Harrisonburg
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Harrisonburg clear sky chart from 2005-11-06 to 2025-12-03.

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% 5% 5% 5% 4 6% 8% 40%	126601
	Night	13% 7% 2 6% 4 4 5% 4 6% 7% 43%	124083
Transparency	Day	39% 8% 12% 22% 15% 4	130207
	Night	37% 7% 12% 22% 17% 4	128546
Seeing	Day	18% 14% 25% 23% 14% 5%	16371
	Night	16% 11% 25% 32% 14% 2	26141

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% 8% 3 7% 4 4 4 3 4 5% 47%	4056
	11th to 20th	Day	11% 8% 3 7% 5% 3 3 3 4 4 47%	4060
	21st to end	Day	14% 7% 2 7% 4 4 4 3 5% 5% 46%	4466
February	1st to 10th	Day	12% 9% 3 8% 5% 4 4 3 6% 6% 40%	1800
	11th to 20th	Day	16% 8% 2 6% 4 4 4 4 5% 6% 40%	1753
	21st to end	Day	16% 8% 2 6% 4 4 5% 3 5% 6% 41%	1710
March	1st to 10th	Day	15% 7% 3 7% 4 5% 5% 3 4 6% 41%	4935
	11th to 20th	Day	9% 6% 2 6% 3 3 4 3 5% 7% 51%	4980
	21st to end	Day	14% 8% 3 7% 5% 5% 4 4 4 6% 40%	5465
April	1st to 10th	Day	12% 7% 2 6% 5% 5% 4 4 5% 7% 44%	2346
	11th to 20th	Day	13% 6% 2 5% 5% 4 4 3 5% 6% 46%	2374
	21st to end	Day	15% 7% 2 7% 5% 5% 5% 3 5% 6% 42%	2474
May	1st to 10th	Day	14% 6% 2 8% 6% 5% 7% 4 7% 9% 31%	5862
	11th to 20th	Day	20% 8% 2 7% 5% 5% 5% 4 6% 8% 31%	5861
	21st to end	Day	15% 6% 2 7% 5% 5% 5% 4 6% 9% 36%	6741
June	1st to 10th	Day	11% 5% 2 6% 4 5% 5% 5% 7% 9% 42%	2800
	11th to 20th	Day	10% 5% 3 8% 5% 5% 6% 5% 7% 9% 37%	2800
	21st to end	Day	7% 5% 2 5% 5% 5% 5% 5% 8% 12% 42%	2800
July	1st to 10th	Day	7% 4 2 7% 6% 5% 6% 5% 8% 10% 40%	6039
	11th to 20th	Day	8% 6% 3 8% 6% 7% 7% 7% 9% 12% 28%	5900
	21st to end	Day	9% 6% 2 7% 5% 6% 6% 5% 8% 10% 34%	6424
August	1st to 10th	Day	13% 6% 3 7% 6% 6% 6% 6% 8% 10% 30%	4234
	11th to 20th	Day	8% 5% 2 7% 6% 6% 6% 5% 8% 10% 37%	4143
	21st to end	Day	8% 4 2 5% 4 5% 5% 5% 7% 10% 45%	4312
September	1st to 10th	Day	12% 6% 2 5% 4 5% 5% 4 5% 6% 47%	2400
	11th to 20th	Day	12% 5% 2 6% 4 5% 5% 3 6% 7% 45%	2287
	21st to end	Day	21% 7% 2 6% 5% 5% 4 3 5% 6% 35%	2148
October	1st to 10th	Day	14% 6% 2 5% 5% 4 3 3 5% 7% 47%	2000
	11th to 20th	Day	13% 6% 3 6% 4 3 3 3 5% 7% 49%	2000
	21st to end	Day	13% 5% 2 5% 3 3 4 4 5% 6% 50%	2200
November	1st to 10th	Day	11% 6% 1 4 3 3 4 3 5% 6% 54%	2057
	11th to 20th	Day	12% 6% 2 4 5% 3 4 3 5% 6% 49%	1837
	21st to end	Day	13% 5% 2 5% 4 5% 4 4 4 6% 48%	1680
December	1st to 10th	Day	11% 6% 2 5% 3 4 5% 3 5% 6% 50%	2616
	11th to 20th	Day	18% 7% 2 5% 3 3 3 3 5% 7% 44%	2990
	21st to end	Day	18% 8% 3 6% 4 4 5% 4 6% 7% 35%	4051
January	1st to 10th	Night	10% 7% 3 6% 3 3 4 3 5% 5% 50%	5889
	11th to 20th	Night	12% 8% 2 5% 4 3 3 2 5% 5% 53%	5900
	21st to end	Night	14% 8% 2 5% 3 3 3 3 4 6% 50%	6490
February	1st to 10th	Night	12% 9% 2 6% 4 4 5% 4 5% 6% 43%	2600
	11th to 20th	Night	18% 8% 2 6% 4 4 4 4 5% 6% 40%	2533
	21st to end	Night	13% 8% 3 6% 4 3 4 4 6% 7% 42%	1915
