West Union
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the West Union clear sky chart from 2005-11-06 to 2026-06-16.

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% 8% 3 7% 5% 4 5% 4 6% 8% 38%	137248
	Night	14% 9% 3 7% 4 4 4 3 5% 7% 39%	134995
Transparency	Day	29% 37% 8% 16% 10% 1	140835
	Night	29% 36% 7% 16% 11% 1	139485
Seeing	Day	16% 9% 25% 24% 18% 7%	17757
	Night	17% 8% 27% 31% 15% 2	27604

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	12% 10% 7% 10% 5% 4 5% 3 5% 6% 31%	4691
	11th to 20th	Day	10% 11% 5% 14% 7% 7% 6% 4 5% 5% 27%	4710
	21st to end	Day	18% 14% 4 11% 6% 5% 4 3 4 4 26%	5532
February	1st to 10th	Day	16% 11% 6% 9% 4 4 4 4 5% 6% 31%	2071
	11th to 20th	Day	16% 9% 4 10% 5% 4 4 4 4 7% 32%	2048
	21st to end	Day	15% 12% 3 9% 4 4 4 3 4 7% 34%	1730
March	1st to 10th	Day	18% 13% 4 7% 4 3 3 2 3 4 39%	4580
	11th to 20th	Day	11% 10% 2 6% 3 3 4 4 5% 9% 43%	4763
	21st to end	Day	16% 10% 3 7% 6% 5% 5% 4 6% 7% 32%	6099
April	1st to 10th	Day	15% 9% 3 7% 5% 4 4 4 5% 7% 37%	2520
	11th to 20th	Day	15% 8% 3 7% 5% 5% 5% 3 5% 6% 40%	2494
	21st to end	Day	17% 7% 2 7% 5% 4 5% 3 6% 7% 37%	2588
May	1st to 10th	Day	16% 7% 3 8% 4 5% 5% 5% 7% 10% 30%	7616
	11th to 20th	Day	15% 8% 2 7% 5% 4 4 3 5% 7% 41%	7626
	21st to end	Day	13% 8% 4 8% 6% 6% 6% 5% 7% 9% 29%	8903
June	1st to 10th	Day	10% 6% 2 5% 5% 4 4 4 6% 9% 45%	2940
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 5% 5% 8% 46%	2871
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 4 7% 10% 43%	2800
July	1st to 10th	Day	10% 5% 2 5% 4 4 4 4 7% 11% 44%	6360
	11th to 20th	Day	7% 6% 2 8% 6% 6% 6% 5% 8% 12% 36%	6360
	21st to end	Day	10% 6% 2 8% 5% 5% 7% 5% 8% 9% 33%	6924
August	1st to 10th	Day	7% 4 2 4 3 3 4 4 6% 10% 52%	4130
	11th to 20th	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 9% 42%	3849
	21st to end	Day	5% 4 2 5% 4 4 5% 4 6% 9% 53%	4312
September	1st to 10th	Day	11% 5% 2 5% 5% 4 5% 4 6% 7% 47%	2400
	11th to 20th	Day	8% 4 2 6% 4 3 5% 5% 6% 8% 50%	2238
	21st to end	Day	11% 5% 2 5% 3 4 4 4 5% 7% 51%	2200
October	1st to 10th	Day	10% 4 2 5% 4 4 3 3 5% 7% 53%	2200
	11th to 20th	Day	14% 6% 2 5% 5% 4 4 3 5% 5% 47%	2020
	21st to end	Day	19% 6% 3 7% 4 3 3 3 5% 6% 42%	1990
November	1st to 10th	Day	10% 6% 1 6% 5% 3 3 3 5% 7% 51%	1852
	11th to 20th	Day	16% 8% 3 7% 4 3 3 2 4 6% 42%	1890
	21st to end	Day	18% 10% 4 7% 4 4 4 3 4 5% 37%	1890
December	1st to 10th	Day	11% 10% 4 8% 4 4 4 3 4 5% 44%	2893
	11th to 20th	Day	15% 12% 4 10% 6% 4 5% 4 5% 6% 29%	3088
	21st to end	Day	29% 15% 6% 10% 5% 3 4 3 4 4 17%	4070
January	1st to 10th	Night	15% 12% 7% 11% 4 4 4 3 4 5% 32%	8232
	11th to 20th	Night	12% 14% 5% 10% 5% 6% 6% 3 3 5% 30%	8280
	21st to end	Night	18% 10% 5% 8% 5% 4 4 3 5% 6% 31%	8667
February	1st to 10th	Night	18% 10% 4 9% 5% 4 4 4 5% 6% 30%	2546
	11th to 20th	Night	16% 10% 3 8% 6% 4 4 4 5% 6% 35%	2458
