34% 31% 6% 1	795
May	1st to 10th	Night	24% 9% 30% 28% 8% 1	600
	11th to 20th	Night	24% 7% 33% 30% 6%	464
	21st to end	Night	21% 3 19% 39% 17% 2	433
June	1st to 10th	Night	19% 4 23% 37% 16% 1	400
	11th to 20th	Night	16% 4 22% 40% 19%	400
	21st to end	Night	10% 2 21% 51% 16%	400
July	1st to 10th	Night	13% 5% 15% 46% 19% 3	390
	11th to 20th	Night	12% 4 14% 43% 26% 2	388
	21st to end	Night	11% 3 19% 45% 21%	438
August	1st to 10th	Night	13% 2 14% 48% 21% 2	580
	11th to 20th	Night	13% 3 15% 41% 25% 3	790
	21st to end	Night	13% 3 16% 40% 23% 4	880
September	1st to 10th	Night	16% 4 22% 36% 20% 3	800
	11th to 20th	Night	16% 3 18% 35% 23% 5%	793
	21st to end	Night	23% 4 21% 31% 18% 2	760
October	1st to 10th	Night	23% 5% 25% 26% 17% 4	760
	11th to 20th	Night	17% 8% 28% 34% 12% 1	760
	21st to end	Night	22% 6% 36% 25% 11% 1	836
November	1st to 10th	Night	15% 8% 35% 29% 12% 1	780
	11th to 20th	Night	20% 12% 33% 26% 9% 1	800
	21st to end	Night	16% 13% 37% 26% 8% 1	800
December	1st to 10th	Night	24% 13% 35% 20% 7% 1	800
	11th to 20th	Night	22% 10% 38% 26% 4	800
	21st to end	Night	21% 10% 38% 27% 3	880

Page generated at 2025-09-16 01:49:16. Version ac07

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