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 2026-10-14.

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	9% 8% 3 6% 4 4 4 3 5% 7% 47%	153023
	Night	11% 9% 3 7% 4 4 4 4 5% 6% 43%	147449
Transparency	Day	37% 3 11% 24% 18% 7%	152749
	Night	42% 3 11% 22% 16% 6%	147305
Seeing	Day	15% 6% 20% 28% 22% 9%	17143
	Night	18% 5% 16% 36% 22% 3	26330

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	17% 21% 8% 11% 5% 4 5% 4 5% 6% 14%	4723
	11th to 20th	Day	10% 9% 5% 10% 5% 4 4 4 5% 7% 37%	4716
	21st to end	Day	14% 13% 4 11% 6% 5% 4 3 5% 6% 30%	5186
February	1st to 10th	Day	19% 17% 5% 10% 5% 4 4 4 6% 7% 20%	1691
	11th to 20th	Day	17% 19% 5% 10% 6% 5% 5% 3 4 6% 22%	2013
	21st to end	Day	15% 18% 4 8% 6% 5% 5% 4 5% 6% 24%	1730
March	1st to 10th	Day	9% 12% 3 7% 6% 6% 6% 5% 8% 8% 29%	4618
	11th to 20th	Day	10% 12% 4 8% 5% 4 4 4 6% 7% 36%	4829
	21st to end	Day	15% 16% 4 10% 6% 6% 6% 4 6% 7% 20%	6004
April	1st to 10th	Day	13% 13% 4 9% 5% 5% 5% 5% 6% 7% 28%	2520
	11th to 20th	Day	11% 12% 3 9% 5% 5% 5% 5% 6% 6% 33%	2559
	21st to end	Day	13% 9% 3 8% 6% 6% 5% 4 5% 7% 34%	2730
May	1st to 10th	Day	6% 6% 2 4 4 4 4 3 5% 8% 54%	7869
	11th to 20th	Day	8% 7% 3 6% 6% 5% 5% 4 7% 9% 39%	8160
	21st to end	Day	7% 7% 3 7% 5% 5% 5% 4 6% 8% 44%	8993
June	1st to 10th	Day	13% 9% 3 6% 5% 4 5% 4 5% 7% 38%	3055
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 4 6% 7% 42%	3150
	21st to end	Day	6% 5% 2 4 3 3 4 3 5% 6% 58%	3053
July	1st to 10th	Day	3 2 2 2 2 3 2 4 5% 75%	8220
	11th to 20th	Day	3 2 1 3 2 2 3 2 3 4 74%	8220
	21st to end	Day	2 1 1 2 2 2 3 3 4 6% 75%	8916
August	1st to 10th	Day	3 2 1 2 2 2 2 2 3 6% 76%	6441
	11th to 20th	Day	3 2 1 3 2 3 3 3 5% 7% 68%	6011
	21st to end	Day	3 2 1 3 3 3 3 3 5% 8% 66%	6176
September	1st to 10th	Day	4 2 1 4 2 3 3 2 5% 7% 66%	2520
	11th to 20th	Day	8% 5% 2 6% 4 4 4 4 5% 7% 50%	2383
	21st to end	Day	8% 5% 2 7% 3 4 4 3 6% 8% 50%	2310
October	1st to 10th	Day	9% 6% 2 4 3 2 3 3 4 7% 58%	3623
	11th to 20th	Day	10% 7% 3 7% 4 4 5% 4 6% 7% 45%	3095
	21st to end	Day	16% 12% 3 7% 4 5% 4 4 5% 6% 35%	3200
November	1st to 10th	Day	23% 10% 3 7% 6% 4 4 4 5% 7% 26%	1854
	11th to 20th	Day	20% 15% 4 10% 6% 6% 5% 3 5% 6% 21%	1850
	21st to end	Day	19% 14% 5% 8% 6% 5% 5% 5% 6% 7% 22%	1600
December	1st to 10th	Day	15% 13% 6% 8% 5% 4 4 3 6% 8% 27%	2360
	11th to 20th	Day	20% 19% 6% 10% 6% 5% 5% 4 4 5% 16%	2712
	21st to end	Day	20% 17% 5% 9% 5% 5% 4 3 6% 7% 19%	3933
January	1st to 10th	Night	20% 18% 6% 11% 6% 5% 5% 4 5% 6% 15%	8172
	11th to 20th	Night	10% 10% 3 9% 4 5% 5% 4 6% 7% 38%	8182
	21st to end	Night	13% 11% 4 8% 5% 5% 5% 4 5% 6% 35%	8994
February	1st to 10th	Night	19% 18% 5% 8% 5% 4 5% 4 5% 4 24%	2925
	11th to 20th	Night	16% 16% 4 8% 4 4 4 3 5% 6% 29%	2494
	21st to end	Night	15% 17% 3 9% 5% 6% 4 4 5% 6% 26%	2076
