Longview
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Longview clear sky chart from 2005-11-06 to 2026-07-31.

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	13% 6% 2 6% 4 4 4 3 5% 7% 45%	138175
	Night	13% 6% 2 5% 3 3 3 3 5% 7% 50%	135143
Transparency	Day	37% 5% 11% 22% 19% 7%	141798
	Night	34% 4 10% 20% 22% 10%	139587
Seeing	Day	14% 9% 22% 27% 22% 6%	17490
	Night	14% 8% 26% 32% 18% 2	27246

By Month

Percentage Cloud Cover SE (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	16% 8% 3 5% 3 3 3 3 4 6% 47%	5289
	11th to 20th	Day	11% 4 1 4 3 3 3 3 4 6% 58%	5310
	21st to end	Day	25% 9% 2 6% 3 2 2 2 3 3 41%	5812
February	1st to 10th	Day	21% 9% 3 6% 3 4 4 2 3 4 42%	1890
	11th to 20th	Day	22% 8% 2 7% 4 3 3 3 4 4 40%	2013
	21st to end	Day	21% 7% 2 5% 3 3 3 2 4 3 46%	1903
March	1st to 10th	Day	18% 10% 3 7% 3 3 3 3 4 6% 41%	5046
	11th to 20th	Day	14% 6% 2 5% 3 3 3 3 4 6% 52%	5090
	21st to end	Day	15% 8% 3 6% 4 4 4 3 5% 6% 43%	5583
April	1st to 10th	Day	14% 8% 2 5% 4 4 3 3 5% 6% 44%	2347
	11th to 20th	Day	15% 8% 3 5% 4 4 4 4 6% 6% 41%	2494
	21st to end	Day	14% 6% 2 5% 5% 5% 5% 4 6% 6% 42%	2520
May	1st to 10th	Day	23% 8% 3 6% 4 4 4 3 5% 6% 35%	7320
	11th to 20th	Day	13% 6% 3 7% 5% 5% 5% 3 5% 7% 40%	7627
	21st to end	Day	13% 7% 3 7% 5% 5% 5% 5% 7% 9% 34%	8392
June	1st to 10th	Day	7% 6% 2 7% 5% 5% 6% 5% 8% 10% 41%	2730
	11th to 20th	Day	6% 4 1 5% 6% 5% 6% 5% 7% 11% 44%	2730
	21st to end	Day	5% 4 2 4 5% 4 6% 4 8% 10% 49%	2730
July	1st to 10th	Day	9% 5% 2 7% 5% 6% 6% 6% 8% 12% 34%	6030
	11th to 20th	Day	4 3 1 4 4 5% 5% 4 6% 10% 54%	6030
	21st to end	Day	10% 6% 2 6% 4 5% 4 4 6% 9% 43%	6553
August	1st to 10th	Day	2 1 1 3 2 2 3 3 5% 8% 69%	4234
	11th to 20th	Day	4 3 1 4 3 3 4 4 7% 10% 57%	4040
	21st to end	Day	7% 4 2 6% 5% 5% 6% 5% 9% 11% 41%	4312
September	1st to 10th	Day	8% 6% 3 6% 5% 4 5% 4 5% 8% 47%	2310
	11th to 20th	Day	10% 6% 2 6% 5% 4 5% 3 6% 8% 45%	2200
	21st to end	Day	7% 3 1 4 4 3 5% 3 5% 7% 58%	2200
October	1st to 10th	Day	6% 3 1 5% 3 3 3 3 5% 5% 62%	2021
	11th to 20th	Day	13% 4 1 4 4 3 3 3 4 5% 57%	2000
	21st to end	Day	14% 5% 2 4 3 3 3 2 3 4 56%	2200
November	1st to 10th	Day	13% 6% 2 6% 3 3 4 2 5% 5% 52%	2058
	11th to 20th	Day	10% 6% 3 5% 4 3 3 2 4 5% 54%	2100
