Langley
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Langley clear sky chart from 2005-11-06 to 2024-09-14.

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	23% 9% 3 6% 4 4 4 3 4 6% 35%	101189
	Night	28% 9% 3 6% 4 4 3 3 4 5% 31%	96783
Transparency	Day	26% 50% 8% 10% 5% 1	100939
	Night	29% 50% 7% 9% 5% 1	96679
Seeing	Day	26% 10% 21% 25% 14% 4	15125
	Night	33% 8% 17% 23% 17% 2	23301

By Month

Percentage Cloud Cover NW (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	43% 13% 3 7% 4 4 4 3 4 4 11%	2219
	11th to 20th	Day	30% 14% 4 6% 4 3 3 2 3 5% 26%	2229
	21st to end	Day	56% 11% 3 5% 3 3 2 2 3 3 10%	2805
February	1st to 10th	Day	36% 11% 3 8% 5% 4 3 2 4 6% 17%	1710
	11th to 20th	Day	33% 10% 3 7% 5% 5% 4 3 3 5% 22%	1665
	21st to end	Day	27% 15% 3 8% 6% 4 3 3 5% 4 22%	1441
March	1st to 10th	Day	26% 16% 3 8% 5% 5% 5% 3 5% 6% 19%	3221
	11th to 20th	Day	23% 9% 3 6% 4 4 5% 3 5% 7% 32%	3490
	21st to end	Day	29% 12% 3 8% 5% 4 4 4 4 5% 22%	3942
April	1st to 10th	Day	29% 11% 3 7% 6% 5% 4 4 5% 6% 20%	2280
	11th to 20th	Day	23% 10% 3 8% 6% 5% 4 3 5% 6% 26%	2326
	21st to end	Day	25% 10% 3 8% 5% 4 5% 4 5% 6% 25%	2536
May	1st to 10th	Day	17% 8% 2 6% 4 4 4 4 4 5% 43%	4420
	11th to 20th	Day	14% 7% 2 6% 4 4 5% 4 7% 9% 39%	4377
	21st to end	Day	28% 10% 3 7% 4 3 4 4 5% 6% 25%	4860
June	1st to 10th	Day	23% 8% 3 7% 5% 4 4 4 5% 6% 31%	2850
	11th to 20th	Day	23% 8% 3 9% 5% 4 5% 4 6% 7% 27%	2850
	21st to end	Day	15% 7% 3 7% 5% 5% 4 4 6% 7% 38%	2850
July	1st to 10th	Day	9% 3 1 4 3 2 3 2 4 5% 64%	4740
	11th to 20th	Day	6% 3 1 3 2 2 2 2 3 5% 70%	4740
	21st to end	Day	14% 4 2 4 3 3 2 3 4 6% 55%	4828
August	1st to 10th	Day	7% 3 1 3 2 2 2 2 4 5% 68%	4382
	11th to 20th	Day	14% 6% 2 6% 3 3 4 4 5% 6% 47%	4026
	21st to end	Day	18% 7% 2 6% 4 4 4 3 5% 7% 41%	4399
September	1st to 10th	Day	15% 7% 3 6% 4 3 3 2 4 6% 46%	2280
	11th to 20th	Day	19% 8% 3 7% 4 4 3 3 4 6% 39%	2198
	21st to end	Day	25% 7% 3 6% 3 4 4 2 5% 6% 36%	1897
October	1st to 10th	Day	26% 7% 3 7% 3 4 3 3 4 7% 33%	1800
	11th to 20th	Day	31% 9% 4 7% 4 3 4 3 5% 7% 23%	1800
	21st to end	Day	36% 8% 3 6% 4 4 3 2 5% 5% 23%	1838
November	1st to 10th	Day	40% 8% 2 7% 5% 4 4 3 5% 5% 17%	1488
	11th to 20th	Day	37% 12% 3 5% 4 4 3 3 3 4 21%	1520
