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 2024-11-06.

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 7% 5% 5% 5% 4 6% 8% 40%	105621
	Night	6% 9% 3 8% 5% 5% 5% 4 6% 8% 40%	102039
Transparency	Day	39% 7% 16% 25% 11% 2	105367
	Night	40% 7% 15% 24% 12% 2	101939
Seeing	Day	13% 10% 29% 29% 15% 4	15360
	Night	11% 7% 31% 35% 14% 2	24406

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 10% 6% 12% 6% 5% 5% 5% 7% 9% 34%	2219
	11th to 20th	Day	3 9% 3 11% 6% 6% 5% 5% 8% 8% 37%	2269
	21st to end	Day	3 9% 3 8% 5% 5% 6% 4 6% 10% 41%	2794
February	1st to 10th	Day	3 11% 4 13% 6% 6% 6% 5% 7% 7% 32%	1520
	11th to 20th	Day	3 12% 5% 10% 8% 6% 6% 6% 6% 8% 30%	1660
	21st to end	Day	2 12% 6% 12% 7% 7% 5% 4 7% 7% 32%	1536
March	1st to 10th	Day	2 9% 5% 13% 8% 7% 5% 4 6% 7% 34%	3138
	11th to 20th	Day	6% 10% 3 6% 4 3 4 3 6% 9% 46%	3449
	21st to end	Day	7% 21% 7% 13% 5% 4 4 3 5% 5% 25%	3881
April	1st to 10th	Day	9% 14% 5% 9% 6% 5% 5% 4 5% 8% 30%	2327
	11th to 20th	Day	11% 12% 5% 9% 5% 4 5% 5% 6% 8% 31%	2440
	21st to end	Day	13% 11% 3 7% 5% 4 5% 5% 6% 8% 32%	2470
May	1st to 10th	Day	16% 15% 4 7% 5% 4 5% 4 5% 6% 29%	4301
	11th to 20th	Day	10% 7% 3 9% 6% 5% 6% 5% 8% 9% 31%	4694
	21st to end	Day	17% 8% 3 9% 7% 7% 7% 4 5% 7% 25%	5161
June	1st to 10th	Day	17% 8% 3 7% 5% 5% 6% 5% 7% 8% 30%	2850
	11th to 20th	Day	20% 8% 3 7% 5% 6% 6% 4 7% 7% 27%	2850
	21st to end	Day	9% 5% 2 5% 4 5% 6% 5% 7% 10% 42%	2850
July	1st to 10th	Day	7% 5% 2 5% 4 4 5% 4 7% 7% 50%	4740
	11th to 20th	Day	7% 3 2 4 2 3 4 4 7% 10% 54%	4740
	21st to end	Day	3 3 1 4 3 3 4 4 6% 9% 60%	4989
August	1st to 10th	Day	9% 5% 3 5% 3 4 5% 5% 8% 10% 43%	4401
	11th to 20th	Day	7% 5% 2 5% 3 4 5% 4 8% 10% 46%	4062
	21st to end	Day	9% 4 1 5% 3 3 4 4 6% 7% 54%	4521
September	1st to 10th	Day	11% 5% 2 6% 4 4 4 3 5% 6% 51%	2350
	11th to 20th	Day	14% 8% 2 5% 4 3 4 3 5% 7% 45%	2090
	21st to end	Day	9% 8% 2 6% 4 5% 4 4 6% 8% 45%	2090
October	1st to 10th	Day	7% 7% 2 7% 4 4 4 4 7% 9% 45%	3362
	11th to 20th	Day	5% 7% 2 6% 4 4 4 4 8% 11% 44%	2914
	21st to end	Day	6% 8% 3 6% 5% 5% 5% 4 6% 7% 46%	3101
November	1st to 10th	Day	8% 15% 4 9% 6% 4 4 4 5% 8% 33%	1609
	11th to 20th	Day	6% 11% 3 8% 4 3 5% 4 7% 7% 41%	1459
	21st to end	Day	4 12% 4 8% 4 5% 5% 5% 6% 10% 38%	1330
December	1st to 10th	Day	4 10% 4 11% 7% 6% 4 4 6% 8% 35%	1330
	11th to 20th	Day	2 11% 6% 13% 6% 6% 5% 4 7% 7% 33%	1682
	21st to end	Day	4 8% 3 9% 5% 4 4 5% 5% 9% 45%	2442
January	1st to 10th	Night	3 11% 5% 12% 8% 6% 7% 5% 7% 9% 28%	4726
	11th to 20th	Night	3 7% 5% 14% 6% 5% 7% 6% 6% 8% 34%	4658
	21st to end	Night	4 11% 4 10% 6% 5% 5% 4 6% 8% 38%	4862
February	1st to 10th	Night	4 11% 4 10% 7% 7% 6% 5% 6% 8% 33%	2660
	11th to 20th	Night	3 14% 5% 10% 5% 6% 5% 5% 6% 8% 32%	2405
	21st to end	Night	3 10% 5% 11% 8% 7% 4 4 5% 7% 35%	1917
