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 2025-04-26.

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% 7% 42%	113663
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	114942
Transparency	Day	39% 5% 11% 24% 17% 4	117301
	Night	39% 5% 11% 24% 18% 4	119444
Seeing	Day	20% 12% 28% 23% 13% 4	15515
	Night	18% 12% 29% 29% 12% 1	25150

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 4 3 4 4 4 6% 6% 49%	3671
	11th to 20th	Day	14% 9% 3 7% 4 3 3 2 4 4 46%	4060
	21st to end	Day	16% 8% 2 6% 4 4 4 3 4 6% 42%	4466
February	1st to 10th	Day	14% 7% 3 7% 4 5% 5% 4 6% 6% 38%	1800
	11th to 20th	Day	18% 8% 2 6% 4 4 4 4 6% 6% 38%	1808
	21st to end	Day	18% 9% 2 6% 4 5% 4 3 5% 5% 40%	1650
March	1st to 10th	Day	19% 5% 2 4 4 4 4 3 5% 6% 43%	4841
	11th to 20th	Day	12% 5% 2 6% 4 3 4 4 5% 6% 49%	4980
	21st to end	Day	16% 6% 2 6% 5% 4 4 4 6% 6% 41%	5556
April	1st to 10th	Day	15% 7% 2 6% 4 4 4 3 7% 8% 41%	2400
	11th to 20th	Day	13% 6% 2 6% 4 4 3 4 5% 6% 46%	2374
	21st to end	Day	18% 7% 2 5% 4 4 4 3 6% 8% 41%	2340
May	1st to 10th	Day	24% 7% 3 6% 4 4 4 4 6% 8% 30%	4014
	11th to 20th	Day	23% 7% 3 6% 4 5% 5% 4 7% 7% 30%	4298
	21st to end	Day	11% 5% 2 6% 4 4 5% 4 6% 9% 45%	4823
June	1st to 10th	Day	11% 6% 2 6% 5% 4 4 4 7% 8% 43%	2660
	11th to 20th	Day	11% 6% 2 6% 4 5% 5% 4 7% 8% 41%	2660
	21st to end	Day	10% 4 2 4 5% 5% 5% 4 8% 10% 44%	2660
July	1st to 10th	Day	6% 4 2 5% 5% 5% 6% 5% 8% 10% 43%	4440
	11th to 20th	Day	10% 5% 2 5% 5% 5% 5% 5% 8% 10% 41%	4324
	21st to end	Day	12% 7% 2 7% 5% 5% 6% 5% 7% 8% 35%	4466
August	1st to 10th	Day	17% 6% 2 6% 5% 6% 5% 5% 7% 9% 33%	4104
	11th to 20th	Day	10% 5% 2 7% 5% 5% 6% 5% 8% 8% 40%	3867
	21st to end	Day	11% 5% 2 6% 3 4 5% 4 6% 7% 48%	4180
September	1st to 10th	Day	14% 6% 2 5% 3 5% 4 4 5% 7% 45%	2280
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 6% 8% 47%	2280
	21st to end	Day	20% 8% 2 6% 3 4 4 4 6% 6% 37%	2000
October	1st to 10th	Day	18% 5% 2 4 4 3 3 2 5% 6% 46%	1900
	11th to 20th	Day	18% 4 1 5% 3 3 4 3 5% 6% 49%	1900
	21st to end	Day	13% 6% 2 5% 3 4 4 3 5% 6% 49%	2090
November	1st to 10th	Day	12% 5% 2 4 4 3 4 3 6% 7% 51%	1846
	11th to 20th	Day	14% 6% 2 4 3 3 4 4 6% 6% 49%	1775
	21st to end	Day	14% 6% 3 4 4 5% 4 3 5% 5% 47%	1600
December	1st to 10th	Day	13% 5% 2 6% 4 3 4 3 6% 7% 47%	2600
	11th to 20th	Day	18% 8% 2 5% 4 3 4 3 5% 6% 42%	2990
	21st to end	Day	17% 8% 2 7% 4 4 4 3 5% 8% 37%	3960
January	1st to 10th	Night	12% 7% 2 6% 4 3 4 3 5% 6% 48%	6257
	11th to 20th	Night	14% 7% 2 5% 3 3 3 2 4 5% 51%	5900
	21st to end	Night	16% 8% 2 5% 3 3 3 3 4 5% 48%	6490
February	1st to 10th	Night	15% 7% 2 6% 5% 4 5% 4 5% 6% 41%	2600
	11th to 20th	Night	19% 7% 2 6% 4 3 3 3 5% 5% 43%	2477
	21st to end	Night	16% 8% 2 7% 3 4 3 3 4 6% 42%	1980
March	1st to 10th	Night	17% 6% 2 5% 3 3 3 3 4 5% 48%	5009