	21st to end	Day	39% 12% 3 6% 4 3 4 3 5% 4 17%	1520
December	1st to 10th	Day	35% 12% 3 5% 4 3 3 4 3 4 24%	1520
	11th to 20th	Day	36% 15% 4 7% 5% 4 5% 2 4 4 13%	1700
	21st to end	Day	40% 14% 4 10% 5% 3 3 2 3 3 12%	2442
January	1st to 10th	Night	41% 13% 3 8% 4 4 3 2 3 3 14%	4726
	11th to 20th	Night	34% 8% 2 5% 3 3 3 3 5% 8% 26%	4667
	21st to end	Night	52% 11% 3 6% 3 3 2 2 2 3 12%	4848
February	1st to 10th	Night	37% 12% 3 6% 4 3 3 2 4 5% 21%	2470
	11th to 20th	Night	31% 12% 3 7% 5% 4 3 2 4 4 25%	2402
	21st to end	Night	30% 13% 3 6% 4 4 4 3 4 5% 26%	2012
March	1st to 10th	Night	29% 16% 3 6% 5% 4 4 3 4 5% 21%	3650
	11th to 20th	Night	26% 7% 2 6% 4 8% 4 4 4 6% 30%	3489
	21st to end	Night	36% 12% 4 8% 5% 4 3 3 3 4 20%	3710
April	1st to 10th	Night	30% 11% 2 7% 6% 4 4 4 5% 5% 23%	1900
	11th to 20th	Night	23% 10% 3 8% 5% 4 4 3 5% 5% 29%	1800
	21st to end	Night	27% 10% 3 7% 5% 5% 5% 3 4 6% 25%	1644
May	1st to 10th	Night	19% 7% 2 6% 3 3 3 2 3 4 46%	2540
	11th to 20th	Night	13% 5% 2 5% 4 3 4 4 7% 6% 46%	2516
	21st to end	Night	26% 9% 3 8% 5% 4 4 3 5% 5% 27%	2712
June	1st to 10th	Night	23% 8% 2 6% 5% 4 4 3 5% 6% 32%	1330
	11th to 20th	Night	19% 10% 4 9% 5% 5% 5% 3 6% 6% 28%	1330
	21st to end	Night	15% 6% 2 7% 4 4 4 3 5% 7% 42%	1330
July	1st to 10th	Night	6% 2 1 4 3 2 2 2 3 6% 67%	2220
	11th to 20th	Night	5% 2 1 2 2 2 2 1 3 3 76%	2220
	21st to end	Night	9% 5% 2 5% 3 3 4 3 4 5% 59%	2721
August	1st to 10th	Night	5% 2 1 3 2 2 2 3 4 6% 71%	2547
	11th to 20th	Night	12% 5% 2 5% 4 4 4 3 5% 7% 50%	2812
	21st to end	Night	14% 5% 2 6% 4 4 5% 3 6% 9% 41%	3271
September	1st to 10th	Night	10% 3 1 4 3 4 5% 4 5% 6% 55%	1900
	11th to 20th	Night	20% 7% 2 5% 5% 3 3 2 4 6% 43%	1840
	21st to end	Night	22% 5% 3 8% 3 4 3 3 4 6% 39%	2058
October	1st to 10th	Night	22% 7% 2 6% 5% 4 4 3 3 4 39%	2160
	11th to 20th	Night	31% 7% 3 6% 5% 3 4 3 4 5% 29%	2160
	21st to end	Night	35% 6% 2 5% 3 4 4 3 4 5% 29%	2513
November	1st to 10th	Night	41% 9% 2 6% 6% 4 4 2 3 5% 18%	2590
	11th to 20th	Night	38% 10% 3 5% 4 3 4 3 3 3 23%	2660
	21st to end	Night	38% 10% 3 7% 6% 4 4 3 3 3 20%	2660
December	1st to 10th	Night	33% 9% 3 6% 4 4 3 4 4 5% 24%	2660
	11th to 20th	Night	44% 14% 3 8% 4 3 3 2 3 3 13%	3501
	21st to end	Night	41% 13% 3 8% 4 3 3 2 3 3 16%	5214

