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 2024-07-03.

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	14% 6% 2 6% 4 4 4 4 6% 8% 44%	89564
	Night	15% 7% 2 6% 4 4 4 3 5% 7% 43%	89587
Transparency	Day	19% 54% 6% 12% 8% 1	89323
	Night	21% 53% 5% 12% 8% 1	89432
Seeing	Day	18% 9% 27% 25% 15% 6%	14620
	Night	17% 9% 27% 31% 14% 2	24097

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% 6% 3 5% 3 3 3 3 4 7% 48%	2517
	11th to 20th	Day	15% 9% 3 6% 4 5% 4 3 5% 5% 41%	2817
	21st to end	Day	16% 9% 3 8% 4 4 4 3 5% 6% 39%	3124
February	1st to 10th	Day	13% 8% 3 6% 4 5% 5% 4 6% 7% 39%	1710
	11th to 20th	Day	18% 7% 2 4 3 3 3 3 6% 7% 43%	1791
	21st to end	Day	18% 8% 3 6% 4 4 4 3 5% 7% 38%	1570
March	1st to 10th	Day	22% 6% 2 6% 3 3 4 3 5% 8% 38%	3263
	11th to 20th	Day	11% 4 1 4 3 3 4 3 5% 8% 54%	3480
	21st to end	Day	20% 7% 2 6% 4 3 3 2 5% 7% 41%	3964
April	1st to 10th	Day	12% 6% 2 5% 4 3 4 4 5% 8% 46%	2280
	11th to 20th	Day	11% 6% 2 5% 4 4 5% 3 5% 6% 51%	2254
	21st to end	Day	15% 6% 2 5% 4 3 3 3 6% 8% 46%	2280
May	1st to 10th	Day	21% 8% 3 6% 4 4 4 4 6% 8% 33%	3826
	11th to 20th	Day	19% 7% 2 7% 4 4 4 4 6% 8% 35%	4389
	21st to end	Day	9% 4 2 5% 4 3 4 4 7% 10% 48%	4823
June	1st to 10th	Day	9% 4 2 5% 4 4 5% 5% 7% 10% 45%	2660
	11th to 20th	Day	9% 5% 1 5% 4 4 5% 4 6% 9% 48%	2660
	21st to end	Day	8% 4 2 5% 4 3 4 4 8% 10% 48%	2660
July	1st to 10th	Day	8% 5% 2 6% 5% 4 5% 3 6% 10% 45%	2548
	11th to 20th	Day	5% 3 2 4 4 4 5% 5% 9% 11% 48%	2514
	21st to end	Day	7% 4 1 6% 5% 5% 5% 6% 10% 10% 41%	2547
August	1st to 10th	Day	9% 4 2 6% 3 4 6% 4 8% 11% 42%	2327
	11th to 20th	Day	9% 6% 2 7% 5% 5% 5% 4 6% 9% 43%	2135
	21st to end	Day	9% 4 2 6% 4 4 5% 4 7% 9% 45%	2376
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 6% 8% 45%	2160
	11th to 20th	Day	10% 7% 2 6% 4 3 3 3 5% 7% 49%	2160
	21st to end	Day	17% 8% 2 7% 4 3 4 3 6% 6% 40%	1933
October	1st to 10th	Day	15% 6% 2 6% 4 3 3 3 5% 6% 46%	1800
	11th to 20th	Day	17% 6% 2 4 2 2 3 2 4 6% 52%	1800
	21st to end	Day	14% 5% 2 6% 4 3 4 3 4 7% 48%	1980
November	1st to 10th	Day	13% 4 2 4 3 4 4 3 5% 7% 52%	1703
	11th to 20th	Day	14% 5% 2 5% 3 3 4 3 6% 7% 48%	1710
	21st to end	Day	15% 5% 3 5% 3 3 3 3 5% 5% 50%	1601
December	1st to 10th	Day	17% 5% 3 5% 4 4 5% 3 6% 7% 41%	1520
	11th to 20th	Day	16% 8% 2 6% 4 3 2 2 4 5% 48%	1910
	21st to end	Day	16% 7% 2 6% 4 4 5% 4 5% 7% 40%	2772
January	1st to 10th	Night	15% 7% 3 6% 4 4 4 3 4 6% 44%	4429
	11th to 20th	Night	17% 6% 3 7% 4 4 3 3 3 6% 44%	4138
	21st to end	Night	18% 10% 3 6% 4 3 3 2 4 5% 41%	4532
February	1st to 10th	Night	16% 7% 3 6% 4 4 3 3 5% 7% 44%	2470
	11th to 20th	Night	18% 6% 3 5% 4 3 3 3 6% 6% 42%	2274
	21st to end	Night	19% 8% 2 5% 2 3 3 3 5% 6% 44%	1884
March	1st to 10th	Night	22% 9% 3 6% 4 4 4 3 4 5% 38%	3588
