Hillsdale Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hillsdale Lake clear sky chart from 2005-11-06 to 2026-07-16.

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	11% 7% 3 6% 4 4 4 3 5% 7% 46%	137589
	Night	11% 7% 3 6% 4 4 4 3 5% 6% 49%	135408
Transparency	Day	37% 5% 12% 26% 18% 3	141200
	Night	35% 5% 11% 26% 19% 4	139864
Seeing	Day	14% 10% 29% 28% 16% 4	17967
	Night	14% 9% 32% 33% 12% 1	27787

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	13% 10% 3 9% 4 4 4 3 4 5% 41%	5289
	11th to 20th	Day	6% 6% 3 9% 6% 6% 5% 3 6% 6% 44%	5310
	21st to end	Day	12% 12% 4 8% 4 3 4 3 4 4 42%	5812
February	1st to 10th	Day	14% 11% 4 7% 4 3 4 2 4 6% 40%	1890
	11th to 20th	Day	13% 9% 3 7% 5% 4 4 4 4 5% 42%	1849
	21st to end	Day	10% 8% 3 7% 4 4 3 3 6% 6% 46%	1798
March	1st to 10th	Day	12% 8% 3 6% 3 3 3 3 3 5% 51%	5046
	11th to 20th	Day	9% 5% 2 5% 3 3 3 2 5% 6% 56%	5090
	21st to end	Day	11% 10% 5% 8% 5% 5% 5% 3 5% 7% 36%	5583
April	1st to 10th	Day	14% 7% 3 7% 5% 3 3 3 5% 5% 45%	2410
	11th to 20th	Day	15% 7% 2 6% 5% 4 4 3 5% 6% 42%	2494
	21st to end	Day	18% 9% 3 8% 4 4 4 3 4 6% 36%	2677
May	1st to 10th	Day	15% 7% 2 6% 4 4 4 3 5% 6% 44%	7617
	11th to 20th	Day	12% 6% 2 5% 4 3 4 3 5% 8% 46%	7627
	21st to end	Day	20% 7% 2 6% 4 4 4 3 5% 7% 38%	8392
June	1st to 10th	Day	8% 4 2 5% 4 4 5% 5% 8% 9% 47%	2914
	11th to 20th	Day	7% 5% 2 5% 5% 4 5% 4 7% 10% 47%	2940
	21st to end	Day	7% 4 2 5% 4 4 5% 3 6% 9% 50%	2833
July	1st to 10th	Day	8% 6% 2 6% 5% 5% 5% 4 5% 8% 46%	6030
	11th to 20th	Day	8% 5% 2 6% 3 4 4 4 6% 10% 49%	5966
	21st to end	Day	7% 4 2 6% 5% 5% 6% 5% 7% 11% 42%	6422
August	1st to 10th	Day	5% 4 2 5% 4 4 5% 3 6% 9% 54%	4234
	11th to 20th	Day	10% 5% 2 6% 4 4 4 4 6% 7% 48%	4171
	21st to end	Day	5% 4 2 4 3 4 4 3 6% 8% 57%	4416
September	1st to 10th	Day	13% 4 2 5% 3 3 3 3 4 7% 53%	2393
	11th to 20th	Day	9% 5% 2 6% 4 5% 4 4 6% 8% 49%	2200
	21st to end	Day	10% 4 2 5% 3 3 4 3 5% 6% 55%	2200
October	1st to 10th	Day	11% 6% 2 4 3 3 3 3 5% 7% 54%	2000
	11th to 20th	Day	12% 6% 2 5% 2 2 3 2 4 5% 57%	2000
	21st to end	Day	17% 7% 2 5% 3 3 3 2 4 5% 49%	2200
November	1st to 10th	Day	10% 6% 1 5% 2 3 3 2 4 6% 56%	2053
	11th to 20th	Day	14% 6% 2 4 3 2 3 3 4 7% 53%	1862
	21st to end	Day	16% 7% 3 6% 3 3 3 3 4 6% 47%	1680
December	1st to 10th	Day	7% 6% 2 6% 4 3 3 3 5% 6% 55%	2690
	11th to 20th	Day	10% 7% 3 7% 4 3 3 3 5% 5% 50%	3088
	21st to end	Day	18% 16% 8% 10% 4 4 4 2 4 5% 26%	4413
January	1st to 10th	Night	11% 10% 4 12% 4 4 4 2 4 5% 40%	7634
	11th to 20th	Night	7% 7% 2 8% 5% 6% 4 3 4 5% 49%	7680
	21st to end	Night	13% 10% 4 8% 4 4 4 3 4 6% 40%	8402
February	1st to 10th	Night	11% 11% 2 7% 5% 4 4 3 5% 6% 41%	2730
	11th to 20th	Night	12% 9% 3 7% 4 4 3 3 4 7% 43%	2663
	21st to end	Night	12% 7% 3 5% 5% 4 4 3 4 6% 48%	2021
