Hanover College
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hanover College clear sky chart from 2005-11-06 to 2025-10-04.

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	14% 8% 3 7% 5% 4 5% 4 6% 7% 37%	126163
	Night	15% 9% 3 7% 5% 4 4 3 5% 7% 38%	123307
Transparency	Day	28% 39% 7% 15% 10% 1	129805
	Night	29% 39% 7% 14% 10% 1	127736
Seeing	Day	18% 14% 27% 22% 13% 5%	16643
	Night	18% 11% 27% 28% 14% 2	26344

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	15% 16% 8% 8% 4 3 4 3 5% 6% 29%	3617
	11th to 20th	Day	14% 13% 5% 14% 4 5% 4 2 3 4 32%	3680
	21st to end	Day	21% 12% 6% 7% 5% 4 4 3 5% 5% 28%	4466
February	1st to 10th	Day	17% 13% 4 10% 4 3 3 2 5% 5% 33%	1991
	11th to 20th	Day	17% 10% 4 9% 5% 5% 4 3 4 5% 34%	1948
	21st to end	Day	16% 14% 4 8% 4 3 3 4 4 6% 32%	1650
March	1st to 10th	Day	20% 12% 4 7% 4 3 3 2 4 4 36%	4480
	11th to 20th	Day	13% 9% 3 6% 4 4 4 3 6% 8% 41%	4904
	21st to end	Day	17% 8% 3 7% 5% 4 5% 5% 6% 7% 34%	5551
April	1st to 10th	Day	16% 9% 2 6% 4 5% 4 4 6% 7% 37%	2400
	11th to 20th	Day	17% 8% 3 6% 4 5% 4 4 5% 6% 39%	2374
	21st to end	Day	17% 7% 3 7% 4 4 4 4 6% 8% 36%	2588
May	1st to 10th	Day	20% 9% 3 7% 5% 5% 5% 5% 7% 9% 26%	5862
	11th to 20th	Day	19% 8% 3 7% 5% 4 4 3 5% 7% 34%	5861
	21st to end	Day	14% 6% 3 7% 6% 6% 6% 5% 8% 10% 31%	6433
June	1st to 10th	Day	10% 5% 2 6% 6% 5% 5% 4 5% 8% 44%	2687
	11th to 20th	Day	8% 5% 2 6% 5% 5% 6% 4 6% 8% 44%	2800
	21st to end	Day	8% 6% 2 6% 5% 5% 5% 4 6% 10% 43%	2800
July	1st to 10th	Day	11% 6% 2 6% 4 4 5% 4 6% 10% 41%	6360
	11th to 20th	Day	9% 6% 2 7% 5% 5% 6% 5% 7% 10% 38%	6324
	21st to end	Day	10% 7% 3 10% 6% 6% 7% 5% 8% 9% 30%	6424
August	1st to 10th	Day	7% 5% 2 6% 4 4 5% 4 7% 10% 47%	4234
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 5% 8% 10% 41%	4021
	21st to end	Day	5% 4 2 5% 3 4 4 3 6% 8% 56%	4312
September	1st to 10th	Day	12% 5% 2 6% 4 4 4 3 5% 7% 48%	2324
	11th to 20th	Day	8% 4 2 6% 4 5% 5% 4 5% 8% 49%	2200
	21st to end	Day	14% 6% 2 5% 4 4 3 3 5% 6% 48%	2200
October	1st to 10th	Day	10% 4 2 5% 4 4 4 3 4 5% 54%	2123
	11th to 20th	Day	12% 6% 2 6% 4 3 4 3 5% 7% 47%	1990
	21st to end	Day	18% 9% 2 6% 4 4 4 3 5% 5% 40%	1885
November	1st to 10th	Day	11% 6% 2 5% 3 3 3 3 4 6% 54%	1762
	11th to 20th	Day	16% 11% 3 6% 4 4 3 3 6% 6% 39%	1800
	21st to end	Day	19% 9% 4 6% 4 3 3 3 4 6% 39%	1800
December	1st to 10th	Day	18% 9% 3 6% 4 3 3 2 4 4 43%	2930
	11th to 20th	Day	16% 15% 5% 9% 5% 5% 5% 4 4 5% 28%	3373
	21st to end	Day	23% 18% 5% 8% 5% 4 4 3 4 5% 21%	4009
January	1st to 10th	Night	16% 16% 5% 13% 5% 5% 5% 3 5% 6% 22%	6348
	11th to 20th	Night	15% 15% 6% 12% 7% 5% 4 2 3 4 26%	6292
	21st to end	Night	20% 12% 4 7% 5% 3 4 3 4 5% 33%	6490
February	1st to 10th	Night	21% 13% 4 8% 5% 4 4 4 5% 5% 28%	2405
	11th to 20th	Night	18% 12% 3 9% 5% 4 4 3 4 5% 33%	2338