March	1st to 10th	Night	15% 7% 2 6% 4 4 3 3 4 6% 46%	4918
	11th to 20th	Night	9% 5% 2 6% 3 4 3 3 5% 8% 51%	4980
	21st to end	Night	12% 7% 3 7% 5% 4 4 3 4 6% 45%	5467
April	1st to 10th	Night	14% 7% 3 6% 5% 4 5% 3 6% 6% 42%	2052
	11th to 20th	Night	12% 6% 2 6% 4 3 5% 4 6% 8% 45%	1975
	21st to end	Night	16% 5% 2 5% 3 4 4 4 6% 7% 45%	1942
May	1st to 10th	Night	16% 8% 3 8% 5% 5% 5% 4 6% 8% 32%	4033
	11th to 20th	Night	21% 8% 3 7% 4 4 5% 4 6% 7% 31%	4032
	21st to end	Night	15% 7% 3 6% 5% 5% 5% 4 6% 9% 36%	4111
June	1st to 10th	Night	9% 6% 2 6% 5% 3 5% 4 6% 10% 44%	1600
	11th to 20th	Night	9% 6% 2 7% 3 4 6% 4 7% 9% 43%	1600
	21st to end	Night	8% 4 2 6% 4 4 6% 6% 7% 9% 44%	1600
July	1st to 10th	Night	7% 5% 2 7% 5% 6% 5% 5% 7% 10% 40%	3913
	11th to 20th	Night	9% 9% 4 10% 6% 7% 6% 6% 8% 9% 27%	4057
	21st to end	Night	7% 6% 3 8% 6% 6% 6% 5% 8% 10% 33%	4427
August	1st to 10th	Night	14% 7% 2 7% 6% 6% 5% 4 7% 10% 32%	2915
	11th to 20th	Night	9% 7% 3 9% 5% 6% 6% 5% 7% 7% 36%	2875
	21st to end	Night	10% 5% 2 6% 4 5% 5% 4 6% 10% 45%	3582
September	1st to 10th	Night	11% 6% 2 5% 4 4 4 4 5% 8% 48%	2000
	11th to 20th	Night	11% 6% 2 7% 4 4 5% 4 6% 6% 45%	2109
	21st to end	Night	20% 8% 3 5% 4 4 3 3 4 5% 39%	2250
October	1st to 10th	Night	15% 6% 2 6% 3 4 4 3 5% 7% 45%	2400
	11th to 20th	Night	12% 6% 2 5% 4 4 4 3 5% 6% 49%	2400
	21st to end	Night	13% 6% 2 5% 4 3 4 3 5% 5% 51%	2640
November	1st to 10th	Night	11% 6% 2 5% 3 4 4 3 4 5% 52%	2461
	11th to 20th	Night	12% 7% 2 5% 4 3 5% 3 5% 6% 48%	2776
	21st to end	Night	13% 6% 2 5% 4 4 4 3 5% 6% 48%	2940
December	1st to 10th	Night	10% 6% 2 5% 4 4 5% 4 4 6% 50%	4609
	11th to 20th	Night	17% 7% 3 6% 4 4 4 4 5% 5% 41%	5205
	21st to end	Night	15% 7% 2 6% 5% 5% 6% 5% 6% 7% 36%	6887

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	39% 4 7% 20% 22% 8%	4057
	11th to 20th	Day	39% 5% 7% 18% 20% 11%	4060
	21st to end	Day	39% 5% 8% 24% 20% 4	4466
February	1st to 10th	Day	43% 6% 11% 22% 15% 3	1800
	11th to 20th	Day	44% 6% 10% 20% 17% 4	1762
	21st to end	Day	43% 7% 10% 17% 16% 6%	1710
March	1st to 10th	Day	42% 6% 8% 20% 16% 9%	4935
	11th to 20th	Day	31% 6% 9% 26% 21% 7%	4980
	21st to end	Day	44% 7% 9% 16% 18% 6%	5465
April	1st to 10th	Day	40% 7% 11% 19% 17% 7%	2346
	11th to 20th	Day	37% 7% 10% 21% 20% 5%	2374
	21st to end	Day	43% 7% 11% 22% 13% 4	2474
May	1st to 10th	Day	45% 12% 13% 20% 10% 2	5897
	11th to 20th	Day	50% 9% 11% 19% 10% 1	5865
	21st to end	Day	41% 10% 14% 18% 14% 2	6756
June	1st to 10th	Day	36% 10% 15% 23% 15% 1	2800
	11th to 20th	Day	41% 11% 15% 20% 12% 2	2800
	21st to end	Day	32% 12% 18% 24% 13% 1	2800
July	1st to 10th	Day	35% 12% 17% 21% 12% 4	5974
	11th to 20th	Day	41% 14% 18% 20% 5% 1	5831
	21st to end	Day	41% 10% 13% 24% 13%	6240
August	1st to 10th	Day	44% 11% 18% 20% 7%	4221
	11th to 20th	Day	37% 11% 13% 26% 12% 1	4182
	21st to end	Day	32% 10% 17% 27% 12% 1	4312
