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 2024-03-18.

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% 9% 4 7% 6% 3 5% 5% 5% 7% 36%	78554
	Night	14% 10% 4 7% 5% 3 4 4 5% 6% 37%	81287
Transparency	Day	47% 7% 13% 22% 11% 1	78254
	Night	47% 6% 12% 22% 12% 1	81107
Seeing	Day	16% 11% 29% 25% 14% 4	14943
	Night	17% 10% 31% 30% 11% 1	25038

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	11% 15% 9% 11% 7% 4 4 4 4 4 27%	2527
	11th to 20th	Day	10% 14% 5% 12% 7% 4 6% 10% 6% 7% 20%	2530
	21st to end	Day	17% 18% 10% 10% 5% 3 4 3 3 6% 21%	2888
February	1st to 10th	Day	11% 12% 6% 9% 5% 4 5% 5% 5% 6% 32%	1710
	11th to 20th	Day	12% 12% 4 6% 7% 4 5% 4 5% 6% 35%	1770
	21st to end	Day	13% 13% 4 10% 8% 4 5% 4 5% 5% 29%	1570
March	1st to 10th	Day	15% 10% 4 7% 5% 3 4 4 5% 7% 36%	1900
	11th to 20th	Day	13% 9% 3 7% 6% 4 5% 4 5% 7% 38%	1986
	21st to end	Day	20% 10% 4 6% 6% 4 4 4 4 6% 31%	2344
April	1st to 10th	Day	18% 12% 5% 6% 6% 3 4 4 5% 6% 31%	2160
	11th to 20th	Day	19% 10% 4 6% 6% 4 4 4 5% 6% 33%	2134
	21st to end	Day	20% 10% 3 5% 6% 3 4 5% 5% 6% 33%	2227
May	1st to 10th	Day	17% 8% 4 6% 6% 4 5% 6% 7% 7% 30%	2438
	11th to 20th	Day	18% 7% 3 6% 5% 4 5% 4 5% 7% 36%	2479
	21st to end	Day	16% 6% 2 6% 5% 3 6% 5% 6% 7% 39%	2725
June	1st to 10th	Day	10% 5% 2 5% 5% 3 5% 4 6% 8% 45%	2520
	11th to 20th	Day	12% 6% 3 5% 5% 4 5% 5% 5% 8% 42%	2520
	21st to end	Day	12% 5% 2 5% 5% 3 5% 6% 6% 9% 41%	2520
July	1st to 10th	Day	7% 3 2 4 5% 3 7% 6% 8% 10% 45%	2520
	11th to 20th	Day	8% 4 2 5% 5% 3 5% 7% 6% 10% 45%	2520
	21st to end	Day	8% 5% 2 5% 5% 3 5% 5% 6% 9% 47%	2743
August	1st to 10th	Day	7% 4 2 6% 6% 4 5% 6% 8% 9% 43%	2488
	11th to 20th	Day	12% 5% 3 5% 5% 3 4 5% 5% 9% 43%	2191
	21st to end	Day	11% 4 3 5% 5% 3 4 5% 6% 8% 46%	2376
September	1st to 10th	Day	14% 5% 2 5% 6% 3 5% 4 6% 6% 44%	2160
	11th to 20th	Day	13% 4 2 5% 4 3 4 5% 5% 8% 47%	2106
	21st to end	Day	14% 7% 3 6% 5% 3 3 4 5% 7% 44%	1980
October	1st to 10th	Day	14% 6% 2 6% 5% 3 4 4 4 7% 45%	1830
	11th to 20th	Day	21% 9% 3 6% 5% 3 4 4 6% 8% 32%	1800
	21st to end	Day	18% 11% 3 6% 5% 2 4 3 4 6% 38%	1807
November	1st to 10th	Day	13% 8% 3 5% 4 3 4 4 5% 6% 44%	1672
	11th to 20th	Day	14% 11% 5% 8% 7% 4 3 4 5% 6% 32%	1672
	21st to end	Day	14% 11% 5% 7% 4 3 4 3 4 6% 37%	1520
December	1st to 10th	Day	13% 13% 6% 8% 7% 3 4 5% 4 6% 30%	1520
	11th to 20th	Day	14% 14% 5% 8% 6% 3 5% 5% 4 6% 29%	1918
	21st to end	Day	20% 19% 9% 12% 7% 3 4 4 3 4 16%	2783
January	1st to 10th	Night	12% 14% 9% 10% 5% 3 4 3 3 4 32%	4428
	11th to 20th	Night	12% 13% 7% 12% 8% 4 5% 6% 6% 5% 21%	4440
	21st to end	Night	16% 19% 9% 10% 5% 3 4 3 3 4 23%	4775
February	1st to 10th	Night	9% 14% 6% 9% 6% 4 5% 4 5% 5% 33%	2470
	11th to 20th	Night	11% 9% 4 8% 6% 4 4 5% 4 6% 40%	2297
	21st to end	Night	16% 13% 4 6% 4 3 4 4 4 6% 36%	1884
