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 2024-05-13.

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% 41%	83482
	Night	17% 7% 2 6% 4 4 4 3 5% 6% 42%	86020
Transparency	Day	42% 6% 12% 23% 16% 2	83187
	Night	42% 5% 11% 23% 17% 3	85849
Seeing	Day	20% 11% 30% 24% 12% 4	14384
	Night	19% 8% 31% 29% 11% 1	23697

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	15% 8% 3 6% 3 4 4 3 4 4 46%	2517
	11th to 20th	Day	16% 10% 3 6% 4 4 4 3 5% 5% 40%	2520
	21st to end	Day	22% 13% 3 8% 3 3 3 2 4 5% 32%	3059
February	1st to 10th	Day	16% 9% 3 6% 4 4 4 2 4 7% 41%	1738
	11th to 20th	Day	16% 8% 3 6% 4 3 4 3 5% 6% 42%	1848
	21st to end	Day	17% 10% 3 7% 4 4 5% 3 4 5% 39%	1570
March	1st to 10th	Day	28% 8% 2 5% 4 3 4 3 3 5% 37%	3116
	11th to 20th	Day	9% 4 2 6% 5% 4 5% 4 5% 7% 48%	3341
	21st to end	Day	20% 8% 2 5% 4 3 3 3 4 5% 43%	4132
April	1st to 10th	Day	17% 8% 3 5% 5% 4 4 3 5% 7% 39%	2280
	11th to 20th	Day	15% 6% 2 5% 5% 4 4 4 6% 7% 41%	2254
	21st to end	Day	18% 6% 2 5% 4 4 5% 4 6% 7% 40%	2285
May	1st to 10th	Day	20% 8% 3 6% 4 4 4 4 5% 8% 33%	2470
	11th to 20th	Day	21% 6% 2 5% 4 4 5% 4 5% 7% 37%	2457
	21st to end	Day	16% 6% 2 5% 4 3 4 3 6% 8% 41%	2725
June	1st to 10th	Day	16% 5% 2 4 5% 5% 5% 5% 7% 10% 37%	2520
	11th to 20th	Day	15% 6% 3 6% 5% 4 5% 4 6% 9% 37%	2520
	21st to end	Day	10% 4 3 6% 5% 5% 6% 5% 7% 9% 40%	2520
July	1st to 10th	Day	11% 5% 2 6% 5% 6% 6% 5% 7% 10% 38%	2520
	11th to 20th	Day	8% 6% 2 5% 5% 5% 6% 6% 6% 10% 41%	2520
	21st to end	Day	11% 6% 3 7% 5% 5% 6% 4 7% 10% 35%	2744
August	1st to 10th	Day	9% 6% 2 6% 5% 5% 6% 5% 8% 9% 39%	2322
	11th to 20th	Day	12% 5% 3 6% 4 5% 5% 4 6% 10% 40%	2126
	21st to end	Day	14% 5% 2 6% 4 4 5% 4 7% 8% 41%	2376
September	1st to 10th	Day	16% 5% 2 5% 3 4 4 4 5% 8% 44%	2160
	11th to 20th	Day	13% 5% 2 5% 4 3 4 4 4 7% 48%	2080
	21st to end	Day	18% 7% 2 6% 4 3 5% 3 6% 6% 40%	1980
October	1st to 10th	Day	22% 5% 1 5% 2 3 5% 3 5% 6% 42%	1886
	11th to 20th	Day	16% 6% 2 5% 4 3 3 4 4 6% 49%	1800
	21st to end	Day	19% 7% 2 6% 2 3 3 3 5% 6% 44%	1850
November	1st to 10th	Day	14% 5% 2 6% 4 3 3 3 5% 6% 50%	1671
	11th to 20th	Day	16% 5% 2 5% 4 3 3 3 5% 5% 47%	1710
	21st to end	Day	16% 6% 1 4 5% 4 4 3 5% 7% 46%	1667
December	1st to 10th	Day	17% 7% 3 7% 4 3 3 3 6% 7% 40%	1520
	11th to 20th	Day	19% 7% 2 4 3 3 3 3 5% 5% 45%	1906
	21st to end	Day	17% 7% 3 6% 5% 5% 5% 4 5% 7% 37%	2772
January	1st to 10th	Night	17% 6% 2 4 3 3 3 3 4 5% 47%	4429
	11th to 20th	Night	18% 10% 2 6% 4 3 3 2 4 4 45%	4440
	21st to end	Night	23% 10% 3 6% 4 3 3 3 5% 5% 35%	4596
February	1st to 10th	Night	17% 9% 3 6% 4 3 4 3 5% 6% 42%	2441
	11th to 20th	Night	18% 8% 3 5% 4 3 4 4 4 6% 42%	2218
	21st to end	Night	19% 7% 2 5% 4 3 3 3 4 6% 43%	1884
March	1st to 10th	Night	23% 12% 3 6% 4 3 2 2 3 5% 38%	3724