March	1st to 10th	Night	14% 8% 3 5% 3 2 3 3 4 5% 51%	5029
	11th to 20th	Night	7% 6% 2 4 3 3 3 3 6% 7% 57%	5090
	21st to end	Night	13% 8% 3 6% 4 4 4 3 6% 8% 41%	5588
April	1st to 10th	Night	12% 6% 2 6% 4 3 3 3 6% 6% 49%	2210
	11th to 20th	Night	13% 7% 3 6% 4 4 3 3 3 7% 47%	2075
	21st to end	Night	16% 6% 3 6% 4 4 4 3 4 7% 42%	1937
May	1st to 10th	Night	15% 6% 2 5% 3 3 4 3 4 6% 47%	5266
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 5% 7% 50%	5265
	21st to end	Night	17% 6% 2 5% 3 3 3 3 4 6% 46%	5793
June	1st to 10th	Night	7% 3 1 4 3 4 5% 4 7% 10% 52%	1696
	11th to 20th	Night	5% 4 2 5% 3 4 4 4 7% 10% 52%	1680
	21st to end	Night	5% 2 1 4 3 3 4 4 6% 9% 59%	1777
July	1st to 10th	Night	9% 5% 2 6% 3 3 4 3 6% 8% 51%	4168
	11th to 20th	Night	5% 4 2 5% 3 4 5% 4 6% 10% 54%	4122
	21st to end	Night	7% 5% 2 5% 3 4 5% 4 8% 12% 44%	4449
August	1st to 10th	Night	3 1 1 3 3 4 5% 4 7% 9% 60%	2926
	11th to 20th	Night	9% 5% 2 6% 3 3 4 3 5% 7% 52%	2888
	21st to end	Night	4 3 2 6% 4 4 4 3 5% 7% 60%	3483
September	1st to 10th	Night	10% 4 2 5% 3 3 3 3 4 6% 57%	2002
	11th to 20th	Night	9% 5% 2 4 3 3 4 4 6% 7% 54%	2200
	21st to end	Night	7% 4 1 5% 3 3 5% 3 4 6% 58%	2200
October	1st to 10th	Night	10% 4 1 3 3 3 4 3 4 5% 59%	2380
	11th to 20th	Night	12% 5% 2 5% 3 2 3 2 3 5% 58%	2400
	21st to end	Night	16% 7% 2 5% 3 3 3 2 3 4 52%	2640
November	1st to 10th	Night	9% 6% 2 4 3 3 3 2 4 5% 61%	2461
	11th to 20th	Night	10% 6% 2 5% 3 3 3 3 5% 7% 54%	2755
	21st to end	Night	15% 7% 2 5% 3 3 2 2 4 4 52%	2940
December	1st to 10th	Night	7% 6% 2 5% 3 3 3 4 5% 6% 56%	4720
	11th to 20th	Night	12% 7% 3 6% 3 3 4 3 5% 6% 46%	5339
	21st to end	Night	19% 15% 5% 9% 5% 4 3 3 3 4 31%	6799

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	46% 4 10% 27% 12% 2	5290
	11th to 20th	Day	42% 3 10% 25% 16% 4	5310
	21st to end	Day	46% 3 11% 19% 17% 3	5812
February	1st to 10th	Day	46% 5% 12% 24% 11% 2	1890
	11th to 20th	Day	45% 5% 12% 22% 14% 3	1858
	21st to end	Day	39% 4 10% 24% 19% 4	1798
March	1st to 10th	Day	37% 3 8% 20% 23% 8%	5046
	11th to 20th	Day	29% 5% 9% 21% 27% 8%	5090
	21st to end	Day	46% 6% 15% 26% 6% 1	5585
April	1st to 10th	Day	41% 5% 12% 23% 17% 3	2410
	11th to 20th	Day	42% 6% 12% 21% 18% 1	2494
	21st to end	Day	50% 5% 11% 23% 10% 1	2677
May	1st to 10th	Day	41% 6% 9% 25% 18% 1	7655
	11th to 20th	Day	35% 6% 11% 24% 20% 3	7631
	21st to end	Day	46% 6% 11% 24% 13% 1	8405
June	1st to 10th	Day	30% 10% 18% 26% 16% 2	2914
	11th to 20th	Day	31% 9% 14% 28% 16% 2	2940
	21st to end	Day	29% 7% 16% 28% 17% 3	2833
July	1st to 10th	Day	35% 6% 13% 24% 20% 2	5966
	11th to 20th	Day	31% 5% 14% 32% 16% 2	5897
	21st to end	Day	33% 6% 16% 34% 10% 1	6240
August	1st to 10th	Day	27% 7% 15% 28% 21% 3	4220
	11th to 20th	Day	35% 5% 18% 26% 15% 1	4210
	21st to end	Day	26% 5% 15% 35% 15% 4	4415