	21st to end	Day	18% 8% 2 5% 3 3 3 3 4 5% 46%	1963
December	1st to 10th	Day	19% 6% 2 5% 3 3 3 2 4 5% 48%	3030
	11th to 20th	Day	19% 7% 3 6% 4 4 3 3 4 6% 41%	3481
	21st to end	Day	17% 9% 3 7% 4 3 3 2 4 5% 44%	4587
January	1st to 10th	Night	15% 7% 2 5% 3 3 3 2 3 4 51%	7634
	11th to 20th	Night	10% 4 1 3 2 2 2 2 4 6% 63%	7680
	21st to end	Night	27% 9% 2 5% 3 3 3 2 3 4 38%	8402
February	1st to 10th	Night	17% 7% 2 5% 3 3 3 3 4 6% 47%	2730
	11th to 20th	Night	20% 8% 3 5% 3 3 3 3 3 4 45%	2508
	21st to end	Night	17% 7% 3 5% 3 3 3 3 3 5% 48%	1903
March	1st to 10th	Night	21% 8% 2 5% 3 3 3 3 4 6% 42%	5029
	11th to 20th	Night	13% 5% 2 5% 3 3 4 3 5% 7% 51%	5090
	21st to end	Night	15% 7% 2 6% 4 4 4 3 5% 6% 46%	5588
April	1st to 10th	Night	11% 4 2 5% 4 5% 4 4 5% 7% 49%	2273
	11th to 20th	Night	12% 6% 2 5% 5% 4 4 4 5% 6% 47%	2075
	21st to end	Night	12% 6% 2 5% 3 4 3 3 5% 7% 50%	2100
May	1st to 10th	Night	19% 10% 3 7% 4 4 4 3 5% 7% 35%	5531
	11th to 20th	Night	13% 7% 2 7% 5% 4 4 3 5% 7% 43%	5265
	21st to end	Night	13% 6% 2 6% 4 4 3 4 7% 9% 41%	5793
June	1st to 10th	Night	4 4 1 5% 4 4 4 3 7% 10% 53%	1890
	11th to 20th	Night	5% 3 1 5% 3 3 3 4 6% 8% 59%	1890
	21st to end	Night	4 4 1 4 4 3 4 5% 7% 9% 57%	1890
July	1st to 10th	Night	7% 4 2 6% 5% 5% 6% 4 8% 13% 41%	4168
	11th to 20th	Night	3 2 1 4 3 3 4 4 6% 11% 58%	4167
	21st to end	Night	8% 5% 2 6% 4 4 4 4 6% 9% 49%	4539
August	1st to 10th	Night	2 1 2 2 2 2 3 4 7% 75%	2926
	11th to 20th	Night	3 2 1 3 3 2 3 4 6% 12% 60%	3013
	21st to end	Night	4 3 1 5% 3 5% 5% 5% 8% 10% 51%	3586
September	1st to 10th	Night	7% 5% 2 5% 4 5% 4 4 5% 7% 53%	2085
	11th to 20th	Night	10% 4 1 4 3 4 4 3 6% 8% 52%	2200
	21st to end	Night	6% 3 1 4 2 3 4 3 5% 8% 61%	2200
October	1st to 10th	Night	4 3 1 4 2 3 4 2 5% 6% 65%	2357
	11th to 20th	Night	11% 4 1 3 2 3 2 3 3 5% 62%	2400
	21st to end	Night	14% 4 2 4 2 3 2 2 3 5% 58%	2640
November	1st to 10th	Night	10% 5% 2 5% 3 3 3 2 4 6% 57%	2460
	11th to 20th	Night	9% 4 3 5% 4 3 3 3 4 5% 58%	2520
	21st to end	Night	15% 6% 3 5% 3 3 3 3 4 5% 50%	2652
December	1st to 10th	Night	18% 7% 2 5% 3 3 3 2 3 4 49%	4380
	11th to 20th	Night	18% 7% 2 6% 4 4 3 3 3 4 46%	4946
	21st to end	Night	15% 9% 3 6% 4 3 3 2 3 4 49%	6633

