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 2026-07-31.

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 5% 3 3 4 3 5% 7% 53%	138318
	Night	10% 4 2 4 3 3 3 3 5% 7% 57%	135039
Transparency	Day	30% 5% 9% 24% 24% 7%	142035
	Night	27% 4 9% 24% 26% 10%	139389
Seeing	Day	11% 8% 25% 30% 20% 6%	17648
	Night	11% 8% 30% 35% 15% 1	27459

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% 5% 2 4 3 3 3 3 5% 6% 56%	5289
	11th to 20th	Day	5% 5% 2 3 2 2 2 2 4 5% 69%	5310
	21st to end	Day	15% 8% 2 5% 3 3 3 3 4 5% 49%	5812
February	1st to 10th	Day	13% 8% 2 5% 3 3 3 3 4 6% 50%	2006
	11th to 20th	Day	14% 6% 2 5% 3 4 3 2 4 7% 50%	2049
	21st to end	Day	16% 8% 2 5% 2 2 2 2 3 4 54%	1730
March	1st to 10th	Day	14% 5% 2 5% 3 3 4 3 4 5% 54%	5002
	11th to 20th	Day	8% 5% 2 5% 3 4 3 3 4 6% 58%	5090
	21st to end	Day	12% 8% 2 6% 4 4 4 3 5% 7% 46%	5583
April	1st to 10th	Day	12% 5% 1 5% 3 3 3 3 4 7% 52%	2310
	11th to 20th	Day	12% 5% 2 6% 4 4 4 3 6% 6% 49%	2423
	21st to end	Day	14% 6% 2 6% 4 3 4 4 4 6% 46%	2730
May	1st to 10th	Day	16% 8% 3 6% 4 4 4 3 5% 6% 43%	7617
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 5% 7% 47%	7627
	21st to end	Day	13% 7% 3 8% 5% 5% 5% 4 6% 7% 39%	8392
June	1st to 10th	Day	7% 5% 2 6% 4 4 5% 4 6% 9% 47%	2730
	11th to 20th	Day	6% 4 2 4 4 4 5% 4 5% 10% 52%	2730
	21st to end	Day	5% 3 1 4 3 3 5% 4 5% 8% 59%	2730
July	1st to 10th	Day	8% 6% 3 7% 5% 5% 5% 4 7% 9% 42%	6030
	11th to 20th	Day	4 3 2 5% 4 4 5% 4 6% 8% 56%	6030
	21st to end	Day	3 2 1 4 3 3 4 4 5% 8% 63%	6553
August	1st to 10th	Day	3 3 1 4 3 4 4 3 5% 8% 62%	4234
	11th to 20th	Day	4 2 2 4 3 4 4 4 7% 10% 56%	4171
	21st to end	Day	3 2 1 4 4 4 5% 4 6% 9% 59%	4268
September	1st to 10th	Day	9% 4 2 5% 3 4 3 3 5% 7% 56%	2200
	11th to 20th	Day	10% 5% 2 7% 4 5% 4 4 5% 7% 49%	2200
	21st to end	Day	8% 3 1 4 3 3 4 3 4 7% 60%	2200
October	1st to 10th	Day	6% 3 1 4 3 3 3 3 5% 6% 62%	2162
	11th to 20th	Day	9% 4 1 3 3 2 2 2 3 5% 66%	2000
	21st to end	Day	13% 5% 2 4 2 2 2 2 3 5% 59%	2200
November	1st to 10th	Day	10% 6% 1 4 3 2 3 3 4 6% 59%	2032
	11th to 20th	Day	7% 4 1 3 2 2 2 2 3 5% 67%	1890
	21st to end	Day	14% 4 2 4 3 2 3 2 4 5% 58%	1890
December	1st to 10th	Day	12% 7% 2 5% 3 2 3 2 4 6% 54%	3030
	11th to 20th	Day	14% 6% 2 5% 3 3 3 3 4 4 54%	3481
	21st to end	Day	19% 8% 2 5% 3 3 3 2 3 5% 47%	4587
January	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 5% 7% 53%	7634
	11th to 20th	Night	4 3 1 3 2 2 2 2 3 5% 74%	7680
	21st to end	Night	18% 7% 2 5% 3 3 3 3 4 5% 47%	8402
February	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 4 5% 54%	2630
	11th to 20th	Night	14% 6% 2 5% 3 3 4 3 4 7% 51%	2458
	21st to end	Night	11% 7% 3 4 3 3 2 3 3 5% 57%	2076
March	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 4 6% 52%	5074
