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 2025-03-12.

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	8% 9% 3 7% 5% 4 4 3 5% 6% 46%	112659
	Night	9% 11% 3 7% 4 4 4 3 5% 6% 42%	115057
Transparency	Day	36% 8% 12% 20% 15% 8%	112403
	Night	39% 8% 14% 21% 14% 5%	114902
Seeing	Day	14% 17% 30% 25% 12% 3	14741
	Night	15% 11% 25% 32% 16% 2	24068

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	10% 20% 8% 13% 7% 6% 6% 4 7% 5% 15%	3639
	11th to 20th	Day	10% 14% 4 10% 6% 5% 5% 3 5% 7% 31%	3640
	21st to end	Day	10% 10% 4 8% 5% 5% 4 3 5% 6% 39%	4004
February	1st to 10th	Day	13% 17% 5% 9% 7% 5% 4 4 6% 6% 24%	1702
	11th to 20th	Day	12% 20% 6% 11% 5% 5% 4 3 5% 4 26%	1911
	21st to end	Day	8% 16% 6% 11% 8% 5% 4 4 5% 7% 25%	1650
March	1st to 10th	Day	6% 13% 5% 11% 7% 6% 6% 5% 7% 7% 28%	3238
	11th to 20th	Day	6% 11% 4 8% 5% 4 4 3 5% 6% 44%	3249
	21st to end	Day	10% 17% 6% 13% 8% 7% 7% 5% 6% 5% 16%	4165
April	1st to 10th	Day	11% 18% 4 9% 6% 5% 4 3 6% 6% 28%	2280
	11th to 20th	Day	11% 13% 4 7% 5% 5% 4 4 5% 6% 35%	2249
	21st to end	Day	11% 13% 4 7% 5% 4 5% 5% 5% 6% 34%	2468
May	1st to 10th	Day	10% 11% 3 8% 6% 5% 4 4 5% 6% 39%	4227
	11th to 20th	Day	8% 5% 2 4 3 3 3 3 4 7% 58%	4377
	21st to end	Day	9% 11% 4 9% 5% 4 4 4 6% 8% 37%	4813
June	1st to 10th	Day	15% 9% 2 6% 4 4 5% 4 6% 8% 37%	2660
	11th to 20th	Day	13% 8% 2 5% 4 4 4 3 5% 6% 46%	2660
	21st to end	Day	6% 3 2 4 3 3 4 3 4 7% 62%	2660
July	1st to 10th	Day	1 1 2 2 2 2 3 3 4 79%	4420
	11th to 20th	Day	1 2 1 2 2 2 3 2 4 4 78%	4420
	21st to end	Day	1 1 1 3 2 2 3 2 4 5% 77%	4790
August	1st to 10th	Day	3 3 1 5% 3 3 4 3 4 5% 65%	4400
	11th to 20th	Day	2 2 1 3 2 3 3 3 4 6% 71%	3990
	21st to end	Day	3 2 1 3 2 2 4 3 4 6% 70%	4180
September	1st to 10th	Day	4 3 1 3 2 2 2 3 3 4 71%	2280
	11th to 20th	Day	7% 4 2 5% 4 4 4 2 4 7% 56%	2180
	21st to end	Day	8% 7% 2 5% 3 3 3 2 4 6% 55%	2090
October	1st to 10th	Day	7% 4 2 5% 3 3 2 2 3 4 65%	3366
	11th to 20th	Day	9% 7% 2 7% 4 4 4 3 5% 8% 49%	3052
	21st to end	Day	12% 10% 4 10% 5% 5% 6% 4 5% 7% 33%	3101
November	1st to 10th	Day	18% 14% 3 7% 5% 4 4 4 4 7% 30%	1764
	11th to 20th	Day	13% 17% 4 9% 6% 5% 5% 4 6% 7% 23%	1782
	21st to end	Day	11% 14% 4 8% 6% 4 4 3 6% 8% 32%	1600
December	1st to 10th	Day	7% 12% 4 6% 4 4 3 3 4 6% 46%	2620
	11th to 20th	Day	11% 16% 4 11% 6% 5% 6% 5% 6% 8% 21%	3028
	21st to end	Day	12% 16% 5% 10% 6% 6% 4 4 5% 8% 22%	4004
January	1st to 10th	Night	12% 19% 7% 13% 7% 6% 5% 4 7% 7% 14%	6306
	11th to 20th	Night	12% 15% 5% 10% 5% 4 4 3 5% 8% 30%	6320
	21st to end	Night	12% 11% 3 7% 4 4 5% 4 6% 7% 37%	6952
February	1st to 10th	Night	14% 18% 5% 10% 7% 5% 5% 3 5% 6% 22%	2695
	11th to 20th	Night	11% 18% 5% 9% 5% 4 4 3 5% 6% 29%	2376
	21st to end	Night	10% 18% 6% 11% 8% 5% 4 3 5% 5% 25%	1980
March	1st to 10th	Night	7% 11% 5% 10% 5% 5% 5% 4 6% 9% 34%	3856
