Portage
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Portage clear sky chart from 2005-11-06 to 2026-10-29.

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% 8% 3 7% 5% 4 5% 4 5% 7% 39%	150063
	Night	13% 9% 4 8% 5% 4 4 3 5% 6% 39%	142359
Transparency	Day	44% 4 12% 25% 13% 1	153532
	Night	45% 4 11% 23% 14% 2	146966
Seeing	Day	17% 11% 30% 25% 13% 4	18693
	Night	17% 10% 31% 30% 11% 1	28472

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	10% 12% 6% 13% 7% 7% 5% 3 4 5% 26%	4691
	11th to 20th	Day	6% 13% 6% 11% 4 4 7% 6% 7% 6% 29%	4710
	21st to end	Day	10% 12% 6% 8% 5% 5% 5% 3 3 5% 36%	5373
February	1st to 10th	Day	11% 13% 5% 10% 5% 5% 5% 4 5% 6% 32%	1890
	11th to 20th	Day	11% 13% 4 7% 6% 5% 5% 3 5% 6% 36%	1962
	21st to end	Day	12% 14% 4 11% 7% 5% 5% 3 5% 5% 30%	1730
March	1st to 10th	Day	15% 7% 2 5% 4 3 3 3 4 7% 47%	4580
	11th to 20th	Day	12% 6% 3 7% 5% 5% 4 3 5% 7% 43%	4920
	21st to end	Day	25% 11% 4 8% 4 3 4 3 4 5% 30%	6023
April	1st to 10th	Day	18% 13% 4 7% 5% 4 4 3 6% 6% 31%	2520
	11th to 20th	Day	18% 10% 3 7% 5% 4 4 3 6% 6% 33%	2494
	21st to end	Day	19% 10% 3 6% 5% 4 4 4 6% 6% 33%	2599
May	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 4 6% 7% 42%	7951
	11th to 20th	Day	14% 5% 2 7% 5% 5% 5% 3 5% 7% 43%	8222
	21st to end	Day	15% 6% 2 6% 4 4 4 4 6% 7% 40%	9047
June	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 4 6% 8% 44%	2940
	11th to 20th	Day	12% 6% 3 6% 5% 5% 5% 4 5% 8% 41%	2940
	21st to end	Day	13% 5% 2 6% 4 4 5% 5% 7% 9% 40%	2940
July	1st to 10th	Day	10% 5% 2 7% 5% 5% 6% 5% 7% 10% 38%	8280
	11th to 20th	Day	6% 4 1 5% 4 4 4 5% 5% 10% 52%	8280
	21st to end	Day	10% 5% 2 6% 5% 4 5% 4 6% 8% 44%	8990
August	1st to 10th	Day	11% 6% 2 7% 5% 5% 4 4 7% 8% 41%	6421
	11th to 20th	Day	15% 6% 2 5% 4 4 4 4 6% 9% 43%	5651
	21st to end	Day	11% 5% 2 6% 4 4 4 4 6% 9% 45%	6080
September	1st to 10th	Day	12% 5% 2 6% 4 5% 4 4 6% 6% 45%	2520
	11th to 20th	Day	13% 5% 2 6% 3 4 4 4 5% 8% 46%	2457
	21st to end	Day	13% 7% 3 7% 5% 4 4 3 5% 7% 44%	2310
October	1st to 10th	Day	13% 6% 2 6% 4 4 4 2 4 7% 48%	2134
	11th to 20th	Day	19% 9% 2 7% 5% 3 5% 3 6% 7% 35%	2100
	21st to end	Day	16% 10% 3 7% 4 3 3 3 5% 6% 39%	2080
November	1st to 10th	Day	12% 9% 3 6% 4 4 4 3 5% 6% 43%	1852
	11th to 20th	Day	13% 13% 4 9% 5% 5% 4 3 5% 6% 32%	1848
	21st to end	Day	14% 13% 5% 9% 4 5% 3 3 4 6% 34%	1680
December	1st to 10th	Day	8% 10% 6% 10% 6% 6% 4 5% 5% 7% 33%	2690
	11th to 20th	Day	13% 16% 5% 11% 6% 5% 5% 3 5% 6% 27%	3088
	21st to end	Day	20% 20% 8% 13% 6% 4 4 3 3 4 15%	4070
January	1st to 10th	Night	11% 13% 7% 12% 6% 4 4 3 4 5% 33%	8232
	11th to 20th	Night	9% 14% 6% 13% 6% 6% 7% 5% 5% 5% 23%	8280
	21st to end	Night	9% 13% 6% 10% 5% 5% 5% 4 5% 5% 32%	8837
February	1st to 10th	Night	10% 15% 5% 9% 5% 5% 4 3 5% 5% 33%	2730
	11th to 20th	Night	10% 10% 4 9% 5% 4 4 3 4 6% 40%	2545
