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 2025-03-27.

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	15% 11% 4 10% 6% 5% 5% 4 5% 6% 30%	113785
	Night	14% 11% 4 10% 6% 5% 5% 4 5% 6% 31%	114224
Transparency	Day	54% 7% 11% 16% 10% 2	113545
	Night	54% 8% 11% 18% 8% 2	114110
Seeing	Day	24% 11% 23% 25% 14% 4	15487
	Night	25% 6% 17% 31% 19% 2	24007

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	23% 17% 5% 16% 9% 7% 5% 3 5% 3 6%	3639
	11th to 20th	Day	17% 14% 4 12% 7% 6% 5% 4 6% 7% 19%	3616
	21st to end	Day	17% 13% 5% 11% 6% 6% 4 3 5% 6% 25%	4303
February	1st to 10th	Day	19% 17% 5% 11% 8% 6% 5% 5% 5% 6% 13%	1800
	11th to 20th	Day	21% 18% 6% 11% 6% 6% 5% 4 4 4 16%	1755
	21st to end	Day	21% 18% 4 11% 8% 6% 5% 3 4 4 15%	1540
March	1st to 10th	Day	13% 13% 5% 16% 9% 8% 7% 4 6% 4 14%	3391
	11th to 20th	Day	17% 13% 4 9% 6% 5% 4 3 4 4 31%	3600
	21st to end	Day	18% 14% 5% 14% 8% 7% 6% 4 6% 5% 14%	3975
April	1st to 10th	Day	21% 16% 4 13% 5% 5% 5% 4 5% 5% 15%	2280
	11th to 20th	Day	21% 13% 4 10% 6% 5% 5% 3 5% 4 22%	2281
	21st to end	Day	18% 14% 5% 10% 7% 6% 5% 4 5% 5% 20%	2470
May	1st to 10th	Day	17% 9% 3 8% 5% 4 5% 4 5% 5% 35%	4389
	11th to 20th	Day	11% 8% 3 8% 6% 5% 6% 4 7% 8% 34%	4377
	21st to end	Day	21% 11% 5% 11% 7% 6% 6% 4 6% 5% 19%	4813
June	1st to 10th	Day	19% 12% 4 8% 7% 5% 5% 4 5% 6% 27%	2660
	11th to 20th	Day	17% 12% 4 11% 6% 6% 7% 4 6% 6% 21%	2828
	21st to end	Day	10% 8% 2 8% 7% 5% 5% 4 5% 7% 39%	2850
July	1st to 10th	Day	5% 5% 2 6% 3 3 2 3 3 4 64%	4740
	11th to 20th	Day	5% 4 1 4 3 3 2 3 4 5% 66%	4434
	21st to end	Day	8% 5% 2 5% 3 3 4 3 4 6% 57%	4790
August	1st to 10th	Day	4 4 1 4 3 3 4 3 6% 7% 59%	4189
	11th to 20th	Day	6% 6% 3 9% 5% 6% 6% 6% 7% 8% 37%	4026
	21st to end	Day	12% 7% 3 7% 5% 4 4 3 5% 7% 44%	4344
September	1st to 10th	Day	7% 6% 3 7% 4 4 4 4 5% 7% 49%	2280
	11th to 20th	Day	9% 9% 3 8% 6% 5% 6% 5% 6% 7% 37%	2275
	21st to end	Day	13% 7% 3 9% 5% 5% 4 4 6% 7% 36%	2000
October	1st to 10th	Day	12% 7% 3 7% 4 5% 5% 4 6% 7% 39%	3180
	11th to 20th	Day	16% 9% 4 11% 6% 6% 5% 5% 6% 9% 23%	3161
	21st to end	Day	17% 10% 4 11% 6% 6% 6% 5% 7% 7% 20%	3337
November	1st to 10th	Day	20% 15% 6% 13% 8% 5% 5% 5% 5% 5% 14%	1613
	11th to 20th	Day	26% 13% 4 11% 5% 5% 6% 4 6% 4 16%	1600
	21st to end	Day	21% 15% 4 10% 7% 6% 4 4 6% 5% 17%	1600
December	1st to 10th	Day	14% 10% 4 11% 7% 7% 6% 5% 7% 6% 24%	2620
	11th to 20th	Day	20% 16% 6% 14% 8% 8% 6% 4 6% 4 9%	3025
	21st to end	Day	21% 17% 6% 14% 9% 7% 6% 4 5% 4 8%	4004
January	1st to 10th	Night	22% 17% 7% 16% 8% 6% 4 3 4 3 8%	6306
	11th to 20th	Night	15% 13% 4 11% 7% 5% 5% 4 5% 9% 23%	6282
	21st to end	Night	18% 13% 4 9% 6% 5% 4 3 5% 8% 25%	6662
February	1st to 10th	Night	18% 14% 5% 13% 6% 6% 6% 4 5% 6% 16%	2600
	11th to 20th	Night	18% 14% 5% 11% 7% 5% 4 4 5% 6% 22%	2532
