Pontiac
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Pontiac clear sky chart from 2005-11-06 to 2026-03-18.

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	16% 10% 4 9% 5% 5% 4 4 5% 7% 31%	131443
	Night	16% 12% 4 10% 6% 5% 4 3 5% 6% 29%	130046
Transparency	Day	35% 37% 8% 14% 6%	134921
	Night	37% 37% 7% 12% 6% 1	134655
Seeing	Day	24% 17% 31% 19% 8% 2	16055
	Night	23% 14% 29% 24% 9% 1	25297

By Month

Percentage Cloud Cover NE (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	13% 16% 6% 15% 7% 5% 5% 3 5% 5% 19%	4661
	11th to 20th	Day	11% 21% 7% 14% 7% 5% 4 3 5% 5% 18%	4650
	21st to end	Day	7% 15% 7% 16% 9% 8% 5% 3 4 5% 19%	5382
February	1st to 10th	Day	13% 15% 5% 13% 8% 5% 5% 3 5% 6% 23%	1890
	11th to 20th	Day	12% 16% 5% 12% 6% 5% 5% 3 5% 6% 26%	1843
	21st to end	Day	18% 14% 4 10% 6% 6% 4 4 5% 4 25%	1716
March	1st to 10th	Day	19% 14% 4 11% 6% 5% 4 3 5% 6% 24%	4618
	11th to 20th	Day	13% 12% 5% 10% 6% 5% 5% 4 5% 7% 28%	5057
	21st to end	Day	18% 12% 3 7% 6% 4 3 2 4 5% 36%	5738
April	1st to 10th	Day	21% 13% 4 6% 5% 4 4 3 4 5% 31%	2400
	11th to 20th	Day	22% 11% 3 7% 4 4 3 3 5% 4 35%	2374
	21st to end	Day	22% 10% 4 7% 5% 4 4 4 3 5% 32%	2556
May	1st to 10th	Day	22% 9% 3 8% 5% 5% 5% 4 5% 6% 28%	6346
	11th to 20th	Day	16% 9% 3 8% 5% 5% 5% 4 6% 8% 32%	6318
	21st to end	Day	21% 8% 3 7% 5% 4 4 4 5% 6% 32%	6949
June	1st to 10th	Day	20% 9% 3 6% 5% 5% 5% 4 5% 7% 30%	2800
	11th to 20th	Day	15% 6% 2 6% 4 5% 5% 4 7% 7% 39%	2800
	21st to end	Day	18% 8% 3 7% 5% 5% 5% 5% 6% 8% 32%	2800
July	1st to 10th	Day	13% 6% 3 8% 5% 5% 5% 4 7% 9% 34%	6360
	11th to 20th	Day	11% 5% 2 6% 4 5% 5% 4 6% 8% 44%	6360
	21st to end	Day	9% 5% 2 5% 4 4 4 4 6% 10% 46%	6926
August	1st to 10th	Day	14% 6% 3 7% 5% 5% 5% 5% 6% 9% 35%	4473
	11th to 20th	Day	16% 7% 2 6% 4 4 4 4 7% 10% 36%	4151
	21st to end	Day	12% 5% 2 6% 4 5% 4 5% 8% 10% 39%	4312
September	1st to 10th	Day	14% 6% 2 5% 4 4 5% 4 6% 7% 43%	2400
	11th to 20th	Day	12% 6% 1 6% 4 4 5% 3 6% 7% 46%	2400
	21st to end	Day	18% 6% 2 7% 4 4 4 3 5% 8% 39%	2127
October	1st to 10th	Day	21% 7% 2 6% 5% 4 5% 4 5% 7% 36%	2000
	11th to 20th	Day	22% 10% 3 7% 5% 5% 4 3 5% 8% 27%	2000
	21st to end	Day	26% 9% 3 9% 4 5% 4 3 4 5% 28%	2050
November	1st to 10th	Day	17% 10% 3 6% 5% 5% 5% 4 6% 7% 32%	1832
	11th to 20th	Day	18% 13% 4 10% 6% 4 5% 4 5% 6% 25%	1680
	21st to end	Day	22% 16% 4 10% 6% 5% 6% 3 4 5% 19%	1680
December	1st to 10th	Day	21% 21% 6% 11% 5% 5% 5% 4 4 5% 13%	2680
	11th to 20th	Day	17% 18% 6% 11% 6% 4 4 3 4 4 24%	3066
	21st to end	Day	22% 18% 5% 11% 5% 5% 4 3 4 4 19%	4048
January	1st to 10th	Night	13% 18% 8% 16% 7% 4 4 3 4 4 18%	8234
	11th to 20th	Night	11% 19% 6% 15% 7% 5% 5% 3 4 5% 18%	8235
	21st to end	Night	9% 17% 6% 13% 8% 7% 6% 4 4 5% 21%	8783
February	1st to 10th	Night	12% 14% 6% 13% 7% 5% 5% 4 5% 6% 24%	2730
