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 2024-01-16.

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% 8% 3 7% 6% 3 5% 5% 6% 8% 38%	74642
	Night	7% 9% 4 7% 6% 4 5% 5% 6% 8% 40%	75162
Transparency	Day	42% 7% 16% 23% 11% 2	74328
	Night	41% 7% 16% 23% 12% 2	74994
Seeing	Day	13% 10% 30% 29% 15% 4	14196
	Night	11% 7% 31% 35% 14% 2	23466

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	3 10% 5% 8% 7% 4 5% 5% 7% 9% 36%	1329
	11th to 20th	Day	4 9% 5% 7% 6% 3 5% 5% 7% 10% 39%	1337
	21st to end	Day	4 11% 4 9% 7% 4 6% 5% 6% 9% 35%	1584
February	1st to 10th	Day	3 9% 5% 11% 7% 4 7% 6% 7% 7% 33%	1440
	11th to 20th	Day	4 11% 6% 8% 9% 4 6% 7% 6% 9% 29%	1571
	21st to end	Day	3 11% 6% 11% 8% 4 6% 5% 6% 8% 32%	1448
March	1st to 10th	Day	4 9% 6% 10% 8% 4 6% 5% 6% 8% 34%	1800
	11th to 20th	Day	4 10% 4 7% 6% 3 6% 5% 7% 9% 39%	1952
	21st to end	Day	8% 14% 4 10% 8% 4 6% 5% 7% 7% 29%	2209
April	1st to 10th	Day	9% 13% 6% 8% 8% 4 6% 5% 5% 8% 29%	2204
	11th to 20th	Day	12% 12% 5% 7% 6% 3 5% 5% 6% 8% 31%	2310
	21st to end	Day	14% 10% 3 6% 5% 3 5% 6% 7% 8% 32%	2340
May	1st to 10th	Day	14% 11% 4 7% 6% 3 6% 6% 7% 7% 29%	2438
	11th to 20th	Day	14% 7% 3 6% 6% 4 5% 5% 6% 8% 35%	2654
	21st to end	Day	22% 6% 3 6% 7% 4 5% 6% 5% 8% 29%	2917
June	1st to 10th	Day	18% 7% 3 6% 6% 4 6% 6% 7% 8% 29%	2700
	11th to 20th	Day	21% 8% 3 6% 6% 4 6% 5% 6% 8% 27%	2700
	21st to end	Day	10% 4 2 5% 4 4 6% 6% 6% 10% 43%	2700
July	1st to 10th	Day	9% 5% 2 5% 5% 3 6% 6% 7% 8% 44%	2700
	11th to 20th	Day	10% 4 2 5% 3 3 5% 5% 7% 9% 47%	2700
	21st to end	Day	5% 4 2 4 4 3 5% 5% 6% 8% 56%	2856
August	1st to 10th	Day	11% 4 2 4 4 2 4 6% 6% 9% 47%	2507
	11th to 20th	Day	9% 4 2 4 3 2 4 5% 7% 9% 50%	2321
	21st to end	Day	11% 4 2 4 4 3 4 5% 6% 8% 50%	2574
September	1st to 10th	Day	11% 5% 2 5% 4 3 4 4 5% 6% 51%	2227
	11th to 20th	Day	14% 7% 2 4 4 2 4 5% 5% 7% 46%	1980
	21st to end	Day	9% 8% 3 5% 4 3 4 5% 6% 9% 45%	1980
October	1st to 10th	Day	11% 11% 3 7% 5% 3 3 5% 6% 7% 40%	1908
	11th to 20th	Day	9% 10% 3 7% 6% 3 5% 4 6% 9% 39%	1666
	21st to end	Day	7% 11% 4 6% 5% 4 5% 6% 7% 8% 37%	1782
November	1st to 10th	Day	8% 14% 5% 8% 6% 3 5% 4 5% 8% 33%	1565
	11th to 20th	Day	6% 10% 4 7% 5% 2 5% 5% 7% 8% 42%	1459
	21st to end	Day	4 11% 5% 7% 4 4 5% 6% 6% 10% 38%	1330
December	1st to 10th	Day	4 9% 4 10% 7% 5% 4 6% 5% 9% 36%	1330
	11th to 20th	Day	2 11% 6% 11% 8% 4 6% 6% 6% 7% 34%	1682
	21st to end	Day	4 7% 4 8% 6% 3 5% 6% 5% 9% 45%	2442
January	1st to 10th	Night	3 11% 5% 9% 9% 5% 7% 6% 6% 9% 30%	2836
	11th to 20th	Night	5% 7% 5% 9% 7% 4 6% 6% 7% 10% 35%	2749
	21st to end	Night	4 9% 5% 8% 7% 4 6% 6% 6% 8% 37%	2772