	21st to end	Night	19% 15% 5% 11% 7% 6% 5% 4 5% 5% 19%	2090
March	1st to 10th	Night	11% 12% 5% 13% 8% 8% 7% 5% 6% 6% 19%	3701
	11th to 20th	Night	12% 10% 3 9% 5% 5% 5% 4 4 6% 36%	3599
	21st to end	Night	18% 19% 5% 13% 7% 6% 5% 3 4 4 16%	3816
April	1st to 10th	Night	18% 12% 5% 11% 8% 7% 6% 3 4 6% 21%	1900
	11th to 20th	Night	16% 12% 4 9% 6% 5% 4 4 5% 7% 28%	1847
	21st to end	Night	17% 11% 4 9% 5% 4 5% 4 5% 7% 29%	1710
May	1st to 10th	Night	17% 8% 3 7% 5% 4 3 3 4 7% 41%	2572
	11th to 20th	Night	8% 7% 3 6% 4 4 4 4 6% 8% 46%	2516
	21st to end	Night	9% 10% 3 9% 7% 6% 5% 4 5% 7% 34%	2762
June	1st to 10th	Night	13% 9% 3 7% 6% 5% 5% 4 4 8% 37%	1520
	11th to 20th	Night	13% 9% 3 7% 4 5% 4 5% 6% 8% 36%	1345
	21st to end	Night	5% 5% 3 7% 4 4 4 3 4 6% 55%	1330
July	1st to 10th	Night	2 2 1 4 2 3 2 3 3 4 73%	2220
	11th to 20th	Night	3 2 1 4 2 3 2 2 3 4 73%	2508
	21st to end	Night	4 3 1 3 2 3 3 2 4 5% 72%	2762
August	1st to 10th	Night	2 2 1 3 2 2 3 2 3 5% 76%	2745
	11th to 20th	Night	6% 4 2 7% 4 4 4 4 5% 8% 53%	2812
	21st to end	Night	11% 5% 2 5% 3 3 4 3 4 5% 54%	3328
September	1st to 10th	Night	6% 3 2 4 3 3 3 3 5% 5% 62%	1900
	11th to 20th	Night	9% 6% 3 8% 5% 4 4 4 4 6% 45%	1902
	21st to end	Night	9% 6% 3 6% 5% 5% 5% 3 6% 6% 46%	2179
October	1st to 10th	Night	9% 6% 2 7% 5% 5% 4 4 6% 8% 44%	3800
	11th to 20th	Night	13% 7% 3 10% 6% 5% 5% 4 5% 7% 34%	3777
	21st to end	Night	15% 11% 4 11% 7% 6% 6% 4 6% 6% 26%	4227
November	1st to 10th	Night	22% 12% 4 10% 6% 6% 6% 4 5% 6% 19%	2682
	11th to 20th	Night	24% 13% 4 10% 7% 5% 5% 4 5% 4 18%	2800
	21st to end	Night	18% 13% 5% 10% 8% 6% 5% 4 5% 6% 21%	2800
December	1st to 10th	Night	12% 11% 3 11% 6% 6% 5% 5% 8% 9% 25%	4560
	11th to 20th	Night	23% 16% 5% 14% 7% 6% 5% 4 5% 5% 10%	5180
	21st to end	Night	21% 16% 5% 13% 9% 7% 6% 4 5% 4 10%	6952

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	80% 8% 5% 5% 2	3639
	11th to 20th	Day	64% 6% 8% 15% 5% 1	3640
	21st to end	Day	61% 6% 6% 15% 11% 1	4303
February	1st to 10th	Day	71% 6% 10% 9% 4	1800
	11th to 20th	Day	71% 7% 8% 9% 4 1	1764
	21st to end	Day	71% 6% 8% 9% 6%	1540
March	1st to 10th	Day	69% 8% 11% 10% 3	3391
	11th to 20th	Day	56% 5% 10% 25% 3 1	3600
	21st to end	Day	69% 10% 11% 9% 1	3975
April	1st to 10th	Day	69% 8% 12% 10% 1	2280
	11th to 20th	Day	64% 7% 10% 11% 8% 1	2281
	21st to end	Day	64% 8% 11% 11% 5%	2470
May	1st to 10th	Day	49% 8% 13% 20% 10%	4389
	11th to 20th	Day	43% 13% 20% 18% 5%	4380
	21st to end	Day	63% 12% 14% 9% 2	4814
June	1st to 10th	Day	58% 8% 15% 13% 6%	2660
	11th to 20th	Day	61% 8% 15% 12% 4	2828
	21st to end	Day	44% 8% 16% 22% 8% 2	2850
July	1st to 10th	Day	25% 5% 7% 18% 23% 23%	4666
	11th to 20th	Day	22% 4 6% 23% 39% 6%	4358
	21st to end	Day	28% 4 10% 31% 24% 3	4639
August	1st to 10th	Day	24% 6% 9% 29% 27% 5%	4172
	11th to 20th	Day	42% 6% 17% 23% 9% 3	4065
