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 2025-07-06.

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% 9% 3 7% 5% 4 4 4 5% 7% 38%	120121
	Night	13% 10% 4 8% 5% 4 4 3 5% 6% 38%	118895
Transparency	Day	45% 5% 12% 24% 13% 1	123635
	Night	46% 5% 11% 23% 14% 2	123521
Seeing	Day	17% 11% 30% 25% 14% 4	16831
	Night	17% 10% 31% 30% 11% 1	26725

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	8% 12% 6% 14% 8% 7% 5% 3 4 5% 28%	3617
	11th to 20th	Day	8% 13% 6% 10% 4 4 7% 6% 7% 6% 28%	3620
	21st to end	Day	12% 15% 8% 8% 3 4 3 3 3 5% 35%	4143
February	1st to 10th	Day	11% 13% 6% 10% 5% 5% 5% 4 5% 6% 32%	1800
	11th to 20th	Day	12% 13% 4 7% 6% 5% 5% 3 5% 5% 35%	1866
	21st to end	Day	12% 14% 4 11% 7% 5% 5% 3 5% 5% 30%	1650
March	1st to 10th	Day	15% 7% 2 5% 4 3 3 3 4 7% 47%	4480
	11th to 20th	Day	12% 6% 2 7% 5% 5% 4 3 5% 7% 44%	4814
	21st to end	Day	25% 11% 4 8% 4 3 3 3 3 5% 30%	5893
April	1st to 10th	Day	18% 13% 4 7% 5% 4 4 3 5% 6% 31%	2400
	11th to 20th	Day	19% 10% 3 7% 5% 4 4 3 6% 6% 33%	2374
	21st to end	Day	19% 11% 3 6% 5% 4 4 4 6% 6% 33%	2475
May	1st to 10th	Day	15% 7% 2 6% 4 4 4 3 6% 6% 43%	6128
	11th to 20th	Day	15% 5% 2 6% 5% 4 4 3 5% 7% 43%	6319
	21st to end	Day	17% 6% 2 6% 4 5% 5% 4 6% 8% 39%	6935
June	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 4 6% 8% 44%	2800
	11th to 20th	Day	12% 6% 3 6% 5% 5% 5% 4 5% 8% 40%	2800
	21st to end	Day	13% 5% 2 6% 5% 4 5% 5% 7% 9% 40%	2800
July	1st to 10th	Day	11% 5% 3 7% 5% 6% 7% 5% 8% 10% 35%	4511
	11th to 20th	Day	5% 3 1 5% 4 4 5% 5% 6% 10% 51%	4440
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 4 7% 8% 42%	4813
August	1st to 10th	Day	10% 5% 2 7% 5% 5% 5% 5% 7% 8% 41%	4376
	11th to 20th	Day	12% 5% 2 5% 3 4 4 4 5% 8% 49%	3834
	21st to end	Day	11% 5% 2 7% 4 5% 4 4 7% 9% 43%	4180
September	1st to 10th	Day	13% 5% 2 6% 5% 4 4 4 6% 6% 46%	2280
	11th to 20th	Day	13% 4 2 6% 4 4 4 4 5% 8% 47%	2223
	21st to end	Day	13% 7% 2 6% 4 4 3 3 5% 7% 45%	2090
October	1st to 10th	Day	13% 6% 2 6% 4 4 4 2 4 7% 47%	1931
	11th to 20th	Day	20% 9% 2 7% 5% 3 5% 3 6% 7% 34%	1900
	21st to end	Day	17% 11% 3 7% 4 3 3 3 5% 6% 39%	1907
November	1st to 10th	Day	13% 9% 3 6% 4 4 4 3 5% 6% 43%	1762
	11th to 20th	Day	13% 13% 5% 10% 6% 5% 4 3 5% 6% 31%	1760
	21st to end	Day	14% 13% 5% 9% 4 4 3 3 4 6% 35%	1600
December	1st to 10th	Day	8% 10% 6% 10% 6% 6% 5% 5% 5% 7% 33%	2610
	11th to 20th	Day	13% 16% 5% 11% 6% 4 5% 3 5% 5% 27%	3008
	21st to end	Day	20% 20% 8% 13% 5% 4 4 3 3 4 15%	3982
January	1st to 10th	Night	8% 12% 7% 13% 6% 4 4 3 4 5% 34%	6348
	11th to 20th	Night	11% 13% 6% 12% 6% 5% 6% 5% 6% 5% 24%	6360
	21st to end	Night	12% 16% 7% 10% 4 4 4 3 4 5% 31%	6831
February	1st to 10th	Night	10% 15% 5% 10% 5% 5% 4 3 5% 5% 33%	2600
	11th to 20th	Night	10% 10% 4 9% 5% 5% 4 3 4 6% 39%	2421
	21st to end	Night	15% 14% 4 7% 4 4 3 3 5% 5% 36%	1980
