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 2025-03-27.

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%	109593
	Night	10% 5% 2 4 3 3 3 3 4 6% 58%	109969
Transparency	Day	30% 5% 9% 25% 24% 7%	113349
	Night	27% 4 9% 25% 26% 10%	114340
Seeing	Day	11% 8% 25% 29% 21% 6%	15705
	Night	11% 8% 30% 35% 15% 1	25757

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	13% 6% 3 4 3 2 3 2 5% 5% 54%	4077
	11th to 20th	Day	6% 6% 1 3 2 2 2 2 3 5% 67%	4080
	21st to end	Day	15% 8% 2 4 3 3 3 2 4 5% 50%	4488
February	1st to 10th	Day	14% 8% 2 5% 3 3 3 3 4 6% 49%	1910
	11th to 20th	Day	14% 6% 2 5% 3 4 3 2 4 7% 50%	1949
	21st to end	Day	17% 8% 2 6% 3 2 2 2 3 4 53%	1650
March	1st to 10th	Day	14% 6% 2 5% 3 3 4 3 3 5% 53%	3515
	11th to 20th	Day	11% 6% 2 6% 4 4 4 3 4 6% 50%	3590
	21st to end	Day	14% 9% 2 7% 5% 4 4 3 5% 6% 42%	3880
April	1st to 10th	Day	11% 5% 1 5% 3 3 3 3 4 7% 54%	2090
	11th to 20th	Day	12% 5% 2 6% 4 4 4 3 5% 5% 50%	2189
	21st to end	Day	12% 6% 2 5% 4 3 4 4 5% 6% 48%	2470
May	1st to 10th	Day	18% 7% 2 5% 4 3 3 3 5% 5% 43%	4082
	11th to 20th	Day	13% 6% 2 5% 4 3 4 3 4 7% 48%	4082
	21st to end	Day	15% 6% 3 6% 5% 4 5% 4 5% 7% 41%	4477
June	1st to 10th	Day	6% 5% 2 6% 4 4 5% 4 6% 9% 49%	2470
	11th to 20th	Day	6% 4 2 4 4 4 5% 4 5% 10% 53%	2470
	21st to end	Day	5% 3 1 4 3 3 4 4 5% 8% 60%	2470
July	1st to 10th	Day	6% 5% 2 4 4 4 5% 4 7% 9% 50%	4120
	11th to 20th	Day	3 3 1 4 4 4 4 4 6% 9% 59%	4120
	21st to end	Day	4 3 1 5% 4 4 4 4 6% 8% 56%	4465
August	1st to 10th	Day	2 3 1 4 3 4 4 3 5% 8% 62%	4104
	11th to 20th	Day	5% 2 2 4 3 3 4 4 7% 10% 56%	4041
	21st to end	Day	3 2 1 4 4 4 5% 4 6% 9% 59%	4138
September	1st to 10th	Day	9% 3 1 5% 3 4 3 3 5% 6% 56%	2090
	11th to 20th	Day	11% 5% 2 7% 4 5% 4 4 4 6% 48%	2090
	21st to end	Day	8% 3 1 4 3 3 4 3 4 7% 60%	2090
October	1st to 10th	Day	7% 4 1 4 3 3 3 3 5% 6% 61%	2054
	11th to 20th	Day	9% 4 1 3 3 2 2 2 3 5% 66%	1900
	21st to end	Day	13% 5% 2 4 2 2 2 2 3 4 60%	2090
November	1st to 10th	Day	11% 6% 1 4 2 2 3 3 4 6% 57%	1933
	11th to 20th	Day	7% 4 1 3 2 2 2 2 3 5% 67%	1800
	21st to end	Day	14% 4 2 4 3 2 3 2 4 5% 58%	1800
December	1st to 10th	Day	12% 7% 2 5% 3 2 3 2 4 6% 54%	2940
	11th to 20th	Day	14% 6% 2 5% 3 3 3 3 4 4 54%	3391
	21st to end	Day	19% 8% 2 5% 3 3 3 2 3 5% 47%	4488
January	1st to 10th	Night	14% 6% 2 5% 3 3 3 3 4 6% 52%	5888
	11th to 20th	Night	5% 4 1 3 2 2 2 2 3 6% 69%	5900
	21st to end	Night	17% 7% 2 4 2 2 3 2 4 4 52%	6490
February	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 4 5% 53%	2505
	11th to 20th	Night	14% 6% 2 5% 3 3 4 3 4 7% 50%	2338
	21st to end	Night	11% 7% 3 4 3 3 2 3 3 5% 56%	1980
March	1st to 10th	Night	12% 7% 2 6% 4 3 3 3 4 6% 50%	3560
	11th to 20th	Night	10% 4 2 4 3 2 2 2 4 7% 59%	3590
	21st to end	Night	12% 6% 2 5% 4 3 4 3 5% 6% 50%	3889
April	1st to 10th	Night	10% 5% 1 5% 3 3 4 2 6% 8% 54%	2090
	11th to 20th	Night	12% 4 1 5% 4 3 4 2 4 6% 55%	1932