	21st to end	Day	41% 5% 12% 25% 16% 2	4343
September	1st to 10th	Day	34% 7% 13% 24% 17% 5%	2280
	11th to 20th	Day	45% 8% 12% 17% 15% 3	2275
	21st to end	Day	46% 8% 15% 16% 13% 2	2000
October	1st to 10th	Day	43% 5% 10% 31% 11% 1	3180
	11th to 20th	Day	58% 5% 13% 14% 9%	3161
	21st to end	Day	62% 6% 9% 14% 8% 1	3337
November	1st to 10th	Day	71% 7% 7% 11% 2	1613
	11th to 20th	Day	69% 7% 9% 8% 5% 2	1600
	21st to end	Day	68% 7% 6% 9% 10%	1600
December	1st to 10th	Day	58% 8% 6% 14% 13% 1	2620
	11th to 20th	Day	78% 6% 7% 8% 1	3028
	21st to end	Day	78% 6% 9% 5% 2	4004
January	1st to 10th	Night	79% 6% 7% 5% 2 1	6306
	11th to 20th	Night	58% 9% 8% 19% 5% 1	6320
	21st to end	Night	59% 9% 6% 17% 8% 1	6662
February	1st to 10th	Night	68% 8% 9% 10% 4 1	2600
	11th to 20th	Night	65% 7% 10% 10% 7% 1	2534
	21st to end	Night	66% 9% 10% 9% 6%	2090
March	1st to 10th	Night	63% 9% 11% 14% 3	3701
	11th to 20th	Night	48% 10% 16% 23% 3 1	3600
	21st to end	Night	71% 9% 11% 9%	3816
April	1st to 10th	Night	66% 8% 13% 11% 3	1900
	11th to 20th	Night	57% 8% 13% 14% 7% 1	1847
	21st to end	Night	56% 8% 15% 15% 5% 1	1710
May	1st to 10th	Night	45% 11% 12% 23% 8% 1	2572
	11th to 20th	Night	35% 9% 21% 26% 8% 1	2516
	21st to end	Night	49% 16% 19% 13% 3	2762
June	1st to 10th	Night	48% 8% 15% 24% 5%	1520
	11th to 20th	Night	46% 11% 20% 20% 4	1345
	21st to end	Night	32% 7% 23% 28% 8% 3	1330
July	1st to 10th	Night	16% 6% 10% 27% 15% 26%	2185
	11th to 20th	Night	17% 3 8% 26% 40% 5%	2461
	21st to end	Night	18% 4 12% 36% 24% 5%	2674
August	1st to 10th	Night	14% 4 12% 33% 32% 4	2737
	11th to 20th	Night	33% 5% 24% 26% 9% 3	2839
	21st to end	Night	36% 3 15% 32% 13% 1	3328
September	1st to 10th	Night	25% 6% 16% 28% 18% 7%	1900
	11th to 20th	Night	41% 7% 13% 20% 14% 5%	1902
	21st to end	Night	38% 8% 15% 21% 16% 2	2179
October	1st to 10th	Night	39% 7% 12% 35% 7% 1	3800
	11th to 20th	Night	52% 5% 15% 21% 6% 1	3777
	21st to end	Night	59% 6% 9% 16% 9%	4227
November	1st to 10th	Night	65% 8% 10% 12% 5%	2682
	11th to 20th	Night	68% 8% 9% 9% 4 2	2800
	21st to end	Night	65% 7% 8% 9% 9% 1	2800
December	1st to 10th	Night	55% 11% 8% 17% 8%	4560
	11th to 20th	Night	76% 6% 8% 8% 2	5176
	21st to end	Night	76% 8% 7% 7% 1	6952

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	39% 14% 18% 21% 7% 3	200
	11th to 20th	Day	31% 15% 21% 18% 13% 4	200
	21st to end	Day	35% 11% 16% 21% 12% 4	350
February	1st to 10th	Day	42% 11% 20% 15% 11% 3	400
	11th to 20th	Day	41% 12% 25% 14% 7% 1	395
	21st to end	Day	38% 15% 18% 19% 10%	359
March	1st to 10th	Day	31% 14% 26% 21% 8% 1	500
	11th to 20th	Day	34% 13% 19% 17% 13% 4	500
	21st to end	Day	34% 15% 22% 19% 8% 2	531
April	1st to 10th	Day	34% 13% 20% 20% 10% 2	460
	11th to 20th	Day	29% 17% 24% 19% 6% 5%	452
	21st to end	Day	29% 16% 24% 20% 9% 2	460
May	1st to 10th	Day	21% 11% 27% 24% 13% 3	463