	11th to 20th	Night	8% 12% 4 7% 4 4 3 3 4 4 47%	3741
	21st to end	Night	10% 17% 5% 12% 7% 6% 5% 5% 7% 7% 20%	3488
April	1st to 10th	Night	10% 14% 5% 8% 6% 5% 4 3 6% 7% 32%	1900
	11th to 20th	Night	14% 11% 4 6% 5% 4 4 3 4 5% 41%	1879
	21st to end	Night	11% 13% 3 7% 5% 4 4 3 5% 6% 40%	1710
May	1st to 10th	Night	13% 11% 3 7% 6% 3 4 2 5% 5% 42%	2736
	11th to 20th	Night	6% 5% 2 3 4 3 3 3 5% 5% 60%	2516
	21st to end	Night	14% 10% 2 6% 4 4 3 3 4 6% 44%	2762
June	1st to 10th	Night	11% 8% 1 5% 5% 4 5% 4 6% 9% 42%	1520
	11th to 20th	Night	14% 5% 2 5% 4 4 3 3 6% 7% 48%	1520
	21st to end	Night	5% 4 1 4 3 2 3 2 5% 7% 63%	1520
July	1st to 10th	Night	2 1 1 1 1 1 2 2 3 3 84%	2540
	11th to 20th	Night	1 1 1 1 1 2 2 3 5% 84%	2540
	21st to end	Night	2 2 1 2 2 2 4 3 4 5% 75%	2761
August	1st to 10th	Night	2 2 2 2 2 3 3 5% 8% 70%	2529
	11th to 20th	Night	1 1 1 2 2 2 3 3 4 6% 75%	2848
	21st to end	Night	2 2 1 3 2 2 3 2 4 5% 74%	3498
September	1st to 10th	Night	5% 2 1 2 2 2 2 3 4 5% 72%	1900
	11th to 20th	Night	6% 4 2 3 4 4 4 4 6% 6% 59%	1999
	21st to end	Night	9% 5% 2 5% 2 3 3 3 4 6% 58%	2090
October	1st to 10th	Night	7% 4 2 4 3 2 3 3 3 4 65%	3611
	11th to 20th	Night	9% 8% 3 6% 4 4 3 3 4 7% 49%	3869
	21st to end	Night	10% 11% 4 7% 4 5% 5% 4 5% 6% 40%	4457
November	1st to 10th	Night	17% 13% 3 7% 5% 4 4 3 6% 6% 33%	2535
	11th to 20th	Night	17% 14% 4 8% 6% 4 4 4 5% 7% 27%	2615
	21st to end	Night	11% 15% 4 8% 5% 5% 4 3 4 6% 34%	2800
December	1st to 10th	Night	8% 9% 3 6% 4 4 4 4 6% 8% 43%	4560
	11th to 20th	Night	12% 18% 5% 11% 6% 5% 5% 4 7% 8% 17%	5176
	21st to end	Night	13% 17% 5% 10% 6% 6% 6% 4 6% 7% 20%	6952

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	62% 11% 14% 11% 1	3639
	11th to 20th	Day	48% 7% 19% 19% 6%	3640
	21st to end	Day	49% 9% 8% 19% 14% 1	4004
February	1st to 10th	Day	57% 13% 13% 13% 5%	1702
	11th to 20th	Day	58% 9% 13% 13% 7% 1	1921
	21st to end	Day	54% 10% 15% 14% 6% 1	1650
March	1st to 10th	Day	50% 9% 16% 15% 9%	3238
	11th to 20th	Day	39% 8% 10% 37% 7%	3249
	21st to end	Day	59% 12% 14% 11% 3	4165
April	1st to 10th	Day	53% 9% 16% 16% 5% 1	2280
	11th to 20th	Day	47% 9% 13% 17% 11% 2	2252
	21st to end	Day	46% 9% 13% 20% 11% 1	2468
May	1st to 10th	Day	38% 7% 15% 25% 13% 1	4227
	11th to 20th	Day	24% 7% 11% 33% 20% 6%	4380
	21st to end	Day	38% 11% 21% 24% 6%	4814
June	1st to 10th	Day	44% 10% 14% 24% 8%	2660
	11th to 20th	Day	35% 7% 15% 26% 14% 3	2660
	21st to end	Day	22% 6% 11% 31% 24% 6%	2660
July	1st to 10th	Day	10% 3 7% 13% 24% 42%	4351
	11th to 20th	Day	10% 5% 5% 9% 23% 49%	4345
	21st to end	Day	11% 3 4 17% 39% 27%	4639
August	1st to 10th	Day	20% 5% 6% 14% 34% 22%	4384
	11th to 20th	Day	16% 4 5% 27% 35% 14%	4029
	21st to end	Day	15% 4 9% 31% 25% 16%	4179
September	1st to 10th	Day	16% 4 9% 24% 30% 17%	2280
