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 2026-06-16.

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 10% 6% 6% 5% 4 6% 7% 29%	137721
	Night	12% 11% 4 10% 6% 5% 5% 4 6% 7% 29%	134332
Transparency	Day	51% 6% 12% 19% 10% 1	141180
	Night	52% 7% 12% 18% 10% 1	138949
Seeing	Day	24% 19% 30% 18% 7% 2	16658
	Night	21% 17% 29% 24% 8% 1	25694

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	8% 12% 6% 16% 10% 7% 6% 4 5% 6% 20%	4661
	11th to 20th	Day	5% 16% 7% 19% 10% 8% 6% 3 4 4 19%	4650
	21st to end	Day	8% 11% 5% 13% 10% 8% 8% 5% 5% 6% 22%	5124
February	1st to 10th	Day	10% 14% 4 14% 9% 8% 6% 4 6% 5% 21%	1880
	11th to 20th	Day	8% 11% 4 13% 9% 8% 6% 5% 5% 6% 25%	1981
	21st to end	Day	11% 13% 6% 11% 8% 6% 5% 4 5% 5% 25%	1730
March	1st to 10th	Day	15% 13% 4 13% 7% 6% 4 3 5% 5% 25%	4576
	11th to 20th	Day	9% 13% 4 11% 7% 7% 6% 4 5% 6% 28%	4797
	21st to end	Day	13% 11% 4 9% 6% 5% 5% 4 4 5% 32%	6102
April	1st to 10th	Day	16% 13% 3 9% 6% 5% 4 3 4 6% 29%	2520
	11th to 20th	Day	16% 11% 3 9% 6% 5% 4 3 4 5% 33%	2494
	21st to end	Day	19% 11% 3 8% 5% 4 4 3 5% 7% 31%	2666
May	1st to 10th	Day	23% 12% 4 9% 5% 5% 5% 4 5% 6% 22%	7989
	11th to 20th	Day	16% 9% 3 8% 5% 6% 6% 4 6% 7% 29%	8221
	21st to end	Day	19% 9% 3 7% 5% 5% 6% 5% 6% 7% 29%	9060
June	1st to 10th	Day	21% 9% 2 7% 5% 5% 5% 4 6% 7% 29%	2940
	11th to 20th	Day	13% 8% 3 7% 5% 5% 6% 5% 6% 8% 34%	2870
	21st to end	Day	19% 7% 3 8% 6% 5% 5% 5% 6% 8% 29%	2800
July	1st to 10th	Day	11% 6% 2 8% 6% 6% 6% 6% 8% 9% 33%	6360
	11th to 20th	Day	8% 5% 2 7% 5% 6% 7% 6% 8% 9% 36%	6360
	21st to end	Day	11% 6% 2 6% 4 5% 5% 5% 7% 9% 38%	6926
August	1st to 10th	Day	13% 7% 2 8% 6% 6% 6% 5% 7% 9% 31%	4563
	11th to 20th	Day	10% 6% 3 9% 5% 5% 6% 6% 8% 11% 30%	4016
	21st to end	Day	13% 7% 2 7% 5% 5% 5% 5% 7% 9% 38%	4312
September	1st to 10th	Day	13% 6% 1 5% 4 5% 4 4 5% 9% 43%	2400
	11th to 20th	Day	11% 6% 2 6% 4 5% 5% 4 5% 8% 46%	2310
	21st to end	Day	12% 6% 3 7% 5% 5% 5% 5% 6% 8% 38%	2200
October	1st to 10th	Day	18% 8% 4 7% 4 4 4 4 6% 6% 35%	2079
	11th to 20th	Day	12% 12% 3 8% 5% 4 4 5% 5% 7% 34%	1971
	21st to end	Day	19% 12% 3 9% 5% 4 4 5% 5% 5% 29%	1980
November	1st to 10th	Day	10% 13% 4 9% 7% 5% 5% 4 6% 6% 31%	1851
	11th to 20th	Day	12% 11% 3 9% 7% 6% 5% 4 5% 7% 30%	1858
	21st to end	Day	13% 14% 5% 11% 7% 5% 5% 4 5% 6% 26%	1680
December	1st to 10th	Day	9% 18% 8% 19% 7% 6% 5% 3 4 4 15%	2680
	11th to 20th	Day	13% 11% 5% 12% 7% 5% 5% 4 5% 7% 25%	3066
	21st to end	Day	15% 15% 5% 12% 6% 5% 4 4 5% 5% 26%	4048
January	1st to 10th	Night	9% 15% 6% 15% 8% 6% 6% 4 5% 6% 20%	8234
	11th to 20th	Night	6% 14% 8% 18% 10% 7% 6% 4 5% 6% 16%	8235
	21st to end	Night	9% 10% 5% 13% 9% 8% 8% 5% 6% 7% 20%	9057
February	1st to 10th	Night	8% 12% 5% 13% 8% 7% 6% 4 8% 7% 22%	2735
	11th to 20th	Night	8% 11% 4 12% 8% 7% 6% 4 6% 6% 26%	2525
