Au Sable Forks Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Au Sable Forks Observatory clear sky chart from 2005-11-06 to 2025-02-21.

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% 10% 4 9% 6% 5% 5% 4 6% 7% 30%	108541
	Night	12% 11% 4 10% 6% 5% 5% 4 6% 7% 30%	110042
Transparency	Day	50% 7% 13% 20% 10% 1	112046
	Night	51% 7% 13% 18% 10% 1	114688
Seeing	Day	23% 18% 31% 18% 7% 2	14930
	Night	21% 17% 29% 24% 8% 1	24081

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	9% 12% 6% 16% 10% 6% 5% 4 5% 6% 21%	3597
	11th to 20th	Day	7% 16% 6% 19% 10% 6% 5% 3 4 4 20%	3570
	21st to end	Day	9% 12% 5% 13% 7% 6% 7% 5% 6% 6% 23%	3960
February	1st to 10th	Day	10% 14% 4 14% 8% 7% 6% 4 6% 5% 21%	1790
	11th to 20th	Day	8% 11% 4 13% 9% 8% 6% 5% 5% 6% 25%	1884
	21st to end	Day	12% 13% 5% 10% 8% 6% 5% 4 5% 6% 26%	1570
March	1st to 10th	Day	19% 15% 4 10% 6% 6% 4 3 5% 5% 23%	3116
	11th to 20th	Day	8% 15% 5% 13% 8% 7% 7% 4 5% 5% 24%	3278
	21st to end	Day	12% 11% 4 8% 6% 4 4 4 5% 5% 36%	4166
April	1st to 10th	Day	17% 13% 3 8% 6% 5% 4 4 4 6% 30%	2280
	11th to 20th	Day	16% 11% 3 8% 6% 5% 4 3 4 5% 34%	2254
	21st to end	Day	20% 11% 3 8% 5% 4 4 3 5% 6% 31%	2412
May	1st to 10th	Day	18% 13% 4 9% 5% 5% 4 4 5% 6% 28%	4290
	11th to 20th	Day	15% 10% 4 8% 5% 5% 5% 4 6% 7% 29%	4398
	21st to end	Day	13% 6% 2 7% 5% 5% 6% 5% 7% 9% 36%	4837
June	1st to 10th	Day	21% 9% 2 6% 5% 5% 5% 4 6% 8% 28%	2660
	11th to 20th	Day	14% 8% 3 7% 5% 5% 6% 5% 6% 8% 34%	2660
	21st to end	Day	19% 7% 3 8% 6% 5% 5% 5% 6% 7% 28%	2660
July	1st to 10th	Day	10% 6% 2 7% 5% 6% 7% 5% 8% 10% 34%	4440
	11th to 20th	Day	10% 6% 2 7% 5% 6% 7% 6% 9% 9% 34%	4440
	21st to end	Day	11% 6% 3 7% 5% 5% 6% 5% 7% 9% 36%	4814
August	1st to 10th	Day	13% 8% 3 8% 6% 6% 6% 5% 8% 9% 30%	4423
	11th to 20th	Day	10% 6% 3 9% 5% 5% 7% 6% 8% 11% 29%	3891
	21st to end	Day	13% 7% 2 6% 5% 5% 5% 5% 7% 9% 38%	4180
September	1st to 10th	Day	13% 6% 1 6% 4 5% 4 4 6% 9% 42%	2280
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 4 5% 8% 45%	2195
	21st to end	Day	11% 6% 3 7% 5% 5% 5% 5% 7% 8% 38%	2090
October	1st to 10th	Day	19% 9% 4 8% 4 4 4 4 6% 7% 32%	1975
	11th to 20th	Day	13% 12% 3 8% 5% 4 4 5% 5% 6% 33%	1873
	21st to end	Day	19% 12% 3 9% 5% 4 4 5% 5% 5% 29%	1881
November	1st to 10th	Day	10% 13% 4 8% 7% 5% 5% 5% 6% 6% 32%	1761
	11th to 20th	Day	12% 11% 3 9% 7% 6% 5% 4 5% 7% 30%	1770
	21st to end	Day	13% 14% 5% 11% 7% 5% 5% 4 5% 6% 26%	1600
December	1st to 10th	Day	9% 19% 8% 19% 8% 6% 5% 3 4 4 14%	2600
	11th to 20th	Day	13% 12% 5% 12% 7% 5% 5% 4 5% 7% 24%	2986
	21st to end	Day	15% 15% 5% 11% 6% 4 4 4 5% 5% 26%	3960
January	1st to 10th	Night	10% 17% 7% 15% 7% 5% 5% 4 4 6% 19%	6349
	11th to 20th	Night	6% 13% 8% 18% 10% 6% 6% 4 5% 6% 18%	6315
	21st to end	Night	10% 12% 6% 14% 7% 6% 7% 4 6% 6% 20%	6996
February	1st to 10th	Night	9% 12% 5% 13% 8% 7% 6% 4 8% 6% 22%	2605
