UI Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the UI Observatory clear sky chart from 2005-11-06 to 2025-06-10.

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	10% 9% 3 6% 4 4 4 4 5% 7% 43%	122956
	Night	12% 10% 3 7% 4 4 4 4 5% 6% 40%	123977
Transparency	Day	41% 3 11% 23% 15% 6%	122723
	Night	44% 3 11% 22% 14% 5%	123853
Seeing	Day	15% 6% 20% 27% 22% 10%	15347
	Night	18% 5% 16% 36% 22% 4	24704

By Month

Percentage Cloud Cover NW (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	19% 23% 8% 10% 4 4 4 4 4 5% 15%	3639
	11th to 20th	Day	14% 11% 6% 10% 5% 5% 4 4 5% 6% 31%	3616
	21st to end	Day	16% 14% 3 9% 5% 4 4 3 4 5% 33%	4004
February	1st to 10th	Day	19% 17% 5% 10% 5% 4 4 3 5% 6% 20%	1610
	11th to 20th	Day	17% 18% 5% 10% 6% 4 5% 3 4 5% 22%	1915
	21st to end	Day	15% 19% 4 8% 6% 5% 5% 4 5% 6% 23%	1650
March	1st to 10th	Day	9% 12% 3 8% 6% 6% 6% 5% 8% 8% 29%	4518
	11th to 20th	Day	10% 11% 4 8% 5% 4 4 4 6% 7% 37%	4726
	21st to end	Day	15% 16% 4 10% 6% 6% 6% 4 6% 7% 20%	5874
April	1st to 10th	Day	14% 13% 4 9% 5% 5% 5% 5% 7% 8% 27%	2400
	11th to 20th	Day	11% 12% 3 9% 5% 5% 5% 5% 6% 6% 34%	2436
	21st to end	Day	13% 9% 3 8% 7% 6% 5% 4 5% 7% 34%	2600
May	1st to 10th	Day	7% 7% 2 5% 4 4 4 3 6% 8% 50%	6072
	11th to 20th	Day	9% 7% 3 6% 6% 6% 5% 4 6% 8% 40%	6277
	21st to end	Day	7% 6% 2 6% 5% 5% 5% 5% 7% 8% 44%	6903
June	1st to 10th	Day	14% 9% 3 6% 5% 5% 4 4 5% 7% 38%	2898
	11th to 20th	Day	13% 7% 2 6% 5% 5% 5% 4 6% 7% 40%	2850
	21st to end	Day	6% 4 2 4 3 4 4 4 5% 6% 59%	2763
July	1st to 10th	Day	2 2 2 2 2 2 2 4 4 80%	4420
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 4 79%	4420
	21st to end	Day	1 1 1 2 2 2 3 3 4 5% 76%	4790
August	1st to 10th	Day	3 2 1 3 3 2 3 2 4 7% 69%	4401
	11th to 20th	Day	4 3 1 4 2 2 3 2 5% 7% 67%	4086
	21st to end	Day	3 3 1 3 3 3 3 3 4 7% 66%	4252
September	1st to 10th	Day	4 2 1 4 2 3 3 2 5% 7% 68%	2280
	11th to 20th	Day	8% 4 2 6% 4 4 4 5% 5% 7% 50%	2157
	21st to end	Day	8% 5% 2 7% 3 4 4 3 6% 8% 51%	2090
October	1st to 10th	Day	9% 6% 2 5% 3 2 3 3 4 6% 58%	3408
	11th to 20th	Day	10% 6% 3 7% 4 5% 5% 4 6% 7% 45%	2970
	21st to end	Day	16% 12% 3 7% 4 5% 4 4 4 6% 35%	3101
November	1st to 10th	Day	23% 10% 3 7% 6% 5% 4 4 5% 8% 26%	1764
	11th to 20th	Day	20% 15% 4 10% 6% 5% 5% 3 5% 6% 22%	1762
