Wilson Coulee Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wilson Coulee Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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% 8% 3 8% 5% 5% 5% 4 6% 8% 40%	131055
	Night	6% 9% 4 8% 5% 5% 5% 4 6% 8% 39%	127705
Transparency	Day	39% 7% 15% 25% 12% 2	130757
	Night	41% 6% 15% 23% 12% 2	127580
Seeing	Day	13% 10% 29% 29% 15% 4	16666
	Night	11% 7% 31% 35% 14% 2	25675

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	2 9% 7% 13% 8% 8% 5% 4 7% 7% 31%	3179
	11th to 20th	Day	2 8% 3 11% 6% 5% 5% 4 7% 7% 40%	3271
	21st to end	Day	2 7% 3 7% 5% 5% 5% 4 6% 12% 46%	4004
February	1st to 10th	Day	3 11% 4 14% 6% 6% 6% 5% 7% 7% 32%	1600
	11th to 20th	Day	3 12% 5% 10% 8% 7% 6% 6% 6% 8% 30%	1750
	21st to end	Day	2 11% 5% 11% 7% 7% 5% 4 6% 8% 33%	1615
March	1st to 10th	Day	3 9% 4 10% 6% 5% 4 3 5% 11% 39%	4518
	11th to 20th	Day	4 10% 4 9% 5% 4 5% 4 6% 9% 41%	4946
	21st to end	Day	7% 20% 7% 13% 6% 5% 4 4 6% 7% 23%	5568
April	1st to 10th	Day	9% 14% 5% 10% 6% 5% 5% 4 5% 7% 31%	2450
	11th to 20th	Day	11% 13% 5% 8% 5% 4 5% 4 6% 8% 31%	2570
	21st to end	Day	13% 11% 3 7% 5% 4 5% 5% 6% 8% 33%	2600
May	1st to 10th	Day	13% 12% 3 6% 4 3 4 4 6% 6% 39%	6151
	11th to 20th	Day	12% 10% 4 10% 6% 6% 6% 5% 7% 9% 26%	6734
	21st to end	Day	12% 6% 2 8% 6% 6% 6% 4 5% 7% 37%	7405
June	1st to 10th	Day	16% 7% 2 7% 5% 5% 6% 5% 7% 8% 32%	3000
	11th to 20th	Day	20% 8% 3 7% 5% 6% 6% 4 7% 8% 28%	3000
	21st to end	Day	9% 5% 2 5% 5% 5% 6% 5% 7% 10% 42%	3000
July	1st to 10th	Day	7% 5% 2 5% 4 4 5% 4 7% 8% 49%	6780
	11th to 20th	Day	10% 4 2 5% 3 4 4 4 7% 10% 48%	6780
	21st to end	Day	8% 5% 2 5% 3 3 4 3 6% 8% 52%	7157
August	1st to 10th	Day	9% 5% 3 6% 3 4 5% 5% 8% 10% 43%	4541
	11th to 20th	Day	7% 5% 2 5% 3 4 6% 4 8% 10% 46%	4193
	21st to end	Day	9% 4 1 4 3 3 4 4 6% 7% 55%	4664
September	1st to 10th	Day	10% 5% 2 5% 4 4 4 3 5% 6% 52%	2473
	11th to 20th	Day	13% 7% 2 5% 4 3 4 3 5% 7% 47%	2200
	21st to end	Day	8% 7% 2 6% 4 5% 4 4 6% 8% 46%	2200
October	1st to 10th	Day	7% 7% 2 7% 4 4 4 4 7% 9% 45%	3468
	11th to 20th	Day	5% 7% 2 6% 4 4 4 4 8% 11% 44%	3006
	21st to end	Day	6% 8% 3 6% 5% 5% 5% 4 6% 7% 46%	3200
November	1st to 10th	Day	8% 15% 4 9% 5% 4 4 4 5% 8% 34%	1667
	11th to 20th	Day	5% 11% 3 8% 4 4 5% 4 7% 7% 41%	1535
	21st to end	Day	4 12% 4 9% 4 5% 5% 5% 6% 10% 36%	1400
December	1st to 10th	Day	3 10% 4 11% 6% 6% 5% 5% 7% 8% 36%	2290
	11th to 20th	Day	2 9% 5% 11% 6% 6% 6% 5% 7% 9% 35%	2642
	21st to end	Day	4 10% 4 9% 5% 4 5% 4 5% 8% 42%	3498
January	1st to 10th	Night	3 13% 6% 12% 9% 7% 7% 4 6% 7% 27%	6766
	11th to 20th	Night	2 7% 4 13% 7% 6% 6% 5% 6% 8% 35%	6670
	21st to end	Night	3 9% 4 10% 5% 4 4 4 6% 9% 43%	6952
February	1st to 10th	Night	3 11% 5% 10% 7% 7% 6% 5% 6% 8% 32%	2800
	11th to 20th	Night	3 13% 5% 11% 6% 6% 5% 5% 6% 8% 32%	2535
	21st to end	Night	3 10% 5% 11% 8% 7% 4 4 5% 7% 35%	2014