	11th to 20th	Night	8% 11% 5% 12% 8% 7% 6% 4 7% 6% 26%	2402
	21st to end	Night	10% 11% 5% 10% 8% 5% 6% 5% 6% 6% 29%	1885
March	1st to 10th	Night	18% 12% 4 9% 7% 5% 5% 4 5% 6% 27%	3724
	11th to 20th	Night	11% 14% 6% 12% 6% 5% 4 4 5% 6% 27%	3678
	21st to end	Night	11% 11% 5% 10% 6% 5% 4 3 4 4 37%	3466
April	1st to 10th	Night	14% 10% 4 9% 6% 6% 5% 3 5% 7% 31%	1900
	11th to 20th	Night	16% 11% 2 7% 6% 6% 4 4 5% 7% 31%	1875
	21st to end	Night	16% 10% 3 7% 5% 4 4 4 5% 6% 35%	1767
May	1st to 10th	Night	20% 14% 3 8% 5% 5% 4 4 6% 6% 24%	2679
	11th to 20th	Night	16% 10% 4 8% 4 4 4 3 5% 7% 34%	2495
	21st to end	Night	11% 7% 2 7% 4 5% 5% 4 6% 9% 39%	2738
June	1st to 10th	Night	18% 6% 3 6% 5% 4 5% 4 6% 9% 34%	1520
	11th to 20th	Night	15% 6% 3 7% 4 4 4 4 6% 8% 39%	1520
	21st to end	Night	14% 7% 2 7% 5% 4 5% 4 6% 8% 38%	1520
July	1st to 10th	Night	10% 7% 3 8% 6% 6% 5% 5% 6% 7% 37%	2520
	11th to 20th	Night	10% 5% 2 5% 4 4 5% 4 6% 8% 47%	2520
	21st to end	Night	8% 5% 2 7% 6% 5% 5% 4 7% 7% 43%	2739
August	1st to 10th	Night	12% 8% 2 7% 5% 4 5% 4 6% 8% 40%	2506
	11th to 20th	Night	10% 7% 3 8% 6% 4 5% 4 7% 7% 40%	2901
	21st to end	Night	9% 5% 2 6% 4 5% 5% 4 6% 9% 46%	3476
September	1st to 10th	Night	10% 5% 2 6% 5% 5% 4 3 6% 8% 47%	1900
	11th to 20th	Night	12% 6% 2 5% 3 4 3 3 5% 8% 49%	1985
	21st to end	Night	14% 5% 2 5% 5% 4 5% 4 6% 8% 43%	2090
October	1st to 10th	Night	18% 8% 3 7% 4 4 5% 4 5% 7% 35%	2203
	11th to 20th	Night	15% 8% 3 7% 6% 6% 5% 4 5% 7% 36%	2305
	21st to end	Night	20% 10% 3 8% 5% 4 5% 4 6% 7% 28%	2717
November	1st to 10th	Night	12% 10% 4 9% 6% 6% 5% 5% 6% 7% 31%	2537
	11th to 20th	Night	12% 11% 4 10% 6% 5% 5% 4 6% 6% 32%	2625
	21st to end	Night	14% 12% 5% 10% 6% 6% 5% 4 7% 6% 26%	2800
December	1st to 10th	Night	11% 15% 7% 17% 8% 6% 6% 4 6% 6% 14%	4580
	11th to 20th	Night	14% 15% 5% 12% 7% 5% 5% 3 5% 5% 24%	5208
	21st to end	Night	16% 15% 5% 12% 6% 5% 4 3 5% 5% 24%	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% 8% 9% 18% 4 1	3598
	11th to 20th	Day	66% 6% 9% 9% 8% 2	3600
	21st to end	Day	57% 6% 13% 13% 10% 1	3960
February	1st to 10th	Day	62% 8% 8% 15% 5% 2	1790
	11th to 20th	Day	57% 8% 10% 16% 9% 1	1893
	21st to end	Day	57% 6% 11% 14% 9% 2	1570
March	1st to 10th	Day	61% 7% 12% 12% 7% 2	3119
	11th to 20th	Day	56% 10% 12% 12% 8% 2	3278
	21st to end	Day	48% 6% 10% 15% 19% 3	4166
April	1st to 10th	Day	55% 6% 9% 18% 10% 3	2280
	11th to 20th	Day	51% 6% 9% 18% 15% 2	2254
	21st to end	Day	54% 5% 10% 16% 11% 3	2412
May	1st to 10th	Day	54% 8% 11% 17% 9% 1	4304
	11th to 20th	Day	48% 9% 17% 15% 9% 1	4402
	21st to end	Day	40% 9% 14% 24% 11% 2	4837
June	1st to 10th	Day	51% 8% 13% 16% 11% 1	2660
	11th to 20th	Day	45% 7% 13% 22% 12% 2	2660
	21st to end	Day	52% 8% 11% 19% 9% 1	2660
July	1st to 10th	Day	42% 7% 14% 25% 11%	4370
	11th to 20th	Day	40% 8% 16% 29% 7%	4365
	21st to end	Day	42% 8% 13% 28% 9%	4661
