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 2025-02-06.

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% 7% 39%	108326
	Night	15% 9% 3 7% 4 4 4 3 5% 6% 40%	110086
Transparency	Day	25% 45% 7% 14% 9% 1	111958
	Night	26% 42% 6% 14% 10% 1	114595
Seeing	Day	16% 8% 24% 25% 18% 8%	15881
	Night	17% 8% 27% 31% 15% 2	25830

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	14% 11% 8% 11% 5% 4 5% 3 5% 6% 27%	3617
	11th to 20th	Day	12% 11% 6% 14% 7% 7% 6% 5% 4 5% 23%	3620
	21st to end	Day	20% 16% 4 9% 4 4 4 3 3 4 28%	4276
February	1st to 10th	Day	17% 11% 6% 9% 4 4 5% 3 5% 6% 31%	1922
	11th to 20th	Day	16% 9% 4 10% 5% 4 4 4 5% 7% 33%	1848
	21st to end	Day	16% 12% 3 9% 4 4 4 4 4 7% 33%	1570
March	1st to 10th	Day	21% 15% 4 7% 4 3 3 2 3 4 34%	3120
	11th to 20th	Day	13% 8% 2 6% 3 3 4 4 5% 7% 44%	3259
	21st to end	Day	15% 9% 3 7% 6% 5% 5% 5% 6% 7% 32%	4165
April	1st to 10th	Day	14% 9% 3 7% 4 4 4 4 5% 7% 38%	2280
	11th to 20th	Day	15% 7% 3 6% 5% 5% 4 3 5% 6% 40%	2254
	21st to end	Day	18% 7% 2 7% 4 4 5% 3 6% 7% 37%	2342
May	1st to 10th	Day	16% 8% 3 7% 4 5% 5% 5% 7% 10% 30%	4082
	11th to 20th	Day	18% 7% 2 6% 4 4 4 3 5% 7% 39%	4081
	21st to end	Day	9% 5% 3 7% 5% 5% 5% 5% 6% 9% 41%	4750
June	1st to 10th	Day	10% 6% 2 5% 5% 4 4 4 6% 9% 45%	2660
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 5% 5% 8% 48%	2660
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 4 7% 10% 43%	2660
July	1st to 10th	Day	13% 5% 2 6% 4 4 5% 4 6% 11% 40%	4440
	11th to 20th	Day	5% 4 1 6% 5% 5% 5% 5% 7% 12% 44%	4440
	21st to end	Day	10% 6% 2 8% 5% 6% 7% 5% 8% 9% 34%	4813
August	1st to 10th	Day	7% 4 2 4 3 3 4 4 7% 10% 52%	4003
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 9% 42%	3729
	21st to end	Day	5% 4 2 5% 4 4 5% 4 6% 9% 53%	4180
September	1st to 10th	Day	11% 5% 2 6% 5% 4 5% 4 5% 7% 46%	2280
	11th to 20th	Day	9% 4 2 6% 4 3 5% 5% 6% 8% 49%	2126
	21st to end	Day	11% 5% 2 5% 3 4 4 4 5% 7% 51%	2090
October	1st to 10th	Day	11% 4 2 5% 4 3 3 3 5% 7% 53%	2090
	11th to 20th	Day	14% 6% 2 5% 5% 3 4 3 5% 5% 47%	1919
	21st to end	Day	19% 6% 3 7% 4 3 4 3 4 6% 41%	1891
November	1st to 10th	Day	10% 5% 1 6% 5% 3 3 3 5% 7% 52%	1762
	11th to 20th	Day	17% 8% 3 7% 4 3 3 2 4 6% 42%	1800
	21st to end	Day	17% 10% 4 7% 4 4 4 3 4 5% 38%	1800
December	1st to 10th	Day	12% 10% 4 7% 4 4 4 3 4 5% 45%	2807
	11th to 20th	Day	15% 12% 5% 10% 6% 4 5% 4 5% 6% 28%	3008
	21st to end	Day	29% 15% 6% 10% 5% 3 4 2 4 4 17%	3982
January	1st to 10th	Night	16% 12% 8% 11% 4 4 4 3 4 6% 28%	6348
	11th to 20th	Night	15% 16% 5% 10% 5% 6% 6% 3 3 4 26%	6360
	21st to end	Night	21% 11% 5% 8% 6% 4 3 3 4 6% 30%	6700
February	1st to 10th	Night	19% 10% 4 9% 5% 4 4 4 5% 5% 30%	2366
	11th to 20th	Night	14% 10% 3 8% 6% 4 4 4 5% 6% 36%	2218
	21st to end	Night	19% 11% 3 8% 4 3 4 2 5% 5% 35%	1884
