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 2024-04-25.

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	15% 8% 3 6% 5% 3 4 4 5% 7% 38%	83472
	Night	15% 9% 3 6% 5% 3 4 4 5% 7% 38%	85660
Transparency	Day	32% 34% 7% 15% 10% 2	83185
	Night	33% 33% 7% 15% 10% 2	85467
Seeing	Day	19% 14% 27% 23% 13% 5%	14591
	Night	18% 11% 27% 28% 14% 2	24507

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% 7% 7% 5% 3 4 4 5% 6% 29%	2527
	11th to 20th	Day	18% 11% 5% 11% 7% 4 5% 3 4 4 28%	2564
	21st to end	Day	28% 14% 8% 6% 4 3 3 3 4 5% 22%	3124
February	1st to 10th	Day	17% 12% 5% 9% 4 2 3 3 5% 6% 33%	1891
	11th to 20th	Day	17% 9% 4 7% 6% 4 4 4 4 6% 35%	1848
	21st to end	Day	17% 14% 4 7% 5% 2 4 5% 5% 7% 31%	1570
March	1st to 10th	Day	26% 13% 3 6% 5% 2 3 3 4 5% 31%	3120
	11th to 20th	Day	14% 7% 3 5% 6% 3 4 3 5% 7% 43%	3432
	21st to end	Day	16% 7% 3 6% 6% 3 5% 5% 6% 7% 36%	3875
April	1st to 10th	Day	15% 8% 3 5% 5% 4 4 5% 6% 7% 37%	2280
	11th to 20th	Day	17% 7% 3 5% 5% 3 5% 4 5% 6% 40%	2254
	21st to end	Day	18% 6% 3 6% 4 3 4 4 5% 8% 37%	2382
May	1st to 10th	Day	19% 7% 3 6% 5% 3 5% 5% 5% 7% 35%	2340
	11th to 20th	Day	18% 7% 3 6% 6% 3 5% 5% 5% 7% 35%	2301
	21st to end	Day	9% 5% 2 6% 7% 5% 7% 6% 7% 10% 35%	2531
June	1st to 10th	Day	10% 5% 3 6% 6% 3 5% 5% 5% 8% 45%	2417
	11th to 20th	Day	9% 4 2 4 6% 4 6% 5% 7% 8% 45%	2520
	21st to end	Day	9% 5% 2 5% 5% 3 5% 5% 6% 11% 43%	2520
July	1st to 10th	Day	12% 6% 3 6% 5% 4 6% 5% 8% 9% 37%	2520
	11th to 20th	Day	8% 5% 3 6% 5% 3 5% 6% 7% 11% 41%	2512
	21st to end	Day	10% 5% 2 6% 4 3 5% 5% 7% 9% 43%	2547
August	1st to 10th	Day	7% 5% 2 5% 5% 4 6% 6% 7% 9% 43%	2327
	11th to 20th	Day	9% 6% 2 6% 5% 4 5% 5% 7% 9% 41%	2225
	21st to end	Day	7% 5% 2 5% 5% 3 5% 5% 6% 9% 48%	2376
September	1st to 10th	Day	13% 5% 2 6% 5% 3 4 4 5% 7% 47%	2092
	11th to 20th	Day	9% 4 2 5% 6% 4 5% 5% 5% 8% 48%	1980
	21st to end	Day	13% 5% 2 4 5% 3 3 3 5% 6% 51%	1980
October	1st to 10th	Day	11% 4 2 5% 5% 3 4 4 4 5% 53%	1980
	11th to 20th	Day	13% 6% 2 5% 5% 2 4 5% 5% 7% 46%	1886
	21st to end	Day	19% 9% 2 6% 4 4 3 4 5% 5% 39%	1786
November	1st to 10th	Day	10% 5% 2 4 3 2 3 4 4 6% 56%	1672
	11th to 20th	Day	16% 10% 3 6% 4 3 4 3 5% 6% 40%	1710
	21st to end	Day	19% 7% 3 5% 5% 2 3 3 4 6% 41%	1710
December	1st to 10th	Day	22% 10% 5% 8% 5% 3 4 3 4 5% 31%	1710
	11th to 20th	Day	15% 15% 5% 8% 5% 3 4 4 3 5% 31%	2153
	21st to end	Day	21% 20% 5% 6% 6% 3 4 4 3 5% 22%	2810
January	1st to 10th	Night	18% 16% 5% 8% 5% 3 5% 5% 5% 6% 25%	4428
	11th to 20th	Night	16% 15% 6% 12% 9% 3 4 3 3 4 25%	4400
	21st to end	Night	26% 16% 5% 7% 5% 2 3 3 4 5% 25%	4532
February	1st to 10th	Night	21% 12% 4 7% 5% 3 4 5% 4 6% 29%	2285
	11th to 20th	Night	17% 11% 4 8% 6% 3 4 5% 4 5% 34%	2218
	21st to end	Night	21% 12% 5% 8% 4 3 4 4 4 5% 31%	1884
