Montgomery County Astrophysical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Montgomery County Astrophysical Observatory clear sky chart from 2005-11-06 to 2026-09-14.

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% 41%	149015
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 42%	142062
Transparency	Day	40% 5% 11% 24% 16% 3	152610
	Night	40% 5% 11% 24% 17% 4	146539
Seeing	Day	20% 12% 28% 24% 13% 4	17384
	Night	18% 12% 29% 29% 11% 1	26832

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	15% 5% 2 6% 4 4 4 4 6% 6% 45%	4763
	11th to 20th	Day	14% 8% 3 6% 4 3 3 2 4 4 49%	5280
	21st to end	Day	15% 7% 2 6% 4 5% 4 4 5% 8% 41%	5780
February	1st to 10th	Day	14% 7% 2 7% 4 4 5% 4 6% 7% 39%	1890
	11th to 20th	Day	18% 8% 2 6% 4 4 5% 4 6% 6% 38%	1901
	21st to end	Day	18% 9% 3 6% 4 5% 4 3 5% 5% 40%	1730
March	1st to 10th	Day	19% 5% 2 5% 4 4 4 3 5% 6% 43%	4949
	11th to 20th	Day	12% 5% 2 6% 4 3 4 4 5% 6% 49%	5090
	21st to end	Day	16% 6% 2 6% 5% 4 4 4 6% 6% 41%	5679
April	1st to 10th	Day	15% 7% 2 6% 4 4 4 3 6% 8% 42%	2520
	11th to 20th	Day	12% 6% 2 6% 4 4 3 4 5% 6% 46%	2494
	21st to end	Day	18% 7% 2 5% 4 4 4 3 6% 7% 41%	2520
May	1st to 10th	Day	18% 7% 3 6% 4 4 4 4 6% 9% 35%	7492
	11th to 20th	Day	19% 5% 2 5% 4 4 5% 4 7% 9% 36%	8039
	21st to end	Day	23% 6% 2 6% 4 4 4 3 5% 7% 37%	9045
June	1st to 10th	Day	11% 5% 2 6% 5% 4 4 4 7% 8% 44%	2940
	11th to 20th	Day	11% 6% 2 6% 4 5% 5% 4 7% 8% 41%	2940
	21st to end	Day	9% 5% 2 4 4 4 5% 4 7% 10% 44%	2940
July	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 4 7% 9% 42%	8280
	11th to 20th	Day	9% 6% 2 7% 5% 6% 6% 6% 9% 11% 33%	8066
	21st to end	Day	9% 5% 2 7% 6% 5% 6% 5% 8% 10% 38%	8340
August	1st to 10th	Day	15% 6% 2 8% 6% 6% 6% 6% 8% 10% 29%	6014
	11th to 20th	Day	11% 5% 2 7% 5% 5% 6% 5% 8% 9% 37%	5700
	21st to end	Day	9% 4 2 5% 3 4 5% 4 7% 9% 47%	6080
September	1st to 10th	Day	13% 5% 2 5% 3 4 5% 4 5% 7% 46%	2520
	11th to 20th	Day	13% 5% 2 6% 4 4 4 3 6% 8% 46%	2436
	21st to end	Day	20% 8% 2 7% 3 4 4 4 5% 6% 36%	2105
October	1st to 10th	Day	17% 5% 2 4 4 3 3 2 5% 7% 47%	2000
	11th to 20th	Day	17% 4 2 5% 2 3 4 3 5% 6% 49%	2000
	21st to end	Day	13% 5% 2 6% 3 4 4 3 5% 6% 49%	2200
November	1st to 10th	Day	12% 4 2 4 4 3 4 3 6% 7% 51%	1940
	11th to 20th	Day	14% 6% 2 4 3 3 4 4 6% 6% 49%	1864
	21st to end	Day	14% 6% 3 5% 4 5% 4 3 5% 5% 46%	1680
December	1st to 10th	Day	13% 5% 2 6% 4 3 4 3 6% 7% 47%	2680
	11th to 20th	Day	18% 8% 2 5% 4 3 4 3 5% 6% 42%	3070
	21st to end	Day	17% 8% 2 7% 4 4 4 3 5% 8% 37%	4048
January	1st to 10th	Night	11% 7% 2 7% 5% 4 4 4 6% 6% 43%	8113
	11th to 20th	Night	14% 7% 2 6% 4 3 3 2 4 5% 50%	7680
	21st to end	Night	14% 7% 2 5% 3 3 3 3 5% 6% 48%	8401
February	1st to 10th	Night	14% 7% 2 6% 5% 5% 5% 4 5% 6% 41%	2730
	11th to 20th	Night	19% 7% 2 6% 4 3 3 3 5% 5% 42%	2604