September	1st to 10th	Day	32% 5% 13% 26% 18% 7%	2393
	11th to 20th	Day	32% 7% 11% 25% 22% 2	2200
	21st to end	Day	29% 6% 9% 27% 25% 3	2200
October	1st to 10th	Day	19% 3 7% 22% 41% 7%	3260
	11th to 20th	Day	20% 2 5% 26% 38% 9%	3240
	21st to end	Day	32% 4 10% 25% 25% 4	3538
November	1st to 10th	Day	30% 5% 11% 30% 19% 5%	2053
	11th to 20th	Day	34% 3 11% 27% 21% 4	1862
	21st to end	Day	40% 3 10% 27% 19% 1	1680
December	1st to 10th	Day	31% 3 12% 37% 16% 1	2690
	11th to 20th	Day	36% 5% 11% 32% 15% 1	3085
	21st to end	Day	61% 4 10% 17% 7% 1	4413
January	1st to 10th	Night	48% 4 12% 24% 11% 1	7635
	11th to 20th	Night	39% 4 8% 26% 20% 4	7680
	21st to end	Night	46% 3 11% 19% 16% 4	8402
February	1st to 10th	Night	44% 5% 12% 22% 15% 3	2730
	11th to 20th	Night	42% 5% 11% 23% 16% 3	2664
	21st to end	Night	37% 4 9% 22% 21% 7%	2021
March	1st to 10th	Night	36% 4 8% 22% 25% 5%	5029
	11th to 20th	Night	26% 4 8% 22% 33% 8%	5090
	21st to end	Night	41% 7% 16% 23% 11% 2	5588
April	1st to 10th	Night	35% 6% 11% 22% 22% 4	2210
	11th to 20th	Night	39% 4 9% 26% 18% 4	2075
	21st to end	Night	42% 6% 12% 25% 14% 1	1937
May	1st to 10th	Night	38% 6% 11% 26% 17% 2	5293
	11th to 20th	Night	35% 5% 12% 24% 19% 5%	5266
	21st to end	Night	40% 6% 10% 25% 17% 1	5805
June	1st to 10th	Night	25% 9% 17% 30% 16% 3	1696
	11th to 20th	Night	25% 8% 15% 30% 20% 2	1680
	21st to end	Night	22% 5% 15% 32% 22% 4	1777
July	1st to 10th	Night	30% 6% 14% 25% 22% 3	4125
	11th to 20th	Night	26% 5% 15% 34% 17% 2	4071
	21st to end	Night	30% 6% 16% 36% 11% 1	4320
August	1st to 10th	Night	18% 7% 19% 29% 20% 7%	2916
	11th to 20th	Night	33% 4 14% 28% 19% 2	2915
	21st to end	Night	24% 5% 12% 37% 17% 4	3483
September	1st to 10th	Night	29% 5% 10% 27% 20% 8%	2002
	11th to 20th	Night	29% 6% 12% 25% 24% 4	2200
	21st to end	Night	26% 5% 8% 25% 30% 5%	2200
October	1st to 10th	Night	18% 3 5% 20% 46% 8%	3887
	11th to 20th	Night	22% 3 6% 25% 31% 13%	3899
	21st to end	Night	33% 3 7% 29% 25% 3	4252
November	1st to 10th	Night	28% 3 11% 28% 24% 6%	2461
	11th to 20th	Night	31% 4 9% 27% 24% 5%	2755
	21st to end	Night	37% 3 9% 29% 19% 3	2940
December	1st to 10th	Night	29% 4 14% 33% 19% 1	4720
	11th to 20th	Night	38% 4 11% 27% 17% 2	5341
	21st to end	Night	58% 4 9% 21% 7% 1	6799

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	18% 15% 35% 26% 5%	330
	11th to 20th	Day	19% 20% 40% 19% 2	330
	21st to end	Day	14% 11% 50% 22% 3	363
February	1st to 10th	Day	22% 16% 39% 20% 3	330
	11th to 20th	Day	21% 16% 41% 17% 5%	330
	21st to end	Day	14% 23% 43% 16% 3	273
March	1st to 10th	Day	19% 17% 42% 17% 4	330
	11th to 20th	Day	21% 18% 33% 21% 6%	330
	21st to end	Day	21% 25% 38% 13% 2 1	363
April	1st to 10th	Day	21% 19% 36% 18% 6% 1	430
	11th to 20th	Day	18% 17% 40% 19% 5%	532
	21st to end	Day	21% 16% 35% 21% 6% 1	697
