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 2024-02-15.

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	17% 11% 4 9% 7% 4 5% 5% 5% 6% 27%	78775
	Night	16% 11% 4 8% 7% 4 5% 5% 5% 6% 29%	80257
Transparency	Day	58% 7% 11% 15% 8% 1	78496
	Night	56% 8% 11% 16% 7% 1	80116
Seeing	Day	24% 11% 23% 26% 14% 4	14050
	Night	25% 6% 16% 31% 19% 2	22606

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	25% 17% 5% 13% 10% 5% 6% 4 4 4 8%	2539
	11th to 20th	Day	24% 17% 5% 7% 6% 4 5% 4 4 5% 18%	2516
	21st to end	Day	23% 17% 7% 12% 9% 5% 5% 4 4 4 11%	2995
February	1st to 10th	Day	20% 16% 6% 10% 9% 4 6% 5% 4 6% 13%	1710
	11th to 20th	Day	21% 16% 6% 9% 8% 5% 6% 5% 4 5% 16%	1612
	21st to end	Day	22% 16% 4 9% 10% 5% 5% 4 4 5% 16%	1381
March	1st to 10th	Day	20% 15% 5% 11% 11% 4 6% 5% 4 4 14%	1883
	11th to 20th	Day	26% 15% 6% 8% 8% 3 5% 4 4 5% 16%	1980
	21st to end	Day	24% 13% 6% 12% 9% 5% 7% 5% 5% 5% 10%	2222
April	1st to 10th	Day	21% 15% 6% 11% 7% 4 6% 5% 5% 5% 15%	2160
	11th to 20th	Day	23% 12% 5% 9% 7% 4 6% 5% 5% 4 21%	2159
	21st to end	Day	18% 13% 5% 9% 8% 5% 5% 5% 5% 6% 21%	2340
May	1st to 10th	Day	15% 9% 4 8% 7% 4 5% 5% 5% 6% 31%	2502
	11th to 20th	Day	17% 10% 4 8% 8% 4 6% 6% 7% 7% 24%	2477
	21st to end	Day	20% 9% 5% 9% 7% 4 6% 6% 6% 6% 23%	2723
June	1st to 10th	Day	20% 11% 4 7% 8% 4 5% 4 5% 6% 26%	2520
	11th to 20th	Day	18% 11% 5% 10% 7% 5% 7% 5% 5% 7% 20%	2679
	21st to end	Day	10% 8% 3 7% 7% 4 5% 5% 5% 7% 39%	2700
July	1st to 10th	Day	8% 6% 3 8% 5% 3 4 4 4 6% 49%	2700
	11th to 20th	Day	9% 6% 3 5% 5% 3 3 5% 5% 7% 49%	2534
	21st to end	Day	7% 4 2 3 3 2 4 4 4 6% 60%	2741
August	1st to 10th	Day	8% 7% 2 5% 5% 3 4 4 5% 9% 49%	2386
	11th to 20th	Day	7% 4 2 4 5% 3 4 5% 6% 7% 54%	2298
	21st to end	Day	8% 6% 3 7% 6% 4 4 5% 5% 8% 43%	2475
September	1st to 10th	Day	7% 6% 2 6% 5% 3 4 5% 5% 7% 49%	2160
	11th to 20th	Day	9% 8% 3 8% 7% 4 5% 6% 5% 7% 38%	2156
	21st to end	Day	13% 7% 3 7% 7% 4 4 5% 6% 7% 36%	1895
October	1st to 10th	Day	17% 10% 4 8% 6% 5% 5% 5% 5% 6% 30%	1800
	11th to 20th	Day	17% 9% 3 9% 8% 4 5% 6% 5% 9% 25%	1800
	21st to end	Day	21% 11% 4 10% 7% 4 5% 4 5% 6% 23%	1922
November	1st to 10th	Day	21% 14% 6% 11% 9% 4 5% 5% 5% 4 14%	1531
	11th to 20th	Day	26% 12% 4 10% 6% 4 5% 6% 5% 5% 17%	1520
	21st to end	Day	22% 14% 5% 9% 8% 5% 5% 4 5% 6% 18%	1520
December	1st to 10th	Day	19% 12% 6% 11% 7% 4 6% 5% 5% 4 20%	1520
	11th to 20th	Day	20% 14% 6% 11% 9% 5% 6% 5% 7% 5% 11%	1925
	21st to end	Day	20% 17% 6% 11% 10% 5% 6% 5% 5% 4 10%	2794
January	1st to 10th	Night	23% 18% 7% 13% 10% 4 5% 4 3 3 10%	4406
	11th to 20th	Night	21% 14% 5% 10% 7% 4 5% 4 5% 6% 19%	4382
	21st to end	Night	23% 17% 6% 11% 9% 4 5% 5% 4 4 12%	4663
February	1st to 10th	Night	19% 13% 5% 11% 8% 4 7% 5% 5% 6% 16%	2470
	11th to 20th	Night	18% 13% 5% 10% 8% 4 5% 5% 5% 6% 22%	2329