March	1st to 10th	Night	2 9% 4 14% 9% 7% 7% 5% 6% 8% 31%	3736
	11th to 20th	Night	4 9% 3 6% 4 5% 4 4 6% 8% 47%	3529
	21st to end	Night	10% 21% 7% 12% 7% 5% 3 3 6% 6% 22%	3771
April	1st to 10th	Night	9% 15% 4 8% 6% 4 5% 4 5% 7% 32%	1853
	11th to 20th	Night	11% 12% 5% 10% 6% 4 4 4 4 7% 32%	1691
	21st to end	Night	11% 11% 3 8% 6% 5% 5% 4 6% 8% 35%	1710
May	1st to 10th	Night	17% 14% 3 9% 5% 4 4 4 4 6% 31%	2662
	11th to 20th	Night	10% 8% 3 8% 5% 5% 6% 4 6% 10% 35%	2199
	21st to end	Night	15% 9% 3 8% 6% 5% 6% 5% 7% 7% 30%	2414
June	1st to 10th	Night	14% 8% 3 7% 5% 5% 5% 4 7% 7% 36%	1330
	11th to 20th	Night	18% 9% 2 6% 4 5% 5% 5% 6% 8% 31%	1330
	21st to end	Night	8% 5% 2 4 3 4 5% 4 8% 9% 47%	1330
July	1st to 10th	Night	7% 5% 2 7% 6% 5% 4 3 5% 7% 50%	2220
	11th to 20th	Night	5% 3 1 4 4 4 5% 5% 7% 10% 54%	2220
	21st to end	Night	5% 3 2 4 3 3 3 3 5% 8% 61%	2561
August	1st to 10th	Night	10% 6% 2 5% 4 4 4 4 7% 9% 43%	2529
	11th to 20th	Night	6% 5% 2 6% 4 4 5% 5% 7% 10% 45%	2776
	21st to end	Night	8% 4 1 4 3 3 4 3 6% 9% 55%	3146
September	1st to 10th	Night	10% 6% 2 5% 3 4 3 3 5% 7% 52%	1828
	11th to 20th	Night	14% 7% 2 4 3 2 4 3 5% 6% 50%	2090
	21st to end	Night	9% 7% 2 5% 3 3 4 4 7% 7% 49%	2090
October	1st to 10th	Night	7% 6% 2 6% 3 3 4 3 6% 8% 52%	3614
	11th to 20th	Night	6% 6% 2 5% 4 4 4 4 6% 8% 50%	4010
	21st to end	Night	5% 9% 3 7% 5% 5% 5% 4 7% 7% 46%	4457
November	1st to 10th	Night	7% 13% 5% 8% 5% 5% 5% 4 5% 7% 36%	2573
	11th to 20th	Night	4 10% 4 8% 6% 5% 4 4 6% 8% 42%	2718
	21st to end	Night	4 10% 3 9% 5% 5% 5% 4 6% 10% 39%	2850
December	1st to 10th	Night	4 11% 4 9% 7% 6% 6% 4 6% 8% 35%	2850
	11th to 20th	Night	2 9% 4 11% 8% 6% 6% 5% 7% 9% 34%	3510
	21st to end	Night	3 9% 3 8% 5% 5% 5% 4 6% 8% 44%	5214

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	42% 8% 14% 24% 10% 3	2219
	11th to 20th	Day	43% 6% 11% 19% 17% 4	2290
	21st to end	Day	36% 7% 16% 30% 9% 1	2794
February	1st to 10th	Day	48% 8% 15% 18% 8% 2	1520
	11th to 20th	Day	48% 7% 14% 20% 9% 2	1669
	21st to end	Day	48% 6% 14% 18% 13% 1	1536
March	1st to 10th	Day	45% 9% 14% 18% 13% 1	3138
	11th to 20th	Day	33% 7% 16% 29% 14% 1	3450
	21st to end	Day	58% 8% 13% 17% 4	3881
April	1st to 10th	Day	51% 7% 15% 19% 7% 1	2327
	11th to 20th	Day	48% 7% 14% 21% 9% 1	2440
	21st to end	Day	48% 6% 15% 21% 9% 1	2470
May	1st to 10th	Day	54% 6% 11% 23% 5%	4301
	11th to 20th	Day	42% 9% 16% 24% 9%	4698
	21st to end	Day	52% 9% 14% 17% 7% 1	5162
June	1st to 10th	Day	49% 8% 15% 18% 8% 1	2850
	11th to 20th	Day	52% 7% 16% 16% 7% 2	2850
	21st to end	Day	33% 9% 20% 25% 10% 2	2850
July	1st to 10th	Day	29% 7% 16% 27% 17% 5%	4666
	11th to 20th	Day	24% 6% 20% 34% 15% 2	4659
	21st to end	Day	19% 5% 12% 29% 24% 10%	4831
August	1st to 10th	Day	33% 8% 16% 35% 7% 1	4384
	11th to 20th	Day	30% 8% 25% 29% 7% 1	4101
	21st to end	Day	27% 6% 17% 35% 13% 1	4520
