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 2025-03-27.

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%	109576
	Night	13% 6% 2 5% 3 3 3 3 4 6% 51%	109958
Transparency	Day	37% 5% 10% 21% 20% 7%	113238
	Night	33% 4 10% 21% 22% 10%	114423
Seeing	Day	14% 9% 22% 27% 22% 6%	15564
	Night	14% 8% 26% 32% 18% 2	25553

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	17% 9% 3 6% 3 3 3 2 3 5% 46%	4077
	11th to 20th	Day	14% 5% 2 4 3 2 3 2 4 7% 56%	4080
	21st to end	Day	24% 10% 2 5% 3 2 2 2 2 3 44%	4488
February	1st to 10th	Day	21% 9% 3 6% 3 4 4 2 3 4 41%	1800
	11th to 20th	Day	22% 9% 2 6% 4 4 3 2 4 4 40%	1914
	21st to end	Day	22% 7% 2 5% 4 3 4 2 4 3 44%	1815
March	1st to 10th	Day	17% 11% 3 8% 4 3 4 3 4 6% 37%	3546
	11th to 20th	Day	18% 8% 2 6% 4 4 4 4 5% 6% 39%	3590
	21st to end	Day	17% 9% 3 6% 4 4 4 3 5% 7% 39%	3880
April	1st to 10th	Day	13% 8% 2 5% 4 4 3 3 6% 6% 45%	2123
	11th to 20th	Day	15% 8% 3 5% 4 4 4 4 6% 6% 41%	2254
	21st to end	Day	13% 6% 2 5% 5% 4 5% 4 6% 7% 44%	2280
May	1st to 10th	Day	20% 8% 4 7% 4 4 4 3 5% 7% 34%	3925
	11th to 20th	Day	14% 6% 3 6% 4 4 4 3 5% 7% 45%	4082
	21st to end	Day	13% 8% 3 8% 5% 5% 5% 4 6% 8% 34%	4477
June	1st to 10th	Day	7% 5% 2 6% 5% 5% 6% 4 8% 10% 42%	2470
	11th to 20th	Day	5% 4 1 5% 6% 5% 6% 5% 7% 11% 46%	2470
	21st to end	Day	5% 4 2 4 5% 4 6% 4 8% 10% 48%	2470
July	1st to 10th	Day	9% 5% 2 6% 5% 6% 6% 6% 8% 11% 38%	4120
	11th to 20th	Day	4 3 2 4 4 5% 5% 4 6% 10% 52%	4120
	21st to end	Day	14% 8% 3 8% 5% 5% 4 5% 6% 9% 34%	4465
August	1st to 10th	Day	2 1 1 3 2 2 3 3 5% 8% 69%	4104
	11th to 20th	Day	4 3 1 4 3 3 4 4 7% 10% 57%	3914
	21st to end	Day	7% 4 2 6% 5% 5% 6% 5% 9% 11% 41%	4180
September	1st to 10th	Day	8% 6% 3 7% 5% 4 4 4 5% 8% 47%	2195
	11th to 20th	Day	11% 6% 2 6% 5% 4 5% 3 6% 8% 44%	2090
	21st to end	Day	7% 3 1 4 4 3 5% 3 5% 7% 57%	2090
October	1st to 10th	Day	6% 3 1 5% 3 3 3 3 5% 5% 61%	1920
	11th to 20th	Day	13% 5% 1 4 4 3 3 3 4 5% 56%	1900
	21st to end	Day	15% 5% 2 4 3 3 3 2 3 4 56%	2090
November	1st to 10th	Day	14% 6% 2 6% 4 3 4 2 5% 5% 50%	1958
	11th to 20th	Day	11% 6% 3 5% 4 3 3 2 4 5% 54%	2000
	21st to end	Day	18% 8% 2 5% 3 3 3 3 4 5% 46%	1870
December	1st to 10th	Day	19% 6% 2 5% 3 3 3 2 4 5% 49%	2940
	11th to 20th	Day	19% 7% 3 6% 4 4 3 3 5% 6% 41%	3391
	21st to end	Day	17% 9% 3 7% 4 3 3 2 4 5% 43%	4488
January	1st to 10th	Night	16% 6% 2 4 3 3 3 2 3 4 53%	5888
	11th to 20th	Night	13% 5% 1 3 2 2 2 2 3 6% 61%	5900
	21st to end	Night	26% 9% 2 5% 3 3 2 2 3 4 41%	6490
February	1st to 10th	Night	18% 8% 2 5% 3 3 3 3 4 5% 46%	2600
	11th to 20th	Night	20% 8% 3 5% 3 3 3 3 3 4 45%	2386
	21st to end	Night	17% 8% 3 5% 3 3 3 3 3 5% 47%	1815
March	1st to 10th	Night	20% 9% 3 6% 4 4 3 3 5% 6% 38%	3529
	11th to 20th	Night	17% 7% 2 7% 4 4 4 3 5% 7% 41%	3590
