Winthrop
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Winthrop clear sky chart from 2005-11-06 to 2025-08-05.

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	4 7% 2 6% 4 4 5% 4 6% 8% 49%	130397
	Night	5% 9% 3 7% 4 4 4 4 6% 7% 46%	126355
Transparency	Day	34% 8% 13% 23% 17% 5%	130123
	Night	37% 8% 15% 24% 13% 3	126206
Seeing	Day	13% 20% 33% 24% 9% 2	15552
	Night	12% 11% 33% 30% 12% 1	24086

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	6% 15% 5% 10% 5% 5% 5% 4 7% 8% 30%	3639
	11th to 20th	Day	5% 10% 3 7% 4 4 3 2 5% 7% 49%	3616
	21st to end	Day	10% 16% 4 7% 4 4 4 3 4 7% 39%	4004
February	1st to 10th	Day	3 12% 4 10% 6% 5% 6% 5% 8% 9% 33%	1715
	11th to 20th	Day	3 11% 3 7% 5% 5% 5% 4 8% 9% 39%	1755
	21st to end	Day	3 9% 4 10% 8% 6% 5% 5% 8% 10% 32%	1630
March	1st to 10th	Day	2 8% 2 7% 4 5% 6% 6% 8% 9% 41%	4518
	11th to 20th	Day	2 8% 2 7% 5% 6% 5% 4 7% 8% 44%	4841
	21st to end	Day	7% 10% 3 8% 6% 6% 6% 5% 7% 8% 34%	5854
April	1st to 10th	Day	6% 9% 3 7% 7% 7% 7% 5% 7% 9% 33%	2400
	11th to 20th	Day	4 7% 3 8% 6% 6% 6% 6% 7% 9% 39%	2372
	21st to end	Day	4 7% 3 7% 5% 6% 6% 5% 7% 9% 42%	2695
May	1st to 10th	Day	4 4 2 5% 4 4 5% 5% 6% 8% 54%	6320
	11th to 20th	Day	4 6% 3 8% 6% 5% 5% 4 7% 9% 43%	6277
	21st to end	Day	8% 7% 2 6% 5% 5% 6% 4 6% 8% 44%	6903
June	1st to 10th	Day	8% 6% 3 6% 5% 6% 6% 6% 7% 8% 41%	2800
	11th to 20th	Day	7% 6% 2 7% 5% 5% 6% 5% 7% 9% 42%	2880
	21st to end	Day	5% 4 2 6% 4 5% 5% 3 6% 8% 53%	3000
July	1st to 10th	Day	2 2 1 2 2 2 3 2 4 5% 76%	6555
	11th to 20th	Day	1 2 1 2 2 2 2 2 4 4 78%	6320
	21st to end	Day	2 2 1 3 3 3 4 2 5% 7% 68%	6874
August	1st to 10th	Day	1 2 1 3 3 3 4 4 6% 7% 67%	4458
	11th to 20th	Day	2 1 1 4 4 4 5% 4 7% 8% 60%	4187
	21st to end	Day	5% 4 2 4 4 3 5% 3 5% 7% 58%	4220
September	1st to 10th	Day	4 3 1 4 3 2 3 3 4 7% 66%	2280
	11th to 20th	Day	4 4 2 6% 4 4 5% 4 6% 8% 52%	2279
	21st to end	Day	6% 6% 2 6% 4 3 4 4 5% 6% 54%	2047
October	1st to 10th	Day	4 2 1 5% 4 4 5% 4 5% 7% 58%	3180
	11th to 20th	Day	5% 5% 2 6% 5% 5% 6% 5% 6% 8% 46%	3139
	21st to end	Day	6% 7% 2 6% 5% 5% 6% 4 6% 8% 45%	3101
November	1st to 10th	Day	8% 10% 3 6% 5% 5% 6% 5% 8% 9% 35%	1689
	11th to 20th	Day	5% 15% 4 8% 6% 6% 4 4 5% 8% 35%	1600
	21st to end	Day	3 13% 6% 11% 6% 5% 6% 4 6% 8% 33%	1600
December	1st to 10th	Day	3 8% 3 7% 5% 5% 5% 5% 8% 8% 43%	2620
	11th to 20th	Day	5% 16% 6% 10% 6% 6% 6% 4 6% 6% 29%	3025
	21st to end	Day	7% 17% 7% 12% 5% 5% 6% 4 5% 6% 26%	4004
January	1st to 10th	Night	5% 18% 7% 15% 6% 6% 4 4 5% 5% 24%	6306
	11th to 20th	Night	5% 12% 3 7% 4 4 4 4 6% 7% 44%	6282
	21st to end	Night	9% 17% 5% 9% 5% 4 4 4 5% 5% 33%	6952
February	1st to 10th	Night	6% 13% 3 8% 6% 7% 5% 5% 7% 8% 33%	2683
	11th to 20th	Night	4 10% 3 7% 4 5% 6% 4 7% 8% 42%	2532
	21st to end	Night	4 11% 5% 10% 6% 5% 5% 5% 9% 8% 34%	2000
