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 2024-01-31.

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	14% 7% 3 6% 5% 3 5% 5% 5% 7% 39%	74975
	Night	15% 8% 3 6% 5% 3 4 4 5% 7% 41%	75171
Transparency	Day	29% 37% 7% 15% 10% 2	74677
	Night	29% 37% 7% 15% 10% 2	74990
Seeing	Day	16% 8% 24% 25% 19% 8%	14487
	Night	18% 8% 27% 31% 15% 2	24460

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% 10% 6% 8% 6% 3 5% 4 4 6% 31%	1517
	11th to 20th	Day	20% 12% 6% 9% 6% 4 3 4 4 5% 28%	1520
	21st to end	Day	17% 14% 6% 7% 5% 3 4 4 4 6% 31%	1781
February	1st to 10th	Day	18% 11% 6% 9% 4 3 4 4 5% 6% 30%	1775
	11th to 20th	Day	17% 8% 5% 9% 6% 3 5% 5% 4 7% 31%	1748
	21st to end	Day	16% 12% 4 8% 5% 3 5% 4 4 7% 32%	1480
March	1st to 10th	Day	18% 8% 3 6% 5% 4 4 4 4 6% 38%	1800
	11th to 20th	Day	18% 10% 4 7% 5% 3 4 5% 4 7% 34%	1843
	21st to end	Day	21% 11% 4 7% 6% 3 4 5% 4 6% 29%	2376
April	1st to 10th	Day	14% 8% 3 6% 5% 3 4 4 5% 8% 39%	2160
	11th to 20th	Day	16% 7% 3 6% 5% 4 5% 4 5% 6% 40%	2134
	21st to end	Day	19% 7% 2 6% 6% 3 5% 4 5% 7% 37%	2218
May	1st to 10th	Day	17% 7% 4 6% 5% 4 5% 6% 5% 7% 34%	2340
	11th to 20th	Day	18% 7% 3 6% 6% 3 4 4 5% 7% 38%	2301
	21st to end	Day	9% 5% 3 6% 7% 4 6% 7% 6% 9% 37%	2679
June	1st to 10th	Day	10% 5% 2 5% 5% 3 4 5% 6% 10% 46%	2520
	11th to 20th	Day	8% 4 2 5% 5% 4 6% 6% 5% 8% 47%	2520
	21st to end	Day	8% 5% 2 5% 5% 3 6% 5% 7% 11% 43%	2520
July	1st to 10th	Day	10% 4 2 5% 5% 3 6% 7% 7% 12% 40%	2520
	11th to 20th	Day	6% 4 2 5% 5% 3 5% 6% 7% 11% 45%	2520
	21st to end	Day	9% 4 2 4 5% 3 6% 6% 7% 9% 45%	2743
August	1st to 10th	Day	8% 4 2 4 5% 3 5% 6% 7% 10% 45%	2282
	11th to 20th	Day	9% 5% 2 4 5% 4 5% 6% 7% 9% 42%	2125
	21st to end	Day	6% 4 2 4 5% 3 5% 6% 6% 8% 51%	2376
September	1st to 10th	Day	12% 5% 2 5% 5% 3 5% 5% 5% 7% 44%	2160
	11th to 20th	Day	9% 4 2 5% 5% 3 4 6% 6% 8% 49%	2015
	21st to end	Day	10% 4 2 4 4 2 4 5% 5% 7% 53%	1980
October	1st to 10th	Day	11% 4 2 4 5% 3 3 4 4 7% 51%	1980
	11th to 20th	Day	15% 6% 2 5% 6% 3 4 4 5% 6% 45%	1818
	21st to end	Day	20% 6% 3 6% 5% 2 4 4 4 6% 41%	1792
November	1st to 10th	Day	11% 4 2 5% 5% 2 4 3 5% 7% 52%	1672
	11th to 20th	Day	17% 7% 4 6% 4 2 4 3 4 7% 42%	1710
	21st to end	Day	18% 10% 4 6% 4 2 5% 4 4 5% 39%	1710
December	1st to 10th	Day	18% 11% 5% 7% 5% 3 4 4 3 5% 34%	1639
	11th to 20th	Day	18% 11% 6% 9% 5% 3 4 4 5% 6% 29%	1918
	21st to end	Day	25% 16% 6% 9% 6% 3 4 4 4 5% 18%	2783
January	1st to 10th	Night	16% 9% 5% 8% 5% 3 5% 4 4 6% 35%	2648
	11th to 20th	Night	18% 11% 6% 9% 7% 3 4 3 3 5% 31%	2660
	21st to end	Night	17% 11% 5% 7% 6% 3 4 4 5% 5% 32%	2800
February	1st to 10th	Night	20% 10% 5% 9% 6% 3 4 4 5% 5% 29%	2182
	11th to 20th	Night	16% 9% 4 8% 7% 3 4 5% 5% 6% 35%	2098
	21st to end	Night	20% 11% 3 7% 4 2 4 4 5% 6% 35%	1776
