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 2024-02-25.

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 5% 5% 3 5% 5% 6% 7% 42%	78459
	Night	15% 6% 2 5% 5% 3 4 4 5% 7% 44%	80826
Transparency	Day	40% 5% 12% 24% 16% 3	78169
	Night	40% 5% 11% 24% 17% 3	80635
Seeing	Day	19% 11% 27% 24% 13% 5%	13967
	Night	18% 12% 29% 29% 12% 1	23614

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 4 3 4 5% 5% 6% 46%	2563
	11th to 20th	Day	15% 11% 4 6% 6% 3 4 3 4 5% 40%	2840
	21st to end	Day	20% 9% 3 6% 5% 4 5% 4 4 6% 36%	3124
February	1st to 10th	Day	15% 6% 3 6% 5% 4 5% 6% 7% 7% 38%	1710
	11th to 20th	Day	17% 6% 3 5% 5% 3 4 5% 6% 6% 40%	1715
	21st to end	Day	18% 8% 3 5% 4 4 4 4 5% 5% 39%	1520
March	1st to 10th	Day	13% 5% 2 5% 4 2 4 3 5% 6% 49%	1939
	11th to 20th	Day	17% 6% 3 6% 5% 2 4 5% 5% 6% 41%	1980
	21st to end	Day	17% 6% 4 7% 6% 2 5% 4 5% 6% 39%	2214
April	1st to 10th	Day	13% 6% 3 5% 5% 3 5% 4 6% 8% 43%	2160
	11th to 20th	Day	13% 5% 3 5% 5% 3 4 4 5% 6% 47%	2134
	21st to end	Day	19% 6% 2 5% 4 3 4 4 5% 8% 40%	2160
May	1st to 10th	Day	14% 7% 3 5% 4 4 4 5% 7% 8% 38%	2299
	11th to 20th	Day	18% 6% 3 5% 6% 3 5% 5% 6% 7% 35%	2420
	21st to end	Day	14% 5% 3 6% 5% 3 5% 5% 6% 9% 40%	2725
June	1st to 10th	Day	11% 5% 2 5% 6% 3 5% 5% 7% 9% 43%	2520
	11th to 20th	Day	12% 6% 3 5% 5% 3 5% 5% 7% 8% 40%	2520
	21st to end	Day	10% 4 2 3 5% 3 6% 5% 7% 10% 44%	2520
July	1st to 10th	Day	10% 5% 2 5% 5% 4 5% 5% 6% 8% 43%	2520
	11th to 20th	Day	7% 4 1 5% 5% 3 6% 7% 8% 10% 45%	2460
	21st to end	Day	10% 5% 2 5% 6% 4 7% 7% 7% 10% 37%	2547
August	1st to 10th	Day	11% 6% 2 5% 6% 4 6% 7% 7% 9% 36%	2327
	11th to 20th	Day	12% 5% 2 6% 6% 3 6% 6% 7% 8% 39%	2210
	21st to end	Day	12% 4 2 5% 5% 4 5% 6% 6% 8% 42%	2376
September	1st to 10th	Day	15% 5% 3 5% 4 3 4 5% 5% 8% 44%	2160
	11th to 20th	Day	12% 5% 2 4 5% 3 4 4 6% 8% 47%	2160
	21st to end	Day	18% 7% 3 5% 4 2 4 5% 6% 6% 39%	1895
October	1st to 10th	Day	18% 5% 2 4 5% 2 3 3 5% 7% 46%	1800
	11th to 20th	Day	19% 4 2 4 3 2 4 4 5% 6% 47%	1800
	21st to end	Day	14% 6% 2 5% 4 3 4 4 4 6% 48%	1980
November	1st to 10th	Day	13% 4 2 4 4 2 3 4 5% 7% 51%	1752
	11th to 20th	Day	15% 5% 2 4 3 2 4 5% 6% 6% 49%	1687
	21st to end	Day	15% 5% 3 4 5% 3 4 4 5% 5% 47%	1520
December	1st to 10th	Day	17% 6% 2 4 5% 3 5% 5% 6% 8% 39%	1520
	11th to 20th	Day	16% 6% 2 5% 4 2 4 4 5% 7% 44%	1910
	21st to end	Day	17% 7% 2 6% 5% 3 6% 4 6% 8% 36%	2772
January	1st to 10th	Night	15% 7% 2 5% 5% 3 4 4 5% 7% 43%	4365
	11th to 20th	Night	19% 7% 2 4 4 3 3 3 3 6% 45%	4120
	21st to end	Night	21% 9% 2 5% 3 2 3 3 4 6% 41%	4532
February	1st to 10th	Night	14% 7% 2 6% 5% 4 4 5% 5% 6% 41%	2470
	11th to 20th	Night	19% 6% 2 5% 5% 2 3 4 4 6% 43%	2350
	21st to end	Night	17% 7% 3 6% 4 2 4 4 5% 6% 41%	1831
March	1st to 10th	Night	14% 4 2 5% 4 3 5% 5% 5% 6% 49%	2018
