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 2024-01-12.

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	14% 10% 4 8% 6% 3 5% 5% 5% 7% 33%	74846
	Night	15% 11% 5% 8% 6% 3 4 4 5% 7% 33%	74890
Transparency	Day	51% 4 11% 21% 12% 1	74549
	Night	52% 4 11% 20% 12% 1	74708
Seeing	Day	18% 13% 28% 24% 13% 4	14586
	Night	19% 10% 29% 29% 11% 1	24553

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	16% 14% 8% 13% 10% 5% 5% 5% 5% 5% 16%	1517
	11th to 20th	Day	15% 18% 9% 12% 10% 4 5% 4 4 5% 15%	1448
	21st to end	Day	14% 19% 10% 14% 9% 4 4 5% 4 5% 12%	1592
February	1st to 10th	Day	15% 14% 7% 13% 13% 5% 5% 4 4 4 16%	1696
	11th to 20th	Day	15% 11% 7% 14% 10% 4 6% 4 4 5% 20%	1748
	21st to end	Day	16% 14% 9% 12% 6% 4 4 4 4 6% 20%	1480
March	1st to 10th	Day	12% 12% 5% 9% 8% 4 5% 5% 5% 8% 26%	1800
	11th to 20th	Day	13% 10% 6% 9% 6% 4 5% 5% 4 7% 33%	1928
	21st to end	Day	19% 12% 5% 8% 7% 3 4 4 5% 6% 27%	2246
April	1st to 10th	Day	17% 13% 5% 6% 6% 3 5% 5% 4 7% 29%	2160
	11th to 20th	Day	16% 11% 3 5% 5% 3 4 5% 5% 6% 36%	2177
	21st to end	Day	18% 9% 4 6% 5% 2 5% 4 5% 7% 34%	2340
May	1st to 10th	Day	17% 8% 3 7% 6% 3 5% 5% 6% 8% 31%	2340
	11th to 20th	Day	18% 8% 3 6% 5% 3 4 4 5% 6% 37%	2301
	21st to end	Day	12% 6% 3 5% 5% 3 5% 5% 5% 9% 41%	2678
June	1st to 10th	Day	10% 5% 3 6% 6% 3 5% 5% 6% 8% 44%	2520
	11th to 20th	Day	11% 5% 2 5% 5% 3 5% 5% 6% 9% 44%	2520
	21st to end	Day	10% 6% 3 5% 6% 3 5% 6% 7% 9% 40%	2520
July	1st to 10th	Day	7% 4 2 5% 5% 3 5% 5% 6% 10% 48%	2520
	11th to 20th	Day	9% 4 3 4 5% 3 4 5% 6% 10% 46%	2520
	21st to end	Day	7% 4 2 5% 5% 3 5% 6% 7% 10% 45%	2735
August	1st to 10th	Day	8% 5% 2 5% 5% 4 6% 7% 7% 10% 42%	2327
	11th to 20th	Day	10% 5% 2 5% 7% 3 6% 5% 6% 8% 43%	2303
	21st to end	Day	10% 4 3 6% 6% 3 5% 5% 5% 8% 46%	2394
September	1st to 10th	Day	11% 6% 3 6% 6% 3 5% 5% 5% 6% 44%	2124
	11th to 20th	Day	12% 5% 2 4 5% 3 4 5% 5% 9% 46%	1980
	21st to end	Day	11% 7% 3 7% 6% 3 5% 4 5% 10% 40%	1980
October	1st to 10th	Day	15% 7% 3 6% 6% 3 5% 5% 5% 6% 41%	1980
	11th to 20th	Day	20% 9% 3 8% 6% 4 5% 6% 5% 7% 28%	1777
	21st to end	Day	22% 13% 4 6% 7% 3 6% 3 5% 6% 25%	1782
November	1st to 10th	Day	13% 9% 3 7% 5% 4 4 4 6% 6% 38%	1672
	11th to 20th	Day	16% 16% 6% 9% 7% 4 4 5% 5% 5% 24%	1710
	21st to end	Day	20% 16% 7% 8% 6% 4 3 4 4 6% 21%	1710
December	1st to 10th	Day	15% 19% 10% 11% 7% 2 5% 3 5% 4 19%	1620
	11th to 20th	Day	13% 18% 10% 14% 7% 3 5% 4 5% 5% 16%	1918
	21st to end	Day	21% 23% 7% 12% 8% 3 3 3 4 6% 10%	2783
January	1st to 10th	Night	16% 15% 8% 12% 9% 4 5% 4 4 5% 18%	2648
	11th to 20th	Night	17% 15% 9% 11% 10% 4 5% 4 4 5% 17%	2541
	21st to end	Night	14% 15% 9% 15% 9% 5% 5% 4 4 5% 14%	2763
February	1st to 10th	Night	15% 17% 6% 11% 10% 4 5% 4 4 6% 17%	2263
	11th to 20th	Night	14% 13% 7% 11% 8% 5% 4 4 4 6% 23%	2098