	21st to end	Night	18% 10% 3 8% 4 3 4 2 5% 5% 37%	2076
March	1st to 10th	Night	19% 13% 4 7% 4 3 3 2 3 5% 36%	5493
	11th to 20th	Night	13% 6% 2 6% 4 3 3 3 5% 7% 47%	5430
	21st to end	Night	17% 8% 3 8% 5% 4 5% 4 6% 6% 34%	5101
April	1st to 10th	Night	13% 7% 3 7% 5% 4 3 3 5% 7% 42%	2100
	11th to 20th	Night	16% 7% 2 5% 4 4 4 3 5% 7% 43%	2075
	21st to end	Night	15% 7% 2 7% 5% 5% 5% 4 5% 7% 37%	2030
May	1st to 10th	Night	17% 9% 3 8% 5% 5% 5% 3 5% 8% 33%	5237
	11th to 20th	Night	14% 6% 2 6% 4 4 4 3 6% 8% 41%	5234
	21st to end	Night	13% 8% 3 9% 5% 5% 5% 4 7% 9% 32%	5260
June	1st to 10th	Night	10% 5% 3 6% 4 3 4 3 5% 8% 49%	1680
	11th to 20th	Night	9% 5% 2 5% 4 3 4 4 6% 8% 50%	1640
	21st to end	Night	6% 5% 1 5% 4 4 5% 5% 6% 11% 47%	1600
July	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 7% 8% 48%	3618
	11th to 20th	Night	6% 6% 2 6% 5% 5% 5% 4 8% 9% 44%	3617
	21st to end	Night	7% 6% 2 8% 5% 5% 5% 5% 8% 10% 38%	3947
August	1st to 10th	Night	5% 4 1 5% 4 3 5% 4 6% 8% 55%	3000
	11th to 20th	Night	6% 4 2 6% 5% 5% 5% 4 6% 8% 52%	3200
	21st to end	Night	4 3 1 4 3 4 4 4 6% 10% 57%	3586
September	1st to 10th	Night	10% 6% 2 4 3 4 3 3 5% 8% 52%	2000
	11th to 20th	Night	6% 3 2 5% 3 3 4 3 5% 8% 57%	2141
	21st to end	Night	12% 6% 2 5% 4 4 3 3 5% 6% 51%	2200
October	1st to 10th	Night	8% 4 2 4 3 3 4 3 5% 6% 58%	2200
	11th to 20th	Night	14% 6% 2 4 3 3 4 3 5% 5% 52%	2376
	21st to end	Night	18% 7% 2 6% 3 4 4 4 4 6% 42%	2848
November	1st to 10th	Night	11% 6% 3 5% 3 4 3 4 4 6% 52%	2666
	11th to 20th	Night	13% 8% 2 6% 4 3 3 4 5% 6% 45%	2730
	21st to end	Night	19% 9% 4 6% 4 4 3 3 4 5% 38%	2730
December	1st to 10th	Night	15% 10% 3 8% 4 3 3 3 4 5% 42%	4508
	11th to 20th	Night	17% 9% 4 8% 4 4 4 3 4 6% 35%	5339
	21st to end	Night	30% 16% 4 8% 4 4 4 2 4 4 20%	7150

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	43% 22% 9% 20% 5% 1	4692
	11th to 20th	Day	45% 23% 9% 12% 9% 2	4710
	21st to end	Day	50% 23% 6% 14% 5% 3	5532
February	1st to 10th	Day	22% 59% 6% 8% 4 1	2071
	11th to 20th	Day	23% 58% 3 9% 6%	2058
	21st to end	Day	18% 59% 4 11% 7% 1	1730
March	1st to 10th	Day	39% 28% 3 13% 14% 3	4580
	11th to 20th	Day	24% 30% 7% 27% 11% 1	4763
	21st to end	Day	34% 32% 9% 18% 7% 1	6101
April	1st to 10th	Day	23% 60% 5% 8% 3	2520
	11th to 20th	Day	18% 60% 5% 12% 4 1	2494
	21st to end	Day	18% 59% 4 11% 5% 2	2588
May	1st to 10th	Day	36% 27% 12% 16% 9%	7651
	11th to 20th	Day	34% 25% 9% 20% 11%	7630
	21st to end	Day	40% 26% 9% 14% 10% 1	8917
June	1st to 10th	Day	15% 60% 5% 11% 8% 2	2940
	11th to 20th	Day	17% 61% 3 9% 8% 2	2871
	21st to end	Day	15% 63% 7% 8% 6% 1	2800
July	1st to 10th	Day	25% 28% 8% 24% 14% 1	6291
	11th to 20th	Day	30% 32% 11% 19% 9%	6285
	21st to end	Day	33% 30% 13% 16% 7%	6728
August	1st to 10th	Day	16% 38% 10% 24% 10%	4116
	11th to 20th	Day	24% 40% 10% 17% 10%	3885
