Portage (MI)
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Portage (MI) 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	13% 10% 4 9% 5% 4 4 4 5% 7% 33%	126042
	Night	15% 12% 5% 10% 5% 4 4 3 5% 6% 31%	123235
Transparency	Day	49% 4 11% 22% 12% 1	129596
	Night	52% 4 10% 20% 12% 1	127755
Seeing	Day	17% 13% 29% 23% 13% 4	16979
	Night	19% 10% 29% 29% 11% 1	26761

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	12% 17% 9% 21% 6% 4 4 3 4 5% 14%	3617
	11th to 20th	Day	12% 15% 8% 20% 13% 10% 7% 3 2 3 6%	3620
	21st to end	Day	19% 17% 8% 16% 6% 4 4 3 4 4 15%	4003
February	1st to 10th	Day	14% 15% 6% 14% 11% 6% 4 3 4 4 18%	1885
	11th to 20th	Day	14% 13% 6% 16% 9% 5% 5% 2 4 4 20%	1948
	21st to end	Day	15% 15% 7% 14% 6% 5% 3 4 5% 6% 21%	1650
March	1st to 10th	Day	14% 12% 5% 13% 6% 5% 4 3 4 6% 29%	4480
	11th to 20th	Day	10% 9% 5% 8% 5% 5% 5% 4 6% 8% 34%	4858
	21st to end	Day	18% 16% 7% 10% 5% 5% 4 3 5% 5% 23%	5641
April	1st to 10th	Day	17% 15% 3 7% 6% 4 4 4 5% 6% 29%	2400
	11th to 20th	Day	16% 11% 3 6% 4 4 4 3 5% 6% 36%	2423
	21st to end	Day	17% 9% 3 7% 5% 4 5% 3 6% 7% 35%	2600
May	1st to 10th	Day	20% 9% 3 7% 5% 4 5% 4 5% 7% 33%	5862
	11th to 20th	Day	14% 7% 3 7% 5% 4 4 4 6% 8% 38%	5861
	21st to end	Day	15% 6% 3 6% 4 3 4 4 5% 8% 42%	6832
June	1st to 10th	Day	10% 6% 2 6% 5% 5% 4 4 6% 8% 44%	2800
	11th to 20th	Day	10% 5% 2 6% 5% 4 5% 5% 6% 10% 42%	2800
	21st to end	Day	10% 6% 2 6% 6% 4 5% 5% 7% 8% 41%	2800
July	1st to 10th	Day	9% 6% 2 7% 5% 5% 5% 5% 7% 10% 40%	6360
	11th to 20th	Day	9% 5% 2 6% 4 4 4 4 7% 10% 45%	6360
	21st to end	Day	8% 6% 2 6% 4 4 5% 4 7% 10% 43%	6889
August	1st to 10th	Day	11% 5% 2 5% 4 4 5% 4 5% 9% 45%	4234
	11th to 20th	Day	10% 6% 2 6% 5% 4 5% 4 6% 7% 46%	4174
	21st to end	Day	8% 4 2 6% 4 5% 5% 5% 6% 9% 46%	4345
September	1st to 10th	Day	10% 6% 2 7% 5% 4 5% 4 5% 6% 46%	2359
	11th to 20th	Day	11% 4 2 5% 4 3 4 3 5% 9% 49%	2178
	21st to end	Day	12% 7% 3 7% 5% 5% 4 3 5% 9% 39%	2090
October	1st to 10th	Day	14% 7% 2 7% 5% 4 4 4 5% 6% 43%	2090
	11th to 20th	Day	18% 9% 3 9% 5% 5% 5% 4 5% 7% 30%	1875
	21st to end	Day	21% 13% 3 7% 6% 4 5% 2 5% 7% 26%	1881
November	1st to 10th	Day	13% 10% 3 8% 6% 4 4 3 6% 6% 37%	1762
	11th to 20th	Day	16% 17% 5% 11% 6% 5% 4 4 5% 5% 23%	1800
	21st to end	Day	19% 18% 7% 10% 6% 5% 3 3 4 6% 20%	1800
December	1st to 10th	Day	10% 20% 9% 16% 6% 5% 5% 3 5% 4 17%	2775
	11th to 20th	Day	17% 21% 12% 14% 7% 3 3 3 3 4 13%	3008
	21st to end	Day	24% 22% 6% 14% 6% 4 3 2 3 5% 10%	3982
January	1st to 10th	Night	13% 18% 9% 21% 6% 4 4 3 4 4 15%	6348
	11th to 20th	Night	12% 17% 9% 21% 9% 8% 6% 3 4 3 8%	6360
	21st to end	Night	20% 15% 7% 15% 6% 6% 5% 3 3 4 16%	6973
February	1st to 10th	Night	14% 18% 5% 12% 9% 5% 3 3 4 6% 20%	2514
	11th to 20th	Night	13% 15% 6% 13% 8% 6% 4 3 4 6% 23%	2338