March	1st to 10th	Night	20% 14% 4 7% 4 4 4 2 4 5% 33%	3733
	11th to 20th	Night	13% 7% 2 5% 4 3 3 3 5% 6% 49%	3714
	21st to end	Night	18% 7% 3 6% 5% 4 4 4 5% 6% 38%	3480
April	1st to 10th	Night	13% 7% 3 7% 5% 4 3 3 6% 7% 42%	1900
	11th to 20th	Night	16% 7% 2 5% 4 4 3 3 5% 7% 43%	1875
	21st to end	Night	16% 8% 2 7% 5% 5% 5% 4 5% 7% 36%	1837
May	1st to 10th	Night	15% 10% 3 8% 5% 4 5% 3 5% 8% 33%	2813
	11th to 20th	Night	17% 6% 2 6% 4 4 4 3 5% 7% 42%	2812
	21st to end	Night	10% 6% 2 7% 4 5% 5% 4 6% 9% 41%	2812
June	1st to 10th	Night	9% 5% 3 6% 4 3 5% 3 5% 8% 50%	1520
	11th to 20th	Night	8% 5% 2 5% 4 3 4 5% 6% 9% 51%	1520
	21st to end	Night	7% 5% 1 5% 4 4 6% 5% 6% 11% 47%	1520
July	1st to 10th	Night	11% 5% 2 5% 4 4 3 3 7% 9% 48%	2530
	11th to 20th	Night	5% 4 2 5% 4 4 5% 4 7% 8% 53%	2530
	21st to end	Night	8% 7% 2 8% 5% 5% 5% 4 7% 9% 38%	2750
August	1st to 10th	Night	6% 4 1 5% 4 3 5% 4 6% 8% 55%	2907
	11th to 20th	Night	6% 4 2 6% 5% 5% 5% 4 6% 8% 51%	3100
	21st to end	Night	4 4 1 4 3 4 4 4 6% 9% 57%	3476
September	1st to 10th	Night	10% 6% 2 4 3 4 3 3 5% 8% 52%	1900
	11th to 20th	Night	7% 4 2 5% 3 3 4 3 5% 8% 56%	2034
	21st to end	Night	11% 6% 2 5% 4 4 3 3 5% 7% 50%	2090
October	1st to 10th	Night	8% 4 2 4 3 3 4 2 5% 6% 57%	2090
	11th to 20th	Night	14% 6% 2 4 3 3 3 3 5% 5% 52%	2257
	21st to end	Night	17% 8% 2 6% 3 4 4 4 5% 6% 42%	2705
November	1st to 10th	Night	11% 6% 2 5% 3 4 3 3 4 6% 52%	2536
	11th to 20th	Night	13% 8% 2 6% 4 3 3 4 5% 6% 45%	2600
	21st to end	Night	19% 10% 4 6% 4 4 3 3 4 6% 38%	2600
December	1st to 10th	Night	15% 10% 3 8% 4 3 3 3 4 5% 43%	4374
	11th to 20th	Night	18% 10% 4 8% 4 4 4 3 4 6% 35%	5199
	21st to end	Night	30% 16% 4 8% 4 4 4 2 4 4 20%	6996

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% 28% 6% 18% 5% 1	3618
	11th to 20th	Day	47% 29% 8% 8% 6% 2	3620
	21st to end	Day	46% 28% 4 14% 6% 2	4276
February	1st to 10th	Day	19% 63% 5% 7% 4 2	1922
	11th to 20th	Day	19% 64% 2 9% 5%	1858
	21st to end	Day	17% 65% 3 8% 6% 1	1570
March	1st to 10th	Day	38% 40% 3 9% 8% 2	3120
	11th to 20th	Day	19% 41% 7% 21% 12% 1	3259
	21st to end	Day	26% 44% 9% 16% 5%	4165
April	1st to 10th	Day	19% 66% 4 8% 3	2280
	11th to 20th	Day	15% 66% 4 10% 3 1	2254
	21st to end	Day	16% 64% 4 8% 5% 3	2342
May	1st to 10th	Day	26% 44% 11% 15% 4	4120
	11th to 20th	Day	28% 42% 6% 21% 3	4085
	21st to end	Day	21% 42% 7% 14% 15% 1	4763
June	1st to 10th	Day	12% 65% 4 11% 6% 2	2660
	11th to 20th	Day	15% 65% 3 8% 7% 1	2660
	21st to end	Day	14% 65% 7% 8% 5% 1	2660
July	1st to 10th	Day	26% 37% 8% 20% 9% 1	4371
	11th to 20th	Day	18% 40% 9% 21% 12%	4365
	21st to end	Day	30% 39% 11% 11% 9% 1	4662
August	1st to 10th	Day	16% 39% 10% 23% 10% 1	3989
	11th to 20th	Day	24% 40% 9% 17% 10%	3765
	21st to end	Day	17% 39% 12% 23% 8% 1	4179