March	1st to 10th	Night	15% 8% 3 7% 4 4 4 3 5% 5% 43%	5373
	11th to 20th	Night	11% 8% 2 5% 4 4 4 4 6% 8% 45%	5176
	21st to end	Night	22% 12% 4 8% 5% 4 4 3 4 5% 30%	5036
April	1st to 10th	Night	20% 10% 2 6% 4 3 5% 3 4 6% 37%	2000
	11th to 20th	Night	21% 8% 2 6% 5% 4 3 2 6% 5% 38%	1975
	21st to end	Night	18% 8% 3 6% 4 3 4 3 5% 7% 38%	1923
May	1st to 10th	Night	17% 7% 2 6% 4 3 4 3 5% 7% 41%	3775
	11th to 20th	Night	10% 5% 2 5% 4 4 5% 4 6% 7% 48%	3596
	21st to end	Night	18% 7% 2 6% 5% 5% 5% 4 5% 7% 36%	3942
June	1st to 10th	Night	9% 5% 2 5% 5% 5% 4 4 7% 8% 47%	1600
	11th to 20th	Night	11% 7% 2 6% 4 3 4 4 6% 9% 45%	1600
	21st to end	Night	10% 5% 2 6% 5% 3 4 4 5% 9% 47%	1600
July	1st to 10th	Night	15% 6% 2 6% 2 4 4 3 6% 9% 44%	2570
	11th to 20th	Night	5% 3 1 4 4 4 4 4 6% 7% 58%	2530
	21st to end	Night	8% 4 2 5% 4 4 5% 4 7% 9% 49%	2750
August	1st to 10th	Night	11% 4 1 6% 4 4 5% 3 6% 9% 48%	2547
	11th to 20th	Night	11% 6% 2 5% 4 4 4 3 5% 7% 50%	2996
	21st to end	Night	9% 5% 2 4 4 4 4 3 6% 8% 50%	3476
September	1st to 10th	Night	10% 4 2 5% 4 3 4 3 5% 7% 53%	1900
	11th to 20th	Night	13% 4 2 5% 3 3 5% 3 5% 5% 52%	1938
	21st to end	Night	11% 5% 2 6% 5% 4 4 3 5% 6% 50%	2090
October	1st to 10th	Night	13% 6% 2 6% 3 4 3 3 5% 7% 49%	2247
	11th to 20th	Night	18% 6% 2 6% 4 4 4 3 5% 6% 42%	2280
	21st to end	Night	18% 9% 3 6% 4 4 5% 4 6% 7% 35%	2689
November	1st to 10th	Night	13% 9% 2 7% 4 4 4 4 5% 5% 44%	2536
	11th to 20th	Night	14% 12% 4 9% 6% 4 4 3 5% 6% 34%	2635
	21st to end	Night	16% 11% 3 8% 4 4 4 3 5% 5% 36%	2800
December	1st to 10th	Night	8% 9% 4 8% 5% 5% 5% 3 6% 7% 39%	4580
	11th to 20th	Night	15% 17% 6% 10% 5% 4 4 3 4 4 27%	5199
	21st to end	Night	20% 18% 7% 11% 5% 4 4 3 4 4 19%	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	60% 4 12% 18% 7%	3618
	11th to 20th	Day	54% 5% 11% 13% 17% 1	3620
	21st to end	Day	53% 3 12% 19% 12%	4143
February	1st to 10th	Day	53% 6% 14% 17% 10% 1	1800
	11th to 20th	Day	50% 4 12% 22% 9% 2	1875
	21st to end	Day	56% 5% 10% 17% 10% 3	1650
March	1st to 10th	Day	40% 4 11% 28% 13% 4	4480
	11th to 20th	Day	40% 4 14% 28% 13% 1	4814
	21st to end	Day	58% 4 9% 15% 11% 2	5895
April	1st to 10th	Day	54% 6% 10% 20% 10% 1	2400
	11th to 20th	Day	51% 5% 10% 20% 12% 2	2374
	21st to end	Day	50% 7% 11% 19% 11% 2	2475
May	1st to 10th	Day	41% 6% 10% 25% 16% 2	6163
	11th to 20th	Day	42% 6% 9% 26% 16%	6323
	21st to end	Day	42% 6% 10% 27% 14% 1	6949
June	1st to 10th	Day	36% 7% 14% 27% 14% 1	2800
	11th to 20th	Day	41% 6% 12% 24% 16% 1	2800
	21st to end	Day	38% 8% 15% 24% 13% 1	2800
July	1st to 10th	Day	43% 5% 15% 28% 8% 2	4442
	11th to 20th	Day	27% 5% 16% 34% 17% 1	4365
	21st to end	Day	38% 7% 13% 22% 18% 2	4662
August	1st to 10th	Day	39% 6% 15% 28% 11% 1	4361
	11th to 20th	Day	34% 4 15% 34% 12% 1	3872
