Washington Island
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Washington Island clear sky chart from 2005-11-06 to 2025-09-19.

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	12% 9% 3 8% 5% 5% 4 4 5% 7% 38%	126464
	Night	14% 11% 4 9% 5% 5% 4 3 5% 6% 34%	122833
Transparency	Day	45% 5% 14% 24% 12% 1	129932
	Night	50% 4 13% 21% 10% 1	127452
Seeing	Day	16% 6% 25% 31% 18% 5%	17246
	Night	19% 7% 26% 31% 15% 2	27054

By Month

Percentage Cloud Cover NE (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	9% 12% 7% 14% 7% 6% 5% 4 5% 6% 25%	3617
	11th to 20th	Day	14% 12% 4 12% 8% 10% 6% 4 6% 6% 19%	3589
	21st to end	Day	14% 14% 4 9% 5% 5% 5% 4 5% 7% 28%	3982
February	1st to 10th	Day	9% 11% 3 10% 7% 6% 5% 4 6% 7% 33%	1669
	11th to 20th	Day	10% 12% 3 8% 5% 5% 5% 4 7% 7% 33%	1948
	21st to end	Day	11% 11% 4 10% 6% 6% 5% 5% 7% 6% 31%	1650
March	1st to 10th	Day	12% 8% 2 5% 5% 4 5% 3 6% 8% 42%	4478
	11th to 20th	Day	10% 9% 4 11% 6% 5% 4 4 6% 8% 32%	4699
	21st to end	Day	21% 11% 4 9% 5% 4 4 3 5% 6% 30%	5877
April	1st to 10th	Day	16% 11% 3 7% 5% 5% 5% 4 6% 8% 30%	2400
	11th to 20th	Day	18% 8% 3 7% 4 4 4 4 5% 7% 36%	2418
	21st to end	Day	16% 8% 2 8% 4 4 4 4 6% 8% 37%	2600
May	1st to 10th	Day	11% 6% 3 6% 4 4 4 3 5% 6% 47%	5929
	11th to 20th	Day	10% 8% 3 7% 5% 4 4 4 5% 7% 42%	6319
	21st to end	Day	11% 8% 2 7% 4 4 4 3 5% 8% 43%	6949
June	1st to 10th	Day	10% 6% 1 6% 5% 4 4 4 6% 8% 45%	2800
	11th to 20th	Day	11% 6% 3 6% 4 4 5% 4 6% 8% 42%	2800
	21st to end	Day	12% 6% 2 6% 4 4 4 4 7% 9% 42%	2800
July	1st to 10th	Day	11% 6% 2 5% 4 4 4 4 6% 10% 46%	6359
	11th to 20th	Day	6% 4 2 5% 3 4 4 3 6% 9% 54%	6360
	21st to end	Day	8% 5% 2 5% 4 4 4 4 6% 9% 49%	6926
August	1st to 10th	Day	7% 5% 2 6% 5% 5% 5% 5% 6% 10% 44%	4544
	11th to 20th	Day	12% 6% 2 5% 2 3 3 3 5% 7% 51%	4174
	21st to end	Day	11% 6% 2 6% 4 4 4 4 5% 7% 48%	4345
September	1st to 10th	Day	12% 5% 2 6% 4 5% 4 4 5% 8% 44%	2400
	11th to 20th	Day	11% 6% 2 5% 4 4 4 4 6% 9% 45%	2216
	21st to end	Day	13% 6% 2 7% 4 4 4 4 8% 8% 38%	2090
October	1st to 10th	Day	17% 7% 2 7% 4 4 3 3 4 8% 41%	2071
	11th to 20th	Day	22% 12% 4 8% 5% 4 4 3 4 6% 28%	1781
	21st to end	Day	21% 12% 4 9% 6% 5% 4 4 6% 5% 24%	1881
November	1st to 10th	Day	16% 10% 3 7% 6% 5% 4 4 5% 7% 34%	1762
	11th to 20th	Day	17% 15% 4 10% 6% 5% 5% 4 6% 6% 22%	1800
