Blackwater National Wildlife Refuge
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blackwater National Wildlife Refuge clear sky chart from 2005-11-06 to 2026-02-01.

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%	127365
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 44%	124645
Transparency	Day	24% 39% 8% 15% 11% 2	130954
	Night	25% 38% 8% 15% 12% 3	129125
Seeing	Day	18% 10% 28% 24% 14% 6%	16644
	Night	18% 10% 27% 31% 14% 2	26188

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	14% 4 2 5% 3 3 3 3 5% 7% 51%	3677
	11th to 20th	Day	14% 8% 3 6% 3 4 3 2 4 4 48%	4127
	21st to end	Day	14% 7% 2 7% 4 4 4 4 5% 6% 44%	4565
February	1st to 10th	Day	14% 8% 3 6% 4 5% 5% 4 6% 7% 39%	1800
	11th to 20th	Day	18% 8% 2 5% 3 3 3 3 5% 7% 43%	1888
	21st to end	Day	18% 8% 3 6% 3 4 4 3 5% 7% 40%	1650
March	1st to 10th	Day	18% 5% 2 5% 3 3 4 3 6% 8% 43%	4679
	11th to 20th	Day	11% 6% 2 5% 3 3 4 3 4 7% 53%	4980
	21st to end	Day	17% 7% 2 6% 4 3 3 3 5% 7% 43%	5684
April	1st to 10th	Day	13% 6% 2 5% 4 4 4 4 5% 8% 46%	2400
	11th to 20th	Day	11% 6% 2 5% 4 4 5% 3 5% 6% 50%	2374
	21st to end	Day	14% 6% 2 5% 4 3 3 3 6% 8% 46%	2400
May	1st to 10th	Day	19% 7% 2 6% 3 4 4 4 6% 9% 35%	5494
	11th to 20th	Day	18% 7% 2 6% 4 4 5% 3 6% 10% 35%	6309
	21st to end	Day	14% 5% 2 6% 4 4 4 4 6% 9% 42%	6934
June	1st to 10th	Day	9% 4 2 5% 4 4 5% 5% 6% 10% 45%	2800
	11th to 20th	Day	10% 4 1 5% 4 4 5% 4 6% 9% 48%	2800
	21st to end	Day	8% 3 2 5% 4 3 4 5% 7% 11% 49%	2800
July	1st to 10th	Day	7% 4 2 6% 4 4 5% 4 8% 12% 44%	6360
	11th to 20th	Day	9% 6% 2 7% 5% 5% 6% 6% 9% 12% 32%	6326
	21st to end	Day	7% 4 2 7% 5% 5% 6% 6% 9% 11% 39%	6424
August	1st to 10th	Day	15% 7% 2 6% 4 4 5% 4 8% 11% 35%	4234
	11th to 20th	Day	9% 6% 2 7% 5% 5% 5% 5% 6% 9% 41%	3859
	21st to end	Day	8% 4 2 5% 4 4 4 4 6% 10% 51%	4312
September	1st to 10th	Day	12% 5% 2 5% 3 4 4 4 6% 8% 46%	2400
	11th to 20th	Day	11% 7% 2 6% 4 3 3 3 5% 8% 48%	2400
	21st to end	Day	19% 9% 2 7% 3 3 4 3 6% 6% 38%	2146
October	1st to 10th	Day	14% 5% 2 6% 4 3 3 3 5% 6% 48%	2000
	11th to 20th	Day	17% 6% 2 4 2 2 3 2 3 5% 53%	2000
	21st to end	Day	13% 5% 2 6% 4 3 3 3 4 7% 50%	2200
November	1st to 10th	Day	12% 5% 2 5% 3 4 4 3 5% 7% 52%	1886
	11th to 20th	Day	13% 5% 2 4 3 3 5% 3 5% 7% 49%	1890
	21st to end	Day	15% 6% 3 5% 3 3 3 2 5% 5% 50%	1769
December	1st to 10th	Day	12% 5% 3 6% 3 3 4 3 5% 6% 50%	2680
	11th to 20th	Day	18% 8% 2 6% 4 3 2 2 3 5% 47%	3070
	21st to end	Day	13% 6% 2 6% 4 4 4 4 5% 8% 44%	4048
January	1st to 10th	Night	12% 7% 3 6% 4 3 4 3 4 6% 48%	6489
	11th to 20th	Night	13% 6% 3 6% 4 3 3 2 3 5% 52%	6048
	21st to end	Night	14% 8% 2 6% 4 3 4 3 4 5% 46%	6633
February	1st to 10th	Night	16% 7% 3 5% 4 4 3 3 5% 7% 43%	2601
	11th to 20th	Night	19% 6% 3 5% 4 3 3 3 5% 6% 42%	2397
	21st to end	Night	18% 8% 2 5% 3 3 4 3 5% 6% 44%	1980