	21st to end	Night	13% 7% 3 7% 4 4 4 3 5% 6% 46%	3889
April	1st to 10th	Night	11% 5% 2 5% 4 5% 4 4 5% 7% 49%	2057
	11th to 20th	Night	12% 6% 2 5% 5% 4 4 4 5% 6% 47%	1875
	21st to end	Night	12% 5% 2 5% 3 3 3 2 4 8% 52%	1900
May	1st to 10th	Night	15% 9% 3 8% 5% 4 4 4 5% 7% 38%	2958
	11th to 20th	Night	11% 7% 2 7% 5% 3 4 3 4 7% 47%	2823
	21st to end	Night	12% 5% 2 6% 4 4 4 4 7% 9% 43%	3090
June	1st to 10th	Night	4 4 1 5% 3 4 4 4 7% 10% 54%	1710
	11th to 20th	Night	3 3 1 4 3 4 3 4 6% 9% 61%	1710
	21st to end	Night	4 4 1 4 4 3 4 5% 7% 8% 57%	1710
July	1st to 10th	Night	7% 4 2 4 4 4 5% 4 7% 12% 48%	2850
	11th to 20th	Night	3 2 1 4 3 3 4 5% 7% 12% 56%	2850
	21st to end	Night	11% 7% 2 7% 4 4 5% 4 6% 9% 41%	3098
August	1st to 10th	Night	2 1 2 2 2 2 3 4 7% 75%	2836
	11th to 20th	Night	3 3 1 3 3 2 3 4 6% 12% 60%	2919
	21st to end	Night	4 3 1 5% 3 5% 5% 5% 8% 10% 52%	3476
September	1st to 10th	Night	7% 5% 2 5% 3 4 4 4 5% 7% 54%	1981
	11th to 20th	Night	11% 5% 1 4 3 4 4 3 5% 8% 52%	2090
	21st to end	Night	6% 3 1 4 2 3 4 3 5% 8% 61%	2090
October	1st to 10th	Night	5% 3 1 4 3 3 4 3 5% 7% 64%	2239
	11th to 20th	Night	12% 4 1 3 2 2 2 3 3 5% 61%	2280
	21st to end	Night	15% 4 2 4 2 3 2 2 3 5% 58%	2508
November	1st to 10th	Night	11% 6% 2 5% 3 3 3 2 4 6% 56%	2340
	11th to 20th	Night	9% 4 3 6% 4 3 3 3 4 5% 57%	2400
	21st to end	Night	15% 6% 3 5% 3 3 3 3 4 5% 51%	2525
December	1st to 10th	Night	18% 6% 2 5% 3 3 3 3 3 5% 49%	4250
	11th to 20th	Night	18% 8% 2 6% 4 4 3 3 3 4 46%	4816
	21st to end	Night	15% 9% 3 6% 4 3 3 2 3 4 49%	6490

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	42% 2 6% 25% 20% 3	4078
	11th to 20th	Day	31% 3 6% 21% 31% 8%	4080
	21st to end	Day	48% 3 3 13% 21% 13%	4488
February	1st to 10th	Day	49% 3 5% 15% 18% 10%	1800
	11th to 20th	Day	49% 3 5% 18% 18% 7%	1924
	21st to end	Day	45% 4 6% 13% 21% 11%	1815
March	1st to 10th	Day	48% 5% 7% 14% 20% 6%	3546
	11th to 20th	Day	45% 5% 8% 20% 16% 6%	3590
	21st to end	Day	45% 5% 8% 19% 21% 3	3880
April	1st to 10th	Day	40% 5% 9% 21% 17% 8%	2123
	11th to 20th	Day	41% 6% 10% 17% 18% 8%	2254
	21st to end	Day	38% 6% 10% 20% 23% 3	2280
May	1st to 10th	Day	48% 7% 11% 20% 10% 4	3960
	11th to 20th	Day	38% 6% 11% 21% 23% 1	4086
	21st to end	Day	44% 8% 16% 19% 11% 3	4489
June	1st to 10th	Day	33% 8% 16% 28% 14% 1	2470
	11th to 20th	Day	30% 8% 17% 26% 16% 3	2470
	21st to end	Day	29% 8% 15% 29% 17% 3	2470
July	1st to 10th	Day	36% 9% 23% 24% 8%	4056
	11th to 20th	Day	26% 6% 17% 36% 15%	4051
	21st to end	Day	45% 7% 14% 24% 9%	4325
August	1st to 10th	Day	13% 5% 15% 33% 29% 4	4090
	11th to 20th	Day	21% 6% 20% 38% 14% 1	3953
	21st to end	Day	33% 8% 19% 24% 15% 1	4179
September	1st to 10th	Day	35% 7% 12% 19% 16% 11%	2195
	11th to 20th	Day	37% 7% 12% 18% 18% 8%	2090