March	1st to 10th	Night	2 8% 3 12% 7% 6% 6% 4 6% 10% 35%	5376
	11th to 20th	Night	3 9% 4 10% 6% 5% 5% 4 6% 8% 40%	5052
	21st to end	Night	9% 19% 6% 12% 6% 5% 4 3 6% 7% 23%	5406
April	1st to 10th	Night	8% 15% 4 8% 6% 4 5% 4 5% 7% 32%	1950
	11th to 20th	Night	11% 12% 5% 10% 6% 4 4 4 4 7% 33%	1781
	21st to end	Night	10% 11% 3 8% 6% 5% 5% 4 6% 9% 35%	1800
May	1st to 10th	Night	13% 12% 3 8% 5% 4 4 3 4 7% 38%	3804
	11th to 20th	Night	13% 10% 4 9% 4 5% 6% 4 6% 10% 30%	3159
	21st to end	Night	10% 7% 2 7% 5% 5% 5% 4 6% 7% 40%	3470
June	1st to 10th	Night	14% 8% 3 7% 5% 5% 5% 4 7% 7% 37%	1400
	11th to 20th	Night	18% 9% 3 6% 4 5% 5% 5% 6% 8% 32%	1400
	21st to end	Night	8% 5% 2 4 4 4 5% 4 8% 9% 47%	1400
July	1st to 10th	Night	8% 6% 2 6% 5% 4 4 4 5% 7% 49%	3180
	11th to 20th	Night	8% 5% 2 5% 4 4 4 4 6% 8% 50%	3180
	21st to end	Night	11% 5% 2 5% 3 3 3 3 5% 8% 52%	3684
August	1st to 10th	Night	10% 7% 2 5% 4 4 4 4 7% 9% 43%	2609
	11th to 20th	Night	6% 5% 2 6% 4 4 5% 5% 6% 10% 45%	2865
	21st to end	Night	8% 4 1 4 3 3 4 3 6% 8% 56%	3245
September	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 6% 7% 53%	1925
	11th to 20th	Night	14% 7% 2 4 3 2 4 3 6% 6% 51%	2200
	21st to end	Night	9% 7% 2 5% 3 3 4 4 7% 7% 50%	2200
October	1st to 10th	Night	7% 6% 2 6% 3 3 4 3 6% 7% 52%	3728
	11th to 20th	Night	6% 6% 2 5% 4 4 4 4 6% 8% 51%	4138
	21st to end	Night	4 9% 3 6% 5% 5% 5% 4 7% 7% 47%	4600
November	1st to 10th	Night	7% 13% 4 8% 5% 5% 5% 4 5% 7% 36%	2656
	11th to 20th	Night	4 10% 4 8% 5% 5% 4 4 6% 8% 41%	2862
	21st to end	Night	4 10% 3 9% 5% 5% 5% 4 6% 10% 38%	3000
December	1st to 10th	Night	2 9% 4 8% 6% 5% 7% 6% 8% 10% 36%	4890
	11th to 20th	Night	2 10% 5% 11% 8% 6% 6% 5% 7% 9% 31%	5550
	21st to end	Night	4 11% 4 9% 5% 5% 5% 4 6% 8% 40%	7458

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	49% 7% 10% 22% 10% 2	3179
	11th to 20th	Day	41% 5% 13% 21% 18% 3	3292
	21st to end	Day	31% 6% 15% 33% 14% 1	4004
February	1st to 10th	Day	49% 8% 14% 18% 9% 2	1600
	11th to 20th	Day	48% 7% 14% 20% 9% 2	1759
	21st to end	Day	47% 6% 14% 19% 14% 1	1615
March	1st to 10th	Day	39% 6% 15% 22% 16% 1	4518
	11th to 20th	Day	39% 6% 13% 26% 15% 1	4947
	21st to end	Day	58% 8% 13% 15% 6%	5568
April	1st to 10th	Day	50% 7% 15% 20% 8% 1	2450
	11th to 20th	Day	48% 6% 14% 21% 9% 1	2570
	21st to end	Day	47% 6% 15% 22% 9% 1	2600
May	1st to 10th	Day	44% 5% 10% 23% 16% 2	6151
	11th to 20th	Day	49% 9% 14% 21% 7%	6738
	21st to end	Day	41% 8% 16% 24% 11% 1	7406
June	1st to 10th	Day	47% 8% 15% 20% 9% 1	3000
	11th to 20th	Day	51% 7% 15% 18% 8% 2	3000
	21st to end	Day	33% 9% 20% 25% 10% 2	3000
July	1st to 10th	Day	28% 7% 18% 27% 16% 3	6706
	11th to 20th	Day	31% 6% 21% 29% 12% 1	6699
	21st to end	Day	29% 6% 15% 26% 17% 7%	6955
August	1st to 10th	Day	33% 8% 16% 35% 7% 1	4524
	11th to 20th	Day	30% 8% 25% 29% 7% 1	4232
