Winlock
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Winlock clear sky chart from 2005-11-06 to 2026-07-31.

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	13% 10% 4 9% 6% 5% 5% 4 6% 6% 32%	143556
	Night	13% 10% 4 10% 6% 5% 5% 4 5% 6% 32%	138198
Transparency	Day	52% 6% 12% 18% 10% 2	143275
	Night	53% 7% 12% 18% 8% 1	138061
Seeing	Day	24% 10% 23% 26% 13% 4	17308
	Night	25% 6% 17% 32% 18% 2	25553

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	21% 17% 6% 16% 9% 7% 6% 3 4 3 6%	4723
	11th to 20th	Day	13% 12% 4 12% 7% 5% 4 4 8% 8% 23%	4716
	21st to end	Day	16% 12% 5% 11% 6% 6% 5% 4 6% 6% 23%	5579
February	1st to 10th	Day	19% 17% 5% 11% 8% 6% 5% 5% 5% 6% 13%	1890
	11th to 20th	Day	20% 17% 6% 12% 7% 6% 5% 4 4 4 16%	1845
	21st to end	Day	22% 18% 4 11% 8% 6% 5% 3 4 4 15%	1615
March	1st to 10th	Day	12% 12% 5% 15% 10% 9% 8% 4 6% 5% 15%	4836
	11th to 20th	Day	17% 15% 5% 12% 7% 6% 4 3 4 4 23%	5110
	21st to end	Day	15% 13% 6% 16% 8% 7% 7% 5% 6% 5% 11%	5730
April	1st to 10th	Day	20% 16% 4 12% 6% 5% 6% 4 6% 6% 16%	2520
	11th to 20th	Day	20% 13% 4 10% 6% 5% 5% 4 5% 4 24%	2525
	21st to end	Day	17% 13% 5% 10% 7% 6% 5% 4 5% 5% 22%	2730
May	1st to 10th	Day	11% 7% 2 6% 5% 4 5% 4 6% 7% 43%	8138
	11th to 20th	Day	10% 9% 4 10% 7% 6% 7% 5% 7% 8% 26%	8160
	21st to end	Day	13% 7% 3 8% 5% 5% 5% 4 7% 9% 33%	8993
June	1st to 10th	Day	18% 11% 4 8% 6% 5% 5% 3 5% 6% 29%	2940
	11th to 20th	Day	16% 11% 4 10% 6% 6% 6% 4 6% 7% 24%	3126
	21st to end	Day	10% 9% 3 9% 7% 5% 5% 4 5% 7% 38%	3150
July	1st to 10th	Day	5% 4 2 5% 3 3 3 2 4 5% 65%	6930
	11th to 20th	Day	4 3 1 3 2 2 2 2 4 5% 71%	6489
	21st to end	Day	6% 5% 2 4 3 3 4 3 5% 6% 59%	7015
August	1st to 10th	Day	4 4 1 4 4 3 4 3 6% 7% 58%	4322
	11th to 20th	Day	6% 6% 3 9% 6% 6% 6% 6% 7% 8% 37%	4156
	21st to end	Day	12% 7% 3 7% 5% 4 4 3 5% 7% 44%	4481
September	1st to 10th	Day	7% 7% 3 7% 4 4 4 4 5% 7% 48%	2400
	11th to 20th	Day	9% 8% 3 8% 6% 5% 5% 5% 6% 7% 38%	2395
	21st to end	Day	12% 7% 3 9% 5% 5% 4 4 6% 7% 37%	2105
October	1st to 10th	Day	12% 7% 3 7% 4 5% 5% 4 6% 7% 39%	3280
	11th to 20th	Day	16% 9% 4 11% 6% 6% 5% 5% 6% 9% 24%	3261
	21st to end	Day	17% 10% 4 11% 7% 6% 6% 5% 7% 7% 20%	3444
November	1st to 10th	Day	20% 15% 6% 13% 8% 5% 5% 5% 5% 5% 14%	1695
	11th to 20th	Day	26% 13% 4 11% 5% 5% 6% 4 6% 4 15%	1680
	21st to end	Day	21% 15% 4 10% 7% 6% 4 4 6% 5% 17%	1680
December	1st to 10th	Day	14% 10% 4 11% 7% 7% 6% 5% 7% 6% 24%	2700
	11th to 20th	Day	20% 16% 6% 14% 8% 8% 6% 4 5% 4 9%	3105
	21st to end	Day	20% 17% 6% 14% 9% 7% 6% 4 5% 4 9%	4092
January	1st to 10th	Night	21% 17% 7% 16% 9% 7% 5% 4 4 3 8%	8172
	11th to 20th	Night	13% 11% 4 10% 6% 5% 4 4 8% 10% 25%	8182
	21st to end	Night	16% 13% 4 10% 6% 5% 5% 4 6% 9% 22%	8617
February	1st to 10th	Night	17% 14% 4 13% 6% 6% 6% 4 6% 6% 16%	2730
	11th to 20th	Night	18% 14% 5% 11% 7% 5% 4 4 5% 6% 22%	2662