	21st to end	Night	19% 11% 6% 10% 6% 3 5% 4 5% 5% 26%	1776
March	1st to 10th	Night	16% 10% 5% 8% 6% 3 4 4 4 7% 32%	2160
	11th to 20th	Night	13% 8% 3 5% 5% 4 5% 5% 5% 6% 39%	2031
	21st to end	Night	21% 8% 5% 7% 6% 3 4 4 4 7% 32%	2124
April	1st to 10th	Night	14% 9% 4 6% 5% 4 5% 4 6% 7% 35%	1800
	11th to 20th	Night	19% 8% 4 5% 6% 3 4 5% 5% 5% 37%	1732
	21st to end	Night	18% 7% 4 6% 5% 2 3 4 6% 7% 38%	1620
May	1st to 10th	Night	16% 10% 4 7% 6% 3 5% 4 4 8% 34%	1620
	11th to 20th	Night	20% 7% 2 5% 4 3 3 4 4 8% 40%	1592
	21st to end	Night	12% 6% 3 5% 6% 3 4 4 7% 10% 41%	1595
June	1st to 10th	Night	10% 4 2 4 5% 2 4 4 7% 10% 48%	1440
	11th to 20th	Night	10% 5% 2 5% 5% 2 5% 5% 7% 9% 45%	1440
	21st to end	Night	9% 4 2 5% 5% 3 5% 5% 5% 11% 44%	1440
July	1st to 10th	Night	6% 4 2 4 5% 3 4 5% 6% 9% 52%	1440
	11th to 20th	Night	8% 4 2 5% 5% 2 3 5% 6% 9% 52%	1440
	21st to end	Night	6% 5% 2 4 4 3 5% 5% 7% 9% 50%	1583
August	1st to 10th	Night	8% 3 2 4 4 2 5% 6% 6% 10% 49%	1611
	11th to 20th	Night	9% 4 3 5% 5% 4 4 5% 6% 8% 48%	1598
	21st to end	Night	9% 5% 2 5% 5% 2 4 5% 5% 6% 52%	1959
September	1st to 10th	Night	10% 6% 2 5% 4 2 4 5% 6% 8% 48%	1818
	11th to 20th	Night	10% 5% 3 5% 3 2 3 4 4 7% 52%	1980
	21st to end	Night	14% 7% 3 5% 5% 2 4 5% 4 6% 44%	1980
October	1st to 10th	Night	16% 6% 2 6% 4 2 3 4 5% 6% 45%	1980
	11th to 20th	Night	18% 10% 4 6% 6% 4 4 3 4 6% 34%	2180
	21st to end	Night	24% 10% 3 7% 5% 3 5% 3 4 6% 31%	2574
November	1st to 10th	Night	14% 10% 4 7% 5% 3 4 4 4 6% 39%	2406
	11th to 20th	Night	16% 14% 5% 10% 6% 3 5% 4 5% 6% 26%	2470
	21st to end	Night	19% 14% 5% 8% 7% 3 4 4 4 6% 26%	2470
December	1st to 10th	Night	17% 18% 9% 12% 7% 3 4 4 4 5% 17%	2555
	11th to 20th	Night	15% 18% 10% 13% 7% 4 4 4 4 4 18%	3279
	21st to end	Night	26% 20% 7% 10% 7% 3 4 4 4 5% 11%	4884

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	71% 4 10% 13% 3	1518
	11th to 20th	Day	73% 4 11% 8% 3	1448
	21st to end	Day	74% 3 9% 10% 3	1592
February	1st to 10th	Day	72% 2 9% 12% 4 1	1696
	11th to 20th	Day	66% 4 8% 15% 7% 1	1758
	21st to end	Day	66% 3 11% 14% 5% 1	1480
March	1st to 10th	Day	57% 4 11% 20% 8% 1	1800
	11th to 20th	Day	53% 3 10% 20% 12% 2	1928
	21st to end	Day	59% 4 10% 20% 6% 1	2246
April	1st to 10th	Day	55% 3 9% 21% 11%	2160
	11th to 20th	Day	50% 3 9% 22% 13% 2	2177
	21st to end	Day	50% 3 12% 18% 15% 2	2340
May	1st to 10th	Day	52% 4 12% 23% 10% 1	2340
	11th to 20th	Day	48% 3 10% 22% 14% 1	2305
	21st to end	Day	40% 5% 13% 26% 14% 2	2678
June	1st to 10th	Day	37% 4 12% 28% 17% 2	2520
	11th to 20th	Day	37% 4 12% 26% 19% 2	2520
	21st to end	Day	40% 5% 15% 22% 16% 2	2520
July	1st to 10th	Day	33% 4 11% 28% 19% 5%	2451
	11th to 20th	Day	34% 4 12% 29% 17% 3	2445
	21st to end	Day	34% 5% 13% 25% 19% 3	2583
August	1st to 10th	Day	35% 5% 16% 28% 14% 2	2314
