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 2024-11-28.

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% 10% 3 8% 5% 5% 5% 4 6% 7% 34%	101241
	Night	14% 10% 4 8% 5% 4 4 4 5% 7% 35%	98278
Transparency	Day	47% 5% 12% 22% 13% 1	104855
	Night	47% 4 12% 23% 13% 1	102791
Seeing	Day	17% 9% 29% 24% 14% 6%	15432
	Night	19% 8% 28% 31% 13% 2	25070

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% 17% 6% 12% 5% 4 4 4 4 6% 22%	2527
	11th to 20th	Day	15% 18% 6% 13% 8% 9% 6% 4 5% 5% 12%	2530
	21st to end	Day	27% 19% 6% 10% 6% 4 4 3 3 4 14%	3106
February	1st to 10th	Day	14% 14% 5% 13% 7% 6% 4 4 5% 6% 21%	1710
	11th to 20th	Day	13% 14% 5% 11% 8% 5% 5% 3 5% 6% 26%	1770
	21st to end	Day	13% 16% 5% 10% 6% 5% 4 3 5% 6% 26%	1570
March	1st to 10th	Day	17% 17% 5% 9% 5% 4 3 3 4 6% 27%	3216
	11th to 20th	Day	12% 12% 4 10% 7% 5% 5% 3 5% 7% 31%	3480
	21st to end	Day	19% 13% 4 9% 5% 4 5% 3 4 5% 29%	3815
April	1st to 10th	Day	15% 12% 3 8% 5% 4 4 5% 5% 6% 31%	2188
	11th to 20th	Day	16% 10% 3 7% 6% 4 4 4 6% 7% 34%	2428
	21st to end	Day	17% 9% 3 8% 5% 5% 4 3 5% 6% 34%	2470
May	1st to 10th	Day	16% 8% 3 7% 5% 4 5% 4 6% 7% 36%	4082
	11th to 20th	Day	15% 8% 2 7% 5% 5% 5% 4 6% 8% 34%	4081
	21st to end	Day	9% 6% 2 8% 5% 4 4 4 5% 8% 45%	4476
June	1st to 10th	Day	9% 6% 3 7% 5% 5% 5% 4 6% 9% 40%	2794
	11th to 20th	Day	9% 5% 2 7% 4 5% 5% 4 7% 8% 42%	2850
	21st to end	Day	8% 7% 3 6% 6% 5% 6% 5% 8% 10% 37%	2793
July	1st to 10th	Day	10% 5% 2 7% 5% 5% 6% 5% 7% 10% 37%	4440
	11th to 20th	Day	6% 5% 2 5% 4 4 5% 5% 8% 10% 45%	4440
	21st to end	Day	5% 6% 3 6% 6% 6% 7% 5% 8% 9% 40%	4508
August	1st to 10th	Day	12% 5% 2 6% 4 5% 6% 4 7% 8% 41%	4104
	11th to 20th	Day	8% 5% 3 7% 5% 5% 6% 4 9% 9% 40%	4044
	21st to end	Day	8% 5% 3 8% 5% 5% 6% 5% 7% 9% 38%	4532
September	1st to 10th	Day	10% 6% 3 7% 6% 4 5% 4 6% 9% 41%	2139
	11th to 20th	Day	9% 6% 1 5% 4 4 4 5% 6% 9% 46%	2090
	21st to end	Day	14% 7% 3 8% 6% 4 5% 4 6% 8% 36%	2090
October	1st to 10th	Day	11% 8% 2 6% 5% 4 5% 4 6% 6% 43%	2019
	11th to 20th	Day	16% 10% 4 7% 5% 5% 6% 3 5% 6% 35%	1900
	21st to end	Day	18% 12% 3 8% 5% 4 5% 3 5% 7% 30%	1952
November	1st to 10th	Day	11% 10% 3 8% 5% 4 4 4 6% 7% 38%	1762
	11th to 20th	Day	14% 11% 3 8% 6% 5% 5% 4 6% 6% 31%	1800
	21st to end	Day	17% 13% 5% 9% 5% 4 5% 3 4 6% 29%	1606
December	1st to 10th	Day	13% 15% 5% 13% 7% 5% 5% 3 6% 6% 22%	1520
	11th to 20th	Day	15% 16% 5% 9% 5% 3 5% 4 6% 6% 24%	1782
	21st to end	Day	25% 21% 6% 10% 5% 5% 4 3 4 6% 12%	2627
January	1st to 10th	Night	16% 15% 5% 11% 6% 5% 4 3 4 6% 26%	4428
	11th to 20th	Night	13% 17% 7% 14% 7% 6% 6% 5% 5% 5% 17%	4440
	21st to end	Night	22% 17% 8% 11% 4 4 3 3 4 4 19%	4535