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	48% 41% 5% 3 1 1	2219
	11th to 20th	Day	37% 38% 6% 10% 5% 4	2250
	21st to end	Day	56% 39% 2 3	2805
February	1st to 10th	Day	26% 65% 2 2 3 2	1710
	11th to 20th	Day	25% 65% 3 4 3 1	1674
	21st to end	Day	22% 66% 4 5% 2 2	1441
March	1st to 10th	Day	37% 45% 10% 5% 2	3221
	11th to 20th	Day	27% 43% 9% 20% 1	3490
	21st to end	Day	38% 44% 9% 6% 3	3942
April	1st to 10th	Day	23% 67% 4 5% 1	2280
	11th to 20th	Day	22% 66% 4 5% 2 1	2326
	21st to end	Day	22% 66% 4 5% 3	2536
May	1st to 10th	Day	23% 44% 11% 16% 6%	4420
	11th to 20th	Day	22% 46% 19% 11% 2	4380
	21st to end	Day	38% 46% 11% 4 1	4861
June	1st to 10th	Day	18% 66% 7% 6% 2	2850
	11th to 20th	Day	19% 66% 5% 7% 3	2850
	21st to end	Day	17% 65% 7% 8% 3	2850
July	1st to 10th	Day	13% 37% 5% 16% 20% 9%	4666
	11th to 20th	Day	8% 36% 7% 29% 19% 1	4659
	21st to end	Day	22% 38% 16% 17% 6% 1	4675
August	1st to 10th	Day	9% 38% 8% 31% 14% 1	4365
	11th to 20th	Day	30% 40% 16% 11% 3 1	4065
	21st to end	Day	30% 39% 11% 12% 9%	4398
September	1st to 10th	Day	19% 62% 6% 8% 5% 1	2280
	11th to 20th	Day	21% 63% 4 6% 6% 1	2198
	21st to end	Day	18% 64% 7% 7% 4	1897
October	1st to 10th	Day	21% 63% 5% 8% 3	1800
	11th to 20th	Day	26% 63% 3 6% 2	1800
	21st to end	Day	23% 63% 4 6% 4	1838
November	1st to 10th	Day	29% 60% 6% 5% 1	1488
	11th to 20th	Day	24% 61% 4 5% 3 2	1520
	21st to end	Day	24% 61% 5% 5% 5% 1	1520
December	1st to 10th	Day	27% 61% 2 6% 4	1520
	11th to 20th	Day	38% 50% 7% 2 2 1	1703
	21st to end	Day	50% 41% 4 3 1 1	2442
January	1st to 10th	Night	48% 40% 5% 5% 2 1	4726
	11th to 20th	Night	32% 39% 7% 12% 8% 3	4708
	21st to end	Night	55% 39% 2 3	4848
February	1st to 10th	Night	25% 65% 2 3 3 2	2470
	11th to 20th	Night	26% 64% 3 4 2 1	2404
	21st to end	Night	19% 65% 5% 8% 2 1	2012
March	1st to 10th	Night	37% 44% 7% 6% 5%	3650
	11th to 20th	Night	23% 45% 11% 19% 2	3490
	21st to end	Night	43% 42% 8% 5% 1	3710
April	1st to 10th	Night	22% 66% 4 6% 1	1900
	11th to 20th	Night	21% 66% 4 5% 2 1	1800
	21st to end	Night	22% 66% 4 5% 3	1644
May	1st to 10th	Night	25% 41% 7% 19% 7% 1	2540
	11th to 20th	Night	20% 45% 19% 13% 3	2516
	21st to end	Night	39% 44% 10% 6% 1	2712
June	1st to 10th	Night	17% 66% 7% 8% 2 1	1330
	11th to 20th	Night	19% 67% 5% 7% 3	1330
	21st to end	Night	15% 64% 8% 9% 3	1330
July	1st to 10th	Night	9% 35% 5% 24% 18% 9%	2185
	11th to 20th	Night	7% 35% 5% 30% 22% 1	2178
	21st to end	Night	21% 36% 10% 19% 13% 1	2635
August	1st to 10th	Night	6% 36% 9% 30% 18% 1	2539
	11th to 20th	Night	28% 39% 17% 12% 3 1	2839
	21st to end	Night	28% 40% 12% 12% 7%	3271
September	1st to 10th	Night	16% 62% 8% 8% 6% 1	1900
	11th to 20th	Night	21% 63% 4 5% 6% 1	1840
	21st to end	Night	18% 64% 5% 8% 4	2058
October	1st to 10th	Night	19% 64% 6% 8% 4	2160
	11th to 20th	Night	24% 63% 4 6% 3	2160
	21st to end	Night	22% 62% 4 8% 4	2513
November	1st to 10th	Night	29% 59% 6% 4 1	2590
	11th to 20th	Night	26% 60% 4 5% 3 2	2660
	21st to end	Night	27% 61% 3 3 5% 1	2660
December	1st to 10th	Night	26% 60% 3 5% 5% 1	2660
	11th to 20th	Night	42% 50% 3 2 1 1	3497
	21st to end	Night	48% 39% 5% 5% 2	5214