	21st to end	Night	12% 4 2 4 4 3 3 3 5% 8% 53%	1710
May	1st to 10th	Night	18% 7% 2 5% 3 3 4 2 4 5% 45%	2823
	11th to 20th	Night	13% 5% 2 5% 4 4 4 3 5% 6% 49%	2823
	21st to end	Night	10% 5% 2 5% 3 4 4 3 5% 8% 51%	3090
June	1st to 10th	Night	5% 2 1 3 3 2 4 4 6% 12% 58%	1710
	11th to 20th	Night	5% 3 1 3 3 3 4 3 6% 10% 60%	1710
	21st to end	Night	3 2 1 2 3 2 3 3 5% 10% 65%	1710
July	1st to 10th	Night	5% 3 2 5% 4 3 4 4 6% 9% 56%	2850
	11th to 20th	Night	2 2 1 2 1 2 3 2 6% 10% 69%	2850
	21st to end	Night	3 3 1 4 3 3 3 2 5% 8% 64%	3098
August	1st to 10th	Night	1 2 1 2 2 3 3 3 5% 8% 70%	2836
	11th to 20th	Night	3 2 1 3 3 3 3 3 6% 9% 66%	2798
	21st to end	Night	2 2 1 3 2 2 3 3 6% 9% 67%	3520
September	1st to 10th	Night	7% 4 2 4 3 3 4 3 5% 6% 60%	2090
	11th to 20th	Night	9% 4 2 6% 4 4 4 3 5% 6% 53%	2090
	21st to end	Night	7% 3 1 3 2 2 3 3 6% 8% 62%	2090
October	1st to 10th	Night	7% 3 1 3 3 3 3 3 5% 6% 64%	2105
	11th to 20th	Night	9% 3 2 4 3 2 4 2 3 4 65%	2280
	21st to end	Night	14% 4 2 3 2 2 2 2 3 4 62%	2508
November	1st to 10th	Night	11% 4 1 3 2 3 3 2 3 5% 63%	2360
	11th to 20th	Night	7% 3 1 5% 2 3 4 3 4 6% 64%	2600
	21st to end	Night	12% 5% 2 4 2 3 3 3 4 6% 57%	2600
December	1st to 10th	Night	9% 6% 2 4 2 2 3 2 4 5% 61%	4250
	11th to 20th	Night	14% 6% 2 5% 3 3 3 2 3 4 56%	4816
	21st to end	Night	17% 7% 2 4 3 2 3 2 4 5% 54%	6490

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	33% 3 8% 28% 24% 4	4078
	11th to 20th	Day	21% 2 8% 22% 40% 6%	4080
	21st to end	Day	37% 3 4 19% 25% 10%	4488
February	1st to 10th	Day	38% 4 7% 20% 23% 8%	1910
	11th to 20th	Day	37% 4 8% 25% 20% 8%	1959
	21st to end	Day	38% 2 4 16% 28% 12%	1650
March	1st to 10th	Day	34% 4 7% 15% 26% 15%	3515
	11th to 20th	Day	35% 6% 10% 23% 19% 6%	3590
	21st to end	Day	41% 7% 7% 23% 17% 4	3880
April	1st to 10th	Day	31% 4 10% 25% 18% 11%	2090
	11th to 20th	Day	35% 6% 9% 21% 21% 9%	2189
	21st to end	Day	35% 6% 10% 24% 20% 5%	2470
May	1st to 10th	Day	41% 6% 9% 20% 16% 8%	4120
	11th to 20th	Day	35% 8% 11% 25% 20% 1	4086
	21st to end	Day	41% 9% 11% 18% 15% 7%	4489
June	1st to 10th	Day	31% 8% 15% 28% 15% 3	2470
	11th to 20th	Day	26% 7% 13% 27% 23% 4	2470
	21st to end	Day	22% 7% 14% 26% 25% 5%	2470
July	1st to 10th	Day	26% 7% 15% 35% 15% 2	4056
	11th to 20th	Day	21% 5% 11% 36% 25% 3	4051
	21st to end	Day	23% 7% 13% 32% 25% 1	4325
August	1st to 10th	Day	18% 5% 12% 24% 30% 10%	4090
	11th to 20th	Day	21% 7% 13% 34% 22% 3	4080
	21st to end	Day	19% 7% 15% 40% 17% 3	4137
September	1st to 10th	Day	28% 5% 10% 22% 23% 12%	2090
	11th to 20th	Day	35% 6% 9% 20% 21% 8%	2090
	21st to end	Day	25% 4 7% 24% 29% 10%	2090
October	1st to 10th	Day	16% 3 6% 13% 38% 24%	3418
	11th to 20th	Day	16% 3 6% 21% 35% 18%	3140
	21st to end	Day	24% 3 7% 30% 27% 9%	3428
November	1st to 10th	Day	29% 4 8% 21% 29% 10%	1933
	11th to 20th	Day	22% 3 7% 27% 28% 14%	1800