	21st to end	Day	17% 38% 12% 23% 9% 1	4311
September	1st to 10th	Day	11% 58% 6% 14% 10% 2	2400
	11th to 20th	Day	11% 58% 7% 11% 13% 1	2238
	21st to end	Day	15% 58% 5% 11% 11%	2200
October	1st to 10th	Day	10% 36% 4 18% 30% 2	3590
	11th to 20th	Day	13% 36% 5% 17% 27% 2	3273
	21st to end	Day	22% 36% 5% 15% 22% 1	3190
November	1st to 10th	Day	18% 53% 4 15% 7% 3	1852
	11th to 20th	Day	19% 54% 3 11% 10% 3	1890
	21st to end	Day	25% 54% 5% 10% 6%	1890
December	1st to 10th	Day	26% 36% 8% 20% 10%	2893
	11th to 20th	Day	39% 32% 6% 17% 5% 1	3085
	21st to end	Day	56% 27% 7% 7% 3	4070
January	1st to 10th	Night	44% 22% 10% 18% 5%	8233
	11th to 20th	Night	45% 22% 7% 11% 12% 3	8280
	21st to end	Night	43% 23% 8% 18% 8% 1	8667
February	1st to 10th	Night	22% 60% 5% 9% 5% 1	2546
	11th to 20th	Night	22% 58% 4 9% 7%	2459
	21st to end	Night	19% 59% 3 10% 8% 1	2076
March	1st to 10th	Night	40% 29% 4 16% 9% 3	5493
	11th to 20th	Night	24% 30% 6% 21% 18% 1	5430
	21st to end	Night	35% 31% 6% 21% 6% 1	5101
April	1st to 10th	Night	21% 60% 4 9% 5% 1	2100
	11th to 20th	Night	17% 59% 5% 9% 8% 1	2075
	21st to end	Night	18% 60% 4 12% 5% 2	2030
May	1st to 10th	Night	38% 26% 11% 17% 7%	5264
	11th to 20th	Night	31% 25% 10% 18% 15%	5235
	21st to end	Night	39% 25% 10% 15% 9% 1	5270
June	1st to 10th	Night	14% 60% 4 12% 9% 2	1680
	11th to 20th	Night	15% 61% 4 9% 9% 2	1640
	21st to end	Night	13% 62% 6% 11% 7% 1	1600
July	1st to 10th	Night	26% 28% 6% 19% 19% 1	3580
	11th to 20th	Night	26% 31% 11% 21% 11%	3572
	21st to end	Night	30% 31% 15% 16% 7% 1	3832
August	1st to 10th	Night	15% 38% 11% 29% 7%	2990
	11th to 20th	Night	19% 38% 11% 19% 12% 1	3230
	21st to end	Night	15% 38% 11% 21% 14% 1	3586
September	1st to 10th	Night	10% 58% 4 14% 12% 2	2000
	11th to 20th	Night	9% 57% 4 13% 15% 1	2141
	21st to end	Night	16% 57% 5% 10% 11% 1	2200
October	1st to 10th	Night	8% 36% 4 15% 35% 2	3590
	11th to 20th	Night	11% 35% 6% 17% 29% 2	3862
	21st to end	Night	18% 36% 5% 16% 20% 4	4598
November	1st to 10th	Night	20% 53% 5% 11% 8% 3	2666
	11th to 20th	Night	18% 54% 4 12% 11% 2	2730
	21st to end	Night	25% 54% 4 11% 6%	2730
December	1st to 10th	Night	29% 34% 7% 18% 11%	4508
	11th to 20th	Night	33% 33% 9% 16% 9%	5341
	21st to end	Night	54% 27% 7% 9% 3 1	7150

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% 17% 33% 21% 6% 1	330
	11th to 20th	Day	26% 18% 35% 19% 3	330
	21st to end	Day	23% 17% 35% 20% 4	363
February	1st to 10th	Day	23% 16% 38% 18% 4	330
	11th to 20th	Day	24% 21% 35% 15% 5% 1	330
	21st to end	Day	26% 13% 39% 18% 4	273
March	1st to 10th	Day	23% 21% 33% 19% 4	330
	11th to 20th	Day	27% 15% 38% 15% 6%	381
	21st to end	Day	27% 17% 33% 17% 6%	594
April	1st to 10th	Day	19% 15% 34% 24% 7% 1	540
	11th to 20th	Day	19% 16% 27% 23% 12% 2	532
	21st to end	Day	21% 12% 35% 21% 8% 3	540