March	1st to 10th	Night	16% 7% 2 5% 3 4 4 3 4 6% 47%	5158
	11th to 20th	Night	11% 6% 2 5% 3 3 3 3 5% 8% 49%	4980
	21st to end	Night	15% 7% 3 6% 4 4 4 3 4 6% 45%	5248
April	1st to 10th	Night	16% 7% 3 7% 4 3 5% 4 6% 6% 41%	2000
	11th to 20th	Night	11% 6% 3 6% 4 3 3 3 5% 7% 47%	1975
	21st to end	Night	15% 5% 2 6% 4 4 5% 4 5% 6% 44%	2000
May	1st to 10th	Night	17% 8% 2 7% 5% 5% 5% 5% 7% 8% 32%	4430
	11th to 20th	Night	19% 7% 2 6% 5% 5% 5% 4 5% 8% 33%	3580
	21st to end	Night	16% 7% 2 7% 4 4 4 4 5% 9% 37%	3920
June	1st to 10th	Night	11% 5% 2 6% 5% 5% 5% 5% 6% 10% 42%	1600
	11th to 20th	Night	10% 6% 2 7% 5% 5% 5% 4 6% 11% 41%	1600
	21st to end	Night	8% 5% 2 6% 5% 5% 5% 4 7% 11% 42%	1600
July	1st to 10th	Night	9% 6% 3 7% 5% 5% 4 5% 7% 11% 38%	3598
	11th to 20th	Night	11% 7% 2 8% 4 5% 5% 5% 9% 12% 33%	3613
	21st to end	Night	8% 7% 3 7% 5% 4 5% 4 7% 10% 39%	4427
August	1st to 10th	Night	14% 8% 3 8% 4 4 4 4 6% 9% 35%	2915
	11th to 20th	Night	11% 7% 3 7% 5% 5% 5% 4 6% 9% 38%	3154
	21st to end	Night	8% 5% 2 6% 4 4 4 3 5% 8% 49%	3586
September	1st to 10th	Night	12% 6% 2 6% 4 4 3 4 5% 7% 48%	2000
	11th to 20th	Night	10% 6% 2 4 3 3 4 4 6% 7% 51%	2000
	21st to end	Night	19% 8% 2 6% 4 4 4 3 6% 6% 35%	2249
October	1st to 10th	Night	16% 6% 2 6% 4 4 4 3 5% 6% 45%	2400
	11th to 20th	Night	17% 5% 2 5% 3 4 3 3 3 5% 50%	2400
	21st to end	Night	16% 5% 2 5% 3 3 4 4 5% 5% 49%	2640
November	1st to 10th	Night	10% 7% 3 5% 3 3 3 2 4 6% 52%	2632
	11th to 20th	Night	16% 7% 3 5% 3 3 4 3 4 6% 47%	2730
	21st to end	Night	12% 5% 2 5% 4 4 3 3 5% 7% 50%	2847
December	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 5% 7% 52%	4720
	11th to 20th	Night	20% 7% 2 6% 4 3 4 4 5% 6% 39%	5345
	21st to end	Night	13% 8% 3 6% 4 4 4 4 6% 8% 42%	7150

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	24% 26% 3 21% 22% 4	3678
	11th to 20th	Day	29% 26% 4 17% 18% 6%	4127
	21st to end	Day	31% 28% 9% 21% 10% 1	4565
February	1st to 10th	Day	18% 63% 5% 9% 4 1	1800
	11th to 20th	Day	19% 60% 3 9% 7% 2	1897
	21st to end	Day	18% 62% 5% 6% 8% 1	1650
March	1st to 10th	Day	32% 29% 5% 13% 14% 7%	4679
	11th to 20th	Day	21% 29% 6% 22% 17% 4	4980
	21st to end	Day	30% 29% 7% 12% 16% 7%	5684
April	1st to 10th	Day	17% 62% 4 8% 7% 1	2400
	11th to 20th	Day	14% 63% 5% 9% 8% 1	2374
	21st to end	Day	14% 62% 5% 11% 7% 1	2400
May	1st to 10th	Day	36% 31% 12% 15% 6% 1	5526
	11th to 20th	Day	36% 31% 10% 17% 7%	6313
	21st to end	Day	30% 31% 10% 16% 12% 1	6949
June	1st to 10th	Day	15% 64% 4 10% 7% 1	2800
	11th to 20th	Day	10% 64% 7% 10% 8% 1	2800
	21st to end	Day	11% 62% 8% 12% 7% 1	2800
July	1st to 10th	Day	24% 30% 21% 13% 10% 2	6290
	11th to 20th	Day	37% 31% 16% 13% 3	6251
	21st to end	Day	28% 29% 10% 17% 15% 1	6240
August	1st to 10th	Day	30% 37% 13% 17% 3	4221
	11th to 20th	Day	23% 39% 9% 22% 7%	3895
	21st to end	Day	16% 38% 9% 21% 15% 1	4312