February	1st to 10th	Night	3 9% 5% 9% 7% 6% 6% 6% 6% 9% 34%	2520
	11th to 20th	Night	4 12% 6% 9% 6% 4 6% 6% 6% 8% 32%	2275
	21st to end	Night	3 9% 6% 11% 9% 5% 6% 5% 5% 6% 35%	1807
March	1st to 10th	Night	3 10% 5% 9% 10% 4 6% 6% 6% 9% 33%	2160
	11th to 20th	Night	4 10% 4 7% 6% 4 6% 6% 6% 9% 38%	2006
	21st to end	Night	6% 12% 6% 9% 7% 4 5% 5% 7% 9% 31%	2146
April	1st to 10th	Night	8% 13% 5% 8% 7% 3 5% 6% 5% 7% 32%	1756
	11th to 20th	Night	12% 11% 5% 9% 7% 3 4 5% 5% 7% 32%	1601
	21st to end	Night	11% 10% 3 7% 7% 3 5% 5% 5% 8% 35%	1620
May	1st to 10th	Night	11% 11% 4 8% 6% 3 4 5% 4 9% 33%	1520
	11th to 20th	Night	12% 7% 2 5% 5% 3 6% 5% 5% 8% 43%	1239
	21st to end	Night	20% 7% 3 5% 5% 3 4 6% 7% 8% 33%	1358
June	1st to 10th	Night	15% 7% 4 5% 6% 3 5% 6% 6% 8% 36%	1260
	11th to 20th	Night	19% 8% 3 5% 5% 3 6% 5% 6% 8% 32%	1260
	21st to end	Night	8% 5% 3 4 4 2 5% 5% 8% 10% 47%	1260
July	1st to 10th	Night	9% 5% 2 5% 5% 3 4 4 5% 8% 50%	1260
	11th to 20th	Night	8% 4 2 5% 5% 3 5% 6% 6% 9% 48%	1260
	21st to end	Night	7% 3 2 3 3 2 3 4 6% 8% 58%	1459
August	1st to 10th	Night	10% 4 2 5% 5% 3 4 5% 6% 9% 48%	1432
	11th to 20th	Night	9% 4 1 3 3 2 4 5% 6% 10% 53%	1580
	21st to end	Night	10% 4 1 3 3 3 3 4 6% 8% 55%	1782
September	1st to 10th	Night	11% 6% 2 4 4 2 4 4 5% 7% 51%	1731
	11th to 20th	Night	14% 7% 2 4 3 2 3 3 5% 6% 51%	1980
	21st to end	Night	10% 7% 2 4 4 3 4 4 6% 8% 48%	1980
October	1st to 10th	Night	11% 9% 3 6% 5% 3 4 4 5% 6% 44%	2052
	11th to 20th	Night	8% 8% 3 6% 5% 3 4 4 6% 9% 44%	2290
	21st to end	Night	7% 10% 3 6% 5% 3 5% 5% 6% 8% 39%	2574
November	1st to 10th	Night	8% 12% 5% 7% 6% 4 5% 5% 5% 7% 37%	2495
	11th to 20th	Night	4 9% 4 7% 6% 4 5% 5% 6% 8% 42%	2718
	21st to end	Night	4 9% 4 8% 5% 4 5% 5% 6% 10% 40%	2850
December	1st to 10th	Night	4 10% 4 8% 8% 5% 6% 6% 6% 9% 35%	2850
	11th to 20th	Night	2 8% 4 10% 9% 4 6% 7% 7% 9% 34%	3510
	21st to end	Night	3 9% 4 7% 6% 4 5% 5% 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	43% 6% 14% 24% 8% 4	1329
	11th to 20th	Day	41% 6% 16% 24% 10% 3	1337
	21st to end	Day	44% 8% 15% 19% 13% 2	1584
February	1st to 10th	Day	47% 8% 15% 19% 8% 3	1440
	11th to 20th	Day	49% 8% 14% 20% 9% 1	1580
	21st to end	Day	49% 6% 14% 17% 13% 1	1448
March	1st to 10th	Day	45% 8% 17% 19% 9% 1	1800
	11th to 20th	Day	40% 7% 14% 25% 12% 2	1953
	21st to end	Day	51% 8% 16% 18% 6% 1	2209
April	1st to 10th	Day	51% 8% 16% 19% 7% 1	2204
	11th to 20th	Day	49% 6% 13% 21% 9% 1	2310
	21st to end	Day	48% 6% 15% 21% 9% 1	2340
May	1st to 10th	Day	50% 7% 16% 20% 7%	2438
	11th to 20th	Day	44% 8% 16% 22% 10% 1	2658
