Culpeper
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Culpeper clear sky chart from 2005-11-06 to 2024-12-02.

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	14% 6% 2 6% 4 4 5% 4 6% 8% 41%	101351
	Night	15% 6% 2 6% 4 4 4 4 5% 7% 43%	98230
Transparency	Day	38% 8% 13% 23% 16% 4	104996
	Night	38% 7% 12% 23% 18% 4	102724
Seeing	Day	18% 12% 26% 25% 14% 5%	15084
	Night	17% 9% 28% 30% 14% 1	24741

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	17% 6% 2 5% 4 3 4 4 4 5% 46%	2749
	11th to 20th	Day	13% 9% 3 8% 4 5% 4 3 5% 4 40%	2840
	21st to end	Day	18% 9% 3 6% 4 4 4 3 5% 5% 39%	3124
February	1st to 10th	Day	12% 9% 3 7% 5% 4 4 3 5% 7% 41%	1710
	11th to 20th	Day	17% 7% 2 7% 4 3 3 4 5% 7% 42%	1679
	21st to end	Day	17% 9% 3 6% 4 4 4 4 6% 5% 38%	1598
March	1st to 10th	Day	21% 7% 3 6% 4 4 4 3 5% 6% 37%	3435
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 6% 7% 50%	3480
	21st to end	Day	21% 8% 2 6% 4 4 4 4 5% 6% 38%	3815
April	1st to 10th	Day	11% 5% 2 5% 4 5% 4 3 5% 7% 47%	2273
	11th to 20th	Day	13% 6% 2 5% 4 4 4 3 5% 6% 48%	2254
	21st to end	Day	17% 7% 2 6% 5% 4 4 3 5% 6% 41%	2280
May	1st to 10th	Day	19% 8% 3 8% 5% 5% 5% 4 6% 8% 30%	4082
	11th to 20th	Day	22% 7% 2 6% 4 4 5% 4 6% 9% 32%	4108
	21st to end	Day	10% 5% 2 5% 4 5% 5% 5% 7% 10% 43%	4823
June	1st to 10th	Day	11% 5% 2 5% 5% 4 5% 5% 6% 10% 43%	2660
	11th to 20th	Day	10% 5% 2 6% 5% 5% 6% 4 7% 11% 40%	2660
	21st to end	Day	8% 4 1 5% 5% 5% 6% 5% 7% 11% 43%	2660
July	1st to 10th	Day	6% 4 2 6% 5% 5% 7% 5% 8% 11% 41%	4412
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 6% 9% 12% 37%	4120
	21st to end	Day	11% 7% 3 8% 6% 5% 5% 5% 7% 9% 35%	4466
August	1st to 10th	Day	14% 5% 2 7% 5% 5% 6% 5% 8% 12% 32%	4104
	11th to 20th	Day	8% 4 2 6% 5% 6% 6% 5% 8% 10% 39%	3926
	21st to end	Day	10% 4 1 5% 4 4 5% 4 6% 8% 49%	4180
September	1st to 10th	Day	13% 5% 3 6% 4 4 4 4 6% 7% 45%	2280
	11th to 20th	Day	12% 6% 2 6% 3 4 4 3 5% 8% 47%	2250
	21st to end	Day	22% 6% 3 5% 4 4 5% 4 5% 5% 38%	1998
October	1st to 10th	Day	15% 6% 2 5% 3 4 3 3 4 6% 47%	1900
	11th to 20th	Day	15% 5% 1 5% 3 4 4 4 4 5% 50%	1900
	21st to end	Day	14% 5% 1 5% 3 4 4 3 4 6% 52%	2090
November	1st to 10th	Day	12% 6% 2 4 4 4 3 3 4 6% 53%	1942
	11th to 20th	Day	14% 5% 2 5% 4 3 4 4 5% 6% 49%	1743
	21st to end	Day	14% 5% 2 4 4 4 4 3 5% 5% 50%	1600
December	1st to 10th	Day	16% 6% 2 5% 3 3 4 4 6% 7% 42%	1528
	11th to 20th	Day	17% 6% 3 4 4 3 3 2 3 6% 49%	1910
	21st to end	Day	19% 6% 2 6% 4 4 5% 4 5% 8% 36%	2772
January	1st to 10th	Night	14% 7% 2 4 3 3 3 3 5% 6% 49%	4192
	11th to 20th	Night	17% 9% 2 4 3 3 3 3 4 5% 48%	4120
	21st to end	Night	18% 8% 3 6% 3 3 3 3 4 5% 43%	4532
February	1st to 10th	Night	15% 7% 2 6% 4 4 4 4 5% 6% 42%	2470
	11th to 20th	Night	19% 8% 2 6% 3 4 3 3 4 6% 43%	2385
	21st to end	Night	16% 8% 3 6% 3 4 3 4 5% 5% 43%	1855
