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 2026-04-02.

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% 6% 2 6% 4 4 4 4 6% 8% 44%	131013
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	130838
Transparency	Day	38% 5% 12% 22% 18% 5%	134619
	Night	38% 5% 10% 22% 19% 6%	135302
Seeing	Day	19% 12% 27% 24% 13% 5%	16917
	Night	17% 11% 28% 30% 13% 2	26892

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% 6% 2 5% 3 3 3 3 4 5% 51%	5258
	11th to 20th	Day	12% 6% 2 5% 3 2 3 2 4 4 57%	5280
	21st to end	Day	15% 8% 2 6% 4 3 3 3 4 4 47%	5780
February	1st to 10th	Day	12% 7% 2 6% 4 4 5% 3 7% 7% 42%	1890
	11th to 20th	Day	21% 6% 2 4 4 3 3 3 4 6% 43%	1854
	21st to end	Day	19% 7% 2 6% 5% 5% 4 3 5% 5% 38%	1823
March	1st to 10th	Day	17% 7% 2 7% 4 4 4 4 5% 7% 40%	5045
	11th to 20th	Day	11% 5% 2 5% 3 3 4 4 5% 7% 51%	5090
	21st to end	Day	16% 6% 2 6% 4 4 5% 4 4 6% 43%	5586
April	1st to 10th	Day	10% 6% 2 5% 4 4 3 3 6% 7% 50%	2370
	11th to 20th	Day	12% 5% 3 6% 4 4 4 3 5% 7% 49%	2374
	21st to end	Day	16% 6% 2 6% 4 4 4 4 5% 7% 43%	2400
May	1st to 10th	Day	12% 6% 2 7% 5% 5% 6% 5% 7% 11% 34%	5855
	11th to 20th	Day	20% 7% 3 7% 5% 4 5% 4 6% 8% 31%	5861
	21st to end	Day	15% 5% 2 5% 4 3 4 4 6% 9% 43%	6602
June	1st to 10th	Day	10% 4 1 5% 5% 4 4 4 7% 10% 45%	2800
	11th to 20th	Day	10% 4 2 6% 5% 6% 5% 4 7% 10% 43%	2800
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 5% 8% 10% 44%	2779
July	1st to 10th	Day	8% 4 2 6% 5% 5% 5% 5% 8% 11% 41%	5900
	11th to 20th	Day	9% 5% 3 7% 5% 6% 7% 5% 10% 14% 29%	5900
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 11% 39%	6424
August	1st to 10th	Day	16% 7% 3 7% 5% 5% 6% 4 7% 11% 31%	4234
	11th to 20th	Day	8% 5% 2 7% 5% 6% 6% 5% 7% 10% 39%	4052
	21st to end	Day	8% 5% 2 5% 3 3 4 3 6% 10% 51%	4312
September	1st to 10th	Day	12% 5% 2 5% 5% 4 4 3 5% 7% 49%	2400
	11th to 20th	Day	14% 6% 2 5% 5% 4 4 4 5% 7% 44%	2316
	21st to end	Day	21% 7% 2 6% 4 3 4 4 5% 5% 37%	2130
October	1st to 10th	Day	15% 5% 2 5% 3 3 4 2 4 6% 51%	2000
	11th to 20th	Day	16% 3 1 5% 4 3 3 2 4 5% 54%	2000
	21st to end	Day	12% 5% 2 5% 3 3 4 2 4 5% 55%	2200
November	1st to 10th	Day	12% 4 2 4 3 3 3 3 4 5% 56%	2057
	11th to 20th	Day	15% 6% 2 5% 3 3 3 3 5% 7% 50%	1951
	21st to end	Day	14% 4 2 4 3 3 4 3 4 7% 51%	1680
December	1st to 10th	Day	12% 6% 2 5% 3 3 4 4 5% 5% 50%	2680
	11th to 20th	Day	19% 6% 2 5% 3 3 3 3 4 5% 48%	3070
	21st to end	Day	16% 6% 2 6% 4 3 4 3 6% 7% 43%	4260
January	1st to 10th	Night	12% 5% 2 5% 3 4 3 3 4 5% 52%	7635
	11th to 20th	Night	14% 5% 2 4 3 3 3 2 3 4 57%	7680
	21st to end	Night	16% 6% 2 5% 3 3 3 2 4 6% 50%	8401
February	1st to 10th	Night	15% 7% 1 5% 3 4 5% 4 5% 6% 45%	2730
	11th to 20th	Night	19% 8% 2 5% 3 3 4 3 5% 6% 42%	2650
	21st to end	Night	15% 7% 2 6% 4 4 3 3 6% 6% 45%	1980
March	1st to 10th	Night	16% 5% 2 5% 4 4 5% 3 5% 6% 45%	5028