	11th to 20th	Night	12% 6% 2 6% 5% 4 3 3 5% 6% 47%	3616
	21st to end	Night	20% 9% 3 6% 3 3 3 2 4 4 42%	3498
April	1st to 10th	Night	17% 9% 4 5% 4 4 4 4 4 5% 41%	1900
	11th to 20th	Night	17% 7% 3 7% 4 4 3 3 4 6% 42%	1875
	21st to end	Night	18% 7% 2 4 3 4 4 4 6% 7% 42%	1895
May	1st to 10th	Night	20% 9% 3 7% 5% 4 4 4 4 7% 33%	1710
	11th to 20th	Night	18% 9% 3 6% 4 4 4 3 5% 8% 36%	1506
	21st to end	Night	17% 8% 2 5% 4 4 5% 3 6% 7% 38%	1550
June	1st to 10th	Night	16% 6% 2 7% 5% 5% 3 3 5% 9% 39%	1440
	11th to 20th	Night	14% 7% 3 6% 5% 4 5% 3 6% 8% 40%	1440
	21st to end	Night	9% 5% 1 6% 5% 5% 5% 5% 8% 10% 41%	1440
July	1st to 10th	Night	10% 6% 3 6% 6% 5% 5% 3 6% 10% 40%	1440
	11th to 20th	Night	8% 7% 3 5% 5% 6% 4 5% 7% 9% 42%	1440
	21st to end	Night	9% 5% 2 6% 5% 4 5% 6% 8% 8% 42%	1574
August	1st to 10th	Night	8% 5% 3 7% 4 4 6% 4 6% 10% 42%	1594
	11th to 20th	Night	11% 6% 3 7% 4 4 5% 4 7% 8% 41%	1774
	21st to end	Night	12% 7% 2 6% 4 4 5% 3 6% 9% 42%	1980
September	1st to 10th	Night	14% 6% 2 6% 4 5% 5% 3 4 6% 44%	1800
	11th to 20th	Night	13% 5% 2 5% 3 4 4 3 5% 6% 51%	1879
	21st to end	Night	18% 8% 2 6% 4 3 3 3 5% 7% 42%	1980
October	1st to 10th	Night	20% 6% 2 5% 4 4 4 3 4 6% 43%	2070
	11th to 20th	Night	15% 5% 2 5% 4 3 4 3 5% 6% 49%	2160
	21st to end	Night	19% 8% 2 5% 3 3 3 3 4 5% 45%	2506
November	1st to 10th	Night	13% 5% 2 5% 4 3 3 2 4 6% 52%	2407
	11th to 20th	Night	17% 7% 2 5% 4 3 3 3 3 5% 47%	2470
	21st to end	Night	13% 5% 2 5% 4 4 4 4 5% 6% 48%	2512
December	1st to 10th	Night	19% 7% 2 5% 4 3 4 4 4 6% 40%	2660
	11th to 20th	Night	19% 5% 2 5% 4 4 3 3 5% 7% 43%	3288
	21st to end	Night	18% 9% 3 6% 4 4 4 4 5% 6% 39%	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	40% 4 10% 27% 16% 3	2518
	11th to 20th	Day	46% 4 10% 23% 14% 3	2520
	21st to end	Day	55% 4 8% 21% 10% 2	3059
February	1st to 10th	Day	45% 3 13% 24% 13% 2	1738
	11th to 20th	Day	43% 5% 9% 25% 16% 3	1857
	21st to end	Day	47% 4 10% 21% 13% 5%	1570
March	1st to 10th	Day	49% 4 9% 26% 10% 2	3119
	11th to 20th	Day	33% 6% 11% 32% 15% 3	3341
	21st to end	Day	45% 4 8% 17% 22% 5%	4132
April	1st to 10th	Day	44% 5% 11% 19% 16% 5%	2280
	11th to 20th	Day	40% 6% 13% 22% 16% 3	2254
	21st to end	Day	42% 6% 11% 19% 18% 4	2285
May	1st to 10th	Day	48% 6% 11% 20% 14% 2	2470
	11th to 20th	Day	46% 6% 11% 21% 15% 1	2461
	21st to end	Day	41% 6% 17% 22% 13% 2	2725
June	1st to 10th	Day	41% 8% 14% 21% 13% 2	2520
	11th to 20th	Day	41% 9% 14% 22% 12% 2	2520
	21st to end	Day	37% 8% 15% 23% 16% 1	2520
July	1st to 10th	Day	40% 9% 15% 20% 15% 2	2450
	11th to 20th	Day	35% 7% 17% 25% 14% 2	2445
	21st to end	Day	40% 8% 11% 25% 14% 2	2591
August	1st to 10th	Day	36% 8% 17% 24% 14% 1	2308
	11th to 20th	Day	39% 8% 16% 22% 14% 2	2126
	21st to end	Day	38% 6% 14% 22% 17% 3	2376
September	1st to 10th	Day	38% 6% 15% 21% 19% 2	2160