	21st to end	Night	19% 13% 6% 9% 8% 5% 5% 5% 5% 5% 20%	1875
March	1st to 10th	Night	19% 12% 6% 9% 7% 4 6% 5% 5% 6% 21%	2075
	11th to 20th	Night	18% 12% 5% 9% 8% 4 8% 5% 4 6% 21%	1979
	21st to end	Night	21% 15% 5% 9% 8% 4 6% 5% 6% 5% 16%	2134
April	1st to 10th	Night	19% 12% 5% 10% 9% 5% 6% 4 4 6% 20%	1800
	11th to 20th	Night	16% 11% 5% 8% 7% 4 5% 5% 5% 7% 27%	1749
	21st to end	Night	17% 10% 5% 7% 6% 4 5% 5% 5% 7% 29%	1620
May	1st to 10th	Night	11% 8% 4 7% 6% 4 4 4 6% 8% 38%	1458
	11th to 20th	Night	14% 9% 3 6% 6% 4 5% 7% 7% 8% 32%	1416
	21st to end	Night	12% 7% 4 7% 7% 3 6% 5% 5% 8% 35%	1552
June	1st to 10th	Night	14% 7% 3 7% 7% 4 6% 5% 4 8% 36%	1440
	11th to 20th	Night	15% 8% 3 6% 6% 4 4 5% 5% 8% 35%	1275
	21st to end	Night	6% 5% 3 6% 4 3 4 3 4 6% 56%	1260
July	1st to 10th	Night	4 3 2 5% 4 3 3 5% 4 6% 60%	1260
	11th to 20th	Night	4 3 1 5% 3 3 3 4 4 6% 64%	1422
	21st to end	Night	2 2 1 2 2 1 2 3 3 5% 76%	1576
August	1st to 10th	Night	3 3 1 3 4 2 4 3 4 7% 67%	1550
	11th to 20th	Night	4 2 1 3 3 2 3 3 4 7% 67%	1602
	21st to end	Night	6% 5% 2 5% 5% 2 4 3 4 5% 58%	1880
September	1st to 10th	Night	7% 3 2 3 4 2 3 4 4 5% 63%	1800
	11th to 20th	Night	9% 5% 4 6% 7% 3 5% 4 4 7% 46%	1801
	21st to end	Night	9% 6% 3 5% 6% 3 5% 4 5% 6% 46%	2064
October	1st to 10th	Night	14% 7% 3 7% 6% 4 4 4 5% 6% 41%	2160
	11th to 20th	Night	16% 7% 3 8% 7% 4 5% 5% 4 6% 36%	2160
	21st to end	Night	17% 10% 3 8% 6% 3 5% 4 5% 6% 32%	2433
November	1st to 10th	Night	23% 11% 5% 8% 7% 4 6% 5% 5% 6% 20%	2544
	11th to 20th	Night	24% 11% 5% 8% 8% 4 5% 5% 5% 5% 19%	2660
	21st to end	Night	18% 11% 5% 9% 9% 5% 5% 5% 5% 6% 22%	2660
December	1st to 10th	Night	19% 13% 5% 9% 7% 4 6% 5% 5% 6% 22%	2660
	11th to 20th	Night	21% 14% 5% 11% 9% 5% 6% 7% 5% 5% 15%	3280
	21st to end	Night	20% 16% 6% 11% 10% 5% 6% 5% 5% 4 11%	4862

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% 4 3 1	2539
	11th to 20th	Day	68% 7% 10% 10% 3 2	2540
	21st to end	Day	77% 6% 5% 7% 4 1	2995
February	1st to 10th	Day	72% 6% 10% 8% 3	1710
	11th to 20th	Day	71% 7% 8% 10% 4 1	1621
	21st to end	Day	71% 6% 8% 9% 6%	1381
March	1st to 10th	Day	70% 7% 8% 10% 4	1883
	11th to 20th	Day	71% 6% 8% 12% 3 1	1980
	21st to end	Day	76% 8% 8% 8%	2222
April	1st to 10th	Day	69% 8% 12% 10% 1	2160
	11th to 20th	Day	66% 6% 9% 11% 6% 1	2159
	21st to end	Day	64% 7% 11% 11% 5%	2340
May	1st to 10th	Day	53% 6% 15% 19% 6%	2502
	11th to 20th	Day	56% 9% 15% 14% 5%	2480
	21st to end	Day	61% 9% 13% 14% 3	2724
June	1st to 10th	Day	59% 8% 15% 13% 5%	2520
	11th to 20th	Day	62% 8% 15% 11% 3	2679
	21st to end	Day	43% 9% 16% 22% 8% 2	2700
July	1st to 10th	Day	38% 7% 11% 24% 16% 3	2626
	11th to 20th	Day	34% 7% 10% 26% 20% 2	2458
	21st to end	Day	25% 5% 10% 33% 23% 4	2589
August	1st to 10th	Day	33% 8% 14% 25% 17% 3	2369
	11th to 20th	Day	28% 8% 14% 30% 15% 5%	2298
