Union
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Union clear sky chart from 2005-11-06 to 2026-07-16.

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	7% 8% 3 7% 4 4 4 3 5% 6% 47%	146887
	Night	9% 10% 3 7% 4 4 4 3 5% 7% 44%	143098
Transparency	Day	35% 7% 12% 23% 17% 7%	141616
	Night	38% 7% 13% 23% 14% 5%	139373
Seeing	Day	14% 18% 30% 24% 12% 3	16509
	Night	15% 11% 25% 32% 16% 2	25657

By Month

Percentage Cloud Cover SW (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	9% 18% 8% 14% 7% 7% 7% 4 7% 5% 15%	4722
	11th to 20th	Day	8% 10% 3 8% 5% 4 4 4 6% 8% 40%	4740
	21st to end	Day	8% 9% 4 9% 6% 5% 5% 4 6% 6% 40%	5187
February	1st to 10th	Day	12% 16% 5% 10% 6% 5% 4 4 6% 7% 24%	1787
	11th to 20th	Day	11% 19% 6% 10% 5% 5% 4 3 5% 5% 27%	2009
	21st to end	Day	8% 16% 6% 11% 7% 5% 4 4 5% 7% 26%	1730
March	1st to 10th	Day	6% 11% 4 12% 7% 6% 6% 5% 7% 7% 29%	4618
	11th to 20th	Day	5% 14% 5% 10% 5% 5% 6% 4 5% 7% 34%	4746
	21st to end	Day	9% 17% 5% 12% 8% 7% 7% 5% 6% 5% 19%	6101
April	1st to 10th	Day	10% 17% 4 9% 6% 5% 4 3 6% 7% 30%	2520
	11th to 20th	Day	11% 12% 4 7% 5% 5% 4 4 5% 6% 36%	2489
	21st to end	Day	11% 13% 3 7% 5% 4 5% 4 5% 6% 36%	2728
May	1st to 10th	Day	6% 7% 2 6% 4 4 4 4 5% 6% 53%	7846
	11th to 20th	Day	8% 7% 3 7% 4 4 4 3 6% 7% 46%	8160
	21st to end	Day	6% 7% 2 6% 3 3 4 4 6% 8% 51%	8992
June	1st to 10th	Day	14% 8% 2 6% 4 4 5% 4 6% 8% 39%	2940
	11th to 20th	Day	12% 8% 2 5% 4 4 4 3 5% 6% 48%	2940
	21st to end	Day	6% 3 2 4 3 4 4 3 5% 7% 61%	2940
July	1st to 10th	Day	2 2 1 3 2 2 3 3 4 4 75%	6459
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 3 81%	6391
	21st to end	Day	2 1 1 3 2 3 3 3 5% 7% 71%	6880
August	1st to 10th	Day	4 4 2 5% 3 4 5% 3 5% 6% 59%	6210
	11th to 20th	Day	3 2 1 3 2 2 3 2 4 6% 73%	5747
	21st to end	Day	4 3 1 4 3 3 4 3 5% 6% 64%	5984
September	1st to 10th	Day	4 3 1 3 2 3 3 3 4 5% 69%	2400
	11th to 20th	Day	7% 4 2 5% 4 4 4 2 4 7% 56%	2295
	21st to end	Day	8% 6% 2 5% 3 3 3 2 4 6% 56%	2200
October	1st to 10th	Day	7% 4 2 5% 3 3 2 2 3 5% 65%	3472
	11th to 20th	Day	8% 7% 2 7% 4 4 4 3 5% 8% 49%	3149
	21st to end	Day	12% 10% 4 10% 5% 5% 6% 4 5% 7% 33%	3200
November	1st to 10th	Day	17% 13% 3 6% 5% 4 4 4 5% 7% 31%	1854
	11th to 20th	Day	13% 17% 4 9% 6% 5% 5% 4 6% 7% 23%	1871
