George Mason Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the George Mason Observatory 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	15% 6% 2 6% 4 4 4 4 6% 8% 41%	101423
	Night	15% 6% 2 6% 4 4 4 4 5% 7% 43%	98164
Transparency	Day	38% 7% 12% 24% 16% 3	105062
	Night	38% 7% 11% 23% 17% 4	102665
Seeing	Day	18% 10% 26% 25% 15% 6%	15076
	Night	17% 9% 29% 31% 13% 1	24633

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% 5% 2 4 3 4 3 3 5% 6% 48%	2617
	11th to 20th	Day	15% 10% 3 7% 3 4 4 3 4 5% 40%	2840
	21st to end	Day	19% 9% 2 7% 5% 4 4 3 4 6% 38%	3124
February	1st to 10th	Day	15% 7% 2 8% 5% 4 5% 3 5% 7% 39%	1710
	11th to 20th	Day	18% 8% 2 4 4 3 3 4 6% 7% 41%	1712
	21st to end	Day	19% 8% 3 5% 4 5% 4 3 4 6% 40%	1570
March	1st to 10th	Day	22% 7% 2 5% 4 4 5% 4 5% 7% 36%	3395
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 5% 6% 50%	3480
	21st to end	Day	20% 7% 2 6% 4 4 4 3 4 6% 39%	3868
April	1st to 10th	Day	13% 6% 2 7% 4 4 4 3 5% 8% 44%	2280
	11th to 20th	Day	13% 6% 2 5% 5% 4 3 3 5% 6% 48%	2254
	21st to end	Day	18% 6% 2 6% 4 4 4 4 6% 6% 41%	2280
May	1st to 10th	Day	23% 8% 3 6% 5% 4 5% 4 6% 8% 30%	4014
	11th to 20th	Day	22% 7% 3 6% 4 4 5% 4 6% 9% 30%	4257
	21st to end	Day	11% 5% 2 5% 4 4 5% 4 7% 9% 45%	4823
June	1st to 10th	Day	11% 5% 1 5% 5% 5% 5% 4 7% 9% 44%	2660
	11th to 20th	Day	10% 6% 2 6% 4 5% 5% 5% 7% 8% 43%	2660
	21st to end	Day	9% 4 2 5% 4 5% 4 6% 7% 10% 44%	2660
July	1st to 10th	Day	6% 3 2 6% 5% 5% 6% 6% 8% 11% 42%	4440
	11th to 20th	Day	10% 5% 2 5% 5% 5% 5% 5% 9% 11% 38%	4261
	21st to end	Day	11% 7% 3 7% 6% 6% 5% 5% 7% 8% 36%	4466
August	1st to 10th	Day	16% 6% 2 6% 5% 5% 5% 5% 7% 11% 33%	4104
	11th to 20th	Day	10% 4 2 6% 5% 5% 6% 5% 8% 9% 41%	3868
	21st to end	Day	10% 4 2 5% 4 4 4 3 6% 8% 48%	4180
September	1st to 10th	Day	14% 5% 2 5% 4 4 5% 4 6% 7% 45%	2280
	11th to 20th	Day	11% 6% 2 5% 4 3 4 3 6% 8% 48%	2280
	21st to end	Day	21% 7% 2 7% 4 3 4 3 6% 6% 37%	1994
October	1st to 10th	Day	17% 6% 3 5% 4 3 3 2 5% 6% 47%	1900
	11th to 20th	Day	17% 4 2 4 2 4 3 3 4 6% 51%	1900
	21st to end	Day	14% 6% 2 4 4 3 3 3 5% 6% 50%	2090
November	1st to 10th	Day	12% 5% 2 4 4 3 4 3 5% 7% 52%	1868
	11th to 20th	Day	16% 4 1 3 3 3 4 4 5% 6% 50%	1778
	21st to end	Day	15% 5% 2 5% 4 4 4 4 5% 6% 48%	1600
December	1st to 10th	Day	15% 7% 3 5% 5% 4 5% 4 7% 6% 41%	1528
	11th to 20th	Day	16% 6% 2 5% 3 4 4 3 4 8% 45%	1910
	21st to end	Day	18% 7% 2 7% 4 4 4 4 6% 8% 36%	2772
January	1st to 10th	Night	15% 7% 2 5% 4 3 3 3 5% 6% 47%	4324
	11th to 20th	Night	18% 8% 2 5% 3 3 3 2 4 5% 46%	4120
	21st to end	Night	19% 9% 3 6% 4 3 3 2 4 6% 42%	4532
February	1st to 10th	Night	15% 7% 3 6% 5% 4 4 4 6% 6% 41%	2470
	11th to 20th	Night	19% 7% 3 7% 3 3 3 3 5% 5% 44%	2352
	21st to end	Night	17% 8% 3 6% 3 3 3 3 5% 6% 43%	1884