	21st to end	Day	27% 6% 17% 35% 14% 1	4663
September	1st to 10th	Day	31% 6% 16% 26% 15% 5%	2473
	11th to 20th	Day	37% 5% 15% 27% 14% 2	2200
	21st to end	Day	35% 6% 14% 30% 12% 3	2200
October	1st to 10th	Day	34% 6% 17% 35% 8% 1	3468
	11th to 20th	Day	32% 6% 20% 26% 14% 3	3006
	21st to end	Day	36% 6% 11% 32% 13% 2	3200
November	1st to 10th	Day	49% 7% 12% 20% 11% 2	1667
	11th to 20th	Day	41% 6% 17% 24% 11% 2	1535
	21st to end	Day	41% 9% 17% 23% 9% 3	1400
December	1st to 10th	Day	45% 6% 14% 25% 8% 2	2290
	11th to 20th	Day	45% 7% 21% 19% 7% 1	2644
	21st to end	Day	39% 5% 12% 23% 18% 4	3498
January	1st to 10th	Night	55% 7% 11% 21% 6% 2	6766
	11th to 20th	Night	45% 6% 13% 19% 13% 5%	6711
	21st to end	Night	36% 6% 14% 26% 16% 1	6952
February	1st to 10th	Night	47% 6% 13% 19% 12% 2	2800
	11th to 20th	Night	48% 7% 13% 19% 11% 1	2536
	21st to end	Night	47% 5% 12% 21% 12% 2	2014
March	1st to 10th	Night	41% 8% 17% 20% 11% 2	5376
	11th to 20th	Night	39% 8% 15% 23% 14% 1	5052
	21st to end	Night	59% 6% 13% 13% 8% 1	5406
April	1st to 10th	Night	48% 7% 12% 21% 11% 1	1950
	11th to 20th	Night	50% 6% 13% 21% 9% 1	1781
	21st to end	Night	46% 6% 15% 22% 9% 2	1800
May	1st to 10th	Night	47% 5% 10% 21% 15% 2	3801
	11th to 20th	Night	47% 7% 14% 25% 6% 1	3159
	21st to end	Night	40% 7% 17% 26% 9% 1	3470
June	1st to 10th	Night	44% 6% 16% 23% 11% 1	1400
	11th to 20th	Night	47% 7% 17% 19% 9% 2	1400
	21st to end	Night	30% 7% 24% 26% 12% 1	1400
July	1st to 10th	Night	34% 6% 16% 24% 19% 2	3145
	11th to 20th	Night	30% 7% 21% 31% 8% 3	3138
	21st to end	Night	31% 6% 16% 26% 15% 6%	3581
August	1st to 10th	Night	35% 7% 16% 28% 13% 2	2601
	11th to 20th	Night	32% 9% 23% 26% 10% 1	2892
	21st to end	Night	25% 5% 19% 34% 14% 2	3245
September	1st to 10th	Night	30% 5% 18% 27% 16% 4	1925
	11th to 20th	Night	33% 5% 16% 29% 13% 4	2200
	21st to end	Night	32% 6% 18% 26% 14% 4	2200
October	1st to 10th	Night	29% 6% 15% 29% 19% 2	3728
	11th to 20th	Night	31% 5% 19% 24% 18% 2	4138
	21st to end	Night	36% 5% 11% 33% 14% 1	4600
November	1st to 10th	Night	46% 6% 14% 20% 12% 2	2656
	11th to 20th	Night	39% 6% 17% 21% 15% 2	2862
	21st to end	Night	41% 6% 17% 22% 11% 4	3000
December	1st to 10th	Night	42% 9% 19% 20% 8% 2	4890
	11th to 20th	Night	47% 7% 21% 17% 7%	5547
	21st to end	Night	40% 5% 11% 25% 16% 3	7458

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	3 10% 28% 36% 15% 10%	200
	11th to 20th	Day	9% 8% 36% 32% 13% 4	200
	21st to end	Day	7% 10% 24% 45% 10% 3	220
February	1st to 10th	Day	8% 13% 37% 29% 11% 3	200
	11th to 20th	Day	6% 7% 38% 34% 13% 2	297
	21st to end	Day	6% 13% 34% 32% 14% 1	379
March	1st to 10th	Day	8% 16% 39% 24% 11% 3	500
	11th to 20th	Day	8% 13% 33% 31% 12% 2	500
	21st to end	Day	13% 19% 32% 23% 10% 3	550
April	1st to 10th	Day	13% 18% 36% 24% 8% 2	500
	11th to 20th	Day	17% 16% 29% 26% 10% 2	492