	21st to end	Night	16% 8% 2 7% 4 4 3 3 4 6% 42%	2076
March	1st to 10th	Night	17% 6% 2 5% 3 3 3 3 4 5% 47%	5121
	11th to 20th	Night	11% 5% 3 5% 3 4 4 3 5% 7% 50%	5090
	21st to end	Night	12% 8% 3 6% 4 5% 4 3 5% 6% 44%	5481
April	1st to 10th	Night	15% 8% 3 5% 5% 4 5% 4 6% 7% 39%	2100
	11th to 20th	Night	13% 6% 2 6% 4 4 4 4 6% 7% 44%	2075
	21st to end	Night	15% 5% 2 6% 4 4 4 4 6% 8% 43%	2100
May	1st to 10th	Night	20% 8% 3 7% 5% 5% 4 4 6% 8% 33%	5415
	11th to 20th	Night	20% 7% 3 6% 4 3 4 4 6% 9% 35%	4817
	21st to end	Night	24% 7% 2 6% 4 4 4 3 5% 7% 33%	5108
June	1st to 10th	Night	10% 5% 2 6% 4 4 5% 4 7% 9% 46%	1680
	11th to 20th	Night	12% 6% 3 6% 5% 4 5% 5% 5% 8% 41%	1680
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 5% 6% 11% 42%	1680
July	1st to 10th	Night	11% 7% 3 7% 5% 5% 6% 4 6% 9% 39%	4678
	11th to 20th	Night	9% 7% 3 9% 6% 7% 6% 5% 8% 9% 32%	4873
	21st to end	Night	10% 6% 2 7% 5% 5% 5% 4 7% 9% 40%	5736
August	1st to 10th	Night	15% 7% 3 9% 5% 6% 6% 4 7% 9% 29%	4135
	11th to 20th	Night	11% 7% 3 9% 5% 5% 6% 4 6% 8% 34%	4299
	21st to end	Night	10% 6% 2 6% 4 4 4 4 6% 8% 45%	5045
September	1st to 10th	Night	14% 5% 3 5% 3 3 4 4 5% 7% 47%	2100
	11th to 20th	Night	11% 6% 3 6% 4 4 3 3 4 7% 49%	2031
	21st to end	Night	20% 7% 2 7% 4 4 4 3 4 6% 39%	2291
October	1st to 10th	Night	18% 6% 2 5% 3 3 3 3 4 5% 47%	2400
	11th to 20th	Night	16% 5% 1 4 3 3 4 2 4 5% 53%	2400
	21st to end	Night	15% 6% 1 4 3 3 4 4 5% 6% 49%	2640
November	1st to 10th	Night	11% 6% 2 5% 4 3 3 3 4 6% 54%	2577
	11th to 20th	Night	14% 6% 2 5% 4 3 3 3 4 6% 49%	2751
	21st to end	Night	13% 5% 2 5% 4 4 4 4 6% 7% 47%	2940
December	1st to 10th	Night	11% 5% 2 6% 4 4 4 3 5% 7% 50%	4720
	11th to 20th	Night	21% 6% 2 5% 4 4 4 3 5% 6% 41%	5345
	21st to end	Night	16% 7% 2 6% 4 5% 5% 4 6% 7% 37%	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	39% 4 6% 27% 19% 5%	4764
	11th to 20th	Day	40% 3 6% 23% 19% 9%	5280
	21st to end	Day	42% 3 9% 31% 12% 3	5780
February	1st to 10th	Day	43% 5% 12% 25% 13% 2	1890
	11th to 20th	Day	45% 4 9% 24% 16% 3	1910
	21st to end	Day	46% 3 10% 22% 15% 4	1730
March	1st to 10th	Day	41% 5% 9% 22% 17% 7%	4949
	11th to 20th	Day	34% 5% 9% 27% 19% 5%	5090
	21st to end	Day	43% 4 9% 20% 18% 6%	5679
April	1st to 10th	Day	40% 5% 12% 21% 18% 5%	2520
	11th to 20th	Day	37% 5% 10% 23% 20% 5%	2494
	21st to end	Day	42% 4 10% 23% 18% 3	2520
May	1st to 10th	Day	45% 5% 13% 21% 15% 1	7526
	11th to 20th	Day	43% 6% 11% 28% 12% 1	8043
	21st to end	Day	47% 4 10% 19% 17% 3	9061
June	1st to 10th	Day	36% 6% 14% 26% 17% 2	2940
	11th to 20th	Day	38% 7% 13% 25% 14% 2	2940
	21st to end	Day	33% 7% 15% 26% 18% 2	2940
July	1st to 10th	Day	37% 8% 15% 22% 14% 3	8210
	11th to 20th	Day	41% 8% 13% 26% 10% 1	7991
	21st to end	Day	39% 7% 9% 24% 20% 1	8156
August	1st to 10th	Day	48% 9% 16% 21% 7%	6001
	11th to 20th	Day	40% 7% 15% 26% 12% 1	5739