March	1st to 10th	Night	2 8% 3 7% 5% 4 4 5% 8% 12% 42%	5376
	11th to 20th	Night	3 10% 2 6% 5% 4 5% 4 5% 7% 50%	5141
	21st to end	Night	6% 11% 3 9% 6% 6% 6% 6% 7% 8% 32%	5108
April	1st to 10th	Night	4 8% 2 8% 5% 5% 6% 5% 8% 11% 40%	2000
	11th to 20th	Night	3 6% 2 6% 5% 5% 6% 4 7% 8% 48%	1979
	21st to end	Night	4 6% 2 6% 5% 4 3 4 7% 8% 51%	1702
May	1st to 10th	Night	4 4 1 4 3 3 3 3 5% 8% 61%	3640
	11th to 20th	Night	4 5% 2 6% 3 4 5% 4 6% 7% 53%	3616
	21st to end	Night	6% 4 2 4 4 3 4 4 8% 11% 51%	3972
June	1st to 10th	Night	5% 5% 2 5% 5% 5% 5% 4 8% 9% 47%	1600
	11th to 20th	Night	3 4 2 5% 4 5% 5% 5% 7% 8% 53%	1512
	21st to end	Night	3 3 1 3 2 4 3 4 8% 7% 62%	1400
July	1st to 10th	Night	1 1 1 3 2 2 2 2 3 5% 78%	3400
	11th to 20th	Night	1 1 2 1 2 2 3 3 5% 81%	3640
	21st to end	Night	1 2 1 3 2 2 3 3 4 6% 74%	3969
August	1st to 10th	Night	1 2 1 3 2 3 4 4 4 6% 71%	2561
	11th to 20th	Night	1 2 1 3 2 3 2 3 6% 9% 70%	2643
	21st to end	Night	5% 2 1 3 3 4 4 4 5% 7% 64%	3456
September	1st to 10th	Night	3 2 1 3 2 3 4 3 4 7% 68%	1900
	11th to 20th	Night	5% 3 1 5% 3 4 3 3 6% 7% 60%	1900
	21st to end	Night	5% 4 2 5% 4 4 3 4 7% 8% 55%	2130
October	1st to 10th	Night	4 4 1 4 3 3 4 4 6% 7% 59%	3800
	11th to 20th	Night	8% 5% 2 6% 4 4 5% 4 6% 7% 49%	3797
	21st to end	Night	7% 7% 2 6% 5% 5% 4 4 5% 8% 46%	4457
November	1st to 10th	Night	9% 13% 3 7% 4 5% 5% 4 7% 8% 34%	2609
	11th to 20th	Night	6% 14% 3 7% 5% 6% 5% 5% 6% 8% 35%	2800
	21st to end	Night	4 16% 5% 10% 6% 5% 5% 4 6% 6% 32%	2800
December	1st to 10th	Night	4 8% 3 6% 6% 5% 5% 4 6% 8% 45%	4560
	11th to 20th	Night	5% 17% 6% 12% 7% 6% 7% 5% 6% 7% 20%	5180
	21st to end	Night	9% 21% 5% 10% 5% 4 5% 3 5% 6% 26%	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	53% 10% 12% 19% 6%	3639
	11th to 20th	Day	38% 6% 18% 27% 10% 2	3640
	21st to end	Day	49% 7% 13% 22% 9%	4004
February	1st to 10th	Day	49% 10% 16% 16% 7% 2	1715
	11th to 20th	Day	43% 11% 18% 18% 9% 1	1764
	21st to end	Day	49% 12% 16% 16% 7% 1	1630
March	1st to 10th	Day	37% 9% 18% 15% 18% 3	4518
	11th to 20th	Day	36% 7% 13% 32% 9% 1	4841
	21st to end	Day	46% 11% 19% 15% 9% 1	5854
April	1st to 10th	Day	49% 10% 16% 18% 7% 1	2400
	11th to 20th	Day	43% 10% 16% 18% 11% 2	2372
	21st to end	Day	40% 11% 13% 22% 12% 2	2695
May	1st to 10th	Day	28% 7% 12% 26% 25% 3	6320
	11th to 20th	Day	36% 7% 17% 25% 13% 1	6280
	21st to end	Day	38% 9% 14% 28% 11% 1	6904
June	1st to 10th	Day	43% 9% 10% 19% 15% 4	2800
	11th to 20th	Day	39% 10% 13% 21% 15% 2	2880
	21st to end	Day	30% 7% 15% 27% 18% 4	3000
July	1st to 10th	Day	13% 4 9% 23% 28% 24%	6485
	11th to 20th	Day	12% 3 5% 22% 32% 26%	6245
	21st to end	Day	16% 4 7% 29% 30% 13%	6681
August	1st to 10th	Day	16% 6% 7% 26% 34% 12%	4441
	11th to 20th	Day	19% 6% 16% 34% 21% 4	4229
	21st to end	Day	27% 6% 14% 25% 23% 5%	4219