	21st to end	Night	9% 11% 5% 11% 8% 6% 6% 5% 6% 6% 28%	2076
March	1st to 10th	Night	16% 11% 4 10% 8% 7% 5% 4 5% 5% 25%	5484
	11th to 20th	Night	10% 11% 5% 11% 6% 5% 5% 4 5% 8% 31%	5378
	21st to end	Night	13% 11% 5% 12% 7% 5% 4 3 5% 4 30%	5072
April	1st to 10th	Night	15% 11% 4 9% 6% 6% 5% 3 4 7% 31%	2100
	11th to 20th	Night	16% 12% 3 7% 6% 6% 4 4 5% 7% 30%	2075
	21st to end	Night	15% 10% 3 7% 6% 4 4 4 5% 6% 36%	1953
May	1st to 10th	Night	22% 14% 3 8% 5% 5% 4 4 6% 6% 22%	4965
	11th to 20th	Night	15% 10% 3 7% 4 4 4 4 6% 9% 33%	4639
	21st to end	Night	16% 9% 3 8% 5% 5% 6% 4 6% 8% 31%	5114
June	1st to 10th	Night	18% 7% 3 6% 5% 4 5% 4 6% 9% 35%	1680
	11th to 20th	Night	14% 6% 3 7% 4 4 4 4 6% 8% 39%	1640
	21st to end	Night	14% 6% 2 7% 5% 4 5% 4 6% 8% 39%	1600
July	1st to 10th	Night	9% 6% 2 7% 5% 5% 5% 5% 7% 10% 39%	3600
	11th to 20th	Night	10% 6% 2 6% 4 5% 5% 4 6% 8% 45%	3600
	21st to end	Night	7% 5% 2 6% 5% 4 5% 4 6% 7% 47%	3926
August	1st to 10th	Night	12% 8% 2 7% 5% 4 5% 4 6% 8% 40%	2586
	11th to 20th	Night	10% 7% 3 8% 6% 4 5% 3 7% 7% 41%	2995
	21st to end	Night	9% 5% 2 5% 4 5% 4 4 6% 9% 46%	3586
September	1st to 10th	Night	10% 6% 2 6% 5% 5% 3 3 6% 8% 48%	2000
	11th to 20th	Night	11% 6% 2 5% 4 4 3 3 5% 8% 49%	2090
	21st to end	Night	14% 6% 2 5% 5% 4 4 4 6% 7% 43%	2200
October	1st to 10th	Night	17% 7% 3 7% 4 3 5% 4 5% 7% 37%	2319
	11th to 20th	Night	15% 8% 3 7% 5% 5% 5% 4 5% 7% 36%	2427
	21st to end	Night	19% 10% 3 8% 5% 4 5% 4 6% 7% 28%	2860
November	1st to 10th	Night	12% 10% 4 9% 6% 6% 5% 5% 6% 6% 31%	2667
	11th to 20th	Night	12% 11% 4 10% 6% 5% 5% 4 6% 6% 31%	2756
	21st to end	Night	13% 12% 5% 10% 6% 6% 5% 4 7% 6% 25%	2940
December	1st to 10th	Night	11% 15% 7% 17% 8% 6% 6% 4 6% 6% 15%	4720
	11th to 20th	Night	14% 15% 5% 12% 7% 5% 5% 3 5% 5% 24%	5348
	21st to end	Night	15% 15% 5% 12% 6% 5% 4 3 5% 5% 24%	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	62% 7% 11% 16% 3	4662
	11th to 20th	Day	67% 5% 8% 8% 9% 2	4680
	21st to end	Day	59% 5% 11% 14% 10% 1	5124
February	1st to 10th	Day	61% 7% 8% 16% 6% 2	1880
	11th to 20th	Day	57% 8% 10% 16% 9% 1	1990
	21st to end	Day	58% 6% 11% 14% 10% 2	1730
March	1st to 10th	Day	59% 6% 10% 11% 9% 5%	4579
	11th to 20th	Day	53% 8% 14% 14% 8% 3	4797
	21st to end	Day	53% 5% 8% 12% 18% 3	6102
April	1st to 10th	Day	56% 5% 8% 17% 11% 3	2520
	11th to 20th	Day	52% 6% 8% 18% 15% 1	2494
	21st to end	Day	53% 5% 10% 18% 12% 3	2666
May	1st to 10th	Day	61% 6% 9% 14% 9% 1	8001
	11th to 20th	Day	50% 7% 15% 18% 9% 1	8225
	21st to end	Day	51% 7% 11% 22% 9% 1	9061
June	1st to 10th	Day	50% 8% 13% 17% 11% 1	2940
	11th to 20th	Day	45% 7% 13% 22% 11% 2	2870
	21st to end	Day	51% 7% 11% 20% 9% 1	2800
July	1st to 10th	Day	44% 7% 14% 23% 12%	6290
	11th to 20th	Day	39% 8% 15% 26% 11% 1	6285
	21st to end	Day	39% 7% 12% 33% 8%	6728
August	1st to 10th	Day	46% 8% 15% 21% 9% 1	4549
