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 2024-11-03.

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%	101473
	Night	14% 10% 4 7% 5% 4 4 3 5% 6% 39%	97595
Transparency	Day	45% 6% 12% 24% 12% 1	101262
	Night	46% 6% 12% 22% 13% 2	97477
Seeing	Day	16% 11% 29% 25% 14% 4	15928
	Night	17% 10% 31% 30% 11% 1	25771

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% 16% 9% 12% 6% 5% 4 3 4 5% 27%	2527
	11th to 20th	Day	10% 14% 5% 14% 5% 5% 8% 7% 6% 6% 20%	2530
	21st to end	Day	17% 19% 10% 10% 4 4 3 3 4 5% 21%	2888
February	1st to 10th	Day	11% 13% 6% 10% 5% 5% 5% 4 5% 6% 32%	1710
	11th to 20th	Day	12% 13% 4 7% 6% 5% 5% 3 5% 6% 35%	1770
	21st to end	Day	13% 14% 4 11% 7% 5% 4 3 5% 5% 29%	1570
March	1st to 10th	Day	15% 8% 2 6% 4 3 3 3 4 7% 46%	3120
	11th to 20th	Day	11% 7% 3 8% 5% 5% 4 3 5% 7% 43%	3370
	21st to end	Day	23% 12% 3 9% 4 3 4 3 3 5% 29%	4119
April	1st to 10th	Day	18% 13% 4 7% 5% 4 4 3 5% 6% 31%	2280
	11th to 20th	Day	19% 10% 3 7% 5% 4 4 3 5% 6% 33%	2254
	21st to end	Day	19% 10% 3 6% 5% 4 4 3 6% 6% 33%	2351
May	1st to 10th	Day	16% 8% 3 6% 5% 5% 4 4 6% 6% 38%	4264
	11th to 20th	Day	14% 6% 2 6% 4 4 4 3 4 7% 46%	4399
	21st to end	Day	15% 7% 2 6% 4 5% 5% 4 6% 7% 39%	4824
June	1st to 10th	Day	11% 6% 2 6% 5% 4 5% 4 6% 7% 45%	2660
	11th to 20th	Day	12% 6% 3 6% 5% 5% 5% 4 5% 8% 41%	2660
	21st to end	Day	13% 5% 2 6% 4 4 5% 5% 7% 9% 40%	2660
July	1st to 10th	Day	11% 5% 3 7% 5% 6% 7% 5% 8% 10% 35%	4440
	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 2 5% 6% 39%	1907
November	1st to 10th	Day	12% 9% 3 6% 4 4 4 3 5% 6% 44%	1690
	11th to 20th	Day	13% 13% 4 10% 6% 4 4 3 5% 6% 32%	1672
	21st to end	Day	14% 12% 5% 8% 4 5% 3 3 4 6% 36%	1520
December	1st to 10th	Day	13% 14% 5% 9% 7% 5% 3 3 5% 6% 30%	1520
	11th to 20th	Day	14% 16% 4 10% 5% 4 5% 3 5% 6% 29%	1918
	21st to end	Day	19% 20% 8% 13% 6% 4 4 3 3 4 16%	2783
January	1st to 10th	Night	11% 15% 9% 11% 5% 4 4 3 4 4 32%	4428
	11th to 20th	Night	12% 14% 6% 14% 6% 5% 5% 5% 6% 4 21%	4440
	21st to end	Night	16% 20% 9% 10% 5% 4 3 2 4 4 23%	4775
February	1st to 10th	Night	9% 15% 6% 10% 5% 5% 4 3 5% 5% 33%	2470
	11th to 20th	Night	10% 10% 4 9% 5% 5% 4 4 4 6% 40%	2297
	21st to end	Night	16% 14% 4 7% 4 4 3 3 5% 5% 36%	1884
March	1st to 10th	Night	16% 9% 2 6% 3 4 4 3 5% 5% 43%	3733
	11th to 20th	Night	10% 8% 2 5% 4 4 4 4 6% 8% 43%	3620
	21st to end	Night	22% 11% 4 9% 4 4 4 3 4 5% 30%	3512
April	1st to 10th	Night	20% 11% 2 6% 4 3 5% 3 4 5% 36%	1900
	11th to 20th	Night	21% 8% 2 5% 5% 4 3 2 6% 5% 38%	1875
	21st to end	Night	18% 8% 3 6% 4 3 4 3 5% 7% 38%	1827
May	1st to 10th	Night	17% 7% 2 6% 5% 4 4 4 6% 8% 36%	2633
	11th to 20th	Night	13% 5% 2 6% 4 4 4 4 5% 7% 46%	2506
	21st to end	Night	15% 7% 2 6% 5% 5% 5% 3 5% 8% 41%	2743
June	1st to 10th	Night	8% 5% 2 5% 5% 4 5% 4 7% 8% 47%	1520
	11th to 20th	Night	11% 6% 2 5% 4 3 4 4 6% 9% 46%	1520
	21st to end	Night	9% 5% 2 6% 5% 3 4 4 5% 9% 48%	1520
July	1st to 10th	Night	15% 6% 2 6% 2 4 4 3 6% 9% 44%	2530
	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% 8% 3 7% 4 4 4 4 5% 5% 45%	2433
	11th to 20th	Night	14% 11% 4 9% 6% 4 4 4 5% 6% 33%	2503
	21st to end	Night	16% 10% 3 7% 4 4 4 3 5% 5% 38%	2660
December	1st to 10th	Night	13% 13% 5% 10% 5% 4 4 3 6% 6% 30%	2660