March	1st to 10th	Night	24% 12% 3 6% 5% 3 3 4 4 5% 33%	3733
	11th to 20th	Night	14% 6% 2 5% 4 3 4 4 5% 6% 47%	3527
	21st to end	Night	16% 8% 3 6% 6% 3 4 4 5% 6% 39%	3771
April	1st to 10th	Night	13% 7% 3 6% 5% 3 5% 5% 7% 7% 41%	1900
	11th to 20th	Night	16% 9% 3 4 5% 3 4 4 5% 6% 41%	1875
	21st to end	Night	17% 7% 3 5% 5% 4 5% 4 5% 7% 39%	1675
May	1st to 10th	Night	16% 6% 4 8% 5% 3 5% 5% 5% 8% 35%	1620
	11th to 20th	Night	13% 6% 3 6% 8% 3 5% 5% 6% 8% 39%	1592
	21st to end	Night	7% 4 2 6% 6% 4 5% 7% 7% 12% 39%	1744
June	1st to 10th	Night	10% 5% 2 4 4 3 4 5% 5% 10% 50%	1538
	11th to 20th	Night	6% 4 2 5% 5% 3 5% 5% 6% 9% 50%	1440
	21st to end	Night	6% 3 2 4 4 2 6% 5% 8% 12% 48%	1440
July	1st to 10th	Night	10% 5% 2 5% 5% 3 4 5% 6% 10% 45%	1440
	11th to 20th	Night	6% 3 2 5% 4 4 5% 5% 6% 9% 51%	1444
	21st to end	Night	8% 4 3 4 5% 3 5% 5% 6% 8% 48%	1771
August	1st to 10th	Night	7% 4 2 5% 5% 2 4 5% 7% 10% 48%	1611
	11th to 20th	Night	6% 3 2 6% 5% 4 5% 6% 7% 9% 47%	1673
	21st to end	Night	6% 3 1 4 4 3 4 5% 6% 10% 54%	1980
September	1st to 10th	Night	13% 5% 1 4 4 3 4 4 5% 8% 51%	1849
	11th to 20th	Night	9% 4 2 3 4 2 3 4 6% 8% 53%	1980
	21st to end	Night	12% 4 1 5% 4 3 4 4 4 6% 54%	1980
October	1st to 10th	Night	7% 4 2 5% 4 2 4 4 4 6% 57%	1980
	11th to 20th	Night	15% 6% 2 5% 4 3 4 5% 5% 7% 46%	2074
	21st to end	Night	18% 7% 3 5% 5% 3 3 4 5% 6% 41%	2567
November	1st to 10th	Night	12% 6% 3 4 5% 2 4 4 5% 5% 50%	2406
	11th to 20th	Night	16% 8% 3 5% 4 3 3 3 5% 7% 43%	2470
	21st to end	Night	19% 9% 4 5% 5% 3 4 4 4 6% 38%	2470
December	1st to 10th	Night	19% 11% 4 6% 5% 2 4 5% 4 6% 33%	2470
	11th to 20th	Night	20% 10% 5% 8% 5% 3 4 4 4 6% 33%	3042
	21st to end	Night	20% 19% 5% 8% 7% 4 5% 4 4 5% 19%	4851

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	45% 26% 10% 13% 6%	2528
	11th to 20th	Day	48% 25% 7% 13% 6%	2564
	21st to end	Day	56% 23% 6% 14% 1 1	3124
February	1st to 10th	Day	36% 34% 6% 16% 8% 1	1891
	11th to 20th	Day	34% 35% 7% 14% 8% 2	1858
	21st to end	Day	38% 35% 6% 13% 6% 2	1570
March	1st to 10th	Day	47% 22% 5% 14% 7% 4	3120
	11th to 20th	Day	31% 23% 8% 23% 14% 1	3432
	21st to end	Day	36% 24% 12% 18% 10%	3875
April	1st to 10th	Day	28% 36% 8% 16% 11% 1	2280
	11th to 20th	Day	31% 35% 6% 15% 11% 2	2254
	21st to end	Day	32% 36% 7% 15% 8% 1	2382
May	1st to 10th	Day	34% 38% 6% 15% 6% 1	2340
	11th to 20th	Day	33% 38% 6% 11% 11% 1	2305
	21st to end	Day	25% 40% 12% 12% 10% 1	2531
June	1st to 10th	Day	26% 38% 7% 16% 11% 2	2417
	11th to 20th	Day	21% 39% 9% 17% 13% 1	2520
	21st to end	Day	23% 38% 9% 14% 12% 4	2520
July	1st to 10th	Day	27% 40% 8% 13% 11% 2	2451
	11th to 20th	Day	24% 40% 9% 14% 11% 3	2437
	21st to end	Day	24% 41% 10% 16% 8% 1	2405
August	1st to 10th	Day	22% 39% 8% 16% 13% 1	2314
	11th to 20th	Day	22% 38% 8% 18% 14% 1	2225
	21st to end	Day	21% 38% 8% 15% 12% 6%	2376