	11th to 20th	Night	11% 4 2 5% 3 3 3 3 5% 7% 54%	5090
	21st to end	Night	13% 6% 3 6% 4 4 4 3 5% 7% 47%	5588
April	1st to 10th	Night	14% 6% 2 6% 4 5% 3 4 4 7% 44%	2052
	11th to 20th	Night	11% 6% 1 5% 4 3 4 3 6% 8% 49%	1975
	21st to end	Night	14% 5% 2 5% 4 4 4 4 5% 7% 46%	2000
May	1st to 10th	Night	14% 8% 3 8% 5% 5% 5% 5% 6% 9% 34%	4082
	11th to 20th	Night	23% 7% 3 6% 5% 4 4 3 5% 6% 34%	4032
	21st to end	Night	14% 6% 2 5% 4 4 4 4 6% 9% 41%	4246
June	1st to 10th	Night	10% 5% 2 5% 4 5% 5% 4 7% 9% 43%	1600
	11th to 20th	Night	10% 6% 2 5% 5% 4 5% 4 6% 10% 42%	1600
	21st to end	Night	9% 5% 3 5% 4 4 5% 4 6% 10% 45%	1612
July	1st to 10th	Night	10% 5% 2 8% 5% 5% 6% 5% 8% 10% 37%	4058
	11th to 20th	Night	11% 8% 3 9% 6% 6% 7% 6% 8% 10% 27%	4057
	21st to end	Night	9% 7% 3 8% 6% 7% 7% 5% 8% 9% 32%	4427
August	1st to 10th	Night	17% 7% 3 7% 5% 5% 5% 4 8% 9% 30%	2915
	11th to 20th	Night	8% 6% 3 7% 5% 6% 5% 5% 7% 9% 40%	2964
	21st to end	Night	9% 6% 2 6% 4 5% 5% 4 6% 10% 42%	3586
September	1st to 10th	Night	12% 6% 2 6% 5% 4 4 4 5% 7% 46%	2000
	11th to 20th	Night	14% 6% 2 6% 3 4 4 3 6% 6% 46%	2080
	21st to end	Night	23% 8% 3 6% 4 4 4 3 5% 6% 36%	2270
October	1st to 10th	Night	15% 7% 2 5% 3 5% 3 3 4 6% 48%	2400
	11th to 20th	Night	15% 6% 2 5% 3 3 3 3 5% 6% 50%	2400
	21st to end	Night	13% 5% 2 4 3 3 3 3 4 6% 54%	2640
November	1st to 10th	Night	12% 7% 3 6% 3 3 3 2 4 5% 52%	2461
	11th to 20th	Night	13% 5% 2 5% 3 3 4 3 5% 6% 51%	2661
	21st to end	Night	11% 5% 2 4 4 4 4 4 5% 6% 52%	2940
December	1st to 10th	Night	10% 5% 2 5% 3 4 4 3 5% 6% 54%	4720
	11th to 20th	Night	19% 7% 2 6% 4 4 3 3 4 6% 40%	5345
	21st to end	Night	15% 6% 2 5% 4 4 4 4 6% 7% 44%	6933

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	37% 2 9% 22% 23% 7%	5259
	11th to 20th	Day	32% 3 4 17% 29% 15%	5280
	21st to end	Day	42% 2 6% 27% 19% 4	5780
February	1st to 10th	Day	40% 4 11% 24% 17% 4	1890
	11th to 20th	Day	44% 3 6% 22% 20% 5%	1863
	21st to end	Day	47% 4 11% 16% 17% 6%	1823
March	1st to 10th	Day	44% 4 8% 18% 16% 9%	5045
	11th to 20th	Day	32% 5% 8% 21% 25% 10%	5090
	21st to end	Day	43% 4 10% 19% 18% 6%	5586
April	1st to 10th	Day	33% 4 12% 23% 20% 8%	2370
	11th to 20th	Day	36% 4 10% 23% 21% 6%	2374
	21st to end	Day	40% 5% 11% 23% 16% 4	2400
May	1st to 10th	Day	43% 7% 16% 22% 10% 1	5890
	11th to 20th	Day	51% 6% 11% 20% 11% 1	5865
	21st to end	Day	37% 5% 15% 21% 18% 3	6617
June	1st to 10th	Day	33% 8% 16% 27% 16% 1	2800
	11th to 20th	Day	36% 8% 16% 23% 16% 2	2800
	21st to end	Day	33% 8% 17% 25% 15% 2	2779
July	1st to 10th	Day	35% 8% 22% 19% 13% 3	5835
	11th to 20th	Day	41% 10% 25% 18% 5% 1	5831
	21st to end	Day	37% 8% 15% 24% 15%	6240
August	1st to 10th	Day	47% 6% 17% 23% 6%	4221
	11th to 20th	Day	37% 7% 20% 25% 10% 1	4091
	21st to end	Day	29% 5% 18% 29% 17% 1	4312