	21st to end	Night	19% 15% 5% 11% 7% 6% 5% 4 5% 5% 20%	2191
March	1st to 10th	Night	10% 11% 4 12% 8% 8% 7% 6% 8% 6% 19%	5276
	11th to 20th	Night	13% 13% 4 10% 7% 6% 6% 4 5% 6% 27%	5109
	21st to end	Night	17% 18% 6% 15% 7% 6% 5% 3 5% 5% 13%	5487
April	1st to 10th	Night	17% 12% 5% 11% 8% 7% 6% 3 4 6% 22%	2100
	11th to 20th	Night	15% 11% 4 8% 6% 5% 4 4 5% 7% 30%	2043
	21st to end	Night	16% 10% 4 9% 5% 4 5% 4 5% 7% 31%	1890
May	1st to 10th	Night	10% 5% 2 5% 4 3 4 4 5% 7% 50%	4784
	11th to 20th	Night	10% 9% 4 8% 6% 6% 5% 4 6% 7% 36%	4700
	21st to end	Night	6% 7% 3 8% 6% 5% 5% 4 7% 8% 41%	5178
June	1st to 10th	Night	13% 8% 3 7% 6% 5% 5% 4 4 8% 38%	1680
	11th to 20th	Night	12% 8% 2 7% 4 4 4 4 6% 8% 40%	1487
	21st to end	Night	5% 5% 3 7% 4 4 4 3 4 6% 53%	1470
July	1st to 10th	Night	2 2 1 3 2 3 2 3 3 5% 74%	3250
	11th to 20th	Night	2 2 1 3 2 2 2 2 3 4 77%	3673
	21st to end	Night	3 3 1 3 2 2 3 2 5% 6% 72%	4049
August	1st to 10th	Night	2 2 1 3 2 2 3 2 3 5% 75%	2832
	11th to 20th	Night	6% 4 2 7% 4 4 5% 4 6% 8% 53%	2902
	21st to end	Night	11% 5% 2 5% 3 3 4 3 4 5% 54%	3433
September	1st to 10th	Night	6% 4 2 5% 3 3 3 3 5% 6% 61%	2000
	11th to 20th	Night	9% 6% 3 8% 5% 4 4 4 4 6% 46%	2002
	21st to end	Night	9% 6% 3 7% 5% 5% 5% 3 5% 7% 46%	2294
October	1st to 10th	Night	9% 6% 2 7% 5% 5% 5% 4 6% 8% 44%	3920
	11th to 20th	Night	13% 8% 3 10% 6% 5% 5% 4 5% 7% 34%	3897
	21st to end	Night	14% 11% 4 11% 7% 6% 6% 4 6% 6% 26%	4362
November	1st to 10th	Night	22% 12% 4 10% 6% 6% 6% 4 5% 6% 19%	2820
	11th to 20th	Night	24% 14% 4 10% 7% 5% 5% 4 5% 4 18%	2940
	21st to end	Night	18% 13% 5% 10% 8% 6% 5% 4 5% 6% 21%	2940
December	1st to 10th	Night	13% 11% 3 11% 6% 6% 5% 5% 7% 9% 24%	4700
	11th to 20th	Night	23% 16% 5% 14% 7% 6% 5% 4 5% 5% 10%	5320
	21st to end	Night	21% 16% 5% 13% 9% 7% 6% 4 5% 4 10%	7106

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	81% 7% 5% 5% 2	4723
	11th to 20th	Day	58% 8% 7% 15% 11% 1	4740
	21st to end	Day	62% 7% 6% 14% 11% 1	5580
February	1st to 10th	Day	71% 6% 10% 9% 4	1890
	11th to 20th	Day	71% 6% 7% 10% 4 1	1854
	21st to end	Day	72% 6% 8% 9% 6%	1615
March	1st to 10th	Day	68% 7% 11% 11% 3	4836
	11th to 20th	Day	64% 5% 8% 19% 3	5110
	21st to end	Day	72% 7% 12% 8% 1	5730
April	1st to 10th	Day	68% 8% 12% 11% 1	2520
	11th to 20th	Day	62% 6% 10% 14% 7% 1	2525
	21st to end	Day	62% 7% 12% 13% 6%	2730
May	1st to 10th	Day	39% 6% 14% 29% 11%	8138
	11th to 20th	Day	53% 8% 17% 18% 3	8163
	21st to end	Day	46% 7% 23% 19% 4	8994
June	1st to 10th	Day	55% 7% 14% 15% 7%	2940
	11th to 20th	Day	57% 8% 14% 16% 5%	3126
	21st to end	Day	45% 8% 16% 22% 9% 2	3150
July	1st to 10th	Day	24% 4 11% 26% 20% 16%	6856
	11th to 20th	Day	18% 3 9% 25% 39% 5%	6413
	21st to end	Day	26% 3 18% 34% 18% 2	6822
August	1st to 10th	Day	24% 6% 9% 29% 27% 5%	4305
	11th to 20th	Day	42% 6% 17% 23% 10% 3	4195