	11th to 20th	Day	27% 6% 12% 20% 24% 10%	2180
	21st to end	Day	28% 6% 12% 27% 22% 5%	2090
October	1st to 10th	Day	24% 4 11% 23% 29% 9%	3366
	11th to 20th	Day	33% 6% 16% 28% 16% 2	3052
	21st to end	Day	46% 6% 11% 27% 9%	3101
November	1st to 10th	Day	52% 9% 14% 18% 6% 1	1764
	11th to 20th	Day	54% 12% 17% 15% 2	1782
	21st to end	Day	48% 11% 16% 15% 8% 1	1600
December	1st to 10th	Day	41% 9% 15% 30% 4	2620
	11th to 20th	Day	60% 12% 18% 9% 1	3028
	21st to end	Day	57% 12% 17% 12% 2	4004
January	1st to 10th	Night	62% 12% 15% 9% 1	6306
	11th to 20th	Night	51% 7% 18% 18% 6%	6320
	21st to end	Night	47% 10% 10% 18% 13% 1	6952
February	1st to 10th	Night	58% 11% 14% 13% 3 1	2695
	11th to 20th	Night	53% 9% 13% 16% 8% 1	2377
	21st to end	Night	53% 9% 14% 18% 6% 1	1980
March	1st to 10th	Night	44% 7% 16% 22% 10% 1	3856
	11th to 20th	Night	36% 6% 12% 38% 8%	3741
	21st to end	Night	48% 14% 20% 13% 4	3488
April	1st to 10th	Night	47% 10% 19% 20% 5%	1900
	11th to 20th	Night	41% 9% 16% 22% 10% 2	1879
	21st to end	Night	39% 10% 19% 19% 12% 1	1710
May	1st to 10th	Night	33% 9% 18% 25% 14% 1	2736
	11th to 20th	Night	20% 6% 12% 37% 24% 1	2516
	21st to end	Night	33% 10% 23% 26% 7%	2762
June	1st to 10th	Night	37% 9% 15% 28% 10% 1	1520
	11th to 20th	Night	32% 7% 14% 29% 14% 3	1520
	21st to end	Night	20% 6% 13% 30% 25% 6%	1520
July	1st to 10th	Night	8% 3 4 15% 27% 43%	2500
	11th to 20th	Night	6% 2 6% 14% 22% 50%	2492
	21st to end	Night	12% 3 4 18% 35% 28%	2674
August	1st to 10th	Night	13% 4 6% 17% 43% 18%	2521
	11th to 20th	Night	12% 3 5% 31% 37% 12%	2875
	21st to end	Night	14% 3 9% 32% 27% 15%	3498
September	1st to 10th	Night	15% 3 9% 26% 30% 17%	1900
	11th to 20th	Night	22% 6% 11% 25% 23% 12%	1999
	21st to end	Night	26% 5% 11% 28% 21% 9%	2090
October	1st to 10th	Night	23% 4 9% 23% 30% 12%	3611
	11th to 20th	Night	33% 4 19% 21% 21% 1	3869
	21st to end	Night	41% 5% 10% 30% 13% 1	4457
November	1st to 10th	Night	48% 8% 15% 19% 9% 1	2535
	11th to 20th	Night	51% 9% 15% 20% 5%	2615
	21st to end	Night	46% 10% 16% 16% 11% 1	2800
December	1st to 10th	Night	37% 11% 19% 27% 6% 1	4560
	11th to 20th	Night	62% 10% 15% 12% 2	5176
	21st to end	Night	56% 11% 16% 13% 4	6952

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% 25% 15% 8% 2	200
	11th to 20th	Day	20% 27% 28% 15% 8% 4	200
	21st to end	Day	22% 18% 30% 21% 9% 1	220
February	1st to 10th	Day	23% 25% 36% 14% 3	302
	11th to 20th	Day	22% 27% 29% 17% 4	476
	21st to end	Day	23% 23% 30% 20% 5%	414
March	1st to 10th	Day	19% 28% 32% 17% 3	500
	11th to 20th	Day	14% 25% 31% 20% 10%	465
	21st to end	Day	21% 25% 31% 17% 6%	506
April	1st to 10th	Day	22% 23% 28% 17% 8% 2	460
	11th to 20th	Day	19% 26% 29% 17% 7% 2	452
	21st to end	Day	14% 19% 34% 24% 7% 1	460
May	1st to 10th	Day	14% 20% 31% 24% 9% 3	463