	11th to 20th	Night	8% 3 1 4 2 2 3 2 4 6% 63%	5090
	21st to end	Night	12% 6% 2 5% 3 3 4 3 5% 7% 51%	5588
April	1st to 10th	Night	11% 5% 1 5% 3 3 4 2 6% 8% 53%	2310
	11th to 20th	Night	12% 4 1 4 4 3 4 2 4 6% 55%	2138
	21st to end	Night	12% 4 2 4 4 3 3 3 6% 8% 51%	1890
May	1st to 10th	Night	15% 7% 2 5% 3 3 4 3 5% 7% 46%	5266
	11th to 20th	Night	9% 4 2 4 4 3 4 4 6% 8% 51%	5265
	21st to end	Night	11% 6% 2 5% 4 4 4 3 6% 9% 46%	5793
June	1st to 10th	Night	6% 3 1 3 3 2 4 4 6% 12% 56%	1890
	11th to 20th	Night	5% 3 1 4 3 3 3 3 6% 10% 58%	1890
	21st to end	Night	3 2 1 2 3 3 3 3 5% 10% 65%	1890
July	1st to 10th	Night	5% 4 3 7% 6% 4 5% 4 7% 10% 47%	4168
	11th to 20th	Night	3 2 1 3 2 3 3 3 6% 9% 65%	4167
	21st to end	Night	2 2 1 3 2 2 2 2 5% 8% 69%	4539
August	1st to 10th	Night	1 2 1 2 2 3 3 3 4 8% 70%	2926
	11th to 20th	Night	3 2 1 3 3 3 3 3 6% 9% 65%	2888
	21st to end	Night	3 2 1 3 2 2 3 4 6% 9% 66%	3631
September	1st to 10th	Night	7% 4 1 4 3 3 4 3 5% 6% 60%	2200
	11th to 20th	Night	9% 4 2 6% 4 4 5% 3 5% 6% 53%	2200
	21st to end	Night	7% 3 1 3 2 2 3 3 6% 8% 62%	2200
October	1st to 10th	Night	6% 3 1 3 3 3 3 3 5% 6% 65%	2216
	11th to 20th	Night	9% 3 2 4 3 2 4 2 3 4 65%	2400
	21st to end	Night	14% 4 2 3 2 2 2 2 3 4 61%	2640
November	1st to 10th	Night	10% 4 1 3 2 3 3 2 3 5% 64%	2481
	11th to 20th	Night	7% 3 1 4 2 3 3 3 4 6% 64%	2730
	21st to end	Night	13% 5% 2 3 2 3 3 3 4 6% 57%	2730
December	1st to 10th	Night	9% 5% 2 4 2 2 3 2 4 6% 61%	4380
	11th to 20th	Night	13% 5% 2 5% 3 3 3 2 3 4 57%	4946
	21st to end	Night	16% 7% 2 3 3 2 3 2 4 5% 54%	6633

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	30% 3 7% 28% 28% 4	5290
	11th to 20th	Day	20% 2 8% 19% 43% 8%	5310
	21st to end	Day	38% 3 4 21% 24% 9%	5812
February	1st to 10th	Day	37% 4 7% 21% 23% 8%	2006
	11th to 20th	Day	36% 4 8% 25% 20% 8%	2059
	21st to end	Day	36% 2 4 17% 29% 12%	1730
March	1st to 10th	Day	33% 3 6% 15% 22% 21%	5002
	11th to 20th	Day	29% 5% 7% 18% 21% 20%	5090
	21st to end	Day	37% 6% 9% 24% 20% 4	5585
April	1st to 10th	Day	32% 4 10% 24% 19% 10%	2310
	11th to 20th	Day	35% 6% 9% 21% 21% 8%	2423
	21st to end	Day	38% 6% 10% 23% 19% 4	2730
May	1st to 10th	Day	42% 5% 10% 19% 19% 5%	7655
	11th to 20th	Day	36% 6% 9% 22% 23% 3	7631
	21st to end	Day	43% 7% 13% 19% 13% 5%	8405
June	1st to 10th	Day	32% 8% 15% 27% 16% 2	2730
	11th to 20th	Day	27% 7% 14% 27% 22% 3	2730
	21st to end	Day	23% 7% 15% 26% 24% 5%	2730
July	1st to 10th	Day	36% 8% 13% 27% 14% 2	5966
	11th to 20th	Day	24% 6% 12% 32% 23% 3	5961
	21st to end	Day	18% 6% 14% 34% 27% 1	6371
August	1st to 10th	Day	19% 5% 12% 25% 29% 10%	4220
	11th to 20th	Day	21% 7% 13% 34% 22% 3	4210
	21st to end	Day	20% 6% 15% 39% 16% 3	4267
September	1st to 10th	Day	28% 4 10% 23% 23% 12%	2200
	11th to 20th	Day	34% 6% 10% 22% 20% 8%	2200