March	1st to 10th	Night	20% 7% 2 7% 4 3 4 4 6% 5% 38%	3418
	11th to 20th	Night	12% 6% 2 4 4 3 4 4 5% 7% 51%	3480
	21st to end	Night	14% 7% 2 5% 3 4 3 3 5% 6% 48%	3817
April	1st to 10th	Night	14% 8% 2 5% 4 4 4 4 4 6% 44%	1906
	11th to 20th	Night	11% 5% 2 5% 5% 4 4 3 5% 7% 50%	1875
	21st to end	Night	16% 6% 2 5% 4 3 4 4 6% 6% 44%	1900
May	1st to 10th	Night	21% 9% 3 7% 4 4 4 3 6% 7% 32%	2827
	11th to 20th	Night	25% 7% 3 6% 5% 4 4 4 6% 6% 31%	2781
	21st to end	Night	11% 5% 2 5% 5% 5% 4 4 6% 10% 44%	2732
June	1st to 10th	Night	11% 5% 2 5% 4 6% 3 4 8% 9% 45%	1520
	11th to 20th	Night	11% 5% 2 5% 5% 6% 4 4 6% 8% 43%	1520
	21st to end	Night	10% 4 1 5% 5% 4 5% 5% 7% 9% 45%	1520
July	1st to 10th	Night	7% 4 2 7% 5% 5% 7% 5% 9% 10% 39%	2542
	11th to 20th	Night	8% 6% 3 7% 5% 6% 5% 5% 8% 11% 38%	2840
	21st to end	Night	9% 8% 3 8% 6% 5% 6% 5% 7% 8% 34%	3087
August	1st to 10th	Night	15% 6% 3 7% 5% 6% 5% 5% 8% 10% 32%	2825
	11th to 20th	Night	11% 7% 3 9% 5% 5% 5% 3 6% 8% 36%	2871
	21st to end	Night	12% 5% 2 6% 3 4 3 3 5% 8% 49%	3476
September	1st to 10th	Night	15% 6% 1 6% 3 4 5% 3 5% 7% 45%	1900
	11th to 20th	Night	12% 6% 2 6% 3 4 4 3 5% 7% 48%	1926
	21st to end	Night	22% 7% 2 5% 4 3 4 3 4 6% 40%	2180
October	1st to 10th	Night	18% 6% 2 6% 3 4 3 3 3 6% 45%	2280
	11th to 20th	Night	14% 5% 2 5% 3 3 4 3 5% 6% 50%	2280
	21st to end	Night	14% 5% 2 5% 3 3 3 3 6% 6% 51%	2508
November	1st to 10th	Night	11% 6% 3 5% 4 3 3 3 5% 5% 53%	2355
	11th to 20th	Night	14% 6% 2 4 4 4 4 4 4 7% 48%	2655
	21st to end	Night	13% 5% 1 4 4 4 4 4 4 7% 50%	2800
December	1st to 10th	Night	16% 8% 2 6% 5% 3 4 3 5% 6% 42%	2686
	11th to 20th	Night	17% 6% 2 5% 4 4 4 3 4 6% 46%	3285
	21st to end	Night	17% 6% 2 6% 4 4 5% 4 6% 7% 39%	4884

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% 5% 10% 29% 15% 5%	2750
	11th to 20th	Day	43% 6% 10% 18% 16% 7%	2840
	21st to end	Day	46% 7% 9% 21% 15% 3	3124
February	1st to 10th	Day	42% 5% 12% 24% 14% 3	1710
	11th to 20th	Day	42% 5% 10% 22% 18% 4	1688
	21st to end	Day	45% 6% 13% 15% 17% 4	1598
March	1st to 10th	Day	46% 7% 9% 19% 14% 5%	3435
	11th to 20th	Day	30% 6% 10% 24% 24% 6%	3480
	21st to end	Day	45% 6% 11% 18% 15% 4	3815
April	1st to 10th	Day	35% 7% 13% 20% 18% 8%	2273
	11th to 20th	Day	36% 6% 11% 20% 21% 6%	2254
	21st to end	Day	42% 7% 10% 22% 14% 4	2280
May	1st to 10th	Day	47% 11% 11% 19% 10% 2	4120
	11th to 20th	Day	46% 9% 12% 21% 9% 1	4112
	21st to end	Day	32% 10% 18% 20% 19% 2	4837
June	1st to 10th	Day	36% 9% 15% 24% 16% 1	2660
	11th to 20th	Day	37% 10% 14% 24% 13% 2	2660
	21st to end	Day	32% 10% 17% 24% 16% 1	2660
July	1st to 10th	Day	32% 13% 20% 19% 10% 6%	4342
	11th to 20th	Day	34% 11% 19% 26% 9% 1	4051
	21st to end	Day	44% 8% 11% 21% 15% 1	4324
August	1st to 10th	Day	41% 10% 19% 25% 6%	4091
	11th to 20th	Day	36% 10% 15% 24% 14% 1	3965