September	1st to 10th	Day	27% 38% 7% 13% 11% 4	2092
	11th to 20th	Day	23% 37% 7% 16% 14% 3	1980
	21st to end	Day	26% 36% 6% 13% 17% 2	1980
October	1st to 10th	Day	20% 36% 6% 15% 20% 3	1980
	11th to 20th	Day	26% 37% 6% 18% 11% 1	1886
	21st to end	Day	31% 38% 5% 15% 10% 1	1786
November	1st to 10th	Day	15% 39% 7% 20% 15% 3	1672
	11th to 20th	Day	28% 40% 5% 14% 10% 2	1710
	21st to end	Day	28% 39% 6% 14% 12% 1	1710
December	1st to 10th	Day	36% 39% 7% 13% 5%	1710
	11th to 20th	Day	39% 32% 6% 13% 9%	2150
	21st to end	Day	53% 25% 7% 9% 5%	2810
January	1st to 10th	Night	45% 27% 10% 11% 6%	4428
	11th to 20th	Night	51% 25% 7% 10% 7% 1	4400
	21st to end	Night	53% 22% 5% 13% 6%	4532
February	1st to 10th	Night	38% 35% 7% 14% 6%	2285
	11th to 20th	Night	36% 35% 6% 14% 9% 1	2219
	21st to end	Night	39% 35% 7% 11% 6% 2	1884
March	1st to 10th	Night	44% 22% 6% 19% 9% 1	3733
	11th to 20th	Night	28% 24% 10% 21% 15% 2	3527
	21st to end	Night	35% 24% 8% 22% 11% 1	3771
April	1st to 10th	Night	27% 35% 9% 17% 11% 2	1900
	11th to 20th	Night	29% 34% 6% 12% 14% 5%	1875
	21st to end	Night	30% 36% 7% 15% 10% 2	1675
May	1st to 10th	Night	34% 38% 6% 13% 7% 1	1620
	11th to 20th	Night	31% 38% 8% 11% 11% 2	1593
	21st to end	Night	23% 40% 10% 14% 10% 4	1745
June	1st to 10th	Night	20% 39% 8% 17% 14% 2	1538
	11th to 20th	Night	19% 38% 11% 15% 14% 3	1440
	21st to end	Night	17% 40% 12% 14% 13% 5%	1440
July	1st to 10th	Night	22% 40% 9% 14% 11% 4	1400
	11th to 20th	Night	21% 39% 9% 15% 13% 4	1396
	21st to end	Night	21% 40% 8% 20% 9% 1	1672
August	1st to 10th	Night	19% 38% 10% 18% 12% 2	1600
	11th to 20th	Night	18% 38% 8% 20% 14% 1	1673
	21st to end	Night	17% 37% 9% 18% 14% 5%	1980
September	1st to 10th	Night	24% 37% 8% 15% 11% 5%	1849
	11th to 20th	Night	18% 37% 7% 19% 15% 3	1980
	21st to end	Night	24% 37% 4 13% 18% 3	1980
October	1st to 10th	Night	19% 36% 6% 15% 20% 4	1980
	11th to 20th	Night	26% 37% 6% 16% 13% 2	2074
	21st to end	Night	29% 37% 6% 13% 12% 3	2567
November	1st to 10th	Night	18% 40% 6% 16% 16% 3	2406
	11th to 20th	Night	27% 40% 5% 13% 13% 3	2470
	21st to end	Night	30% 39% 6% 13% 11% 1	2470
December	1st to 10th	Night	33% 41% 7% 14% 6%	2470
	11th to 20th	Night	38% 33% 6% 13% 9% 1	3044
	21st to end	Night	54% 26% 8% 8% 4	4851

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	27% 22% 33% 15% 3	270
	11th to 20th	Day	31% 29% 27% 12% 1	270
	21st to end	Day	32% 27% 29% 9% 2	297
February	1st to 10th	Day	29% 29% 27% 13% 1 1	270
	11th to 20th	Day	29% 29% 32% 9% 1 1	270
	21st to end	Day	31% 21% 38% 8% 2	225
March	1st to 10th	Day	26% 31% 33% 9% 1	270
	11th to 20th	Day	27% 23% 33% 12% 5%	425
	21st to end	Day	32% 20% 29% 15% 4 1	506
April	1st to 10th	Day	26% 20% 30% 17% 7% 1	460
	11th to 20th	Day	22% 20% 27% 20% 9% 2	452
	21st to end	Day	25% 19% 33% 16% 6% 1	437