September	1st to 10th	Day	37% 9% 13% 17% 19% 4	2400
	11th to 20th	Day	37% 7% 9% 25% 19% 3	2287
	21st to end	Day	49% 7% 10% 17% 15% 2	2148
October	1st to 10th	Day	32% 5% 9% 20% 28% 5%	3260
	11th to 20th	Day	25% 5% 9% 29% 20% 13%	3240
	21st to end	Day	22% 4 10% 30% 23% 11%	3538
November	1st to 10th	Day	31% 5% 10% 26% 24% 4	2057
	11th to 20th	Day	35% 5% 11% 21% 22% 7%	1837
	21st to end	Day	36% 5% 10% 25% 21% 3	1680
December	1st to 10th	Day	34% 6% 10% 24% 20% 5%	2616
	11th to 20th	Day	39% 6% 12% 23% 15% 4	2986
	21st to end	Day	44% 8% 14% 19% 14% 1	4051
January	1st to 10th	Night	33% 5% 9% 23% 25% 6%	5889
	11th to 20th	Night	33% 4 8% 23% 22% 10%	5900
	21st to end	Night	37% 4 8% 26% 21% 4	6490
February	1st to 10th	Night	41% 7% 12% 23% 15% 2	2600
	11th to 20th	Night	44% 5% 10% 20% 16% 6%	2535
	21st to end	Night	41% 7% 12% 20% 15% 6%	1915
March	1st to 10th	Night	38% 6% 8% 22% 18% 8%	4918
	11th to 20th	Night	27% 6% 11% 26% 24% 6%	4980
	21st to end	Night	38% 6% 12% 20% 19% 5%	5467
April	1st to 10th	Night	41% 7% 11% 19% 18% 4	2052
	11th to 20th	Night	35% 8% 14% 20% 19% 4	1975
	21st to end	Night	38% 7% 13% 25% 14% 3	1942
May	1st to 10th	Night	47% 10% 14% 18% 8% 2	4060
	11th to 20th	Night	49% 9% 11% 23% 9%	4033
	21st to end	Night	41% 11% 16% 23% 7% 2	4118
June	1st to 10th	Night	34% 9% 16% 26% 14% 1	1600
	11th to 20th	Night	35% 12% 17% 23% 13% 1	1600
	21st to end	Night	31% 12% 17% 20% 18% 2	1600
July	1st to 10th	Night	36% 13% 19% 18% 13% 2	3870
	11th to 20th	Night	48% 13% 16% 16% 6% 1	4006
	21st to end	Night	40% 12% 17% 21% 10%	4297
August	1st to 10th	Night	44% 10% 17% 22% 6%	2904
	11th to 20th	Night	42% 10% 14% 23% 11%	2902
	21st to end	Night	33% 9% 17% 29% 11% 1	3582
September	1st to 10th	Night	34% 10% 15% 21% 17% 4	2000
	11th to 20th	Night	37% 7% 10% 26% 18% 2	2109
	21st to end	Night	47% 7% 10% 19% 15% 2	2250
October	1st to 10th	Night	40% 6% 10% 20% 20% 4	3920
	11th to 20th	Night	25% 6% 11% 20% 30% 8%	3899
	21st to end	Night	22% 4 11% 30% 26% 7%	4252
November	1st to 10th	Night	33% 6% 10% 22% 23% 5%	2461
	11th to 20th	Night	36% 6% 8% 22% 22% 6%	2776
	21st to end	Night	37% 6% 10% 23% 20% 4	2940
December	1st to 10th	Night	31% 6% 12% 27% 21% 3	4609
	11th to 20th	Night	43% 6% 11% 18% 17% 5%	5208
	21st to end	Night	41% 8% 14% 21% 15% 1	6887

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	31% 27% 26% 13% 3	300
	11th to 20th	Day	27% 32% 26% 14% 2	300
	21st to end	Day	23% 33% 29% 11% 2 1	330
February	1st to 10th	Day	25% 32% 30% 11% 2	300
	11th to 20th	Day	40% 26% 28% 5%	300
	21st to end	Day	31% 22% 28% 17% 2	379
March	1st to 10th	Day	22% 26% 27% 20% 5%	500
	11th to 20th	Day	23% 25% 28% 17% 5% 2	500
	21st to end	Day	23% 22% 30% 19% 5% 1	550
April	1st to 10th	Day	27% 23% 26% 17% 7% 1	500
	11th to 20th	Day	22% 16% 29% 18% 11% 3	492
	21st to end	Day	23% 18% 28% 21% 9% 2	500
