Washington(MI)
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Washington(MI) clear sky chart from 2005-11-06 to 2024-01-14.

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% 9% 4 7% 6% 4 5% 5% 6% 8% 34%	74649
	Night	15% 9% 4 7% 6% 3 5% 5% 5% 7% 35%	75111
Transparency	Day	49% 4 12% 21% 12% 2	74359
	Night	49% 4 12% 22% 12% 1	74922
Seeing	Day	17% 9% 29% 24% 14% 6%	14201
	Night	19% 8% 28% 31% 13% 2	23968

By Month

Percentage Cloud Cover SE (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	15% 13% 5% 12% 8% 4 4 5% 4 7% 23%	1517
	11th to 20th	Day	15% 15% 7% 12% 7% 6% 6% 5% 5% 6% 17%	1464
	21st to end	Day	14% 16% 8% 10% 9% 4 6% 4 4 5% 18%	1777
February	1st to 10th	Day	15% 13% 6% 12% 9% 4 5% 4 5% 6% 20%	1620
	11th to 20th	Day	14% 12% 5% 10% 9% 4 5% 4 5% 6% 24%	1675
	21st to end	Day	14% 15% 6% 9% 7% 4 5% 4 5% 6% 26%	1480
March	1st to 10th	Day	14% 12% 5% 8% 7% 3 5% 5% 5% 7% 28%	1854
	11th to 20th	Day	13% 12% 5% 8% 7% 3 4 4 5% 7% 33%	1980
	21st to end	Day	19% 12% 4 9% 6% 3 5% 4 4 5% 27%	2178
April	1st to 10th	Day	15% 11% 4 8% 6% 4 3 6% 5% 7% 31%	2072
	11th to 20th	Day	16% 10% 3 7% 7% 3 4 5% 6% 7% 33%	2298
	21st to end	Day	18% 8% 3 7% 6% 4 4 4 5% 7% 34%	2340
May	1st to 10th	Day	16% 8% 3 7% 6% 4 5% 5% 6% 7% 33%	2340
	11th to 20th	Day	17% 9% 3 5% 6% 4 4 5% 5% 8% 35%	2301
	21st to end	Day	12% 6% 2 6% 5% 3 5% 6% 6% 9% 40%	2531
June	1st to 10th	Day	9% 5% 3 6% 6% 4 5% 5% 6% 9% 41%	2646
	11th to 20th	Day	10% 5% 3 6% 5% 4 5% 5% 6% 9% 42%	2700
	21st to end	Day	8% 6% 3 5% 6% 3 6% 6% 7% 10% 38%	2647
July	1st to 10th	Day	7% 4 2 5% 6% 3 6% 6% 5% 9% 45%	2520
	11th to 20th	Day	8% 5% 3 4 5% 3 6% 6% 8% 10% 43%	2520
	21st to end	Day	6% 5% 3 5% 7% 4 6% 6% 8% 9% 40%	2575
August	1st to 10th	Day	8% 5% 2 5% 6% 4 7% 6% 7% 10% 40%	2327
	11th to 20th	Day	9% 4 2 6% 7% 4 6% 6% 8% 8% 39%	2303
	21st to end	Day	11% 6% 3 6% 6% 3 5% 5% 6% 9% 41%	2574
September	1st to 10th	Day	11% 5% 4 6% 7% 3 5% 5% 6% 9% 39%	2027
	11th to 20th	Day	10% 5% 2 4 4 3 5% 6% 6% 9% 45%	1980
	21st to end	Day	13% 6% 3 6% 6% 4 5% 5% 6% 9% 37%	1980
October	1st to 10th	Day	11% 8% 2 6% 6% 3 5% 5% 6% 7% 42%	1913
	11th to 20th	Day	17% 9% 4 7% 5% 4 6% 4 4 6% 33%	1800
	21st to end	Day	19% 12% 4 8% 6% 3 4 4 5% 7% 29%	1850
November	1st to 10th	Day	11% 9% 3 7% 6% 3 5% 5% 6% 7% 38%	1672
	11th to 20th	Day	14% 10% 4 7% 7% 4 5% 5% 6% 7% 32%	1710
	21st to end	Day	18% 11% 5% 7% 6% 3 5% 5% 4 6% 30%	1549
December	1st to 10th	Day	13% 14% 6% 12% 8% 4 5% 4 5% 6% 22%	1520
	11th to 20th	Day	16% 15% 5% 8% 6% 3 5% 5% 6% 7% 25%	1782
	21st to end	Day	25% 20% 7% 9% 6% 3 4 4 4 5% 12%	2627
January	1st to 10th	Night	17% 10% 5% 10% 8% 4 5% 5% 4 7% 25%	2648
	11th to 20th	Night	18% 15% 6% 10% 8% 4 4 4 5% 5% 21%	2575
	21st to end	Night	12% 13% 8% 10% 8% 4 5% 5% 5% 6% 24%	2577
February	1st to 10th	Night	15% 15% 5% 10% 8% 4 5% 5% 4 5% 23%	2340