February	1st to 10th	Night	14% 15% 5% 11% 7% 5% 5% 4 5% 5% 23%	2470
	11th to 20th	Night	12% 13% 4 10% 7% 5% 5% 4 6% 6% 28%	2294
	21st to end	Night	18% 12% 3 9% 5% 5% 4 3 5% 6% 29%	1884
March	1st to 10th	Night	15% 14% 3 9% 5% 4 4 3 4 6% 34%	3637
	11th to 20th	Night	9% 8% 3 6% 5% 5% 5% 4 6% 8% 41%	3480
	21st to end	Night	15% 9% 3 8% 5% 4 5% 3 4 5% 38%	3817
April	1st to 10th	Night	16% 11% 3 6% 5% 4 4 4 6% 7% 34%	2009
	11th to 20th	Night	18% 9% 3 5% 4 4 3 4 5% 7% 38%	1701
	21st to end	Night	16% 8% 2 6% 4 3 4 3 6% 8% 39%	1710
May	1st to 10th	Night	16% 9% 2 7% 4 4 3 3 6% 8% 37%	2813
	11th to 20th	Night	14% 7% 3 7% 4 5% 4 3 6% 7% 41%	2812
	21st to end	Night	9% 5% 2 6% 4 4 4 4 5% 9% 49%	3079
June	1st to 10th	Night	10% 5% 2 7% 4 4 4 4 5% 8% 46%	1377
	11th to 20th	Night	9% 6% 2 6% 4 5% 5% 5% 6% 8% 45%	1330
	21st to end	Night	8% 6% 2 6% 4 4 4 3 7% 8% 48%	1379
July	1st to 10th	Night	8% 4 1 4 4 4 4 4 6% 11% 49%	2530
	11th to 20th	Night	9% 3 2 4 3 4 4 3 6% 8% 53%	2530
	21st to end	Night	4 4 2 5% 5% 5% 6% 5% 8% 9% 46%	3046
August	1st to 10th	Night	10% 6% 2 6% 4 3 5% 3 6% 9% 46%	2825
	11th to 20th	Night	8% 5% 3 7% 4 4 5% 4 6% 8% 47%	2785
	21st to end	Night	8% 6% 2 6% 4 4 5% 3 6% 8% 48%	3124
September	1st to 10th	Night	8% 6% 3 6% 4 4 5% 4 5% 8% 47%	2017
	11th to 20th	Night	9% 5% 2 5% 3 3 4 3 5% 8% 54%	2090
	21st to end	Night	15% 6% 2 6% 4 3 5% 3 5% 7% 44%	2090
October	1st to 10th	Night	13% 6% 3 6% 4 3 3 4 5% 7% 47%	2160
	11th to 20th	Night	17% 9% 3 7% 5% 4 4 4 5% 6% 37%	2280
	21st to end	Night	21% 8% 3 8% 4 4 5% 4 6% 6% 32%	2641
November	1st to 10th	Night	12% 8% 3 6% 4 4 4 4 4 8% 43%	2536
	11th to 20th	Night	14% 10% 3 8% 6% 5% 6% 5% 6% 7% 31%	2600
	21st to end	Night	18% 13% 4 8% 5% 4 5% 4 6% 6% 28%	2725
December	1st to 10th	Night	17% 14% 5% 10% 6% 5% 5% 4 5% 6% 23%	2660
	11th to 20th	Night	13% 14% 5% 10% 6% 5% 5% 4 6% 6% 26%	3409
	21st to end	Night	27% 21% 7% 10% 4 4 4 3 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	64% 4 12% 14% 6% 1	2528
	11th to 20th	Day	71% 5% 11% 9% 4	2530
	21st to end	Day	75% 3 11% 7% 4 1	3106
February	1st to 10th	Day	63% 5% 9% 15% 8%	1710
	11th to 20th	Day	60% 3 8% 18% 9% 1	1779
	21st to end	Day	59% 3 12% 17% 8% 2	1570
March	1st to 10th	Day	59% 3 12% 19% 5% 1	3216
	11th to 20th	Day	52% 5% 12% 21% 8% 2	3480
	21st to end	Day	58% 3 12% 14% 10% 2	3815
April	1st to 10th	Day	52% 4 11% 21% 12% 1	2188
	11th to 20th	Day	49% 4 10% 21% 13% 2	2428
	21st to end	Day	50% 4 13% 18% 14% 2	2470
May	1st to 10th	Day	47% 4 11% 24% 12% 2	4120
	11th to 20th	Day	47% 4 15% 24% 9% 1	4085
	21st to end	Day	37% 4 12% 18% 24% 5%	4489
June	1st to 10th	Day	40% 5% 12% 27% 14% 2	2794
	11th to 20th	Day	38% 5% 14% 24% 17% 3	2850