	11th to 20th	Day	38% 5% 13% 29% 13% 2	2303
	21st to end	Day	37% 4 10% 28% 18% 4	2394
September	1st to 10th	Day	41% 3 14% 23% 16% 3	2124
	11th to 20th	Day	35% 4 10% 32% 18% 1	1980
	21st to end	Day	41% 4 10% 26% 18% 1	1980
October	1st to 10th	Day	44% 4 9% 27% 15% 2	1980
	11th to 20th	Day	54% 3 10% 22% 10%	1777
	21st to end	Day	61% 3 10% 18% 7% 1	1782
November	1st to 10th	Day	47% 3 12% 24% 13% 1	1672
	11th to 20th	Day	62% 4 9% 18% 6% 1	1710
	21st to end	Day	66% 3 12% 15% 5%	1710
December	1st to 10th	Day	68% 2 12% 14% 4	1620
	11th to 20th	Day	71% 3 12% 13% 2	1915
	21st to end	Day	78% 3 8% 9% 3	2783
January	1st to 10th	Night	68% 3 9% 15% 4	2648
	11th to 20th	Night	71% 4 11% 10% 5%	2541
	21st to end	Night	72% 3 11% 9% 4 1	2763
February	1st to 10th	Night	69% 3 9% 12% 5% 1	2263
	11th to 20th	Night	63% 3 8% 18% 7% 1	2099
	21st to end	Night	60% 3 12% 17% 7% 2	1776
March	1st to 10th	Night	54% 2 12% 22% 9% 1	2160
	11th to 20th	Night	45% 4 13% 21% 15% 2	2031
	21st to end	Night	54% 3 10% 21% 10% 1	2124
April	1st to 10th	Night	47% 5% 12% 23% 13% 1	1800
	11th to 20th	Night	48% 3 8% 19% 18% 3	1732
	21st to end	Night	46% 4 10% 21% 16% 3	1620
May	1st to 10th	Night	51% 3 11% 22% 11% 2	1620
	11th to 20th	Night	45% 4 10% 21% 17% 2	1593
	21st to end	Night	39% 5% 15% 25% 15% 2	1596
June	1st to 10th	Night	31% 5% 12% 31% 19% 3	1440
	11th to 20th	Night	35% 5% 13% 26% 19% 2	1440
	21st to end	Night	34% 5% 16% 25% 16% 3	1440
July	1st to 10th	Night	29% 5% 15% 27% 21% 4	1400
	11th to 20th	Night	29% 5% 14% 27% 22% 3	1392
	21st to end	Night	29% 5% 15% 26% 21% 4	1495
August	1st to 10th	Night	29% 5% 14% 31% 19% 2	1600
	11th to 20th	Night	34% 5% 14% 28% 18% 2	1598
	21st to end	Night	33% 4 9% 27% 23% 4	1959
September	1st to 10th	Night	34% 5% 12% 27% 19% 3	1818
	11th to 20th	Night	33% 3 7% 33% 22% 1	1980
	21st to end	Night	40% 4 10% 29% 17% 1	1980
October	1st to 10th	Night	40% 4 8% 28% 18% 2	1980
	11th to 20th	Night	52% 3 9% 23% 12% 1	2180
	21st to end	Night	56% 3 9% 21% 10% 1	2574
November	1st to 10th	Night	48% 4 9% 23% 15% 1	2406
	11th to 20th	Night	58% 3 12% 18% 7% 1	2470
	21st to end	Night	60% 4 12% 19% 4	2470
December	1st to 10th	Night	70% 3 9% 12% 5%	2555
	11th to 20th	Night	70% 3 10% 12% 4 1	3281
	21st to end	Night	77% 2 10% 8% 2	4884

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	23% 18% 33% 20% 6% 1	270
	11th to 20th	Day	22% 16% 43% 12% 6% 1	243
	21st to end	Day	22% 19% 35% 20% 3	264
February	1st to 10th	Day	26% 21% 33% 18% 2	240
	11th to 20th	Day	28% 21% 34% 15% 2	240
	21st to end	Day	25% 32% 33% 9% 1 1	198
March	1st to 10th	Day	20% 28% 35% 14% 4	240
	11th to 20th	Day	19% 18% 33% 22% 7% 1	368
	21st to end	Day	28% 18% 29% 18% 6% 1	462
April	1st to 10th	Day	26% 13% 33% 22% 5% 1	420
	11th to 20th	Day	25% 21% 27% 16% 9% 2	412