	21st to end	Night	17% 12% 5% 11% 6% 4 4 3 5% 6% 27%	1980
March	1st to 10th	Night	15% 10% 4 10% 5% 4 4 3 4 7% 34%	5373
	11th to 20th	Night	11% 7% 3 7% 5% 5% 4 3 5% 7% 43%	5101
	21st to end	Night	17% 12% 5% 10% 5% 4 4 3 4 6% 31%	5330
April	1st to 10th	Night	15% 11% 3 7% 4 5% 4 3 6% 7% 35%	2000
	11th to 20th	Night	19% 9% 3 6% 5% 4 4 4 5% 5% 37%	1926
	21st to end	Night	17% 8% 3 7% 5% 3 4 3 6% 7% 39%	1800
May	1st to 10th	Night	18% 10% 3 8% 5% 4 4 3 5% 7% 33%	4033
	11th to 20th	Night	17% 8% 3 6% 4 3 3 3 4 7% 42%	4032
	21st to end	Night	16% 6% 2 6% 5% 4 3 3 5% 7% 42%	4038
June	1st to 10th	Night	11% 4 2 4 5% 3 4 4 7% 10% 47%	1600
	11th to 20th	Night	10% 5% 2 6% 5% 4 4 4 8% 9% 43%	1600
	21st to end	Night	9% 4 2 6% 5% 4 5% 4 6% 11% 44%	1600
July	1st to 10th	Night	8% 5% 2 5% 4 4 5% 4 6% 10% 46%	3618
	11th to 20th	Night	8% 5% 2 6% 4 4 4 3 6% 8% 51%	3617
	21st to end	Night	9% 6% 2 6% 3 3 4 4 6% 9% 47%	3980
August	1st to 10th	Night	9% 5% 2 5% 4 4 5% 4 6% 9% 47%	2915
	11th to 20th	Night	10% 5% 2 6% 4 5% 4 3 6% 7% 47%	2875
	21st to end	Night	8% 6% 2 7% 4 4 4 4 6% 7% 48%	3550
September	1st to 10th	Night	9% 6% 2 5% 4 3 4 3 6% 8% 50%	2020
	11th to 20th	Night	9% 5% 3 5% 3 3 3 3 4 7% 54%	2179
	21st to end	Night	14% 7% 3 6% 4 3 4 4 4 6% 44%	2090
October	1st to 10th	Night	15% 6% 2 6% 4 3 4 3 5% 6% 47%	2090
	11th to 20th	Night	18% 11% 3 7% 5% 5% 4 2 4 6% 36%	2302
	21st to end	Night	23% 10% 3 7% 4 4 5% 3 4 6% 32%	2717
November	1st to 10th	Night	14% 11% 3 8% 5% 4 4 3 4 6% 39%	2536
	11th to 20th	Night	16% 15% 4 11% 5% 4 5% 3 5% 6% 25%	2600
	21st to end	Night	18% 16% 5% 10% 6% 4 4 3 5% 5% 24%	2600
December	1st to 10th	Night	12% 18% 9% 17% 7% 5% 4 3 4 4 16%	4405
	11th to 20th	Night	16% 22% 11% 15% 7% 4 3 2 3 4 13%	5199
	21st to end	Night	26% 21% 6% 11% 6% 4 4 3 4 5% 11%	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	73% 2 10% 13% 2	3618
	11th to 20th	Day	83% 3 6% 6% 1	3620
	21st to end	Day	72% 4 10% 11% 3	4003
February	1st to 10th	Day	70% 3 9% 12% 6% 1	1885
	11th to 20th	Day	66% 4 7% 15% 7% 1	1958
	21st to end	Day	64% 3 12% 14% 6% 1	1650
March	1st to 10th	Day	57% 3 12% 20% 8%	4480
	11th to 20th	Day	47% 3 15% 26% 8% 1	4858
	21st to end	Day	65% 3 8% 15% 8%	5643
April	1st to 10th	Day	56% 4 9% 21% 11%	2400
	11th to 20th	Day	49% 3 9% 23% 13% 2	2423
	21st to end	Day	49% 3 12% 20% 14% 2	2600
May	1st to 10th	Day	51% 4 9% 21% 15% 1	5897
	11th to 20th	Day	44% 3 14% 24% 13% 1	5865
	21st to end	Day	41% 4 10% 25% 19% 1	6846
June	1st to 10th	Day	37% 4 12% 27% 17% 2	2800
	11th to 20th	Day	36% 5% 13% 26% 18% 2	2800
	21st to end	Day	39% 5% 15% 22% 17% 2	2800
July	1st to 10th	Day	37% 7% 12% 25% 17% 2	6291
	11th to 20th	Day	33% 6% 13% 29% 17% 1	6285
	21st to end	Day	35% 7% 14% 24% 19% 2	6693
August	1st to 10th	Day	35% 6% 15% 27% 16% 1	4221
	11th to 20th	Day	36% 6% 11% 31% 15% 1	4213