	21st to end	Day	41% 5% 12% 25% 16% 2	4480
September	1st to 10th	Day	35% 7% 14% 24% 16% 4	2400
	11th to 20th	Day	44% 7% 12% 18% 15% 3	2395
	21st to end	Day	45% 8% 14% 17% 13% 2	2105
October	1st to 10th	Day	43% 5% 10% 31% 10% 1	3280
	11th to 20th	Day	58% 5% 13% 15% 9%	3261
	21st to end	Day	62% 6% 10% 14% 8% 1	3444
November	1st to 10th	Day	71% 7% 8% 11% 2	1695
	11th to 20th	Day	70% 7% 9% 8% 5% 2	1680
	21st to end	Day	68% 7% 6% 9% 10%	1680
December	1st to 10th	Day	59% 8% 6% 13% 12% 1	2700
	11th to 20th	Day	78% 6% 7% 8% 1	3108
	21st to end	Day	78% 6% 9% 5% 2	4092
January	1st to 10th	Night	80% 6% 7% 6% 1 1	8172
	11th to 20th	Night	53% 13% 8% 18% 8% 1	8220
	21st to end	Night	60% 11% 5% 15% 8% 1	8617
February	1st to 10th	Night	68% 8% 9% 10% 4 1	2730
	11th to 20th	Night	65% 7% 10% 10% 7% 1	2664
	21st to end	Night	66% 9% 10% 10% 6%	2191
March	1st to 10th	Night	61% 8% 13% 13% 4	5276
	11th to 20th	Night	58% 8% 12% 19% 3	5110
	21st to end	Night	73% 7% 11% 9% 1	5487
April	1st to 10th	Night	64% 8% 13% 12% 3	2100
	11th to 20th	Night	56% 8% 13% 15% 8% 1	2043
	21st to end	Night	53% 8% 16% 18% 5% 1	1890
May	1st to 10th	Night	35% 6% 16% 32% 11%	4781
	11th to 20th	Night	46% 7% 18% 22% 5% 1	4700
	21st to end	Night	40% 10% 26% 23% 2	5182
June	1st to 10th	Night	46% 8% 16% 24% 7%	1680
	11th to 20th	Night	43% 10% 19% 24% 5%	1487
	21st to end	Night	33% 7% 22% 28% 8% 3	1470
July	1st to 10th	Night	14% 4 19% 32% 14% 18%	3215
	11th to 20th	Night	14% 3 8% 31% 39% 4	3626
	21st to end	Night	17% 4 21% 37% 18% 4	3937
August	1st to 10th	Night	14% 4 13% 33% 32% 4	2824
	11th to 20th	Night	33% 5% 24% 25% 9% 4	2929
	21st to end	Night	36% 3 15% 31% 14% 1	3433
September	1st to 10th	Night	26% 6% 17% 28% 17% 7%	2000
	11th to 20th	Night	40% 7% 13% 20% 15% 5%	2002
	21st to end	Night	38% 8% 15% 22% 15% 2	2294
October	1st to 10th	Night	39% 7% 13% 34% 7% 1	3920
	11th to 20th	Night	52% 5% 15% 21% 7% 1	3897
	21st to end	Night	59% 6% 9% 16% 9%	4362
November	1st to 10th	Night	66% 8% 9% 12% 5%	2820
	11th to 20th	Night	69% 8% 8% 9% 4 2	2940
	21st to end	Night	66% 7% 8% 9% 9% 1	2940
December	1st to 10th	Night	56% 11% 8% 17% 8%	4700
	11th to 20th	Night	77% 6% 8% 8% 2	5316
	21st to end	Night	76% 8% 7% 7% 1	7106

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	38% 13% 20% 21% 6% 2	220
	11th to 20th	Day	30% 15% 19% 18% 14% 5%	220
	21st to end	Day	35% 11% 17% 21% 12% 4	379
February	1st to 10th	Day	41% 10% 21% 15% 10% 2	430
	11th to 20th	Day	39% 12% 25% 15% 7% 1	425
	21st to end	Day	39% 14% 19% 18% 9%	386
March	1st to 10th	Day	31% 14% 25% 20% 7% 1	540
	11th to 20th	Day	38% 13% 18% 16% 12% 3	540
	21st to end	Day	34% 15% 22% 19% 8% 2	594
April	1st to 10th	Day	32% 12% 21% 22% 11% 2	540
	11th to 20th	Day	28% 16% 24% 20% 8% 5%	532
	21st to end	Day	28% 15% 24% 20% 10% 3	540
May	1st to 10th	Day	19% 11% 27% 25% 14% 4	543