	11th to 20th	Night	17% 6% 3 5% 3 3 4 4 5% 7% 43%	1980
	21st to end	Night	15% 6% 3 6% 5% 3 5% 4 5% 6% 42%	2142
April	1st to 10th	Night	15% 7% 3 5% 5% 3 5% 5% 5% 8% 40%	1800
	11th to 20th	Night	14% 4 3 6% 4 3 4 4 5% 7% 46%	1775
	21st to end	Night	17% 5% 2 5% 4 3 4 5% 5% 8% 43%	1800
May	1st to 10th	Night	16% 7% 2 5% 5% 2 4 5% 5% 8% 40%	1674
	11th to 20th	Night	18% 7% 3 5% 4 3 4 5% 6% 8% 38%	1469
	21st to end	Night	12% 5% 2 4 5% 4 4 4 6% 12% 41%	1550
June	1st to 10th	Night	11% 4 2 5% 4 2 5% 6% 7% 10% 45%	1440
	11th to 20th	Night	11% 6% 3 6% 5% 2 6% 5% 5% 9% 41%	1440
	21st to end	Night	9% 4 2 5% 5% 4 5% 6% 6% 11% 43%	1440
July	1st to 10th	Night	10% 5% 2 5% 6% 5% 8% 6% 7% 9% 39%	1440
	11th to 20th	Night	6% 4 2 5% 5% 3 6% 6% 7% 10% 46%	1496
	21st to end	Night	9% 3 2 5% 6% 3 5% 5% 8% 12% 43%	1771
August	1st to 10th	Night	9% 5% 2 6% 6% 4 7% 6% 8% 8% 40%	1611
	11th to 20th	Night	10% 6% 3 7% 5% 4 5% 5% 6% 7% 42%	1672
	21st to end	Night	10% 4 2 6% 5% 3 5% 5% 6% 9% 44%	1980
September	1st to 10th	Night	15% 5% 3 5% 4 2 4 4 5% 6% 46%	1800
	11th to 20th	Night	9% 6% 3 6% 4 3 4 4 4 7% 51%	1800
	21st to end	Night	17% 6% 2 5% 5% 3 4 4 4 6% 42%	2061
October	1st to 10th	Night	19% 5% 2 4 4 3 4 4 4 5% 46%	2160
	11th to 20th	Night	16% 5% 2 4 4 2 4 3 4 5% 52%	2160
	21st to end	Night	16% 6% 2 4 4 3 4 5% 5% 6% 48%	2376
November	1st to 10th	Night	12% 6% 2 4 4 2 3 3 4 5% 54%	2325
	11th to 20th	Night	15% 5% 2 4 4 2 3 4 4 6% 50%	2489
	21st to end	Night	14% 4 2 4 5% 3 5% 4 6% 7% 47%	2660
December	1st to 10th	Night	17% 7% 3 5% 5% 3 4 4 5% 7% 40%	2660
	11th to 20th	Night	17% 5% 2 5% 5% 3 4 4 5% 6% 43%	3285
	21st to end	Night	16% 7% 2 5% 5% 4 5% 5% 6% 8% 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	37% 6% 7% 32% 15% 4	2564
	11th to 20th	Day	47% 4 9% 21% 15% 4	2840
	21st to end	Day	50% 4 10% 23% 11% 2	3124
February	1st to 10th	Day	44% 5% 13% 24% 13% 2	1710
	11th to 20th	Day	44% 4 9% 25% 16% 3	1724
	21st to end	Day	47% 4 10% 21% 15% 4	1520
March	1st to 10th	Day	37% 3 12% 25% 20% 2	1939
	11th to 20th	Day	44% 4 10% 19% 18% 6%	1980
	21st to end	Day	47% 4 9% 20% 17% 3	2214
April	1st to 10th	Day	39% 5% 12% 21% 18% 5%	2160
	11th to 20th	Day	37% 5% 10% 22% 20% 6%	2134
	21st to end	Day	43% 4 10% 22% 18% 4	2160
May	1st to 10th	Day	41% 6% 14% 20% 17% 2	2299
	11th to 20th	Day	46% 6% 12% 20% 14% 2	2424
	21st to end	Day	40% 5% 16% 25% 11% 2	2725
June	1st to 10th	Day	37% 6% 14% 26% 15% 2	2520
	11th to 20th	Day	40% 8% 13% 24% 14% 2	2520
	21st to end	Day	33% 7% 15% 26% 17% 2	2520
July	1st to 10th	Day	37% 7% 14% 22% 16% 3	2450
	11th to 20th	Day	31% 8% 16% 27% 16% 3	2385
	21st to end	Day	39% 8% 13% 22% 16% 2	2405
August	1st to 10th	Day	41% 7% 16% 24% 11% 1	2314
	11th to 20th	Day	41% 7% 14% 22% 16% 2	2210
	21st to end	Day	37% 7% 15% 25% 14% 3	2376