	21st to end	Day	25% 4 8% 25% 29% 10%	2200
October	1st to 10th	Day	16% 3 6% 13% 39% 24%	3526
	11th to 20th	Day	17% 3 6% 22% 35% 18%	3240
	21st to end	Day	24% 3 7% 29% 27% 9%	3538
November	1st to 10th	Day	27% 4 8% 21% 30% 10%	2032
	11th to 20th	Day	21% 3 7% 27% 29% 13%	1890
	21st to end	Day	30% 3 6% 22% 29% 10%	1890
December	1st to 10th	Day	34% 3 6% 31% 23% 3	3030
	11th to 20th	Day	35% 3 5% 17% 31% 9%	3479
	21st to end	Day	40% 3 5% 24% 21% 6%	4587
January	1st to 10th	Night	31% 3 7% 27% 26% 5%	7635
	11th to 20th	Night	15% 3 8% 20% 40% 15%	7680
	21st to end	Night	40% 3 5% 15% 26% 9%	8402
February	1st to 10th	Night	34% 4 7% 21% 25% 9%	2630
	11th to 20th	Night	35% 4 7% 21% 25% 8%	2458
	21st to end	Night	32% 3 5% 18% 25% 17%	2076
March	1st to 10th	Night	33% 5% 5% 15% 22% 20%	5074
	11th to 20th	Night	22% 3 7% 23% 18% 27%	5090
	21st to end	Night	33% 5% 10% 20% 29% 3	5588
April	1st to 10th	Night	30% 4 10% 21% 21% 13%	2310
	11th to 20th	Night	31% 6% 9% 21% 22% 12%	2138
	21st to end	Night	31% 5% 12% 26% 21% 5%	1890
May	1st to 10th	Night	38% 6% 12% 19% 19% 6%	5293
	11th to 20th	Night	29% 6% 13% 25% 20% 7%	5266
	21st to end	Night	34% 7% 17% 21% 15% 6%	5805
June	1st to 10th	Night	21% 8% 16% 33% 20% 4	1890
	11th to 20th	Night	21% 6% 15% 29% 23% 6%	1890
	21st to end	Night	16% 5% 14% 29% 29% 7%	1890
July	1st to 10th	Night	32% 6% 15% 29% 16% 2	4125
	11th to 20th	Night	15% 6% 12% 36% 28% 3	4116
	21st to end	Night	15% 3 13% 38% 28% 3	4410
August	1st to 10th	Night	13% 5% 8% 28% 36% 10%	2916
	11th to 20th	Night	15% 5% 12% 38% 25% 5%	2915
	21st to end	Night	15% 5% 16% 40% 19% 4	3631
September	1st to 10th	Night	25% 5% 10% 22% 27% 11%	2200
	11th to 20th	Night	31% 6% 9% 22% 22% 10%	2200
	21st to end	Night	20% 4 8% 24% 31% 13%	2200
October	1st to 10th	Night	13% 3 7% 14% 37% 26%	3619
	11th to 20th	Night	18% 3 4 18% 36% 21%	3899
	21st to end	Night	22% 3 6% 30% 27% 12%	4252
November	1st to 10th	Night	25% 4 6% 23% 30% 13%	2481
	11th to 20th	Night	22% 4 7% 23% 29% 14%	2730
	21st to end	Night	30% 4 6% 20% 29% 10%	2730
December	1st to 10th	Night	26% 3 6% 29% 29% 6%	4380
	11th to 20th	Night	33% 3 4 17% 28% 14%	4947
	21st to end	Night	34% 3 6% 24% 25% 7%	6633

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% 25% 7%	330
	11th to 20th	Day	16% 19% 35% 24% 5% 2	330
	21st to end	Day	15% 12% 42% 23% 8% 1	363
February	1st to 10th	Day	20% 12% 37% 24% 6%	330
	11th to 20th	Day	18% 15% 46% 18% 2	330
	21st to end	Day	21% 16% 41% 16% 5%	273
March	1st to 10th	Day	20% 19% 35% 19% 6% 1	330
	11th to 20th	Day	21% 20% 37% 17% 5%	330
	21st to end	Day	17% 13% 45% 19% 5%	363
April	1st to 10th	Day	16% 22% 30% 25% 7% 1	330
	11th to 20th	Day	17% 17% 41% 21% 3	461
	21st to end	Day	16% 17% 37% 21% 7% 1	750