	21st to end	Night	14% 14% 4 7% 4 4 3 3 5% 5% 36%	2076
March	1st to 10th	Night	15% 8% 3 7% 4 4 4 3 5% 5% 43%	5493
	11th to 20th	Night	11% 8% 2 5% 4 4 4 4 6% 8% 45%	5291
	21st to end	Night	22% 11% 4 8% 5% 4 4 3 4 5% 30%	5148
April	1st to 10th	Night	20% 10% 2 6% 5% 3 5% 3 4 5% 36%	2100
	11th to 20th	Night	21% 8% 2 6% 5% 4 3 2 6% 5% 38%	2075
	21st to end	Night	18% 8% 3 6% 4 3 4 3 5% 7% 38%	2019
May	1st to 10th	Night	14% 6% 2 5% 4 3 4 4 5% 8% 45%	4902
	11th to 20th	Night	11% 5% 2 6% 4 4 5% 4 6% 7% 46%	4670
	21st to end	Night	16% 7% 3 7% 4 5% 5% 4 5% 7% 37%	5139
June	1st to 10th	Night	9% 5% 2 5% 5% 4 4 3 6% 8% 47%	1680
	11th to 20th	Night	11% 6% 2 5% 4 4 4 4 6% 9% 45%	1680
	21st to end	Night	10% 5% 2 6% 5% 4 4 4 5% 9% 47%	1680
July	1st to 10th	Night	14% 6% 2 7% 3 4 4 3 6% 9% 42%	4708
	11th to 20th	Night	6% 3 1 4 4 4 4 3 6% 8% 57%	4707
	21st to end	Night	7% 4 2 6% 4 4 5% 4 6% 8% 51%	5117
August	1st to 10th	Night	14% 5% 2 6% 4 4 4 3 5% 7% 46%	3732
	11th to 20th	Night	12% 6% 2 6% 4 4 4 3 5% 8% 45%	4400
	21st to end	Night	9% 4 2 5% 4 4 4 4 6% 8% 49%	5046
September	1st to 10th	Night	10% 4 2 5% 4 3 4 3 5% 8% 52%	2100
	11th to 20th	Night	13% 4 2 5% 3 3 5% 3 5% 6% 51%	2142
	21st to end	Night	11% 5% 2 6% 5% 4 4 3 5% 5% 50%	2310
October	1st to 10th	Night	12% 6% 2 5% 3 4 4 3 5% 7% 50%	2483
	11th to 20th	Night	18% 6% 2 6% 4 4 4 3 5% 6% 42%	2520
	21st to end	Night	17% 9% 3 6% 4 4 5% 4 6% 7% 36%	2935
November	1st to 10th	Night	13% 9% 3 7% 4 4 4 4 5% 5% 44%	2666
	11th to 20th	Night	14% 11% 4 9% 6% 4 4 4 5% 6% 34%	2767
	21st to end	Night	16% 11% 3 9% 4 4 4 3 5% 5% 36%	2940
December	1st to 10th	Night	8% 9% 4 8% 5% 5% 5% 3 6% 7% 38%	4720
	11th to 20th	Night	14% 17% 6% 11% 5% 5% 4 3 4 4 26%	5339
	21st to end	Night	21% 19% 7% 11% 5% 4 4 3 4 4 19%	7150

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	61% 4 14% 15% 6%	4692
	11th to 20th	Day	54% 4 10% 18% 15% 1	4710
	21st to end	Day	52% 3 10% 23% 12%	5373
February	1st to 10th	Day	53% 6% 13% 17% 10% 1	1890
	11th to 20th	Day	49% 4 12% 23% 10% 2	1971
	21st to end	Day	56% 5% 10% 17% 9% 2	1730
March	1st to 10th	Day	40% 4 12% 28% 13% 4	4580
	11th to 20th	Day	40% 4 14% 28% 13% 1	4920
	21st to end	Day	58% 4 10% 16% 11% 2	6025
April	1st to 10th	Day	54% 5% 10% 20% 10%	2520
	11th to 20th	Day	51% 5% 10% 20% 11% 2	2494
	21st to end	Day	50% 7% 12% 20% 11% 2	2599
May	1st to 10th	Day	40% 5% 8% 28% 17% 2	7986
	11th to 20th	Day	42% 5% 8% 27% 18%	8226
	21st to end	Day	43% 5% 9% 24% 16% 3	9061
June	1st to 10th	Day	37% 7% 13% 27% 14% 2	2940
	11th to 20th	Day	41% 6% 12% 25% 16% 1	2940
	21st to end	Day	38% 8% 16% 24% 13% 1	2940
July	1st to 10th	Day	41% 3 16% 29% 9% 1	8211
	11th to 20th	Day	29% 3 12% 34% 21% 1	8205
	21st to end	Day	39% 4 11% 26% 20% 1	8794
August	1st to 10th	Day	40% 4 15% 27% 12% 1	6406
	11th to 20th	Day	40% 3 17% 31% 9% 1	5689