March	1st to 10th	Night	21% 7% 3 6% 5% 4 4 3 4 6% 38%	3454
	11th to 20th	Night	12% 6% 2 5% 3 3 4 4 5% 8% 49%	3480
	21st to end	Night	15% 8% 2 6% 4 3 4 2 4 6% 47%	3763
April	1st to 10th	Night	15% 8% 2 7% 4 4 4 3 5% 7% 41%	1900
	11th to 20th	Night	14% 5% 2 5% 4 4 4 3 5% 7% 47%	1875
	21st to end	Night	17% 6% 2 4 4 4 4 5% 6% 7% 42%	1900
May	1st to 10th	Night	22% 10% 2 7% 4 4 5% 3 6% 7% 31%	2907
	11th to 20th	Night	23% 9% 3 6% 5% 3 4 3 5% 6% 32%	2633
	21st to end	Night	10% 5% 2 5% 5% 4 5% 4 6% 11% 43%	2732
June	1st to 10th	Night	11% 4 2 4 4 4 6% 5% 6% 8% 45%	1520
	11th to 20th	Night	12% 6% 2 6% 4 4 4 4 7% 10% 41%	1520
	21st to end	Night	9% 4 2 5% 5% 5% 4 4 7% 10% 43%	1520
July	1st to 10th	Night	7% 5% 2 8% 4 5% 6% 5% 8% 9% 42%	2520
	11th to 20th	Night	8% 6% 2 7% 5% 6% 6% 5% 7% 11% 38%	2695
	21st to end	Night	11% 7% 3 7% 6% 5% 5% 5% 7% 8% 36%	3087
August	1st to 10th	Night	17% 6% 2 8% 4 6% 5% 5% 6% 10% 33%	2825
	11th to 20th	Night	11% 7% 4 8% 5% 4 5% 5% 7% 9% 37%	2926
	21st to end	Night	10% 5% 2 5% 3 3 4 3 6% 7% 51%	3476
September	1st to 10th	Night	14% 5% 3 6% 3 4 6% 4 5% 7% 44%	1900
	11th to 20th	Night	10% 5% 3 6% 3 3 4 3 5% 8% 49%	1900
	21st to end	Night	20% 7% 3 7% 4 4 3 4 5% 7% 38%	2180
October	1st to 10th	Night	19% 5% 2 5% 4 3 3 3 4 5% 46%	2280
	11th to 20th	Night	15% 4 2 5% 3 3 3 3 5% 5% 52%	2280
	21st to end	Night	14% 5% 2 4 3 4 3 3 5% 7% 50%	2508
November	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 4 5% 54%	2426
	11th to 20th	Night	15% 6% 2 4 4 2 3 3 5% 6% 50%	2620
	21st to end	Night	14% 4 2 4 4 4 4 4 6% 7% 49%	2800
December	1st to 10th	Night	17% 7% 3 6% 4 4 4 4 4 7% 41%	2686
	11th to 20th	Night	17% 5% 2 5% 4 4 3 3 5% 7% 43%	3285
	21st to end	Night	17% 7% 2 6% 4 4 5% 4 6% 7% 37%	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	36% 5% 8% 32% 15% 4	2618
	11th to 20th	Day	45% 5% 9% 21% 15% 5%	2840
	21st to end	Day	48% 6% 10% 23% 12% 2	3124
February	1st to 10th	Day	44% 6% 12% 23% 13% 2	1710
	11th to 20th	Day	42% 5% 10% 24% 17% 3	1721
	21st to end	Day	46% 4 12% 18% 15% 3	1570
March	1st to 10th	Day	46% 7% 10% 21% 13% 3	3395
	11th to 20th	Day	31% 7% 9% 27% 21% 4	3480
	21st to end	Day	45% 6% 10% 20% 15% 3	3868
April	1st to 10th	Day	38% 6% 12% 20% 19% 5%	2280
	11th to 20th	Day	37% 5% 10% 21% 21% 6%	2254
	21st to end	Day	42% 7% 10% 22% 16% 4	2280
May	1st to 10th	Day	50% 9% 10% 20% 10% 1	4051
	11th to 20th	Day	47% 10% 13% 22% 8% 1	4261
	21st to end	Day	32% 10% 15% 24% 17% 2	4837
June	1st to 10th	Day	34% 9% 15% 25% 16% 1	2660
	11th to 20th	Day	36% 9% 13% 25% 15% 2	2660
	21st to end	Day	31% 10% 16% 25% 17% 2	2660
July	1st to 10th	Day	33% 10% 21% 20% 11% 5%	4370
	11th to 20th	Day	35% 9% 17% 28% 10% 1	4187
	21st to end	Day	42% 9% 11% 21% 17% 1	4324
August	1st to 10th	Day	43% 9% 16% 24% 7%	4091
	11th to 20th	Day	36% 9% 13% 27% 15% 1	3907
