Mount Airy Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Airy Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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 5% 4 6% 8% 40%	127028
	Night	14% 7% 2 6% 4 4 4 4 5% 7% 42%	123690
Transparency	Day	39% 7% 13% 23% 16% 3	130622
	Night	39% 6% 12% 23% 17% 3	128165
Seeing	Day	18% 12% 27% 24% 14% 5%	16332
	Night	17% 10% 30% 29% 12% 1	25823

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% 5% 2 5% 4 4 5% 4 5% 7% 46%	3661
	11th to 20th	Day	13% 9% 3 9% 5% 4 4 3 4 5% 43%	4060
	21st to end	Day	15% 9% 3 6% 4 4 4 3 4 6% 42%	4466
February	1st to 10th	Day	14% 8% 2 8% 5% 5% 5% 4 6% 5% 38%	1800
	11th to 20th	Day	18% 8% 2 6% 4 4 4 3 5% 6% 39%	1801
	21st to end	Day	18% 9% 2 7% 4 4 3 3 4 5% 41%	1650
March	1st to 10th	Day	18% 5% 2 5% 4 4 4 4 4 7% 43%	4840
	11th to 20th	Day	11% 7% 2 6% 4 4 4 4 5% 6% 48%	4980
	21st to end	Day	15% 6% 2 6% 5% 5% 5% 4 6% 7% 39%	5556
April	1st to 10th	Day	14% 8% 3 6% 5% 4 5% 3 7% 8% 38%	2400
	11th to 20th	Day	13% 6% 2 6% 4 4 4 3 5% 6% 45%	2374
	21st to end	Day	17% 7% 2 6% 4 4 4 3 5% 7% 41%	2400
May	1st to 10th	Day	21% 7% 3 6% 4 5% 4 4 7% 9% 30%	5805
	11th to 20th	Day	22% 7% 2 7% 4 4 5% 4 6% 9% 30%	6198
	21st to end	Day	16% 5% 2 7% 5% 4 5% 4 6% 9% 38%	6934
June	1st to 10th	Day	11% 5% 2 5% 4 4 5% 4 7% 10% 42%	2800
	11th to 20th	Day	12% 6% 2 6% 5% 4 5% 5% 6% 9% 39%	2800
	21st to end	Day	9% 4 2 5% 4 4 5% 4 7% 11% 46%	2800
July	1st to 10th	Day	7% 5% 2 6% 5% 5% 5% 5% 8% 10% 43%	6360
	11th to 20th	Day	10% 6% 2 7% 6% 6% 6% 5% 8% 11% 33%	6226
	21st to end	Day	9% 6% 2 6% 5% 4 6% 5% 7% 10% 40%	6424
August	1st to 10th	Day	17% 6% 2 6% 4 5% 6% 5% 7% 9% 34%	4234
	11th to 20th	Day	10% 5% 2 7% 5% 5% 6% 5% 7% 10% 39%	4013
	21st to end	Day	11% 5% 2 5% 4 5% 5% 4 6% 8% 47%	4312
September	1st to 10th	Day	14% 6% 2 6% 4 4 4 2 5% 8% 46%	2400
	11th to 20th	Day	12% 6% 2 6% 4 3 4 3 6% 8% 47%	2400
	21st to end	Day	20% 8% 3 6% 4 3 4 4 5% 7% 36%	2102
October	1st to 10th	Day	16% 6% 2 5% 4 3 3 3 5% 8% 46%	2000
	11th to 20th	Day	16% 6% 2 5% 3 4 4 3 4 6% 48%	2000
	21st to end	Day	13% 6% 2 6% 4 3 4 3 5% 5% 49%	2200
November	1st to 10th	Day	11% 6% 2 4 3 3 3 3 6% 8% 51%	1934
	11th to 20th	Day	13% 6% 2 5% 3 4 3 4 5% 6% 48%	1852
	21st to end	Day	14% 7% 3 6% 4 4 4 3 5% 5% 45%	1680
December	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 6% 7% 44%	2616
	11th to 20th	Day	19% 10% 2 6% 4 4 4 3 4 6% 40%	2990
	21st to end	Day	18% 8% 3 7% 4 4 4 3 6% 8% 35%	3960
January	1st to 10th	Night	12% 7% 2 6% 4 4 4 4 5% 7% 45%	6280
	11th to 20th	Night	14% 7% 2 6% 3 3 3 3 4 5% 49%	5900
	21st to end	Night	16% 8% 2 5% 3 3 4 3 4 6% 46%	6490
February	1st to 10th	Night	15% 7% 3 6% 5% 4 4 3 6% 7% 40%	2600
	11th to 20th	Night	18% 8% 2 7% 3 3 4 4 4 7% 40%	2483
	21st to end	Night	16% 9% 2 7% 6% 3 4 3 4 6% 40%	1980