	11th to 20th	Night	11% 5% 3 5% 3 4 4 3 5% 7% 50%	4980
	21st to end	Night	12% 8% 3 6% 4 4 4 3 5% 6% 44%	5362
April	1st to 10th	Night	16% 8% 3 5% 5% 4 5% 4 6% 7% 39%	2000
	11th to 20th	Night	14% 6% 3 6% 3 4 4 4 5% 7% 44%	1975
	21st to end	Night	15% 5% 2 6% 4 4 4 4 6% 8% 42%	1957
May	1st to 10th	Night	24% 8% 3 6% 4 4 3 3 5% 7% 32%	2907
	11th to 20th	Night	24% 8% 3 7% 4 3 4 4 5% 6% 32%	2591
	21st to end	Night	11% 4 2 5% 4 5% 4 4 6% 11% 43%	2732
June	1st to 10th	Night	11% 4 2 5% 4 4 5% 4 7% 9% 45%	1520
	11th to 20th	Night	11% 6% 3 6% 4 4 5% 5% 5% 8% 42%	1520
	21st to end	Night	9% 5% 2 6% 4 5% 5% 4 6% 11% 43%	1520
July	1st to 10th	Night	7% 5% 2 6% 5% 6% 7% 5% 7% 10% 40%	2520
	11th to 20th	Night	7% 6% 3 7% 6% 5% 6% 5% 8% 10% 38%	2632
	21st to end	Night	12% 7% 3 7% 5% 5% 5% 4 7% 8% 38%	3087
August	1st to 10th	Night	17% 6% 3 7% 5% 5% 5% 4 6% 9% 33%	2825
	11th to 20th	Night	10% 7% 3 9% 5% 5% 6% 4 7% 8% 37%	2929
	21st to end	Night	11% 4 2 5% 4 4 4 4 6% 9% 48%	3476
September	1st to 10th	Night	15% 6% 3 5% 3 3 4 4 5% 6% 46%	1900
	11th to 20th	Night	9% 6% 3 6% 3 3 3 3 4 7% 51%	1900
	21st to end	Night	20% 6% 2 7% 4 4 4 3 4 6% 40%	2176
October	1st to 10th	Night	19% 6% 2 5% 3 3 4 3 4 5% 46%	2280
	11th to 20th	Night	15% 5% 1 4 4 3 4 2 4 5% 53%	2280
	21st to end	Night	15% 6% 1 4 3 3 4 4 5% 6% 49%	2508
November	1st to 10th	Night	11% 6% 2 5% 4 3 3 3 4 5% 55%	2451
	11th to 20th	Night	15% 6% 2 5% 4 3 3 3 4 6% 49%	2620
	21st to end	Night	13% 4 2 5% 4 4 4 4 6% 7% 47%	2800
December	1st to 10th	Night	11% 5% 2 5% 4 4 4 3 5% 7% 50%	4580
	11th to 20th	Night	21% 6% 2 5% 4 4 4 3 5% 6% 40%	5205
	21st to end	Night	17% 7% 2 6% 4 5% 5% 4 6% 7% 37%	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	34% 5% 5% 28% 22% 6%	3672
	11th to 20th	Day	42% 3 7% 22% 18% 8%	4060
	21st to end	Day	44% 3 10% 27% 14% 3	4466
February	1st to 10th	Day	44% 5% 12% 25% 12% 2	1800
	11th to 20th	Day	45% 4 9% 24% 16% 3	1817
	21st to end	Day	46% 4 10% 22% 15% 4	1650
March	1st to 10th	Day	40% 4 9% 22% 17% 7%	4841
	11th to 20th	Day	34% 5% 9% 27% 20% 5%	4980
	21st to end	Day	44% 4 9% 19% 18% 6%	5556
April	1st to 10th	Day	41% 5% 11% 20% 18% 5%	2400
	11th to 20th	Day	38% 5% 10% 22% 20% 6%	2374
	21st to end	Day	42% 4 9% 23% 18% 3	2340
May	1st to 10th	Day	52% 5% 11% 20% 11% 1	4051
	11th to 20th	Day	49% 7% 14% 21% 8% 1	4302
	21st to end	Day	35% 6% 15% 25% 16% 3	4837
June	1st to 10th	Day	37% 6% 14% 27% 15% 2	2660
	11th to 20th	Day	38% 8% 13% 25% 14% 2	2660
	21st to end	Day	33% 7% 15% 26% 18% 2	2660
July	1st to 10th	Day	33% 9% 19% 21% 12% 5%	4370
	11th to 20th	Day	36% 7% 16% 29% 11% 2	4249
	21st to end	Day	44% 7% 10% 22% 16% 1	4324
August	1st to 10th	Day	46% 7% 15% 25% 8% 1	4091
	11th to 20th	Day	38% 7% 11% 28% 15% 1	3906
	21st to end	Day	35% 6% 12% 28% 17% 2	4180
