Montgomery Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Montgomery Observatory clear sky chart from 2005-11-06 to 2024-02-15.

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	10% 5% 2 4 4 3 4 4 5% 7% 53%	78763
	Night	10% 4 2 3 3 2 3 4 4 7% 57%	80372
Transparency	Day	31% 5% 10% 24% 23% 7%	78488
	Night	28% 5% 9% 23% 26% 9%	80167
Seeing	Day	12% 7% 24% 29% 21% 7%	14227
	Night	11% 8% 30% 35% 15% 1	24292

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	10% 7% 2 5% 4 2 3 4 5% 6% 54%	2847
	11th to 20th	Day	8% 5% 1 3 3 1 2 3 4 6% 64%	2850
	21st to end	Day	17% 9% 3 5% 4 2 3 3 3 4 48%	3135
February	1st to 10th	Day	13% 7% 2 4 4 3 3 4 4 6% 49%	1814
	11th to 20th	Day	15% 5% 2 4 4 2 4 3 4 7% 50%	1789
	21st to end	Day	18% 8% 3 5% 3 2 2 2 3 4 51%	1480
March	1st to 10th	Day	15% 6% 2 4 4 3 4 5% 4 6% 48%	1961
	11th to 20th	Day	15% 7% 2 5% 4 3 4 3 3 6% 49%	1980
	21st to end	Day	13% 6% 4 4 4 3 4 3 4 7% 47%	2178
April	1st to 10th	Day	11% 4 2 4 4 3 3 4 4 7% 54%	1980
	11th to 20th	Day	12% 5% 2 5% 5% 3 4 4 5% 6% 50%	2072
	21st to end	Day	12% 5% 2 4 4 2 4 5% 4 7% 49%	2340
May	1st to 10th	Day	14% 6% 3 4 4 2 3 4 5% 6% 49%	2340
	11th to 20th	Day	14% 6% 2 5% 4 3 5% 4 4 7% 45%	2302
	21st to end	Day	13% 5% 3 5% 6% 3 5% 6% 5% 8% 42%	2531
June	1st to 10th	Day	6% 4 2 6% 4 3 5% 5% 6% 9% 50%	2340
	11th to 20th	Day	6% 3 2 4 5% 3 5% 5% 5% 10% 54%	2340
	21st to end	Day	6% 3 2 4 3 2 4 4 5% 8% 60%	2340
July	1st to 10th	Day	7% 3 2 4 6% 3 6% 7% 9% 10% 43%	2340
	11th to 20th	Day	4 2 1 2 4 3 5% 4 6% 9% 60%	2340
	21st to end	Day	4 2 1 4 5% 3 5% 7% 7% 9% 53%	2546
August	1st to 10th	Day	2 2 2 4 5% 4 5% 6% 6% 9% 55%	2328
	11th to 20th	Day	7% 3 2 4 5% 3 5% 6% 7% 9% 49%	2301
	21st to end	Day	4 2 2 4 4 3 5% 5% 7% 10% 55%	2362
September	1st to 10th	Day	8% 3 2 4 4 3 4 4 5% 7% 57%	1980
	11th to 20th	Day	11% 5% 2 5% 6% 4 5% 5% 4 6% 46%	1980
	21st to end	Day	8% 3 1 3 4 3 3 4 4 7% 60%	1980
October	1st to 10th	Day	7% 4 2 4 4 3 3 4 5% 7% 59%	1946
	11th to 20th	Day	10% 4 1 3 3 2 2 2 3 5% 65%	1800
	21st to end	Day	14% 5% 2 3 3 1 2 3 3 5% 59%	1980
November	1st to 10th	Day	10% 5% 2 3 3 2 2 3 4 7% 59%	1835
	11th to 20th	Day	7% 4 1 3 3 2 2 3 3 5% 67%	1710
	21st to end	Day	15% 4 2 4 3 2 2 2 4 6% 57%	1710
December	1st to 10th	Day	11% 5% 2 5% 5% 2 3 3 5% 7% 53%	1710
	11th to 20th	Day	14% 6% 3 5% 4 3 4 3 3 5% 50%	2161
	21st to end	Day	17% 7% 2 4 4 2 4 4 4 6% 46%	3135
January	1st to 10th	Night	12% 7% 2 4 3 2 3 4 4 6% 52%	4108
	11th to 20th	Night	7% 5% 2 3 2 2 2 3 3 6% 65%	4120
	21st to end	Night	19% 7% 3 4 3 2 3 3 3 4 50%	4532
February	1st to 10th	Night	13% 6% 2 4 4 2 3 4 4 5% 53%	2380
	11th to 20th	Night	14% 5% 3 5% 3 2 3 3 3 7% 51%	2152
	21st to end	Night	12% 6% 3 4 3 3 2 3 3 5% 54%	1776
March	1st to 10th	Night	13% 6% 2 5% 5% 3 4 3 4 6% 50%	1998