	21st to end	Day	32% 7% 16% 29% 15% 2	4180
September	1st to 10th	Day	38% 7% 11% 18% 20% 5%	2280
	11th to 20th	Day	35% 7% 8% 28% 19% 3	2250
	21st to end	Day	45% 7% 10% 19% 16% 3	1998
October	1st to 10th	Day	36% 5% 7% 18% 29% 5%	3160
	11th to 20th	Day	25% 3 8% 26% 27% 10%	3140
	21st to end	Day	21% 3 10% 29% 24% 12%	3428
November	1st to 10th	Day	32% 5% 10% 25% 24% 5%	1942
	11th to 20th	Day	35% 4 11% 21% 21% 7%	1743
	21st to end	Day	35% 6% 11% 24% 22% 3	1600
December	1st to 10th	Day	40% 6% 13% 24% 12% 4	1528
	11th to 20th	Day	37% 4 9% 31% 13% 6%	1906
	21st to end	Day	42% 8% 13% 25% 11% 1	2772
January	1st to 10th	Night	34% 5% 11% 25% 20% 5%	4192
	11th to 20th	Night	39% 4 10% 22% 20% 6%	4120
	21st to end	Night	43% 5% 8% 24% 18% 2	4532
February	1st to 10th	Night	41% 6% 11% 24% 16% 2	2470
	11th to 20th	Night	43% 3 8% 21% 18% 6%	2387
	21st to end	Night	41% 6% 11% 20% 17% 5%	1855
March	1st to 10th	Night	44% 7% 10% 22% 14% 2	3418
	11th to 20th	Night	29% 7% 10% 22% 26% 5%	3480
	21st to end	Night	36% 7% 10% 22% 19% 5%	3817
April	1st to 10th	Night	39% 6% 11% 18% 21% 5%	1906
	11th to 20th	Night	34% 6% 12% 22% 20% 6%	1875
	21st to end	Night	37% 7% 11% 25% 14% 5%	1900
May	1st to 10th	Night	50% 8% 11% 19% 10% 2	2854
	11th to 20th	Night	48% 10% 12% 20% 9% 1	2782
	21st to end	Night	33% 10% 17% 25% 13% 3	2739
June	1st to 10th	Night	33% 10% 14% 27% 15% 2	1520
	11th to 20th	Night	37% 9% 15% 23% 14% 1	1520
	21st to end	Night	33% 9% 17% 20% 18% 3	1520
July	1st to 10th	Night	36% 12% 22% 16% 12% 3	2502
	11th to 20th	Night	38% 11% 20% 21% 9% 1	2786
	21st to end	Night	43% 10% 15% 19% 13%	2988
August	1st to 10th	Night	44% 9% 16% 22% 7% 1	2814
	11th to 20th	Night	43% 9% 12% 22% 14% 1	2898
	21st to end	Night	33% 7% 14% 29% 16% 1	3476
September	1st to 10th	Night	38% 7% 11% 20% 17% 6%	1900
	11th to 20th	Night	35% 6% 9% 26% 21% 3	1926
	21st to end	Night	45% 6% 10% 19% 17% 3	2180
October	1st to 10th	Night	41% 5% 9% 20% 19% 6%	3800
	11th to 20th	Night	24% 4 10% 21% 31% 10%	3779
	21st to end	Night	21% 5% 9% 31% 27% 8%	4120
November	1st to 10th	Night	33% 4 9% 23% 24% 7%	2355
	11th to 20th	Night	36% 5% 7% 23% 22% 7%	2655
	21st to end	Night	34% 4 11% 24% 22% 5%	2800
December	1st to 10th	Night	42% 5% 11% 23% 15% 4	2686
	11th to 20th	Night	39% 5% 9% 22% 19% 5%	3288
	21st to end	Night	39% 8% 15% 23% 14% 1	4884

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	24% 24% 30% 17% 4	247
	11th to 20th	Day	27% 28% 26% 16% 3	270
	21st to end	Day	23% 26% 38% 10% 3 1	297
February	1st to 10th	Day	25% 24% 36% 13% 2	270
	11th to 20th	Day	36% 24% 29% 9% 1	286
	21st to end	Day	29% 20% 28% 20% 3	382
March	1st to 10th	Day	25% 18% 33% 20% 4	460
	11th to 20th	Day	23% 23% 30% 17% 6% 1	460
	21st to end	Day	25% 20% 33% 16% 6% 1	506
April	1st to 10th	Day	24% 22% 26% 21% 6% 1	460
	11th to 20th	Day	17% 19% 30% 20% 12% 3	452