	11th to 20th	Day	19% 16% 27% 23% 13% 2	488
	21st to end	Day	14% 14% 26% 29% 14% 2	529
June	1st to 10th	Day	14% 15% 28% 24% 14% 4	490
	11th to 20th	Day	11% 19% 33% 25% 9% 2	490
	21st to end	Day	5% 10% 32% 32% 18% 3	490
July	1st to 10th	Day	2 9% 34% 36% 16% 3	486
	11th to 20th	Day	1 11% 38% 32% 16% 2	484
	21st to end	Day	2 8% 32% 33% 19% 5%	531
August	1st to 10th	Day	3 7% 32% 34% 16% 6%	487
	11th to 20th	Day	4 7% 25% 35% 20% 9%	477
	21st to end	Day	6% 11% 28% 31% 17% 7%	539
September	1st to 10th	Day	5% 11% 24% 30% 22% 8%	490
	11th to 20th	Day	10% 9% 27% 31% 19% 4	490
	21st to end	Day	11% 13% 28% 28% 18% 2	490
October	1st to 10th	Day	14% 13% 28% 25% 17% 3	490
	11th to 20th	Day	13% 19% 28% 24% 12% 4	432
	21st to end	Day	19% 20% 26% 24% 7% 4	330
November	1st to 10th	Day	26% 18% 29% 22% 5%	300
	11th to 20th	Day	19% 26% 26% 23% 6% 1	290
	21st to end	Day	18% 19% 38% 20% 7%	200
December	1st to 10th	Day	19% 23% 27% 21% 9% 4	200
	11th to 20th	Day	28% 29% 31% 11% 3	200
	21st to end	Day	22% 26% 31% 15% 4 2	220
January	1st to 10th	Night	20% 19% 31% 19% 10% 2	800
	11th to 20th	Night	22% 14% 29% 24% 9% 1	800
	21st to end	Night	20% 10% 25% 32% 11% 1	880
February	1st to 10th	Night	21% 16% 31% 24% 6% 1	800
	11th to 20th	Night	19% 14% 28% 27% 11% 1	784
	21st to end	Night	16% 20% 30% 22% 9% 2	660
March	1st to 10th	Night	17% 15% 30% 26% 10% 1	800
	11th to 20th	Night	16% 11% 30% 27% 13% 2	766
	21st to end	Night	17% 15% 28% 28% 10% 1	836
April	1st to 10th	Night	14% 12% 27% 31% 13% 2	760
	11th to 20th	Night	16% 10% 34% 25% 13% 2	751
	21st to end	Night	15% 13% 25% 34% 12% 1	570
May	1st to 10th	Night	10% 12% 27% 35% 13% 2	498
	11th to 20th	Night	14% 6% 17% 31% 29% 3	374
	21st to end	Night	12% 7% 20% 35% 25% 2	411
June	1st to 10th	Night	12% 6% 20% 38% 22% 2	380
	11th to 20th	Night	14% 8% 24% 36% 18% 1	380
	21st to end	Night	6% 3 17% 48% 25% 1	380
July	1st to 10th	Night	2 2 17% 47% 29% 2	370
	11th to 20th	Night	2 2 14% 58% 23% 1	368
	21st to end	Night	1 2 15% 53% 28% 2	416
August	1st to 10th	Night	3 2 10% 48% 33% 2	378
	11th to 20th	Night	2 2 16% 45% 31% 5%	574
	21st to end	Night	3 3 18% 43% 29% 3	836
September	1st to 10th	Night	5% 3 17% 42% 29% 5%	760
	11th to 20th	Night	5% 4 15% 41% 29% 7%	760
	21st to end	Night	11% 4 21% 35% 23% 6%	760
October	1st to 10th	Night	12% 4 21% 38% 21% 3	760
	11th to 20th	Night	14% 9% 21% 34% 18% 5%	760
	21st to end	Night	19% 11% 22% 33% 13% 4	836
November	1st to 10th	Night	21% 11% 28% 31% 8% 1	780
	11th to 20th	Night	22% 15% 26% 28% 9% 1	800
	21st to end	Night	17% 15% 28% 25% 12% 3	800
December	1st to 10th	Night	18% 14% 29% 28% 10% 2	800
	11th to 20th	Night	23% 16% 29% 24% 8% 1	800
	21st to end	Night	25% 19% 30% 21% 5% 1	880

Page generated at 2025-03-09 05:08:43. Version ac07

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