September	1st to 10th	Day	32% 6% 15% 26% 16% 5%	2350
	11th to 20th	Day	38% 5% 16% 26% 13% 2	2090
	21st to end	Day	36% 6% 14% 29% 12% 3	2090
October	1st to 10th	Day	34% 7% 17% 34% 7% 1	3362
	11th to 20th	Day	32% 5% 21% 26% 13% 3	2914
	21st to end	Day	36% 7% 11% 33% 12% 2	3101
November	1st to 10th	Day	50% 7% 12% 20% 11% 1	1609
	11th to 20th	Day	40% 6% 17% 24% 10% 2	1459
	21st to end	Day	38% 9% 18% 24% 9% 3	1330
December	1st to 10th	Day	46% 6% 14% 18% 12% 4	1330
	11th to 20th	Day	48% 7% 17% 18% 8% 1	1684
	21st to end	Day	34% 5% 13% 24% 18% 5%	2442
January	1st to 10th	Night	49% 8% 13% 21% 7% 2	4726
	11th to 20th	Night	46% 7% 13% 16% 14% 5%	4699
	21st to end	Night	41% 7% 15% 24% 12% 1	4862
February	1st to 10th	Night	47% 7% 13% 20% 11% 3	2660
	11th to 20th	Night	48% 7% 14% 19% 11% 1	2406
	21st to end	Night	48% 5% 12% 21% 12% 2	1917
March	1st to 10th	Night	46% 10% 12% 22% 9% 1	3736
	11th to 20th	Night	31% 10% 20% 24% 14% 1	3529
	21st to end	Night	62% 7% 12% 14% 5% 1	3771
April	1st to 10th	Night	49% 7% 13% 21% 10% 1	1853
	11th to 20th	Night	51% 6% 14% 20% 9% 1	1691
	21st to end	Night	46% 6% 15% 22% 9% 2	1710
May	1st to 10th	Night	54% 5% 12% 22% 6%	2662
	11th to 20th	Night	40% 8% 15% 28% 8% 1	2199
	21st to end	Night	49% 8% 17% 18% 7% 1	2414
June	1st to 10th	Night	44% 6% 16% 22% 10% 1	1330
	11th to 20th	Night	47% 7% 17% 18% 9% 2	1330
	21st to end	Night	30% 7% 24% 26% 12% 1	1330
July	1st to 10th	Night	34% 6% 16% 23% 19% 3	2185
	11th to 20th	Night	23% 7% 23% 33% 10% 4	2178
	21st to end	Night	22% 6% 10% 31% 22% 9%	2480
August	1st to 10th	Night	35% 7% 15% 28% 13% 2	2521
	11th to 20th	Night	31% 9% 23% 26% 10% 1	2803
	21st to end	Night	25% 5% 20% 34% 14% 2	3146
September	1st to 10th	Night	31% 5% 17% 25% 17% 5%	1828
	11th to 20th	Night	33% 5% 16% 28% 13% 4	2090
	21st to end	Night	32% 6% 18% 26% 14% 4	2090
October	1st to 10th	Night	29% 6% 15% 28% 19% 2	3614
	11th to 20th	Night	31% 5% 19% 25% 18% 2	4010
	21st to end	Night	36% 5% 12% 33% 14%	4457
November	1st to 10th	Night	46% 6% 14% 20% 12% 2	2573
	11th to 20th	Night	39% 6% 17% 21% 15% 2	2718
	21st to end	Night	39% 6% 18% 22% 11% 4	2850
December	1st to 10th	Night	45% 7% 15% 20% 10% 4	2850
	11th to 20th	Night	43% 8% 21% 21% 7% 1	3507
	21st to end	Night	37% 6% 13% 25% 15% 5%	5214

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 9% 29% 34% 14% 11%	180
	11th to 20th	Day	9% 7% 36% 31% 13% 4	180
	21st to end	Day	8% 10% 22% 45% 12% 4	198
February	1st to 10th	Day	9% 13% 36% 29% 9% 3	180
	11th to 20th	Day	6% 8% 37% 33% 13% 2	267
	21st to end	Day	6% 13% 33% 31% 15% 1	348
March	1st to 10th	Day	8% 16% 40% 23% 10% 3	460
	11th to 20th	Day	9% 14% 33% 29% 13% 2	460
	21st to end	Day	13% 19% 32% 22% 10% 3	506
April	1st to 10th	Day	13% 19% 36% 24% 7% 1	460
	11th to 20th	Day	17% 14% 29% 26% 11% 3	452