	21st to end	Day	33% 6% 15% 28% 16% 1	6080
September	1st to 10th	Day	36% 6% 13% 22% 19% 4	2520
	11th to 20th	Day	36% 4 8% 28% 20% 3	2436
	21st to end	Day	47% 5% 8% 19% 18% 2	2105
October	1st to 10th	Day	37% 4 7% 19% 28% 5%	3260
	11th to 20th	Day	26% 4 7% 29% 26% 9%	3240
	21st to end	Day	24% 3 8% 31% 28% 6%	3538
November	1st to 10th	Day	32% 4 12% 26% 21% 4	1940
	11th to 20th	Day	35% 4 10% 25% 22% 4	1864
	21st to end	Day	39% 5% 9% 29% 16% 2	1680
December	1st to 10th	Day	37% 4 14% 29% 13% 4	2680
	11th to 20th	Day	44% 4 12% 28% 10% 2	3066
	21st to end	Day	45% 4 14% 21% 15% 1	4048
January	1st to 10th	Night	40% 4 8% 26% 19% 3	8114
	11th to 20th	Night	37% 3 7% 22% 23% 8%	7680
	21st to end	Night	37% 2 7% 31% 19% 3	8401
February	1st to 10th	Night	42% 4 12% 25% 15% 2	2730
	11th to 20th	Night	44% 4 9% 22% 17% 4	2606
	21st to end	Night	44% 3 10% 22% 16% 5%	2076
March	1st to 10th	Night	40% 4 7% 26% 14% 9%	5121
	11th to 20th	Night	33% 5% 10% 28% 17% 7%	5090
	21st to end	Night	41% 3 9% 22% 20% 5%	5481
April	1st to 10th	Night	43% 5% 9% 20% 19% 4	2100
	11th to 20th	Night	39% 4 12% 22% 18% 5%	2075
	21st to end	Night	38% 6% 11% 25% 16% 4	2100
May	1st to 10th	Night	50% 4 11% 20% 12% 3	5444
	11th to 20th	Night	45% 5% 12% 25% 11% 1	4818
	21st to end	Night	51% 4 11% 18% 13% 2	5115
June	1st to 10th	Night	34% 7% 12% 27% 19% 2	1680
	11th to 20th	Night	40% 7% 13% 24% 14% 2	1680
	21st to end	Night	36% 6% 13% 24% 18% 3	1680
July	1st to 10th	Night	42% 8% 15% 19% 14% 2	4640
	11th to 20th	Night	46% 9% 13% 21% 10% 1	4828
	21st to end	Night	39% 7% 13% 23% 18% 1	5606
August	1st to 10th	Night	49% 8% 16% 18% 8% 1	4124
	11th to 20th	Night	45% 8% 13% 23% 10% 1	4326
	21st to end	Night	36% 6% 12% 29% 16% 1	5045
September	1st to 10th	Night	37% 5% 11% 23% 19% 5%	2100
	11th to 20th	Night	36% 4 8% 27% 21% 3	2031
	21st to end	Night	47% 5% 9% 19% 17% 3	2291
October	1st to 10th	Night	40% 4 9% 22% 20% 5%	3920
	11th to 20th	Night	25% 3 10% 24% 30% 8%	3899
	21st to end	Night	25% 3 9% 32% 25% 7%	4252
November	1st to 10th	Night	33% 3 10% 27% 23% 5%	2577
	11th to 20th	Night	37% 3 8% 24% 22% 5%	2751
	21st to end	Night	36% 4 11% 25% 20% 3	2940
December	1st to 10th	Night	35% 3 13% 28% 19% 2	4720
	11th to 20th	Night	45% 4 11% 19% 17% 4	5348
	21st to end	Night	45% 5% 13% 21% 15% 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	35% 16% 33% 12% 3 1	239
	11th to 20th	Day	32% 25% 30% 11% 2	330
	21st to end	Day	25% 23% 36% 11% 4 1	363
February	1st to 10th	Day	27% 20% 37% 12% 4	330
	11th to 20th	Day	35% 20% 36% 7% 2	388
	21st to end	Day	27% 20% 35% 17% 2	446
March	1st to 10th	Day	28% 21% 31% 16% 4	540
	11th to 20th	Day	28% 20% 29% 18% 3 2	540
	21st to end	Day	22% 21% 34% 17% 5% 1	594
April	1st to 10th	Day	24% 21% 31% 17% 6% 1	540
	11th to 20th	Day	19% 19% 34% 16% 10% 3	532
	21st to end	Day	26% 12% 32% 20% 8% 1	540