	21st to end	Day	38% 4 14% 29% 13% 1	4179
September	1st to 10th	Day	38% 6% 12% 25% 18% 1	2280
	11th to 20th	Day	35% 6% 15% 29% 14% 1	2223
	21st to end	Day	40% 5% 11% 31% 13%	2090
October	1st to 10th	Day	24% 4 6% 41% 24% 1	3204
	11th to 20th	Day	39% 4 8% 30% 17% 2	3140
	21st to end	Day	43% 3 12% 28% 13% 1	3119
November	1st to 10th	Day	42% 5% 15% 23% 14% 2	1762
	11th to 20th	Day	53% 5% 13% 18% 9% 1	1760
	21st to end	Day	51% 5% 15% 22% 7% 1	1600
December	1st to 10th	Day	50% 5% 19% 19% 6% 1	2610
	11th to 20th	Day	59% 4 11% 18% 7% 2	3005
	21st to end	Day	73% 4 7% 10% 6%	3982
January	1st to 10th	Night	54% 4 9% 23% 10%	6348
	11th to 20th	Night	60% 4 8% 11% 16%	6360
	21st to end	Night	57% 3 11% 17% 12%	6831
February	1st to 10th	Night	53% 5% 11% 19% 11% 2	2600
	11th to 20th	Night	47% 5% 10% 24% 12% 1	2422
	21st to end	Night	50% 5% 12% 20% 11% 2	1980
March	1st to 10th	Night	44% 4 12% 24% 10% 6%	5373
	11th to 20th	Night	36% 4 13% 29% 14% 4	5176
	21st to end	Night	58% 4 9% 17% 11% 1	5036
April	1st to 10th	Night	49% 5% 10% 20% 15% 1	2000
	11th to 20th	Night	48% 4 9% 21% 16% 3	1975
	21st to end	Night	43% 8% 12% 23% 13% 2	1923
May	1st to 10th	Night	41% 6% 12% 19% 20% 2	3802
	11th to 20th	Night	34% 7% 16% 21% 20% 2	3597
	21st to end	Night	47% 6% 8% 23% 13% 3	3950
June	1st to 10th	Night	33% 8% 14% 27% 18% 1	1600
	11th to 20th	Night	35% 7% 11% 25% 20% 2	1600
	21st to end	Night	34% 6% 16% 24% 16% 3	1600
July	1st to 10th	Night	39% 4 17% 25% 13% 3	2530
	11th to 20th	Night	26% 4 15% 34% 19% 2	2482
	21st to end	Night	31% 8% 15% 23% 20% 4	2662
August	1st to 10th	Night	33% 5% 16% 25% 19% 2	2538
	11th to 20th	Night	35% 5% 13% 31% 16% 1	3024
	21st to end	Night	33% 4 15% 27% 19% 3	3476
September	1st to 10th	Night	30% 7% 10% 28% 23% 2	1900
	11th to 20th	Night	34% 4 11% 33% 17% 1	1938
	21st to end	Night	34% 6% 11% 29% 19% 1	2090
October	1st to 10th	Night	26% 4 5% 36% 27% 2	3754
	11th to 20th	Night	32% 4 11% 30% 21% 2	3779
	21st to end	Night	45% 4 9% 26% 15% 1	4427
November	1st to 10th	Night	41% 5% 12% 25% 15% 2	2536
	11th to 20th	Night	52% 5% 13% 18% 10% 2	2635
	21st to end	Night	50% 6% 13% 22% 8% 1	2800
December	1st to 10th	Night	44% 4 18% 25% 7% 2	4580
	11th to 20th	Night	62% 4 9% 15% 9% 1	5201
	21st to end	Night	70% 4 7% 13% 6%	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	16% 13% 36% 23% 9% 3	300
	11th to 20th	Day	17% 20% 35% 21% 6%	300
	21st to end	Day	22% 13% 35% 25% 5% 1	330
February	1st to 10th	Day	15% 17% 44% 20% 3 1	300
	11th to 20th	Day	19% 21% 36% 17% 6%	300
	21st to end	Day	20% 18% 43% 18% 2	249
March	1st to 10th	Day	19% 16% 40% 20% 5%	300
	11th to 20th	Day	22% 17% 36% 15% 7% 3	300
	21st to end	Day	28% 12% 31% 22% 7% 1	515
April	1st to 10th	Day	26% 14% 35% 20% 4	500
	11th to 20th	Day	26% 18% 25% 19% 10% 2	492
	21st to end	Day	23% 14% 30% 22% 9% 2	500
