Hampden-Sydney College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hampden-Sydney College Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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	14% 5% 2 5% 5% 3 5% 5% 5% 8% 44%	74630
	Night	14% 6% 2 5% 5% 3 4 4 5% 7% 45%	75168
Transparency	Day	39% 6% 12% 23% 17% 4	74351
	Night	39% 5% 11% 22% 19% 5%	74966
Seeing	Day	19% 11% 26% 24% 14% 5%	13994
	Night	17% 10% 27% 30% 14% 2	23949

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	16% 7% 3 5% 3 2 4 4 5% 5% 46%	1706
	11th to 20th	Day	17% 6% 3 5% 4 2 4 4 5% 6% 44%	1665
	21st to end	Day	17% 6% 2 4 5% 2 4 5% 4 5% 47%	1782
February	1st to 10th	Day	13% 7% 2 5% 5% 3 6% 4 7% 8% 40%	1620
	11th to 20th	Day	20% 6% 2 4 4 2 3 3 4 6% 45%	1582
	21st to end	Day	19% 6% 3 6% 5% 4 4 4 5% 6% 38%	1559
March	1st to 10th	Day	14% 5% 2 4 4 2 4 4 4 6% 50%	1980
	11th to 20th	Day	17% 6% 2 5% 5% 3 5% 6% 5% 7% 40%	1980
	21st to end	Day	17% 7% 3 5% 5% 3 5% 5% 4 6% 40%	2178
April	1st to 10th	Day	10% 5% 2 4 4 3 4 4 6% 8% 52%	2123
	11th to 20th	Day	12% 4 3 4 5% 3 4 4 5% 7% 49%	2134
	21st to end	Day	17% 5% 2 5% 4 3 5% 5% 5% 7% 43%	2160
May	1st to 10th	Day	15% 6% 2 5% 5% 3 5% 5% 6% 9% 40%	2333
	11th to 20th	Day	17% 5% 3 6% 5% 3 6% 5% 5% 8% 38%	2301
	21st to end	Day	13% 6% 3 4 5% 3 5% 7% 6% 9% 39%	2589
June	1st to 10th	Day	11% 3 2 4 5% 3 5% 5% 7% 10% 45%	2520
	11th to 20th	Day	11% 4 2 5% 5% 4 5% 5% 7% 10% 43%	2520
	21st to end	Day	8% 4 2 6% 5% 4 5% 6% 8% 10% 42%	2502
July	1st to 10th	Day	10% 4 2 6% 6% 4 6% 5% 7% 10% 40%	2340
	11th to 20th	Day	6% 3 3 5% 6% 4 7% 6% 8% 14% 39%	2340
	21st to end	Day	8% 5% 3 5% 6% 4 7% 7% 8% 10% 37%	2547
August	1st to 10th	Day	10% 5% 2 6% 7% 4 6% 5% 7% 10% 37%	2327
	11th to 20th	Day	12% 6% 2 6% 7% 4 6% 7% 7% 8% 36%	2241
	21st to end	Day	11% 5% 2 5% 5% 3 5% 5% 7% 10% 40%	2376
September	1st to 10th	Day	12% 5% 2 5% 5% 3 4 4 5% 7% 48%	2160
	11th to 20th	Day	13% 5% 2 4 6% 3 4 5% 5% 7% 46%	2085
	21st to end	Day	21% 6% 2 6% 5% 2 4 5% 5% 5% 39%	1918
October	1st to 10th	Day	16% 4 2 4 4 2 4 3 4 6% 50%	1800
	11th to 20th	Day	17% 3 2 4 5% 2 3 2 4 5% 53%	1800
	21st to end	Day	12% 5% 2 5% 4 2 4 3 4 5% 53%	1980
November	1st to 10th	Day	13% 4 2 3 4 2 4 3 4 5% 56%	1857
	11th to 20th	Day	15% 5% 2 4 4 2 3 4 4 6% 50%	1766
	21st to end	Day	15% 3 2 3 4 2 4 4 4 7% 52%	1520
December	1st to 10th	Day	15% 8% 2 5% 4 3 5% 6% 6% 6% 40%	1520
	11th to 20th	Day	16% 5% 2 4 3 2 3 3 4 4 55%	1910
	21st to end	Day	17% 6% 3 4 4 3 5% 5% 5% 6% 43%	2909
January	1st to 10th	Night	15% 6% 2 5% 5% 2 4 4 4 5% 48%	2459
	11th to 20th	Night	17% 6% 3 5% 5% 3 4 4 5% 7% 42%	2417
	21st to end	Night	16% 5% 2 3 5% 2 3 4 4 6% 49%	2574
February	1st to 10th	Night	16% 6% 1 4 4 3 5% 5% 5% 7% 44%	2340
	11th to 20th	Night	18% 8% 2 5% 4 3 4 3 5% 6% 42%	2262
	21st to end	Night	17% 6% 2 4 5% 3 3 4 5% 6% 46%	1695