	11th to 20th	Night	12% 14% 6% 11% 8% 5% 4 3 4 5% 28%	2663
	21st to end	Night	15% 13% 5% 10% 6% 5% 5% 3 5% 5% 26%	2089
March	1st to 10th	Night	21% 15% 4 9% 5% 4 3 2 3 4 28%	5441
	11th to 20th	Night	12% 13% 4 10% 5% 5% 5% 4 6% 8% 28%	5057
	21st to end	Night	17% 12% 3 9% 5% 4 4 3 4 6% 33%	5193
April	1st to 10th	Night	21% 11% 3 8% 5% 4 4 4 4 6% 31%	2000
	11th to 20th	Night	18% 10% 3 8% 5% 4 4 3 4 6% 35%	1975
	21st to end	Night	20% 10% 3 8% 4 5% 4 3 5% 6% 33%	1860
May	1st to 10th	Night	21% 10% 3 7% 5% 5% 5% 4 7% 8% 26%	3594
	11th to 20th	Night	16% 8% 3 6% 5% 4 4 4 5% 7% 38%	3575
	21st to end	Night	19% 8% 3 8% 5% 4 5% 4 5% 6% 32%	3926
June	1st to 10th	Night	16% 7% 3 7% 5% 5% 5% 4 6% 9% 33%	1600
	11th to 20th	Night	13% 6% 2 7% 4 4 3 4 6% 7% 43%	1600
	21st to end	Night	17% 7% 3 6% 4 5% 5% 4 6% 8% 36%	1600
July	1st to 10th	Night	13% 7% 3 8% 6% 5% 5% 5% 7% 8% 34%	3600
	11th to 20th	Night	11% 7% 2 6% 5% 4 5% 4 6% 9% 42%	3600
	21st to end	Night	9% 5% 2 5% 5% 4 5% 3 6% 8% 48%	3926
August	1st to 10th	Night	12% 6% 3 7% 5% 4 5% 4 6% 9% 39%	2674
	11th to 20th	Night	14% 7% 3 6% 4 4 4 4 5% 8% 41%	2875
	21st to end	Night	14% 5% 2 6% 4 4 5% 4 6% 8% 42%	3576
September	1st to 10th	Night	11% 5% 2 7% 5% 4 4 4 5% 7% 47%	2000
	11th to 20th	Night	14% 5% 2 4 3 3 4 3 5% 7% 50%	2000
	21st to end	Night	18% 7% 2 6% 4 3 3 3 4 6% 43%	2269
October	1st to 10th	Night	18% 7% 3 6% 4 4 4 3 5% 6% 41%	2400
	11th to 20th	Night	20% 8% 3 7% 6% 5% 5% 4 5% 6% 31%	2400
	21st to end	Night	22% 11% 3 8% 5% 4 5% 3 5% 6% 28%	2790
November	1st to 10th	Night	19% 11% 3 7% 6% 4 4 4 6% 6% 32%	2683
	11th to 20th	Night	19% 12% 4 9% 6% 5% 5% 3 5% 5% 28%	2940
	21st to end	Night	21% 14% 4 11% 5% 4 5% 3 5% 6% 21%	2940
December	1st to 10th	Night	19% 18% 5% 11% 7% 5% 5% 4 6% 5% 15%	4720
	11th to 20th	Night	22% 16% 5% 11% 5% 4 3 2 3 4 25%	5348
	21st to end	Night	25% 19% 4 11% 5% 3 3 3 3 4 21%	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	54% 23% 8% 11% 4	4662
	11th to 20th	Day	57% 22% 6% 8% 7%	4680
	21st to end	Day	55% 24% 6% 8% 6% 1	5382
February	1st to 10th	Day	27% 59% 3 7% 3 1	1890
	11th to 20th	Day	26% 58% 4 9% 4	1852
	21st to end	Day	26% 59% 4 4 5% 1	1716
March	1st to 10th	Day	47% 29% 5% 7% 9% 2	4621
	11th to 20th	Day	39% 31% 12% 10% 7% 1	5057
	21st to end	Day	37% 30% 4 14% 14% 1	5738
April	1st to 10th	Day	22% 62% 3 5% 6% 1	2400
	11th to 20th	Day	21% 62% 2 8% 5% 1	2374
	21st to end	Day	21% 62% 3 7% 6%	2556
May	1st to 10th	Day	43% 30% 6% 14% 5% 1	6357
	11th to 20th	Day	37% 32% 11% 14% 7%	6322
	21st to end	Day	40% 30% 9% 14% 8%	6949
June	1st to 10th	Day	20% 62% 4 9% 5% 1	2800
	11th to 20th	Day	18% 62% 5% 9% 5% 1	2800
	21st to end	Day	19% 61% 5% 10% 5%	2800
July	1st to 10th	Day	35% 27% 12% 20% 6%	6290
	11th to 20th	Day	28% 27% 11% 27% 8%	6285
	21st to end	Day	24% 26% 13% 29% 8%	6728