September	1st to 10th	Day	21% 6% 11% 29% 27% 5%	2280
	11th to 20th	Day	31% 8% 11% 24% 22% 5%	2279
	21st to end	Day	31% 8% 15% 25% 17% 4	2047
October	1st to 10th	Day	27% 5% 13% 36% 17% 1	3180
	11th to 20th	Day	35% 9% 13% 24% 18%	3139
	21st to end	Day	37% 8% 12% 26% 16% 2	3101
November	1st to 10th	Day	47% 12% 14% 16% 9% 1	1689
	11th to 20th	Day	50% 9% 16% 16% 8% 1	1600
	21st to end	Day	50% 12% 14% 14% 8% 2	1600
December	1st to 10th	Day	38% 15% 18% 20% 9%	2620
	11th to 20th	Day	56% 11% 15% 12% 7%	3028
	21st to end	Day	60% 9% 11% 13% 6%	4004
January	1st to 10th	Night	60% 9% 11% 15% 4	6306
	11th to 20th	Night	39% 6% 18% 26% 10% 1	6320
	21st to end	Night	53% 10% 13% 17% 6%	6952
February	1st to 10th	Night	48% 10% 16% 16% 8% 2	2683
	11th to 20th	Night	41% 10% 17% 21% 10% 2	2534
	21st to end	Night	46% 12% 15% 17% 9% 1	2000
March	1st to 10th	Night	34% 9% 16% 23% 16% 2	5376
	11th to 20th	Night	35% 6% 15% 28% 15%	5142
	21st to end	Night	48% 13% 16% 18% 5%	5108
April	1st to 10th	Night	38% 13% 18% 22% 7% 1	2000
	11th to 20th	Night	35% 10% 16% 22% 14% 2	1979
	21st to end	Night	32% 11% 15% 28% 13%	1702
May	1st to 10th	Night	22% 7% 13% 31% 25% 1	3637
	11th to 20th	Night	30% 6% 18% 32% 12% 1	3616
	21st to end	Night	24% 11% 24% 32% 8% 1	3972
June	1st to 10th	Night	33% 10% 13% 26% 15% 3	1600
	11th to 20th	Night	30% 9% 16% 26% 17% 1	1512
	21st to end	Night	21% 8% 18% 31% 20% 3	1400
July	1st to 10th	Night	12% 3 9% 29% 27% 20%	3361
	11th to 20th	Night	8% 3 9% 29% 30% 21%	3592
	21st to end	Night	13% 3 9% 34% 28% 13%	3857
August	1st to 10th	Night	16% 5% 7% 30% 31% 11%	2553
	11th to 20th	Night	13% 6% 24% 36% 18% 3	2667
	21st to end	Night	21% 6% 16% 32% 23% 3	3456
September	1st to 10th	Night	19% 7% 11% 31% 27% 5%	1900
	11th to 20th	Night	25% 7% 10% 30% 23% 4	1900
	21st to end	Night	28% 9% 14% 31% 14% 5%	2130
October	1st to 10th	Night	26% 6% 18% 33% 16% 1	3800
	11th to 20th	Night	35% 8% 15% 28% 13% 1	3797
	21st to end	Night	38% 7% 14% 26% 14% 1	4457
November	1st to 10th	Night	48% 11% 16% 14% 10% 2	2609
	11th to 20th	Night	47% 11% 15% 19% 7% 2	2800
	21st to end	Night	52% 11% 13% 15% 7% 2	2800
December	1st to 10th	Night	39% 13% 18% 23% 7%	4560
	11th to 20th	Night	61% 12% 11% 11% 5%	5176
	21st to end	Night	59% 7% 12% 15% 6% 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	16% 21% 22% 26% 12% 4	200
	11th to 20th	Day	16% 15% 27% 27% 13% 3	200
	21st to end	Day	18% 14% 30% 25% 13% 1	220
February	1st to 10th	Day	14% 25% 37% 17% 6% 1	315
	11th to 20th	Day	17% 22% 35% 20% 6% 1	395
	21st to end	Day	13% 24% 37% 20% 6%	404
March	1st to 10th	Day	16% 21% 37% 18% 6% 1	500
	11th to 20th	Day	15% 25% 28% 22% 9% 1	500
	21st to end	Day	16% 25% 35% 17% 5%	550
April	1st to 10th	Day	23% 21% 33% 16% 6% 1	500
	11th to 20th	Day	18% 29% 27% 20% 6% 2	492
	21st to end	Day	13% 27% 32% 19% 7% 2	500