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% 2 8% 25% 22% 3	5290
	11th to 20th	Day	28% 3 6% 26% 30% 7%	5310
	21st to end	Day	50% 3 3 12% 21% 11%	5812
February	1st to 10th	Day	48% 3 5% 16% 19% 10%	1890
	11th to 20th	Day	48% 4 6% 18% 17% 7%	2023
	21st to end	Day	44% 4 6% 13% 22% 11%	1903
March	1st to 10th	Day	46% 4 6% 13% 20% 12%	5046
	11th to 20th	Day	35% 4 6% 16% 19% 20%	5090
	21st to end	Day	42% 4 9% 21% 19% 5%	5585
April	1st to 10th	Day	40% 5% 9% 21% 17% 8%	2347
	11th to 20th	Day	42% 6% 10% 16% 19% 8%	2494
	21st to end	Day	40% 6% 10% 19% 21% 3	2520
May	1st to 10th	Day	49% 5% 10% 22% 11% 3	7355
	11th to 20th	Day	43% 5% 11% 17% 21% 3	7631
	21st to end	Day	44% 7% 15% 19% 13% 2	8405
June	1st to 10th	Day	35% 8% 15% 27% 14% 1	2730
	11th to 20th	Day	32% 8% 16% 26% 16% 3	2730
	21st to end	Day	27% 8% 16% 30% 16% 2	2730
July	1st to 10th	Day	39% 10% 22% 22% 7%	5966
	11th to 20th	Day	25% 6% 19% 37% 13%	5961
	21st to end	Day	37% 6% 14% 30% 12% 1	6371
August	1st to 10th	Day	14% 5% 15% 33% 28% 4	4220
	11th to 20th	Day	21% 6% 20% 39% 14% 1	4079
	21st to end	Day	34% 8% 19% 24% 15% 1	4311
September	1st to 10th	Day	35% 6% 12% 19% 17% 10%	2310
	11th to 20th	Day	36% 7% 12% 20% 18% 7%	2200
	21st to end	Day	25% 6% 10% 23% 25% 11%	2200
October	1st to 10th	Day	14% 3 6% 12% 26% 38%	3294
	11th to 20th	Day	20% 3 5% 18% 31% 24%	3240
	21st to end	Day	27% 3 4 21% 33% 10%	3538
November	1st to 10th	Day	35% 4 8% 18% 26% 9%	2058
	11th to 20th	Day	32% 4 7% 19% 26% 12%	2100
	21st to end	Day	42% 3 6% 17% 24% 8%	1963
December	1st to 10th	Day	40% 2 6% 22% 25% 5%	3030
	11th to 20th	Day	45% 3 6% 14% 23% 10%	3479
	21st to end	Day	44% 3 5% 18% 24% 5%	4587
January	1st to 10th	Night	38% 3 7% 20% 27% 5%	7635
	11th to 20th	Night	25% 2 5% 19% 34% 15%	7680
	21st to end	Night	52% 2 4 13% 18% 11%	8402
February	1st to 10th	Night	40% 3 7% 19% 18% 12%	2730
	11th to 20th	Night	44% 3 7% 17% 21% 8%	2508
	21st to end	Night	40% 4 8% 14% 21% 13%	1903
March	1st to 10th	Night	45% 4 5% 12% 20% 14%	5029
	11th to 20th	Night	33% 4 7% 15% 19% 21%	5090
	21st to end	Night	40% 4 9% 19% 21% 7%	5588
April	1st to 10th	Night	34% 5% 12% 20% 18% 11%	2273
	11th to 20th	Night	36% 6% 10% 18% 20% 11%	2075
	21st to end	Night	34% 4 11% 21% 25% 5%	2100
May	1st to 10th	Night	49% 5% 11% 19% 13% 4	5561
	11th to 20th	Night	42% 5% 12% 17% 21% 4	5266
	21st to end	Night	39% 6% 18% 22% 10% 5%	5805
June	1st to 10th	Night	24% 8% 17% 29% 20% 2	1890
	11th to 20th	Night	22% 6% 17% 30% 20% 4	1890
	21st to end	Night	22% 8% 16% 28% 23% 3	1890
July	1st to 10th	Night	34% 6% 23% 29% 7%	4125
	11th to 20th	Night	20% 5% 24% 35% 15% 1	4116
	21st to end	Night	32% 6% 16% 32% 12% 3	4410
August	1st to 10th	Night	11% 4 14% 33% 32% 6%	2916
	11th to 20th	Night	17% 5% 17% 43% 15% 3	3040
	21st to end	Night	26% 7% 18% 25% 21% 2	3586
September	1st to 10th	Night	29% 7% 12% 21% 20% 10%	2085
	11th to 20th	Night	30% 7% 12% 22% 20% 9%	2200
	21st to end	Night	22% 4 10% 23% 28% 13%	2200
October	1st to 10th	Night	13% 3 5% 15% 19% 45%	3851
	11th to 20th	Night	16% 3 4 17% 33% 27%	3899
	21st to end	Night	26% 3 4 18% 32% 16%	4252
November	1st to 10th	Night	29% 4 9% 18% 27% 13%	2460
	11th to 20th	Night	29% 4 8% 19% 26% 14%	2520
	21st to end	Night	38% 3 7% 18% 24% 10%	2652
December	1st to 10th	Night	40% 3 6% 18% 27% 6%	4380
	11th to 20th	Night	44% 4 7% 13% 23% 10%	4947
	21st to end	Night	40% 3 5% 17% 27% 7%	6633