September	1st to 10th	Day	36% 7% 14% 17% 22% 4	2400
	11th to 20th	Day	39% 5% 9% 23% 20% 3	2316
	21st to end	Day	48% 5% 10% 17% 17% 3	2130
October	1st to 10th	Day	34% 4 7% 20% 30% 6%	3260
	11th to 20th	Day	24% 3 5% 24% 30% 15%	3240
	21st to end	Day	23% 3 10% 27% 25% 14%	3538
November	1st to 10th	Day	32% 4 8% 26% 25% 6%	2057
	11th to 20th	Day	36% 3 9% 23% 21% 7%	1951
	21st to end	Day	34% 4 9% 24% 25% 5%	1680
December	1st to 10th	Day	35% 4 8% 26% 20% 7%	2680
	11th to 20th	Day	40% 3 12% 21% 19% 5%	3066
	21st to end	Day	40% 4 12% 26% 17% 1	4260
January	1st to 10th	Night	34% 3 10% 21% 22% 10%	7636
	11th to 20th	Night	33% 3 4 18% 32% 10%	7680
	21st to end	Night	37% 2 6% 27% 21% 6%	8401
February	1st to 10th	Night	38% 4 11% 25% 18% 4	2730
	11th to 20th	Night	43% 3 8% 19% 20% 6%	2652
	21st to end	Night	39% 4 9% 23% 18% 7%	1980
March	1st to 10th	Night	40% 4 7% 20% 20% 9%	5028
	11th to 20th	Night	30% 4 7% 18% 29% 12%	5090
	21st to end	Night	38% 3 9% 23% 21% 6%	5588
April	1st to 10th	Night	40% 5% 11% 18% 21% 6%	2052
	11th to 20th	Night	33% 4 10% 24% 22% 6%	1975
	21st to end	Night	38% 5% 12% 23% 17% 5%	2000
May	1st to 10th	Night	46% 6% 17% 19% 10% 2	4109
	11th to 20th	Night	50% 6% 11% 21% 11% 2	4033
	21st to end	Night	39% 6% 16% 24% 14% 2	4253
June	1st to 10th	Night	35% 8% 14% 25% 17% 1	1600
	11th to 20th	Night	36% 8% 19% 20% 15% 2	1600
	21st to end	Night	34% 7% 15% 22% 18% 3	1612
July	1st to 10th	Night	41% 9% 19% 17% 12% 2	4015
	11th to 20th	Night	50% 10% 19% 14% 5% 1	4006
	21st to end	Night	46% 8% 14% 20% 11%	4297
August	1st to 10th	Night	48% 7% 18% 21% 6%	2904
	11th to 20th	Night	39% 7% 18% 25% 11% 1	2991
	21st to end	Night	35% 7% 13% 31% 12% 2	3586
September	1st to 10th	Night	38% 7% 12% 19% 18% 6%	2000
	11th to 20th	Night	38% 6% 11% 21% 22% 2	2080
	21st to end	Night	49% 5% 10% 17% 15% 4	2270
October	1st to 10th	Night	40% 4 9% 18% 24% 4	3920
	11th to 20th	Night	25% 3 7% 22% 30% 12%	3899
	21st to end	Night	22% 3 5% 30% 26% 14%	4252
November	1st to 10th	Night	35% 4 9% 21% 26% 6%	2461
	11th to 20th	Night	35% 4 8% 19% 25% 9%	2661
	21st to end	Night	33% 4 8% 25% 24% 7%	2940
December	1st to 10th	Night	32% 3 6% 29% 24% 5%	4720
	11th to 20th	Night	47% 4 8% 18% 16% 7%	5348
	21st to end	Night	39% 4 11% 27% 17% 2	6933

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	30% 25% 26% 13% 4	330
	11th to 20th	Day	29% 29% 33% 7% 1	330
	21st to end	Day	28% 25% 36% 9% 2	363
February	1st to 10th	Day	24% 25% 38% 10% 4	330
	11th to 20th	Day	42% 23% 26% 8% 2	341
	21st to end	Day	30% 20% 32% 16% 2	446
March	1st to 10th	Day	24% 19% 34% 19% 5%	540
	11th to 20th	Day	24% 21% 34% 16% 4 1	540
	21st to end	Day	24% 19% 38% 15% 4 1	594
April	1st to 10th	Day	23% 21% 30% 21% 5%	504
	11th to 20th	Day	22% 14% 35% 20% 7% 3	492
	21st to end	Day	21% 12% 36% 21% 8% 1	500
May	1st to 10th	Day	22% 15% 34% 20% 7% 2	503