	21st to end	Day	20% 15% 7% 10% 6% 4 5% 3 4 5% 20%	1635
December	1st to 10th	Day	13% 17% 8% 14% 8% 6% 6% 4 5% 5% 16%	2610
	11th to 20th	Day	14% 19% 6% 14% 9% 8% 5% 3 4 5% 13%	3004
	21st to end	Day	20% 22% 6% 16% 7% 5% 3 2 3 3 13%	3982
January	1st to 10th	Night	10% 13% 5% 17% 8% 5% 5% 3 4 5% 23%	6348
	11th to 20th	Night	10% 16% 5% 14% 9% 8% 7% 4 5% 5% 16%	6316
	21st to end	Night	9% 13% 5% 10% 6% 5% 5% 4 6% 7% 30%	6996
February	1st to 10th	Night	10% 13% 5% 9% 7% 5% 5% 4 5% 6% 30%	2727
	11th to 20th	Night	10% 12% 5% 9% 5% 5% 5% 4 6% 7% 32%	2338
	21st to end	Night	15% 12% 3 9% 5% 6% 5% 3 6% 7% 29%	1980
March	1st to 10th	Night	15% 9% 3 7% 4 3 3 3 4 7% 43%	5362
	11th to 20th	Night	11% 10% 4 9% 5% 5% 5% 4 6% 9% 33%	5260
	21st to end	Night	20% 10% 3 8% 5% 4 4 4 5% 6% 31%	5083
April	1st to 10th	Night	18% 12% 4 8% 4 5% 4 3 4 6% 32%	2000
	11th to 20th	Night	20% 9% 2 7% 5% 5% 4 3 5% 6% 36%	1930
	21st to end	Night	16% 9% 3 8% 6% 5% 3 3 6% 6% 36%	1800
May	1st to 10th	Night	15% 7% 2 6% 4 3 4 3 4 8% 45%	4010
	11th to 20th	Night	12% 8% 3 8% 5% 4 5% 4 5% 6% 39%	3596
	21st to end	Night	17% 7% 2 6% 4 4 4 3 5% 6% 41%	3949
June	1st to 10th	Night	12% 6% 2 6% 5% 4 4 4 6% 7% 44%	1600
	11th to 20th	Night	14% 6% 4 6% 5% 5% 5% 3 7% 8% 37%	1600
	21st to end	Night	14% 6% 2 6% 4 4 3 4 7% 7% 45%	1600
July	1st to 10th	Night	11% 6% 2 6% 5% 4 5% 4 6% 8% 44%	3620
	11th to 20th	Night	8% 4 2 5% 4 3 3 3 5% 8% 55%	3620
	21st to end	Night	6% 5% 2 6% 4 4 4 4 6% 8% 52%	3948
August	1st to 10th	Night	9% 6% 2 7% 5% 4 4 3 6% 8% 45%	2614
	11th to 20th	Night	12% 4 1 4 4 3 4 4 5% 8% 50%	2875
	21st to end	Night	10% 6% 3 7% 4 4 5% 3 4 6% 49%	3549
September	1st to 10th	Night	11% 6% 2 6% 4 4 3 3 5% 6% 51%	2000
	11th to 20th	Night	14% 6% 2 5% 3 3 4 3 5% 6% 49%	2120
	21st to end	Night	15% 8% 3 7% 4 3 4 3 4 7% 42%	2090
October	1st to 10th	Night	17% 9% 2 7% 4 4 5% 4 4 7% 39%	2107
	11th to 20th	Night	21% 10% 3 7% 4 4 5% 4 5% 5% 32%	2399
	21st to end	Night	24% 13% 4 8% 5% 3 4 3 5% 6% 26%	2717
November	1st to 10th	Night	16% 13% 3 6% 5% 4 4 4 5% 7% 34%	2536
	11th to 20th	Night	17% 17% 5% 11% 6% 5% 4 3 4 5% 24%	2600
	21st to end	Night	19% 17% 6% 9% 5% 5% 4 4 5% 5% 21%	2764
December	1st to 10th	Night	16% 13% 7% 11% 6% 4 4 3 5% 6% 23%	4580
	11th to 20th	Night	14% 17% 7% 16% 7% 8% 5% 3 5% 4 14%	5203
	21st to end	Night	24% 18% 7% 13% 5% 4 3 2 3 4 16%	6996