September	1st to 10th	Day	16% 58% 8% 10% 6% 2	2400
	11th to 20th	Day	17% 58% 5% 11% 8% 1	2400
	21st to end	Day	26% 57% 4 10% 3 1	2146
October	1st to 10th	Day	20% 37% 5% 14% 19% 5%	3260
	11th to 20th	Day	13% 36% 4 17% 20% 10%	3240
	21st to end	Day	10% 35% 5% 22% 19% 9%	3538
November	1st to 10th	Day	17% 54% 5% 13% 9% 3	1886
	11th to 20th	Day	15% 54% 5% 11% 12% 2	1890
	21st to end	Day	16% 54% 4 17% 8% 1	1769
December	1st to 10th	Day	24% 35% 9% 20% 9% 3	2680
	11th to 20th	Day	30% 30% 9% 19% 9% 3	3066
	21st to end	Day	28% 27% 11% 20% 13% 1	4048
January	1st to 10th	Night	28% 26% 4 18% 20% 4	6489
	11th to 20th	Night	26% 26% 4 21% 18% 5%	6048
	21st to end	Night	30% 28% 6% 21% 14% 2	6633
February	1st to 10th	Night	16% 62% 7% 10% 4	2601
	11th to 20th	Night	19% 60% 2 7% 9% 2	2399
	21st to end	Night	16% 61% 4 10% 6% 3	1980
March	1st to 10th	Night	31% 29% 5% 15% 11% 8%	5158
	11th to 20th	Night	23% 29% 8% 19% 16% 5%	4980
	21st to end	Night	29% 30% 6% 15% 15% 4	5248
April	1st to 10th	Night	18% 62% 4 8% 7% 1	2000
	11th to 20th	Night	15% 63% 4 9% 7% 2	1975
	21st to end	Night	17% 62% 5% 9% 7% 1	2000
May	1st to 10th	Night	37% 31% 11% 15% 5% 1	4460
	11th to 20th	Night	36% 31% 10% 15% 6%	3581
	21st to end	Night	34% 30% 10% 16% 9% 2	3927
June	1st to 10th	Night	16% 64% 3 9% 7% 1	1600
	11th to 20th	Night	14% 63% 5% 10% 7% 1	1600
	21st to end	Night	13% 62% 6% 11% 8% 1	1600
July	1st to 10th	Night	32% 31% 14% 13% 9% 2	3560
	11th to 20th	Night	34% 31% 20% 12% 3	3568
	21st to end	Night	31% 29% 11% 16% 14%	4297
August	1st to 10th	Night	35% 37% 10% 13% 4	2904
	11th to 20th	Night	25% 38% 11% 20% 6%	3184
	21st to end	Night	18% 37% 6% 20% 18% 1	3586
September	1st to 10th	Night	15% 58% 8% 11% 5% 2	2000
	11th to 20th	Night	14% 58% 5% 13% 8% 2	2000
	21st to end	Night	25% 57% 5% 8% 4	2249
October	1st to 10th	Night	23% 37% 7% 15% 15% 3	3920
	11th to 20th	Night	12% 36% 7% 13% 23% 9%	3899
	21st to end	Night	10% 35% 6% 23% 19% 7%	4252
November	1st to 10th	Night	18% 53% 4 11% 11% 2	2632
	11th to 20th	Night	17% 54% 4 10% 12% 4	2730
	21st to end	Night	14% 54% 5% 14% 11% 1	2847
December	1st to 10th	Night	20% 34% 10% 19% 14% 2	4720
	11th to 20th	Night	33% 31% 8% 15% 9% 4	5348
	21st to end	Night	31% 27% 11% 17% 13% 1	7150

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	28% 20% 30% 18% 4 1	220
	11th to 20th	Day	28% 26% 32% 10% 4	325
	21st to end	Day	21% 22% 41% 12% 4	363
February	1st to 10th	Day	24% 20% 36% 14% 6%	300
	11th to 20th	Day	30% 20% 34% 13% 2	436
	21st to end	Day	25% 18% 36% 18% 3	414
March	1st to 10th	Day	23% 19% 32% 22% 3 1	500
	11th to 20th	Day	23% 16% 35% 19% 5% 2	500
	21st to end	Day	23% 16% 36% 19% 6% 1	550
April	1st to 10th	Day	23% 15% 37% 17% 6% 2	500
	11th to 20th	Day	17% 15% 33% 20% 12% 2	492
	21st to end	Day	22% 12% 32% 22% 11% 1	500