	21st to end	Day	16% 19% 31% 21% 11% 3	500
May	1st to 10th	Day	18% 11% 28% 25% 13% 4	530
	11th to 20th	Day	19% 8% 25% 27% 17% 4	725
	21st to end	Day	18% 7% 23% 28% 20% 4	789
June	1st to 10th	Day	16% 11% 28% 30% 12% 2	730
	11th to 20th	Day	20% 10% 28% 28% 12% 2	730
	21st to end	Day	12% 7% 26% 32% 19% 5%	730
July	1st to 10th	Day	10% 6% 25% 33% 22% 4	721
	11th to 20th	Day	11% 6% 26% 34% 19% 3	718
	21st to end	Day	7% 7% 29% 31% 21% 5%	702
August	1st to 10th	Day	9% 7% 28% 33% 18% 5%	527
	11th to 20th	Day	9% 6% 28% 33% 18% 6%	517
	21st to end	Day	11% 7% 26% 30% 18% 8%	583
September	1st to 10th	Day	12% 9% 28% 28% 16% 8%	530
	11th to 20th	Day	17% 8% 25% 28% 17% 6%	530
	21st to end	Day	13% 17% 28% 29% 10% 3	530
October	1st to 10th	Day	13% 10% 26% 30% 16% 4	450
	11th to 20th	Day	14% 11% 32% 25% 14% 5%	330
	21st to end	Day	13% 13% 39% 23% 10% 3	363
November	1st to 10th	Day	19% 10% 28% 31% 9% 3	306
	11th to 20th	Day	12% 16% 33% 30% 8% 2	267
	21st to end	Day	9% 13% 29% 31% 11% 9%	200
December	1st to 10th	Day	8% 8% 35% 30% 18% 4	200
	11th to 20th	Day	5% 11% 40% 29% 12% 5%	200
	21st to end	Day	9% 5% 28% 32% 23% 3	220
January	1st to 10th	Night	9% 11% 36% 33% 10% 1	1000
	11th to 20th	Night	8% 8% 37% 35% 11% 1	970
	21st to end	Night	9% 10% 34% 36% 9% 1	880
February	1st to 10th	Night	8% 11% 32% 33% 13% 4	800
	11th to 20th	Night	10% 7% 33% 37% 12% 1	784
	21st to end	Night	7% 6% 31% 36% 17% 3	660
March	1st to 10th	Night	8% 7% 35% 35% 13% 3	800
	11th to 20th	Night	8% 10% 35% 35% 11% 2	800
	21st to end	Night	12% 8% 31% 35% 10% 3	880
April	1st to 10th	Night	13% 11% 31% 31% 12% 1	750
	11th to 20th	Night	15% 10% 36% 30% 9%	593
	21st to end	Night	16% 9% 31% 32% 11%	600
May	1st to 10th	Night	15% 11% 24% 32% 18%	514
	11th to 20th	Night	14% 9% 23% 33% 20% 1	394
	21st to end	Night	20% 6% 22% 35% 15% 1	433
June	1st to 10th	Night	16% 8% 26% 35% 13% 2	400
	11th to 20th	Night	20% 5% 29% 32% 13% 1	400
	21st to end	Night	10% 4 20% 40% 24% 2	400
July	1st to 10th	Night	11% 3 17% 43% 24% 1	390
	11th to 20th	Night	10% 3 20% 43% 22% 2	388
	21st to end	Night	7% 3 21% 43% 23% 2	438
August	1st to 10th	Night	12% 3 14% 39% 31% 2	398
	11th to 20th	Night	9% 4 18% 46% 22% 2	570
	21st to end	Night	9% 5% 23% 41% 22% 1	660
September	1st to 10th	Night	12% 3 26% 35% 21% 3	710
	11th to 20th	Night	18% 3 19% 36% 21% 3	800
	21st to end	Night	11% 6% 34% 36% 10% 3	800
October	1st to 10th	Night	15% 4 28% 36% 14% 3	800
	11th to 20th	Night	12% 7% 32% 33% 13% 3	800
	21st to end	Night	11% 8% 34% 36% 10% 1	880
November	1st to 10th	Night	13% 10% 38% 29% 9% 1	883
	11th to 20th	Night	10% 13% 39% 30% 7% 2	1000
	21st to end	Night	9% 7% 40% 33% 9% 2	1000
December	1st to 10th	Night	8% 8% 35% 34% 12% 3	1000
	11th to 20th	Night	6% 8% 40% 36% 9% 1	1000
	21st to end	Night	9% 7% 35% 34% 12% 2	1100

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