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	58% 6% 15% 17% 5%	3618
	11th to 20th	Day	64% 4 12% 12% 7% 1	3620
	21st to end	Day	56% 3 14% 15% 11% 1	3982
February	1st to 10th	Day	50% 5% 15% 24% 7% 1	1669
	11th to 20th	Day	48% 5% 16% 20% 10% 1	1958
	21st to end	Day	52% 5% 12% 20% 10% 1	1650
March	1st to 10th	Day	40% 4 18% 19% 16% 3	4480
	11th to 20th	Day	49% 4 15% 21% 9% 1	4699
	21st to end	Day	56% 4 10% 20% 9%	5877
April	1st to 10th	Day	51% 5% 12% 21% 11% 1	2400
	11th to 20th	Day	47% 5% 9% 25% 12% 2	2418
	21st to end	Day	45% 5% 11% 21% 16% 2	2600
May	1st to 10th	Day	37% 5% 11% 27% 18% 2	5938
	11th to 20th	Day	41% 5% 10% 27% 15% 1	6323
	21st to end	Day	39% 5% 11% 24% 18% 3	6949
June	1st to 10th	Day	35% 5% 14% 29% 14% 2	2800
	11th to 20th	Day	39% 5% 15% 24% 15% 1	2800
	21st to end	Day	37% 7% 17% 25% 13% 1	2800
July	1st to 10th	Day	34% 5% 16% 32% 12% 1	6291
	11th to 20th	Day	26% 6% 15% 40% 12% 1	6285
	21st to end	Day	30% 6% 15% 31% 17% 1	6728
August	1st to 10th	Day	34% 6% 18% 29% 12% 1	4529
	11th to 20th	Day	32% 4 17% 34% 11%	4213
	21st to end	Day	36% 6% 17% 29% 12% 1	4344
September	1st to 10th	Day	38% 4 17% 29% 11% 1	2400
	11th to 20th	Day	35% 5% 22% 30% 8%	2216
	21st to end	Day	41% 6% 17% 26% 9%	2090
October	1st to 10th	Day	29% 4 14% 34% 19%	3448
	11th to 20th	Day	53% 4 11% 22% 9%	2932
	21st to end	Day	53% 5% 16% 21% 5%	3081
November	1st to 10th	Day	50% 4 14% 21% 10%	1762
	11th to 20th	Day	62% 5% 15% 13% 5%	1800
	21st to end	Day	66% 5% 15% 12% 2	1635
December	1st to 10th	Day	70% 5% 12% 11% 2	2610
	11th to 20th	Day	74% 4 10% 9% 3	3005
	21st to end	Day	77% 3 6% 10% 3	3982
January	1st to 10th	Night	62% 5% 11% 18% 4	6348
	11th to 20th	Night	68% 4 13% 9% 5%	6360
	21st to end	Night	53% 4 16% 18% 8%	6996
February	1st to 10th	Night	55% 4 11% 21% 8% 1	2727
	11th to 20th	Night	49% 5% 13% 22% 9% 2	2339
	21st to end	Night	54% 3 14% 20% 8% 1	1980
March	1st to 10th	Night	42% 3 18% 24% 13% 1	5373
	11th to 20th	Night	46% 5% 17% 20% 10% 1	5261
	21st to end	Night	54% 4 12% 21% 9%	5083
April	1st to 10th	Night	55% 4 8% 19% 13% 2	2000
	11th to 20th	Night	50% 4 8% 22% 13% 3	1930
	21st to end	Night	49% 4 11% 20% 14% 3	1800
May	1st to 10th	Night	39% 4 11% 27% 16% 3	4017
	11th to 20th	Night	44% 6% 9% 24% 15% 3	3597
	21st to end	Night	44% 4 11% 24% 11% 6%	3950
June	1st to 10th	Night	38% 5% 11% 30% 15% 2	1600
	11th to 20th	Night	43% 6% 13% 20% 16% 2	1600
	21st to end	Night	38% 5% 15% 25% 15% 2	1600
July	1st to 10th	Night	37% 5% 14% 28% 13% 3	3580
	11th to 20th	Night	27% 5% 15% 36% 15% 2	3572
	21st to end	Night	30% 5% 15% 27% 22% 2	3832
August	1st to 10th	Night	36% 5% 16% 27% 15% 1	2605
	11th to 20th	Night	32% 6% 16% 32% 14% 1	2902
	21st to end	Night	37% 5% 14% 26% 17% 1	3549
September	1st to 10th	Night	34% 5% 15% 28% 16% 2	2000
	11th to 20th	Night	36% 3 16% 33% 11% 1	2120
	21st to end	Night	43% 4 16% 25% 11% 1	2090
October	1st to 10th	Night	37% 3 11% 35% 13% 1	3510
	11th to 20th	Night	45% 4 13% 23% 13%	3987
	21st to end	Night	53% 3 14% 21% 9%	4467
November	1st to 10th	Night	50% 4 14% 22% 9% 1	2536
	11th to 20th	Night	64% 4 12% 13% 8%	2600
	21st to end	Night	65% 4 15% 14% 2	2764
December	1st to 10th	Night	62% 5% 17% 13% 2	4580
	11th to 20th	Night	73% 4 11% 10% 3	5201
	21st to end	Night	72% 4 9% 9% 5%	6996

