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 2026-06-01.

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 7% 4 4 4 3 5% 6% 34%	135839
	Night	27% 9% 3 7% 4 4 4 3 4 5% 29%	136000
Transparency	Day	33% 39% 10% 12% 5% 1	135548
	Night	37% 37% 9% 11% 5% 1	135869
Seeing	Day	27% 10% 21% 24% 14% 4	17244
	Night	33% 8% 17% 24% 17% 2	25557

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	42% 14% 4 7% 5% 4 4 3 4 4 9%	4125
	11th to 20th	Day	22% 10% 3 6% 4 4 4 3 5% 8% 32%	4177
	21st to end	Day	41% 9% 3 6% 4 3 3 2 4 3 24%	5225
February	1st to 10th	Day	34% 12% 3 9% 5% 4 3 3 4 6% 18%	1890
	11th to 20th	Day	31% 10% 3 7% 5% 5% 4 3 3 5% 23%	1845
	21st to end	Day	27% 15% 3 8% 6% 4 3 3 4 4 21%	1589
March	1st to 10th	Day	27% 15% 3 9% 5% 5% 4 3 5% 6% 18%	4742
	11th to 20th	Day	25% 12% 4 8% 5% 4 5% 3 4 6% 24%	5110
	21st to end	Day	29% 12% 4 9% 5% 5% 5% 4 5% 6% 17%	5780
April	1st to 10th	Day	28% 10% 3 7% 6% 5% 4 4 6% 6% 21%	2520
	11th to 20th	Day	21% 10% 3 8% 6% 6% 4 3 5% 6% 27%	2574
	21st to end	Day	23% 10% 3 8% 5% 4 5% 3 5% 6% 28%	2803
May	1st to 10th	Day	15% 8% 3 7% 4 4 3 3 4 5% 44%	6460
	11th to 20th	Day	19% 9% 3 7% 5% 5% 5% 4 6% 8% 30%	6417
	21st to end	Day	23% 8% 3 7% 4 4 5% 5% 6% 8% 29%	7119
June	1st to 10th	Day	22% 7% 3 7% 5% 4 4 4 5% 6% 33%	3001
	11th to 20th	Day	23% 8% 3 9% 6% 4 5% 4 6% 7% 27%	3000
	21st to end	Day	15% 7% 3 7% 5% 5% 4 4 5% 7% 38%	3000
July	1st to 10th	Day	9% 3 1 4 3 3 3 3 4 6% 60%	6780
	11th to 20th	Day	5% 3 1 3 2 2 2 2 4 5% 71%	6780
	21st to end	Day	11% 4 1 4 2 2 3 3 5% 7% 59%	6926
August	1st to 10th	Day	7% 3 1 3 2 2 2 2 4 6% 67%	4521
	11th to 20th	Day	14% 6% 2 6% 3 3 4 4 5% 6% 46%	4156
	21st to end	Day	17% 7% 2 6% 4 4 4 3 5% 7% 42%	4538
September	1st to 10th	Day	15% 7% 3 6% 4 4 4 3 4 6% 47%	2400
	11th to 20th	Day	18% 8% 3 7% 4 4 3 3 5% 6% 40%	2400
	21st to end	Day	24% 7% 3 7% 3 4 4 3 5% 6% 35%	2106
October	1st to 10th	Day	23% 6% 3 8% 5% 5% 5% 4 6% 8% 29%	3280
	11th to 20th	Day	35% 7% 3 6% 4 3 4 3 5% 6% 23%	3261
	21st to end	Day	32% 11% 4 8% 5% 5% 5% 4 5% 6% 16%	3300
November	1st to 10th	Day	39% 9% 3 8% 4 5% 4 3 5% 5% 16%	1648
	11th to 20th	Day	37% 13% 3 6% 4 4 4 3 3 4 20%	1680
	21st to end	Day	39% 12% 3 6% 4 3 4 3 4 5% 16%	1680
December	1st to 10th	Day	27% 10% 3 7% 5% 4 5% 5% 5% 7% 22%	2700
	11th to 20th	Day	41% 14% 4 7% 5% 3 5% 2 4 3 11%	2731
	21st to end	Day	40% 13% 4 10% 5% 4 3 3 4 3 11%	3575
January	1st to 10th	Night	40% 14% 4 9% 5% 4 4 3 3 3 11%	8770
	11th to 20th	Night	25% 7% 2 5% 4 4 5% 7% 6% 8% 27%	8719
	21st to end	Night	37% 8% 3 6% 4 3 3 3 4 4 25%	8954
February	1st to 10th	Night	35% 12% 3 7% 5% 3 3 2 4 5% 21%	2730
	11th to 20th	Night	31% 12% 3 7% 5% 4 3 2 4 5% 25%	2662
	21st to end	Night	30% 13% 3 6% 4 4 4 3 4 5% 24%	2216
March	1st to 10th	Night	26% 15% 3 7% 5% 4 4 4 5% 5% 22%	5368