August	1st to 10th	Day	30% 36% 12% 16% 6%	4459
	11th to 20th	Day	30% 38% 9% 19% 5%	4190
	21st to end	Day	25% 37% 12% 18% 7%	4312
September	1st to 10th	Day	16% 59% 7% 12% 5%	2400
	11th to 20th	Day	14% 57% 5% 17% 6%	2400
	21st to end	Day	16% 58% 6% 12% 8%	2127
October	1st to 10th	Day	32% 36% 7% 21% 4	3260
	11th to 20th	Day	28% 38% 9% 17% 7%	3240
	21st to end	Day	32% 37% 14% 13% 3	3288
November	1st to 10th	Day	29% 55% 5% 8% 2	1832
	11th to 20th	Day	27% 55% 5% 8% 3	1680
	21st to end	Day	32% 56% 4 3 4	1680
December	1st to 10th	Day	50% 35% 7% 7% 1	2680
	11th to 20th	Day	47% 32% 6% 9% 6%	3066
	21st to end	Day	52% 28% 4 13% 3	4048
January	1st to 10th	Night	55% 23% 9% 10% 4	8234
	11th to 20th	Night	56% 22% 8% 10% 4	8280
	21st to end	Night	54% 23% 5% 8% 9%	8783
February	1st to 10th	Night	25% 59% 4 8% 3	2730
	11th to 20th	Night	25% 58% 3 10% 4	2665
	21st to end	Night	24% 59% 4 7% 6%	2089
March	1st to 10th	Night	46% 29% 5% 9% 9% 3	5451
	11th to 20th	Night	38% 32% 12% 11% 6% 2	5058
	21st to end	Night	39% 29% 6% 16% 8% 2	5193
April	1st to 10th	Night	21% 62% 3 7% 5% 1	2000
	11th to 20th	Night	20% 62% 2 8% 7% 1	1975
	21st to end	Night	20% 62% 4 7% 7% 1	1860
May	1st to 10th	Night	41% 34% 6% 13% 4 1	3600
	11th to 20th	Night	33% 32% 11% 14% 10%	3576
	21st to end	Night	41% 30% 8% 12% 8%	3927
June	1st to 10th	Night	17% 62% 4 11% 5% 1	1600
	11th to 20th	Night	15% 63% 4 10% 8% 1	1600
	21st to end	Night	19% 61% 4 10% 7%	1600
July	1st to 10th	Night	38% 26% 14% 15% 7%	3560
	11th to 20th	Night	29% 27% 11% 26% 8%	3552
	21st to end	Night	25% 26% 10% 29% 10%	3809
August	1st to 10th	Night	28% 38% 10% 18% 7%	2664
	11th to 20th	Night	31% 38% 10% 18% 4	2902
	21st to end	Night	27% 37% 10% 17% 8% 2	3576
September	1st to 10th	Night	13% 59% 7% 15% 6%	2000
	11th to 20th	Night	14% 57% 6% 17% 5% 1	2000
	21st to end	Night	16% 57% 5% 14% 8%	2269
October	1st to 10th	Night	24% 36% 9% 22% 9%	3920
	11th to 20th	Night	27% 37% 11% 17% 7%	3899
	21st to end	Night	31% 37% 13% 13% 6%	4502
November	1st to 10th	Night	28% 54% 7% 8% 3	2683
	11th to 20th	Night	27% 55% 4 10% 4	2940
	21st to end	Night	30% 55% 6% 6% 3	2940
December	1st to 10th	Night	47% 36% 8% 8% 2	4720
	11th to 20th	Night	47% 31% 4 10% 9%	5348
	21st to end	Night	52% 26% 3 10% 9%	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	21% 22% 38% 16% 3	220
	11th to 20th	Day	25% 19% 40% 13% 3 1	220
	21st to end	Day	21% 21% 35% 18% 5%	289
February	1st to 10th	Day	28% 25% 32% 12% 3	330
	11th to 20th	Day	26% 29% 38% 7% 1	330
	21st to end	Day	31% 24% 31% 13% 1	432
March	1st to 10th	Day	29% 22% 33% 12% 4	540
	11th to 20th	Day	31% 21% 31% 13% 3 1	528
	21st to end	Day	31% 19% 31% 14% 4 1	550
April	1st to 10th	Day	34% 23% 24% 15% 4 1	500
	11th to 20th	Day	36% 20% 22% 13% 8% 1	492
	21st to end	Day	32% 22% 30% 11% 5% 1	500