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% 10% 31% 30% 12% 1	300
	11th to 20th	Day	20% 10% 35% 25% 8% 2	300
	21st to end	Day	22% 6% 28% 32% 12% 1	330
February	1st to 10th	Day	12% 9% 39% 27% 9% 3	300
	11th to 20th	Day	14% 8% 44% 26% 8% 1	300
	21st to end	Day	16% 10% 42% 26% 7%	249
March	1st to 10th	Day	13% 6% 39% 31% 10% 1	300
	11th to 20th	Day	15% 10% 30% 32% 11% 2	375
	21st to end	Day	22% 7% 27% 28% 13% 2	550
April	1st to 10th	Day	21% 7% 27% 30% 13% 2	500
	11th to 20th	Day	20% 8% 30% 28% 11% 3	492
	21st to end	Day	19% 5% 29% 29% 16% 3	500
May	1st to 10th	Day	17% 4 27% 35% 13% 3	518
	11th to 20th	Day	16% 3 23% 36% 17% 4	725
	21st to end	Day	14% 4 22% 34% 20% 6%	789
June	1st to 10th	Day	11% 2 24% 37% 22% 4	730
	11th to 20th	Day	15% 3 18% 38% 22% 4	730
	21st to end	Day	14% 4 20% 36% 21% 5%	730
July	1st to 10th	Day	9% 2 14% 37% 30% 8%	721
	11th to 20th	Day	8% 3 18% 37% 29% 7%	718
	21st to end	Day	7% 2 15% 36% 30% 10%	794
August	1st to 10th	Day	7% 3 14% 35% 30% 11%	720
	11th to 20th	Day	13% 5% 10% 28% 32% 11%	519
	21st to end	Day	14% 2 16% 29% 29% 9%	583
September	1st to 10th	Day	17% 4 18% 31% 22% 9%	530
	11th to 20th	Day	12% 3 17% 30% 29% 10%	522
	21st to end	Day	16% 7% 21% 27% 21% 8%	490
October	1st to 10th	Day	19% 7% 25% 30% 16% 4	471
	11th to 20th	Day	24% 10% 31% 24% 10% 1	300
	21st to end	Day	26% 9% 36% 23% 6%	330
November	1st to 10th	Day	17% 11% 33% 28% 10% 1	300
	11th to 20th	Day	19% 13% 37% 23% 7%	300
	21st to end	Day	22% 14% 36% 23% 5%	300
December	1st to 10th	Day	23% 12% 36% 20% 8% 1	300
	11th to 20th	Day	25% 13% 39% 19% 4 1	300
	21st to end	Day	31% 11% 32% 20% 5% 1	330
January	1st to 10th	Night	18% 10% 35% 28% 8% 1	1000
	11th to 20th	Night	22% 10% 33% 25% 9% 1	1000
	21st to end	Night	17% 6% 32% 32% 11% 1	1100
February	1st to 10th	Night	17% 6% 37% 31% 9% 1	992
	11th to 20th	Night	17% 9% 35% 30% 9%	783
	21st to end	Night	20% 6% 35% 33% 6%	660
March	1st to 10th	Night	17% 6% 34% 31% 11% 1	800
	11th to 20th	Night	15% 6% 36% 33% 9% 1	800
	21st to end	Night	19% 6% 27% 35% 12% 2	716
April	1st to 10th	Night	23% 6% 27% 34% 7% 2	600
	11th to 20th	Night	22% 4 25% 33% 14% 1	593
	21st to end	Night	18% 4 27% 37% 14% 1	600
May	1st to 10th	Night	19% 4 24% 36% 15% 2	600
	11th to 20th	Night	17% 4 21% 38% 17% 3	591
	21st to end	Night	13% 4 16% 39% 24% 5%	650
June	1st to 10th	Night	15% 2 18% 40% 22% 3	600
	11th to 20th	Night	17% 3 18% 37% 23% 3	600
	21st to end	Night	15% 4 15% 38% 25% 4	600
July	1st to 10th	Night	9% 2 11% 40% 34% 4	585
	11th to 20th	Night	11% 2 13% 36% 32% 6%	582
	21st to end	Night	9% 4 15% 38% 29% 6%	657
August	1st to 10th	Night	9% 1 12% 39% 32% 7%	597
	11th to 20th	Night	10% 3 12% 32% 35% 8%	594
	21st to end	Night	12% 4 13% 39% 30% 2	660
September	1st to 10th	Night	12% 4 19% 33% 23% 8%	600
	11th to 20th	Night	15% 3 18% 32% 23% 9%	734
	21st to end	Night	18% 6% 18% 29% 24% 5%	760
October	1st to 10th	Night	22% 8% 23% 27% 19% 3	760
	11th to 20th	Night	27% 6% 29% 26% 11%	760
	21st to end	Night	31% 8% 26% 26% 9% 1	836
November	1st to 10th	Night	24% 10% 28% 30% 8%	780
	11th to 20th	Night	26% 15% 30% 21% 8%	800
	21st to end	Night	27% 15% 29% 22% 8%	964
December	1st to 10th	Night	25% 14% 32% 22% 6% 1	1000
	11th to 20th	Night	24% 13% 34% 19% 8% 2	1000
	21st to end	Night	28% 10% 33% 24% 5%	1100

Page generated at 2025-09-16 00:49:10. Version ac07

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