September	1st to 10th	Day	39% 6% 13% 21% 18% 4	2160
	11th to 20th	Day	35% 4 9% 28% 21% 4	2160
	21st to end	Day	44% 5% 8% 21% 20% 2	1895
October	1st to 10th	Day	39% 4 11% 25% 18% 3	1800
	11th to 20th	Day	37% 5% 10% 25% 22% 3	1800
	21st to end	Day	38% 4 8% 27% 21% 2	1980
November	1st to 10th	Day	33% 4 11% 27% 20% 5%	1752
	11th to 20th	Day	35% 4 10% 24% 22% 4	1687
	21st to end	Day	39% 5% 10% 28% 17% 2	1520
December	1st to 10th	Day	43% 5% 13% 25% 13% 2	1520
	11th to 20th	Day	39% 5% 10% 32% 12% 3	1906
	21st to end	Day	45% 5% 13% 24% 11% 1	2772
January	1st to 10th	Night	40% 5% 9% 27% 16% 3	4365
	11th to 20th	Night	41% 4 10% 25% 17% 3	4120
	21st to end	Night	46% 3 8% 27% 15% 1	4532
February	1st to 10th	Night	43% 5% 12% 25% 14% 1	2470
	11th to 20th	Night	43% 4 9% 22% 18% 4	2352
	21st to end	Night	44% 3 9% 23% 16% 5%	1831
March	1st to 10th	Night	36% 5% 11% 27% 19% 3	2018
	11th to 20th	Night	41% 5% 11% 21% 18% 4	1980
	21st to end	Night	43% 4 10% 21% 15% 6%	2142
April	1st to 10th	Night	42% 5% 9% 20% 19% 4	1800
	11th to 20th	Night	38% 4 12% 22% 19% 6%	1775
	21st to end	Night	39% 6% 10% 25% 14% 5%	1800
May	1st to 10th	Night	42% 5% 12% 23% 13% 4	1674
	11th to 20th	Night	44% 6% 11% 19% 17% 2	1470
	21st to end	Night	37% 6% 20% 22% 13% 2	1551
June	1st to 10th	Night	34% 7% 13% 27% 17% 2	1440
	11th to 20th	Night	40% 7% 13% 24% 14% 3	1440
	21st to end	Night	35% 6% 14% 24% 19% 2	1440
July	1st to 10th	Night	40% 8% 17% 18% 13% 4	1400
	11th to 20th	Night	31% 8% 15% 26% 17% 2	1448
	21st to end	Night	31% 9% 17% 26% 16% 1	1672
August	1st to 10th	Night	38% 8% 16% 22% 14% 1	1600
	11th to 20th	Night	40% 7% 13% 23% 14% 2	1672
	21st to end	Night	36% 7% 14% 24% 16% 3	1980
September	1st to 10th	Night	39% 5% 11% 22% 18% 5%	1800
	11th to 20th	Night	34% 5% 9% 27% 22% 4	1800
	21st to end	Night	44% 4 9% 21% 18% 4	2061
October	1st to 10th	Night	41% 5% 9% 24% 18% 3	2160
	11th to 20th	Night	36% 3 9% 29% 21% 3	2160
	21st to end	Night	37% 4 8% 26% 21% 4	2376
November	1st to 10th	Night	33% 3 9% 28% 22% 5%	2325
	11th to 20th	Night	37% 3 8% 24% 22% 5%	2489
	21st to end	Night	35% 4 11% 26% 20% 3	2660
December	1st to 10th	Night	43% 4 12% 24% 14% 2	2660
	11th to 20th	Night	41% 5% 9% 24% 18% 3	3288
	21st to end	Night	44% 5% 15% 24% 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	31% 16% 35% 13% 3 1	195
	11th to 20th	Day	31% 23% 31% 13% 3	270
	21st to end	Day	25% 22% 35% 12% 4 1	297
February	1st to 10th	Day	24% 20% 40% 11% 4	270
	11th to 20th	Day	32% 20% 39% 7% 2	322
	21st to end	Day	27% 20% 34% 17% 2	362
March	1st to 10th	Day	24% 24% 30% 19% 3	420
	11th to 20th	Day	27% 20% 30% 18% 3 2	420
	21st to end	Day	21% 21% 32% 19% 6% 2	462
April	1st to 10th	Day	21% 20% 33% 18% 6% 1	420
	11th to 20th	Day	18% 17% 33% 17% 12% 3	412
	21st to end	Day	28% 11% 34% 18% 7% 2	420
May	1st to 10th	Day	24% 16% 28% 20% 8% 4	423