	21st to end	Day	33% 6% 12% 30% 16% 2	4344
September	1st to 10th	Day	38% 3 14% 26% 16% 3	2359
	11th to 20th	Day	33% 4 9% 32% 21% 2	2178
	21st to end	Day	43% 4 10% 25% 17% 1	2090
October	1st to 10th	Day	30% 3 9% 40% 18% 1	3480
	11th to 20th	Day	40% 4 11% 25% 20%	3091
	21st to end	Day	49% 3 10% 26% 10% 2	3081
November	1st to 10th	Day	47% 3 12% 24% 13% 1	1762
	11th to 20th	Day	62% 4 9% 18% 6% 1	1800
	21st to end	Day	67% 3 11% 14% 4	1800
December	1st to 10th	Day	68% 5% 14% 10% 2	2775
	11th to 20th	Day	78% 2 9% 10% 1	3005
	21st to end	Day	78% 3 7% 10% 2	3982
January	1st to 10th	Night	74% 2 7% 13% 4 1	6348
	11th to 20th	Night	80% 3 8% 7% 3	6360
	21st to end	Night	72% 3 12% 9% 4	6973
February	1st to 10th	Night	67% 3 9% 14% 7% 1	2514
	11th to 20th	Night	63% 3 8% 19% 7% 1	2339
	21st to end	Night	59% 3 12% 18% 7% 2	1980
March	1st to 10th	Night	50% 3 12% 25% 8% 1	5373
	11th to 20th	Night	41% 4 12% 30% 12% 2	5101
	21st to end	Night	56% 2 9% 18% 13% 1	5330
April	1st to 10th	Night	48% 4 11% 23% 13% 1	2000
	11th to 20th	Night	48% 3 8% 20% 18% 3	1926
	21st to end	Night	46% 4 10% 23% 15% 2	1800
May	1st to 10th	Night	53% 3 9% 19% 13% 3	4060
	11th to 20th	Night	44% 4 10% 24% 17% 2	4033
	21st to end	Night	43% 3 9% 23% 20% 2	4046
June	1st to 10th	Night	32% 4 12% 31% 19% 3	1600
	11th to 20th	Night	36% 5% 13% 26% 19% 2	1600
	21st to end	Night	34% 6% 16% 25% 17% 3	1600
July	1st to 10th	Night	33% 6% 13% 28% 16% 5%	3580
	11th to 20th	Night	31% 6% 14% 25% 22% 2	3572
	21st to end	Night	34% 6% 13% 22% 23% 2	3865
August	1st to 10th	Night	32% 6% 14% 31% 16% 1	2904
	11th to 20th	Night	34% 5% 12% 32% 16% 1	2902
	21st to end	Night	35% 6% 9% 30% 18% 2	3550
September	1st to 10th	Night	32% 5% 11% 29% 20% 3	2020
	11th to 20th	Night	31% 3 8% 33% 24% 1	2179
	21st to end	Night	41% 3 9% 28% 17% 1	2090
October	1st to 10th	Night	27% 4 9% 35% 25% 1	3480
	11th to 20th	Night	41% 4 9% 24% 22%	3825
	21st to end	Night	42% 3 8% 30% 17% 1	4467
November	1st to 10th	Night	48% 4 9% 24% 15% 1	2536
	11th to 20th	Night	58% 3 12% 18% 8% 2	2600
	21st to end	Night	61% 4 12% 19% 4	2600
December	1st to 10th	Night	70% 5% 12% 10% 3	4405
	11th to 20th	Night	76% 3 8% 10% 2	5201
	21st to end	Night	77% 3 9% 9% 3	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	21% 20% 34% 19% 6% 1	300
	11th to 20th	Day	22% 19% 41% 12% 5% 1	300
	21st to end	Day	23% 19% 34% 21% 3	330
February	1st to 10th	Day	25% 23% 30% 17% 4 1	300
	11th to 20th	Day	28% 20% 33% 16% 2	300
	21st to end	Day	24% 31% 33% 9% 2	249
March	1st to 10th	Day	20% 26% 38% 13% 3	300
	11th to 20th	Day	22% 19% 33% 19% 6% 1	443
	21st to end	Day	28% 19% 31% 17% 5% 1	550
April	1st to 10th	Day	27% 17% 32% 19% 5% 1	500
	11th to 20th	Day	26% 21% 28% 15% 8% 2	492
	21st to end	Day	24% 14% 33% 19% 10% 1	500