May	1st to 10th	Day	20% 11% 33% 24% 10% 1	613
	11th to 20th	Day	19% 12% 30% 24% 12% 4	725
	21st to end	Day	19% 8% 29% 26% 12% 6%	789
June	1st to 10th	Day	10% 10% 28% 32% 17% 3	730
	11th to 20th	Day	15% 7% 32% 25% 18% 3	730
	21st to end	Day	13% 10% 29% 29% 16% 3	730
July	1st to 10th	Day	7% 4 20% 33% 26% 11%	697
	11th to 20th	Day	8% 6% 26% 31% 21% 7%	668
	21st to end	Day	9% 4 24% 34% 23% 7%	739
August	1st to 10th	Day	8% 4 21% 31% 27% 9%	676
	11th to 20th	Day	13% 6% 17% 28% 23% 13%	548
	21st to end	Day	12% 6% 23% 29% 21% 9%	539
September	1st to 10th	Day	16% 7% 23% 28% 19% 7%	490
	11th to 20th	Day	12% 5% 22% 29% 23% 9%	490
	21st to end	Day	18% 9% 26% 25% 17% 5%	490
October	1st to 10th	Day	18% 11% 27% 27% 14% 4	331
	11th to 20th	Day	23% 15% 33% 20% 8%	300
	21st to end	Day	25% 14% 35% 18% 6% 1	330
November	1st to 10th	Day	16% 17% 37% 21% 8% 2	300
	11th to 20th	Day	17% 19% 38% 19% 7%	300
	21st to end	Day	24% 14% 40% 17% 5%	300
December	1st to 10th	Day	16% 18% 43% 16% 5% 1	300
	11th to 20th	Day	22% 16% 35% 19% 8% 1	300
	21st to end	Day	26% 17% 36% 16% 4 1	330
January	1st to 10th	Night	15% 15% 38% 25% 8% 1	1000
	11th to 20th	Night	19% 13% 37% 25% 5% 1	1000
	21st to end	Night	20% 12% 35% 27% 6% 1	1100
February	1st to 10th	Night	18% 12% 40% 24% 7%	1000
	11th to 20th	Night	17% 14% 37% 26% 6%	866
	21st to end	Night	22% 12% 39% 22% 4	660
March	1st to 10th	Night	22% 12% 34% 25% 8% 1	800
	11th to 20th	Night	17% 13% 35% 28% 7%	692
	21st to end	Night	23% 10% 29% 30% 7% 1	660
April	1st to 10th	Night	22% 12% 37% 25% 4	600
	11th to 20th	Night	22% 13% 29% 30% 6%	593
	21st to end	Night	20% 11% 33% 28% 7% 1	600
May	1st to 10th	Night	21% 10% 32% 32% 6% 1	600
	11th to 20th	Night	18% 7% 33% 34% 8%	591
	21st to end	Night	15% 7% 28% 35% 14% 1	650
June	1st to 10th	Night	11% 5% 27% 44% 12% 1	600
	11th to 20th	Night	14% 4 31% 38% 13%	600
	21st to end	Night	11% 4 27% 41% 16%	600
July	1st to 10th	Night	9% 4 15% 41% 29% 3	570
	11th to 20th	Night	8% 3 23% 43% 22% 2	552
	21st to end	Night	7% 3 17% 48% 24% 2	624
August	1st to 10th	Night	7% 3 17% 48% 24% 2	567
	11th to 20th	Night	10% 4 15% 39% 26% 5%	564
	21st to end	Night	11% 3 18% 45% 22% 2	627
September	1st to 10th	Night	11% 4 23% 36% 20% 5%	570
	11th to 20th	Night	14% 4 23% 34% 22% 3	608
	21st to end	Night	13% 8% 27% 34% 18% 1	760
October	1st to 10th	Night	15% 8% 28% 32% 17% 1	760
	11th to 20th	Night	20% 11% 33% 28% 8%	760
	21st to end	Night	23% 11% 31% 27% 8% 1	836
November	1st to 10th	Night	16% 11% 38% 25% 9% 1	780
	11th to 20th	Night	19% 14% 39% 22% 5%	835
	21st to end	Night	21% 14% 34% 24% 7%	1000
December	1st to 10th	Night	20% 13% 41% 21% 6%	1000
	11th to 20th	Night	23% 12% 36% 23% 6% 1	1000
	21st to end	Night	27% 13% 33% 24% 4	1100

Page generated at 2025-07-03 00:30:13. Version ac07

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