	21st to end	Day	25% 6% 10% 22% 25% 12%	2090
October	1st to 10th	Day	15% 3 6% 13% 25% 39%	3193
	11th to 20th	Day	20% 2 5% 17% 31% 25%	3140
	21st to end	Day	27% 3 4 21% 33% 10%	3428
November	1st to 10th	Day	36% 4 9% 18% 24% 9%	1958
	11th to 20th	Day	33% 4 7% 18% 27% 12%	2000
	21st to end	Day	42% 3 6% 16% 25% 8%	1870
December	1st to 10th	Day	40% 2 6% 22% 25% 5%	2940
	11th to 20th	Day	45% 3 6% 14% 22% 10%	3389
	21st to end	Day	44% 4 5% 18% 25% 5%	4488
January	1st to 10th	Night	36% 3 6% 23% 27% 5%	5888
	11th to 20th	Night	28% 2 5% 19% 33% 13%	5900
	21st to end	Night	50% 2 5% 12% 18% 13%	6490
February	1st to 10th	Night	40% 4 7% 19% 18% 12%	2600
	11th to 20th	Night	44% 3 6% 17% 21% 9%	2386
	21st to end	Night	41% 4 8% 14% 20% 13%	1815
March	1st to 10th	Night	48% 5% 6% 13% 17% 10%	3529
	11th to 20th	Night	43% 5% 10% 16% 19% 7%	3590
	21st to end	Night	38% 4 10% 18% 23% 5%	3889
April	1st to 10th	Night	33% 5% 12% 20% 18% 12%	2057
	11th to 20th	Night	36% 6% 10% 18% 19% 12%	1875
	21st to end	Night	32% 4 10% 21% 27% 5%	1900
May	1st to 10th	Night	43% 8% 11% 20% 13% 5%	2988
	11th to 20th	Night	37% 6% 13% 19% 22% 3	2824
	21st to end	Night	36% 8% 15% 25% 13% 3	3099
June	1st to 10th	Night	22% 8% 17% 30% 20% 2	1710
	11th to 20th	Night	20% 6% 18% 31% 21% 5%	1710
	21st to end	Night	22% 8% 15% 27% 25% 3	1710
July	1st to 10th	Night	29% 6% 24% 33% 8%	2805
	11th to 20th	Night	20% 5% 21% 37% 16% 2	2796
	21st to end	Night	40% 6% 15% 28% 10% 1	2999
August	1st to 10th	Night	10% 4 14% 33% 32% 7%	2826
	11th to 20th	Night	17% 5% 17% 43% 15% 3	2946
	21st to end	Night	26% 7% 18% 25% 22% 2	3476
September	1st to 10th	Night	29% 7% 13% 22% 19% 10%	1981
	11th to 20th	Night	30% 7% 12% 21% 20% 10%	2090
	21st to end	Night	22% 4 10% 22% 29% 13%	2090
October	1st to 10th	Night	13% 3 5% 14% 19% 46%	3733
	11th to 20th	Night	16% 3 4 16% 34% 28%	3779
	21st to end	Night	26% 3 4 18% 32% 16%	4120
November	1st to 10th	Night	31% 4 9% 17% 27% 12%	2340
	11th to 20th	Night	30% 4 8% 18% 26% 15%	2400
	21st to end	Night	37% 3 7% 18% 24% 11%	2525
December	1st to 10th	Night	39% 3 6% 18% 28% 6%	4250
	11th to 20th	Night	44% 4 7% 13% 23% 10%	4817
	21st to end	Night	40% 3 5% 17% 27% 7%	6490

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% 16% 38% 16% 5% 1	300
	11th to 20th	Day	24% 21% 33% 17% 4	300
	21st to end	Day	25% 22% 33% 16% 3	330
February	1st to 10th	Day	29% 16% 35% 13% 6%	300
	11th to 20th	Day	35% 19% 34% 11% 1	300
	21st to end	Day	34% 23% 31% 11% 1	249
March	1st to 10th	Day	24% 23% 32% 17% 4	300
	11th to 20th	Day	23% 27% 34% 13% 3	300
	21st to end	Day	20% 22% 36% 18% 4	315
April	1st to 10th	Day	22% 15% 41% 19% 4	303
	11th to 20th	Day	22% 18% 29% 21% 8% 1	452
	21st to end	Day	17% 15% 34% 23% 8% 2	460
May	1st to 10th	Day	13% 13% 35% 30% 9%	558