	11th to 20th	Day	43% 9% 19% 24% 6%	4057
	21st to end	Day	41% 7% 16% 24% 12% 1	4312
September	1st to 10th	Day	37% 6% 13% 31% 12% 1	2400
	11th to 20th	Day	36% 5% 13% 31% 15% 1	2310
	21st to end	Day	41% 9% 15% 23% 11% 2	2200
October	1st to 10th	Day	41% 5% 13% 27% 13%	3378
	11th to 20th	Day	44% 5% 15% 21% 15%	3187
	21st to end	Day	43% 5% 15% 28% 8% 1	3180
November	1st to 10th	Day	51% 7% 13% 18% 10% 1	1851
	11th to 20th	Day	52% 7% 12% 15% 11% 3	1858
	21st to end	Day	58% 6% 13% 17% 5% 1	1680
December	1st to 10th	Day	72% 5% 8% 12% 3	2680
	11th to 20th	Day	56% 6% 12% 17% 9%	3066
	21st to end	Day	58% 6% 9% 15% 11% 1	4048
January	1st to 10th	Night	63% 6% 12% 12% 6% 1	8234
	11th to 20th	Night	67% 6% 11% 9% 7% 1	8280
	21st to end	Night	60% 6% 11% 12% 11% 1	9057
February	1st to 10th	Night	57% 8% 13% 13% 8% 1	2735
	11th to 20th	Night	54% 8% 12% 17% 8% 1	2527
	21st to end	Night	54% 7% 11% 17% 9% 1	2076
March	1st to 10th	Night	59% 6% 9% 13% 11% 2	5494
	11th to 20th	Night	50% 8% 16% 16% 9% 2	5379
	21st to end	Night	54% 6% 8% 17% 13% 2	5072
April	1st to 10th	Night	52% 6% 9% 18% 13% 1	2100
	11th to 20th	Night	53% 6% 10% 17% 12% 1	2075
	21st to end	Night	48% 6% 10% 19% 14% 3	1953
May	1st to 10th	Night	60% 6% 10% 15% 9% 1	4971
	11th to 20th	Night	47% 7% 12% 23% 9% 1	4640
	21st to end	Night	48% 7% 12% 20% 12% 1	5115
June	1st to 10th	Night	46% 9% 11% 21% 12% 2	1680
	11th to 20th	Night	40% 9% 14% 22% 13% 1	1640
	21st to end	Night	42% 9% 14% 23% 11% 1	1600
July	1st to 10th	Night	39% 6% 17% 26% 11%	3560
	11th to 20th	Night	36% 8% 15% 27% 14%	3552
	21st to end	Night	34% 8% 16% 32% 10%	3809
August	1st to 10th	Night	42% 8% 16% 25% 9%	2576
	11th to 20th	Night	40% 9% 18% 24% 9%	3020
	21st to end	Night	33% 8% 17% 29% 13% 1	3586
September	1st to 10th	Night	34% 8% 15% 29% 13% 1	2000
	11th to 20th	Night	34% 5% 15% 32% 13%	2090
	21st to end	Night	37% 8% 15% 25% 14% 1	2200
October	1st to 10th	Night	44% 6% 12% 26% 11% 1	3787
	11th to 20th	Night	42% 5% 13% 24% 14% 2	3950
	21st to end	Night	47% 5% 15% 24% 9% 1	4610
November	1st to 10th	Night	51% 6% 14% 18% 9% 2	2667
	11th to 20th	Night	51% 7% 13% 18% 9% 3	2756
	21st to end	Night	55% 8% 14% 16% 6%	2940
December	1st to 10th	Night	68% 6% 10% 13% 3	4720
	11th to 20th	Night	60% 5% 9% 13% 12%	5348
	21st to end	Night	62% 6% 9% 13% 8% 1	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	20% 27% 32% 15% 6%	220
	11th to 20th	Day	27% 26% 34% 11% 2	220
	21st to end	Day	25% 23% 36% 13% 2 1	242
February	1st to 10th	Day	34% 31% 27% 6% 2	325
	11th to 20th	Day	32% 26% 34% 7% 1	469
	21st to end	Day	30% 23% 33% 12% 1	446
March	1st to 10th	Day	29% 26% 29% 14% 3	540
	11th to 20th	Day	32% 24% 28% 13% 2 1	540
	21st to end	Day	28% 30% 25% 11% 5% 1	594
April	1st to 10th	Day	37% 19% 26% 13% 4 1	540
	11th to 20th	Day	33% 23% 25% 12% 4 2	532