May	1st to 10th	Day	14% 11% 36% 31% 8%	753
	11th to 20th	Day	16% 8% 33% 27% 14% 2	775
	21st to end	Day	14% 7% 30% 30% 16% 4	844
June	1st to 10th	Day	7% 4 20% 35% 28% 6%	780
	11th to 20th	Day	7% 3 21% 41% 23% 5%	780
	21st to end	Day	4 3 16% 39% 29% 9%	780
July	1st to 10th	Day	5% 1 9% 35% 35% 14%	771
	11th to 20th	Day	3 2 8% 32% 42% 13%	768
	21st to end	Day	3 1 5% 31% 43% 17%	845
August	1st to 10th	Day	3 1 8% 38% 40% 10%	726
	11th to 20th	Day	7% 2 9% 35% 38% 10%	715
	21st to end	Day	5% 1 5% 33% 37% 18%	565
September	1st to 10th	Day	8% 1 7% 29% 40% 15%	330
	11th to 20th	Day	6% 1 7% 35% 35% 16%	330
	21st to end	Day	12% 3 12% 31% 31% 12%	330
October	1st to 10th	Day	4 4 21% 36% 25% 10%	330
	11th to 20th	Day	10% 8% 29% 32% 19% 2	330
	21st to end	Day	21% 9% 26% 28% 13% 2	363
November	1st to 10th	Day	15% 7% 37% 28% 10% 3	330
	11th to 20th	Day	13% 8% 37% 28% 12% 2	330
	21st to end	Day	18% 11% 37% 25% 7% 2	330
December	1st to 10th	Day	12% 12% 35% 29% 10% 2	330
	11th to 20th	Day	17% 14% 36% 26% 7% 1	330
	21st to end	Day	21% 13% 35% 26% 5%	363
January	1st to 10th	Night	14% 11% 38% 30% 7%	1050
	11th to 20th	Night	12% 13% 36% 33% 6%	1050
	21st to end	Night	14% 10% 37% 32% 6%	1155
February	1st to 10th	Night	16% 13% 39% 29% 4	950
	11th to 20th	Night	17% 17% 38% 25% 4	824
	21st to end	Night	15% 15% 42% 26% 2	692
March	1st to 10th	Night	15% 16% 35% 30% 4	651
	11th to 20th	Night	14% 13% 40% 29% 4	630
	21st to end	Night	13% 12% 44% 26% 4	693
April	1st to 10th	Night	12% 13% 43% 28% 4	630
	11th to 20th	Night	13% 13% 44% 26% 5%	623
	21st to end	Night	13% 10% 43% 27% 7% 1	630
May	1st to 10th	Night	11% 10% 39% 34% 5% 1	630
	11th to 20th	Night	12% 7% 33% 33% 14% 1	621
	21st to end	Night	8% 4 33% 37% 16% 2	683
June	1st to 10th	Night	5% 3 22% 43% 25% 2	630
	11th to 20th	Night	5% 2 25% 52% 16%	630
	21st to end	Night	3 1 18% 48% 27% 3	630
July	1st to 10th	Night	5% 1 10% 44% 37% 3	615
	11th to 20th	Night	2 1 8% 44% 41% 3	612
	21st to end	Night	2 1 8% 43% 43% 3	687
August	1st to 10th	Night	2 1 7% 50% 39% 1	597
	11th to 20th	Night	6% 1 9% 44% 36% 3	594
	21st to end	Night	3 1 6% 43% 41% 6%	660
September	1st to 10th	Night	7% 3 11% 45% 33% 3	600
	11th to 20th	Night	10% 1 12% 42% 32% 3	600
	21st to end	Night	8% 2 16% 44% 27% 3	600
October	1st to 10th	Night	7% 2 23% 41% 25% 2	616
	11th to 20th	Night	10% 7% 31% 38% 14%	800
	21st to end	Night	16% 9% 30% 32% 12% 1	880
November	1st to 10th	Night	11% 6% 34% 37% 11% 1	841
	11th to 20th	Night	8% 12% 34% 35% 11% 1	1050
	21st to end	Night	16% 11% 36% 28% 9%	1050
December	1st to 10th	Night	13% 10% 36% 34% 7%	1050
	11th to 20th	Night	17% 12% 32% 32% 6%	1050
	21st to end	Night	16% 12% 37% 29% 6%	1155

Page generated at 2026-07-28 02:25:20. Version ac07

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