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	40% 17% 19% 12% 8% 4	180
	11th to 20th	Day	47% 13% 14% 16% 8% 2	180
	21st to end	Day	45% 14% 16% 13% 11% 1	209
February	1st to 10th	Day	43% 11% 20% 18% 6% 2	370
	11th to 20th	Day	37% 13% 21% 20% 8% 1	365
	21st to end	Day	34% 17% 25% 14% 7% 1	321
March	1st to 10th	Day	34% 15% 21% 19% 9% 1	460
	11th to 20th	Day	35% 12% 18% 19% 14% 2	460
	21st to end	Day	38% 15% 19% 19% 8% 1	506
April	1st to 10th	Day	32% 14% 23% 19% 10% 1	460
	11th to 20th	Day	27% 13% 26% 21% 10% 3	452
	21st to end	Day	28% 14% 27% 18% 12% 2	460
May	1st to 10th	Day	20% 11% 24% 30% 11% 3	463
	11th to 20th	Day	23% 7% 22% 29% 15% 5%	488
	21st to end	Day	23% 13% 26% 23% 11% 4	550
June	1st to 10th	Day	23% 11% 26% 27% 11% 2	680
	11th to 20th	Day	24% 12% 27% 26% 9% 2	680
	21st to end	Day	18% 10% 23% 29% 17% 3	680
July	1st to 10th	Day	14% 7% 26% 32% 18% 4	671
	11th to 20th	Day	9% 8% 22% 39% 17% 5%	668
	21st to end	Day	11% 4 19% 36% 22% 8%	556
August	1st to 10th	Day	13% 6% 17% 34% 22% 9%	487
	11th to 20th	Day	10% 3 17% 37% 25% 8%	477
	21st to end	Day	15% 10% 22% 29% 17% 6%	539
September	1st to 10th	Day	18% 8% 23% 29% 16% 6%	490
	11th to 20th	Day	23% 9% 18% 28% 19% 3	463
	21st to end	Day	24% 10% 20% 23% 17% 5%	450
October	1st to 10th	Day	30% 7% 19% 21% 18% 5%	450
	11th to 20th	Day	38% 8% 16% 18% 16% 4	450
	21st to end	Day	39% 12% 19% 16% 11% 3	362
November	1st to 10th	Day	39% 16% 16% 18% 11%	180
	11th to 20th	Day	47% 8% 14% 11% 14% 6%	180
	21st to end	Day	46% 8% 22% 7% 9% 8%	180
December	1st to 10th	Day	44% 14% 13% 12% 11% 5%	180
	11th to 20th	Day	51% 10% 16% 15% 7% 1	180
	21st to end	Day	49% 10% 17% 19% 5% 1	198
January	1st to 10th	Night	49% 13% 16% 14% 7% 1	850
	11th to 20th	Night	46% 9% 15% 22% 8%	841
	21st to end	Night	43% 11% 16% 21% 9%	836
February	1st to 10th	Night	42% 11% 22% 18% 6%	760
	11th to 20th	Night	37% 10% 21% 24% 7%	744
	21st to end	Night	36% 10% 23% 21% 9% 1	628
March	1st to 10th	Night	35% 11% 20% 20% 13% 1	760
	11th to 20th	Night	40% 8% 19% 22% 10% 1	760
	21st to end	Night	40% 9% 18% 22% 10% 1	836
April	1st to 10th	Night	34% 9% 18% 26% 12% 2	760
	11th to 20th	Night	27% 9% 25% 25% 12% 2	672
	21st to end	Night	33% 7% 20% 27% 12% 1	504
May	1st to 10th	Night	20% 7% 17% 30% 24% 2	380
	11th to 20th	Night	20% 4 9% 33% 30% 4	374
	21st to end	Night	21% 5% 13% 33% 26% 2	411
June	1st to 10th	Night	27% 5% 13% 29% 23% 2	380
	11th to 20th	Night	23% 8% 19% 28% 19% 3	380
	21st to end	Night	15% 5% 15% 30% 32% 3	380
July	1st to 10th	Night	12% 2 11% 32% 39% 4	370
	11th to 20th	Night	11% 4 8% 33% 38% 6%	368
	21st to end	Night	6% 1 9% 27% 49% 7%	416
August	1st to 10th	Night	13% 1 5% 22% 49% 10%	383
	11th to 20th	Night	8% 2 6% 27% 45% 11%	564
	21st to end	Night	14% 4 12% 29% 37% 5%	700
September	1st to 10th	Night	14% 2 11% 37% 32% 5%	760
	11th to 20th	Night	22% 4 14% 21% 33% 6%	736
	21st to end	Night	27% 5% 14% 29% 24% 2	720
October	1st to 10th	Night	26% 5% 15% 27% 24% 3	720
	11th to 20th	Night	37% 6% 15% 23% 16% 2	720
	21st to end	Night	38% 8% 20% 23% 9% 2	792
November	1st to 10th	Night	46% 9% 20% 22% 4	740
	11th to 20th	Night	45% 8% 22% 16% 9% 1	760
	21st to end	Night	43% 11% 18% 16% 10% 1	760
December	1st to 10th	Night	37% 8% 21% 23% 10%	760
	11th to 20th	Night	51% 11% 20% 14% 3	841
	21st to end	Night	49% 12% 22% 14% 3	935

Page generated at 2024-09-11 07:00:03. Version ac07

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