May	1st to 10th	Day	19% 13% 35% 23% 9% 1	753
	11th to 20th	Day	18% 12% 30% 25% 12% 3	775
	21st to end	Day	16% 10% 29% 25% 15% 4	844
June	1st to 10th	Day	7% 5% 22% 34% 24% 7%	780
	11th to 20th	Day	7% 5% 27% 37% 20% 4	780
	21st to end	Day	7% 7% 33% 35% 17% 2	780
July	1st to 10th	Day	8% 4 15% 32% 30% 11%	771
	11th to 20th	Day	5% 3 16% 39% 30% 7%	744
	21st to end	Day	8% 3 16% 34% 31% 8%	794
August	1st to 10th	Day	7% 3 15% 40% 29% 6%	726
	11th to 20th	Day	8% 3 16% 37% 31% 6%	715
	21st to end	Day	9% 2 13% 30% 35% 10%	648
September	1st to 10th	Day	14% 2 13% 35% 29% 8%	523
	11th to 20th	Day	5% 3 18% 38% 28% 7%	330
	21st to end	Day	13% 6% 22% 36% 16% 7%	330
October	1st to 10th	Day	12% 5% 29% 28% 18% 8%	330
	11th to 20th	Day	12% 15% 37% 25% 10% 1	330
	21st to end	Day	23% 13% 35% 22% 5% 1	363
November	1st to 10th	Day	16% 10% 38% 27% 7% 2	330
	11th to 20th	Day	15% 16% 34% 23% 11% 1	330
	21st to end	Day	22% 12% 36% 22% 8%	330
December	1st to 10th	Day	9% 15% 42% 26% 5% 3	330
	11th to 20th	Day	16% 16% 37% 22% 8% 1	330
	21st to end	Day	21% 19% 37% 20% 2	363
January	1st to 10th	Night	13% 13% 40% 29% 6%	1050
	11th to 20th	Night	14% 14% 41% 26% 4 1	1050
	21st to end	Night	13% 13% 41% 29% 3	1155
February	1st to 10th	Night	17% 11% 44% 25% 3	1050
	11th to 20th	Night	17% 15% 40% 25% 3	1029
	21st to end	Night	17% 13% 44% 23% 3	637
March	1st to 10th	Night	16% 11% 44% 25% 4	630
	11th to 20th	Night	19% 11% 38% 28% 4	630
	21st to end	Night	23% 13% 37% 24% 3	693
April	1st to 10th	Night	15% 12% 42% 26% 5%	630
	11th to 20th	Night	17% 15% 35% 28% 5%	623
	21st to end	Night	19% 12% 37% 27% 6%	630
May	1st to 10th	Night	18% 7% 34% 32% 8%	630
	11th to 20th	Night	15% 7% 31% 33% 13%	621
	21st to end	Night	13% 6% 29% 36% 15% 2	683
June	1st to 10th	Night	7% 4 19% 44% 24% 1	630
	11th to 20th	Night	5% 3 28% 45% 19% 1	630
	21st to end	Night	7% 3 31% 46% 13%	630
July	1st to 10th	Night	7% 2 13% 45% 31% 2	615
	11th to 20th	Night	4 1 15% 47% 29% 3	597
	21st to end	Night	8% 1 13% 43% 33% 2	657
August	1st to 10th	Night	4 1 16% 48% 29% 2	597
	11th to 20th	Night	8% 2 11% 45% 32% 3	594
	21st to end	Night	7% 1 13% 40% 38% 2	660
September	1st to 10th	Night	11% 2 19% 42% 25% 2	600
	11th to 20th	Night	10% 2 20% 39% 26% 4	600
	21st to end	Night	7% 5% 21% 44% 21% 3	600
October	1st to 10th	Night	12% 4 26% 36% 20% 2	780
	11th to 20th	Night	16% 7% 40% 30% 8% 1	800
	21st to end	Night	19% 11% 34% 28% 8%	880
November	1st to 10th	Night	12% 8% 38% 31% 11% 1	821
	11th to 20th	Night	12% 13% 37% 30% 8%	1050
	21st to end	Night	19% 12% 34% 29% 6%	1050
December	1st to 10th	Night	16% 11% 39% 28% 6%	1050
	11th to 20th	Night	17% 11% 40% 27% 5%	1050
	21st to end	Night	19% 14% 36% 28% 4	1155

Page generated at 2026-07-13 02:14:52. Version ac07

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