March	1st to 10th	Night	16% 6% 2 5% 4 4 4 3 4 5% 46%	5010
	11th to 20th	Night	10% 5% 2 6% 4 4 4 3 5% 7% 49%	4980
	21st to end	Night	11% 8% 2 6% 4 4 4 3 5% 7% 44%	5376
April	1st to 10th	Night	15% 8% 3 6% 4 6% 4 4 6% 7% 38%	2000
	11th to 20th	Night	12% 7% 2 7% 5% 4 4 3 5% 8% 43%	1975
	21st to end	Night	16% 5% 2 5% 4 3 5% 4 6% 7% 43%	2000
May	1st to 10th	Night	23% 9% 3 7% 4 4 5% 4 6% 8% 28%	4133
	11th to 20th	Night	23% 8% 3 6% 4 4 4 4 7% 8% 31%	3679
	21st to end	Night	18% 6% 2 6% 5% 4 5% 4 7% 8% 35%	3920
June	1st to 10th	Night	11% 6% 2 5% 4 5% 5% 4 5% 10% 44%	1600
	11th to 20th	Night	11% 6% 3 8% 4 6% 5% 5% 6% 8% 40%	1600
	21st to end	Night	9% 6% 2 5% 5% 6% 4 4 7% 9% 44%	1600
July	1st to 10th	Night	7% 6% 3 7% 5% 5% 6% 5% 7% 9% 40%	3598
	11th to 20th	Night	9% 7% 3 9% 6% 6% 6% 5% 8% 9% 30%	3726
	21st to end	Night	9% 6% 2 6% 4 4 5% 4 7% 10% 42%	4427
August	1st to 10th	Night	17% 6% 2 7% 4 5% 5% 5% 6% 10% 33%	2915
	11th to 20th	Night	9% 7% 3 8% 5% 6% 6% 4 6% 8% 38%	3002
	21st to end	Night	11% 6% 2 5% 3 4 4 4 6% 8% 47%	3586
September	1st to 10th	Night	13% 6% 2 5% 4 3 4 4 5% 7% 47%	2000
	11th to 20th	Night	10% 5% 2 6% 4 3 4 2 4 7% 51%	2000
	21st to end	Night	19% 7% 2 6% 5% 4 4 3 5% 6% 38%	2292
October	1st to 10th	Night	19% 5% 2 5% 3 3 3 3 4 5% 47%	2400
	11th to 20th	Night	15% 5% 2 5% 2 3 3 3 4 6% 52%	2400
	21st to end	Night	15% 5% 2 5% 3 4 4 3 5% 7% 48%	2640
November	1st to 10th	Night	11% 5% 2 5% 4 3 3 2 4 7% 53%	2582
	11th to 20th	Night	14% 7% 2 5% 3 4 4 3 5% 5% 48%	2766
	21st to end	Night	13% 4 2 6% 4 5% 4 4 5% 7% 45%	2940
December	1st to 10th	Night	10% 6% 2 6% 4 4 4 3 5% 7% 48%	4609
	11th to 20th	Night	20% 7% 2 6% 4 4 4 3 5% 6% 39%	5205
	21st to end	Night	16% 9% 2 6% 4 4 5% 4 6% 8% 34%	6996

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% 10% 26% 18% 5%	3662
	11th to 20th	Day	42% 5% 9% 21% 17% 6%	4060
	21st to end	Day	42% 6% 11% 24% 15% 3	4466
February	1st to 10th	Day	45% 7% 12% 24% 11% 1	1800
	11th to 20th	Day	45% 5% 10% 24% 14% 2	1810
	21st to end	Day	45% 5% 11% 20% 14% 4	1650
March	1st to 10th	Day	39% 6% 10% 22% 16% 6%	4840
	11th to 20th	Day	34% 6% 12% 25% 18% 6%	4980
	21st to end	Day	42% 6% 10% 18% 18% 5%	5556
April	1st to 10th	Day	41% 6% 13% 19% 16% 4	2400
	11th to 20th	Day	38% 6% 12% 21% 18% 5%	2374
	21st to end	Day	41% 6% 10% 21% 18% 3	2400
May	1st to 10th	Day	47% 9% 14% 19% 9% 2	5839
	11th to 20th	Day	47% 9% 13% 22% 9% 1	6202
	21st to end	Day	40% 8% 13% 22% 15% 1	6949
June	1st to 10th	Day	35% 8% 15% 25% 15% 2	2800
	11th to 20th	Day	38% 11% 13% 23% 13% 2	2800
	21st to end	Day	31% 9% 16% 25% 18% 1	2800
July	1st to 10th	Day	33% 10% 17% 20% 16% 3	6290
	11th to 20th	Day	40% 11% 17% 22% 8% 1	6151
	21st to end	Day	36% 8% 11% 23% 20% 1	6240
August	1st to 10th	Day	44% 9% 15% 24% 8%	4221
	11th to 20th	Day	37% 8% 13% 27% 14% 1	4052
	21st to end	Day	34% 7% 13% 27% 17% 2	4312