	21st to end	Day	40% 6% 13% 24% 15% 2	2793
July	1st to 10th	Day	41% 6% 13% 22% 16% 2	4371
	11th to 20th	Day	30% 6% 14% 34% 15% 1	4365
	21st to end	Day	37% 7% 15% 17% 23% 2	4365
August	1st to 10th	Day	39% 6% 13% 26% 16% 1	4091
	11th to 20th	Day	37% 6% 12% 33% 11% 1	4083
	21st to end	Day	39% 6% 13% 27% 14% 1	4531
September	1st to 10th	Day	39% 6% 13% 26% 14% 2	2139
	11th to 20th	Day	33% 5% 11% 33% 16% 1	2090
	21st to end	Day	45% 4 13% 25% 12% 1	2090
October	1st to 10th	Day	31% 4 12% 32% 19% 1	3357
	11th to 20th	Day	41% 3 11% 26% 18%	3140
	21st to end	Day	46% 4 14% 23% 12% 1	3189
November	1st to 10th	Day	44% 5% 11% 26% 13% 1	1762
	11th to 20th	Day	53% 4 13% 19% 10% 1	1800
	21st to end	Day	57% 4 12% 19% 7% 1	1606
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 12% 16% 7% 1	4428
	11th to 20th	Night	67% 5% 12% 11% 4	4440
	21st to end	Night	69% 2 10% 14% 4	4535
February	1st to 10th	Night	62% 5% 9% 15% 8% 1	2470
	11th to 20th	Night	56% 4 10% 19% 10% 1	2296
	21st to end	Night	55% 3 11% 20% 10% 1	1884
March	1st to 10th	Night	52% 3 12% 25% 7%	3637
	11th to 20th	Night	40% 5% 14% 21% 19% 1	3480
	21st to end	Night	49% 3 10% 24% 13% 1	3817
April	1st to 10th	Night	49% 4 11% 22% 13% 2	2009
	11th to 20th	Night	46% 4 11% 19% 18% 2	1701
	21st to end	Night	44% 4 11% 21% 17% 3	1710
May	1st to 10th	Night	46% 3 11% 22% 14% 4	2840
	11th to 20th	Night	42% 6% 10% 28% 13% 2	2813
	21st to end	Night	33% 4 13% 22% 21% 7%	3087
June	1st to 10th	Night	35% 5% 10% 31% 18% 3	1377
	11th to 20th	Night	35% 5% 13% 25% 19% 2	1330
	21st to end	Night	33% 5% 13% 26% 22% 1	1379
July	1st to 10th	Night	29% 4 13% 32% 15% 6%	2490
	11th to 20th	Night	28% 4 14% 35% 18% 2	2482
	21st to end	Night	30% 7% 18% 22% 20% 3	2949
August	1st to 10th	Night	35% 5% 14% 28% 16% 1	2814
	11th to 20th	Night	33% 6% 12% 35% 12% 2	2812
	21st to end	Night	34% 6% 12% 34% 14% 1	3124
September	1st to 10th	Night	34% 5% 13% 28% 17% 2	2017
	11th to 20th	Night	29% 4 12% 32% 21% 2	2090
	21st to end	Night	40% 4 12% 31% 12% 1	2090
October	1st to 10th	Night	26% 3 10% 32% 27% 1	3602
	11th to 20th	Night	43% 3 11% 24% 18% 1	3779
	21st to end	Night	41% 3 12% 29% 13% 1	4341
November	1st to 10th	Night	40% 5% 12% 27% 16% 1	2536
	11th to 20th	Night	51% 4 13% 21% 9% 1	2600
	21st to end	Night	55% 4 14% 19% 7%	2725
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% 13% 43% 16% 4 1	270
	21st to end	Day	27% 10% 38% 19% 5% 1	297
February	1st to 10th	Day	23% 18% 39% 14% 4 1	270
	11th to 20th	Day	23% 17% 40% 17% 2	270
	21st to end	Day	22% 20% 44% 12% 3	225
March	1st to 10th	Day	26% 17% 35% 17% 4 1	327
	11th to 20th	Day	23% 12% 32% 25% 7% 2	460
	21st to end	Day	28% 11% 33% 22% 5% 1	506