March	1st to 10th	Night	9% 10% 3 8% 5% 5% 6% 4 6% 8% 37%	5496
	11th to 20th	Night	12% 13% 4 8% 4 4 3 3 5% 5% 39%	5390
	21st to end	Night	14% 11% 4 10% 6% 7% 6% 5% 7% 8% 22%	5211
April	1st to 10th	Night	12% 11% 3 7% 5% 6% 5% 5% 6% 8% 32%	2100
	11th to 20th	Night	14% 9% 3 7% 5% 4 4 3 6% 6% 40%	2010
	21st to end	Night	10% 9% 3 7% 6% 5% 4 4 6% 8% 38%	1890
May	1st to 10th	Night	7% 4 2 4 4 4 4 4 6% 8% 54%	5052
	11th to 20th	Night	9% 6% 2 6% 5% 5% 5% 6% 7% 9% 40%	4700
	21st to end	Night	9% 6% 3 6% 4 4 5% 4 6% 8% 46%	5178
June	1st to 10th	Night	11% 6% 2 6% 5% 4 4 3 6% 10% 43%	1559
	11th to 20th	Night	10% 6% 2 6% 5% 4 4 4 6% 7% 45%	1470
	21st to end	Night	7% 5% 2 5% 3 4 4 4 6% 6% 54%	1558
July	1st to 10th	Night	2 2 1 2 2 1 2 2 5% 6% 75%	4740
	11th to 20th	Night	2 1 1 2 2 2 2 2 4 5% 76%	4740
	21st to end	Night	3 2 1 2 2 2 2 2 4 6% 74%	5152
August	1st to 10th	Night	3 2 1 3 2 2 2 2 4 4 76%	3709
	11th to 20th	Night	2 2 1 3 3 4 4 4 5% 6% 67%	4051
	21st to end	Night	4 3 1 4 3 4 4 3 5% 6% 64%	4976
September	1st to 10th	Night	5% 3 1 2 2 3 3 3 5% 8% 64%	2100
	11th to 20th	Night	8% 4 2 5% 4 4 4 4 5% 7% 54%	2236
	21st to end	Night	9% 5% 2 5% 3 3 4 3 5% 7% 54%	2310
October	1st to 10th	Night	6% 3 2 4 3 3 3 4 6% 6% 60%	3793
	11th to 20th	Night	10% 6% 2 6% 4 4 5% 4 5% 7% 48%	4124
	21st to end	Night	13% 7% 2 6% 5% 5% 5% 4 5% 7% 42%	4600
November	1st to 10th	Night	19% 11% 3 7% 5% 5% 4 3 5% 7% 29%	2665
	11th to 20th	Night	21% 12% 4 8% 5% 5% 4 3 5% 6% 28%	2767
	21st to end	Night	19% 12% 3 8% 5% 4 4 4 6% 6% 29%	2995
December	1st to 10th	Night	12% 10% 3 6% 5% 3 4 3 6% 7% 40%	5040
	11th to 20th	Night	21% 17% 5% 11% 6% 6% 5% 4 6% 6% 15%	5700
	21st to end	Night	19% 16% 5% 9% 5% 5% 4 3 4 5% 22%	7294

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	70% 5% 12% 12% 1	4723
	11th to 20th	Day	46% 4 15% 25% 9% 1	4740
	21st to end	Day	56% 3 13% 18% 10%	5187
February	1st to 10th	Day	63% 5% 15% 11% 5%	1691
	11th to 20th	Day	63% 4 12% 15% 6%	2023
	21st to end	Day	60% 4 9% 20% 6% 1	1730
March	1st to 10th	Day	51% 4 8% 23% 12% 2	4618
	11th to 20th	Day	45% 3 13% 30% 8%	4829
	21st to end	Day	64% 3 14% 15% 4	6004
April	1st to 10th	Day	53% 4 16% 21% 6%	2520
	11th to 20th	Day	48% 4 12% 21% 13% 2	2559
	21st to end	Day	48% 4 11% 25% 12% 1	2730
May	1st to 10th	Day	29% 3 9% 37% 22% 2	7869
	11th to 20th	Day	38% 6% 13% 28% 14% 2	8163
	21st to end	Day	37% 3 18% 30% 12% 1	8994
June	1st to 10th	Day	46% 4 11% 24% 13% 1	3055
	11th to 20th	Day	41% 3 12% 26% 16% 3	3150
	21st to end	Day	27% 3 10% 32% 24% 4	3053
July	1st to 10th	Day	13% 2 4 25% 31% 24%	8151
	11th to 20th	Day	15% 2 4 16% 31% 32%	8145
	21st to end	Day	11% 3 4 27% 36% 20%	8723
August	1st to 10th	Day	13% 1 3 14% 42% 27%	6424
	11th to 20th	Day	16% 3 7% 37% 30% 6%	6051
	21st to end	Day	17% 3 8% 35% 30% 6%	6175