	21st to end	Day	16% 20% 32% 21% 10% 2	460
May	1st to 10th	Day	19% 10% 28% 25% 14% 4	488
	11th to 20th	Day	18% 8% 24% 28% 18% 4	675
	21st to end	Day	19% 7% 23% 26% 20% 4	734
June	1st to 10th	Day	17% 10% 28% 30% 12% 3	680
	11th to 20th	Day	21% 10% 28% 26% 12% 2	680
	21st to end	Day	12% 8% 27% 31% 18% 5%	680
July	1st to 10th	Day	10% 6% 25% 34% 22% 4	671
	11th to 20th	Day	11% 7% 26% 34% 20% 3	668
	21st to end	Day	6% 6% 31% 31% 21% 5%	652
August	1st to 10th	Day	9% 7% 28% 34% 17% 4	487
	11th to 20th	Day	9% 6% 27% 34% 18% 7%	477
	21st to end	Day	12% 7% 28% 30% 16% 7%	539
September	1st to 10th	Day	13% 9% 30% 28% 14% 6%	490
	11th to 20th	Day	18% 8% 26% 28% 15% 4	490
	21st to end	Day	14% 18% 25% 30% 10% 3	490
October	1st to 10th	Day	14% 11% 26% 30% 17% 4	414
	11th to 20th	Day	14% 11% 30% 25% 14% 5%	300
	21st to end	Day	14% 12% 38% 23% 10% 3	330
November	1st to 10th	Day	19% 11% 28% 29% 9% 3	285
	11th to 20th	Day	12% 15% 32% 30% 9% 2	241
	21st to end	Day	7% 13% 31% 29% 11% 9%	180
December	1st to 10th	Day	7% 8% 34% 30% 18% 3	180
	11th to 20th	Day	4 11% 43% 28% 10% 4	180
	21st to end	Day	9% 5% 29% 31% 23% 4	198
January	1st to 10th	Night	9% 10% 36% 34% 10% 1	950
	11th to 20th	Night	8% 8% 36% 36% 12% 1	922
	21st to end	Night	9% 10% 33% 37% 10% 1	836
February	1st to 10th	Night	8% 11% 32% 33% 13% 4	760
	11th to 20th	Night	10% 8% 32% 37% 12% 1	744
	21st to end	Night	6% 6% 30% 37% 18% 3	628
March	1st to 10th	Night	8% 7% 35% 34% 13% 3	760
	11th to 20th	Night	9% 10% 35% 34% 11% 2	760
	21st to end	Night	12% 8% 31% 36% 11% 3	836
April	1st to 10th	Night	13% 11% 32% 31% 12% 1	713
	11th to 20th	Night	16% 9% 35% 31% 9%	563
	21st to end	Night	17% 9% 31% 31% 11%	570
May	1st to 10th	Night	16% 10% 24% 31% 18%	489
	11th to 20th	Night	13% 8% 23% 34% 20% 1	374
	21st to end	Night	21% 7% 23% 33% 16% 1	411
June	1st to 10th	Night	16% 9% 26% 35% 13% 2	380
	11th to 20th	Night	21% 6% 28% 31% 13% 1	380
	21st to end	Night	10% 4 20% 40% 24% 2	380
July	1st to 10th	Night	10% 3 18% 44% 24% 1	370
	11th to 20th	Night	10% 3 20% 43% 23% 2	368
	21st to end	Night	6% 3 21% 44% 23% 2	416
August	1st to 10th	Night	12% 3 14% 39% 30% 2	378
	11th to 20th	Night	9% 4 18% 45% 23% 2	541
	21st to end	Night	9% 5% 23% 40% 22% 1	627
September	1st to 10th	Night	13% 3 27% 35% 19% 3	674
	11th to 20th	Night	19% 4 20% 35% 20% 2	760
	21st to end	Night	12% 6% 33% 35% 11% 3	760
October	1st to 10th	Night	15% 4 27% 37% 14% 3	760
	11th to 20th	Night	12% 6% 32% 33% 14% 3	760
	21st to end	Night	11% 7% 33% 37% 11% 1	836
November	1st to 10th	Night	14% 10% 38% 29% 9% 1	855
	11th to 20th	Night	10% 13% 39% 31% 7% 1	950
	21st to end	Night	9% 7% 40% 32% 10% 2	950
December	1st to 10th	Night	8% 8% 35% 34% 12% 3	950
	11th to 20th	Night	5% 8% 40% 36% 9% 1	950
	21st to end	Night	9% 7% 35% 35% 12% 2	1045

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