	11th to 20th	Night	14% 5% 3 4 4 2 3 3 4 6% 53%	1980
	21st to end	Night	12% 4 2 4 3 2 4 4 5% 6% 55%	2178
April	1st to 10th	Night	10% 4 2 4 3 2 4 3 5% 8% 54%	1980
	11th to 20th	Night	11% 4 1 4 4 2 4 3 4 6% 55%	1830
	21st to end	Night	11% 3 2 3 4 2 3 4 5% 8% 53%	1620
May	1st to 10th	Night	10% 4 2 4 4 2 6% 3 5% 7% 52%	1620
	11th to 20th	Night	10% 5% 2 3 4 2 4 4 5% 8% 52%	1593
	21st to end	Night	7% 3 1 4 4 3 4 4 6% 10% 53%	1745
June	1st to 10th	Night	5% 2 2 2 3 2 3 4 6% 12% 59%	1620
	11th to 20th	Night	5% 2 1 3 4 3 3 4 6% 11% 59%	1620
	21st to end	Night	3 2 1 2 3 2 3 3 5% 11% 65%	1620
July	1st to 10th	Night	5% 2 2 4 5% 2 4 5% 6% 11% 54%	1620
	11th to 20th	Night	2 2 1 2 2 2 3 4 6% 9% 68%	1620
	21st to end	Night	2 2 1 3 3 2 3 3 6% 11% 65%	1772
August	1st to 10th	Night	2 2 1 2 2 3 4 5% 6% 11% 62%	1611
	11th to 20th	Night	5% 2 1 3 4 2 4 6% 7% 9% 57%	1600
	21st to end	Night	3 2 1 3 3 2 3 5% 7% 10% 62%	1990
September	1st to 10th	Night	7% 3 2 4 3 2 4 4 5% 6% 61%	1980
	11th to 20th	Night	10% 4 2 5% 5% 3 4 4 5% 6% 51%	1980
	21st to end	Night	7% 3 1 2 3 1 3 4 6% 8% 62%	1980
October	1st to 10th	Night	7% 3 1 3 3 2 3 4 5% 7% 63%	1994
	11th to 20th	Night	10% 3 2 3 3 2 3 2 3 3 65%	2160
	21st to end	Night	14% 4 1 3 3 2 2 2 3 4 61%	2376
November	1st to 10th	Night	9% 4 2 3 3 2 3 3 4 5% 64%	2239
	11th to 20th	Night	7% 3 2 4 3 2 4 3 3 5% 64%	2470
	21st to end	Night	13% 5% 2 3 2 2 4 3 4 6% 56%	2470
December	1st to 10th	Night	11% 4 2 3 3 3 2 3 5% 6% 58%	2470
	11th to 20th	Night	12% 5% 2 5% 4 2 4 3 3 5% 54%	3036
	21st to end	Night	15% 6% 2 3 3 2 3 3 4 5% 55%	4532

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	31% 4 9% 24% 27% 5%	2848
	11th to 20th	Day	24% 3 11% 23% 35% 5%	2850
	21st to end	Day	42% 4 4 22% 23% 6%	3135
February	1st to 10th	Day	37% 4 7% 21% 23% 7%	1814
	11th to 20th	Day	37% 4 7% 24% 20% 8%	1799
	21st to end	Day	40% 3 4 17% 26% 11%	1480
March	1st to 10th	Day	37% 4 7% 18% 19% 14%	1961
	11th to 20th	Day	39% 5% 8% 21% 21% 7%	1980
	21st to end	Day	38% 5% 9% 22% 20% 5%	2178
April	1st to 10th	Day	31% 5% 11% 25% 18% 10%	1980
	11th to 20th	Day	35% 6% 9% 22% 20% 9%	2072
	21st to end	Day	35% 6% 11% 24% 20% 5%	2340
May	1st to 10th	Day	35% 6% 11% 24% 19% 5%	2340
	11th to 20th	Day	37% 8% 11% 25% 18% 1	2306
	21st to end	Day	40% 8% 13% 22% 15% 2	2531
June	1st to 10th	Day	30% 8% 15% 28% 15% 3	2340
	11th to 20th	Day	26% 8% 14% 27% 22% 3	2340
	21st to end	Day	23% 7% 13% 26% 26% 5%	2340
July	1st to 10th	Day	30% 9% 20% 27% 13% 1	2276
	11th to 20th	Day	19% 6% 13% 32% 26% 4	2271
	21st to end	Day	23% 8% 16% 34% 19% 1	2405
August	1st to 10th	Day	23% 8% 14% 28% 23% 4	2314
	11th to 20th	Day	29% 8% 14% 24% 22% 3	2301
	21st to end	Day	22% 7% 14% 32% 20% 5%	2362
September	1st to 10th	Day	27% 5% 11% 23% 23% 12%	1980