	21st to end	Day	19% 14% 4 8% 6% 5% 4 5% 6% 7% 22%	1524
December	1st to 10th	Day	14% 12% 6% 8% 5% 4 4 3 7% 8% 28%	2290
	11th to 20th	Day	20% 19% 6% 10% 6% 5% 5% 4 4 5% 16%	2642
	21st to end	Day	20% 17% 5% 9% 5% 5% 4 3 6% 7% 19%	3848
January	1st to 10th	Night	21% 19% 7% 10% 6% 5% 4 4 5% 5% 14%	6306
	11th to 20th	Night	13% 12% 4 9% 4 5% 5% 3 5% 7% 33%	6282
	21st to end	Night	15% 12% 4 7% 5% 4 4 3 5% 5% 36%	6952
February	1st to 10th	Night	19% 18% 5% 8% 5% 4 5% 4 5% 4 23%	2786
	11th to 20th	Night	16% 15% 4 8% 4 4 4 3 5% 6% 30%	2373
	21st to end	Night	15% 18% 3 9% 5% 6% 4 4 5% 6% 26%	1980
March	1st to 10th	Night	9% 10% 3 8% 5% 5% 6% 4 6% 8% 37%	5376
	11th to 20th	Night	11% 13% 4 8% 4 4 3 3 5% 5% 40%	5273
	21st to end	Night	14% 11% 4 10% 7% 7% 6% 5% 7% 8% 22%	5099
April	1st to 10th	Night	13% 12% 3 7% 6% 6% 5% 5% 6% 8% 32%	2000
	11th to 20th	Night	14% 9% 3 7% 5% 3 4 3 6% 6% 41%	1913
	21st to end	Night	11% 9% 3 8% 6% 4 4 4 6% 8% 38%	1800
May	1st to 10th	Night	9% 4 2 5% 5% 4 4 3 5% 7% 52%	3884
	11th to 20th	Night	10% 6% 2 7% 5% 5% 5% 5% 6% 8% 42%	3616
	21st to end	Night	10% 6% 2 5% 3 4 5% 3 5% 7% 49%	3972
June	1st to 10th	Night	11% 6% 2 6% 5% 4 4 3 6% 9% 43%	1481
	11th to 20th	Night	11% 7% 2 7% 5% 4 4 4 5% 7% 44%	1330
	21st to end	Night	6% 4 2 5% 3 4 4 3 5% 6% 56%	1410
July	1st to 10th	Night	2 1 2 1 1 2 2 3 3 82%	2540
	11th to 20th	Night	1 1 2 1 2 2 2 4 4 80%	2540
	21st to end	Night	3 2 1 2 2 2 2 2 3 5% 76%	2762
August	1st to 10th	Night	4 3 1 3 2 2 3 3 5% 6% 67%	2529
	11th to 20th	Night	3 2 1 4 3 4 4 3 5% 5% 66%	2751
	21st to end	Night	4 3 1 4 3 3 3 3 4 6% 65%	3423
September	1st to 10th	Night	5% 3 1 2 2 2 3 3 4 7% 66%	1900
	11th to 20th	Night	7% 4 2 5% 4 4 4 4 5% 7% 54%	2023
	21st to end	Night	10% 5% 2 5% 3 3 3 3 5% 7% 53%	2090
October	1st to 10th	Night	6% 3 2 3 3 3 3 3 6% 6% 60%	3569
	11th to 20th	Night	10% 6% 2 6% 4 4 5% 4 5% 7% 48%	3962
	21st to end	Night	13% 7% 2 6% 5% 5% 5% 3 5% 7% 42%	4457
November	1st to 10th	Night	20% 12% 3 7% 5% 5% 4 3 5% 7% 29%	2535
	11th to 20th	Night	21% 12% 4 8% 5% 5% 4 3 5% 6% 28%	2635
	21st to end	Night	18% 12% 3 8% 5% 4 4 4 6% 6% 30%	2852
December	1st to 10th	Night	12% 9% 3 6% 5% 4 5% 4 6% 7% 41%	4890
	11th to 20th	Night	21% 17% 5% 11% 6% 6% 5% 4 6% 6% 15%	5550
	21st to end	Night	20% 16% 5% 9% 5% 5% 4 3 4 5% 22%	7136