September	1st to 10th	Day	19% 3 10% 30% 27% 11%	2520
	11th to 20th	Day	32% 4 12% 26% 21% 5%	2383
	21st to end	Day	33% 3 15% 30% 16% 3	2310
October	1st to 10th	Day	28% 3 12% 34% 22% 2	3623
	11th to 20th	Day	37% 5% 18% 27% 13%	3095
	21st to end	Day	48% 3 12% 28% 9% 1	3200
November	1st to 10th	Day	58% 4 14% 20% 4 1	1854
	11th to 20th	Day	64% 5% 13% 14% 3 1	1850
	21st to end	Day	61% 5% 14% 14% 5% 1	1600
December	1st to 10th	Day	53% 6% 17% 20% 4 1	2360
	11th to 20th	Day	71% 4 13% 10% 1	2714
	21st to end	Day	63% 4 17% 13% 3	3933
January	1st to 10th	Night	70% 4 12% 13% 1	8172
	11th to 20th	Night	45% 4 17% 23% 9% 2	8220
	21st to end	Night	51% 3 13% 22% 10%	8994
February	1st to 10th	Night	63% 4 14% 14% 5% 1	2925
	11th to 20th	Night	56% 3 15% 18% 7%	2495
	21st to end	Night	60% 3 12% 17% 7% 1	2076
March	1st to 10th	Night	46% 3 8% 24% 18% 1	5496
	11th to 20th	Night	48% 2 9% 32% 9%	5391
	21st to end	Night	59% 3 12% 22% 4	5211
April	1st to 10th	Night	49% 4 13% 24% 9%	2100
	11th to 20th	Night	46% 3 10% 24% 14% 3	2010
	21st to end	Night	44% 4 11% 22% 16% 2	1890
May	1st to 10th	Night	30% 2 7% 33% 27% 3	5049
	11th to 20th	Night	35% 5% 11% 29% 18% 2	4700
	21st to end	Night	37% 4 16% 27% 16%	5182
June	1st to 10th	Night	38% 5% 9% 28% 17% 2	1559
	11th to 20th	Night	37% 3 9% 26% 20% 5%	1470
	21st to end	Night	30% 3 8% 25% 26% 7%	1558
July	1st to 10th	Night	10% 2 3 21% 34% 29%	4700
	11th to 20th	Night	12% 3 4 15% 29% 38%	4692
	21st to end	Night	13% 2 2 21% 36% 27%	5040
August	1st to 10th	Night	13% 2 2 8% 38% 36%	3701
	11th to 20th	Night	18% 3 6% 28% 36% 8%	4077
	21st to end	Night	21% 3 5% 27% 34% 10%	4976
September	1st to 10th	Night	20% 2 6% 26% 29% 16%	2100
	11th to 20th	Night	30% 3 8% 26% 26% 8%	2236
	21st to end	Night	31% 3 12% 30% 18% 7%	2310
October	1st to 10th	Night	24% 3 9% 32% 29% 3	3793
	11th to 20th	Night	36% 4 15% 23% 21% 1	4124
	21st to end	Night	41% 4 9% 32% 13% 1	4600
November	1st to 10th	Night	54% 4 13% 22% 6% 1	2665
	11th to 20th	Night	58% 3 15% 18% 5% 1	2767
	21st to end	Night	55% 4 15% 20% 6%	2995
December	1st to 10th	Night	42% 6% 21% 25% 6%	5040
	11th to 20th	Night	70% 4 13% 12% 1	5697
	21st to end	Night	64% 3 15% 14% 4	7294

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% 16% 4	220
	11th to 20th	Day	24% 4 22% 30% 15% 6%	220
	21st to end	Day	26% 3 26% 31% 14%	242
February	1st to 10th	Day	26% 11% 29% 25% 8% 3	231
	11th to 20th	Day	27% 11% 19% 28% 13% 2	516
	21st to end	Day	23% 11% 25% 28% 10% 3	446
March	1st to 10th	Day	19% 11% 26% 29% 13% 3	540
	11th to 20th	Day	20% 11% 19% 25% 18% 7%	540
	21st to end	Day	20% 11% 26% 25% 15% 3	594
April	1st to 10th	Day	19% 10% 20% 26% 20% 6%	540
	11th to 20th	Day	17% 10% 29% 25% 15% 5%	532
	21st to end	Day	14% 8% 21% 29% 22% 7%	540
May	1st to 10th	Day	12% 4 21% 30% 26% 6%	543