September	1st to 10th	Day	11% 61% 5% 12% 9% 2	2280
	11th to 20th	Day	11% 61% 7% 10% 11% 1	2126
	21st to end	Day	13% 61% 5% 12% 9%	2090
October	1st to 10th	Day	9% 37% 4 18% 30% 2	3480
	11th to 20th	Day	13% 37% 5% 17% 27% 2	3172
	21st to end	Day	21% 37% 5% 14% 22% 1	3091
November	1st to 10th	Day	17% 56% 4 14% 6% 3	1762
	11th to 20th	Day	18% 57% 3 10% 10% 3	1800
	21st to end	Day	24% 57% 5% 9% 5%	1800
December	1st to 10th	Day	25% 37% 8% 20% 10%	2807
	11th to 20th	Day	39% 33% 6% 17% 4 1	3005
	21st to end	Day	56% 27% 7% 7% 3	3982
January	1st to 10th	Night	45% 28% 7% 14% 5%	6348
	11th to 20th	Night	48% 27% 6% 8% 8% 3	6360
	21st to end	Night	42% 28% 5% 16% 8% 1	6700
February	1st to 10th	Night	20% 64% 4 7% 4 1	2366
	11th to 20th	Night	18% 64% 3 8% 7%	2219
	21st to end	Night	18% 64% 3 7% 7% 1	1884
March	1st to 10th	Night	37% 41% 4 12% 5% 1	3733
	11th to 20th	Night	17% 42% 6% 17% 19% 1	3714
	21st to end	Night	25% 43% 5% 21% 5% 1	3480
April	1st to 10th	Night	18% 66% 3 8% 4 1	1900
	11th to 20th	Night	14% 65% 4 8% 7% 1	1875
	21st to end	Night	16% 65% 4 8% 4 3	1837
May	1st to 10th	Night	29% 44% 11% 12% 3	2840
	11th to 20th	Night	25% 42% 9% 19% 6%	2813
	21st to end	Night	24% 42% 7% 14% 12% 2	2820
June	1st to 10th	Night	11% 65% 3 12% 7% 2	1520
	11th to 20th	Night	12% 65% 4 8% 8% 2	1520
	21st to end	Night	12% 64% 6% 10% 6% 2	1520
July	1st to 10th	Night	23% 38% 6% 17% 15% 1	2490
	11th to 20th	Night	15% 39% 8% 23% 15% 1	2482
	21st to end	Night	29% 39% 12% 11% 8% 1	2662
August	1st to 10th	Night	15% 39% 10% 29% 6%	2897
	11th to 20th	Night	19% 39% 10% 18% 12% 1	3130
	21st to end	Night	15% 39% 11% 20% 13% 1	3476
September	1st to 10th	Night	9% 61% 4 13% 11% 2	1900
	11th to 20th	Night	9% 60% 4 13% 13% 2	2034
	21st to end	Night	14% 60% 5% 10% 10% 1	2090
October	1st to 10th	Night	8% 37% 4 15% 34% 2	3480
	11th to 20th	Night	11% 36% 5% 17% 29% 1	3743
	21st to end	Night	17% 37% 5% 16% 19% 4	4455
November	1st to 10th	Night	20% 56% 4 11% 7% 3	2536
	11th to 20th	Night	17% 56% 4 10% 11% 2	2600
	21st to end	Night	23% 57% 4 10% 6%	2600
December	1st to 10th	Night	28% 35% 7% 18% 11%	4374
	11th to 20th	Night	33% 33% 9% 15% 9%	5201
	21st to end	Night	54% 27% 7% 9% 3 1	6996

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% 34% 20% 6% 1	300
	11th to 20th	Day	27% 15% 34% 20% 3	300
	21st to end	Day	24% 16% 35% 21% 5%	330
February	1st to 10th	Day	25% 17% 36% 18% 4	285
	11th to 20th	Day	21% 20% 35% 17% 6% 1	270
	21st to end	Day	28% 12% 37% 18% 4	225
March	1st to 10th	Day	23% 19% 34% 21% 4	270
	11th to 20th	Day	25% 16% 36% 17% 6%	316
	21st to end	Day	28% 15% 33% 17% 7%	506
April	1st to 10th	Day	18% 15% 33% 25% 8% 1	460
	11th to 20th	Day	19% 14% 27% 24% 14% 2	452
	21st to end	Day	23% 13% 35% 18% 8% 3	460
May	1st to 10th	Day	20% 13% 27% 25% 11% 3	463