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	71% 4 13% 11% 1	3639
	11th to 20th	Day	53% 4 15% 22% 6%	3640
	21st to end	Day	55% 2 11% 19% 11%	4004
February	1st to 10th	Day	64% 5% 14% 11% 5%	1610
	11th to 20th	Day	63% 4 12% 16% 5%	1925
	21st to end	Day	61% 4 9% 18% 6% 1	1650
March	1st to 10th	Day	52% 3 8% 23% 12% 2	4518
	11th to 20th	Day	45% 3 14% 30% 8%	4726
	21st to end	Day	64% 3 14% 15% 4	5874
April	1st to 10th	Day	54% 4 15% 20% 7%	2400
	11th to 20th	Day	48% 4 12% 21% 13% 2	2436
	21st to end	Day	50% 3 11% 24% 11% 1	2600
May	1st to 10th	Day	32% 3 10% 32% 22% 1	6072
	11th to 20th	Day	40% 5% 12% 28% 13% 2	6280
	21st to end	Day	37% 3 17% 29% 13% 1	6904
June	1st to 10th	Day	46% 4 11% 25% 12% 1	2898
	11th to 20th	Day	43% 3 13% 25% 14% 2	2850
	21st to end	Day	26% 3 10% 32% 25% 4	2763
July	1st to 10th	Day	11% 2 4 17% 29% 36%	4351
	11th to 20th	Day	11% 2 4 11% 27% 45%	4345
	21st to end	Day	11% 3 4 20% 35% 27%	4639
August	1st to 10th	Day	17% 2 5% 17% 41% 18%	4384
	11th to 20th	Day	19% 2 6% 39% 26% 8%	4126
	21st to end	Day	19% 3 9% 33% 28% 7%	4251
September	1st to 10th	Day	18% 3 9% 30% 28% 12%	2280
	11th to 20th	Day	32% 3 12% 26% 21% 6%	2157
	21st to end	Day	32% 3 15% 31% 15% 3	2090
October	1st to 10th	Day	29% 2 12% 33% 22% 2	3408
	11th to 20th	Day	37% 4 18% 27% 13%	2970
	21st to end	Day	49% 3 11% 27% 8% 1	3101
November	1st to 10th	Day	58% 4 14% 19% 4 1	1764
	11th to 20th	Day	64% 5% 13% 14% 3 1	1762
	21st to end	Day	61% 5% 14% 14% 5% 1	1524
December	1st to 10th	Day	52% 6% 18% 20% 4 1	2290
	11th to 20th	Day	72% 4 13% 10% 1	2644
	21st to end	Day	64% 4 17% 13% 3	3848
January	1st to 10th	Night	73% 3 11% 12% 1	6306
	11th to 20th	Night	51% 3 18% 19% 8% 1	6320
	21st to end	Night	52% 2 13% 22% 11%	6952
February	1st to 10th	Night	63% 4 14% 13% 5% 1	2786
	11th to 20th	Night	56% 3 15% 18% 7%	2374
	21st to end	Night	60% 3 12% 16% 7% 1	1980
March	1st to 10th	Night	46% 3 8% 24% 18% 1	5376
	11th to 20th	Night	47% 2 9% 32% 9%	5274
	21st to end	Night	59% 4 12% 21% 3	5099
April	1st to 10th	Night	50% 4 13% 24% 9%	2000
	11th to 20th	Night	46% 3 10% 24% 14% 3	1913
	21st to end	Night	45% 4 11% 22% 16% 2	1800
May	1st to 10th	Night	33% 1 7% 29% 26% 3	3881
	11th to 20th	Night	39% 4 10% 28% 16% 3	3616
	21st to end	Night	36% 2 16% 29% 17%	3972
June	1st to 10th	Night	38% 5% 9% 29% 17% 2	1481
	11th to 20th	Night	40% 3 9% 27% 17% 4	1330
	21st to end	Night	29% 3 8% 26% 27% 7%	1410
July	1st to 10th	Night	10% 1 3 16% 29% 41%	2500
	11th to 20th	Night	8% 3 4 11% 21% 53%	2492
	21st to end	Night	13% 2 2 15% 29% 39%	2674
August	1st to 10th	Night	18% 3 3 11% 41% 23%	2521
	11th to 20th	Night	19% 3 5% 28% 33% 12%	2777
	21st to end	Night	21% 3 6% 26% 30% 13%	3423
September	1st to 10th	Night	19% 2 6% 27% 30% 17%	1900
	11th to 20th	Night	30% 3 8% 25% 26% 8%	2023
	21st to end	Night	32% 2 12% 30% 17% 7%	2090
October	1st to 10th	Night	24% 3 9% 32% 28% 3	3569
	11th to 20th	Night	36% 4 15% 23% 21% 1	3962
	21st to end	Night	41% 4 9% 32% 14% 1	4457
November	1st to 10th	Night	54% 4 13% 22% 6% 1	2535
	11th to 20th	Night	57% 3 15% 19% 5% 1	2635
	21st to end	Night	54% 3 15% 21% 6%	2852
December	1st to 10th	Night	40% 6% 22% 26% 6%	4890
	11th to 20th	Night	70% 4 13% 12% 1	5547
	21st to end	Night	64% 3 15% 13% 5%	7136