	21st to end	Day	23% 12% 36% 18% 10% 1	460
May	1st to 10th	Day	23% 16% 27% 19% 11% 3	463
	11th to 20th	Day	22% 11% 28% 24% 11% 4	487
	21st to end	Day	18% 5% 26% 28% 17% 6%	529
June	1st to 10th	Day	13% 6% 26% 31% 19% 5%	490
	11th to 20th	Day	12% 8% 26% 28% 19% 7%	490
	21st to end	Day	10% 5% 27% 34% 19% 5%	490
July	1st to 10th	Day	10% 3 18% 32% 26% 11%	485
	11th to 20th	Day	5% 2 17% 36% 29% 12%	484
	21st to end	Day	12% 3 19% 31% 24% 10%	532
August	1st to 10th	Day	12% 4 16% 30% 27% 11%	486
	11th to 20th	Day	11% 3 18% 30% 26% 12%	479
	21st to end	Day	9% 3 19% 32% 22% 15%	539
September	1st to 10th	Day	16% 5% 18% 28% 20% 14%	490
	11th to 20th	Day	14% 3 21% 27% 22% 13%	490
	21st to end	Day	28% 10% 22% 25% 12% 5%	490
October	1st to 10th	Day	20% 7% 22% 29% 16% 7%	490
	11th to 20th	Day	15% 13% 27% 26% 17% 3	490
	21st to end	Day	15% 13% 32% 28% 9% 4	539
November	1st to 10th	Day	15% 13% 30% 26% 13% 3	488
	11th to 20th	Day	20% 12% 31% 27% 8% 2	343
	21st to end	Day	21% 15% 34% 24% 7%	200
December	1st to 10th	Day	19% 19% 40% 16% 7% 1	182
	11th to 20th	Day	21% 17% 34% 19% 8%	180
	21st to end	Day	23% 20% 30% 19% 7% 1	198
January	1st to 10th	Night	20% 16% 38% 23% 3	760
	11th to 20th	Night	19% 15% 33% 28% 5%	760
	21st to end	Night	18% 15% 39% 26% 2	836
February	1st to 10th	Night	20% 12% 38% 27% 3	760
	11th to 20th	Night	25% 18% 36% 19% 1	743
	21st to end	Night	19% 19% 38% 21% 2	628
March	1st to 10th	Night	18% 17% 37% 25% 3	760
	11th to 20th	Night	21% 17% 34% 21% 5% 1	760
	21st to end	Night	18% 15% 38% 22% 6%	836
April	1st to 10th	Night	18% 18% 35% 25% 5%	760
	11th to 20th	Night	14% 13% 35% 28% 9% 1	751
	21st to end	Night	18% 11% 33% 31% 7% 1	760
May	1st to 10th	Night	20% 9% 28% 28% 14% 1	584
	11th to 20th	Night	20% 4 28% 31% 16%	543
	21st to end	Night	14% 2 20% 39% 23% 2	411
June	1st to 10th	Night	10% 4 21% 43% 21% 2	380
	11th to 20th	Night	12% 4 19% 42% 22% 1	380
	21st to end	Night	9% 2 19% 48% 21% 2	380
July	1st to 10th	Night	11% 3 15% 39% 29% 3	379
	11th to 20th	Night	6% 1 13% 40% 36% 5%	552
	21st to end	Night	9% 3 15% 38% 31% 4	624
August	1st to 10th	Night	11% 2 12% 43% 29% 3	567
	11th to 20th	Night	12% 3 13% 39% 29% 4	611
	21st to end	Night	12% 3 14% 37% 30% 4	836
September	1st to 10th	Night	16% 3 14% 34% 29% 4	760
	11th to 20th	Night	13% 3 16% 34% 31% 3	760
	21st to end	Night	26% 4 19% 30% 19% 2	760
October	1st to 10th	Night	20% 5% 19% 30% 21% 4	760
	11th to 20th	Night	16% 8% 27% 32% 16% 1	760
	21st to end	Night	16% 8% 32% 31% 13% 1	836
November	1st to 10th	Night	15% 8% 33% 31% 12% 1	780
	11th to 20th	Night	17% 10% 34% 31% 8% 1	800
	21st to end	Night	15% 11% 35% 29% 10% 1	800
December	1st to 10th	Night	20% 10% 35% 26% 8%	768
	11th to 20th	Night	18% 8% 40% 30% 5%	760
	21st to end	Night	20% 11% 32% 34% 4	836

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