	21st to end	Day	33% 22% 27% 10% 6% 1	540
May	1st to 10th	Day	32% 20% 31% 10% 5% 2	543
	11th to 20th	Day	22% 20% 33% 18% 5% 2	567
	21st to end	Day	21% 20% 31% 20% 7% 1	617
June	1st to 10th	Day	26% 15% 32% 20% 5% 1	570
	11th to 20th	Day	19% 18% 35% 17% 9% 1	550
	21st to end	Day	21% 19% 34% 19% 6% 2	530
July	1st to 10th	Day	15% 13% 36% 24% 12% 1	525
	11th to 20th	Day	12% 10% 39% 27% 10% 1	524
	21st to end	Day	15% 11% 35% 29% 8% 2	576
August	1st to 10th	Day	12% 10% 33% 29% 14% 3	526
	11th to 20th	Day	13% 10% 32% 27% 13% 6%	519
	21st to end	Day	14% 14% 29% 28% 10% 5%	583
September	1st to 10th	Day	16% 12% 29% 22% 14% 6%	530
	11th to 20th	Day	12% 12% 25% 26% 16% 8%	530
	21st to end	Day	17% 13% 33% 22% 10% 5%	530
October	1st to 10th	Day	24% 14% 25% 20% 14% 2	530
	11th to 20th	Day	22% 21% 28% 20% 8% 1	513
	21st to end	Day	31% 15% 31% 19% 3 1	462
November	1st to 10th	Day	33% 15% 25% 19% 7%	425
	11th to 20th	Day	29% 22% 28% 13% 6% 2	398
	21st to end	Day	33% 23% 29% 13% 2	220
December	1st to 10th	Day	26% 25% 30% 13% 5%	220
	11th to 20th	Day	35% 23% 32% 8% 2	220
	21st to end	Day	28% 22% 32% 12% 4 1	242
January	1st to 10th	Night	23% 21% 35% 18% 3	840
	11th to 20th	Night	26% 20% 33% 18% 3	840
	21st to end	Night	15% 22% 37% 20% 5%	924
February	1st to 10th	Night	18% 22% 38% 19% 3	840
	11th to 20th	Night	22% 21% 36% 19% 2	823
	21st to end	Night	20% 22% 35% 20% 3	692
March	1st to 10th	Night	27% 19% 31% 20% 2	840
	11th to 20th	Night	27% 24% 27% 17% 4 1	840
	21st to end	Night	25% 21% 29% 19% 5% 1	924
April	1st to 10th	Night	27% 21% 29% 20% 4	840
	11th to 20th	Night	25% 24% 26% 18% 6%	831
	21st to end	Night	24% 19% 29% 21% 6%	693
May	1st to 10th	Night	29% 14% 29% 22% 6%	530
	11th to 20th	Night	18% 18% 32% 26% 6% 1	414
	21st to end	Night	15% 16% 30% 28% 11%	455
June	1st to 10th	Night	19% 11% 28% 31% 10%	420
	11th to 20th	Night	16% 10% 27% 34% 11%	410
	21st to end	Night	16% 10% 31% 31% 11% 1	400
July	1st to 10th	Night	12% 8% 27% 38% 14% 1	390
	11th to 20th	Night	11% 5% 26% 42% 14% 2	388
	21st to end	Night	11% 5% 25% 45% 13%	438
August	1st to 10th	Night	12% 5% 23% 40% 19% 1	398
	11th to 20th	Night	11% 6% 21% 39% 21% 2	660
	21st to end	Night	12% 7% 24% 38% 17% 1	880
September	1st to 10th	Night	12% 9% 25% 37% 15% 2	800
	11th to 20th	Night	13% 7% 24% 30% 23% 4	800
	21st to end	Night	17% 9% 29% 25% 18% 2	800
October	1st to 10th	Night	23% 12% 23% 27% 14% 1	800
	11th to 20th	Night	22% 14% 26% 28% 9% 1	800
	21st to end	Night	27% 15% 29% 24% 5%	880
November	1st to 10th	Night	22% 19% 29% 22% 7%	820
	11th to 20th	Night	23% 19% 31% 18% 7% 2	840
	21st to end	Night	24% 22% 29% 20% 5% 1	840
December	1st to 10th	Night	25% 22% 31% 18% 4	840
	11th to 20th	Night	26% 21% 31% 18% 3 1	840
	21st to end	Night	27% 22% 31% 16% 3	924

Page generated at 2026-06-13 02:31:39. Version ac07

Copyright © 2026 Attilla Danko.