	11th to 20th	Night	14% 15% 6% 10% 6% 5% 4 3 4 4 28%	3279
	21st to end	Night	22% 17% 6% 10% 6% 4 3 3 4 4 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	40% 5% 10% 28% 14% 2	3120
	11th to 20th	Day	40% 6% 12% 29% 12% 1	3370
	21st to end	Day	58% 6% 10% 17% 8% 2	4119
April	1st to 10th	Day	54% 6% 10% 19% 10% 1	2280
	11th to 20th	Day	51% 6% 9% 20% 12% 2	2254
	21st to end	Day	49% 8% 11% 19% 11% 2	2351
May	1st to 10th	Day	43% 8% 11% 21% 16% 1	4302
	11th to 20th	Day	38% 7% 9% 31% 14%	4403
	21st to end	Day	41% 8% 11% 22% 16% 2	4837
June	1st to 10th	Day	36% 7% 14% 27% 15% 2	2660
	11th to 20th	Day	40% 6% 12% 24% 16% 1	2660
	21st to end	Day	38% 9% 15% 24% 13% 1	2660
July	1st to 10th	Day	43% 5% 15% 27% 8% 2	4371
	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	37% 6% 10% 31% 14% 1	1931
	11th to 20th	Day	49% 6% 11% 23% 10% 1	1900
	21st to end	Day	48% 5% 12% 23% 12%	1907
November	1st to 10th	Day	40% 5% 15% 23% 15% 2	1690
	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	43% 6% 11% 25% 9% 5%	3733
	11th to 20th	Night	37% 6% 11% 27% 17% 3	3620
	21st to end	Night	57% 6% 8% 18% 10% 2	3512
April	1st to 10th	Night	49% 5% 10% 20% 14% 1	1900
	11th to 20th	Night	47% 4 9% 20% 17% 3	1875
	21st to end	Night	42% 9% 12% 22% 13% 2	1827
May	1st to 10th	Night	43% 8% 15% 16% 18%	2660
	11th to 20th	Night	36% 9% 12% 24% 18% 1	2507
	21st to end	Night	40% 9% 10% 19% 18% 4	2751
June	1st to 10th	Night	33% 8% 14% 26% 18% 1	1520
	11th to 20th	Night	34% 8% 11% 26% 21% 2	1520
	21st to end	Night	33% 6% 16% 25% 16% 3	1520
July	1st to 10th	Night	39% 4 17% 24% 13% 3	2490
	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	35% 6% 8% 31% 19% 2	2247
	11th to 20th	Night	41% 6% 13% 23% 16% 1	2280
	21st to end	Night	48% 6% 12% 22% 12% 1	2689
November	1st to 10th	Night	41% 5% 11% 25% 16% 2	2433
	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	21% 17% 35% 16% 7% 3	270
	21st to end	Day	27% 12% 30% 23% 8% 1	473
April	1st to 10th	Day	26% 13% 36% 21% 4	460
	11th to 20th	Day	26% 17% 26% 19% 10% 2	452
	21st to end	Day	22% 14% 28% 23% 10% 3	460
May	1st to 10th	Day	21% 12% 33% 23% 10% 1	567
	11th to 20th	Day	19% 11% 31% 24% 11% 3	675
	21st to end	Day	19% 8% 28% 26% 13% 6%	734
June	1st to 10th	Day	10% 9% 28% 32% 18% 3	680
	11th to 20th	Day	14% 8% 32% 25% 18% 4	680
	21st to end	Day	13% 9% 29% 29% 17% 4	680
July	1st to 10th	Day	8% 4 20% 31% 26% 11%	671
	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	14% 16% 37% 22% 8% 2	276
	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% 35% 29% 7%	658
	21st to end	Night	22% 10% 29% 30% 7% 1	627
April	1st to 10th	Night	22% 12% 38% 24% 4	570
	11th to 20th	Night	22% 13% 29% 30% 6%	563
	21st to end	Night	20% 11% 33% 28% 7% 1	570
May	1st to 10th	Night	21% 10% 32% 31% 6% 1	570
	11th to 20th	Night	19% 6% 33% 34% 8%	561
	21st to end	Night	14% 7% 29% 34% 15% 1	617
June	1st to 10th	Night	11% 6% 27% 44% 13% 1	570
	11th to 20th	Night	14% 4 31% 38% 13%	570
	21st to end	Night	10% 4 27% 42% 17%	570
July	1st to 10th	Night	9% 4 15% 40% 29% 3	555
	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% 10% 38% 25% 10% 1	749
	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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