	11th to 20th	Day	17% 7% 19% 25% 17% 14%	568
	21st to end	Day	13% 4 17% 24% 27% 16%	617
June	1st to 10th	Day	14% 5% 16% 26% 26% 12%	685
	11th to 20th	Day	14% 7% 23% 30% 20% 6%	780
	21st to end	Day	7% 4 18% 29% 30% 12%	683
July	1st to 10th	Day	3 2 16% 30% 35% 13%	566
	11th to 20th	Day	4 3 21% 30% 28% 13%	564
	21st to end	Day	2 3 16% 28% 35% 16%	619
August	1st to 10th	Day	6% 3 17% 28% 28% 18%	567
	11th to 20th	Day	4 3 11% 30% 32% 20%	557
	21st to end	Day	9% 4 18% 25% 29% 16%	627
September	1st to 10th	Day	6% 2 17% 27% 30% 17%	570
	11th to 20th	Day	9% 5% 16% 28% 26% 15%	570
	21st to end	Day	10% 6% 16% 30% 28% 8%	570
October	1st to 10th	Day	13% 7% 17% 31% 24% 8%	570
	11th to 20th	Day	13% 9% 18% 29% 22% 9%	423
	21st to end	Day	24% 6% 21% 32% 14% 3	363
November	1st to 10th	Day	24% 9% 26% 31% 11%	330
	11th to 20th	Day	25% 9% 25% 26% 13% 3	308
	21st to end	Day	23% 5% 25% 32% 12% 2	220
December	1st to 10th	Day	31% 6% 19% 23% 14% 7%	220
	11th to 20th	Day	33% 9% 26% 23% 7% 3	220
	21st to end	Day	32% 7% 26% 24% 8% 3	242
January	1st to 10th	Night	30% 8% 18% 30% 12% 2	840
	11th to 20th	Night	24% 7% 16% 35% 15% 3	840
	21st to end	Night	20% 5% 17% 38% 18% 2	924
February	1st to 10th	Night	26% 8% 20% 33% 11% 2	840
	11th to 20th	Night	24% 6% 17% 34% 17% 2	824
	21st to end	Night	23% 10% 19% 33% 14% 2	692
March	1st to 10th	Night	19% 5% 20% 36% 17% 3	840
	11th to 20th	Night	18% 7% 21% 31% 21% 2	840
	21st to end	Night	22% 5% 19% 38% 14% 2	924
April	1st to 10th	Night	19% 5% 16% 38% 19% 2	840
	11th to 20th	Night	17% 6% 23% 34% 16% 3	762
	21st to end	Night	15% 5% 18% 37% 23% 3	630
May	1st to 10th	Night	11% 4 20% 36% 26% 3	540
	11th to 20th	Night	12% 5% 12% 34% 28% 9%	414
	21st to end	Night	14% 4 14% 33% 30% 6%	455
June	1st to 10th	Night	11% 5% 14% 36% 29% 6%	420
	11th to 20th	Night	11% 6% 24% 35% 21% 4	420
	21st to end	Night	8% 3 14% 39% 31% 5%	420
July	1st to 10th	Night	4 1 11% 38% 40% 6%	410
	11th to 20th	Night	2 2 16% 44% 33% 3	408
	21st to end	Night	2 2 12% 38% 41% 5%	460
August	1st to 10th	Night	5% 2 8% 38% 40% 6%	418
	11th to 20th	Night	3 1 10% 37% 41% 8%	584
	21st to end	Night	6% 2 15% 39% 35% 4	870
September	1st to 10th	Night	6% 1 12% 41% 31% 8%	840
	11th to 20th	Night	9% 2 10% 35% 36% 7%	840
	21st to end	Night	11% 3 15% 38% 29% 5%	840
October	1st to 10th	Night	12% 2 14% 44% 25% 3	840
	11th to 20th	Night	17% 6% 13% 34% 25% 5%	812
	21st to end	Night	19% 4 19% 38% 16% 5%	880
November	1st to 10th	Night	24% 5% 16% 39% 16% 1	820
	11th to 20th	Night	26% 7% 18% 33% 14% 2	840
	21st to end	Night	23% 6% 15% 34% 19% 3	895
December	1st to 10th	Night	25% 5% 15% 37% 17% 2	1050
	11th to 20th	Night	29% 6% 17% 35% 12% 1	1050
	21st to end	Night	32% 9% 20% 29% 9% 1	1008

Page generated at 2026-10-11 02:06:30. Version ac07

Copyright © 2026 Attilla Danko.