Mason Neck State Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mason Neck State Park clear sky chart from 2005-11-06 to 2026-10-29.

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% 42%	149381
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 43%	142689
Transparency	Day	38% 6% 12% 24% 17% 4	152976
	Night	39% 5% 11% 23% 18% 4	147165
Seeing	Day	18% 11% 26% 25% 14% 5%	17626
	Night	18% 10% 29% 29% 13% 1	27114

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 6% 4 4 4 3 5% 6% 47%	4875
	11th to 20th	Day	14% 7% 3 5% 3 3 3 2 3 4 51%	5280
	21st to end	Day	14% 6% 2 7% 4 4 4 3 6% 7% 44%	5780
February	1st to 10th	Day	15% 6% 3 7% 4 5% 5% 3 5% 7% 42%	1890
	11th to 20th	Day	20% 7% 2 6% 4 3 4 3 5% 6% 41%	1910
	21st to end	Day	17% 8% 3 7% 4 4 4 3 4 6% 40%	1730
March	1st to 10th	Day	18% 6% 2 5% 3 4 4 4 5% 7% 41%	4971
	11th to 20th	Day	11% 6% 2 5% 3 4 3 3 4 6% 51%	5090
	21st to end	Day	16% 7% 2 6% 5% 4 4 3 5% 6% 42%	5674
April	1st to 10th	Day	13% 6% 2 6% 4 4 4 3 5% 7% 45%	2520
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 4 7% 49%	2494
	21st to end	Day	16% 6% 2 5% 4 4 4 4 6% 7% 42%	2520
May	1st to 10th	Day	16% 7% 3 6% 4 4 5% 4 6% 9% 36%	7438
	11th to 20th	Day	17% 6% 2 5% 4 4 4 4 7% 10% 39%	7956
	21st to end	Day	21% 6% 2 6% 4 4 4 3 5% 7% 38%	9045
June	1st to 10th	Day	10% 4 2 6% 4 4 5% 4 6% 9% 45%	2940
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 6% 10% 44%	2940
	21st to end	Day	9% 3 1 5% 4 5% 5% 5% 8% 10% 44%	2940
July	1st to 10th	Day	8% 5% 2 6% 4 4 5% 4 7% 10% 44%	8280
	11th to 20th	Day	9% 6% 3 7% 4 5% 6% 5% 10% 13% 32%	7945
	21st to end	Day	8% 5% 2 7% 5% 5% 6% 5% 8% 11% 39%	8340
August	1st to 10th	Day	12% 6% 3 7% 5% 6% 6% 5% 9% 11% 30%	6014
	11th to 20th	Day	10% 5% 2 7% 5% 5% 6% 5% 8% 10% 37%	5676
	21st to end	Day	8% 3 2 5% 4 5% 5% 5% 7% 10% 47%	6080
September	1st to 10th	Day	13% 4 2 4 4 4 5% 3 5% 8% 48%	2520
	11th to 20th	Day	11% 6% 2 5% 3 4 4 3 6% 8% 48%	2520
	21st to end	Day	20% 7% 3 7% 4 3 4 3 5% 7% 35%	2208
October	1st to 10th	Day	16% 4 2 5% 3 3 4 3 5% 6% 49%	2100
	11th to 20th	Day	17% 4 2 4 3 3 3 2 5% 6% 52%	2100
	21st to end	Day	14% 5% 2 5% 3 4 3 3 4 6% 51%	2280
November	1st to 10th	Day	11% 5% 1 4 3 4 3 3 6% 7% 53%	1962
	11th to 20th	Day	14% 6% 1 5% 2 3 4 3 5% 6% 51%	1885
	21st to end	Day	15% 5% 2 5% 3 4 4 4 4 5% 49%	1680
December	1st to 10th	Day	11% 5% 2 7% 3 3 4 3 5% 8% 48%	2680
	11th to 20th	Day	17% 7% 3 6% 3 2 3 3 4 6% 45%	3070
	21st to end	Day	15% 8% 3 6% 4 4 4 4 6% 8% 39%	4048
January	1st to 10th	Night	12% 6% 2 6% 4 4 5% 4 6% 6% 45%	8016
	11th to 20th	Night	14% 6% 2 5% 3 3 3 2 4 5% 52%	7680
	21st to end	Night	13% 6% 2 6% 4 4 4 3 4 5% 48%	8401
February	1st to 10th	Night	15% 7% 2 6% 4 4 4 3 5% 7% 42%	2730
	11th to 20th	Night	20% 7% 2 6% 4 3 4 2 4 5% 43%	2595
	21st to end	Night	16% 9% 2 5% 4 4 3 3 5% 6% 43%	2076