	21st to end	Night	20% 12% 4 8% 3 4 4 3 4 5% 32%	1980
March	1st to 10th	Night	21% 12% 3 6% 3 3 3 3 4 5% 36%	5373
	11th to 20th	Night	13% 6% 2 6% 4 4 4 3 5% 6% 46%	5055
	21st to end	Night	16% 9% 3 8% 6% 5% 4 4 5% 6% 33%	5407
April	1st to 10th	Night	14% 7% 2 7% 4 4 5% 3 6% 6% 40%	2000
	11th to 20th	Night	15% 9% 3 5% 4 4 4 3 5% 5% 41%	1975
	21st to end	Night	16% 8% 3 6% 5% 5% 5% 4 5% 7% 38%	1810
May	1st to 10th	Night	19% 10% 3 9% 5% 5% 5% 4 5% 8% 28%	4033
	11th to 20th	Night	16% 7% 2 7% 6% 5% 5% 4 6% 9% 33%	4032
	21st to end	Night	12% 7% 3 7% 5% 5% 5% 4 6% 9% 37%	4421
June	1st to 10th	Night	10% 6% 2 5% 3 4 4 3 6% 9% 48%	1708
	11th to 20th	Night	6% 5% 2 6% 5% 4 5% 4 7% 9% 48%	1600
	21st to end	Night	5% 3 2 5% 4 4 4 5% 8% 13% 48%	1600
July	1st to 10th	Night	8% 6% 2 6% 5% 4 4 4 5% 9% 47%	3618
	11th to 20th	Night	7% 5% 2 7% 4 5% 5% 4 7% 10% 44%	3634
	21st to end	Night	9% 6% 3 7% 5% 6% 6% 5% 8% 10% 36%	4427
August	1st to 10th	Night	7% 5% 2 6% 4 3 4 3 6% 10% 50%	2915
	11th to 20th	Night	6% 5% 2 6% 4 4 5% 4 6% 9% 47%	3024
	21st to end	Night	4 2 1 4 3 3 4 3 5% 10% 62%	3586
September	1st to 10th	Night	11% 4 1 5% 4 3 3 3 5% 7% 52%	2055
	11th to 20th	Night	9% 5% 2 4 4 3 3 3 6% 9% 54%	2200
	21st to end	Night	14% 5% 1 6% 4 4 3 3 4 6% 51%	2200
October	1st to 10th	Night	7% 4 2 5% 3 4 4 3 4 6% 58%	2124
	11th to 20th	Night	14% 6% 2 5% 4 4 3 4 5% 6% 47%	2190
	21st to end	Night	17% 7% 3 6% 4 4 3 3 5% 6% 42%	2710
November	1st to 10th	Night	12% 7% 3 5% 4 3 4 3 5% 5% 49%	2536
	11th to 20th	Night	15% 9% 3 6% 4 4 3 3 5% 7% 42%	2600
	21st to end	Night	18% 11% 3 6% 5% 4 3 3 4 6% 37%	2600
December	1st to 10th	Night	17% 12% 3 8% 4 3 3 3 4 5% 39%	4250
	11th to 20th	Night	20% 10% 5% 8% 4 4 3 3 5% 6% 32%	4822
	21st to end	Night	24% 18% 5% 10% 5% 4 4 3 4 4 19%	6949

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	43% 27% 7% 15% 7%	3618
	11th to 20th	Day	43% 26% 5% 13% 10% 2	3680
	21st to end	Day	45% 29% 5% 14% 5% 3	4466
February	1st to 10th	Day	21% 62% 5% 7% 5% 1	1991
	11th to 20th	Day	22% 60% 3 8% 6%	1958
	21st to end	Day	18% 61% 4 10% 6% 1	1650
March	1st to 10th	Day	41% 29% 2 11% 11% 6%	4480
	11th to 20th	Day	27% 29% 6% 24% 12% 1	4904
	21st to end	Day	33% 31% 9% 14% 11% 1	5553
April	1st to 10th	Day	21% 63% 4 8% 4	2400
	11th to 20th	Day	17% 63% 4 9% 5% 1	2374
	21st to end	Day	17% 63% 4 11% 5% 1	2588
May	1st to 10th	Day	38% 33% 10% 11% 8%	5897
	11th to 20th	Day	36% 31% 8% 18% 7%	5865
	21st to end	Day	34% 32% 11% 14% 9% 1	6447
June	1st to 10th	Day	16% 63% 4 9% 7% 1	2687
	11th to 20th	Day	17% 64% 4 8% 8%	2800
	21st to end	Day	15% 63% 7% 8% 7% 1	2800
July	1st to 10th	Day	27% 29% 8% 19% 17% 1	6291
	11th to 20th	Day	29% 31% 10% 20% 10%	6249
	21st to end	Day	38% 30% 9% 13% 10% 1	6240
August	1st to 10th	Day	18% 40% 10% 25% 6%	4221