	11th to 20th	Night	13% 12% 5% 9% 8% 4 6% 6% 5% 6% 27%	2170
	21st to end	Night	19% 12% 4 8% 6% 4 4 4 5% 6% 30%	1776
March	1st to 10th	Night	15% 10% 4 7% 6% 4 5% 5% 4 6% 34%	2106
	11th to 20th	Night	13% 8% 3 6% 5% 4 6% 5% 6% 8% 38%	1980
	21st to end	Night	19% 9% 4 6% 6% 3 5% 5% 4 5% 34%	2178
April	1st to 10th	Night	16% 9% 3 6% 5% 3 4 5% 6% 7% 35%	1904
	11th to 20th	Night	17% 8% 3 5% 4 4 4 4 5% 7% 38%	1611
	21st to end	Night	17% 7% 2 5% 5% 3 4 4 5% 8% 39%	1620
May	1st to 10th	Night	15% 8% 3 7% 6% 3 4 5% 6% 7% 34%	1620
	11th to 20th	Night	17% 5% 3 6% 4 3 4 4 6% 7% 41%	1592
	21st to end	Night	9% 4 3 5% 5% 3 5% 5% 6% 9% 46%	1744
June	1st to 10th	Night	10% 4 2 6% 5% 3 4 6% 5% 9% 47%	1305
	11th to 20th	Night	9% 5% 2 5% 5% 3 5% 5% 6% 8% 46%	1260
	21st to end	Night	8% 6% 2 6% 5% 3 4 4 7% 8% 49%	1306
July	1st to 10th	Night	7% 4 2 4 4 3 5% 4 5% 9% 53%	1440
	11th to 20th	Night	8% 3 2 4 4 3 5% 4 7% 8% 52%	1440
	21st to end	Night	5% 4 2 4 4 3 5% 6% 7% 9% 50%	1743
August	1st to 10th	Night	6% 4 2 4 5% 2 5% 5% 7% 10% 50%	1611
	11th to 20th	Night	6% 4 3 4 7% 3 5% 5% 6% 8% 49%	1598
	21st to end	Night	8% 5% 2 5% 4 3 4 4 6% 7% 52%	1782
September	1st to 10th	Night	9% 6% 3 6% 5% 3 5% 5% 5% 8% 46%	1910
	11th to 20th	Night	9% 5% 2 4 4 3 4 4 5% 8% 53%	1980
	21st to end	Night	14% 5% 2 6% 4 2 4 4 5% 7% 45%	1980
October	1st to 10th	Night	14% 6% 3 5% 5% 2 4 5% 4 7% 45%	2046
	11th to 20th	Night	17% 8% 3 6% 6% 3 4 5% 5% 6% 36%	2160
	21st to end	Night	22% 8% 3 7% 6% 3 5% 5% 5% 6% 31%	2502
November	1st to 10th	Night	12% 7% 3 5% 5% 3 4 4 4 8% 43%	2406
	11th to 20th	Night	15% 9% 4 7% 6% 4 5% 6% 6% 7% 31%	2470
	21st to end	Night	19% 11% 4 7% 6% 3 5% 5% 5% 6% 29%	2627
December	1st to 10th	Night	17% 13% 6% 9% 7% 4 5% 5% 5% 7% 23%	2660
	11th to 20th	Night	13% 14% 5% 9% 7% 4 5% 5% 6% 6% 26%	3409
	21st to end	Night	27% 21% 7% 9% 6% 3 4 4 4 4 14%	5035

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	63% 3 11% 14% 7% 1	1518
	11th to 20th	Day	67% 5% 12% 11% 5%	1464
	21st to end	Day	69% 4 8% 12% 7% 1	1777
February	1st to 10th	Day	65% 5% 9% 14% 7%	1620
	11th to 20th	Day	61% 3 9% 18% 8% 1	1684
	21st to end	Day	60% 2 11% 17% 8% 2	1480
March	1st to 10th	Day	56% 4 13% 17% 8% 1	1854
	11th to 20th	Day	51% 4 10% 20% 12% 3	1980
	21st to end	Day	59% 3 9% 19% 8% 1	2178
April	1st to 10th	Day	53% 4 11% 21% 12% 1	2072
	11th to 20th	Day	50% 4 10% 20% 14% 2	2298
	21st to end	Day	50% 4 13% 17% 13% 2	2340
May	1st to 10th	Day	50% 4 11% 22% 11% 2	2340
	11th to 20th	Day	49% 3 12% 20% 14% 2	2305
	21st to end	Day	40% 5% 14% 23% 15% 2	2531
June	1st to 10th	Day	39% 4 12% 27% 14% 2	2646
	11th to 20th	Day	38% 5% 14% 24% 16% 3	2700
	21st to end	Day	39% 6% 13% 24% 16% 2	2647
July	1st to 10th	Day	35% 5% 10% 27% 19% 3	2451
	11th to 20th	Day	34% 5% 14% 27% 18% 2	2445
	21st to end	Day	36% 6% 14% 23% 18% 3	2432