March	1st to 10th	Night	16% 6% 2 5% 4 4 4 3 4 6% 47%	5085
	11th to 20th	Night	11% 5% 2 5% 3 3 4 4 6% 8% 50%	5090
	21st to end	Night	13% 7% 2 6% 4 4 4 4 5% 7% 45%	5499
April	1st to 10th	Night	15% 7% 2 7% 5% 4 4 3 6% 7% 42%	2100
	11th to 20th	Night	12% 5% 2 5% 4 4 4 3 5% 8% 47%	2075
	21st to end	Night	17% 5% 2 4 3 4 5% 4 5% 8% 43%	2100
May	1st to 10th	Night	17% 9% 3 7% 5% 4 5% 4 5% 8% 33%	5459
	11th to 20th	Night	19% 6% 2 6% 5% 4 4 3 6% 8% 37%	4901
	21st to end	Night	22% 8% 3 7% 4 4 4 3 5% 7% 33%	5108
June	1st to 10th	Night	10% 5% 1 5% 4 5% 5% 4 8% 9% 44%	1680
	11th to 20th	Night	11% 6% 2 6% 4 5% 4 5% 7% 9% 40%	1680
	21st to end	Night	9% 5% 2 5% 5% 5% 5% 5% 7% 9% 42%	1680
July	1st to 10th	Night	10% 6% 3 8% 4 5% 5% 4 6% 9% 40%	4678
	11th to 20th	Night	10% 7% 3 9% 6% 6% 6% 5% 7% 10% 30%	4994
	21st to end	Night	8% 7% 3 7% 5% 5% 6% 5% 7% 9% 37%	5736
August	1st to 10th	Night	14% 8% 3 8% 5% 5% 5% 4 8% 11% 29%	4135
	11th to 20th	Night	10% 7% 3 9% 6% 6% 6% 5% 7% 8% 32%	4322
	21st to end	Night	9% 6% 3 6% 4 4 5% 4 5% 8% 45%	5045
September	1st to 10th	Night	12% 5% 2 5% 4 4 5% 4 6% 7% 46%	2100
	11th to 20th	Night	11% 6% 2 5% 3 4 4 3 6% 9% 47%	2100
	21st to end	Night	20% 7% 3 7% 4 3 4 4 5% 7% 35%	2406
October	1st to 10th	Night	17% 6% 2 5% 3 4 3 3 4 6% 46%	2520
	11th to 20th	Night	17% 4 2 5% 3 3 3 3 4 5% 52%	2520
	21st to end	Night	14% 4 1 5% 3 3 3 3 5% 7% 51%	2737
November	1st to 10th	Night	13% 5% 3 5% 3 3 4 3 4 6% 53%	2554
	11th to 20th	Night	15% 6% 1 5% 3 3 3 3 4 6% 50%	2732
	21st to end	Night	12% 5% 2 4 4 4 4 4 5% 7% 49%	2940
December	1st to 10th	Night	10% 5% 2 5% 3 3 3 4 5% 6% 53%	4720
	11th to 20th	Night	20% 6% 2 5% 3 4 4 3 5% 6% 41%	5345
	21st to end	Night	16% 7% 2 6% 4 4 5% 4 6% 8% 38%	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	38% 4 8% 25% 19% 6%	4876
	11th to 20th	Day	38% 2 6% 21% 23% 11%	5280
	21st to end	Day	41% 3 8% 31% 14% 3	5780
February	1st to 10th	Day	43% 5% 11% 25% 14% 2	1890
	11th to 20th	Day	43% 4 8% 23% 18% 3	1919
	21st to end	Day	45% 6% 12% 18% 16% 4	1730
March	1st to 10th	Day	42% 4 9% 20% 17% 8%	4971
	11th to 20th	Day	34% 4 10% 24% 22% 7%	5090
	21st to end	Day	42% 5% 11% 18% 17% 7%	5674
April	1st to 10th	Day	38% 5% 11% 20% 20% 6%	2520
	11th to 20th	Day	36% 5% 10% 22% 21% 6%	2494
	21st to end	Day	40% 6% 10% 23% 16% 4	2520
May	1st to 10th	Day	44% 7% 12% 21% 16% 1	7471
	11th to 20th	Day	41% 7% 11% 27% 14% 1	7960
	21st to end	Day	45% 7% 10% 19% 17% 3	9061
June	1st to 10th	Day	34% 7% 14% 25% 18% 2	2940
	11th to 20th	Day	35% 8% 14% 25% 17% 2	2940
	21st to end	Day	31% 9% 14% 26% 17% 2	2940
July	1st to 10th	Day	34% 9% 19% 22% 13% 4	8210
	11th to 20th	Day	40% 10% 15% 25% 10% 1	7871
	21st to end	Day	36% 8% 12% 22% 21% 1	8156
August	1st to 10th	Day	45% 9% 18% 22% 6%	6001
	11th to 20th	Day	39% 9% 17% 24% 12% 1	5715
	21st to end	Day	30% 7% 17% 28% 17% 2	6080