	21st to end	Day	38% 7% 13% 28% 12% 2	2475
September	1st to 10th	Day	34% 7% 13% 25% 16% 5%	2160
	11th to 20th	Day	45% 8% 12% 17% 15% 3	2156
	21st to end	Day	46% 9% 14% 15% 14% 2	1895
October	1st to 10th	Day	55% 6% 9% 20% 10% 1	1800
	11th to 20th	Day	56% 7% 15% 14% 8% 1	1800
	21st to end	Day	62% 6% 9% 14% 9%	1922
November	1st to 10th	Day	72% 8% 7% 11% 3	1531
	11th to 20th	Day	69% 7% 9% 8% 5% 2	1520
	21st to end	Day	67% 7% 5% 9% 11%	1520
December	1st to 10th	Day	67% 7% 6% 11% 8% 1	1520
	11th to 20th	Day	74% 9% 7% 9% 2	1928
	21st to end	Day	76% 8% 8% 5% 3	2794
January	1st to 10th	Night	79% 7% 6% 5% 2 1	4406
	11th to 20th	Night	65% 7% 9% 14% 3 1	4420
	21st to end	Night	75% 7% 6% 7% 4 1	4663
February	1st to 10th	Night	69% 8% 9% 10% 3 1	2470
	11th to 20th	Night	65% 7% 10% 10% 7% 1	2331
	21st to end	Night	67% 8% 9% 9% 6%	1875
March	1st to 10th	Night	63% 8% 11% 14% 4	2075
	11th to 20th	Night	63% 9% 10% 13% 4 1	1980
	21st to end	Night	69% 10% 11% 9% 1	2134
April	1st to 10th	Night	66% 8% 12% 11% 3	1800
	11th to 20th	Night	59% 8% 12% 14% 6% 1	1749
	21st to end	Night	55% 8% 15% 15% 5% 1	1620
May	1st to 10th	Night	46% 8% 17% 20% 8% 1	1458
	11th to 20th	Night	50% 8% 16% 19% 4 2	1416
	21st to end	Night	49% 12% 17% 18% 5%	1552
June	1st to 10th	Night	48% 8% 16% 24% 4	1440
	11th to 20th	Night	48% 11% 21% 18% 3	1275
	21st to end	Night	31% 7% 23% 27% 9% 3	1260
July	1st to 10th	Night	26% 9% 17% 26% 16% 6%	1225
	11th to 20th	Night	24% 5% 14% 31% 23% 3	1375
	21st to end	Night	14% 5% 11% 40% 22% 8%	1488
August	1st to 10th	Night	20% 7% 20% 30% 19% 5%	1542
	11th to 20th	Night	19% 7% 18% 35% 16% 5%	1602
	21st to end	Night	29% 5% 13% 36% 15% 2	1880
September	1st to 10th	Night	25% 6% 16% 29% 17% 7%	1800
	11th to 20th	Night	40% 7% 12% 20% 15% 5%	1801
	21st to end	Night	38% 8% 14% 21% 16% 2	2064
October	1st to 10th	Night	44% 8% 11% 25% 11% 1	2160
	11th to 20th	Night	50% 6% 12% 21% 9% 1	2160
	21st to end	Night	55% 6% 10% 17% 12%	2433
November	1st to 10th	Night	65% 8% 10% 12% 5%	2544
	11th to 20th	Night	67% 8% 9% 9% 5% 2	2660
	21st to end	Night	64% 8% 8% 10% 10% 1	2660
December	1st to 10th	Night	63% 9% 9% 12% 7% 1	2660
	11th to 20th	Night	70% 8% 9% 10% 3	3276
	21st to end	Night	73% 10% 7% 7% 2	4862

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% 16% 21% 7% 3	180
	11th to 20th	Day	33% 17% 19% 17% 12% 2	180
	21st to end	Day	38% 12% 17% 20% 11% 3	321
February	1st to 10th	Day	43% 9% 19% 15% 11% 2	370
	11th to 20th	Day	40% 12% 24% 16% 7% 1	348
	21st to end	Day	35% 16% 20% 18% 11%	302
March	1st to 10th	Day	31% 15% 25% 20% 8% 1	420
	11th to 20th	Day	34% 14% 20% 17% 12% 3	420
	21st to end	Day	34% 15% 22% 19% 8% 2	462
April	1st to 10th	Day	35% 14% 20% 20% 9% 2	420
	11th to 20th	Day	31% 16% 24% 18% 6% 6%	412
	21st to end	Day	28% 16% 25% 19% 10% 2	420