	11th to 20th	Night	29% 10% 3 7% 4 6% 4 4 4 5% 23%	5109
	21st to end	Night	38% 12% 3 8% 4 4 3 3 3 4 17%	5436
April	1st to 10th	Night	29% 11% 3 7% 6% 4 4 4 5% 5% 23%	2100
	11th to 20th	Night	21% 10% 3 8% 5% 4 5% 4 5% 6% 29%	1992
	21st to end	Night	26% 9% 3 6% 5% 5% 5% 3 4 6% 29%	1817
May	1st to 10th	Night	17% 8% 3 6% 3 3 3 2 3 4 47%	3720
	11th to 20th	Night	18% 8% 3 7% 4 4 4 3 6% 6% 36%	3696
	21st to end	Night	20% 8% 3 9% 6% 5% 5% 3 5% 6% 29%	3981
June	1st to 10th	Night	22% 8% 2 6% 5% 4 4 4 5% 6% 34%	1400
	11th to 20th	Night	18% 10% 3 9% 5% 5% 5% 3 6% 6% 29%	1400
	21st to end	Night	16% 7% 2 7% 4 4 4 3 5% 7% 41%	1400
July	1st to 10th	Night	9% 3 2 4 3 3 2 2 3 6% 63%	3180
	11th to 20th	Night	4 2 1 2 2 2 2 2 3 4 78%	3180
	21st to end	Night	6% 4 2 4 2 3 3 3 4 5% 63%	3914
August	1st to 10th	Night	6% 2 1 3 2 2 2 3 4 6% 70%	2627
	11th to 20th	Night	12% 5% 2 5% 4 3 4 3 5% 7% 50%	2902
	21st to end	Night	14% 5% 2 6% 4 4 5% 3 6% 9% 42%	3374
September	1st to 10th	Night	10% 4 1 4 3 3 5% 4 5% 7% 55%	2000
	11th to 20th	Night	19% 7% 2 5% 5% 3 4 3 5% 6% 43%	2000
	21st to end	Night	23% 6% 3 8% 4 4 3 3 4 6% 38%	2289
October	1st to 10th	Night	20% 8% 3 8% 6% 5% 5% 4 5% 6% 31%	3920
	11th to 20th	Night	32% 7% 2 6% 4 3 4 4 5% 6% 28%	3897
	21st to end	Night	32% 9% 3 7% 4 5% 4 3 4 5% 23%	4498
November	1st to 10th	Night	40% 9% 2 6% 5% 4 4 3 3 5% 18%	2870
	11th to 20th	Night	38% 10% 3 6% 4 3 4 3 3 3 22%	2940
	21st to end	Night	38% 10% 3 8% 6% 4 4 3 3 3 19%	2940
December	1st to 10th	Night	28% 9% 3 7% 6% 6% 7% 5% 5% 5% 19%	4700
	11th to 20th	Night	43% 13% 3 8% 4 4 3 3 3 4 12%	5676
	21st to end	Night	39% 14% 4 9% 4 4 3 3 3 4 14%	7623

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	64% 24% 5% 5% 1	4125
	11th to 20th	Day	35% 24% 7% 23% 10% 2	4198
	21st to end	Day	53% 22% 4 16% 4 1	5226
February	1st to 10th	Day	29% 59% 3 3 4 2	1890
	11th to 20th	Day	28% 59% 4 6% 3 1	1854
	21st to end	Day	28% 60% 4 5% 2 1	1589
March	1st to 10th	Day	48% 32% 9% 9% 2	4742
	11th to 20th	Day	45% 31% 7% 15% 1	5110
	21st to end	Day	51% 32% 10% 6% 2	5780
April	1st to 10th	Day	26% 61% 6% 6% 1	2520
	11th to 20th	Day	25% 61% 5% 6% 2 1	2574
	21st to end	Day	24% 60% 5% 7% 3	2803
May	1st to 10th	Day	29% 32% 12% 20% 8%	6460
	11th to 20th	Day	39% 34% 16% 10% 1	6420
	21st to end	Day	37% 34% 15% 12% 2	7120
June	1st to 10th	Day	18% 63% 8% 8% 3	3001
	11th to 20th	Day	21% 63% 6% 7% 3	3000
	21st to end	Day	19% 62% 8% 8% 3	3000
July	1st to 10th	Day	19% 28% 14% 17% 16% 6%	6706
	11th to 20th	Day	10% 26% 16% 27% 19% 2	6699
	21st to end	Day	20% 28% 24% 21% 6%	6731
August	1st to 10th	Day	10% 37% 8% 30% 13% 1	4504
	11th to 20th	Day	31% 39% 16% 11% 3 1	4195
	21st to end	Day	29% 38% 11% 12% 9% 1	4537
September	1st to 10th	Day	20% 60% 7% 9% 4 1	2400