	11th to 20th	Day	27% 11% 22% 24% 11% 3	568
	21st to end	Day	21% 10% 25% 26% 14% 4	617
June	1st to 10th	Day	23% 11% 24% 28% 11% 3	570
	11th to 20th	Day	20% 15% 31% 25% 7% 2	756
	21st to end	Day	13% 9% 28% 32% 14% 4	780
July	1st to 10th	Day	11% 5% 27% 37% 15% 5%	771
	11th to 20th	Day	10% 6% 25% 35% 18% 6%	579
	21st to end	Day	7% 3 20% 38% 24% 6%	616
August	1st to 10th	Day	12% 6% 16% 40% 20% 7%	527
	11th to 20th	Day	5% 5% 18% 37% 26% 8%	517
	21st to end	Day	13% 6% 27% 31% 17% 5%	583
September	1st to 10th	Day	14% 6% 23% 35% 18% 4	530
	11th to 20th	Day	15% 9% 21% 33% 18% 4	530
	21st to end	Day	18% 10% 23% 30% 16% 3	530
October	1st to 10th	Day	22% 11% 24% 24% 15% 3	530
	11th to 20th	Day	26% 10% 23% 21% 16% 5%	530
	21st to end	Day	30% 13% 23% 19% 11% 4	518
November	1st to 10th	Day	32% 13% 25% 19% 9% 2	245
	11th to 20th	Day	34% 12% 19% 18% 11% 6%	220
	21st to end	Day	32% 14% 23% 14% 11% 7%	220
December	1st to 10th	Day	32% 15% 18% 12% 19% 5%	220
	11th to 20th	Day	40% 12% 26% 14% 6% 2	220
	21st to end	Day	42% 10% 21% 13% 12% 1	242
January	1st to 10th	Night	41% 9% 17% 19% 12% 2	840
	11th to 20th	Night	33% 8% 14% 25% 18% 2	840
	21st to end	Night	31% 5% 15% 28% 18% 3	924
February	1st to 10th	Night	33% 7% 17% 28% 13% 2	840
	11th to 20th	Night	32% 8% 18% 28% 14% 1	824
	21st to end	Night	34% 10% 18% 27% 11% 1	692
March	1st to 10th	Night	29% 9% 18% 29% 14% 1	840
	11th to 20th	Night	30% 10% 17% 28% 13% 1	840
	21st to end	Night	33% 10% 20% 25% 11% 1	924
April	1st to 10th	Night	27% 8% 20% 28% 15% 2	840
	11th to 20th	Night	24% 6% 24% 30% 14% 2	795
	21st to end	Night	27% 4 15% 36% 16% 2	630
May	1st to 10th	Night	17% 5% 23% 34% 20% 1	441
	11th to 20th	Night	21% 3 19% 35% 20% 2	414
	21st to end	Night	13% 4 19% 39% 23% 2	455
June	1st to 10th	Night	18% 4 19% 39% 19% 1	420
	11th to 20th	Night	17% 4 23% 40% 15% 1	420
	21st to end	Night	12% 2 19% 45% 21%	420
July	1st to 10th	Night	7% 2 16% 48% 25% 1	410
	11th to 20th	Night	7% 3 19% 47% 22% 2	408
	21st to end	Night	3 2 15% 47% 30% 2	458
August	1st to 10th	Night	6% 2 8% 43% 37% 4	530
	11th to 20th	Night	9% 2 7% 37% 41% 5%	594
	21st to end	Night	12% 3 14% 40% 29% 2	770
September	1st to 10th	Night	11% 2 15% 43% 27% 3	800
	11th to 20th	Night	14% 3 13% 32% 32% 5%	800
	21st to end	Night	15% 5% 18% 39% 21% 2	800
October	1st to 10th	Night	19% 4 17% 34% 24% 3	800
	11th to 20th	Night	24% 5% 17% 30% 21% 4	800
	21st to end	Night	27% 7% 18% 31% 16% 1	880
November	1st to 10th	Night	34% 7% 17% 31% 10% 1	820
	11th to 20th	Night	39% 6% 18% 22% 14% 1	840
	21st to end	Night	31% 8% 15% 27% 16% 3	840
December	1st to 10th	Night	32% 6% 14% 30% 17% 2	840
	11th to 20th	Night	37% 8% 18% 25% 12% 1	840
	21st to end	Night	39% 11% 19% 22% 9% 1	924

Page generated at 2026-07-28 00:25:19. Version ac07

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