May	1st to 10th	Day	20% 11% 26% 26% 13% 4	423
	11th to 20th	Day	28% 10% 21% 25% 12% 3	448
	21st to end	Day	22% 10% 24% 27% 14% 3	485
June	1st to 10th	Day	25% 10% 23% 27% 11% 3	450
	11th to 20th	Day	21% 16% 30% 24% 8% 1	609
	21st to end	Day	11% 10% 28% 33% 14% 4	630
July	1st to 10th	Day	12% 5% 26% 37% 14% 5%	621
	11th to 20th	Day	11% 5% 25% 35% 19% 5%	457
	21st to end	Day	7% 3 20% 41% 24% 4	487
August	1st to 10th	Day	13% 6% 15% 40% 20% 7%	447
	11th to 20th	Day	5% 5% 18% 38% 27% 8%	437
	21st to end	Day	12% 6% 28% 31% 17% 5%	495
September	1st to 10th	Day	14% 6% 25% 36% 16% 3	450
	11th to 20th	Day	16% 9% 20% 33% 19% 4	450
	21st to end	Day	19% 11% 21% 30% 16% 3	450
October	1st to 10th	Day	22% 12% 23% 23% 15% 4	450
	11th to 20th	Day	25% 9% 23% 21% 17% 5%	450
	21st to end	Day	29% 14% 23% 17% 12% 4	437
November	1st to 10th	Day	30% 14% 24% 19% 11% 1	201
	11th to 20th	Day	31% 13% 20% 16% 13% 7%	180
	21st to end	Day	32% 16% 21% 13% 10% 8%	180
December	1st to 10th	Day	29% 17% 18% 12% 19% 4	180
	11th to 20th	Day	36% 12% 27% 16% 7% 3	180
	21st to end	Day	42% 10% 20% 15% 11% 2	198
January	1st to 10th	Night	41% 9% 16% 19% 13% 2	760
	11th to 20th	Night	35% 8% 14% 24% 17% 2	760
	21st to end	Night	33% 6% 15% 27% 17% 3	836
February	1st to 10th	Night	34% 8% 16% 28% 14% 1	760
	11th to 20th	Night	31% 9% 18% 28% 14%	722
	21st to end	Night	34% 9% 18% 26% 12% 1	592
March	1st to 10th	Night	30% 9% 17% 28% 15% 1	720
	11th to 20th	Night	28% 10% 18% 28% 14% 1	720
	21st to end	Night	33% 9% 18% 26% 12% 1	792
April	1st to 10th	Night	28% 8% 21% 27% 14% 2	720
	11th to 20th	Night	26% 6% 22% 29% 15% 2	681
	21st to end	Night	28% 4 16% 35% 16% 2	540
May	1st to 10th	Night	17% 5% 24% 33% 21% 1	378
	11th to 20th	Night	21% 3 16% 35% 21% 2	354
	21st to end	Night	15% 3 17% 39% 24% 2	389
June	1st to 10th	Night	18% 4 16% 39% 21% 1	360
	11th to 20th	Night	19% 5% 19% 40% 16% 1	360
	21st to end	Night	12% 3 18% 46% 22% 1	360
July	1st to 10th	Night	7% 3 13% 48% 27% 1	350
	11th to 20th	Night	7% 3 16% 48% 24% 2	348
	21st to end	Night	3 2 12% 49% 32% 2	394
August	1st to 10th	Night	6% 2 8% 42% 37% 4	476
	11th to 20th	Night	7% 2 7% 37% 42% 5%	534
	21st to end	Night	11% 3 14% 39% 30% 2	692
September	1st to 10th	Night	11% 3 15% 43% 26% 2	720
	11th to 20th	Night	14% 4 12% 32% 34% 5%	720
	21st to end	Night	15% 5% 16% 39% 22% 2	720
October	1st to 10th	Night	19% 4 16% 33% 24% 3	720
	11th to 20th	Night	23% 5% 17% 29% 22% 4	720
	21st to end	Night	27% 7% 17% 29% 18% 2	792
November	1st to 10th	Night	34% 7% 16% 32% 10% 1	740
	11th to 20th	Night	36% 7% 19% 23% 14% 2	760
	21st to end	Night	29% 8% 15% 27% 16% 4	760
December	1st to 10th	Night	31% 6% 15% 30% 17% 1	760
	11th to 20th	Night	36% 8% 17% 25% 12% 1	760
	21st to end	Night	39% 11% 18% 22% 9% 1	836

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