March	1st to 10th	Night	16% 11% 4 5% 5% 3 4 4 5% 6% 38%	2280
	11th to 20th	Night	13% 8% 3 6% 5% 3 4 5% 6% 8% 40%	2156
	21st to end	Night	18% 9% 3 7% 5% 2 5% 4 5% 6% 34%	2012
April	1st to 10th	Night	20% 10% 3 5% 6% 2 5% 4 5% 6% 35%	1800
	11th to 20th	Night	22% 8% 2 5% 5% 3 4 3 5% 6% 37%	1775
	21st to end	Night	18% 8% 3 6% 4 2 4 4 5% 7% 38%	1731
May	1st to 10th	Night	19% 8% 2 5% 5% 2 5% 5% 5% 8% 35%	1518
	11th to 20th	Night	17% 6% 2 4 6% 4 4 5% 4 8% 40%	1416
	21st to end	Night	12% 5% 2 5% 5% 4 4 5% 6% 9% 42%	1551
June	1st to 10th	Night	9% 4 3 5% 5% 4 5% 5% 6% 9% 47%	1440
	11th to 20th	Night	11% 6% 2 5% 5% 3 4 4 6% 9% 46%	1440
	21st to end	Night	9% 5% 2 5% 5% 2 4 4 5% 10% 49%	1440
July	1st to 10th	Night	8% 4 1 4 3 3 3 4 6% 10% 54%	1440
	11th to 20th	Night	7% 3 1 3 5% 3 5% 4 5% 7% 55%	1440
	21st to end	Night	6% 3 1 4 4 3 4 5% 7% 10% 53%	1575
August	1st to 10th	Night	6% 3 2 5% 6% 3 6% 5% 6% 10% 49%	1448
	11th to 20th	Night	10% 5% 2 4 5% 3 3 4 4 8% 52%	1708
	21st to end	Night	10% 4 3 4 4 2 4 3 5% 7% 53%	1980
September	1st to 10th	Night	11% 4 2 5% 5% 2 4 4 5% 8% 52%	1800
	11th to 20th	Night	14% 4 2 4 3 2 4 4 5% 5% 53%	1836
	21st to end	Night	11% 5% 2 5% 5% 3 4 4 5% 6% 50%	1980
October	1st to 10th	Night	13% 6% 2 5% 4 3 4 4 5% 7% 48%	2128
	11th to 20th	Night	19% 6% 2 5% 6% 3 4 4 5% 6% 40%	2160
	21st to end	Night	19% 9% 3 5% 5% 3 4 5% 6% 7% 34%	2547
November	1st to 10th	Night	13% 8% 3 6% 5% 3 4 4 5% 5% 45%	2406
	11th to 20th	Night	14% 10% 4 8% 7% 3 4 4 5% 6% 33%	2503
	21st to end	Night	16% 10% 3 6% 5% 3 4 4 5% 5% 38%	2660
December	1st to 10th	Night	13% 13% 5% 9% 7% 3 5% 4 6% 6% 30%	2660
	11th to 20th	Night	14% 14% 7% 9% 6% 4 5% 4 4 5% 29%	3279
	21st to end	Night	22% 16% 6% 9% 7% 4 4 4 4 5% 20%	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	61% 6% 13% 14% 6%	2528
	11th to 20th	Day	62% 6% 11% 16% 5%	2530
	21st to end	Day	67% 4 10% 14% 5%	2888
February	1st to 10th	Day	52% 6% 14% 17% 9% 1	1710
	11th to 20th	Day	49% 5% 13% 22% 9% 2	1779
	21st to end	Day	57% 5% 10% 16% 10% 3	1570
March	1st to 10th	Day	49% 6% 12% 22% 9% 1	1900
	11th to 20th	Day	46% 6% 13% 22% 12% 1	1986
	21st to end	Day	55% 6% 11% 20% 7% 1	2344
April	1st to 10th	Day	54% 6% 10% 19% 10% 1	2160
	11th to 20th	Day	52% 6% 9% 20% 12% 2	2134
	21st to end	Day	50% 8% 11% 18% 11% 1	2227
May	1st to 10th	Day	49% 9% 14% 19% 8% 1	2438
	11th to 20th	Day	47% 7% 12% 22% 12% 1	2483
	21st to end	Day	43% 8% 12% 22% 13% 2	2725
June	1st to 10th	Day	36% 7% 15% 27% 14% 2	2520
	11th to 20th	Day	40% 7% 13% 24% 16% 1	2520
	21st to end	Day	37% 9% 16% 24% 13% 1	2520
July	1st to 10th	Day	31% 8% 15% 30% 13% 3	2451
	11th to 20th	Day	33% 9% 15% 25% 16% 2	2445
	21st to end	Day	31% 9% 16% 26% 15% 3	2591
August	1st to 10th	Day	33% 10% 18% 28% 10% 1	2473
	11th to 20th	Day	37% 7% 16% 27% 12% 1	2191