	11th to 20th	Night	13% 5% 2 5% 3 3 4 3 5% 8% 49%	3480
	21st to end	Night	17% 8% 3 6% 3 3 3 2 4 5% 45%	3668
April	1st to 10th	Night	16% 7% 2 7% 4 3 5% 4 6% 6% 41%	1900
	11th to 20th	Night	11% 6% 3 6% 4 3 4 3 5% 7% 48%	1875
	21st to end	Night	15% 5% 2 6% 4 4 5% 4 5% 6% 44%	1900
May	1st to 10th	Night	19% 8% 2 7% 4 5% 5% 4 6% 7% 33%	3084
	11th to 20th	Night	20% 8% 3 6% 6% 4 5% 4 4 7% 33%	2500
	21st to end	Night	10% 5% 2 6% 4 4 5% 5% 6% 10% 44%	2732
June	1st to 10th	Night	11% 5% 2 5% 4 5% 5% 5% 6% 10% 42%	1520
	11th to 20th	Night	9% 5% 2 7% 5% 5% 5% 4 5% 11% 42%	1520
	21st to end	Night	8% 5% 2 6% 5% 5% 5% 4 7% 11% 41%	1520
July	1st to 10th	Night	9% 5% 3 7% 6% 5% 4 5% 7% 10% 41%	1456
	11th to 20th	Night	5% 5% 2 6% 4 4 5% 6% 8% 10% 44%	1442
	21st to end	Night	6% 6% 3 6% 5% 4 6% 5% 7% 11% 41%	1771
August	1st to 10th	Night	7% 5% 1 8% 5% 5% 4 4 7% 10% 42%	1611
	11th to 20th	Night	9% 7% 3 6% 5% 5% 5% 5% 7% 9% 39%	1745
	21st to end	Night	9% 6% 2 7% 5% 5% 5% 4 6% 9% 43%	1980
September	1st to 10th	Night	12% 6% 2 6% 4 4 3 4 5% 7% 48%	1800
	11th to 20th	Night	9% 6% 2 4 3 3 4 4 6% 7% 53%	1800
	21st to end	Night	18% 7% 2 6% 5% 4 4 3 6% 6% 38%	2023
October	1st to 10th	Night	16% 6% 2 6% 4 4 4 3 5% 6% 44%	2160
	11th to 20th	Night	17% 5% 2 5% 3 4 3 3 3 4 49%	2160
	21st to end	Night	17% 5% 2 5% 3 3 4 4 5% 5% 47%	2376
November	1st to 10th	Night	11% 7% 3 5% 3 3 3 2 4 6% 51%	2375
	11th to 20th	Night	16% 7% 3 5% 3 3 3 3 4 6% 47%	2470
	21st to end	Night	12% 5% 2 5% 4 4 3 3 5% 7% 50%	2575
December	1st to 10th	Night	18% 7% 2 6% 4 4 4 3 6% 7% 39%	2660
	11th to 20th	Night	18% 7% 2 6% 4 4 3 3 5% 6% 43%	3285
	21st to end	Night	14% 8% 3 6% 4 4 4 4 6% 8% 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	23% 38% 4 22% 11% 3	2518
	11th to 20th	Day	29% 38% 4 14% 12% 4	2817
	21st to end	Day	32% 40% 8% 14% 6% 1	3124
February	1st to 10th	Day	16% 67% 4 9% 4 1	1710
	11th to 20th	Day	17% 64% 3 9% 6% 2	1800
	21st to end	Day	18% 65% 6% 4 7% 1	1570
March	1st to 10th	Day	34% 41% 4 11% 7% 3	3263
	11th to 20th	Day	13% 41% 4 23% 17% 2	3480
	21st to end	Day	28% 40% 5% 11% 12% 3	3964
April	1st to 10th	Day	15% 65% 4 8% 7% 1	2280
	11th to 20th	Day	12% 66% 4 9% 8% 1	2254
	21st to end	Day	14% 65% 4 9% 6% 1	2280
May	1st to 10th	Day	35% 42% 6% 13% 3 1	3861
	11th to 20th	Day	34% 41% 10% 12% 3	4393
	21st to end	Day	18% 42% 9% 16% 13% 2	4837
June	1st to 10th	Day	13% 67% 4 9% 6%	2660
	11th to 20th	Day	8% 67% 6% 10% 8% 1	2660
	21st to end	Day	10% 64% 7% 11% 6% 1	2660
July	1st to 10th	Day	15% 63% 7% 8% 7% 1	2478
	11th to 20th	Day	10% 64% 8% 12% 6% 1	2439
	21st to end	Day	13% 63% 5% 10% 7% 1	2405
August	1st to 10th	Day	13% 65% 9% 10% 4	2314
	11th to 20th	Day	12% 66% 6% 10% 6%	2135
	21st to end	Day	11% 64% 7% 9% 8% 1	2376
September	1st to 10th	Day	14% 64% 8% 9% 4 1	2160