September	1st to 10th	Day	38% 6% 12% 20% 19% 4	2280
	11th to 20th	Day	35% 5% 8% 28% 21% 4	2280
	21st to end	Day	46% 5% 8% 20% 19% 2	2000
October	1st to 10th	Day	38% 4 7% 18% 28% 5%	3160
	11th to 20th	Day	25% 4 7% 29% 26% 9%	3140
	21st to end	Day	24% 3 9% 31% 28% 6%	3428
November	1st to 10th	Day	33% 4 11% 26% 21% 5%	1846
	11th to 20th	Day	35% 4 10% 24% 22% 4	1775
	21st to end	Day	39% 5% 10% 29% 17% 2	1600
December	1st to 10th	Day	36% 4 14% 29% 14% 4	2600
	11th to 20th	Day	44% 4 12% 28% 10% 2	2986
	21st to end	Day	45% 4 14% 21% 15% 1	3960
January	1st to 10th	Night	37% 4 6% 25% 24% 4	6257
	11th to 20th	Night	36% 4 8% 25% 21% 7%	5900
	21st to end	Night	39% 2 8% 29% 19% 3	6490
February	1st to 10th	Night	43% 4 12% 25% 14% 1	2600
	11th to 20th	Night	43% 4 9% 22% 18% 4	2479
	21st to end	Night	44% 3 9% 22% 16% 5%	1980
March	1st to 10th	Night	39% 4 7% 26% 14% 9%	5009
	11th to 20th	Night	33% 5% 10% 28% 17% 7%	4980
	21st to end	Night	41% 3 8% 22% 21% 5%	5362
April	1st to 10th	Night	43% 5% 9% 20% 19% 4	2000
	11th to 20th	Night	39% 4 11% 21% 18% 5%	1975
	21st to end	Night	39% 6% 10% 25% 15% 5%	1957
May	1st to 10th	Night	52% 4 11% 20% 10% 2	2936
	11th to 20th	Night	51% 6% 11% 20% 10% 1	2592
	21st to end	Night	35% 7% 18% 25% 13% 3	2739
June	1st to 10th	Night	34% 7% 12% 28% 17% 2	1520
	11th to 20th	Night	38% 7% 13% 26% 14% 3	1520
	21st to end	Night	35% 6% 14% 24% 18% 3	1520
July	1st to 10th	Night	38% 8% 20% 18% 13% 4	2480
	11th to 20th	Night	38% 8% 17% 25% 11% 1	2584
	21st to end	Night	42% 7% 13% 21% 16% 1	2988
August	1st to 10th	Night	46% 7% 17% 21% 8% 1	2814
	11th to 20th	Night	43% 7% 10% 27% 12% 1	2956
	21st to end	Night	33% 6% 12% 31% 17% 2	3476
September	1st to 10th	Night	39% 5% 10% 22% 19% 5%	1900
	11th to 20th	Night	34% 4 8% 28% 22% 4	1900
	21st to end	Night	46% 4 9% 20% 17% 3	2176
October	1st to 10th	Night	41% 4 9% 22% 19% 5%	3800
	11th to 20th	Night	25% 3 10% 24% 30% 8%	3779
	21st to end	Night	24% 3 9% 32% 25% 7%	4120
November	1st to 10th	Night	33% 3 10% 27% 23% 5%	2451
	11th to 20th	Night	37% 3 8% 24% 22% 5%	2620
	21st to end	Night	36% 4 11% 25% 20% 3	2800
December	1st to 10th	Night	35% 3 12% 28% 19% 2	4580
	11th to 20th	Night	46% 4 11% 19% 16% 4	5208
	21st to end	Night	45% 5% 13% 21% 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	35% 16% 34% 12% 3 1	217
	11th to 20th	Day	31% 23% 31% 12% 2	300
	21st to end	Day	24% 23% 36% 12% 4 1	330
February	1st to 10th	Day	27% 21% 38% 10% 4	300
	11th to 20th	Day	36% 20% 37% 6% 2	355
	21st to end	Day	27% 20% 34% 17% 2	414
March	1st to 10th	Day	26% 22% 31% 17% 4	500
	11th to 20th	Day	27% 21% 30% 18% 3 2	500
	21st to end	Day	22% 20% 33% 18% 5% 1	550
April	1st to 10th	Day	24% 21% 32% 16% 6% 1	500
	11th to 20th	Day	19% 18% 34% 17% 10% 3	492
	21st to end	Day	27% 11% 33% 19% 8% 2	480