	11th to 20th	Day	20% 11% 32% 24% 11% 2	447
	21st to end	Day	17% 6% 23% 29% 20% 5%	485
June	1st to 10th	Day	15% 7% 27% 26% 19% 6%	450
	11th to 20th	Day	17% 6% 25% 30% 17% 5%	450
	21st to end	Day	10% 7% 30% 30% 18% 6%	450
July	1st to 10th	Day	13% 3 18% 31% 22% 12%	445
	11th to 20th	Day	5% 2 20% 34% 30% 9%	444
	21st to end	Day	13% 3 23% 31% 24% 6%	488
August	1st to 10th	Day	13% 4 19% 28% 25% 12%	446
	11th to 20th	Day	12% 4 16% 32% 25% 11%	439
	21st to end	Day	11% 3 18% 34% 22% 13%	495
September	1st to 10th	Day	18% 7% 17% 24% 22% 13%	450
	11th to 20th	Day	14% 5% 18% 32% 19% 13%	450
	21st to end	Day	24% 8% 23% 28% 11% 5%	450
October	1st to 10th	Day	24% 6% 20% 26% 20% 6%	450
	11th to 20th	Day	18% 13% 26% 26% 14% 3	450
	21st to end	Day	19% 13% 34% 23% 9% 2	495
November	1st to 10th	Day	17% 9% 35% 23% 13% 2	405
	11th to 20th	Day	21% 14% 34% 20% 10% 1	347
	21st to end	Day	23% 18% 33% 14% 11% 1	180
December	1st to 10th	Day	23% 18% 34% 19% 3 2	180
	11th to 20th	Day	26% 21% 27% 19% 7%	180
	21st to end	Day	25% 16% 35% 17% 7% 1	198
January	1st to 10th	Night	20% 19% 41% 17% 3	760
	11th to 20th	Night	24% 17% 32% 24% 3	760
	21st to end	Night	21% 19% 36% 22% 2	836
February	1st to 10th	Night	19% 16% 36% 27% 2	760
	11th to 20th	Night	25% 22% 36% 16% 1	743
	21st to end	Night	24% 18% 37% 19% 2	611
March	1st to 10th	Night	20% 17% 38% 22% 2	720
	11th to 20th	Night	23% 19% 33% 19% 4 1	720
	21st to end	Night	19% 17% 39% 20% 4 1	792
April	1st to 10th	Night	20% 22% 32% 22% 4	720
	11th to 20th	Night	16% 14% 36% 26% 7% 1	711
	21st to end	Night	20% 12% 34% 27% 7%	720
May	1st to 10th	Night	21% 8% 31% 27% 12% 1	594
	11th to 20th	Night	22% 6% 26% 35% 11%	408
	21st to end	Night	15% 5% 22% 39% 17% 1	389
June	1st to 10th	Night	12% 3 27% 35% 22% 1	360
	11th to 20th	Night	14% 3 24% 41% 16% 2	360
	21st to end	Night	9% 3 24% 45% 19% 1	360
July	1st to 10th	Night	10% 3 15% 42% 27% 3	350
	11th to 20th	Night	5% 3 14% 41% 34% 3	404
	21st to end	Night	9% 2 16% 43% 26% 4	591
August	1st to 10th	Night	11% 3 10% 44% 30% 2	537
	11th to 20th	Night	13% 3 11% 44% 25% 4	608
	21st to end	Night	11% 3 15% 39% 27% 4	792
September	1st to 10th	Night	18% 3 20% 29% 27% 4	720
	11th to 20th	Night	10% 5% 15% 36% 30% 4	720
	21st to end	Night	20% 5% 23% 33% 19% 1	720
October	1st to 10th	Night	21% 7% 19% 29% 20% 4	720
	11th to 20th	Night	18% 11% 27% 29% 15% 1	720
	21st to end	Night	20% 11% 32% 25% 12%	792
November	1st to 10th	Night	15% 10% 31% 32% 10% 1	740
	11th to 20th	Night	18% 17% 31% 27% 7% 1	760
	21st to end	Night	16% 16% 33% 28% 7% 1	760
December	1st to 10th	Night	23% 17% 32% 21% 7% 1	760
	11th to 20th	Night	19% 14% 38% 24% 5%	760
	21st to end	Night	19% 16% 34% 28% 3	836

Page generated at 2024-02-22 06:05:42. Version ac07

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