	21st to end	Day	11% 14% 4 8% 6% 4 4 3 6% 8% 32%	1680
December	1st to 10th	Day	8% 13% 4 6% 4 4 3 3 4 5% 45%	2700
	11th to 20th	Day	11% 15% 4 11% 6% 5% 5% 5% 6% 8% 21%	3108
	21st to end	Day	12% 16% 5% 10% 6% 6% 4 4 5% 8% 23%	4092
January	1st to 10th	Night	11% 19% 6% 13% 7% 6% 6% 4 7% 7% 14%	8172
	11th to 20th	Night	9% 12% 4 8% 4 3 4 3 5% 8% 40%	8220
	21st to end	Night	10% 9% 3 7% 4 5% 5% 5% 7% 8% 37%	8994
February	1st to 10th	Night	14% 17% 5% 10% 6% 5% 5% 3 5% 6% 23%	2830
	11th to 20th	Night	11% 18% 5% 9% 5% 4 4 3 5% 6% 29%	2498
	21st to end	Night	10% 18% 6% 10% 7% 5% 4 3 5% 6% 26%	2076
March	1st to 10th	Night	7% 10% 4 10% 5% 5% 5% 4 6% 9% 36%	5496
	11th to 20th	Night	8% 15% 5% 9% 5% 4 4 3 4 4 38%	5456
	21st to end	Night	10% 16% 5% 12% 6% 6% 5% 4 6% 6% 24%	5116
April	1st to 10th	Night	10% 14% 5% 8% 6% 5% 4 3 6% 7% 32%	2100
	11th to 20th	Night	13% 10% 3 6% 4 4 4 3 4 5% 42%	2079
	21st to end	Night	10% 13% 3 7% 4 4 4 3 5% 6% 42%	1890
May	1st to 10th	Night	8% 6% 2 4 4 3 3 2 4 6% 58%	5087
	11th to 20th	Night	8% 9% 2 5% 4 5% 4 3 5% 6% 50%	4700
	21st to end	Night	9% 7% 2 5% 3 3 3 4 5% 7% 52%	5181
June	1st to 10th	Night	10% 8% 1 5% 5% 4 5% 4 5% 8% 45%	1680
	11th to 20th	Night	13% 5% 2 5% 4 4 4 3 5% 6% 50%	1680
	21st to end	Night	5% 5% 1 5% 3 3 3 2 5% 7% 61%	1680
July	1st to 10th	Night	1 1 1 2 2 2 3 2 4 5% 77%	3720
	11th to 20th	Night	1 1 1 1 1 2 2 2 5% 85%	3680
	21st to end	Night	3 2 1 2 1 2 3 3 4 7% 72%	3970
August	1st to 10th	Night	2 2 1 3 2 3 4 3 6% 8% 66%	3582
	11th to 20th	Night	1 1 1 2 2 3 3 3 3 5% 75%	4102
	21st to end	Night	3 3 1 4 3 3 3 2 4 6% 67%	5015
September	1st to 10th	Night	4 2 1 2 2 2 2 3 5% 6% 71%	2000
	11th to 20th	Night	6% 4 2 4 4 4 4 4 6% 6% 59%	2104
	21st to end	Night	8% 5% 2 5% 3 3 3 3 4 5% 59%	2200
October	1st to 10th	Night	7% 4 2 4 3 2 3 3 3 4 65%	3725
	11th to 20th	Night	9% 8% 2 7% 4 4 3 3 4 7% 48%	3992
	21st to end	Night	10% 11% 4 7% 4 5% 5% 4 5% 6% 41%	4600
November	1st to 10th	Night	16% 13% 3 7% 5% 4 4 3 6% 7% 33%	2665
	11th to 20th	Night	17% 14% 4 8% 6% 4 4 4 5% 7% 27%	2746
	21st to end	Night	11% 15% 4 8% 5% 5% 4 3 5% 7% 33%	2940
December	1st to 10th	Night	9% 10% 3 7% 4 4 4 4 5% 8% 43%	4700
	11th to 20th	Night	12% 18% 5% 11% 6% 5% 5% 4 7% 8% 18%	5316
	21st to end	Night	13% 17% 5% 10% 6% 6% 6% 4 6% 8% 20%	7106