May	1st to 10th	Day	27% 18% 32% 15% 6% 2	503
	11th to 20th	Day	20% 18% 31% 22% 7% 2	527
	21st to end	Day	24% 16% 30% 20% 8% 2	573
June	1st to 10th	Day	28% 12% 33% 20% 7% 1	530
	11th to 20th	Day	17% 16% 32% 23% 9% 3	530
	21st to end	Day	22% 15% 33% 20% 9% 2	530
July	1st to 10th	Day	11% 11% 33% 32% 10% 2	525
	11th to 20th	Day	14% 10% 35% 25% 12% 3	524
	21st to end	Day	13% 10% 35% 28% 11% 3	576
August	1st to 10th	Day	13% 9% 30% 30% 14% 4	526
	11th to 20th	Day	18% 6% 26% 30% 14% 7%	519
	21st to end	Day	17% 11% 30% 24% 13% 5%	583
September	1st to 10th	Day	15% 14% 30% 23% 12% 6%	530
	11th to 20th	Day	14% 9% 24% 24% 19% 10%	530
	21st to end	Day	19% 14% 29% 20% 12% 5%	530
October	1st to 10th	Day	24% 16% 25% 22% 10% 3	530
	11th to 20th	Day	27% 19% 32% 15% 6% 1	530
	21st to end	Day	29% 18% 30% 17% 5%	500
November	1st to 10th	Day	29% 17% 31% 18% 5% 1	406
	11th to 20th	Day	28% 23% 33% 13% 4	220
	21st to end	Day	31% 22% 33% 10% 3 1	220
December	1st to 10th	Day	32% 15% 37% 14% 2	220
	11th to 20th	Day	25% 28% 36% 8% 2	220
	21st to end	Day	31% 21% 33% 12% 2 1	242
January	1st to 10th	Night	27% 16% 35% 19% 3	840
	11th to 20th	Night	28% 19% 28% 20% 5%	840
	21st to end	Night	18% 16% 38% 23% 5% 1	924
February	1st to 10th	Night	19% 21% 34% 22% 4	840
	11th to 20th	Night	22% 21% 37% 18% 4	823
	21st to end	Night	23% 21% 29% 23% 4	692
March	1st to 10th	Night	25% 18% 34% 19% 4	840
	11th to 20th	Night	25% 21% 30% 18% 5%	829
	21st to end	Night	24% 18% 33% 19% 7%	880
April	1st to 10th	Night	27% 17% 29% 23% 5%	800
	11th to 20th	Night	25% 19% 29% 20% 7%	791
	21st to end	Night	27% 16% 28% 22% 7%	660
May	1st to 10th	Night	24% 11% 28% 28% 9% 1	400
	11th to 20th	Night	24% 11% 26% 30% 9%	394
	21st to end	Night	16% 11% 35% 28% 10%	433
June	1st to 10th	Night	20% 9% 29% 34% 8%	400
	11th to 20th	Night	17% 9% 27% 35% 13% 1	400
	21st to end	Night	19% 7% 28% 32% 14%	400
July	1st to 10th	Night	12% 5% 28% 38% 16%	390
	11th to 20th	Night	14% 3 26% 41% 16% 1	388
	21st to end	Night	10% 5% 28% 41% 15% 2	438
August	1st to 10th	Night	12% 5% 22% 39% 21% 1	447
	11th to 20th	Night	12% 3 22% 37% 22% 3	594
	21st to end	Night	15% 6% 26% 35% 17% 2	870
September	1st to 10th	Night	13% 8% 29% 30% 19% 1	800
	11th to 20th	Night	16% 6% 23% 29% 23% 3	800
	21st to end	Night	21% 8% 26% 25% 18% 2	800
October	1st to 10th	Night	22% 13% 26% 23% 15% 1	800
	11th to 20th	Night	25% 14% 30% 23% 7% 1	800
	21st to end	Night	29% 15% 28% 24% 5%	880
November	1st to 10th	Night	26% 16% 27% 24% 7% 1	820
	11th to 20th	Night	27% 17% 32% 16% 9%	840
	21st to end	Night	30% 20% 25% 20% 5%	840
December	1st to 10th	Night	28% 20% 29% 19% 4	840
	11th to 20th	Night	27% 19% 30% 18% 5% 1	840
	21st to end	Night	34% 17% 30% 15% 3	924

Page generated at 2026-03-15 02:23:56. Version ac07

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