	11th to 20th	Day	22% 10% 29% 23% 13% 3	527
	21st to end	Day	20% 6% 25% 27% 16% 5%	573
June	1st to 10th	Day	10% 8% 27% 31% 19% 5%	530
	11th to 20th	Day	12% 6% 25% 33% 18% 7%	530
	21st to end	Day	10% 4 20% 38% 21% 6%	530
July	1st to 10th	Day	10% 3 16% 38% 23% 9%	525
	11th to 20th	Day	6% 3 15% 38% 28% 11%	524
	21st to end	Day	10% 3 20% 32% 26% 9%	576
August	1st to 10th	Day	15% 4 20% 27% 25% 9%	526
	11th to 20th	Day	12% 4 17% 31% 26% 9%	519
	21st to end	Day	11% 2 18% 31% 23% 15%	583
September	1st to 10th	Day	14% 5% 20% 27% 21% 13%	530
	11th to 20th	Day	17% 5% 16% 31% 21% 10%	530
	21st to end	Day	29% 7% 22% 25% 12% 4	530
October	1st to 10th	Day	19% 5% 25% 26% 18% 7%	530
	11th to 20th	Day	17% 12% 23% 27% 18% 3	530
	21st to end	Day	18% 13% 30% 26% 10% 3	583
November	1st to 10th	Day	16% 12% 32% 25% 11% 4	535
	11th to 20th	Day	20% 16% 31% 22% 10% 1	441
	21st to end	Day	22% 21% 28% 20% 7% 2	220
December	1st to 10th	Day	21% 21% 37% 14% 5% 1	220
	11th to 20th	Day	26% 16% 31% 21% 4 1	220
	21st to end	Day	27% 16% 34% 16% 6% 1	292
January	1st to 10th	Night	20% 20% 36% 21% 3	840
	11th to 20th	Night	20% 22% 33% 23% 2	840
	21st to end	Night	20% 17% 39% 22% 2	924
February	1st to 10th	Night	19% 17% 37% 23% 4	840
	11th to 20th	Night	26% 20% 37% 16% 1	823
	21st to end	Night	22% 16% 37% 22% 3	692
March	1st to 10th	Night	19% 19% 34% 25% 3	840
	11th to 20th	Night	22% 16% 35% 21% 5% 1	840
	21st to end	Night	18% 16% 40% 22% 4	924
April	1st to 10th	Night	18% 19% 36% 21% 5%	805
	11th to 20th	Night	14% 14% 36% 28% 7%	791
	21st to end	Night	16% 9% 39% 31% 6%	800
May	1st to 10th	Night	17% 10% 35% 28% 9% 1	623
	11th to 20th	Night	19% 7% 25% 34% 14% 1	591
	21st to end	Night	16% 2 20% 39% 22% 2	578
June	1st to 10th	Night	11% 2 19% 44% 24% 1	400
	11th to 20th	Night	12% 5% 16% 42% 23% 3	400
	21st to end	Night	10% 2 18% 46% 22% 2	412
July	1st to 10th	Night	12% 2 12% 39% 31% 4	585
	11th to 20th	Night	6% 2 10% 41% 36% 5%	582
	21st to end	Night	8% 2 17% 40% 30% 3	657
August	1st to 10th	Night	14% 2 12% 40% 28% 4	597
	11th to 20th	Night	14% 3 10% 39% 30% 3	644
	21st to end	Night	11% 2 15% 36% 31% 5%	880
September	1st to 10th	Night	14% 3 13% 35% 29% 6%	800
	11th to 20th	Night	16% 2 15% 33% 29% 4	800
	21st to end	Night	27% 4 17% 32% 18% 2	800
October	1st to 10th	Night	18% 5% 19% 28% 25% 6%	800
	11th to 20th	Night	17% 8% 28% 30% 17% 1	800
	21st to end	Night	15% 11% 27% 33% 13% 1	880
November	1st to 10th	Night	15% 9% 28% 33% 12% 2	820
	11th to 20th	Night	17% 13% 31% 29% 10%	840
	21st to end	Night	14% 13% 33% 29% 10% 1	840
December	1st to 10th	Night	21% 12% 36% 25% 5% 1	840
	11th to 20th	Night	21% 11% 35% 29% 5%	840
	21st to end	Night	21% 15% 30% 31% 3	924

Page generated at 2026-03-30 02:18:39. Version ac07

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