March	1st to 10th	Night	13% 4 2 4 4 2 4 3 5% 6% 51%	1980
	11th to 20th	Night	18% 5% 3 6% 5% 3 5% 4 5% 7% 40%	1980
	21st to end	Night	15% 6% 4 6% 5% 2 4 3 5% 7% 42%	2178
April	1st to 10th	Night	14% 6% 2 5% 5% 4 4 4 4 8% 45%	1836
	11th to 20th	Night	11% 6% 2 4 4 3 4 4 5% 9% 50%	1775
	21st to end	Night	16% 5% 2 4 5% 3 5% 4 5% 6% 46%	1800
May	1st to 10th	Night	13% 6% 2 5% 4 3 5% 6% 6% 8% 44%	1641
	11th to 20th	Night	17% 6% 2 4 5% 3 4 5% 5% 7% 41%	1592
	21st to end	Night	14% 5% 2 5% 4 4 5% 5% 5% 10% 42%	1680
June	1st to 10th	Night	11% 4 2 5% 4 3 6% 5% 7% 10% 43%	1440
	11th to 20th	Night	10% 5% 3 5% 6% 3 5% 5% 6% 10% 42%	1440
	21st to end	Night	10% 5% 3 5% 4 3 5% 5% 5% 11% 45%	1450
July	1st to 10th	Night	9% 6% 2 6% 6% 3 5% 6% 7% 10% 42%	1620
	11th to 20th	Night	4 4 2 5% 5% 4 6% 6% 9% 10% 45%	1620
	21st to end	Night	7% 4 2 4 6% 5% 7% 6% 8% 11% 40%	1771
August	1st to 10th	Night	11% 4 2 6% 7% 4 5% 6% 8% 10% 38%	1611
	11th to 20th	Night	12% 6% 2 7% 6% 4 5% 5% 7% 8% 38%	1642
	21st to end	Night	10% 5% 2 5% 6% 4 7% 6% 8% 12% 37%	1980
September	1st to 10th	Night	13% 5% 2 6% 5% 3 4 5% 5% 7% 45%	1800
	11th to 20th	Night	13% 5% 2 5% 4 3 4 4 6% 6% 48%	1872
	21st to end	Night	22% 7% 3 5% 5% 3 4 3 5% 6% 38%	2042
October	1st to 10th	Night	16% 6% 2 5% 5% 3 4 4 4 6% 47%	2160
	11th to 20th	Night	15% 5% 2 5% 4 2 3 4 4 6% 49%	2160
	21st to end	Night	13% 5% 2 3 4 2 4 4 5% 6% 53%	2376
November	1st to 10th	Night	12% 7% 3 5% 4 2 4 3 3 5% 52%	2221
	11th to 20th	Night	14% 5% 1 4 4 2 4 4 5% 6% 52%	2407
	21st to end	Night	12% 4 2 4 4 3 4 4 5% 7% 51%	2660
December	1st to 10th	Night	17% 6% 3 5% 4 3 4 4 4 6% 42%	2660
	11th to 20th	Night	17% 6% 2 4 5% 3 3 3 3 6% 48%	3285
	21st to end	Night	16% 7% 3 5% 5% 3 4 5% 6% 7% 40%	4742

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	41% 4 8% 23% 19% 5%	1707
	11th to 20th	Day	42% 4 9% 21% 20% 3	1665
	21st to end	Day	40% 3 7% 24% 20% 5%	1782
February	1st to 10th	Day	41% 5% 11% 24% 15% 4	1620
	11th to 20th	Day	43% 3 6% 22% 21% 5%	1591
	21st to end	Day	48% 5% 11% 14% 16% 6%	1559
March	1st to 10th	Day	37% 3 9% 23% 22% 6%	1980
	11th to 20th	Day	43% 6% 10% 18% 17% 7%	1980
	21st to end	Day	47% 5% 9% 20% 15% 4	2178
April	1st to 10th	Day	30% 4 12% 24% 21% 8%	2123
	11th to 20th	Day	36% 4 10% 22% 21% 6%	2134
	21st to end	Day	41% 5% 11% 21% 16% 5%	2160
May	1st to 10th	Day	41% 7% 15% 21% 14% 3	2333
	11th to 20th	Day	44% 7% 14% 24% 10% 2	2305
	21st to end	Day	40% 7% 16% 24% 12%	2589
June	1st to 10th	Day	33% 8% 16% 27% 15% 1	2520
	11th to 20th	Day	36% 8% 16% 22% 16% 2	2520
	21st to end	Day	35% 9% 16% 25% 14% 2	2502
July	1st to 10th	Day	38% 9% 15% 20% 16% 3	2275
	11th to 20th	Day	33% 9% 20% 24% 12% 2	2271
	21st to end	Day	38% 9% 15% 24% 14%	2405
August	1st to 10th	Day	41% 8% 16% 25% 9% 1	2314
	11th to 20th	Day	42% 8% 16% 21% 10% 1	2241