	11th to 20th	Day	22% 58% 5% 8% 6%	2400
	21st to end	Day	22% 58% 8% 8% 4	2106
October	1st to 10th	Day	38% 37% 11% 12% 2	3280
	11th to 20th	Day	43% 37% 7% 8% 6%	3261
	21st to end	Day	46% 38% 5% 7% 3	3300
November	1st to 10th	Day	33% 55% 6% 5% 1	1648
	11th to 20th	Day	30% 56% 4 5% 3 2	1680
	21st to end	Day	29% 55% 5% 5% 5% 1	1680
December	1st to 10th	Day	37% 38% 6% 14% 5%	2700
	11th to 20th	Day	56% 32% 7% 3 1	2734
	21st to end	Day	61% 30% 5% 3 1	3575
January	1st to 10th	Night	61% 24% 6% 6% 2	8770
	11th to 20th	Night	32% 26% 11% 18% 12% 2	8760
	21st to end	Night	48% 23% 7% 15% 6%	8954
February	1st to 10th	Night	28% 59% 3 3 4 3	2730
	11th to 20th	Night	29% 58% 3 5% 3 1	2664
	21st to end	Night	25% 59% 5% 7% 3 1	2216
March	1st to 10th	Night	45% 32% 10% 9% 6%	5368
	11th to 20th	Night	42% 32% 10% 15% 2	5110
	21st to end	Night	54% 30% 10% 5% 1	5436
April	1st to 10th	Night	26% 60% 5% 8% 1	2100
	11th to 20th	Night	23% 61% 6% 6% 3 1	1992
	21st to end	Night	22% 60% 7% 7% 4	1817
May	1st to 10th	Night	30% 29% 8% 24% 8%	3717
	11th to 20th	Night	36% 33% 17% 12% 2	3696
	21st to end	Night	41% 32% 13% 13% 1	3981
June	1st to 10th	Night	17% 63% 8% 8% 3 1	1400
	11th to 20th	Night	20% 64% 6% 9% 3	1400
	21st to end	Night	18% 61% 9% 9% 3	1400
July	1st to 10th	Night	18% 26% 14% 23% 14% 6%	3145
	11th to 20th	Night	7% 26% 16% 28% 21% 1	3138
	21st to end	Night	19% 27% 20% 23% 11%	3804
August	1st to 10th	Night	7% 35% 10% 30% 17% 1	2619
	11th to 20th	Night	28% 38% 17% 12% 3 1	2929
	21st to end	Night	28% 38% 12% 12% 8% 1	3374
September	1st to 10th	Night	16% 60% 9% 9% 5% 1	2000
	11th to 20th	Night	22% 58% 6% 7% 6% 1	2000
	21st to end	Night	22% 58% 6% 10% 4	2289
October	1st to 10th	Night	40% 37% 12% 8% 2	3920
	11th to 20th	Night	40% 36% 7% 11% 5%	3897
	21st to end	Night	44% 38% 6% 9% 3	4498
November	1st to 10th	Night	34% 54% 6% 5% 1	2870
	11th to 20th	Night	31% 55% 4 5% 3 2	2940
	21st to end	Night	31% 56% 3 3 5% 1	2940
December	1st to 10th	Night	38% 40% 9% 10% 3	4700
	11th to 20th	Night	56% 32% 5% 5% 1	5672
	21st to end	Night	58% 29% 6% 6% 1	7623

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% 16% 18% 14% 9% 4	220
	11th to 20th	Day	41% 10% 13% 21% 10% 4	220
	21st to end	Day	43% 13% 15% 14% 13% 3	255
February	1st to 10th	Day	41% 11% 19% 19% 7% 2	430
	11th to 20th	Day	36% 13% 20% 20% 10% 1	425
	21st to end	Day	36% 17% 24% 14% 8% 1	373
March	1st to 10th	Day	35% 15% 20% 19% 10% 1	540
	11th to 20th	Day	38% 13% 17% 17% 13% 2	540
	21st to end	Day	37% 14% 20% 20% 8% 1	594
April	1st to 10th	Day	31% 13% 23% 20% 12% 2	540
	11th to 20th	Day	25% 12% 26% 22% 12% 3	532
	21st to end	Day	26% 14% 27% 18% 13% 3	540
May	1st to 10th	Day	19% 11% 24% 30% 13% 3	543
	11th to 20th	Day	24% 10% 22% 27% 13% 4	568