	11th to 20th	Day	37% 6% 9% 19% 19% 8%	1980
	21st to end	Day	25% 4 8% 24% 29% 10%	1980
October	1st to 10th	Day	26% 5% 10% 21% 24% 14%	1946
	11th to 20th	Day	24% 3 7% 19% 32% 15%	1800
	21st to end	Day	30% 3 8% 22% 25% 12%	1980
November	1st to 10th	Day	27% 4 8% 22% 29% 10%	1835
	11th to 20th	Day	22% 3 8% 27% 29% 12%	1710
	21st to end	Day	31% 3 6% 21% 28% 10%	1710
December	1st to 10th	Day	32% 4 10% 27% 22% 5%	1710
	11th to 20th	Day	38% 3 7% 17% 26% 9%	2159
	21st to end	Day	39% 4 7% 22% 24% 4	3135
January	1st to 10th	Night	32% 4 7% 21% 29% 7%	4108
	11th to 20th	Night	22% 4 13% 22% 32% 7%	4120
	21st to end	Night	39% 3 5% 18% 26% 9%	4532
February	1st to 10th	Night	35% 4 7% 21% 24% 8%	2380
	11th to 20th	Night	35% 4 7% 21% 25% 8%	2152
	21st to end	Night	34% 3 6% 18% 22% 16%	1776
March	1st to 10th	Night	35% 5% 5% 18% 19% 17%	1998
	11th to 20th	Night	34% 4 9% 22% 22% 9%	1980
	21st to end	Night	31% 6% 12% 23% 23% 6%	2178
April	1st to 10th	Night	29% 4 11% 21% 21% 13%	1980
	11th to 20th	Night	31% 5% 9% 22% 20% 13%	1830
	21st to end	Night	29% 5% 11% 26% 23% 5%	1620
May	1st to 10th	Night	31% 6% 11% 21% 22% 8%	1620
	11th to 20th	Night	31% 7% 17% 25% 19% 2	1594
	21st to end	Night	25% 9% 16% 29% 18% 3	1746
June	1st to 10th	Night	18% 8% 16% 34% 21% 4	1620
	11th to 20th	Night	20% 6% 16% 29% 23% 6%	1620
	21st to end	Night	16% 5% 12% 27% 32% 8%	1620
July	1st to 10th	Night	23% 7% 17% 30% 21% 2	1575
	11th to 20th	Night	13% 5% 12% 35% 31% 5%	1566
	21st to end	Night	15% 4 15% 35% 29% 3	1673
August	1st to 10th	Night	15% 7% 12% 32% 27% 7%	1601
	11th to 20th	Night	21% 7% 13% 26% 29% 5%	1600
	21st to end	Night	17% 5% 13% 32% 25% 7%	1990
September	1st to 10th	Night	24% 5% 11% 22% 28% 10%	1980
	11th to 20th	Night	33% 6% 9% 18% 22% 11%	1980
	21st to end	Night	20% 4 8% 24% 31% 13%	1980
October	1st to 10th	Night	22% 5% 11% 22% 25% 15%	1994
	11th to 20th	Night	26% 4 5% 15% 30% 20%	2160
	21st to end	Night	29% 3 6% 20% 26% 16%	2376
November	1st to 10th	Night	25% 4 6% 24% 29% 13%	2239
	11th to 20th	Night	23% 4 8% 23% 29% 13%	2470
	21st to end	Night	31% 4 6% 19% 28% 11%	2470
December	1st to 10th	Night	28% 3 8% 26% 24% 11%	2470
	11th to 20th	Night	34% 3 5% 17% 29% 11%	3037
	21st to end	Night	33% 3 7% 21% 27% 9%	4532

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	16% 14% 38% 24% 7%	270
	11th to 20th	Day	17% 18% 33% 24% 5% 2	270
	21st to end	Day	15% 11% 41% 23% 8% 1	297
February	1st to 10th	Day	20% 14% 37% 23% 6%	270
	11th to 20th	Day	21% 14% 44% 20% 1 1	252
	21st to end	Day	28% 17% 37% 15% 4	198
March	1st to 10th	Day	19% 20% 34% 18% 8% 1	240
	11th to 20th	Day	23% 18% 35% 17% 7%	240
	21st to end	Day	19% 13% 47% 17% 4	264
April	1st to 10th	Day	15% 22% 30% 26% 8%	240
	11th to 20th	Day	16% 15% 43% 22% 4	350