	11th to 20th	Day	21% 39% 9% 22% 8% 1	4060
	21st to end	Day	14% 38% 9% 25% 12% 1	4311
September	1st to 10th	Day	13% 58% 5% 12% 11% 2	2324
	11th to 20th	Day	12% 58% 7% 11% 11% 1	2200
	21st to end	Day	17% 57% 4 11% 10%	2200
October	1st to 10th	Day	11% 37% 3 22% 26% 1	3513
	11th to 20th	Day	15% 38% 6% 18% 22% 1	3282
	21st to end	Day	19% 37% 5% 17% 19% 2	3085
November	1st to 10th	Day	18% 56% 5% 13% 7% 1	1762
	11th to 20th	Day	20% 56% 2 11% 9% 2	1800
	21st to end	Day	23% 57% 5% 10% 4	1800
December	1st to 10th	Day	27% 36% 6% 19% 12%	2930
	11th to 20th	Day	40% 33% 5% 14% 7% 1	3370
	21st to end	Day	51% 27% 7% 9% 5%	4009
January	1st to 10th	Night	49% 28% 6% 9% 7%	6348
	11th to 20th	Night	49% 26% 5% 9% 7% 3	6292
	21st to end	Night	39% 29% 4 17% 10% 1	6490
February	1st to 10th	Night	22% 62% 4 7% 4 1	2405
	11th to 20th	Night	22% 61% 4 7% 6%	2339
	21st to end	Night	20% 61% 3 8% 6% 1	1980
March	1st to 10th	Night	40% 29% 4 18% 7% 3	5373
	11th to 20th	Night	24% 31% 7% 20% 17% 1	5055
	21st to end	Night	36% 30% 6% 18% 10% 1	5407
April	1st to 10th	Night	20% 63% 5% 8% 4 1	2000
	11th to 20th	Night	17% 63% 4 9% 6% 1	1975
	21st to end	Night	18% 62% 4 10% 5% 1	1810
May	1st to 10th	Night	39% 33% 8% 13% 6% 1	4060
	11th to 20th	Night	34% 32% 11% 19% 4	4033
	21st to end	Night	32% 32% 12% 16% 8% 1	4429
June	1st to 10th	Night	13% 62% 4 10% 8% 2	1708
	11th to 20th	Night	14% 63% 5% 8% 8% 2	1600
	21st to end	Night	11% 62% 8% 10% 9% 1	1600
July	1st to 10th	Night	25% 29% 7% 20% 16% 2	3580
	11th to 20th	Night	27% 30% 12% 22% 10%	3589
	21st to end	Night	30% 30% 12% 18% 9% 2	4297
August	1st to 10th	Night	20% 39% 9% 27% 6%	2904
	11th to 20th	Night	22% 38% 9% 21% 10%	3051
	21st to end	Night	10% 37% 10% 26% 16% 1	3586
September	1st to 10th	Night	11% 58% 6% 12% 11% 2	2055
	11th to 20th	Night	11% 58% 5% 12% 13% 2	2200
	21st to end	Night	15% 58% 5% 11% 11% 1	2200
October	1st to 10th	Night	11% 37% 4 14% 34% 1	3514
	11th to 20th	Night	12% 37% 6% 15% 29% 2	3637
	21st to end	Night	18% 37% 5% 15% 21% 5%	4460
November	1st to 10th	Night	21% 57% 4 10% 7% 1	2536
	11th to 20th	Night	19% 57% 3 10% 9% 2	2600
	21st to end	Night	25% 56% 5% 10% 4	2600
December	1st to 10th	Night	30% 37% 7% 15% 10%	4250
	11th to 20th	Night	37% 33% 9% 12% 8%	4824
	21st to end	Night	55% 27% 6% 8% 3	6949

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	29% 21% 32% 15% 3	300
	11th to 20th	Day	31% 28% 28% 12% 1	300
	21st to end	Day	30% 28% 30% 9% 2	330
February	1st to 10th	Day	28% 29% 29% 12% 1 1	300
	11th to 20th	Day	31% 29% 31% 8% 1 1	300
	21st to end	Day	29% 22% 39% 8% 2	249
March	1st to 10th	Day	26% 30% 33% 9% 2	300
	11th to 20th	Day	27% 24% 32% 12% 5%	463
	21st to end	Day	33% 21% 28% 15% 3 1	550
April	1st to 10th	Day	27% 20% 29% 17% 6% 1	500
	11th to 20th	Day	23% 20% 28% 19% 8% 2	492
	21st to end	Day	24% 18% 34% 16% 6% 1	500