	21st to end	Day	34% 8% 15% 24% 17% 2	2376
September	1st to 10th	Day	39% 7% 12% 24% 18% 1	2160
	11th to 20th	Day	35% 6% 15% 29% 13% 1	2106
	21st to end	Day	41% 5% 11% 31% 12%	1980
October	1st to 10th	Day	39% 7% 11% 29% 13% 1	1830
	11th to 20th	Day	50% 7% 11% 22% 9% 1	1800
	21st to end	Day	49% 5% 12% 21% 12%	1807
November	1st to 10th	Day	41% 5% 15% 23% 15% 2	1672
	11th to 20th	Day	53% 6% 13% 18% 9% 1	1672
	21st to end	Day	49% 6% 16% 22% 7% 1	1520
December	1st to 10th	Day	55% 7% 15% 14% 8% 1	1520
	11th to 20th	Day	55% 5% 16% 14% 6% 3	1915
	21st to end	Day	72% 6% 8% 9% 5%	2783
January	1st to 10th	Night	57% 6% 10% 19% 8%	4428
	11th to 20th	Night	62% 6% 10% 14% 8%	4440
	21st to end	Night	66% 5% 9% 15% 5%	4775
February	1st to 10th	Night	52% 6% 10% 20% 11% 2	2470
	11th to 20th	Night	47% 6% 11% 24% 12% 1	2298
	21st to end	Night	50% 5% 12% 20% 11% 2	1884
March	1st to 10th	Night	48% 6% 12% 22% 10% 3	2280
	11th to 20th	Night	42% 6% 12% 23% 15% 2	2156
	21st to end	Night	51% 5% 10% 21% 11% 1	2012
April	1st to 10th	Night	50% 6% 10% 19% 14% 1	1800
	11th to 20th	Night	48% 4 9% 20% 16% 3	1775
	21st to end	Night	43% 9% 12% 21% 13% 2	1731
May	1st to 10th	Night	47% 7% 13% 18% 13% 1	1518
	11th to 20th	Night	43% 7% 14% 21% 14% 1	1417
	21st to end	Night	38% 8% 13% 24% 14% 2	1552
June	1st to 10th	Night	33% 8% 14% 27% 18% 1	1440
	11th to 20th	Night	34% 8% 11% 25% 21% 1	1440
	21st to end	Night	31% 6% 17% 26% 16% 3	1440
July	1st to 10th	Night	25% 7% 15% 33% 18% 3	1400
	11th to 20th	Night	28% 7% 13% 28% 21% 3	1392
	21st to end	Night	24% 9% 17% 29% 16% 5%	1487
August	1st to 10th	Night	29% 9% 18% 25% 17% 3	1439
	11th to 20th	Night	32% 7% 15% 29% 17% 1	1708
	21st to end	Night	30% 6% 14% 27% 20% 4	1980
September	1st to 10th	Night	31% 8% 11% 27% 22% 2	1800
	11th to 20th	Night	34% 4 12% 33% 16% 1	1836
	21st to end	Night	35% 6% 11% 29% 18% 1	1980
October	1st to 10th	Night	36% 6% 9% 30% 18% 2	2128
	11th to 20th	Night	43% 7% 13% 21% 16% 1	2160
	21st to end	Night	48% 6% 12% 22% 12% 1	2547
November	1st to 10th	Night	41% 5% 11% 25% 16% 2	2406
	11th to 20th	Night	51% 6% 13% 18% 10% 1	2503
	21st to end	Night	48% 6% 14% 23% 8% 1	2660
December	1st to 10th	Night	54% 6% 13% 19% 8% 1	2660
	11th to 20th	Night	59% 6% 10% 17% 7% 1	3281
	21st to end	Night	67% 5% 9% 14% 4	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	17% 12% 36% 22% 9% 3	270
	11th to 20th	Day	18% 19% 34% 23% 6%	270
	21st to end	Day	22% 11% 34% 27% 6% 1	297
February	1st to 10th	Day	14% 16% 44% 21% 4 1	270
	11th to 20th	Day	19% 21% 36% 17% 7%	270
	21st to end	Day	18% 18% 43% 19% 2	225
March	1st to 10th	Day	19% 16% 40% 20% 5%	270
	11th to 20th	Day	20% 17% 35% 17% 8% 3	261
	21st to end	Day	27% 11% 30% 23% 8% 1	430
April	1st to 10th	Day	24% 14% 35% 22% 5%	420
	11th to 20th	Day	25% 17% 25% 19% 11% 2	412