	21st to end	Day	36% 4 14% 34% 12% 1	6079
September	1st to 10th	Day	38% 6% 12% 26% 17% 1	2520
	11th to 20th	Day	36% 5% 15% 29% 13% 1	2457
	21st to end	Day	41% 4 10% 30% 13% 1	2310
October	1st to 10th	Day	24% 4 6% 41% 24% 1	3407
	11th to 20th	Day	39% 4 8% 30% 18% 2	3340
	21st to end	Day	43% 3 11% 28% 14% 1	3292
November	1st to 10th	Day	42% 4 14% 23% 15% 2	1852
	11th to 20th	Day	53% 5% 13% 20% 9% 1	1848
	21st to end	Day	52% 5% 15% 21% 6% 1	1680
December	1st to 10th	Day	51% 5% 18% 19% 6% 1	2690
	11th to 20th	Day	58% 4 11% 18% 7% 2	3085
	21st to end	Day	73% 4 7% 10% 6%	4070
January	1st to 10th	Night	55% 3 11% 22% 8%	8233
	11th to 20th	Night	63% 4 8% 12% 13% 1	8280
	21st to end	Night	54% 3 10% 20% 13%	8837
February	1st to 10th	Night	52% 5% 11% 20% 10% 2	2730
	11th to 20th	Night	46% 5% 10% 25% 12% 1	2546
	21st to end	Night	50% 4 12% 21% 11% 2	2076
March	1st to 10th	Night	44% 4 12% 24% 10% 6%	5493
	11th to 20th	Night	37% 4 13% 29% 13% 4	5291
	21st to end	Night	57% 4 9% 17% 11% 1	5148
April	1st to 10th	Night	50% 5% 9% 20% 15% 1	2100
	11th to 20th	Night	48% 4 8% 21% 16% 3	2075
	21st to end	Night	43% 8% 12% 23% 13% 2	2019
May	1st to 10th	Night	38% 5% 10% 24% 22% 1	4929
	11th to 20th	Night	36% 6% 14% 22% 20% 1	4671
	21st to end	Night	46% 5% 7% 22% 15% 4	5149
June	1st to 10th	Night	33% 7% 13% 26% 18% 1	1680
	11th to 20th	Night	35% 7% 11% 25% 21% 2	1680
	21st to end	Night	35% 6% 16% 25% 16% 3	1680
July	1st to 10th	Night	41% 3 14% 29% 12% 2	4670
	11th to 20th	Night	27% 3 12% 33% 24% 1	4662
	21st to end	Night	31% 4 14% 29% 20% 2	5002
August	1st to 10th	Night	39% 4 13% 25% 19% 1	3723
	11th to 20th	Night	40% 4 16% 28% 12%	4428
	21st to end	Night	34% 3 14% 31% 17% 2	5046
September	1st to 10th	Night	31% 7% 11% 28% 21% 1	2100
	11th to 20th	Night	35% 4 12% 33% 16% 1	2142
	21st to end	Night	35% 6% 10% 29% 19% 1	2310
October	1st to 10th	Night	27% 3 5% 36% 27% 2	3990
	11th to 20th	Night	33% 4 10% 30% 21% 3	4019
	21st to end	Night	45% 4 9% 26% 16% 1	4673
November	1st to 10th	Night	42% 5% 11% 25% 15% 2	2666
	11th to 20th	Night	51% 5% 12% 19% 10% 2	2767
	21st to end	Night	50% 5% 13% 23% 8% 1	2940
December	1st to 10th	Night	45% 4 17% 25% 7% 2	4720
	11th to 20th	Night	62% 4 9% 15% 9% 1	5341
	21st to end	Night	70% 4 8% 13% 6%	7150

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	18% 13% 37% 22% 8% 3	330
	11th to 20th	Day	17% 21% 35% 21% 6%	330
	21st to end	Day	21% 14% 35% 24% 5% 1	363
February	1st to 10th	Day	14% 17% 44% 20% 3 1	330
	11th to 20th	Day	19% 21% 36% 18% 5%	330
	21st to end	Day	20% 18% 44% 17% 1	273
March	1st to 10th	Day	20% 17% 38% 19% 5%	330
	11th to 20th	Day	24% 16% 37% 14% 6% 3	330
	21st to end	Day	27% 13% 31% 21% 7% 1	557
April	1st to 10th	Day	26% 15% 35% 19% 4	540
	11th to 20th	Day	27% 18% 25% 18% 10% 2	532
	21st to end	Day	22% 14% 29% 22% 9% 2	540