August	1st to 10th	Day	47% 7% 16% 21% 9% 1	4409
	11th to 20th	Day	43% 9% 19% 23% 6%	3932
	21st to end	Day	41% 7% 16% 24% 12% 1	4180
September	1st to 10th	Day	38% 7% 13% 29% 13% 1	2280
	11th to 20th	Day	36% 5% 13% 30% 14% 1	2195
	21st to end	Day	41% 9% 16% 23% 10% 2	2090
October	1st to 10th	Day	42% 5% 13% 27% 12%	3274
	11th to 20th	Day	45% 5% 16% 21% 14%	3089
	21st to end	Day	43% 5% 15% 28% 8% 1	3081
November	1st to 10th	Day	50% 8% 13% 18% 10% 1	1761
	11th to 20th	Day	52% 7% 12% 15% 12% 3	1770
	21st to end	Day	58% 7% 13% 16% 5% 1	1600
December	1st to 10th	Day	72% 5% 8% 12% 2	2600
	11th to 20th	Day	57% 6% 12% 16% 9%	2986
	21st to end	Day	58% 6% 9% 15% 11% 1	3960
January	1st to 10th	Night	64% 7% 12% 11% 6%	6349
	11th to 20th	Night	65% 7% 12% 10% 6% 1	6360
	21st to end	Night	61% 6% 11% 10% 11% 1	6996
February	1st to 10th	Night	57% 8% 13% 13% 8% 1	2605
	11th to 20th	Night	54% 8% 13% 16% 9% 1	2404
	21st to end	Night	54% 7% 12% 17% 9% 2	1885
March	1st to 10th	Night	56% 8% 11% 15% 9% 2	3734
	11th to 20th	Night	54% 8% 11% 16% 9% 1	3679
	21st to end	Night	48% 6% 9% 19% 15% 3	3466
April	1st to 10th	Night	52% 6% 9% 18% 13% 1	1900
	11th to 20th	Night	52% 7% 10% 18% 12% 1	1875
	21st to end	Night	49% 6% 11% 18% 13% 3	1767
May	1st to 10th	Night	54% 8% 10% 16% 9% 2	2685
	11th to 20th	Night	48% 8% 13% 20% 10% 1	2496
	21st to end	Night	38% 10% 14% 20% 16% 2	2739
June	1st to 10th	Night	46% 9% 12% 21% 10% 2	1520
	11th to 20th	Night	40% 9% 14% 22% 13% 1	1520
	21st to end	Night	43% 9% 14% 23% 10% 1	1520
July	1st to 10th	Night	43% 7% 13% 23% 13% 1	2480
	11th to 20th	Night	34% 8% 16% 32% 10%	2472
	21st to end	Night	37% 9% 17% 25% 11%	2651
August	1st to 10th	Night	42% 8% 16% 25% 9%	2496
	11th to 20th	Night	40% 9% 18% 23% 9%	2926
	21st to end	Night	34% 8% 17% 28% 13% 1	3476
September	1st to 10th	Night	34% 8% 16% 28% 13% 1	1900
	11th to 20th	Night	35% 5% 15% 31% 13% 1	1985
	21st to end	Night	37% 8% 15% 25% 13% 1	2090
October	1st to 10th	Night	45% 6% 13% 26% 10%	3671
	11th to 20th	Night	42% 5% 13% 24% 13% 2	3828
	21st to end	Night	47% 5% 15% 24% 9% 1	4467
November	1st to 10th	Night	50% 7% 14% 18% 10% 2	2537
	11th to 20th	Night	50% 7% 12% 18% 9% 3	2625
	21st to end	Night	55% 8% 14% 17% 6%	2800
December	1st to 10th	Night	68% 6% 11% 12% 2	4580
	11th to 20th	Night	60% 5% 9% 13% 12%	5208
	21st to end	Night	62% 6% 9% 13% 8% 1	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	20% 27% 32% 15% 7%	200
	11th to 20th	Day	25% 26% 34% 13% 2 1	200
	21st to end	Day	24% 23% 36% 13% 3 1	220
February	1st to 10th	Day	37% 32% 26% 4 1	295
	11th to 20th	Day	32% 26% 34% 6% 1	432
	21st to end	Day	31% 22% 32% 13% 2	382
March	1st to 10th	Day	28% 23% 30% 15% 4	460
	11th to 20th	Day	29% 26% 28% 13% 2 1	460
	21st to end	Day	28% 28% 25% 12% 6% 1	506
April	1st to 10th	Day	34% 20% 28% 15% 4 1	460
	11th to 20th	Day	33% 23% 24% 13% 5% 3	452