	21st to end	Day	38% 7% 18% 21% 13% 3	2376
September	1st to 10th	Day	37% 7% 13% 18% 21% 4	2160
	11th to 20th	Day	38% 6% 9% 23% 21% 3	2085
	21st to end	Day	47% 5% 9% 18% 18% 3	1918
October	1st to 10th	Day	37% 4 8% 26% 21% 4	1800
	11th to 20th	Day	36% 4 7% 26% 23% 5%	1800
	21st to end	Day	35% 3 7% 25% 22% 8%	1980
November	1st to 10th	Day	32% 4 8% 25% 26% 6%	1857
	11th to 20th	Day	37% 3 9% 22% 21% 8%	1766
	21st to end	Day	33% 4 9% 24% 25% 5%	1520
December	1st to 10th	Day	43% 6% 10% 21% 16% 4	1520
	11th to 20th	Day	34% 3 8% 26% 20% 8%	1906
	21st to end	Day	41% 5% 9% 27% 16% 2	2909
January	1st to 10th	Night	40% 4 8% 21% 21% 7%	2459
	11th to 20th	Night	43% 5% 9% 21% 18% 4	2417
	21st to end	Night	38% 3 7% 24% 23% 5%	2574
February	1st to 10th	Night	39% 4 11% 25% 17% 4	2340
	11th to 20th	Night	43% 4 8% 20% 19% 7%	2264
	21st to end	Night	40% 3 9% 23% 19% 7%	1695
March	1st to 10th	Night	35% 4 9% 24% 22% 6%	1980
	11th to 20th	Night	44% 5% 8% 20% 18% 5%	1980
	21st to end	Night	43% 4 10% 20% 18% 5%	2178
April	1st to 10th	Night	39% 5% 10% 18% 22% 6%	1836
	11th to 20th	Night	33% 4 10% 24% 22% 6%	1775
	21st to end	Night	39% 4 12% 22% 17% 5%	1800
May	1st to 10th	Night	38% 6% 15% 23% 15% 4	1641
	11th to 20th	Night	43% 7% 13% 21% 13% 3	1593
	21st to end	Night	38% 6% 17% 24% 14% 1	1681
June	1st to 10th	Night	34% 8% 14% 27% 15% 1	1440
	11th to 20th	Night	36% 9% 18% 20% 14% 2	1440
	21st to end	Night	35% 8% 15% 21% 18% 3	1450
July	1st to 10th	Night	37% 9% 16% 18% 15% 5%	1575
	11th to 20th	Night	30% 11% 21% 24% 12% 3	1566
	21st to end	Night	34% 9% 20% 25% 12%	1672
August	1st to 10th	Night	40% 9% 17% 22% 11% 1	1600
	11th to 20th	Night	41% 9% 16% 21% 12% 1	1642
	21st to end	Night	39% 9% 17% 22% 11% 3	1980
September	1st to 10th	Night	39% 8% 12% 19% 17% 6%	1800
	11th to 20th	Night	37% 6% 11% 21% 22% 2	1872
	21st to end	Night	47% 5% 9% 18% 17% 4	2042
October	1st to 10th	Night	40% 5% 8% 24% 20% 3	2160
	11th to 20th	Night	36% 5% 7% 24% 23% 6%	2160
	21st to end	Night	33% 4 7% 24% 25% 7%	2376
November	1st to 10th	Night	36% 4 9% 20% 25% 7%	2221
	11th to 20th	Night	34% 4 8% 20% 24% 9%	2407
	21st to end	Night	33% 4 8% 25% 23% 7%	2660
December	1st to 10th	Night	43% 4 6% 24% 18% 4	2660
	11th to 20th	Night	39% 4 7% 21% 22% 7%	3288
	21st to end	Night	42% 5% 9% 24% 18% 3	4742

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% 25% 27% 14% 5%	270
	11th to 20th	Day	28% 26% 36% 9% 1	255
	21st to end	Day	28% 23% 37% 10% 2	264
February	1st to 10th	Day	27% 25% 35% 8% 4	240
	11th to 20th	Day	39% 24% 28% 8% 1	249
	21st to end	Day	32% 19% 32% 14% 3	346
March	1st to 10th	Day	23% 21% 34% 18% 4	420
	11th to 20th	Day	24% 22% 32% 16% 4 1	420
	21st to end	Day	25% 18% 36% 16% 4 1	462
April	1st to 10th	Day	21% 20% 30% 23% 6%	420
	11th to 20th	Day	22% 14% 33% 20% 8% 4	412