April	1st to 10th	Day	17% 13% 38% 21% 9% 1	460
	11th to 20th	Day	25% 15% 30% 16% 8% 6%	452
	21st to end	Day	24% 10% 29% 22% 11% 3	460
May	1st to 10th	Day	23% 8% 30% 24% 12% 3	463
	11th to 20th	Day	19% 8% 28% 24% 14% 7%	487
	21st to end	Day	14% 5% 23% 29% 20% 9%	529
June	1st to 10th	Day	12% 8% 29% 31% 16% 4	648
	11th to 20th	Day	11% 6% 32% 29% 18% 4	680
	21st to end	Day	14% 6% 29% 29% 18% 5%	680
July	1st to 10th	Day	9% 5% 21% 31% 21% 13%	671
	11th to 20th	Day	11% 5% 24% 31% 19% 10%	668
	21st to end	Day	9% 4 22% 30% 24% 11%	561
August	1st to 10th	Day	11% 2 20% 26% 24% 15%	486
	11th to 20th	Day	11% 3 17% 30% 24% 15%	479
	21st to end	Day	12% 3 19% 31% 22% 13%	539
September	1st to 10th	Day	12% 4 25% 23% 23% 13%	490
	11th to 20th	Day	9% 3 19% 24% 29% 16%	490
	21st to end	Day	18% 7% 23% 23% 20% 8%	490
October	1st to 10th	Day	18% 8% 25% 23% 17% 10%	490
	11th to 20th	Day	21% 11% 32% 24% 9% 3	490
	21st to end	Day	24% 13% 36% 20% 5% 1	401
November	1st to 10th	Day	17% 17% 35% 22% 7% 2	300
	11th to 20th	Day	22% 18% 35% 16% 7% 1	300
	21st to end	Day	24% 16% 35% 20% 5%	209
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	25% 13% 32% 26% 4 1	950
	21st to end	Night	20% 10% 32% 30% 7%	839
February	1st to 10th	Night	21% 13% 38% 25% 3	760
	11th to 20th	Night	20% 15% 33% 27% 5%	743
	21st to end	Night	25% 15% 37% 21% 2	628
March	1st to 10th	Night	20% 11% 35% 28% 6%	760
	11th to 20th	Night	17% 15% 36% 26% 6% 1	760
	21st to end	Night	22% 10% 31% 27% 8% 1	836
April	1st to 10th	Night	22% 11% 33% 28% 5% 1	679
	11th to 20th	Night	23% 8% 31% 28% 8% 2	563
	21st to end	Night	20% 5% 31% 33% 10%	570
May	1st to 10th	Night	19% 4 29% 37% 10% 1	570
	11th to 20th	Night	22% 4 25% 31% 16% 1	561
	21st to end	Night	10% 2 22% 42% 21% 2	617
June	1st to 10th	Night	13% 4 24% 46% 14%	399
	11th to 20th	Night	11% 4 21% 51% 12% 1	380
	21st to end	Night	11% 2 21% 47% 19%	429
July	1st to 10th	Night	8% 1 18% 40% 28% 4	555
	11th to 20th	Night	11% 2 16% 41% 26% 3	552
	21st to end	Night	6% 2 18% 48% 24% 2	624
August	1st to 10th	Night	8% 1 17% 38% 32% 3	567
	11th to 20th	Night	8% 1 17% 38% 31% 4	564
	21st to end	Night	10% 2 17% 36% 32% 3	627
September	1st to 10th	Night	11% 5% 18% 33% 27% 5%	697
	11th to 20th	Night	11% 3 15% 35% 29% 6%	760
	21st to end	Night	18% 4 20% 32% 21% 5%	760
October	1st to 10th	Night	17% 6% 25% 31% 19% 3	760
	11th to 20th	Night	23% 9% 28% 29% 9% 2	760
	21st to end	Night	24% 10% 32% 29% 5%	836
November	1st to 10th	Night	18% 10% 31% 27% 13% 1	780
	11th to 20th	Night	21% 13% 32% 25% 8% 1	800
	21st to end	Night	26% 12% 33% 23% 6%	789
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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