	11th to 20th	Day	27% 11% 22% 25% 13% 4	488
	21st to end	Day	22% 11% 23% 27% 13% 3	529
June	1st to 10th	Day	25% 10% 23% 28% 11% 3	490
	11th to 20th	Day	21% 15% 31% 24% 8% 1	658
	21st to end	Day	12% 10% 28% 32% 15% 4	680
July	1st to 10th	Day	11% 6% 27% 38% 14% 5%	671
	11th to 20th	Day	10% 5% 24% 35% 19% 6%	497
	21st to end	Day	8% 3 20% 40% 24% 4	531
August	1st to 10th	Day	12% 6% 15% 40% 20% 7%	487
	11th to 20th	Day	5% 5% 17% 37% 27% 8%	477
	21st to end	Day	13% 6% 27% 30% 17% 5%	539
September	1st to 10th	Day	14% 6% 24% 36% 18% 3	490
	11th to 20th	Day	16% 9% 21% 33% 18% 3	490
	21st to end	Day	18% 10% 23% 30% 16% 3	490
October	1st to 10th	Day	22% 12% 23% 24% 16% 4	490
	11th to 20th	Day	25% 10% 23% 21% 16% 5%	490
	21st to end	Day	30% 14% 23% 18% 12% 4	477
November	1st to 10th	Day	32% 14% 23% 18% 10% 2	223
	11th to 20th	Day	33% 13% 19% 16% 13% 7%	200
	21st to end	Day	32% 15% 22% 14% 11% 8%	200
December	1st to 10th	Day	28% 16% 18% 13% 21% 6%	200
	11th to 20th	Day	39% 12% 27% 15% 6% 3	200
	21st to end	Day	44% 10% 20% 14% 10% 1	220
January	1st to 10th	Night	40% 9% 17% 20% 12% 2	800
	11th to 20th	Night	34% 8% 14% 25% 17% 2	800
	21st to end	Night	31% 5% 15% 27% 18% 3	880
February	1st to 10th	Night	34% 8% 17% 28% 13% 1	800
	11th to 20th	Night	32% 8% 18% 28% 14%	784
	21st to end	Night	34% 10% 18% 27% 11% 1	660
March	1st to 10th	Night	29% 9% 17% 29% 15% 1	800
	11th to 20th	Night	29% 10% 17% 29% 14% 1	800
	21st to end	Night	33% 10% 19% 25% 11% 1	862
April	1st to 10th	Night	28% 8% 21% 27% 14% 2	760
	11th to 20th	Night	25% 6% 23% 29% 14% 2	719
	21st to end	Night	28% 5% 16% 35% 15% 2	570
May	1st to 10th	Night	18% 6% 24% 32% 20% 1	399
	11th to 20th	Night	21% 3 18% 35% 21% 2	374
	21st to end	Night	14% 4 18% 39% 23% 2	411
June	1st to 10th	Night	18% 4 18% 39% 20% 1	380
	11th to 20th	Night	19% 4 21% 40% 16% 1	380
	21st to end	Night	12% 2 18% 46% 22% 1	380
July	1st to 10th	Night	7% 3 14% 49% 26% 1	370
	11th to 20th	Night	7% 3 16% 48% 23% 2	368
	21st to end	Night	4 2 14% 47% 31% 2	416
August	1st to 10th	Night	6% 2 8% 43% 37% 4	503
	11th to 20th	Night	8% 2 7% 36% 41% 5%	564
	21st to end	Night	12% 3 14% 40% 30% 2	731
September	1st to 10th	Night	11% 2 14% 43% 27% 2	760
	11th to 20th	Night	15% 3 12% 32% 33% 5%	760
	21st to end	Night	15% 5% 17% 40% 21% 2	760
October	1st to 10th	Night	19% 4 17% 34% 24% 3	760
	11th to 20th	Night	24% 5% 17% 29% 21% 4	760
	21st to end	Night	27% 7% 17% 30% 17% 2	836
November	1st to 10th	Night	33% 7% 16% 32% 11% 1	780
	11th to 20th	Night	38% 7% 19% 22% 14% 2	800
	21st to end	Night	30% 8% 15% 26% 17% 4	800
December	1st to 10th	Night	30% 6% 14% 30% 18% 2	800
	11th to 20th	Night	36% 8% 17% 25% 12% 1	800
	21st to end	Night	39% 11% 19% 21% 9% 1	880

Page generated at 2025-03-24 06:04:30. Version ac07

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