	11th to 20th	Day	19% 7% 24% 25% 16% 9%	487
	21st to end	Day	11% 5% 26% 27% 19% 13%	686
June	1st to 10th	Day	13% 5% 20% 29% 25% 7%	680
	11th to 20th	Day	10% 4 21% 32% 26% 8%	680
	21st to end	Day	11% 3 25% 28% 24% 10%	680
July	1st to 10th	Day	12% 4 12% 26% 25% 22%	671
	11th to 20th	Day	7% 2 15% 32% 29% 15%	668
	21st to end	Day	12% 1 13% 29% 32% 12%	739
August	1st to 10th	Day	10% 2 12% 26% 32% 18%	496
	11th to 20th	Day	13% 3 14% 29% 30% 12%	479
	21st to end	Day	6% 1 10% 26% 36% 20%	539
September	1st to 10th	Day	11% 3 14% 24% 35% 13%	490
	11th to 20th	Day	8% 1 10% 28% 37% 16%	490
	21st to end	Day	14% 4 23% 23% 22% 13%	490
October	1st to 10th	Day	10% 3 23% 32% 23% 9%	490
	11th to 20th	Day	13% 13% 34% 23% 13% 5%	319
	21st to end	Day	27% 12% 31% 23% 6%	330
November	1st to 10th	Day	11% 11% 33% 26% 14% 4	300
	11th to 20th	Day	19% 19% 30% 19% 10% 3	300
	21st to end	Day	24% 11% 30% 27% 8%	300
December	1st to 10th	Day	17% 17% 37% 20% 6% 3	300
	11th to 20th	Day	22% 12% 36% 22% 7% 1	300
	21st to end	Day	32% 15% 29% 18% 5% 1	330
January	1st to 10th	Night	21% 13% 35% 27% 4	1000
	11th to 20th	Night	26% 12% 37% 22% 4	1000
	21st to end	Night	21% 12% 37% 27% 3	1100
February	1st to 10th	Night	24% 11% 37% 25% 3	806
	11th to 20th	Night	20% 15% 36% 26% 3	743
	21st to end	Night	26% 11% 37% 24% 2	628
March	1st to 10th	Night	22% 12% 35% 26% 5%	760
	11th to 20th	Night	22% 11% 36% 25% 6%	760
	21st to end	Night	22% 9% 34% 29% 6%	641
April	1st to 10th	Night	15% 11% 33% 35% 5%	570
	11th to 20th	Night	19% 10% 26% 33% 12%	563
	21st to end	Night	21% 6% 32% 33% 6% 1	570
May	1st to 10th	Night	18% 6% 31% 34% 11% 1	570
	11th to 20th	Night	17% 4 24% 36% 17% 2	561
	21st to end	Night	11% 3 19% 39% 26% 2	617
June	1st to 10th	Night	11% 3 19% 42% 24% 1	570
	11th to 20th	Night	9% 2 17% 47% 23% 2	570
	21st to end	Night	7% 2 21% 40% 28% 2	570
July	1st to 10th	Night	11% 1 11% 32% 37% 7%	555
	11th to 20th	Night	6% 3 12% 41% 33% 5%	552
	21st to end	Night	8% 2 10% 42% 34% 5%	624
August	1st to 10th	Night	8% 1 9% 40% 37% 5%	567
	11th to 20th	Night	7% 1 10% 42% 36% 4	564
	21st to end	Night	7% 1 11% 33% 43% 6%	627
September	1st to 10th	Night	11% 3 12% 36% 33% 5%	570
	11th to 20th	Night	7% 2 13% 35% 37% 6%	704
	21st to end	Night	14% 5% 18% 32% 27% 4	760
October	1st to 10th	Night	9% 4 21% 38% 24% 4	760
	11th to 20th	Night	18% 9% 29% 28% 15% 2	760
	21st to end	Night	20% 10% 34% 28% 8%	836
November	1st to 10th	Night	14% 8% 30% 33% 13% 1	780
	11th to 20th	Night	16% 10% 34% 31% 9%	800
	21st to end	Night	25% 11% 29% 27% 8%	800
December	1st to 10th	Night	24% 7% 38% 25% 5% 1	872
	11th to 20th	Night	24% 8% 32% 28% 6% 1	1000
	21st to end	Night	32% 13% 30% 22% 4	1100

Page generated at 2025-02-03 02:05:53. Version ac07

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