	11th to 20th	Day	14% 7% 27% 35% 15% 3	675
	21st to end	Day	12% 7% 25% 35% 16% 5%	734
June	1st to 10th	Day	7% 3 15% 37% 31% 8%	680
	11th to 20th	Day	5% 2 16% 39% 31% 7%	680
	21st to end	Day	5% 1 13% 35% 34% 12%	680
July	1st to 10th	Day	8% 2 11% 32% 36% 11%	671
	11th to 20th	Day	3 1 6% 31% 46% 13%	668
	21st to end	Day	8% 2 5% 30% 42% 13%	739
August	1st to 10th	Day	5% 1 5% 26% 49% 14%	676
	11th to 20th	Day	8% 1 3 34% 42% 12%	590
	21st to end	Day	9% 1 6% 23% 40% 21%	539
September	1st to 10th	Day	10% 1 7% 28% 40% 14%	405
	11th to 20th	Day	6% 4 9% 32% 34% 15%	300
	21st to end	Day	12% 2 13% 36% 24% 14%	300
October	1st to 10th	Day	6% 3 20% 41% 23% 6%	300
	11th to 20th	Day	12% 8% 29% 34% 14% 4	300
	21st to end	Day	22% 12% 26% 23% 13% 3	330
November	1st to 10th	Day	17% 9% 31% 30% 12% 1	300
	11th to 20th	Day	14% 11% 33% 34% 7% 2	300
	21st to end	Day	20% 18% 32% 20% 9%	300
December	1st to 10th	Day	19% 15% 37% 20% 8% 2	300
	11th to 20th	Day	25% 16% 35% 17% 6% 1	300
	21st to end	Day	23% 18% 38% 16% 5% 1	330
January	1st to 10th	Night	19% 11% 34% 28% 7%	1000
	11th to 20th	Night	22% 15% 32% 26% 6%	1000
	21st to end	Night	22% 11% 35% 28% 4	1100
February	1st to 10th	Night	23% 12% 35% 26% 5%	1000
	11th to 20th	Night	26% 14% 34% 23% 3	831
	21st to end	Night	22% 16% 38% 20% 4	495
March	1st to 10th	Night	21% 11% 34% 29% 5%	600
	11th to 20th	Night	17% 14% 36% 28% 5%	600
	21st to end	Night	15% 12% 39% 28% 6%	647
April	1st to 10th	Night	12% 14% 41% 26% 6%	570
	11th to 20th	Night	15% 13% 36% 25% 10% 1	563
	21st to end	Night	12% 10% 40% 26% 11% 1	570
May	1st to 10th	Night	11% 10% 38% 30% 11% 1	570
	11th to 20th	Night	10% 6% 31% 35% 16% 2	561
	21st to end	Night	9% 4 26% 37% 20% 3	617
June	1st to 10th	Night	4 2 11% 44% 36% 3	570
	11th to 20th	Night	3 2 16% 46% 31% 1	570
	21st to end	Night	4 1 14% 36% 38% 6%	570
July	1st to 10th	Night	5% 2 10% 38% 38% 7%	555
	11th to 20th	Night	1 1 6% 39% 46% 6%	552
	21st to end	Night	3 8% 38% 42% 8%	624
August	1st to 10th	Night	3 1 4 39% 46% 7%	567
	11th to 20th	Night	5% 1 6% 38% 44% 6%	564
	21st to end	Night	6% 1 6% 36% 45% 6%	627
September	1st to 10th	Night	9% 2 7% 32% 43% 6%	570
	11th to 20th	Night	12% 2 8% 32% 39% 7%	570
	21st to end	Night	8% 2 9% 41% 36% 5%	570
October	1st to 10th	Night	7% 2 14% 40% 32% 5%	719
	11th to 20th	Night	14% 5% 24% 36% 18% 3	760
	21st to end	Night	16% 7% 27% 31% 18% 1	836
November	1st to 10th	Night	13% 5% 25% 39% 17% 1	780
	11th to 20th	Night	10% 10% 31% 38% 11% 1	800
	21st to end	Night	16% 12% 32% 27% 12%	925
December	1st to 10th	Night	17% 11% 32% 33% 7%	1000
	11th to 20th	Night	23% 10% 29% 33% 6%	1000
	21st to end	Night	20% 12% 32% 29% 6%	1100

Page generated at 2025-03-24 08:04:32. Version ac07

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