	21st to end	Day	32% 7% 14% 27% 18% 2	4180
September	1st to 10th	Day	37% 8% 12% 19% 20% 4	2280
	11th to 20th	Day	34% 5% 10% 26% 20% 4	2280
	21st to end	Day	45% 7% 10% 18% 18% 2	1994
October	1st to 10th	Day	38% 4 7% 17% 29% 5%	3160
	11th to 20th	Day	24% 4 7% 29% 26% 9%	3140
	21st to end	Day	22% 3 10% 30% 27% 9%	3428
November	1st to 10th	Day	32% 5% 12% 26% 20% 5%	1868
	11th to 20th	Day	34% 5% 11% 24% 21% 6%	1778
	21st to end	Day	37% 5% 9% 28% 18% 2	1600
December	1st to 10th	Day	41% 7% 12% 25% 12% 3	1528
	11th to 20th	Day	38% 5% 9% 32% 12% 4	1906
	21st to end	Day	43% 8% 12% 26% 10% 1	2772
January	1st to 10th	Night	37% 6% 9% 26% 18% 4	4324
	11th to 20th	Night	40% 4 9% 23% 19% 4	4120
	21st to end	Night	44% 5% 8% 26% 16% 1	4532
February	1st to 10th	Night	41% 6% 12% 24% 15% 2	2470
	11th to 20th	Night	43% 4 9% 22% 17% 5%	2354
	21st to end	Night	42% 5% 11% 21% 16% 5%	1884
March	1st to 10th	Night	45% 6% 10% 24% 12% 2	3454
	11th to 20th	Night	30% 7% 9% 28% 22% 4	3480
	21st to end	Night	39% 6% 8% 23% 19% 5%	3763
April	1st to 10th	Night	41% 6% 9% 18% 21% 4	1900
	11th to 20th	Night	36% 6% 12% 21% 20% 6%	1875
	21st to end	Night	38% 7% 12% 24% 14% 5%	1900
May	1st to 10th	Night	50% 9% 10% 18% 10% 2	2936
	11th to 20th	Night	51% 9% 10% 20% 9% 1	2634
	21st to end	Night	32% 11% 16% 24% 12% 3	2739
June	1st to 10th	Night	34% 10% 11% 27% 16% 2	1520
	11th to 20th	Night	37% 9% 14% 24% 13% 2	1520
	21st to end	Night	33% 10% 15% 21% 18% 3	1520
July	1st to 10th	Night	36% 10% 21% 17% 13% 3	2480
	11th to 20th	Night	37% 9% 20% 22% 10% 1	2646
	21st to end	Night	42% 9% 14% 20% 15% 1	2988
August	1st to 10th	Night	46% 8% 16% 21% 8% 1	2814
	11th to 20th	Night	42% 8% 12% 24% 13% 1	2953
	21st to end	Night	31% 7% 12% 29% 19% 2	3476
September	1st to 10th	Night	39% 7% 10% 20% 18% 6%	1900
	11th to 20th	Night	33% 7% 8% 27% 21% 4	1900
	21st to end	Night	45% 7% 10% 18% 16% 3	2180
October	1st to 10th	Night	41% 4 10% 20% 19% 6%	3800
	11th to 20th	Night	24% 4 11% 21% 31% 9%	3779
	21st to end	Night	22% 4 9% 31% 26% 8%	4120
November	1st to 10th	Night	33% 4 9% 25% 22% 6%	2426
	11th to 20th	Night	36% 4 9% 23% 22% 6%	2620
	21st to end	Night	34% 5% 12% 24% 21% 4	2800
December	1st to 10th	Night	43% 6% 10% 23% 15% 3	2686
	11th to 20th	Night	40% 5% 11% 24% 17% 4	3288
	21st to end	Night	42% 10% 14% 22% 11% 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% 16% 37% 17% 4 1	209
	11th to 20th	Day	30% 20% 29% 18% 4	270
	21st to end	Day	21% 21% 35% 16% 6% 1	297
February	1st to 10th	Day	22% 20% 37% 16% 4	270
	11th to 20th	Day	30% 24% 33% 12% 3	319
	21st to end	Day	28% 16% 31% 21% 3	382
March	1st to 10th	Day	22% 18% 33% 22% 5%	460
	11th to 20th	Day	22% 19% 34% 18% 7% 1	460
	21st to end	Day	23% 15% 32% 22% 7% 1	506
April	1st to 10th	Day	21% 17% 32% 22% 7% 1	460
	11th to 20th	Day	16% 16% 28% 23% 13% 4	452