	11th to 20th	Day	13% 64% 4 9% 7% 2	2160
	21st to end	Day	21% 62% 3 9% 3 1	1933
October	1st to 10th	Day	14% 64% 5% 11% 6% 1	1800
	11th to 20th	Day	11% 63% 4 11% 10% 2	1800
	21st to end	Day	12% 62% 3 11% 9% 2	1980
November	1st to 10th	Day	16% 59% 5% 11% 7% 3	1703
	11th to 20th	Day	13% 60% 4 9% 11% 2	1710
	21st to end	Day	14% 59% 3 15% 7% 1	1601
December	1st to 10th	Day	21% 60% 5% 8% 4 2	1520
	11th to 20th	Day	21% 48% 4 17% 8% 3	1906
	21st to end	Day	28% 39% 9% 17% 8%	2772
January	1st to 10th	Night	27% 37% 5% 17% 11% 3	4429
	11th to 20th	Night	28% 37% 4 18% 11% 2	4138
	21st to end	Night	31% 39% 3 16% 9% 1	4532
February	1st to 10th	Night	15% 65% 6% 10% 4 1	2470
	11th to 20th	Night	17% 64% 2 7% 9% 2	2276
	21st to end	Night	15% 64% 4 9% 6% 2	1884
March	1st to 10th	Night	36% 41% 5% 11% 6% 1	3588
	11th to 20th	Night	18% 40% 6% 20% 14% 2	3480
	21st to end	Night	26% 40% 4 13% 13% 4	3668
April	1st to 10th	Night	16% 65% 3 8% 7% 1	1900
	11th to 20th	Night	14% 66% 4 9% 6% 2	1875
	21st to end	Night	16% 65% 4 9% 6% 1	1900
May	1st to 10th	Night	34% 42% 7% 13% 4	3114
	11th to 20th	Night	32% 43% 9% 13% 2	2501
	21st to end	Night	22% 41% 12% 14% 10% 2	2739
June	1st to 10th	Night	15% 67% 3 9% 7%	1520
	11th to 20th	Night	12% 66% 4 10% 7% 1	1520
	21st to end	Night	12% 65% 5% 10% 7% 2	1520
July	1st to 10th	Night	17% 63% 8% 6% 5% 2	1416
	11th to 20th	Night	11% 63% 9% 11% 6%	1394
	21st to end	Night	12% 64% 7% 11% 6%	1672
August	1st to 10th	Night	15% 64% 7% 7% 7%	1600
	11th to 20th	Night	11% 66% 8% 10% 5%	1745
	21st to end	Night	12% 64% 6% 9% 10% 1	1980
September	1st to 10th	Night	13% 64% 9% 10% 3 2	1800
	11th to 20th	Night	11% 64% 4 11% 8% 2	1800
	21st to end	Night	20% 64% 4 8% 4	2023
October	1st to 10th	Night	15% 64% 4 9% 6% 1	2160
	11th to 20th	Night	13% 63% 3 10% 10% 2	2160
	21st to end	Night	14% 62% 3 11% 8% 2	2376
November	1st to 10th	Night	17% 59% 4 11% 7% 2	2375
	11th to 20th	Night	15% 59% 3 8% 12% 3	2470
	21st to end	Night	12% 60% 5% 13% 10% 1	2575
December	1st to 10th	Night	22% 59% 5% 9% 3 2	2660
	11th to 20th	Night	21% 49% 4 13% 11% 3	3288
	21st to end	Night	29% 38% 11% 16% 6% 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	23% 19% 33% 18% 4 2	180
	11th to 20th	Day	26% 25% 32% 11% 5%	265
	21st to end	Day	20% 19% 42% 13% 5%	297
February	1st to 10th	Day	21% 19% 39% 15% 7%	270
	11th to 20th	Day	27% 20% 37% 14% 2	399
	21st to end	Day	25% 18% 36% 19% 3	382
March	1st to 10th	Day	22% 18% 33% 23% 3 1	460
	11th to 20th	Day	22% 17% 35% 19% 4 2	460
	21st to end	Day	23% 14% 35% 20% 6% 1	506
April	1st to 10th	Day	21% 15% 37% 18% 7% 2	460
	11th to 20th	Day	15% 13% 34% 22% 13% 2	452
	21st to end	Day	23% 12% 32% 22% 10% 2	460
May	1st to 10th	Day	24% 13% 26% 22% 11% 4	463
	11th to 20th	Day	19% 7% 31% 29% 10% 3	487