	11th to 20th	Day	35% 5% 10% 23% 24% 3	2080
	21st to end	Day	43% 5% 10% 18% 21% 3	1980
October	1st to 10th	Day	43% 5% 11% 21% 18% 2	1886
	11th to 20th	Day	37% 4 10% 24% 24% 2	1800
	21st to end	Day	41% 4 10% 23% 20% 2	1850
November	1st to 10th	Day	35% 4 12% 27% 20% 2	1671
	11th to 20th	Day	39% 4 9% 22% 24% 3	1710
	21st to end	Day	39% 4 11% 30% 13% 3	1667
December	1st to 10th	Day	44% 4 15% 22% 15% 1	1520
	11th to 20th	Day	41% 4 12% 26% 15% 2	1906
	21st to end	Day	44% 8% 11% 24% 13% 1	2772
January	1st to 10th	Night	38% 3 10% 31% 14% 4	4429
	11th to 20th	Night	44% 3 11% 25% 14% 3	4440
	21st to end	Night	50% 4 7% 23% 13% 2	4596
February	1st to 10th	Night	43% 4 15% 22% 14% 2	2441
	11th to 20th	Night	44% 3 9% 19% 19% 5%	2220
	21st to end	Night	42% 3 9% 22% 19% 4	1884
March	1st to 10th	Night	50% 3 8% 23% 12% 3	3734
	11th to 20th	Night	37% 6% 12% 25% 17% 3	3617
	21st to end	Night	47% 3 6% 16% 23% 5%	3498
April	1st to 10th	Night	44% 5% 9% 20% 18% 5%	1900
	11th to 20th	Night	43% 4 10% 19% 21% 3	1875
	21st to end	Night	41% 5% 9% 20% 21% 4	1895
May	1st to 10th	Night	50% 7% 8% 18% 15% 2	1710
	11th to 20th	Night	46% 5% 11% 17% 18% 3	1507
	21st to end	Night	45% 6% 15% 19% 13% 2	1551
June	1st to 10th	Night	43% 7% 10% 21% 18% 1	1440
	11th to 20th	Night	42% 7% 13% 24% 13% 2	1440
	21st to end	Night	34% 8% 16% 24% 15% 2	1440
July	1st to 10th	Night	41% 8% 15% 22% 13% 2	1400
	11th to 20th	Night	35% 7% 14% 27% 14% 2	1392
	21st to end	Night	34% 8% 15% 26% 14% 3	1486
August	1st to 10th	Night	36% 8% 18% 21% 15% 1	1584
	11th to 20th	Night	38% 7% 16% 24% 14% 1	1774
	21st to end	Night	38% 7% 14% 20% 18% 3	1980
September	1st to 10th	Night	41% 6% 12% 22% 18% 2	1800
	11th to 20th	Night	33% 4 12% 22% 25% 4	1879
	21st to end	Night	43% 5% 6% 23% 21% 2	1980
October	1st to 10th	Night	43% 6% 10% 22% 17% 2	2070
	11th to 20th	Night	35% 5% 11% 28% 17% 3	2160
	21st to end	Night	42% 4 9% 23% 20% 2	2506
November	1st to 10th	Night	34% 4 11% 25% 21% 4	2407
	11th to 20th	Night	41% 4 8% 23% 20% 5%	2470
	21st to end	Night	36% 4 13% 27% 18% 2	2512
December	1st to 10th	Night	45% 4 12% 23% 15% 1	2660
	11th to 20th	Night	40% 4 13% 20% 18% 4	3288
	21st to end	Night	45% 5% 13% 24% 12% 1	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	22% 16% 42% 15% 4 1	180
	11th to 20th	Day	30% 27% 26% 14% 4	180
	21st to end	Day	24% 17% 37% 18% 3 1	279
February	1st to 10th	Day	24% 20% 38% 15% 2 1	298
	11th to 20th	Day	25% 18% 40% 15% 1	456
	21st to end	Day	28% 19% 32% 18% 3	382
March	1st to 10th	Day	24% 17% 36% 18% 5%	460
	11th to 20th	Day	19% 20% 39% 16% 5% 1	460
	21st to end	Day	27% 20% 31% 17% 4 1	506
April	1st to 10th	Day	23% 19% 34% 16% 6% 1	460
	11th to 20th	Day	21% 15% 35% 20% 8% 2	452
	21st to end	Day	25% 14% 35% 19% 5% 2	460
May	1st to 10th	Day	27% 12% 32% 19% 8% 2	463