	21st to end	Day	51% 8% 15% 20% 6% 1	2918
June	1st to 10th	Day	50% 8% 15% 18% 7% 1	2700
	11th to 20th	Day	52% 7% 16% 16% 7% 2	2700
	21st to end	Day	33% 9% 21% 25% 10% 2	2700
July	1st to 10th	Day	34% 8% 20% 24% 12% 2	2626
	11th to 20th	Day	33% 7% 17% 27% 14% 2	2619
	21st to end	Day	24% 8% 18% 31% 16% 3	2697
August	1st to 10th	Day	32% 8% 19% 28% 11% 2	2490
	11th to 20th	Day	29% 8% 22% 28% 10% 3	2321
	21st to end	Day	31% 7% 17% 28% 15% 2	2574
September	1st to 10th	Day	33% 6% 16% 26% 15% 5%	2227
	11th to 20th	Day	38% 5% 15% 27% 14% 2	1980
	21st to end	Day	37% 6% 14% 29% 12% 3	1980
October	1st to 10th	Day	42% 7% 15% 24% 10% 2	1908
	11th to 20th	Day	41% 5% 17% 24% 10% 3	1666
	21st to end	Day	42% 9% 13% 24% 11% 2	1782
November	1st to 10th	Day	50% 7% 12% 20% 11% 2	1565
	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	47% 8% 13% 19% 10% 3	2836
	11th to 20th	Night	43% 7% 16% 22% 8% 4	2749
	21st to end	Night	41% 7% 15% 21% 14% 2	2772
February	1st to 10th	Night	46% 7% 14% 20% 11% 3	2520
	11th to 20th	Night	48% 7% 14% 20% 10% 1	2276
	21st to end	Night	49% 6% 11% 20% 12% 3	1807
March	1st to 10th	Night	45% 8% 15% 23% 7% 1	2160
	11th to 20th	Night	41% 7% 15% 23% 12% 1	2006
	21st to end	Night	48% 7% 17% 18% 9% 1	2146
April	1st to 10th	Night	49% 7% 12% 21% 10% 1	1756
	11th to 20th	Night	51% 6% 14% 20% 9% 1	1601
	21st to end	Night	46% 6% 15% 22% 9% 2	1620
May	1st to 10th	Night	48% 6% 14% 20% 11% 1	1520
	11th to 20th	Night	39% 6% 15% 29% 10% 1	1239
	21st to end	Night	48% 7% 17% 20% 7% 1	1358
June	1st to 10th	Night	45% 6% 17% 22% 10%	1260
	11th to 20th	Night	47% 7% 17% 19% 9% 2	1260
	21st to end	Night	31% 7% 24% 26% 11% 1	1260
July	1st to 10th	Night	32% 7% 20% 27% 14% 1	1225
	11th to 20th	Night	32% 7% 20% 26% 11% 3	1218
	21st to end	Night	22% 8% 16% 31% 20% 3	1378
August	1st to 10th	Night	33% 7% 18% 28% 12% 3	1424
	11th to 20th	Night	25% 8% 21% 30% 13% 2	1580
	21st to end	Night	27% 5% 16% 30% 18% 4	1782
September	1st to 10th	Night	32% 5% 18% 26% 17% 3	1731
	11th to 20th	Night	33% 5% 15% 28% 14% 5%	1980
	21st to end	Night	33% 6% 18% 25% 14% 4	1980
October	1st to 10th	Night	40% 6% 13% 25% 15% 1	2052
	11th to 20th	Night	35% 7% 17% 25% 12% 3	2290
	21st to end	Night	41% 6% 16% 25% 11% 1	2574
November	1st to 10th	Night	46% 6% 14% 20% 12% 2	2495
	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	8% 6% 35% 32% 15% 4	170
	21st to end	Day	9% 11% 23% 43% 11% 4	176
February	1st to 10th	Day	8% 14% 41% 28% 8% 2	160
	11th to 20th	Day	7% 9% 39% 31% 13% 1	238
	21st to end	Day	5% 11% 33% 33% 16% 1	314
March	1st to 10th	Day	8% 17% 41% 21% 10% 3	420