	21st to end	Day	34% 22% 26% 11% 6% 2	460
May	1st to 10th	Day	28% 20% 33% 11% 6% 2	463
	11th to 20th	Day	20% 19% 34% 20% 5% 2	487
	21st to end	Day	19% 19% 31% 22% 8% 1	529
June	1st to 10th	Day	26% 15% 33% 19% 5% 1	490
	11th to 20th	Day	18% 18% 35% 18% 10% 1	490
	21st to end	Day	21% 19% 33% 19% 6% 2	490
July	1st to 10th	Day	14% 13% 35% 25% 12% 1	485
	11th to 20th	Day	12% 11% 39% 27% 10% 1	484
	21st to end	Day	15% 11% 35% 30% 8% 2	532
August	1st to 10th	Day	13% 10% 33% 27% 14% 3	486
	11th to 20th	Day	13% 10% 33% 27% 13% 5%	479
	21st to end	Day	14% 14% 29% 28% 10% 5%	539
September	1st to 10th	Day	17% 13% 29% 23% 13% 6%	490
	11th to 20th	Day	13% 13% 24% 25% 16% 9%	490
	21st to end	Day	15% 13% 33% 23% 11% 5%	490
October	1st to 10th	Day	26% 15% 25% 20% 13% 2	490
	11th to 20th	Day	22% 21% 27% 20% 8% 1	475
	21st to end	Day	32% 14% 31% 18% 3 1	429
November	1st to 10th	Day	31% 15% 26% 20% 8% 1	395
	11th to 20th	Day	30% 22% 28% 14% 6% 1	370
	21st to end	Day	32% 24% 29% 14% 2	200
December	1st to 10th	Day	27% 25% 29% 14% 5% 1	200
	11th to 20th	Day	35% 22% 32% 9% 3	200
	21st to end	Day	28% 22% 32% 12% 5% 1	220
January	1st to 10th	Night	23% 21% 35% 18% 3	800
	11th to 20th	Night	25% 21% 33% 18% 3	800
	21st to end	Night	15% 22% 38% 21% 5%	880
February	1st to 10th	Night	19% 23% 38% 18% 2	800
	11th to 20th	Night	23% 22% 36% 18% 2	783
	21st to end	Night	20% 23% 34% 19% 3	629
March	1st to 10th	Night	26% 18% 31% 22% 3	760
	11th to 20th	Night	26% 24% 28% 17% 4 1	760
	21st to end	Night	24% 20% 29% 21% 5% 1	836
April	1st to 10th	Night	25% 20% 30% 21% 5%	760
	11th to 20th	Night	23% 25% 26% 18% 7% 1	751
	21st to end	Night	25% 19% 27% 22% 6%	627
May	1st to 10th	Night	28% 14% 29% 23% 6%	480
	11th to 20th	Night	17% 18% 32% 26% 6% 1	374
	21st to end	Night	13% 15% 30% 29% 12%	411
June	1st to 10th	Night	20% 11% 28% 30% 11%	380
	11th to 20th	Night	16% 9% 27% 35% 12% 1	380
	21st to end	Night	17% 10% 30% 31% 11% 1	380
July	1st to 10th	Night	12% 8% 28% 37% 15% 1	370
	11th to 20th	Night	11% 5% 26% 41% 14% 2	368
	21st to end	Night	11% 5% 25% 44% 14%	416
August	1st to 10th	Night	12% 5% 24% 39% 19% 1	378
	11th to 20th	Night	11% 6% 21% 39% 20% 3	626
	21st to end	Night	12% 7% 24% 37% 18% 1	836
September	1st to 10th	Night	12% 9% 25% 37% 15% 2	760
	11th to 20th	Night	13% 7% 24% 29% 22% 4	760
	21st to end	Night	17% 9% 29% 24% 18% 2	760
October	1st to 10th	Night	23% 13% 24% 26% 14% 1	760
	11th to 20th	Night	23% 15% 25% 28% 10% 1	760
	21st to end	Night	27% 16% 28% 23% 6%	836
November	1st to 10th	Night	22% 19% 29% 22% 8% 1	780
	11th to 20th	Night	23% 19% 31% 19% 7% 2	800
	21st to end	Night	24% 22% 29% 19% 6% 1	800
December	1st to 10th	Night	26% 22% 30% 18% 4	800
	11th to 20th	Night	25% 21% 30% 19% 3 1	800
	21st to end	Night	26% 22% 32% 17% 3	880

Page generated at 2025-02-18 03:26:33. Version ac07

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