March	1st to 10th	Night	18% 8% 4 6% 4 3 5% 4 4 5% 39%	2160
	11th to 20th	Night	18% 8% 4 6% 5% 3 4 3 5% 7% 38%	2116
	21st to end	Night	22% 7% 4 6% 5% 3 4 4 5% 6% 34%	1994
April	1st to 10th	Night	12% 6% 3 6% 6% 3 4 4 6% 7% 43%	1800
	11th to 20th	Night	16% 7% 3 5% 4 3 3 4 5% 8% 43%	1775
	21st to end	Night	17% 7% 3 6% 5% 4 5% 5% 5% 7% 36%	1741
May	1st to 10th	Night	16% 8% 3 6% 6% 3 5% 4 5% 8% 37%	1620
	11th to 20th	Night	14% 6% 3 6% 6% 4 4 5% 6% 7% 41%	1592
	21st to end	Night	9% 5% 3 7% 5% 4 6% 5% 7% 10% 40%	1594
June	1st to 10th	Night	9% 5% 3 5% 4 3 4 4 5% 8% 50%	1440
	11th to 20th	Night	8% 4 2 5% 4 2 4 5% 6% 9% 52%	1440
	21st to end	Night	7% 4 2 4 5% 3 6% 6% 6% 11% 47%	1440
July	1st to 10th	Night	7% 5% 3 4 4 3 4 4 8% 10% 48%	1440
	11th to 20th	Night	6% 4 2 4 5% 3 4 4 8% 7% 53%	1440
	21st to end	Night	6% 5% 1 5% 4 3 4 4 5% 9% 54%	1575
August	1st to 10th	Night	7% 5% 1 4 6% 2 5% 5% 7% 9% 49%	1651
	11th to 20th	Night	6% 3 2 5% 6% 3 5% 6% 5% 9% 50%	1776
	21st to end	Night	6% 4 2 4 3 3 3 5% 5% 8% 56%	1980
September	1st to 10th	Night	11% 6% 2 4 4 3 4 3 5% 9% 51%	1800
	11th to 20th	Night	7% 3 2 4 4 2 3 5% 5% 8% 56%	1926
	21st to end	Night	11% 5% 2 4 5% 3 3 3 5% 7% 52%	1980
October	1st to 10th	Night	8% 4 2 4 4 3 4 3 5% 6% 56%	1980
	11th to 20th	Night	15% 6% 2 4 4 2 3 4 5% 5% 50%	2138
	21st to end	Night	18% 8% 3 5% 5% 3 4 5% 4 6% 41%	2562
November	1st to 10th	Night	11% 5% 3 5% 3 3 4 4 4 6% 52%	2406
	11th to 20th	Night	13% 7% 3 5% 5% 2 4 4 4 7% 46%	2470
	21st to end	Night	19% 9% 4 6% 5% 3 4 3 4 5% 39%	2470
December	1st to 10th	Night	17% 11% 4 7% 6% 3 4 4 4 8% 33%	2538
	11th to 20th	Night	18% 10% 4 7% 5% 3 4 4 3 5% 37%	3279
	21st to end	Night	26% 17% 5% 7% 6% 3 4 4 3 4 21%	4884

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	34% 39% 7% 12% 7%	1518
	11th to 20th	Day	36% 40% 6% 12% 5%	1520
	21st to end	Day	39% 36% 7% 14% 3 1	1781
February	1st to 10th	Day	36% 36% 6% 15% 6%	1775
	11th to 20th	Day	33% 38% 8% 13% 6% 2	1758
	21st to end	Day	37% 36% 5% 15% 5% 2	1480
March	1st to 10th	Day	31% 37% 6% 17% 9%	1800
	11th to 20th	Day	33% 37% 9% 13% 7% 1	1843
	21st to end	Day	38% 38% 6% 14% 4	2376
April	1st to 10th	Day	25% 37% 8% 17% 12% 1	2160
	11th to 20th	Day	31% 36% 6% 16% 10% 2	2134
	21st to end	Day	31% 38% 8% 17% 6% 1	2218
May	1st to 10th	Day	34% 37% 8% 14% 6%	2340
	11th to 20th	Day	30% 38% 8% 10% 14% 1	2305
	21st to end	Day	27% 40% 9% 12% 10% 2	2679
June	1st to 10th	Day	23% 38% 9% 16% 11% 3	2520
	11th to 20th	Day	19% 38% 9% 16% 15% 3	2520
	21st to end	Day	22% 39% 9% 15% 12% 4	2520
July	1st to 10th	Day	24% 39% 10% 14% 10% 3	2451
	11th to 20th	Day	21% 39% 9% 18% 10% 2	2445
	21st to end	Day	22% 40% 11% 18% 8% 2	2591
August	1st to 10th	Day	22% 39% 7% 21% 11% 1	2268
	11th to 20th	Day	20% 37% 9% 22% 11% 1	2125
	21st to end	Day	17% 37% 7% 18% 14% 7%	2376