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	23% 17% 38% 16% 5% 1	330
	11th to 20th	Day	23% 20% 35% 17% 5%	330
	21st to end	Day	25% 22% 33% 16% 3	363
February	1st to 10th	Day	28% 16% 37% 13% 5%	330
	11th to 20th	Day	33% 19% 34% 12% 2	330
	21st to end	Day	31% 23% 33% 12% 1	273
March	1st to 10th	Day	24% 22% 33% 17% 4	330
	11th to 20th	Day	23% 25% 35% 13% 3	330
	21st to end	Day	19% 20% 35% 20% 6%	363
April	1st to 10th	Day	24% 14% 38% 17% 6%	367
	11th to 20th	Day	21% 18% 33% 20% 8% 1	532
	21st to end	Day	19% 14% 34% 23% 8% 2	540
May	1st to 10th	Day	16% 12% 35% 28% 8%	648
	11th to 20th	Day	15% 9% 27% 32% 14% 2	775
	21st to end	Day	12% 6% 25% 36% 15% 5%	844
June	1st to 10th	Day	7% 4 16% 37% 29% 7%	780
	11th to 20th	Day	6% 2 17% 39% 29% 6%	780
	21st to end	Day	5% 1 14% 36% 33% 11%	780
July	1st to 10th	Day	7% 2 10% 32% 37% 11%	771
	11th to 20th	Day	4 2 6% 31% 45% 12%	768
	21st to end	Day	7% 2 5% 30% 42% 14%	845
August	1st to 10th	Day	5% 1 5% 27% 49% 14%	726
	11th to 20th	Day	7% 1 4 33% 43% 12%	636
	21st to end	Day	9% 1 6% 24% 40% 20%	583
September	1st to 10th	Day	10% 1 8% 30% 38% 13%	440
	11th to 20th	Day	6% 3 8% 31% 35% 16%	330
	21st to end	Day	11% 2 15% 35% 24% 14%	330
October	1st to 10th	Day	5% 3 19% 41% 24% 7%	330
	11th to 20th	Day	11% 7% 28% 34% 15% 5%	330
	21st to end	Day	22% 12% 27% 23% 14% 3	363
November	1st to 10th	Day	17% 9% 30% 30% 13% 2	330
	11th to 20th	Day	13% 10% 34% 35% 7% 2	330
	21st to end	Day	19% 19% 32% 20% 9%	330
December	1st to 10th	Day	20% 14% 38% 19% 8% 2	330
	11th to 20th	Day	25% 15% 35% 18% 6% 1	330
	21st to end	Day	22% 18% 36% 19% 4 1	363
January	1st to 10th	Night	19% 11% 34% 28% 7%	1050
	11th to 20th	Night	21% 15% 32% 26% 6%	1050
	21st to end	Night	22% 12% 35% 27% 4	1155
February	1st to 10th	Night	22% 12% 36% 26% 4	1050
	11th to 20th	Night	26% 14% 34% 23% 3	873
	21st to end	Night	21% 16% 39% 20% 4	519
March	1st to 10th	Night	21% 11% 34% 28% 5%	630
	11th to 20th	Night	17% 13% 36% 29% 5%	630
	21st to end	Night	15% 11% 39% 28% 6%	693
April	1st to 10th	Night	12% 15% 39% 27% 6%	630
	11th to 20th	Night	16% 14% 36% 24% 9%	623
	21st to end	Night	13% 10% 39% 28% 10% 1	630
May	1st to 10th	Night	14% 10% 37% 30% 10% 1	630
	11th to 20th	Night	12% 8% 30% 33% 15% 2	621
	21st to end	Night	9% 4 25% 39% 20% 3	683
June	1st to 10th	Night	5% 2 12% 43% 34% 3	630
	11th to 20th	Night	6% 2 17% 45% 29% 1	630
	21st to end	Night	4 1 14% 39% 36% 6%	630
July	1st to 10th	Night	5% 2 9% 37% 40% 7%	615
	11th to 20th	Night	2 1 6% 39% 46% 6%	612
	21st to end	Night	3 1 8% 38% 43% 8%	687
August	1st to 10th	Night	3 1 4 39% 46% 7%	597
	11th to 20th	Night	5% 1 6% 37% 45% 6%	594
	21st to end	Night	7% 1 6% 36% 45% 6%	660
September	1st to 10th	Night	9% 2 8% 33% 42% 6%	600
	11th to 20th	Night	12% 2 8% 31% 40% 7%	600
	21st to end	Night	8% 2 10% 41% 36% 4	600
October	1st to 10th	Night	7% 2 14% 41% 32% 5%	757
	11th to 20th	Night	13% 5% 24% 36% 19% 2	800
	21st to end	Night	15% 8% 27% 32% 17% 1	880
November	1st to 10th	Night	12% 5% 25% 40% 17% 1	820
	11th to 20th	Night	10% 9% 32% 37% 11% 1	840
	21st to end	Night	17% 12% 32% 27% 12%	972
December	1st to 10th	Night	18% 11% 32% 32% 7%	1050
	11th to 20th	Night	23% 10% 29% 33% 6%	1050
	21st to end	Night	21% 12% 32% 29% 6%	1155

Page generated at 2026-07-28 02:25:24. Version ac07

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