September	1st to 10th	Day	37% 7% 12% 21% 20% 4	2400
	11th to 20th	Day	34% 6% 10% 26% 20% 3	2400
	21st to end	Day	46% 6% 10% 18% 18% 2	2102
October	1st to 10th	Day	37% 5% 7% 19% 27% 5%	3260
	11th to 20th	Day	27% 5% 10% 27% 22% 9%	3240
	21st to end	Day	23% 4 11% 29% 29% 4	3538
November	1st to 10th	Day	32% 4 14% 25% 20% 4	1934
	11th to 20th	Day	35% 5% 10% 24% 21% 4	1852
	21st to end	Day	39% 6% 11% 28% 15% 1	1680
December	1st to 10th	Day	37% 5% 16% 28% 10% 4	2616
	11th to 20th	Day	45% 4 13% 27% 10% 2	2986
	21st to end	Day	44% 7% 16% 19% 14% 1	3960
January	1st to 10th	Night	35% 6% 10% 24% 22% 3	6280
	11th to 20th	Night	36% 4 10% 24% 19% 7%	5900
	21st to end	Night	39% 4 9% 28% 18% 2	6490
February	1st to 10th	Night	42% 5% 14% 24% 13% 1	2600
	11th to 20th	Night	43% 4 10% 22% 16% 4	2485
	21st to end	Night	44% 6% 10% 21% 14% 5%	1980
March	1st to 10th	Night	38% 5% 9% 26% 13% 9%	5010
	11th to 20th	Night	32% 6% 12% 28% 16% 6%	4980
	21st to end	Night	39% 5% 10% 20% 21% 4	5376
April	1st to 10th	Night	44% 6% 11% 18% 18% 4	2000
	11th to 20th	Night	39% 6% 12% 20% 18% 5%	1975
	21st to end	Night	38% 7% 12% 23% 16% 4	2000
May	1st to 10th	Night	52% 7% 11% 16% 10% 3	4161
	11th to 20th	Night	47% 9% 13% 21% 10% 1	3680
	21st to end	Night	43% 9% 14% 23% 9% 1	3927
June	1st to 10th	Night	35% 7% 12% 25% 17% 3	1600
	11th to 20th	Night	40% 9% 13% 24% 14% 2	1600
	21st to end	Night	34% 8% 14% 23% 18% 2	1600
July	1st to 10th	Night	40% 10% 15% 17% 15% 3	3560
	11th to 20th	Night	46% 12% 15% 17% 9% 1	3681
	21st to end	Night	35% 9% 13% 22% 20% 1	4297
August	1st to 10th	Night	45% 9% 17% 21% 8% 1	2904
	11th to 20th	Night	41% 7% 12% 26% 13% 1	3029
	21st to end	Night	33% 7% 13% 29% 15% 2	3586
September	1st to 10th	Night	36% 8% 11% 21% 20% 5%	2000
	11th to 20th	Night	33% 5% 11% 25% 22% 3	2000
	21st to end	Night	46% 7% 10% 17% 17% 3	2292
October	1st to 10th	Night	40% 5% 11% 20% 21% 4	3920
	11th to 20th	Night	25% 5% 10% 24% 29% 8%	3899
	21st to end	Night	25% 4 10% 32% 24% 6%	4252
November	1st to 10th	Night	32% 5% 10% 25% 23% 4	2582
	11th to 20th	Night	37% 5% 9% 23% 21% 5%	2766
	21st to end	Night	37% 5% 12% 24% 18% 3	2940
December	1st to 10th	Night	35% 4 13% 30% 15% 2	4609
	11th to 20th	Night	45% 5% 13% 18% 16% 3	5208
	21st to end	Night	43% 8% 14% 20% 15% 1	6996

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	31% 23% 29% 14% 2 1	213
	11th to 20th	Day	28% 26% 33% 11% 2	300
	21st to end	Day	21% 29% 34% 12% 3 1	330
February	1st to 10th	Day	25% 22% 39% 11% 3	300
	11th to 20th	Day	33% 24% 36% 6% 1	348
	21st to end	Day	29% 18% 33% 17% 2	414
March	1st to 10th	Day	21% 24% 30% 20% 4	500
	11th to 20th	Day	25% 20% 31% 18% 4 2	500
	21st to end	Day	23% 19% 33% 19% 6% 1	550
April	1st to 10th	Day	23% 18% 34% 17% 6% 2	500
	11th to 20th	Day	18% 19% 32% 15% 12% 4	492
	21st to end	Day	24% 13% 33% 20% 8% 2	500