May	1st to 10th	Day	20% 14% 27% 26% 11% 3	543
	11th to 20th	Day	19% 8% 26% 24% 14% 8%	567
	21st to end	Day	12% 6% 26% 26% 18% 11%	791
June	1st to 10th	Day	13% 5% 22% 29% 25% 7%	780
	11th to 20th	Day	11% 4 22% 31% 24% 7%	756
	21st to end	Day	10% 3 23% 28% 26% 10%	730
July	1st to 10th	Day	11% 3 11% 26% 26% 22%	721
	11th to 20th	Day	8% 3 15% 32% 29% 14%	718
	21st to end	Day	11% 2 13% 29% 33% 12%	794
August	1st to 10th	Day	10% 2 13% 25% 32% 18%	536
	11th to 20th	Day	12% 2 13% 28% 32% 12%	519
	21st to end	Day	6% 1 11% 26% 37% 19%	583
September	1st to 10th	Day	10% 3 15% 25% 34% 13%	530
	11th to 20th	Day	8% 1 10% 27% 37% 17%	530
	21st to end	Day	14% 4 22% 23% 23% 14%	530
October	1st to 10th	Day	10% 3 21% 32% 24% 9%	530
	11th to 20th	Day	13% 13% 31% 23% 15% 5%	350
	21st to end	Day	28% 13% 31% 22% 6%	363
November	1st to 10th	Day	11% 13% 33% 26% 13% 3	330
	11th to 20th	Day	19% 18% 31% 19% 10% 2	330
	21st to end	Day	25% 12% 29% 26% 8%	330
December	1st to 10th	Day	16% 17% 38% 20% 5% 3	330
	11th to 20th	Day	22% 13% 37% 22% 6% 1	330
	21st to end	Day	31% 15% 30% 18% 4 1	363
January	1st to 10th	Night	21% 13% 35% 27% 4	1050
	11th to 20th	Night	25% 13% 38% 21% 3	1050
	21st to end	Night	20% 13% 37% 26% 3	1155
February	1st to 10th	Night	23% 11% 38% 26% 3	866
	11th to 20th	Night	22% 15% 35% 26% 3	823
	21st to end	Night	25% 11% 38% 24% 2	692
March	1st to 10th	Night	23% 14% 33% 25% 4	840
	11th to 20th	Night	22% 11% 36% 25% 6%	840
	21st to end	Night	22% 10% 34% 28% 6%	709
April	1st to 10th	Night	16% 11% 34% 34% 5%	630
	11th to 20th	Night	19% 11% 27% 31% 11%	623
	21st to end	Night	20% 7% 32% 34% 7% 1	630
May	1st to 10th	Night	18% 6% 31% 34% 11% 1	630
	11th to 20th	Night	16% 4 26% 36% 16% 2	621
	21st to end	Night	11% 4 20% 39% 25% 2	683
June	1st to 10th	Night	11% 3 20% 41% 24% 1	630
	11th to 20th	Night	10% 2 18% 45% 23% 2	615
	21st to end	Night	7% 2 21% 40% 29% 2	600
July	1st to 10th	Night	11% 1 10% 33% 38% 7%	585
	11th to 20th	Night	6% 3 13% 41% 33% 5%	582
	21st to end	Night	8% 2 10% 41% 35% 5%	657
August	1st to 10th	Night	8% 1 9% 40% 37% 5%	597
	11th to 20th	Night	6% 1 10% 41% 38% 5%	594
	21st to end	Night	6% 1 10% 35% 42% 6%	660
September	1st to 10th	Night	11% 3 12% 36% 33% 5%	600
	11th to 20th	Night	7% 2 13% 35% 37% 6%	741
	21st to end	Night	14% 5% 18% 31% 27% 5%	800
October	1st to 10th	Night	9% 5% 20% 38% 24% 4	800
	11th to 20th	Night	17% 9% 29% 29% 15% 2	800
	21st to end	Night	21% 10% 34% 27% 8%	880
November	1st to 10th	Night	14% 8% 31% 33% 13% 1	820
	11th to 20th	Night	16% 11% 34% 30% 8%	840
	21st to end	Night	25% 12% 29% 27% 8%	840
December	1st to 10th	Night	24% 7% 38% 25% 5% 1	916
	11th to 20th	Night	24% 9% 32% 28% 6% 1	1050
	21st to end	Night	32% 13% 30% 22% 3	1155

Page generated at 2026-06-13 01:31:40. Version ac07

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