September	1st to 10th	Day	28% 37% 6% 14% 12% 3	2160
	11th to 20th	Day	23% 37% 5% 19% 13% 2	2015
	21st to end	Day	21% 36% 5% 16% 19% 3	1980
October	1st to 10th	Day	19% 36% 6% 18% 19% 3	1980
	11th to 20th	Day	27% 37% 6% 15% 13% 2	1818
	21st to end	Day	29% 37% 5% 19% 10% 1	1792
November	1st to 10th	Day	18% 41% 7% 19% 11% 4	1672
	11th to 20th	Day	28% 40% 6% 15% 9% 3	1710
	21st to end	Day	29% 39% 6% 15% 11% 1	1710
December	1st to 10th	Day	33% 40% 9% 13% 5%	1639
	11th to 20th	Day	41% 32% 5% 17% 5%	1915
	21st to end	Day	56% 26% 7% 7% 3	2783
January	1st to 10th	Night	33% 40% 7% 15% 5%	2648
	11th to 20th	Night	36% 40% 6% 12% 5%	2660
	21st to end	Night	38% 35% 7% 14% 6%	2800
February	1st to 10th	Night	38% 37% 8% 12% 5% 1	2182
	11th to 20th	Night	33% 37% 7% 13% 9% 2	2099
	21st to end	Night	34% 36% 9% 15% 5% 2	1776
March	1st to 10th	Night	30% 36% 5% 17% 10% 1	2160
	11th to 20th	Night	32% 36% 10% 13% 7% 1	2116
	21st to end	Night	36% 36% 9% 13% 4 1	1994
April	1st to 10th	Night	23% 37% 7% 19% 11% 2	1800
	11th to 20th	Night	27% 36% 6% 14% 13% 4	1775
	21st to end	Night	31% 38% 6% 15% 9% 1	1741
May	1st to 10th	Night	32% 37% 8% 15% 7%	1620
	11th to 20th	Night	30% 38% 6% 13% 13% 1	1593
	21st to end	Night	23% 40% 9% 13% 10% 4	1595
June	1st to 10th	Night	24% 38% 5% 15% 17% 2	1440
	11th to 20th	Night	18% 37% 9% 17% 15% 4	1440
	21st to end	Night	18% 38% 10% 13% 15% 5%	1440
July	1st to 10th	Night	20% 41% 10% 15% 11% 3	1400
	11th to 20th	Night	20% 39% 9% 16% 12% 3	1392
	21st to end	Night	20% 39% 9% 19% 11% 1	1487
August	1st to 10th	Night	19% 39% 9% 16% 14% 2	1641
	11th to 20th	Night	18% 38% 7% 20% 15% 1	1776
	21st to end	Night	16% 37% 7% 18% 16% 6%	1980
September	1st to 10th	Night	25% 36% 6% 17% 11% 5%	1800
	11th to 20th	Night	19% 36% 5% 18% 18% 4	1926
	21st to end	Night	23% 36% 5% 15% 16% 5%	1980
October	1st to 10th	Night	18% 37% 6% 14% 22% 4	1980
	11th to 20th	Night	25% 36% 4 19% 12% 3	2138
	21st to end	Night	30% 37% 5% 15% 11% 2	2562
November	1st to 10th	Night	18% 41% 6% 18% 16% 3	2406
	11th to 20th	Night	25% 40% 6% 15% 13% 2	2470
	21st to end	Night	28% 39% 7% 14% 10% 1	2470
December	1st to 10th	Night	33% 40% 7% 15% 5%	2538
	11th to 20th	Night	36% 33% 7% 14% 9%	3281
	21st to end	Night	55% 26% 6% 8% 4 1	4884

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% 33% 20% 6% 1	270
	11th to 20th	Day	29% 14% 33% 20% 3	270
	21st to end	Day	25% 16% 33% 20% 5%	294
February	1st to 10th	Day	28% 17% 36% 15% 4	240
	11th to 20th	Day	23% 20% 34% 17% 6%	240
	21st to end	Day	29% 12% 37% 18% 4 1	198
March	1st to 10th	Day	23% 18% 36% 20% 4	240
	11th to 20th	Day	28% 13% 35% 17% 6%	283
	21st to end	Day	29% 15% 32% 17% 7%	462
April	1st to 10th	Day	17% 13% 34% 26% 9% 1	420
	11th to 20th	Day	19% 14% 26% 24% 15% 2	412
	21st to end	Day	24% 12% 34% 19% 8% 3	420