	21st to end	Day	21% 14% 29% 23% 10% 3	420
May	1st to 10th	Day	21% 12% 33% 23% 10% 1	521
	11th to 20th	Day	20% 11% 31% 24% 10% 3	625
	21st to end	Day	19% 8% 28% 25% 13% 7%	679
June	1st to 10th	Day	10% 8% 27% 33% 18% 3	630
	11th to 20th	Day	14% 8% 30% 26% 19% 4	630
	21st to end	Day	11% 9% 28% 31% 17% 4	630
July	1st to 10th	Day	7% 3 19% 32% 26% 12%	621
	11th to 20th	Day	9% 6% 26% 32% 20% 7%	618
	21st to end	Day	8% 4 25% 34% 22% 7%	684
August	1st to 10th	Day	7% 4 21% 32% 28% 9%	626
	11th to 20th	Day	13% 6% 15% 29% 24% 14%	505
	21st to end	Day	12% 7% 23% 29% 20% 10%	495
September	1st to 10th	Day	17% 7% 22% 28% 19% 7%	450
	11th to 20th	Day	13% 5% 22% 29% 22% 9%	450
	21st to end	Day	19% 9% 26% 24% 17% 5%	450
October	1st to 10th	Day	19% 10% 25% 27% 15% 4	300
	11th to 20th	Day	24% 16% 33% 18% 8%	270
	21st to end	Day	26% 13% 34% 20% 6% 1	297
November	1st to 10th	Day	15% 16% 37% 22% 9% 2	270
	11th to 20th	Day	16% 19% 37% 20% 7%	270
	21st to end	Day	23% 14% 39% 19% 6%	270
December	1st to 10th	Day	17% 17% 43% 16% 6% 1	270
	11th to 20th	Day	22% 15% 34% 20% 9% 1	270
	21st to end	Day	25% 17% 39% 16% 3	297
January	1st to 10th	Night	15% 15% 38% 24% 8% 1	950
	11th to 20th	Night	19% 13% 36% 25% 5% 1	950
	21st to end	Night	20% 11% 35% 27% 6% 1	1045
February	1st to 10th	Night	18% 11% 40% 24% 7%	950
	11th to 20th	Night	17% 13% 37% 26% 6%	822
	21st to end	Night	23% 13% 39% 21% 4	628
March	1st to 10th	Night	21% 12% 33% 25% 8% 1	760
	11th to 20th	Night	17% 12% 34% 29% 7%	652
	21st to end	Night	22% 10% 29% 30% 8% 1	594
April	1st to 10th	Night	21% 12% 38% 24% 4	540
	11th to 20th	Night	23% 12% 27% 31% 7%	533
	21st to end	Night	20% 11% 33% 28% 8% 1	540
May	1st to 10th	Night	22% 9% 32% 31% 6% 1	540
	11th to 20th	Night	19% 6% 33% 33% 8%	531
	21st to end	Night	14% 7% 29% 34% 15% 1	584
June	1st to 10th	Night	11% 5% 27% 44% 13% 1	540
	11th to 20th	Night	13% 4 30% 38% 14%	540
	21st to end	Night	9% 4 26% 43% 18%	540
July	1st to 10th	Night	8% 4 15% 40% 30% 3	525
	11th to 20th	Night	7% 3 23% 43% 22% 2	522
	21st to end	Night	6% 3 17% 48% 24% 2	591
August	1st to 10th	Night	7% 3 17% 48% 25% 2	537
	11th to 20th	Night	11% 4 14% 39% 26% 5%	534
	21st to end	Night	11% 3 18% 44% 22% 2	594
September	1st to 10th	Night	11% 5% 23% 37% 20% 5%	540
	11th to 20th	Night	14% 4 23% 33% 21% 4	576
	21st to end	Night	13% 8% 27% 33% 18% 1	720
October	1st to 10th	Night	15% 8% 28% 31% 18% 1	720
	11th to 20th	Night	21% 11% 32% 28% 8%	720
	21st to end	Night	23% 10% 31% 27% 8% 1	792
November	1st to 10th	Night	16% 10% 38% 25% 10% 1	740
	11th to 20th	Night	18% 15% 39% 22% 5%	793
	21st to end	Night	21% 14% 34% 24% 7%	950
December	1st to 10th	Night	20% 13% 40% 21% 6%	950
	11th to 20th	Night	22% 12% 36% 23% 6% 1	950
	21st to end	Night	27% 14% 33% 23% 3	1045

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