	21st to end	Day	24% 12% 35% 20% 8% 1	420
May	1st to 10th	Day	23% 14% 34% 20% 7% 2	423
	11th to 20th	Day	20% 8% 31% 24% 13% 4	447
	21st to end	Day	20% 4 24% 29% 18% 6%	485
June	1st to 10th	Day	12% 7% 24% 33% 20% 5%	450
	11th to 20th	Day	12% 6% 26% 31% 17% 8%	450
	21st to end	Day	11% 4 22% 37% 19% 6%	450
July	1st to 10th	Day	11% 4 16% 37% 22% 10%	445
	11th to 20th	Day	5% 2 16% 40% 29% 9%	444
	21st to end	Day	10% 3 20% 32% 26% 8%	488
August	1st to 10th	Day	13% 4 19% 26% 27% 9%	446
	11th to 20th	Day	13% 4 15% 30% 27% 10%	439
	21st to end	Day	11% 2 19% 28% 23% 17%	495
September	1st to 10th	Day	16% 5% 16% 26% 22% 14%	450
	11th to 20th	Day	14% 5% 16% 32% 21% 11%	450
	21st to end	Day	28% 7% 21% 26% 13% 4	450
October	1st to 10th	Day	22% 5% 24% 25% 18% 6%	450
	11th to 20th	Day	17% 11% 23% 28% 18% 3	450
	21st to end	Day	17% 12% 31% 27% 10% 3	495
November	1st to 10th	Day	17% 12% 29% 25% 13% 4	455
	11th to 20th	Day	20% 15% 30% 22% 11% 1	376
	21st to end	Day	19% 20% 29% 22% 6% 3	180
December	1st to 10th	Day	20% 22% 36% 16% 6% 1	180
	11th to 20th	Day	27% 16% 31% 21% 4 1	180
	21st to end	Day	26% 15% 34% 18% 6% 1	238
January	1st to 10th	Night	21% 19% 36% 21% 3	760
	11th to 20th	Night	21% 19% 32% 25% 3	744
	21st to end	Night	20% 17% 38% 23% 2	792
February	1st to 10th	Night	20% 17% 37% 22% 4	720
	11th to 20th	Night	26% 19% 38% 16% 1	703
	21st to end	Night	23% 14% 37% 23% 3	592
March	1st to 10th	Night	18% 19% 35% 26% 2	720
	11th to 20th	Night	22% 16% 35% 20% 6% 1	720
	21st to end	Night	19% 14% 39% 23% 5%	792
April	1st to 10th	Night	18% 19% 35% 23% 6%	720
	11th to 20th	Night	14% 14% 34% 29% 8%	711
	21st to end	Night	18% 9% 37% 31% 6%	720
May	1st to 10th	Night	16% 10% 35% 28% 10% 1	561
	11th to 20th	Night	19% 7% 25% 34% 14% 1	531
	21st to end	Night	16% 2 18% 40% 24% 2	520
June	1st to 10th	Night	11% 2 17% 43% 26% 1	360
	11th to 20th	Night	12% 4 16% 41% 24% 3	360
	21st to end	Night	11% 2 19% 46% 21% 2	370
July	1st to 10th	Night	12% 2 12% 38% 32% 4	525
	11th to 20th	Night	5% 1 10% 42% 36% 5%	522
	21st to end	Night	8% 2 17% 41% 31% 2	591
August	1st to 10th	Night	13% 2 12% 40% 29% 4	537
	11th to 20th	Night	15% 3 10% 39% 30% 3	578
	21st to end	Night	11% 2 15% 34% 31% 6%	792
September	1st to 10th	Night	15% 3 13% 32% 31% 7%	720
	11th to 20th	Night	15% 2 15% 33% 30% 5%	720
	21st to end	Night	26% 4 17% 32% 20% 2	720
October	1st to 10th	Night	18% 5% 19% 28% 24% 6%	720
	11th to 20th	Night	17% 8% 28% 30% 17% 1	720
	21st to end	Night	16% 11% 27% 33% 12% 2	792
November	1st to 10th	Night	16% 8% 29% 33% 13% 2	740
	11th to 20th	Night	17% 12% 30% 29% 11%	760
	21st to end	Night	14% 13% 31% 30% 11% 1	760
December	1st to 10th	Night	22% 13% 34% 25% 6% 1	760
	11th to 20th	Night	20% 10% 34% 30% 5%	760
	21st to end	Night	22% 15% 29% 31% 3	836

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