	11th to 20th	Day	22% 9% 35% 23% 8% 2	456
	21st to end	Day	22% 5% 23% 28% 17% 6%	485
June	1st to 10th	Day	17% 6% 27% 30% 15% 4	450
	11th to 20th	Day	15% 7% 34% 28% 13% 4	450
	21st to end	Day	13% 4 29% 34% 17% 4	450
July	1st to 10th	Day	13% 2 22% 33% 21% 8%	445
	11th to 20th	Day	9% 3 20% 36% 24% 8%	444
	21st to end	Day	13% 5% 26% 34% 18% 5%	488
August	1st to 10th	Day	11% 3 23% 37% 20% 7%	446
	11th to 20th	Day	13% 4 18% 34% 23% 8%	439
	21st to end	Day	12% 3 23% 32% 19% 10%	495
September	1st to 10th	Day	18% 6% 26% 24% 17% 10%	450
	11th to 20th	Day	13% 4 25% 30% 16% 11%	450
	21st to end	Day	23% 7% 26% 26% 14% 5%	450
October	1st to 10th	Day	26% 8% 20% 25% 16% 6%	450
	11th to 20th	Day	17% 11% 29% 27% 14% 3	450
	21st to end	Day	21% 11% 34% 21% 10% 3	430
November	1st to 10th	Day	15% 11% 32% 25% 13% 2	365
	11th to 20th	Day	19% 16% 33% 22% 10%	370
	21st to end	Day	19% 10% 36% 24% 8% 2	327
December	1st to 10th	Day	24% 19% 36% 11% 8% 1	180
	11th to 20th	Day	29% 19% 31% 15% 6%	180
	21st to end	Day	26% 13% 32% 23% 5% 1	198
January	1st to 10th	Night	20% 12% 42% 21% 4	760
	11th to 20th	Night	26% 12% 34% 24% 4	760
	21st to end	Night	19% 14% 42% 22% 3	836
February	1st to 10th	Night	21% 11% 39% 25% 3	760
	11th to 20th	Night	24% 16% 42% 17% 1	743
	21st to end	Night	23% 16% 36% 21% 4	628
March	1st to 10th	Night	21% 12% 40% 23% 4	760
	11th to 20th	Night	22% 12% 40% 21% 4 1	760
	21st to end	Night	22% 9% 39% 24% 6%	836
April	1st to 10th	Night	22% 12% 37% 23% 6%	760
	11th to 20th	Night	20% 9% 34% 31% 6%	751
	21st to end	Night	21% 8% 34% 31% 6% 1	755
May	1st to 10th	Night	25% 8% 29% 29% 8% 1	570
	11th to 20th	Night	23% 7% 33% 30% 6%	427
	21st to end	Night	21% 2 18% 39% 18% 2	389
June	1st to 10th	Night	19% 4 21% 37% 18% 1	360
	11th to 20th	Night	15% 4 21% 41% 19%	360
	21st to end	Night	9% 2 20% 53% 16%	360
July	1st to 10th	Night	14% 4 15% 45% 21% 3	350
	11th to 20th	Night	12% 3 14% 43% 26% 2	348
	21st to end	Night	10% 2 20% 45% 22% 1	394
August	1st to 10th	Night	12% 2 14% 48% 23% 3	520
	11th to 20th	Night	13% 3 14% 41% 25% 3	710
	21st to end	Night	14% 3 14% 41% 24% 4	792
September	1st to 10th	Night	16% 3 21% 36% 21% 4	720
	11th to 20th	Night	16% 3 18% 35% 23% 6%	720
	21st to end	Night	22% 4 21% 32% 19% 2	720
October	1st to 10th	Night	23% 5% 24% 26% 17% 4	720
	11th to 20th	Night	17% 8% 27% 35% 12% 1	720
	21st to end	Night	23% 7% 36% 24% 10% 1	792
November	1st to 10th	Night	15% 8% 34% 29% 12% 1	740
	11th to 20th	Night	20% 12% 32% 26% 10% 1	760
	21st to end	Night	16% 11% 38% 26% 8% 1	760
December	1st to 10th	Night	25% 13% 35% 19% 8% 1	760
	11th to 20th	Night	21% 10% 39% 26% 4	760
	21st to end	Night	21% 10% 40% 26% 3	836

Page generated at 2024-05-10 12:38:57. Version ac07

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