August	1st to 10th	Day	37% 6% 14% 29% 13% 2	2314
	11th to 20th	Day	39% 6% 14% 27% 12% 2	2303
	21st to end	Day	39% 5% 11% 26% 17% 1	2574
September	1st to 10th	Day	41% 6% 13% 25% 14% 2	2027
	11th to 20th	Day	34% 5% 11% 34% 14% 1	1980
	21st to end	Day	44% 5% 13% 26% 12% 1	1980
October	1st to 10th	Day	41% 5% 12% 28% 13% 2	1913
	11th to 20th	Day	52% 4 13% 21% 11% 1	1800
	21st to end	Day	55% 5% 13% 19% 8%	1850
November	1st to 10th	Day	44% 5% 12% 25% 13% 1	1672
	11th to 20th	Day	52% 3 13% 20% 10% 1	1710
	21st to end	Day	56% 4 12% 20% 7% 1	1549
December	1st to 10th	Day	63% 3 10% 18% 5%	1520
	11th to 20th	Day	59% 4 15% 16% 6%	1778
	21st to end	Day	75% 3 11% 7% 4	2627
January	1st to 10th	Night	60% 4 10% 18% 8% 1	2648
	11th to 20th	Night	65% 3 12% 13% 6%	2575
	21st to end	Night	60% 3 12% 17% 7% 1	2577
February	1st to 10th	Night	63% 5% 9% 15% 7% 1	2340
	11th to 20th	Night	57% 4 10% 18% 10% 1	2172
	21st to end	Night	56% 3 11% 20% 10% 1	1776
March	1st to 10th	Night	51% 4 10% 23% 11% 1	2106
	11th to 20th	Night	45% 4 11% 23% 15% 2	1980
	21st to end	Night	53% 4 8% 23% 11% 1	2178
April	1st to 10th	Night	48% 4 11% 22% 14% 2	1904
	11th to 20th	Night	46% 4 11% 19% 18% 2	1611
	21st to end	Night	44% 4 10% 21% 17% 3	1620
May	1st to 10th	Night	48% 4 12% 22% 12% 3	1620
	11th to 20th	Night	43% 4 10% 21% 19% 3	1593
	21st to end	Night	34% 5% 14% 26% 17% 3	1745
June	1st to 10th	Night	35% 5% 10% 31% 17% 3	1305
	11th to 20th	Night	35% 5% 13% 26% 19% 2	1260
	21st to end	Night	33% 5% 13% 26% 22% 1	1306
July	1st to 10th	Night	29% 4 11% 30% 23% 4	1400
	11th to 20th	Night	30% 4 12% 34% 17% 3	1392
	21st to end	Night	27% 6% 19% 25% 19% 4	1646
August	1st to 10th	Night	28% 5% 17% 33% 15% 2	1600
	11th to 20th	Night	31% 6% 14% 34% 13% 3	1598
	21st to end	Night	30% 5% 13% 31% 19% 1	1782
September	1st to 10th	Night	35% 5% 14% 28% 15% 2	1910
	11th to 20th	Night	31% 4 12% 32% 20% 1	1980
	21st to end	Night	39% 4 12% 32% 13% 1	1980
October	1st to 10th	Night	39% 4 11% 26% 19% 1	2046
	11th to 20th	Night	49% 4 13% 21% 13% 1	2160
	21st to end	Night	53% 4 11% 22% 10% 1	2502
November	1st to 10th	Night	40% 5% 12% 27% 16% 1	2406
	11th to 20th	Night	51% 4 14% 22% 9% 1	2470
	21st to end	Night	55% 4 14% 20% 7%	2627
December	1st to 10th	Night	62% 4 12% 17% 6%	2660
	11th to 20th	Night	58% 4 14% 16% 7% 1	3412
	21st to end	Night	75% 3 11% 8% 3	5035

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	21% 19% 36% 16% 9%	270
	11th to 20th	Day	24% 12% 43% 16% 4 1	249
	21st to end	Day	23% 11% 40% 19% 5% 1	264
February	1st to 10th	Day	25% 18% 41% 13% 3	240
	11th to 20th	Day	25% 17% 42% 14% 3	240
	21st to end	Day	24% 20% 43% 11% 2	198
March	1st to 10th	Day	25% 19% 36% 16% 4 1	294
	11th to 20th	Day	22% 11% 33% 25% 7% 2	420
	21st to end	Day	29% 11% 30% 23% 6% 1	462
April	1st to 10th	Day	17% 12% 38% 21% 10% 1	420
	11th to 20th	Day	25% 13% 30% 16% 9% 7%	412