	21st to end	Day	19% 4 23% 28% 18% 8%	529
June	1st to 10th	Day	13% 5% 28% 31% 17% 7%	490
	11th to 20th	Day	11% 6% 24% 30% 21% 9%	490
	21st to end	Day	10% 3 26% 35% 20% 6%	490
July	1st to 10th	Day	11% 2 19% 31% 25% 12%	453
	11th to 20th	Day	5% 13% 35% 33% 14%	444
	21st to end	Day	10% 1 18% 33% 26% 11%	488
August	1st to 10th	Day	12% 2 18% 32% 26% 10%	446
	11th to 20th	Day	12% 2 14% 30% 28% 13%	439
	21st to end	Day	10% 2 17% 31% 25% 15%	495
September	1st to 10th	Day	16% 3 18% 25% 20% 18%	450
	11th to 20th	Day	14% 2 20% 29% 20% 14%	450
	21st to end	Day	27% 7% 22% 24% 14% 6%	450
October	1st to 10th	Day	19% 5% 24% 24% 19% 9%	450
	11th to 20th	Day	19% 9% 26% 25% 17% 4	450
	21st to end	Day	19% 9% 33% 24% 12% 3	495
November	1st to 10th	Day	18% 10% 27% 25% 15% 5%	381
	11th to 20th	Day	19% 14% 33% 21% 12% 2	370
	21st to end	Day	20% 14% 31% 21% 11% 3	261
December	1st to 10th	Day	18% 19% 38% 17% 6% 2	180
	11th to 20th	Day	24% 18% 33% 17% 8%	180
	21st to end	Day	23% 14% 27% 27% 8% 1	198
January	1st to 10th	Night	19% 18% 35% 23% 5%	760
	11th to 20th	Night	21% 13% 33% 27% 6%	760
	21st to end	Night	20% 16% 36% 25% 2	836
February	1st to 10th	Night	20% 12% 36% 28% 3	760
	11th to 20th	Night	23% 19% 34% 21% 1	743
	21st to end	Night	22% 17% 35% 23% 3	628
March	1st to 10th	Night	20% 15% 35% 24% 6%	760
	11th to 20th	Night	21% 14% 34% 22% 6% 1	760
	21st to end	Night	21% 12% 33% 28% 6%	836
April	1st to 10th	Night	19% 14% 40% 22% 6%	760
	11th to 20th	Night	14% 11% 35% 32% 8%	751
	21st to end	Night	19% 9% 31% 32% 9%	760
May	1st to 10th	Night	18% 7% 31% 33% 10% 1	740
	11th to 20th	Night	17% 6% 27% 40% 11%	379
	21st to end	Night	13% 2 19% 41% 21% 2	411
June	1st to 10th	Night	13% 3 21% 42% 20% 1	380
	11th to 20th	Night	9% 2 19% 45% 22% 2	380
	21st to end	Night	12% 1 18% 47% 20% 2	380
July	1st to 10th	Night	13% 1 12% 42% 30% 3	354
	11th to 20th	Night	5% 1 7% 44% 39% 3	350
	21st to end	Night	7% 2 15% 40% 32% 4	591
August	1st to 10th	Night	10% 1 10% 41% 34% 4	537
	11th to 20th	Night	13% 2 12% 39% 31% 4	681
	21st to end	Night	11% 2 9% 38% 33% 6%	792
September	1st to 10th	Night	16% 2 13% 35% 29% 6%	720
	11th to 20th	Night	12% 2 14% 35% 31% 6%	720
	21st to end	Night	22% 4 18% 32% 21% 3	720
October	1st to 10th	Night	19% 5% 17% 31% 22% 5%	720
	11th to 20th	Night	19% 9% 23% 30% 18% 1	720
	21st to end	Night	20% 10% 27% 28% 14% 2	792
November	1st to 10th	Night	14% 7% 31% 31% 14% 3	740
	11th to 20th	Night	21% 14% 27% 28% 10% 1	760
	21st to end	Night	15% 15% 29% 32% 9% 1	760
December	1st to 10th	Night	22% 14% 31% 23% 9% 1	760
	11th to 20th	Night	20% 12% 36% 26% 7%	760
	21st to end	Night	20% 11% 32% 33% 5%	836

Page generated at 2024-06-29 22:08:54. Version ac07

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