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	28% 9% 22% 21% 17% 4	200
	11th to 20th	Day	26% 4 23% 28% 13% 7%	200
	21st to end	Day	27% 4 25% 30% 14%	220
February	1st to 10th	Day	27% 11% 29% 25% 8% 1	210
	11th to 20th	Day	27% 11% 18% 28% 13% 2	478
	21st to end	Day	23% 11% 23% 29% 11% 3	414
March	1st to 10th	Day	19% 9% 26% 29% 13% 3	500
	11th to 20th	Day	20% 9% 18% 26% 19% 8%	500
	21st to end	Day	20% 10% 26% 26% 15% 3	550
April	1st to 10th	Day	20% 9% 21% 26% 18% 6%	500
	11th to 20th	Day	16% 10% 29% 24% 15% 6%	492
	21st to end	Day	14% 8% 22% 29% 22% 6%	500
May	1st to 10th	Day	13% 5% 21% 28% 27% 6%	503
	11th to 20th	Day	17% 7% 18% 26% 17% 15%	528
	21st to end	Day	13% 4 16% 23% 28% 17%	573
June	1st to 10th	Day	15% 4 15% 26% 27% 13%	636
	11th to 20th	Day	15% 6% 23% 29% 21% 6%	680
	21st to end	Day	6% 3 17% 29% 31% 13%	593
July	1st to 10th	Day	3 2 15% 29% 36% 14%	486
	11th to 20th	Day	4 3 19% 30% 30% 14%	484
	21st to end	Day	2 3 17% 28% 34% 17%	531
August	1st to 10th	Day	6% 2 16% 28% 29% 19%	487
	11th to 20th	Day	4 3 10% 27% 34% 22%	477
	21st to end	Day	9% 3 18% 24% 29% 17%	539
September	1st to 10th	Day	6% 2 18% 26% 31% 17%	490
	11th to 20th	Day	10% 5% 16% 27% 27% 16%	490
	21st to end	Day	11% 6% 17% 31% 27% 8%	490
October	1st to 10th	Day	15% 7% 15% 31% 24% 8%	490
	11th to 20th	Day	14% 7% 17% 30% 23% 10%	376
	21st to end	Day	25% 6% 20% 33% 14% 3	330
November	1st to 10th	Day	24% 6% 26% 31% 12%	300
	11th to 20th	Day	25% 9% 25% 24% 14% 3	280
	21st to end	Day	24% 4 23% 34% 14% 3	200
December	1st to 10th	Day	29% 6% 20% 24% 15% 8%	200
	11th to 20th	Day	32% 8% 26% 24% 8% 3	200
	21st to end	Day	33% 7% 24% 25% 8% 3	220
January	1st to 10th	Night	31% 8% 17% 30% 13% 2	800
	11th to 20th	Night	25% 7% 16% 34% 14% 3	800
	21st to end	Night	21% 5% 17% 36% 18% 2	880
February	1st to 10th	Night	27% 8% 20% 33% 10% 2	800
	11th to 20th	Night	24% 6% 18% 34% 16% 2	784
	21st to end	Night	23% 10% 17% 33% 14% 2	660
March	1st to 10th	Night	20% 4 19% 36% 18% 3	800
	11th to 20th	Night	17% 7% 20% 32% 22% 2	800
	21st to end	Night	22% 5% 19% 38% 15% 2	880
April	1st to 10th	Night	20% 5% 16% 38% 19% 2	800
	11th to 20th	Night	18% 6% 23% 34% 16% 3	725
	21st to end	Night	15% 5% 19% 37% 22% 3	600
May	1st to 10th	Night	12% 4 21% 35% 25% 3	514
	11th to 20th	Night	13% 4 12% 33% 29% 9%	394
	21st to end	Night	14% 4 12% 32% 31% 6%	433
June	1st to 10th	Night	11% 5% 13% 36% 28% 6%	399
	11th to 20th	Night	12% 6% 22% 33% 23% 4	380
	21st to end	Night	8% 2 14% 39% 32% 5%	380
July	1st to 10th	Night	4 1 11% 38% 39% 6%	370
	11th to 20th	Night	2 1 15% 45% 33% 3	368
	21st to end	Night	2 2 12% 37% 42% 6%	416
August	1st to 10th	Night	5% 2 8% 38% 40% 7%	378
	11th to 20th	Night	3 1 10% 35% 42% 9%	528
	21st to end	Night	5% 2 15% 38% 36% 4	787
September	1st to 10th	Night	6% 1 12% 41% 31% 8%	760
	11th to 20th	Night	9% 2 11% 34% 37% 8%	760
	21st to end	Night	12% 2 14% 38% 28% 5%	760
October	1st to 10th	Night	13% 2 13% 44% 25% 3	760
	11th to 20th	Night	17% 5% 12% 35% 26% 5%	760
	21st to end	Night	19% 4 18% 38% 16% 5%	836
November	1st to 10th	Night	25% 5% 15% 39% 16% 1	780
	11th to 20th	Night	26% 7% 19% 33% 14% 2	800
	21st to end	Night	22% 6% 15% 33% 20% 3	852
December	1st to 10th	Night	23% 4 15% 38% 17% 2	1000
	11th to 20th	Night	29% 5% 16% 36% 12% 1	1000
	21st to end	Night	33% 9% 19% 29% 9% 1	960

Page generated at 2025-06-07 07:28:18. Version ac07

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