	21st to end	Day	21% 13% 26% 23% 12% 4	640
June	1st to 10th	Day	21% 11% 25% 29% 11% 2	731
	11th to 20th	Day	23% 12% 27% 27% 10% 2	730
	21st to end	Day	19% 10% 22% 29% 18% 3	730
July	1st to 10th	Day	14% 7% 25% 33% 17% 4	721
	11th to 20th	Day	9% 8% 23% 40% 16% 5%	718
	21st to end	Day	11% 4 19% 36% 22% 8%	601
August	1st to 10th	Day	13% 6% 17% 33% 22% 9%	527
	11th to 20th	Day	11% 4 16% 36% 24% 8%	517
	21st to end	Day	14% 9% 22% 29% 18% 7%	583
September	1st to 10th	Day	17% 8% 21% 30% 16% 8%	530
	11th to 20th	Day	22% 8% 18% 27% 21% 4	530
	21st to end	Day	24% 10% 21% 24% 16% 5%	530
October	1st to 10th	Day	29% 6% 20% 22% 18% 5%	530
	11th to 20th	Day	38% 9% 16% 18% 15% 5%	530
	21st to end	Day	40% 12% 17% 17% 11% 3	434
November	1st to 10th	Day	40% 15% 16% 20% 10%	220
	11th to 20th	Day	47% 10% 14% 12% 12% 5%	220
	21st to end	Day	46% 8% 21% 8% 10% 7%	220
December	1st to 10th	Day	44% 13% 12% 12% 12% 7%	220
	11th to 20th	Day	54% 11% 14% 13% 6% 1	220
	21st to end	Day	49% 11% 16% 17% 7% 1	242
January	1st to 10th	Night	48% 12% 16% 16% 7% 1	950
	11th to 20th	Night	43% 8% 14% 24% 10% 1	939
	21st to end	Night	41% 10% 15% 23% 11% 1	924
February	1st to 10th	Night	41% 11% 22% 19% 7%	840
	11th to 20th	Night	37% 10% 20% 24% 8%	824
	21st to end	Night	37% 10% 23% 21% 8% 1	692
March	1st to 10th	Night	35% 11% 19% 21% 13% 1	840
	11th to 20th	Night	41% 9% 18% 21% 10%	840
	21st to end	Night	39% 9% 18% 23% 10% 1	924
April	1st to 10th	Night	33% 9% 18% 25% 13% 2	840
	11th to 20th	Night	25% 9% 24% 26% 12% 3	744
	21st to end	Night	31% 6% 19% 28% 13% 2	557
May	1st to 10th	Night	19% 7% 16% 32% 25% 2	420
	11th to 20th	Night	21% 5% 10% 32% 29% 3	414
	21st to end	Night	20% 5% 14% 33% 27% 2	455
June	1st to 10th	Night	26% 5% 14% 31% 23% 2	400
	11th to 20th	Night	22% 7% 19% 29% 20% 4	400
	21st to end	Night	17% 5% 14% 29% 32% 3	400
July	1st to 10th	Night	12% 3 11% 32% 39% 3	390
	11th to 20th	Night	10% 3 9% 34% 38% 6%	388
	21st to end	Night	5% 1 9% 27% 49% 8%	438
August	1st to 10th	Night	13% 1 5% 22% 48% 10%	403
	11th to 20th	Night	9% 2 6% 28% 44% 11%	594
	21st to end	Night	13% 4 11% 28% 39% 5%	737
September	1st to 10th	Night	14% 2 10% 36% 33% 6%	800
	11th to 20th	Night	22% 3 14% 23% 33% 6%	800
	21st to end	Night	28% 5% 15% 28% 23% 2	800
October	1st to 10th	Night	26% 5% 16% 27% 24% 3	800
	11th to 20th	Night	36% 6% 15% 24% 16% 2	800
	21st to end	Night	39% 8% 20% 24% 8% 2	880
November	1st to 10th	Night	46% 9% 19% 22% 4	820
	11th to 20th	Night	47% 8% 22% 15% 8% 1	840
	21st to end	Night	43% 10% 18% 18% 10% 1	840
December	1st to 10th	Night	38% 9% 19% 22% 11% 1	840
	11th to 20th	Night	52% 11% 19% 14% 3	939
	21st to end	Night	48% 11% 22% 15% 4	1045

Page generated at 2026-05-28 22:53:30. Version ac07

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