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	63% 10% 15% 11% 1	4723
	11th to 20th	Day	39% 6% 18% 25% 10% 1	4740
	21st to end	Day	46% 8% 10% 23% 12% 1	5187
February	1st to 10th	Day	56% 12% 14% 13% 4	1787
	11th to 20th	Day	57% 9% 13% 13% 7% 1	2019
	21st to end	Day	53% 10% 14% 14% 7% 1	1730
March	1st to 10th	Day	50% 7% 16% 16% 10% 1	4618
	11th to 20th	Day	46% 7% 11% 30% 6%	4746
	21st to end	Day	59% 10% 14% 14% 4	6101
April	1st to 10th	Day	51% 8% 15% 19% 5% 1	2520
	11th to 20th	Day	47% 9% 13% 19% 12% 2	2492
	21st to end	Day	45% 9% 12% 21% 12% 1	2728
May	1st to 10th	Day	28% 4 10% 32% 24% 2	7846
	11th to 20th	Day	33% 5% 13% 32% 15% 3	8163
	21st to end	Day	27% 7% 17% 33% 14% 2	8994
June	1st to 10th	Day	42% 9% 13% 25% 10% 1	2940
	11th to 20th	Day	34% 7% 13% 26% 16% 4	2940
	21st to end	Day	22% 6% 11% 30% 24% 6%	2940
July	1st to 10th	Day	14% 3 5% 17% 30% 31%	6391
	11th to 20th	Day	9% 3 4 9% 32% 41%	6316
	21st to end	Day	13% 3 4 27% 34% 19%	6681
August	1st to 10th	Day	20% 5% 5% 15% 34% 21%	4524
	11th to 20th	Day	15% 3 5% 27% 35% 14%	4158
	21st to end	Day	16% 4 9% 31% 24% 16%	4311
September	1st to 10th	Day	17% 4 9% 24% 30% 16%	2400
	11th to 20th	Day	26% 6% 13% 21% 24% 10%	2295
	21st to end	Day	28% 6% 11% 27% 23% 5%	2200
October	1st to 10th	Day	24% 4 11% 24% 28% 9%	3472
	11th to 20th	Day	33% 5% 15% 29% 16% 2	3149
	21st to end	Day	46% 6% 11% 27% 9%	3200
November	1st to 10th	Day	51% 9% 15% 19% 6% 1	1854
	11th to 20th	Day	54% 12% 17% 15% 2	1871
	21st to end	Day	49% 11% 17% 15% 8% 1	1680
December	1st to 10th	Day	43% 9% 15% 29% 4	2700
	11th to 20th	Day	59% 12% 18% 10% 1	3108
	21st to end	Day	57% 12% 17% 12% 2	4092
January	1st to 10th	Night	61% 11% 17% 10% 1	8172
	11th to 20th	Night	41% 6% 18% 25% 9% 1	8220
	21st to end	Night	45% 9% 11% 21% 13% 1	8994
February	1st to 10th	Night	56% 10% 15% 14% 4 1	2830
	11th to 20th	Night	52% 9% 13% 17% 8% 1	2499
	21st to end	Night	52% 8% 13% 18% 7% 1	2076
March	1st to 10th	Night	42% 6% 15% 21% 13% 1	5496
	11th to 20th	Night	46% 5% 10% 32% 7%	5456
	21st to end	Night	50% 11% 17% 17% 4	5116
April	1st to 10th	Night	46% 9% 18% 22% 5%	2100
	11th to 20th	Night	40% 8% 16% 24% 10% 2	2079
	21st to end	Night	38% 9% 17% 21% 14% 1	1890
May	1st to 10th	Night	23% 5% 11% 35% 24% 2	5085
	11th to 20th	Night	31% 4 13% 34% 18% 1	4700
	21st to end	Night	28% 6% 17% 37% 11%	5182
June	1st to 10th	Night	35% 8% 14% 29% 12% 1	1680
	11th to 20th	Night	31% 7% 13% 28% 17% 4	1680
	21st to end	Night	22% 5% 12% 30% 25% 6%	1680
July	1st to 10th	Night	11% 3 4 19% 30% 33%	3680
	11th to 20th	Night	6% 2 5% 14% 31% 43%	3632
	21st to end	Night	12% 3 3 32% 30% 20%	3857
August	1st to 10th	Night	13% 4 6% 17% 42% 17%	2601
	11th to 20th	Night	12% 3 5% 31% 37% 12%	2966
	21st to end	Night	14% 3 8% 32% 27% 16%	3608
September	1st to 10th	Night	15% 3 9% 27% 30% 16%	2000
	11th to 20th	Night	22% 6% 11% 25% 24% 12%	2104
	21st to end	Night	26% 5% 11% 28% 21% 10%	2200
October	1st to 10th	Night	23% 4 9% 23% 29% 11%	3725
	11th to 20th	Night	33% 4 19% 21% 21% 1	3992
	21st to end	Night	40% 5% 10% 31% 13% 1	4600
November	1st to 10th	Night	48% 7% 16% 19% 8% 1	2665
	11th to 20th	Night	52% 9% 15% 19% 5%	2746
	21st to end	Night	47% 10% 17% 16% 10% 1	2940
December	1st to 10th	Night	37% 11% 19% 26% 6% 1	4700
	11th to 20th	Night	61% 9% 15% 12% 2	5316
	21st to end	Night	56% 11% 16% 13% 4	7106