May	1st to 10th	Day	20% 21% 27% 18% 10% 4	503
	11th to 20th	Day	19% 15% 28% 22% 12% 4	527
	21st to end	Day	17% 10% 24% 25% 16% 8%	573
June	1st to 10th	Day	13% 9% 28% 28% 17% 5%	530
	11th to 20th	Day	12% 9% 26% 28% 17% 7%	530
	21st to end	Day	8% 5% 28% 32% 20% 6%	530
July	1st to 10th	Day	10% 4 19% 30% 27% 10%	525
	11th to 20th	Day	5% 3 18% 36% 27% 11%	524
	21st to end	Day	11% 4 19% 31% 25% 11%	576
August	1st to 10th	Day	13% 6% 21% 26% 21% 12%	526
	11th to 20th	Day	12% 5% 17% 30% 24% 12%	519
	21st to end	Day	8% 5% 17% 33% 22% 15%	583
September	1st to 10th	Day	12% 7% 20% 27% 20% 14%	530
	11th to 20th	Day	13% 4 21% 29% 23% 10%	530
	21st to end	Day	27% 11% 21% 22% 16% 3	530
October	1st to 10th	Day	18% 7% 21% 30% 18% 7%	530
	11th to 20th	Day	15% 17% 24% 24% 17% 3	530
	21st to end	Day	20% 14% 32% 23% 9% 3	583
November	1st to 10th	Day	17% 13% 30% 27% 11% 2	535
	11th to 20th	Day	21% 16% 32% 22% 8% 1	342
	21st to end	Day	22% 19% 35% 19% 5% 1	220
December	1st to 10th	Day	22% 20% 39% 15% 3 1	204
	11th to 20th	Day	24% 19% 31% 18% 8% 1	200
	21st to end	Day	20% 26% 31% 18% 5%	240
January	1st to 10th	Night	19% 19% 35% 23% 4	800
	11th to 20th	Night	21% 16% 31% 29% 3	800
	21st to end	Night	17% 16% 40% 25% 2	880
February	1st to 10th	Night	17% 17% 35% 30% 2	800
	11th to 20th	Night	24% 22% 34% 20% 1	783
	21st to end	Night	20% 19% 37% 22% 2	660
March	1st to 10th	Night	18% 18% 35% 25% 4	800
	11th to 20th	Night	22% 18% 31% 22% 6% 1	800
	21st to end	Night	19% 17% 32% 25% 5% 1	880
April	1st to 10th	Night	18% 21% 32% 25% 5%	800
	11th to 20th	Night	15% 16% 30% 28% 10% 1	791
	21st to end	Night	19% 11% 27% 35% 7% 1	742
May	1st to 10th	Night	19% 8% 24% 32% 16% 1	600
	11th to 20th	Night	19% 4 23% 37% 15% 2	591
	21st to end	Night	15% 3 19% 37% 24% 2	521
June	1st to 10th	Night	10% 3 21% 42% 22% 2	400
	11th to 20th	Night	10% 5% 14% 45% 25% 2	400
	21st to end	Night	7% 3 17% 49% 23% 2	400
July	1st to 10th	Night	11% 3 10% 38% 32% 5%	518
	11th to 20th	Night	7% 3 9% 34% 42% 5%	582
	21st to end	Night	7% 2 11% 39% 35% 6%	657
August	1st to 10th	Night	10% 3 11% 39% 33% 4	597
	11th to 20th	Night	12% 3 11% 36% 35% 3	594
	21st to end	Night	11% 4 13% 37% 31% 4	876
September	1st to 10th	Night	13% 2 15% 35% 30% 5%	800
	11th to 20th	Night	12% 4 15% 35% 30% 5%	800
	21st to end	Night	24% 5% 18% 31% 20% 1	800
October	1st to 10th	Night	17% 7% 18% 30% 24% 4	800
	11th to 20th	Night	14% 10% 27% 33% 16% 1	800
	21st to end	Night	15% 10% 30% 32% 12% 1	880
November	1st to 10th	Night	15% 10% 26% 36% 12% 2	820
	11th to 20th	Night	17% 14% 27% 32% 9% 1	840
	21st to end	Night	16% 14% 32% 30% 8% 1	840
December	1st to 10th	Night	20% 13% 32% 29% 6%	809
	11th to 20th	Night	16% 13% 32% 34% 5%	800
	21st to end	Night	17% 14% 29% 37% 4	880

Page generated at 2025-11-30 01:00:55. Version ac07

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