	21st to end	Day	24% 10% 29% 22% 11% 4	420
May	1st to 10th	Day	23% 8% 30% 24% 12% 4	423
	11th to 20th	Day	20% 9% 28% 23% 13% 7%	447
	21st to end	Day	14% 5% 21% 29% 20% 10%	485
June	1st to 10th	Day	11% 8% 30% 31% 15% 5%	599
	11th to 20th	Day	12% 6% 31% 29% 18% 5%	630
	21st to end	Day	13% 5% 28% 30% 19% 5%	630
July	1st to 10th	Day	8% 4 19% 32% 22% 14%	621
	11th to 20th	Day	11% 5% 24% 32% 19% 10%	618
	21st to end	Day	9% 4 23% 28% 25% 11%	516
August	1st to 10th	Day	10% 2 20% 26% 25% 17%	446
	11th to 20th	Day	11% 3 17% 29% 24% 16%	439
	21st to end	Day	13% 3 19% 30% 22% 14%	495
September	1st to 10th	Day	13% 3 24% 24% 24% 12%	450
	11th to 20th	Day	10% 4 19% 23% 28% 16%	450
	21st to end	Day	16% 8% 22% 24% 22% 8%	450
October	1st to 10th	Day	18% 7% 25% 22% 17% 11%	450
	11th to 20th	Day	22% 12% 31% 24% 8% 2	450
	21st to end	Day	25% 12% 38% 19% 5% 1	365
November	1st to 10th	Day	17% 16% 34% 24% 7% 2	270
	11th to 20th	Day	20% 19% 36% 17% 8% 1	270
	21st to end	Day	24% 14% 36% 21% 5%	194
December	1st to 10th	Day	24% 14% 40% 13% 7% 1	180
	11th to 20th	Day	28% 16% 38% 13% 4 1	180
	21st to end	Day	30% 15% 33% 17% 5%	254
January	1st to 10th	Night	22% 13% 35% 23% 6% 1	950
	11th to 20th	Night	26% 13% 31% 25% 4 1	920
	21st to end	Night	18% 11% 33% 31% 7%	795
February	1st to 10th	Night	21% 13% 38% 25% 3	720
	11th to 20th	Night	21% 15% 33% 26% 5%	703
	21st to end	Night	26% 14% 37% 21% 2	592
March	1st to 10th	Night	20% 11% 34% 28% 6%	720
	11th to 20th	Night	17% 13% 35% 27% 6% 1	720
	21st to end	Night	23% 9% 31% 27% 9% 1	792
April	1st to 10th	Night	22% 12% 32% 29% 5% 1	644
	11th to 20th	Night	23% 7% 31% 29% 9% 2	533
	21st to end	Night	21% 5% 31% 32% 10%	540
May	1st to 10th	Night	19% 4 29% 37% 10% 1	540
	11th to 20th	Night	21% 5% 25% 32% 16% 1	531
	21st to end	Night	10% 2 22% 42% 22% 3	584
June	1st to 10th	Night	12% 3 24% 46% 14%	378
	11th to 20th	Night	11% 3 22% 51% 12% 1	360
	21st to end	Night	11% 2 20% 47% 20%	406
July	1st to 10th	Night	8% 2 17% 39% 30% 4	525
	11th to 20th	Night	11% 2 15% 42% 26% 3	522
	21st to end	Night	6% 2 18% 48% 23% 2	591
August	1st to 10th	Night	9% 1 15% 39% 33% 3	537
	11th to 20th	Night	8% 1 16% 39% 31% 4	534
	21st to end	Night	11% 1 17% 36% 32% 3	594
September	1st to 10th	Night	12% 4 18% 34% 27% 5%	660
	11th to 20th	Night	11% 3 16% 36% 27% 6%	720
	21st to end	Night	17% 4 20% 32% 22% 5%	720
October	1st to 10th	Night	18% 5% 24% 31% 19% 3	720
	11th to 20th	Night	23% 10% 29% 29% 8% 2	720
	21st to end	Night	26% 10% 31% 28% 4	792
November	1st to 10th	Night	17% 10% 30% 28% 14% 1	740
	11th to 20th	Night	21% 13% 33% 25% 8% 1	760
	21st to end	Night	26% 11% 34% 23% 7%	760
December	1st to 10th	Night	24% 11% 36% 22% 6% 1	760
	11th to 20th	Night	22% 13% 36% 25% 4	840
	21st to end	Night	31% 9% 32% 25% 4	1045

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