May	1st to 10th	Day	24% 14% 27% 21% 10% 4	503
	11th to 20th	Day	20% 8% 31% 28% 10% 3	527
	21st to end	Day	21% 5% 23% 27% 17% 8%	573
June	1st to 10th	Day	13% 5% 28% 30% 17% 6%	530
	11th to 20th	Day	12% 6% 25% 30% 20% 8%	530
	21st to end	Day	10% 3 24% 36% 20% 6%	530
July	1st to 10th	Day	11% 2 20% 33% 23% 12%	525
	11th to 20th	Day	6% 16% 35% 32% 12%	524
	21st to end	Day	10% 1 19% 35% 26% 10%	576
August	1st to 10th	Day	13% 2 21% 30% 24% 10%	526
	11th to 20th	Day	12% 2 17% 30% 27% 12%	519
	21st to end	Day	9% 4 18% 31% 23% 14%	583
September	1st to 10th	Day	16% 5% 20% 25% 19% 15%	530
	11th to 20th	Day	16% 2 21% 29% 20% 13%	530
	21st to end	Day	29% 6% 24% 23% 13% 5%	530
October	1st to 10th	Day	17% 6% 24% 25% 18% 10%	530
	11th to 20th	Day	21% 11% 25% 24% 15% 4	530
	21st to end	Day	18% 10% 33% 24% 12% 3	583
November	1st to 10th	Day	18% 12% 28% 24% 14% 4	444
	11th to 20th	Day	20% 13% 33% 21% 10% 1	430
	21st to end	Day	22% 16% 31% 19% 9% 2	309
December	1st to 10th	Day	19% 20% 39% 16% 5% 1	220
	11th to 20th	Day	25% 17% 35% 15% 7%	220
	21st to end	Day	25% 15% 28% 24% 8% 1	242
January	1st to 10th	Night	20% 19% 34% 23% 5%	840
	11th to 20th	Night	22% 14% 33% 25% 5%	840
	21st to end	Night	21% 17% 36% 25% 2	924
February	1st to 10th	Night	20% 12% 37% 27% 3	801
	11th to 20th	Night	25% 20% 33% 21% 1	783
	21st to end	Night	22% 17% 36% 23% 3	660
March	1st to 10th	Night	20% 16% 35% 24% 5%	800
	11th to 20th	Night	21% 14% 35% 22% 6% 1	800
	21st to end	Night	21% 12% 34% 27% 5%	880
April	1st to 10th	Night	20% 15% 40% 21% 5%	800
	11th to 20th	Night	15% 12% 34% 31% 8%	791
	21st to end	Night	19% 9% 32% 32% 9%	800
May	1st to 10th	Night	18% 7% 32% 33% 10% 1	779
	11th to 20th	Night	17% 6% 27% 39% 11%	399
	21st to end	Night	15% 3 20% 40% 20% 2	433
June	1st to 10th	Night	13% 4 21% 42% 19% 1	400
	11th to 20th	Night	10% 3 19% 46% 21% 2	400
	21st to end	Night	11% 1 18% 48% 20% 2	400
July	1st to 10th	Night	12% 1 12% 42% 30% 2	390
	11th to 20th	Night	7% 1 8% 45% 36% 3	391
	21st to end	Night	8% 2 16% 40% 31% 4	657
August	1st to 10th	Night	11% 1 12% 40% 32% 4	597
	11th to 20th	Night	14% 2 12% 38% 30% 4	759
	21st to end	Night	11% 2 11% 39% 31% 6%	880
September	1st to 10th	Night	16% 2 14% 36% 28% 5%	800
	11th to 20th	Night	14% 2 13% 36% 29% 6%	800
	21st to end	Night	23% 4 19% 31% 21% 3	800
October	1st to 10th	Night	19% 5% 17% 31% 23% 6%	800
	11th to 20th	Night	19% 10% 23% 29% 18% 1	800
	21st to end	Night	19% 10% 27% 29% 14% 2	880
November	1st to 10th	Night	15% 8% 31% 31% 13% 2	820
	11th to 20th	Night	21% 14% 28% 27% 9% 1	840
	21st to end	Night	15% 16% 28% 31% 8% 1	840
December	1st to 10th	Night	22% 14% 32% 23% 8% 1	840
	11th to 20th	Night	21% 12% 35% 25% 6%	840
	21st to end	Night	20% 11% 31% 33% 5%	924

Page generated at 2026-01-29 01:21:41. Version ac07

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