May	1st to 10th	Day	23% 16% 34% 16% 7% 4	503
	11th to 20th	Day	20% 14% 29% 22% 10% 4	527
	21st to end	Day	15% 11% 27% 24% 16% 8%	573
June	1st to 10th	Day	12% 8% 29% 28% 17% 5%	617
	11th to 20th	Day	11% 7% 22% 34% 20% 6%	730
	21st to end	Day	10% 8% 26% 28% 21% 7%	730
July	1st to 10th	Day	14% 5% 18% 26% 23% 14%	721
	11th to 20th	Day	8% 6% 25% 29% 22% 10%	708
	21st to end	Day	11% 4 22% 31% 21% 10%	576
August	1st to 10th	Day	8% 6% 19% 29% 25% 14%	526
	11th to 20th	Day	12% 6% 20% 30% 23% 10%	519
	21st to end	Day	9% 3 18% 28% 29% 13%	583
September	1st to 10th	Day	11% 6% 19% 32% 26% 7%	530
	11th to 20th	Day	7% 5% 18% 32% 26% 13%	530
	21st to end	Day	17% 9% 20% 28% 20% 6%	530
October	1st to 10th	Day	10% 8% 25% 33% 20% 5%	502
	11th to 20th	Day	15% 19% 31% 20% 12% 3	490
	21st to end	Day	26% 15% 37% 19% 3	334
November	1st to 10th	Day	13% 21% 34% 23% 8% 2	300
	11th to 20th	Day	27% 19% 29% 21% 3 1	300
	21st to end	Day	31% 16% 31% 19% 3	300
December	1st to 10th	Day	25% 23% 34% 14% 3 1	300
	11th to 20th	Day	26% 19% 39% 12% 5%	300
	21st to end	Day	31% 26% 30% 10% 2	330
January	1st to 10th	Night	23% 20% 34% 20% 4	1000
	11th to 20th	Night	28% 19% 33% 16% 3	971
	21st to end	Night	25% 18% 34% 21% 2	880
February	1st to 10th	Night	27% 15% 35% 21% 2	800
	11th to 20th	Night	25% 20% 36% 18% 1	783
	21st to end	Night	28% 17% 37% 17% 1	660
March	1st to 10th	Night	24% 17% 33% 22% 4	800
	11th to 20th	Night	23% 16% 35% 20% 6% 1	800
	21st to end	Night	23% 14% 31% 25% 5%	858
April	1st to 10th	Night	19% 13% 35% 26% 7%	600
	11th to 20th	Night	20% 13% 32% 25% 9% 1	593
	21st to end	Night	22% 8% 34% 29% 7% 1	600
May	1st to 10th	Night	17% 11% 34% 27% 11% 1	600
	11th to 20th	Night	15% 7% 26% 31% 18% 2	591
	21st to end	Night	10% 4 25% 34% 24% 4	650
June	1st to 10th	Night	12% 3 21% 42% 21% 2	600
	11th to 20th	Night	8% 5% 18% 44% 23% 3	600
	21st to end	Night	7% 2 23% 38% 28% 2	600
July	1st to 10th	Night	11% 3 12% 32% 31% 10%	585
	11th to 20th	Night	5% 3 16% 38% 33% 5%	582
	21st to end	Night	10% 3 12% 40% 30% 5%	657
August	1st to 10th	Night	8% 2 11% 35% 38% 5%	597
	11th to 20th	Night	7% 2 12% 40% 33% 5%	594
	21st to end	Night	7% 2 14% 33% 39% 6%	660
September	1st to 10th	Night	10% 4 14% 35% 32% 4	655
	11th to 20th	Night	9% 2 16% 34% 34% 5%	800
	21st to end	Night	16% 6% 21% 32% 24% 3	800
October	1st to 10th	Night	10% 6% 23% 32% 24% 4	773
	11th to 20th	Night	17% 9% 28% 31% 13% 2	760
	21st to end	Night	21% 11% 33% 23% 11% 1	836
November	1st to 10th	Night	15% 11% 31% 28% 13% 1	780
	11th to 20th	Night	20% 16% 30% 25% 9% 1	800
	21st to end	Night	26% 13% 25% 29% 7%	800
December	1st to 10th	Night	25% 15% 33% 23% 5%	800
	11th to 20th	Night	25% 14% 33% 20% 6% 1	800
	21st to end	Night	30% 17% 31% 19% 3	1079

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