	11th to 20th	Day	9% 15% 34% 27% 13% 2	420
	21st to end	Day	13% 18% 33% 21% 11% 3	462
April	1st to 10th	Day	14% 18% 35% 25% 7% 1	420
	11th to 20th	Day	18% 14% 28% 26% 11% 3	412
	21st to end	Day	17% 19% 31% 21% 10% 2	420
May	1st to 10th	Day	19% 9% 29% 25% 14% 4	446
	11th to 20th	Day	19% 8% 24% 28% 18% 4	625
	21st to end	Day	20% 6% 24% 27% 20% 4	679
June	1st to 10th	Day	17% 9% 28% 30% 13% 3	630
	11th to 20th	Day	22% 10% 28% 26% 12% 2	630
	21st to end	Day	12% 7% 26% 31% 18% 5%	630
July	1st to 10th	Day	10% 6% 25% 34% 22% 4	621
	11th to 20th	Day	11% 7% 27% 33% 19% 3	618
	21st to end	Day	6% 7% 30% 32% 21% 5%	602
August	1st to 10th	Day	9% 7% 30% 33% 17% 4	447
	11th to 20th	Day	9% 6% 28% 34% 16% 7%	437
	21st to end	Day	11% 8% 28% 29% 16% 8%	495
September	1st to 10th	Day	14% 9% 32% 28% 14% 4	450
	11th to 20th	Day	18% 8% 26% 28% 15% 4	450
	21st to end	Day	15% 17% 24% 29% 11% 3	450
October	1st to 10th	Day	14% 11% 25% 28% 17% 4	378
	11th to 20th	Day	15% 11% 30% 24% 15% 5%	270
	21st to end	Day	15% 12% 37% 23% 11% 2	297
November	1st to 10th	Day	20% 11% 29% 29% 8% 3	270
	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% 35% 36% 12% 1	899
	21st to end	Night	9% 10% 33% 36% 10% 1	792
February	1st to 10th	Night	8% 11% 32% 32% 13% 3	720
	11th to 20th	Night	11% 8% 33% 37% 10% 1	704
	21st to end	Night	6% 5% 30% 37% 19% 3	592
March	1st to 10th	Night	9% 6% 36% 34% 13% 3	720
	11th to 20th	Night	8% 10% 35% 33% 12% 2	720
	21st to end	Night	11% 8% 31% 36% 11% 3	792
April	1st to 10th	Night	12% 11% 31% 32% 12% 1	676
	11th to 20th	Night	17% 8% 35% 31% 9%	533
	21st to end	Night	17% 9% 31% 31% 12%	540
May	1st to 10th	Night	16% 10% 25% 31% 18%	464
	11th to 20th	Night	14% 8% 23% 33% 21% 1	354
	21st to end	Night	22% 6% 23% 33% 16% 1	389
June	1st to 10th	Night	17% 7% 25% 35% 14% 2	360
	11th to 20th	Night	21% 6% 28% 31% 13% 1	360
	21st to end	Night	10% 4 18% 41% 24% 2	360
July	1st to 10th	Night	10% 3 17% 43% 25% 1	350
	11th to 20th	Night	10% 3 20% 44% 21% 2	348
	21st to end	Night	7% 3 20% 45% 23% 2	394
August	1st to 10th	Night	12% 3 14% 40% 30% 2	358
	11th to 20th	Night	10% 3 18% 45% 22% 2	512
	21st to end	Night	9% 4 23% 40% 22% 1	594
September	1st to 10th	Night	13% 3 27% 35% 19% 3	638
	11th to 20th	Night	19% 3 21% 35% 20% 3	720
	21st to end	Night	12% 6% 33% 35% 12% 3	720
October	1st to 10th	Night	16% 4 26% 36% 15% 3	720
	11th to 20th	Night	13% 6% 31% 32% 14% 4	720
	21st to end	Night	12% 7% 33% 37% 10% 1	792
November	1st to 10th	Night	14% 10% 38% 29% 9% 1	830
	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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