	21st to end	Day	14% 17% 38% 22% 8% 1	600
May	1st to 10th	Day	14% 10% 36% 31% 8%	603
	11th to 20th	Day	17% 7% 32% 27% 14% 3	625
	21st to end	Day	15% 6% 30% 28% 17% 5%	679
June	1st to 10th	Day	7% 3 17% 36% 31% 7%	630
	11th to 20th	Day	6% 2 20% 41% 25% 5%	630
	21st to end	Day	4 3 14% 39% 29% 11%	630
July	1st to 10th	Day	5% 1 8% 32% 37% 16%	621
	11th to 20th	Day	3 1 7% 31% 43% 15%	618
	21st to end	Day	3 1 5% 30% 44% 18%	684
August	1st to 10th	Day	3 1 8% 39% 39% 10%	626
	11th to 20th	Day	8% 2 9% 34% 37% 10%	615
	21st to end	Day	5% 1 5% 32% 39% 20%	481
September	1st to 10th	Day	7% 1 5% 25% 43% 19%	270
	11th to 20th	Day	7% 1 8% 31% 36% 17%	270
	21st to end	Day	14% 3 10% 30% 31% 12%	270
October	1st to 10th	Day	4 4 23% 38% 21% 10%	270
	11th to 20th	Day	11% 9% 30% 30% 17% 2	270
	21st to end	Day	22% 9% 26% 28% 13% 2	297
November	1st to 10th	Day	16% 6% 37% 27% 11% 3	270
	11th to 20th	Day	14% 9% 37% 27% 12% 2	270
	21st to end	Day	20% 11% 37% 23% 8% 1	270
December	1st to 10th	Day	12% 11% 36% 29% 9% 2	270
	11th to 20th	Day	17% 13% 36% 26% 6%	270
	21st to end	Day	24% 12% 35% 24% 5%	297
January	1st to 10th	Night	13% 11% 37% 31% 7% 1	950
	11th to 20th	Night	13% 13% 35% 32% 7%	950
	21st to end	Night	13% 10% 36% 33% 7%	1045
February	1st to 10th	Night	16% 13% 39% 28% 4	860
	11th to 20th	Night	17% 15% 38% 25% 4 1	722
	21st to end	Night	16% 16% 42% 24% 2	592
March	1st to 10th	Night	15% 15% 36% 30% 4	558
	11th to 20th	Night	16% 12% 39% 29% 4	540
	21st to end	Night	13% 12% 43% 26% 5% 1	594
April	1st to 10th	Night	11% 13% 44% 28% 4	540
	11th to 20th	Night	12% 14% 43% 26% 5%	533
	21st to end	Night	13% 9% 45% 26% 7% 1	540
May	1st to 10th	Night	10% 10% 40% 33% 6% 1	540
	11th to 20th	Night	12% 6% 33% 33% 15% 1	531
	21st to end	Night	7% 3 34% 37% 16% 2	584
June	1st to 10th	Night	5% 2 20% 43% 27% 2	540
	11th to 20th	Night	5% 1 25% 53% 16%	540
	21st to end	Night	3 1 17% 47% 28% 4	540
July	1st to 10th	Night	5% 1 10% 42% 39% 4	525
	11th to 20th	Night	3 1 8% 43% 41% 3	522
	21st to end	Night	2 1 8% 43% 43% 3	591
August	1st to 10th	Night	2 1 8% 50% 39% 1	537
	11th to 20th	Night	7% 9% 43% 37% 4	534
	21st to end	Night	3 1 6% 43% 42% 6%	594
September	1st to 10th	Night	6% 2 11% 44% 34% 3	540
	11th to 20th	Night	11% 1 13% 39% 32% 4	540
	21st to end	Night	9% 2 16% 43% 27% 3	540
October	1st to 10th	Night	7% 3 25% 39% 24% 3	554
	11th to 20th	Night	11% 7% 31% 37% 13%	720
	21st to end	Night	16% 9% 29% 32% 13% 1	792
November	1st to 10th	Night	10% 6% 35% 36% 12% 1	759
	11th to 20th	Night	8% 12% 33% 35% 11% 1	950
	21st to end	Night	16% 10% 35% 29% 9%	950
December	1st to 10th	Night	13% 10% 35% 34% 7%	950
	11th to 20th	Night	18% 12% 33% 32% 6%	950
	21st to end	Night	16% 12% 37% 29% 6%	1045

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