May	1st to 10th	Day	21% 14% 27% 26% 10% 3	423
	11th to 20th	Day	20% 7% 24% 26% 16% 8%	447
	21st to end	Day	11% 4 25% 26% 20% 14%	633
June	1st to 10th	Day	12% 4 20% 30% 27% 7%	630
	11th to 20th	Day	10% 3 21% 31% 27% 8%	630
	21st to end	Day	11% 4 24% 27% 25% 10%	630
July	1st to 10th	Day	11% 3 11% 26% 26% 24%	621
	11th to 20th	Day	8% 2 14% 32% 29% 15%	618
	21st to end	Day	12% 1 13% 28% 33% 12%	684
August	1st to 10th	Day	11% 2 10% 26% 32% 19%	456
	11th to 20th	Day	13% 2 13% 29% 31% 13%	439
	21st to end	Day	6% 1 11% 26% 35% 20%	495
September	1st to 10th	Day	12% 3 15% 23% 35% 13%	450
	11th to 20th	Day	8% 1 10% 29% 36% 16%	450
	21st to end	Day	12% 4 23% 24% 24% 14%	450
October	1st to 10th	Day	11% 3 24% 32% 22% 8%	450
	11th to 20th	Day	14% 13% 35% 23% 13% 2	288
	21st to end	Day	29% 13% 29% 24% 5%	297
November	1st to 10th	Day	10% 10% 33% 27% 15% 4	270
	11th to 20th	Day	19% 19% 28% 21% 11% 3	270
	21st to end	Day	23% 10% 30% 28% 8%	270
December	1st to 10th	Day	17% 16% 36% 20% 6% 4	270
	11th to 20th	Day	23% 11% 36% 23% 7% 1	270
	21st to end	Day	31% 16% 29% 18% 4 1	297
January	1st to 10th	Night	22% 13% 34% 27% 5%	950
	11th to 20th	Night	26% 12% 36% 22% 4	950
	21st to end	Night	21% 13% 36% 27% 3	1043
February	1st to 10th	Night	25% 11% 36% 25% 3	742
	11th to 20th	Night	22% 15% 36% 25% 3	703
	21st to end	Night	27% 11% 36% 23% 2	592
March	1st to 10th	Night	22% 12% 35% 26% 5%	720
	11th to 20th	Night	23% 10% 35% 25% 6%	720
	21st to end	Night	23% 9% 33% 29% 7%	608
April	1st to 10th	Night	15% 11% 32% 36% 6%	540
	11th to 20th	Night	19% 10% 25% 32% 13%	533
	21st to end	Night	22% 6% 30% 34% 7% 1	540
May	1st to 10th	Night	18% 6% 31% 34% 11% 1	540
	11th to 20th	Night	16% 4 25% 37% 17% 2	531
	21st to end	Night	10% 2 19% 38% 28% 2	584
June	1st to 10th	Night	11% 3 19% 42% 25% 1	540
	11th to 20th	Night	9% 1 17% 47% 24% 2	540
	21st to end	Night	7% 2 20% 40% 29% 1	540
July	1st to 10th	Night	11% 1 10% 32% 39% 8%	525
	11th to 20th	Night	6% 3 11% 41% 33% 5%	522
	21st to end	Night	7% 2 10% 41% 35% 5%	591
August	1st to 10th	Night	9% 1 8% 39% 37% 5%	537
	11th to 20th	Night	7% 9% 42% 37% 5%	534
	21st to end	Night	7% 1 11% 32% 42% 6%	594
September	1st to 10th	Night	12% 3 12% 35% 34% 5%	540
	11th to 20th	Night	8% 2 14% 34% 36% 6%	666
	21st to end	Night	13% 4 18% 32% 28% 4	720
October	1st to 10th	Night	9% 5% 21% 37% 24% 4	720
	11th to 20th	Night	18% 8% 30% 28% 14% 2	720
	21st to end	Night	21% 10% 33% 28% 8%	792
November	1st to 10th	Night	14% 7% 31% 33% 14% 1	740
	11th to 20th	Night	16% 11% 33% 31% 9%	760
	21st to end	Night	24% 11% 29% 28% 8%	760
December	1st to 10th	Night	24% 7% 37% 26% 6% 1	828
	11th to 20th	Night	25% 8% 31% 29% 7% 1	950
	21st to end	Night	31% 13% 30% 22% 4	1045

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