	21st to end	Day	24% 11% 31% 21% 11% 2	460
May	1st to 10th	Day	24% 11% 27% 19% 15% 3	463
	11th to 20th	Day	20% 7% 30% 24% 15% 4	487
	21st to end	Day	18% 5% 22% 26% 21% 8%	529
June	1st to 10th	Day	13% 5% 25% 27% 21% 9%	490
	11th to 20th	Day	12% 6% 24% 30% 20% 8%	490
	21st to end	Day	9% 3 26% 36% 18% 8%	490
July	1st to 10th	Day	11% 2 15% 32% 25% 14%	485
	11th to 20th	Day	5% 2 15% 33% 32% 13%	484
	21st to end	Day	12% 2 16% 33% 27% 9%	532
August	1st to 10th	Day	12% 3 14% 29% 29% 12%	486
	11th to 20th	Day	11% 3 17% 28% 28% 12%	479
	21st to end	Day	7% 3 18% 30% 24% 18%	539
September	1st to 10th	Day	15% 4 20% 23% 20% 17%	490
	11th to 20th	Day	13% 3 20% 29% 20% 14%	490
	21st to end	Day	27% 7% 23% 26% 13% 5%	490
October	1st to 10th	Day	19% 7% 20% 26% 20% 8%	490
	11th to 20th	Day	15% 12% 25% 28% 16% 4	490
	21st to end	Day	16% 9% 32% 28% 12% 3	539
November	1st to 10th	Day	14% 12% 31% 28% 12% 3	450
	11th to 20th	Day	19% 11% 34% 23% 11% 2	378
	21st to end	Day	20% 20% 28% 22% 9% 2	200
December	1st to 10th	Day	19% 13% 41% 21% 4 2	182
	11th to 20th	Day	26% 11% 34% 22% 8% 1	180
	21st to end	Day	25% 11% 32% 23% 8% 1	198
January	1st to 10th	Night	20% 17% 41% 19% 3	760
	11th to 20th	Night	22% 12% 36% 26% 4	760
	21st to end	Night	20% 15% 39% 24% 2	836
February	1st to 10th	Night	19% 12% 38% 28% 3	760
	11th to 20th	Night	23% 18% 39% 18% 1	743
	21st to end	Night	23% 18% 36% 21% 3	628
March	1st to 10th	Night	19% 14% 39% 24% 4	760
	11th to 20th	Night	22% 14% 37% 22% 5% 1	760
	21st to end	Night	18% 15% 37% 25% 4	836
April	1st to 10th	Night	18% 17% 38% 23% 4	760
	11th to 20th	Night	15% 11% 35% 30% 8%	751
	21st to end	Night	19% 8% 32% 33% 8% 1	760
May	1st to 10th	Night	19% 7% 30% 31% 12% 1	627
	11th to 20th	Night	19% 4 28% 35% 12%	460
	21st to end	Night	13% 4 21% 40% 21% 1	411
June	1st to 10th	Night	11% 3 23% 40% 22% 1	380
	11th to 20th	Night	12% 5% 22% 39% 21% 2	380
	21st to end	Night	8% 2 22% 51% 17% 1	380
July	1st to 10th	Night	9% 1 19% 40% 27% 4	370
	11th to 20th	Night	6% 2 13% 39% 36% 5%	464
	21st to end	Night	9% 1 16% 41% 30% 3	624
August	1st to 10th	Night	10% 2 14% 42% 30% 3	567
	11th to 20th	Night	12% 2 15% 40% 28% 3	640
	21st to end	Night	11% 2 15% 39% 29% 4	836
September	1st to 10th	Night	15% 3 16% 31% 30% 4	760
	11th to 20th	Night	11% 3 14% 39% 30% 3	760
	21st to end	Night	22% 4 22% 32% 20% 1	760
October	1st to 10th	Night	22% 4 19% 29% 21% 5%	760
	11th to 20th	Night	16% 9% 27% 31% 17%	760
	21st to end	Night	17% 8% 32% 29% 14%	836
November	1st to 10th	Night	13% 8% 32% 33% 11% 2	780
	11th to 20th	Night	17% 12% 33% 30% 9% 1	800
	21st to end	Night	16% 13% 34% 29% 9% 1	800
December	1st to 10th	Night	21% 12% 34% 26% 7%	768
	11th to 20th	Night	18% 9% 39% 28% 5%	760
	21st to end	Night	20% 11% 34% 32% 4	836

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