May	1st to 10th	Day	24% 16% 35% 16% 6% 4	423
	11th to 20th	Day	19% 13% 30% 24% 9% 4	447
	21st to end	Day	14% 8% 25% 26% 18% 9%	485
June	1st to 10th	Day	12% 8% 26% 29% 19% 6%	527
	11th to 20th	Day	11% 7% 23% 33% 19% 7%	630
	21st to end	Day	10% 8% 27% 27% 21% 7%	630
July	1st to 10th	Day	14% 4 17% 26% 24% 16%	621
	11th to 20th	Day	8% 5% 23% 29% 22% 11%	610
	21st to end	Day	13% 3 22% 31% 22% 10%	488
August	1st to 10th	Day	9% 5% 17% 31% 25% 14%	446
	11th to 20th	Day	13% 6% 20% 30% 22% 10%	439
	21st to end	Day	10% 4 18% 27% 29% 13%	495
September	1st to 10th	Day	12% 4 19% 32% 25% 8%	450
	11th to 20th	Day	8% 4 18% 32% 26% 11%	450
	21st to end	Day	15% 9% 20% 29% 20% 6%	450
October	1st to 10th	Day	11% 8% 26% 33% 17% 5%	450
	11th to 20th	Day	16% 19% 32% 21% 10% 1	450
	21st to end	Day	28% 16% 36% 17% 3	301
November	1st to 10th	Day	12% 21% 33% 23% 9% 2	270
	11th to 20th	Day	26% 18% 29% 23% 3 1	270
	21st to end	Day	29% 16% 31% 20% 3	270
December	1st to 10th	Day	26% 23% 33% 15% 3 1	270
	11th to 20th	Day	24% 20% 38% 13% 6%	270
	21st to end	Day	30% 27% 30% 10% 3	297
January	1st to 10th	Night	23% 20% 33% 20% 4	950
	11th to 20th	Night	29% 18% 33% 16% 3	923
	21st to end	Night	25% 18% 33% 21% 2	836
February	1st to 10th	Night	27% 15% 34% 22% 2	760
	11th to 20th	Night	23% 20% 37% 19% 1	743
	21st to end	Night	29% 17% 37% 16% 1	628
March	1st to 10th	Night	23% 18% 34% 21% 4	760
	11th to 20th	Night	23% 15% 35% 19% 6% 1	760
	21st to end	Night	23% 14% 31% 26% 6%	815
April	1st to 10th	Night	17% 13% 36% 27% 7%	570
	11th to 20th	Night	20% 12% 32% 25% 10% 1	563
	21st to end	Night	23% 8% 33% 29% 7% 1	555
May	1st to 10th	Night	16% 11% 34% 26% 11% 1	540
	11th to 20th	Night	16% 7% 27% 31% 17% 2	531
	21st to end	Night	9% 3 23% 34% 26% 4	584
June	1st to 10th	Night	12% 3 19% 42% 21% 2	540
	11th to 20th	Night	7% 4 19% 45% 22% 3	540
	21st to end	Night	7% 2 24% 38% 27% 2	540
July	1st to 10th	Night	12% 3 13% 31% 31% 11%	525
	11th to 20th	Night	5% 3 14% 39% 33% 5%	522
	21st to end	Night	10% 3 13% 39% 30% 5%	591
August	1st to 10th	Night	9% 2 9% 36% 38% 5%	537
	11th to 20th	Night	7% 2 12% 41% 33% 5%	534
	21st to end	Night	7% 2 13% 33% 39% 6%	594
September	1st to 10th	Night	11% 4 14% 35% 32% 5%	589
	11th to 20th	Night	10% 2 17% 32% 34% 5%	720
	21st to end	Night	14% 6% 22% 32% 24% 3	720
October	1st to 10th	Night	11% 6% 24% 32% 23% 4	720
	11th to 20th	Night	18% 9% 28% 31% 13% 2	720
	21st to end	Night	22% 11% 33% 23% 10% 1	792
November	1st to 10th	Night	14% 10% 32% 28% 14% 1	740
	11th to 20th	Night	19% 16% 29% 25% 9% 1	760
	21st to end	Night	25% 12% 25% 30% 7%	760
December	1st to 10th	Night	25% 15% 32% 23% 5%	760
	11th to 20th	Night	25% 14% 33% 21% 6% 1	760
	21st to end	Night	29% 18% 31% 19% 3	1025

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