May	1st to 10th	Day	24% 18% 29% 19% 7% 3	543
	11th to 20th	Day	21% 12% 29% 24% 11% 3	567
	21st to end	Day	20% 8% 24% 27% 17% 4	617
June	1st to 10th	Day	13% 10% 28% 25% 19% 5%	570
	11th to 20th	Day	17% 7% 25% 31% 16% 4	570
	21st to end	Day	11% 8% 28% 30% 18% 6%	570
July	1st to 10th	Day	13% 3 17% 33% 22% 11%	565
	11th to 20th	Day	7% 3 21% 34% 27% 8%	564
	21st to end	Day	11% 5% 23% 33% 23% 6%	620
August	1st to 10th	Day	14% 4 20% 30% 23% 10%	566
	11th to 20th	Day	12% 5% 19% 34% 21% 9%	559
	21st to end	Day	10% 5% 21% 34% 20% 11%	627
September	1st to 10th	Day	15% 7% 20% 29% 19% 10%	570
	11th to 20th	Day	14% 4 20% 32% 18% 11%	542
	21st to end	Day	27% 7% 22% 28% 11% 4	530
October	1st to 10th	Day	22% 6% 20% 26% 18% 7%	530
	11th to 20th	Day	18% 15% 27% 25% 12% 3	530
	21st to end	Day	17% 14% 33% 24% 10% 2	583
November	1st to 10th	Day	16% 12% 36% 23% 11% 2	473
	11th to 20th	Day	22% 14% 35% 19% 9% 1	404
	21st to end	Day	27% 16% 33% 14% 9%	220
December	1st to 10th	Day	23% 20% 37% 16% 3 1	220
	11th to 20th	Day	25% 19% 32% 18% 6%	220
	21st to end	Day	26% 19% 33% 16% 6%	242
January	1st to 10th	Night	20% 19% 41% 17% 3	840
	11th to 20th	Night	23% 19% 31% 23% 3	840
	21st to end	Night	20% 18% 38% 22% 2	924
February	1st to 10th	Night	19% 16% 37% 26% 2	840
	11th to 20th	Night	26% 22% 35% 16% 1	823
	21st to end	Night	24% 17% 37% 20% 2	692
March	1st to 10th	Night	22% 17% 37% 21% 2	840
	11th to 20th	Night	23% 20% 33% 20% 4 1	840
	21st to end	Night	20% 19% 37% 20% 4 1	924
April	1st to 10th	Night	21% 22% 32% 22% 4	840
	11th to 20th	Night	16% 15% 36% 26% 7%	831
	21st to end	Night	19% 12% 34% 28% 7%	840
May	1st to 10th	Night	22% 10% 29% 27% 11%	693
	11th to 20th	Night	22% 6% 28% 33% 10%	477
	21st to end	Night	17% 6% 24% 37% 16% 1	455
June	1st to 10th	Night	11% 5% 27% 35% 21% 1	420
	11th to 20th	Night	14% 5% 25% 40% 15% 1	420
	21st to end	Night	10% 4 24% 44% 19% 1	420
July	1st to 10th	Night	10% 4 17% 41% 26% 3	410
	11th to 20th	Night	6% 3 14% 42% 32% 3	474
	21st to end	Night	10% 3 16% 42% 25% 3	690
August	1st to 10th	Night	13% 3 11% 42% 29% 2	627
	11th to 20th	Night	13% 4 13% 44% 23% 3	711
	21st to end	Night	11% 4 17% 39% 26% 4	924
September	1st to 10th	Night	16% 3 21% 32% 25% 3	840
	11th to 20th	Night	11% 5% 15% 37% 29% 4	813
	21st to end	Night	23% 5% 22% 33% 18% 1	800
October	1st to 10th	Night	20% 7% 20% 29% 21% 4	800
	11th to 20th	Night	18% 12% 26% 29% 15% 1	800
	21st to end	Night	19% 11% 31% 26% 12%	880
November	1st to 10th	Night	15% 11% 31% 32% 10% 1	820
	11th to 20th	Night	19% 17% 32% 26% 6% 1	840
	21st to end	Night	17% 17% 33% 27% 6%	840
December	1st to 10th	Night	22% 17% 33% 22% 6% 1	840
	11th to 20th	Night	20% 14% 37% 24% 5%	840
	21st to end	Night	19% 16% 34% 28% 3	924

Page generated at 2026-09-11 04:11:56. Version ac07

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