September	1st to 10th	Day	35% 6% 14% 20% 21% 4	2520
	11th to 20th	Day	33% 6% 9% 28% 20% 4	2520
	21st to end	Day	47% 5% 10% 18% 17% 2	2208
October	1st to 10th	Day	36% 5% 6% 18% 29% 5%	3360
	11th to 20th	Day	26% 3 7% 29% 27% 9%	3340
	21st to end	Day	22% 3 9% 29% 27% 10%	3618
November	1st to 10th	Day	31% 5% 11% 27% 21% 5%	1962
	11th to 20th	Day	34% 4 10% 24% 21% 6%	1885
	21st to end	Day	38% 4 10% 27% 19% 3	1680
December	1st to 10th	Day	36% 5% 11% 27% 17% 4	2680
	11th to 20th	Day	41% 4 14% 27% 12% 3	3066
	21st to end	Day	42% 5% 14% 22% 16% 1	4048
January	1st to 10th	Night	38% 4 9% 24% 21% 4	8017
	11th to 20th	Night	36% 3 6% 23% 25% 9%	7680
	21st to end	Night	39% 3 6% 29% 19% 4	8401
February	1st to 10th	Night	41% 5% 11% 25% 17% 2	2730
	11th to 20th	Night	45% 3 8% 22% 18% 6%	2597
	21st to end	Night	42% 4 11% 21% 17% 6%	2076
March	1st to 10th	Night	39% 4 8% 24% 17% 9%	5085
	11th to 20th	Night	32% 4 11% 24% 23% 7%	5090
	21st to end	Night	39% 5% 8% 23% 20% 6%	5499
April	1st to 10th	Night	41% 6% 9% 19% 21% 4	2100
	11th to 20th	Night	36% 5% 12% 21% 21% 5%	2075
	21st to end	Night	38% 7% 10% 25% 15% 5%	2100
May	1st to 10th	Night	49% 6% 11% 20% 10% 3	5488
	11th to 20th	Night	44% 6% 12% 24% 13% 1	4902
	21st to end	Night	50% 6% 10% 17% 14% 2	5115
June	1st to 10th	Night	35% 8% 11% 26% 18% 2	1680
	11th to 20th	Night	38% 9% 15% 22% 14% 2	1680
	21st to end	Night	35% 9% 14% 21% 19% 3	1680
July	1st to 10th	Night	42% 9% 15% 20% 13% 2	4640
	11th to 20th	Night	47% 9% 16% 18% 9% 1	4948
	21st to end	Night	39% 8% 14% 21% 16% 1	5606
August	1st to 10th	Night	48% 10% 16% 18% 7%	4124
	11th to 20th	Night	46% 9% 14% 20% 11% 1	4349
	21st to end	Night	37% 6% 12% 27% 18% 1	5045
September	1st to 10th	Night	37% 7% 12% 22% 18% 6%	2100
	11th to 20th	Night	33% 7% 9% 27% 21% 3	2100
	21st to end	Night	47% 6% 11% 17% 15% 3	2406
October	1st to 10th	Night	40% 5% 8% 21% 20% 6%	4040
	11th to 20th	Night	25% 3 10% 21% 32% 9%	4019
	21st to end	Night	22% 3 9% 30% 27% 9%	4349
November	1st to 10th	Night	34% 3 9% 25% 23% 6%	2554
	11th to 20th	Night	36% 4 8% 23% 22% 7%	2732
	21st to end	Night	34% 5% 10% 26% 21% 4	2940
December	1st to 10th	Night	32% 4 11% 27% 22% 3	4720
	11th to 20th	Night	44% 4 11% 20% 16% 5%	5348
	21st to end	Night	43% 5% 15% 21% 14% 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% 18% 31% 17% 5%	259
	11th to 20th	Day	26% 26% 29% 15% 3	330
	21st to end	Day	21% 20% 38% 14% 7%	363
February	1st to 10th	Day	24% 20% 37% 15% 4 1	330
	11th to 20th	Day	32% 22% 33% 12% 1	397
	21st to end	Day	27% 18% 32% 20% 3	446
March	1st to 10th	Day	22% 20% 32% 20% 5%	540
	11th to 20th	Day	25% 19% 32% 17% 5% 1	540
	21st to end	Day	22% 17% 33% 22% 4 1	594
April	1st to 10th	Day	22% 18% 30% 22% 6% 1	540
	11th to 20th	Day	16% 18% 30% 21% 11% 3	532
	21st to end	Day	22% 12% 30% 24% 10% 2	540