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% 29% 26% 13% 7% 2	220
	11th to 20th	Day	19% 26% 27% 17% 7% 4	220
	21st to end	Day	21% 19% 31% 20% 8% 1	242
February	1st to 10th	Day	22% 25% 35% 16% 2	327
	11th to 20th	Day	23% 26% 28% 18% 4	514
	21st to end	Day	25% 22% 30% 19% 4	446
March	1st to 10th	Day	20% 28% 32% 17% 3	540
	11th to 20th	Day	16% 27% 31% 17% 9%	540
	21st to end	Day	21% 25% 30% 18% 5%	594
April	1st to 10th	Day	20% 22% 27% 18% 11% 3	540
	11th to 20th	Day	19% 25% 30% 17% 7% 2	532
	21st to end	Day	14% 18% 33% 25% 8% 2	540
May	1st to 10th	Day	13% 19% 32% 24% 10% 3	543
	11th to 20th	Day	18% 17% 29% 22% 11% 2	568
	21st to end	Day	13% 14% 26% 28% 15% 3	617
June	1st to 10th	Day	14% 16% 27% 23% 14% 5%	570
	11th to 20th	Day	11% 18% 34% 27% 9% 2	570
	21st to end	Day	5% 11% 30% 33% 18% 3	570
July	1st to 10th	Day	2 9% 34% 35% 16% 4	566
	11th to 20th	Day	2 11% 36% 33% 16% 2	545
	21st to end	Day	2 8% 31% 33% 21% 6%	575
August	1st to 10th	Day	4 8% 31% 35% 17% 6%	527
	11th to 20th	Day	3 7% 26% 35% 20% 9%	517
	21st to end	Day	6% 11% 27% 30% 19% 8%	583
September	1st to 10th	Day	5% 10% 23% 30% 23% 9%	530
	11th to 20th	Day	9% 9% 26% 31% 20% 6%	530
	21st to end	Day	11% 13% 27% 28% 18% 3	530
October	1st to 10th	Day	14% 14% 28% 24% 16% 3	530
	11th to 20th	Day	13% 19% 29% 24% 11% 4	469
	21st to end	Day	19% 21% 26% 23% 7% 4	363
November	1st to 10th	Day	25% 18% 31% 21% 4	330
	11th to 20th	Day	19% 26% 25% 23% 6% 1	319
	21st to end	Day	17% 22% 36% 18% 6%	220
December	1st to 10th	Day	20% 25% 25% 20% 8% 3	220
	11th to 20th	Day	28% 30% 30% 10% 3	220
	21st to end	Day	23% 26% 32% 14% 4 2	242
January	1st to 10th	Night	20% 20% 32% 18% 10% 1	840
	11th to 20th	Night	21% 15% 30% 24% 9% 1	840
	21st to end	Night	20% 11% 26% 32% 10% 1	924
February	1st to 10th	Night	21% 16% 31% 25% 5% 1	840
	11th to 20th	Night	19% 14% 29% 26% 11% 1	824
	21st to end	Night	18% 20% 30% 22% 9% 2	692
March	1st to 10th	Night	17% 16% 31% 26% 10% 1	840
	11th to 20th	Night	17% 13% 30% 26% 12% 1	840
	21st to end	Night	18% 15% 29% 28% 9% 1	924
April	1st to 10th	Night	14% 12% 27% 32% 14% 2	840
	11th to 20th	Night	15% 11% 33% 27% 13% 2	831
	21st to end	Night	14% 12% 24% 35% 13% 1	630
May	1st to 10th	Night	10% 11% 27% 36% 14% 2	550
	11th to 20th	Night	13% 7% 19% 31% 27% 2	414
	21st to end	Night	11% 7% 20% 35% 25% 2	455
June	1st to 10th	Night	11% 7% 20% 39% 22% 2	420
	11th to 20th	Night	13% 7% 23% 39% 17% 1	420
	21st to end	Night	6% 4 16% 48% 25% 1	420
July	1st to 10th	Night	2 2 15% 50% 29% 2	410
	11th to 20th	Night	2 3 14% 57% 23% 1	398
	21st to end	Night	1 2 15% 52% 29% 2	438
August	1st to 10th	Night	3 2 10% 49% 34% 2	398
	11th to 20th	Night	2 2 16% 46% 30% 4	605
	21st to end	Night	4 3 18% 42% 30% 3	880
September	1st to 10th	Night	5% 3 16% 42% 30% 5%	800
	11th to 20th	Night	5% 4 15% 41% 29% 6%	800
	21st to end	Night	11% 4 21% 35% 23% 6%	800
October	1st to 10th	Night	12% 4 22% 38% 21% 3	800
	11th to 20th	Night	14% 10% 21% 33% 17% 5%	800
	21st to end	Night	19% 11% 23% 32% 12% 3	880
November	1st to 10th	Night	21% 11% 29% 30% 8% 1	820
	11th to 20th	Night	23% 15% 26% 28% 8% 1	840
	21st to end	Night	18% 17% 27% 25% 11% 3	840
December	1st to 10th	Night	18% 14% 29% 27% 9% 2	840
	11th to 20th	Night	23% 16% 30% 23% 8% 1	840
	21st to end	Night	25% 19% 30% 21% 5% 1	924

Page generated at 2026-07-13 00:14:54. Version ac07

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