	21st to end	Day	25% 13% 33% 18% 10% 1	420
May	1st to 10th	Day	23% 17% 28% 21% 9% 3	423
	11th to 20th	Day	22% 12% 27% 23% 11% 5%	447
	21st to end	Day	13% 9% 27% 27% 19% 5%	632
June	1st to 10th	Day	12% 7% 32% 30% 14% 5%	630
	11th to 20th	Day	14% 9% 27% 30% 16% 4	630
	21st to end	Day	13% 10% 30% 30% 13% 5%	630
July	1st to 10th	Day	7% 4 24% 31% 24% 10%	621
	11th to 20th	Day	11% 5% 26% 28% 21% 7%	618
	21st to end	Day	8% 4 26% 33% 22% 7%	676
August	1st to 10th	Day	10% 4 18% 28% 23% 17%	446
	11th to 20th	Day	12% 4 21% 27% 27% 9%	439
	21st to end	Day	12% 5% 21% 30% 23% 9%	495
September	1st to 10th	Day	15% 6% 22% 26% 22% 9%	450
	11th to 20th	Day	8% 4 22% 26% 29% 10%	450
	21st to end	Day	15% 13% 22% 26% 18% 7%	450
October	1st to 10th	Day	17% 10% 28% 24% 16% 5%	450
	11th to 20th	Day	25% 17% 27% 24% 7%	378
	21st to end	Day	30% 16% 36% 13% 4	297
November	1st to 10th	Day	17% 19% 33% 24% 7% 1	270
	11th to 20th	Day	25% 24% 26% 19% 6%	270
	21st to end	Day	29% 16% 37% 15% 3	270
December	1st to 10th	Day	23% 31% 26% 13% 7%	270
	11th to 20th	Day	20% 23% 39% 15% 3	270
	21st to end	Day	23% 28% 33% 15% 2	297
January	1st to 10th	Night	22% 16% 35% 21% 6%	950
	11th to 20th	Night	24% 14% 35% 23% 3 1	908
	21st to end	Night	20% 12% 36% 28% 4	981
February	1st to 10th	Night	26% 13% 37% 20% 4	720
	11th to 20th	Night	20% 16% 34% 26% 4	703
	21st to end	Night	25% 15% 36% 20% 4	592
March	1st to 10th	Night	21% 14% 32% 26% 7% 1	720
	11th to 20th	Night	18% 11% 39% 25% 7%	720
	21st to end	Night	23% 12% 31% 27% 6% 1	738
April	1st to 10th	Night	17% 14% 35% 27% 6% 1	540
	11th to 20th	Night	23% 10% 34% 26% 5% 1	533
	21st to end	Night	21% 8% 35% 28% 7%	540
May	1st to 10th	Night	20% 9% 34% 29% 8% 1	540
	11th to 20th	Night	22% 7% 27% 31% 12%	531
	21st to end	Night	14% 4 29% 34% 17% 1	584
June	1st to 10th	Night	11% 4 26% 45% 15%	540
	11th to 20th	Night	12% 4 25% 41% 16%	540
	21st to end	Night	11% 3 24% 40% 21% 2	540
July	1st to 10th	Night	8% 2 18% 39% 28% 4	525
	11th to 20th	Night	8% 4 18% 41% 25% 4	522
	21st to end	Night	7% 3 16% 46% 27% 1	591
August	1st to 10th	Night	9% 2 11% 47% 28% 4	537
	11th to 20th	Night	10% 1 16% 40% 29% 4	534
	21st to end	Night	10% 4 17% 37% 29% 4	594
September	1st to 10th	Night	11% 5% 18% 38% 24% 4	558
	11th to 20th	Night	12% 3 18% 35% 28% 4	720
	21st to end	Night	15% 9% 24% 29% 19% 4	720
October	1st to 10th	Night	18% 7% 23% 33% 19% 1	720
	11th to 20th	Night	23% 12% 29% 28% 7% 1	720
	21st to end	Night	29% 13% 31% 21% 6%	792
November	1st to 10th	Night	21% 12% 31% 26% 10%	740
	11th to 20th	Night	23% 17% 32% 23% 4 1	760
	21st to end	Night	24% 11% 36% 24% 4	760
December	1st to 10th	Night	26% 16% 34% 19% 4	845
	11th to 20th	Night	25% 13% 38% 21% 3	950
	21st to end	Night	31% 16% 33% 18% 1	1045

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