May	1st to 10th	Day	20% 13% 33% 23% 10% 1	658
	11th to 20th	Day	19% 13% 30% 23% 12% 3	775
	21st to end	Day	19% 7% 29% 27% 12% 6%	844
June	1st to 10th	Day	11% 9% 27% 34% 17% 3	780
	11th to 20th	Day	15% 9% 33% 24% 17% 3	780
	21st to end	Day	13% 9% 29% 29% 16% 3	780
July	1st to 10th	Day	8% 4 20% 32% 25% 10%	771
	11th to 20th	Day	9% 6% 25% 32% 21% 7%	768
	21st to end	Day	9% 4 25% 35% 21% 6%	849
August	1st to 10th	Day	9% 5% 20% 32% 26% 9%	776
	11th to 20th	Day	15% 5% 17% 29% 22% 12%	636
	21st to end	Day	12% 6% 23% 29% 21% 9%	627
September	1st to 10th	Day	16% 7% 24% 29% 18% 7%	570
	11th to 20th	Day	14% 5% 21% 28% 22% 9%	570
	21st to end	Day	17% 8% 25% 26% 17% 7%	570
October	1st to 10th	Day	17% 11% 29% 26% 14% 3	394
	11th to 20th	Day	25% 13% 34% 19% 8% 1	360
	21st to end	Day	22% 13% 36% 21% 7% 1	387
November	1st to 10th	Day	16% 18% 37% 21% 7% 2	330
	11th to 20th	Day	18% 19% 37% 18% 8%	330
	21st to end	Day	24% 14% 40% 16% 5%	330
December	1st to 10th	Day	17% 18% 42% 17% 5% 1	330
	11th to 20th	Day	21% 17% 36% 18% 7% 1	330
	21st to end	Day	26% 16% 38% 16% 4 1	363
January	1st to 10th	Night	16% 15% 37% 24% 7%	1050
	11th to 20th	Night	19% 14% 37% 24% 5% 1	1050
	21st to end	Night	20% 12% 36% 26% 5% 1	1155
February	1st to 10th	Night	18% 12% 40% 24% 7%	1050
	11th to 20th	Night	17% 14% 36% 26% 6%	910
	21st to end	Night	22% 13% 39% 22% 4	692
March	1st to 10th	Night	22% 12% 34% 24% 8% 1	840
	11th to 20th	Night	18% 13% 35% 27% 7%	727
	21st to end	Night	22% 11% 31% 29% 7% 1	693
April	1st to 10th	Night	23% 13% 37% 23% 4	630
	11th to 20th	Night	22% 13% 29% 29% 6%	623
	21st to end	Night	20% 11% 33% 28% 7% 1	630
May	1st to 10th	Night	20% 10% 32% 32% 6%	630
	11th to 20th	Night	18% 8% 33% 33% 8%	621
	21st to end	Night	15% 6% 29% 34% 14% 1	683
June	1st to 10th	Night	11% 5% 27% 44% 12%	630
	11th to 20th	Night	14% 6% 31% 37% 12%	630
	21st to end	Night	11% 4 27% 41% 17%	630
July	1st to 10th	Night	10% 3 15% 40% 29% 2	615
	11th to 20th	Night	8% 3 22% 43% 22% 2	612
	21st to end	Night	7% 3 16% 49% 23% 2	690
August	1st to 10th	Night	8% 3 16% 48% 24% 2	627
	11th to 20th	Night	11% 4 15% 39% 25% 5%	624
	21st to end	Night	10% 3 19% 44% 22% 2	693
September	1st to 10th	Night	11% 4 23% 36% 20% 5%	630
	11th to 20th	Night	14% 4 22% 34% 22% 3	672
	21st to end	Night	14% 7% 25% 35% 18% 1	840
October	1st to 10th	Night	14% 8% 29% 33% 16% 1	840
	11th to 20th	Night	20% 11% 33% 27% 8%	840
	21st to end	Night	22% 10% 31% 28% 8% 1	913
November	1st to 10th	Night	17% 11% 38% 25% 9% 1	820
	11th to 20th	Night	19% 14% 39% 22% 5%	877
	21st to end	Night	22% 14% 33% 24% 6%	1050
December	1st to 10th	Night	20% 13% 40% 21% 6%	1050
	11th to 20th	Night	22% 13% 36% 23% 6% 1	1050
	21st to end	Night	27% 13% 33% 24% 3	1155

Page generated at 2026-10-26 03:37:49. Version ac07

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