May	1st to 10th	Day	24% 16% 28% 19% 9% 4	503
	11th to 20th	Day	21% 13% 27% 24% 11% 4	527
	21st to end	Day	18% 7% 24% 25% 20% 6%	573
June	1st to 10th	Day	13% 8% 29% 24% 19% 7%	530
	11th to 20th	Day	16% 7% 25% 28% 18% 5%	530
	21st to end	Day	9% 4 25% 35% 20% 7%	530
July	1st to 10th	Day	9% 5% 18% 33% 23% 12%	525
	11th to 20th	Day	6% 3 19% 32% 31% 10%	524
	21st to end	Day	12% 4 18% 35% 25% 7%	576
August	1st to 10th	Day	14% 3 18% 28% 26% 11%	526
	11th to 20th	Day	11% 5% 18% 32% 24% 11%	519
	21st to end	Day	8% 5% 20% 30% 23% 14%	583
September	1st to 10th	Day	16% 6% 21% 24% 21% 12%	530
	11th to 20th	Day	12% 3 21% 30% 22% 11%	530
	21st to end	Day	25% 9% 22% 27% 13% 4	530
October	1st to 10th	Day	19% 7% 20% 26% 20% 7%	530
	11th to 20th	Day	18% 14% 26% 27% 13% 3	530
	21st to end	Day	15% 11% 35% 26% 10% 2	583
November	1st to 10th	Day	14% 14% 31% 27% 11% 2	470
	11th to 20th	Day	21% 13% 33% 23% 9% 1	392
	21st to end	Day	25% 19% 35% 14% 8%	220
December	1st to 10th	Day	24% 20% 34% 18% 3 2	204
	11th to 20th	Day	26% 22% 27% 20% 7%	200
	21st to end	Day	25% 16% 35% 19% 5%	220
January	1st to 10th	Night	19% 19% 41% 18% 3	800
	11th to 20th	Night	21% 17% 33% 25% 3	800
	21st to end	Night	19% 16% 43% 21% 1	880
February	1st to 10th	Night	19% 16% 37% 26% 2	800
	11th to 20th	Night	26% 19% 38% 16% 1	783
	21st to end	Night	24% 17% 39% 17% 3	660
March	1st to 10th	Night	21% 15% 40% 22% 3	800
	11th to 20th	Night	21% 17% 35% 22% 5% 1	800
	21st to end	Night	19% 16% 41% 20% 4 1	880
April	1st to 10th	Night	20% 17% 38% 22% 4	800
	11th to 20th	Night	16% 13% 35% 29% 7% 1	791
	21st to end	Night	19% 10% 36% 28% 7% 1	800
May	1st to 10th	Night	21% 8% 30% 30% 11%	649
	11th to 20th	Night	20% 9% 27% 34% 10%	446
	21st to end	Night	13% 6% 22% 40% 18% 1	433
June	1st to 10th	Night	12% 3 27% 35% 23% 1	400
	11th to 20th	Night	13% 4 23% 43% 17% 1	400
	21st to end	Night	10% 2 22% 45% 20% 1	400
July	1st to 10th	Night	9% 4 17% 41% 27% 3	390
	11th to 20th	Night	8% 2 13% 41% 33% 2	439
	21st to end	Night	9% 2 14% 44% 28% 3	657
August	1st to 10th	Night	11% 3 11% 42% 30% 3	597
	11th to 20th	Night	11% 4 14% 42% 26% 3	669
	21st to end	Night	11% 3 15% 39% 28% 4	880
September	1st to 10th	Night	15% 3 19% 33% 26% 4	800
	11th to 20th	Night	12% 3 17% 38% 27% 4	800
	21st to end	Night	23% 4 21% 33% 18% 1	800
October	1st to 10th	Night	20% 5% 23% 29% 21% 3	800
	11th to 20th	Night	17% 9% 27% 33% 13% 1	800
	21st to end	Night	18% 9% 32% 29% 11%	880
November	1st to 10th	Night	14% 9% 32% 36% 8% 1	820
	11th to 20th	Night	19% 14% 33% 28% 6% 1	840
	21st to end	Night	17% 15% 34% 27% 7%	840
December	1st to 10th	Night	23% 12% 37% 22% 6%	809
	11th to 20th	Night	20% 9% 44% 23% 4	800
	21st to end	Night	18% 13% 38% 28% 2	880

Page generated at 2025-11-30 01:00:52. Version ac07

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