May	1st to 10th	Day	13% 21% 34% 26% 5% 1	503
	11th to 20th	Day	19% 17% 27% 26% 8% 2	528
	21st to end	Day	15% 21% 29% 23% 10% 2	573
June	1st to 10th	Day	15% 23% 33% 20% 8% 1	530
	11th to 20th	Day	15% 26% 34% 17% 6% 2	610
	21st to end	Day	9% 19% 36% 24% 10% 1	730
July	1st to 10th	Day	6% 19% 33% 33% 8% 1	630
	11th to 20th	Day	7% 17% 34% 29% 11% 2	524
	21st to end	Day	3 15% 34% 32% 14% 2	575
August	1st to 10th	Day	6% 11% 34% 30% 15% 4	504
	11th to 20th	Day	4 11% 32% 32% 17% 5%	477
	21st to end	Day	9% 18% 35% 27% 9% 3	539
September	1st to 10th	Day	9% 17% 35% 23% 12% 4	490
	11th to 20th	Day	10% 18% 31% 26% 13% 3	490
	21st to end	Day	13% 21% 31% 24% 9% 2	490
October	1st to 10th	Day	14% 17% 33% 26% 10%	490
	11th to 20th	Day	15% 20% 30% 22% 10% 3	480
	21st to end	Day	18% 19% 33% 20% 8% 2	330
November	1st to 10th	Day	13% 22% 35% 21% 8%	263
	11th to 20th	Day	18% 22% 30% 17% 11% 3	200
	21st to end	Day	17% 15% 31% 22% 12% 5%	200
December	1st to 10th	Day	15% 14% 22% 23% 19% 8%	200
	11th to 20th	Day	12% 16% 36% 29% 7% 2	200
	21st to end	Day	11% 11% 35% 27% 15% 1	220
January	1st to 10th	Night	17% 15% 31% 25% 10% 1	800
	11th to 20th	Night	16% 11% 35% 27% 10% 1	800
	21st to end	Night	14% 10% 32% 31% 11% 1	880
February	1st to 10th	Night	14% 13% 39% 25% 8% 2	800
	11th to 20th	Night	11% 11% 36% 30% 11%	784
	21st to end	Night	15% 14% 32% 26% 12% 2	660
March	1st to 10th	Night	11% 11% 35% 31% 11% 1	800
	11th to 20th	Night	11% 12% 37% 28% 12% 1	800
	21st to end	Night	12% 14% 37% 29% 7% 1	880
April	1st to 10th	Night	10% 13% 35% 31% 10% 1	800
	11th to 20th	Night	9% 16% 36% 29% 9% 2	791
	21st to end	Night	10% 16% 39% 25% 9% 1	502
May	1st to 10th	Night	10% 13% 34% 32% 10%	400
	11th to 20th	Night	11% 10% 31% 31% 16% 1	394
	21st to end	Night	11% 12% 33% 29% 15%	433
June	1st to 10th	Night	10% 14% 35% 31% 10% 1	400
	11th to 20th	Night	7% 16% 38% 29% 10%	400
	21st to end	Night	6% 10% 35% 32% 18%	400
July	1st to 10th	Night	4 8% 32% 41% 14% 1	390
	11th to 20th	Night	4 11% 32% 39% 14%	388
	21st to end	Night	2 7% 31% 40% 21%	438
August	1st to 10th	Night	4 7% 22% 43% 24%	386
	11th to 20th	Night	2 6% 27% 37% 26% 2	457
	21st to end	Night	6% 9% 29% 36% 18% 2	807
September	1st to 10th	Night	5% 9% 29% 33% 20% 3	760
	11th to 20th	Night	7% 8% 24% 31% 25% 6%	760
	21st to end	Night	9% 10% 29% 32% 17% 3	760
October	1st to 10th	Night	12% 9% 28% 31% 18% 2	760
	11th to 20th	Night	13% 12% 28% 31% 14% 2	760
	21st to end	Night	15% 14% 33% 26% 9% 3	836
November	1st to 10th	Night	20% 12% 36% 27% 6%	780
	11th to 20th	Night	20% 11% 36% 22% 10% 2	800
	21st to end	Night	14% 13% 34% 25% 10% 3	800
December	1st to 10th	Night	14% 8% 36% 28% 12% 3	800
	11th to 20th	Night	19% 10% 36% 29% 6% 1	800
	21st to end	Night	17% 13% 36% 29% 5%	880

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