May	1st to 10th	Day	23% 14% 30% 19% 11% 3	543
	11th to 20th	Day	19% 11% 29% 25% 14% 3	567
	21st to end	Day	21% 7% 23% 24% 18% 6%	617
June	1st to 10th	Day	12% 6% 28% 26% 21% 7%	570
	11th to 20th	Day	14% 7% 24% 31% 19% 6%	570
	21st to end	Day	10% 4 24% 34% 21% 7%	570
July	1st to 10th	Day	12% 2 14% 33% 27% 12%	565
	11th to 20th	Day	6% 2 13% 38% 29% 12%	564
	21st to end	Day	10% 3 19% 33% 25% 10%	620
August	1st to 10th	Day	12% 5% 16% 31% 28% 8%	566
	11th to 20th	Day	13% 4 17% 29% 26% 10%	559
	21st to end	Day	8% 3 21% 33% 23% 13%	627
September	1st to 10th	Day	13% 4 22% 27% 20% 14%	570
	11th to 20th	Day	12% 5% 17% 33% 22% 10%	570
	21st to end	Day	26% 9% 24% 25% 12% 4	570
October	1st to 10th	Day	19% 5% 21% 28% 18% 9%	570
	11th to 20th	Day	18% 12% 27% 27% 14% 3	570
	21st to end	Day	15% 12% 31% 28% 10% 4	615
November	1st to 10th	Day	16% 12% 31% 27% 12% 2	485
	11th to 20th	Day	19% 12% 34% 25% 8% 1	425
	21st to end	Day	22% 19% 30% 20% 9% 1	220
December	1st to 10th	Day	20% 13% 44% 17% 5% 2	220
	11th to 20th	Day	24% 15% 34% 21% 5% 1	220
	21st to end	Day	25% 15% 33% 20% 6%	242
January	1st to 10th	Night	20% 19% 38% 20% 3	840
	11th to 20th	Night	22% 15% 35% 24% 5%	840
	21st to end	Night	21% 15% 38% 23% 2	924
February	1st to 10th	Night	20% 14% 36% 26% 3	840
	11th to 20th	Night	26% 17% 38% 17% 1	823
	21st to end	Night	24% 17% 34% 23% 2	692
March	1st to 10th	Night	21% 14% 38% 24% 3	840
	11th to 20th	Night	21% 14% 37% 21% 6% 1	840
	21st to end	Night	18% 15% 38% 25% 3	924
April	1st to 10th	Night	18% 17% 38% 23% 4	840
	11th to 20th	Night	15% 13% 33% 31% 8%	831
	21st to end	Night	19% 8% 34% 31% 8%	840
May	1st to 10th	Night	19% 11% 31% 28% 11% 1	711
	11th to 20th	Night	17% 6% 29% 34% 12% 1	509
	21st to end	Night	17% 4 21% 37% 20% 1	455
June	1st to 10th	Night	10% 5% 23% 40% 20% 2	420
	11th to 20th	Night	14% 4 23% 37% 21% 1	420
	21st to end	Night	10% 3 23% 43% 18% 2	420
July	1st to 10th	Night	10% 2 16% 42% 27% 3	410
	11th to 20th	Night	8% 2 12% 39% 35% 4	516
	21st to end	Night	9% 2 17% 38% 31% 3	690
August	1st to 10th	Night	11% 2 13% 39% 31% 4	627
	11th to 20th	Night	12% 3 16% 39% 27% 3	721
	21st to end	Night	11% 3 16% 35% 29% 5%	924
September	1st to 10th	Night	14% 2 18% 33% 28% 4	840
	11th to 20th	Night	12% 3 15% 37% 30% 3	840
	21st to end	Night	24% 5% 21% 31% 18% 1	840
October	1st to 10th	Night	20% 5% 21% 29% 21% 5%	840
	11th to 20th	Night	18% 9% 26% 29% 17% 1	840
	21st to end	Night	17% 9% 30% 29% 14% 1	913
November	1st to 10th	Night	15% 9% 32% 30% 12% 2	820
	11th to 20th	Night	18% 12% 31% 29% 9%	840
	21st to end	Night	16% 14% 32% 28% 9% 1	840
December	1st to 10th	Night	21% 11% 35% 25% 8%	840
	11th to 20th	Night	19% 11% 38% 26% 5%	840
	21st to end	Night	20% 12% 33% 31% 4	924

Page generated at 2026-10-26 04:37:50. Version ac07

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