May	1st to 10th	Day	25% 16% 28% 20% 8% 3	463
	11th to 20th	Day	20% 11% 30% 23% 12% 3	487
	21st to end	Day	16% 6% 25% 29% 20% 5%	529
June	1st to 10th	Day	14% 8% 28% 25% 19% 6%	490
	11th to 20th	Day	16% 6% 24% 31% 18% 5%	490
	21st to end	Day	10% 7% 30% 30% 17% 5%	490
July	1st to 10th	Day	12% 4 17% 34% 22% 11%	485
	11th to 20th	Day	7% 2 21% 34% 28% 8%	484
	21st to end	Day	13% 4 23% 32% 23% 6%	532
August	1st to 10th	Day	13% 4 19% 29% 24% 11%	486
	11th to 20th	Day	11% 4 18% 33% 23% 10%	479
	21st to end	Day	10% 4 18% 34% 21% 12%	539
September	1st to 10th	Day	17% 8% 18% 25% 21% 12%	490
	11th to 20th	Day	13% 5% 19% 32% 19% 12%	490
	21st to end	Day	27% 8% 23% 27% 10% 5%	490
October	1st to 10th	Day	24% 7% 20% 25% 19% 6%	490
	11th to 20th	Day	18% 13% 26% 27% 13% 4	490
	21st to end	Day	17% 12% 33% 25% 11% 2	539
November	1st to 10th	Day	16% 10% 35% 24% 12% 2	439
	11th to 20th	Day	20% 14% 36% 20% 9% 1	375
	21st to end	Day	26% 18% 33% 14% 10% 1	200
December	1st to 10th	Day	23% 19% 37% 18% 3 2	200
	11th to 20th	Day	26% 20% 30% 18% 7%	200
	21st to end	Day	25% 17% 35% 17% 6%	220
January	1st to 10th	Night	21% 20% 40% 17% 3	800
	11th to 20th	Night	23% 18% 32% 24% 3	800
	21st to end	Night	20% 19% 38% 22% 2	880
February	1st to 10th	Night	19% 16% 36% 26% 2	800
	11th to 20th	Night	27% 22% 35% 15% 1	783
	21st to end	Night	24% 17% 37% 20% 2	660
March	1st to 10th	Night	21% 18% 37% 21% 3	800
	11th to 20th	Night	22% 20% 33% 20% 4 1	800
	21st to end	Night	19% 18% 38% 20% 4 1	880
April	1st to 10th	Night	21% 22% 32% 21% 4	800
	11th to 20th	Night	17% 15% 35% 26% 7% 1	791
	21st to end	Night	18% 12% 35% 27% 8%	783
May	1st to 10th	Night	22% 8% 30% 28% 12%	627
	11th to 20th	Night	22% 6% 27% 35% 11%	431
	21st to end	Night	15% 5% 22% 39% 18% 1	411
June	1st to 10th	Night	12% 4 27% 35% 22% 1	380
	11th to 20th	Night	13% 3 24% 42% 16% 2	380
	21st to end	Night	9% 3 25% 44% 18% 1	380
July	1st to 10th	Night	10% 3 17% 41% 26% 3	370
	11th to 20th	Night	6% 3 14% 42% 33% 3	428
	21st to end	Night	10% 3 16% 43% 26% 3	624
August	1st to 10th	Night	12% 3 10% 43% 30% 2	567
	11th to 20th	Night	12% 3 13% 44% 24% 3	643
	21st to end	Night	11% 3 15% 39% 27% 4	836
September	1st to 10th	Night	17% 3 21% 29% 26% 4	760
	11th to 20th	Night	11% 5% 15% 36% 29% 4	760
	21st to end	Night	23% 5% 22% 31% 18% 1	760
October	1st to 10th	Night	21% 7% 20% 29% 20% 4	760
	11th to 20th	Night	17% 12% 26% 29% 15% 1	760
	21st to end	Night	19% 11% 31% 26% 13%	836
November	1st to 10th	Night	15% 11% 31% 33% 10% 1	780
	11th to 20th	Night	19% 16% 31% 27% 6% 1	800
	21st to end	Night	16% 17% 33% 27% 7% 1	800
December	1st to 10th	Night	23% 17% 32% 22% 6% 1	800
	11th to 20th	Night	20% 14% 37% 24% 5%	800
	21st to end	Night	19% 16% 34% 29% 3	880

Page generated at 2025-04-23 09:25:02. Version ac07

Copyright © 2025 Attilla Danko.