May	1st to 10th	Day	22% 18% 28% 21% 8% 2	503
	11th to 20th	Day	21% 11% 28% 23% 12% 5%	527
	21st to end	Day	14% 11% 28% 26% 17% 4	738
June	1st to 10th	Day	12% 10% 32% 29% 13% 5%	730
	11th to 20th	Day	13% 9% 30% 30% 14% 4	730
	21st to end	Day	12% 11% 30% 30% 13% 4	730
July	1st to 10th	Day	8% 5% 25% 31% 22% 9%	721
	11th to 20th	Day	11% 6% 28% 28% 20% 8%	718
	21st to end	Day	8% 4 26% 32% 23% 7%	784
August	1st to 10th	Day	10% 4 19% 28% 24% 15%	526
	11th to 20th	Day	11% 4 21% 29% 27% 8%	519
	21st to end	Day	10% 5% 21% 30% 23% 10%	583
September	1st to 10th	Day	14% 7% 23% 25% 23% 9%	530
	11th to 20th	Day	8% 4 20% 25% 30% 13%	522
	21st to end	Day	16% 14% 22% 24% 17% 7%	490
October	1st to 10th	Day	16% 10% 28% 26% 15% 5%	490
	11th to 20th	Day	23% 17% 27% 26% 7%	414
	21st to end	Day	28% 17% 36% 15% 4	330
November	1st to 10th	Day	18% 18% 33% 23% 6% 1	300
	11th to 20th	Day	25% 26% 25% 17% 6%	300
	21st to end	Day	28% 18% 37% 13% 3	300
December	1st to 10th	Day	23% 30% 26% 14% 7%	300
	11th to 20th	Day	21% 24% 37% 15% 3	300
	21st to end	Day	25% 27% 30% 15% 3	330
January	1st to 10th	Night	22% 17% 34% 22% 5%	1000
	11th to 20th	Night	24% 15% 36% 22% 3 1	1000
	21st to end	Night	21% 13% 35% 27% 3	1090
February	1st to 10th	Night	25% 14% 37% 21% 4	800
	11th to 20th	Night	20% 16% 34% 26% 4	783
	21st to end	Night	24% 15% 38% 20% 3	660
March	1st to 10th	Night	21% 14% 33% 25% 6% 1	800
	11th to 20th	Night	18% 13% 40% 24% 6%	800
	21st to end	Night	23% 13% 31% 27% 6% 1	820
April	1st to 10th	Night	18% 15% 34% 28% 5% 1	600
	11th to 20th	Night	24% 11% 34% 25% 5% 1	593
	21st to end	Night	21% 10% 36% 27% 7%	600
May	1st to 10th	Night	19% 9% 34% 30% 8% 1	600
	11th to 20th	Night	21% 8% 28% 31% 12%	591
	21st to end	Night	14% 5% 30% 34% 16% 1	650
June	1st to 10th	Night	11% 5% 26% 44% 14% 1	600
	11th to 20th	Night	13% 4 26% 42% 15%	600
	21st to end	Night	10% 3 25% 40% 21% 2	600
July	1st to 10th	Night	9% 2 19% 40% 26% 4	585
	11th to 20th	Night	8% 4 18% 41% 24% 4	582
	21st to end	Night	8% 3 16% 45% 27% 1	657
August	1st to 10th	Night	9% 2 12% 47% 27% 3	597
	11th to 20th	Night	10% 2 16% 41% 27% 4	594
	21st to end	Night	9% 3 17% 37% 30% 4	660
September	1st to 10th	Night	10% 5% 19% 37% 25% 4	620
	11th to 20th	Night	10% 3 17% 36% 29% 4	793
	21st to end	Night	16% 8% 24% 29% 18% 4	760
October	1st to 10th	Night	17% 7% 24% 33% 18% 1	760
	11th to 20th	Night	22% 12% 29% 28% 8% 1	760
	21st to end	Night	28% 13% 31% 22% 6%	836
November	1st to 10th	Night	22% 12% 31% 26% 9%	780
	11th to 20th	Night	23% 17% 32% 23% 4 1	800
	21st to end	Night	24% 12% 36% 23% 4	800
December	1st to 10th	Night	26% 16% 34% 19% 4	890
	11th to 20th	Night	26% 14% 38% 20% 3	1000
	21st to end	Night	31% 16% 33% 18% 2	1100

Page generated at 2025-09-16 01:49:10. Version ac07

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