	21st to end	Day	30% 3 6% 21% 30% 10%	1800
December	1st to 10th	Day	33% 3 6% 31% 22% 3	2940
	11th to 20th	Day	35% 3 5% 16% 31% 10%	3389
	21st to end	Day	41% 3 5% 24% 21% 6%	4488
January	1st to 10th	Night	34% 4 7% 27% 24% 5%	5888
	11th to 20th	Night	18% 3 9% 20% 37% 12%	5900
	21st to end	Night	37% 3 5% 16% 27% 12%	6490
February	1st to 10th	Night	35% 4 7% 20% 25% 9%	2505
	11th to 20th	Night	35% 4 7% 21% 24% 8%	2338
	21st to end	Night	33% 3 6% 17% 24% 17%	1980
March	1st to 10th	Night	35% 5% 5% 17% 21% 17%	3560
	11th to 20th	Night	27% 4 9% 31% 19% 10%	3590
	21st to end	Night	34% 6% 9% 22% 25% 4	3889
April	1st to 10th	Night	29% 4 11% 20% 21% 14%	2090
	11th to 20th	Night	31% 5% 9% 22% 21% 13%	1932
	21st to end	Night	30% 5% 11% 26% 22% 5%	1710
May	1st to 10th	Night	40% 7% 9% 17% 19% 8%	2850
	11th to 20th	Night	34% 7% 14% 25% 19% 1	2824
	21st to end	Night	28% 9% 14% 24% 18% 7%	3099
June	1st to 10th	Night	19% 8% 16% 34% 20% 4	1710
	11th to 20th	Night	20% 6% 15% 29% 25% 6%	1710
	21st to end	Night	16% 5% 13% 29% 30% 7%	1710
July	1st to 10th	Night	24% 6% 14% 36% 17% 3	2805
	11th to 20th	Night	11% 4 10% 44% 27% 3	2796
	21st to end	Night	18% 4 13% 37% 26% 2	2999
August	1st to 10th	Night	13% 5% 8% 27% 36% 11%	2826
	11th to 20th	Night	15% 5% 12% 38% 26% 5%	2825
	21st to end	Night	14% 4 16% 41% 20% 4	3520
September	1st to 10th	Night	25% 5% 10% 21% 27% 12%	2090
	11th to 20th	Night	32% 6% 9% 20% 23% 11%	2090
	21st to end	Night	20% 4 7% 24% 31% 13%	2090
October	1st to 10th	Night	14% 3 7% 14% 36% 27%	3508
	11th to 20th	Night	17% 3 4 18% 36% 21%	3779
	21st to end	Night	22% 3 6% 30% 27% 12%	4120
November	1st to 10th	Night	26% 4 6% 23% 29% 14%	2360
	11th to 20th	Night	23% 4 8% 22% 29% 14%	2600
	21st to end	Night	30% 4 7% 19% 29% 11%	2600
December	1st to 10th	Night	26% 3 6% 29% 29% 6%	4250
	11th to 20th	Night	34% 3 5% 16% 28% 14%	4817
	21st to end	Night	35% 3 6% 24% 25% 7%	6490

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	17% 14% 39% 24% 7%	300
	11th to 20th	Day	17% 18% 34% 25% 5% 2	300
	21st to end	Day	15% 11% 42% 23% 8% 1	330
February	1st to 10th	Day	21% 13% 36% 23% 6%	300
	11th to 20th	Day	19% 15% 46% 18% 2	300
	21st to end	Day	23% 16% 40% 16% 5%	249
March	1st to 10th	Day	19% 19% 35% 20% 6% 1	300
	11th to 20th	Day	21% 20% 37% 15% 6%	300
	21st to end	Day	19% 13% 46% 18% 3	315
April	1st to 10th	Day	16% 22% 30% 24% 7%	270
	11th to 20th	Day	16% 16% 43% 21% 4	387
	21st to end	Day	14% 17% 38% 22% 8% 1	650
May	1st to 10th	Day	14% 10% 37% 30% 8%	653
	11th to 20th	Day	16% 7% 33% 28% 14% 3	675
	21st to end	Day	15% 7% 31% 27% 16% 4	734
June	1st to 10th	Day	7% 3 18% 36% 30% 7%	680
	11th to 20th	Day	6% 2 20% 42% 25% 5%	680
	21st to end	Day	4 3 14% 40% 30% 10%	680
July	1st to 10th	Day	5% 1 9% 33% 36% 15%	671
	11th to 20th	Day	3 2 7% 32% 41% 14%	668
	21st to end	Day	3 1 5% 31% 43% 17%	739
August	1st to 10th	Day	3 1 8% 38% 40% 11%	676
	11th to 20th	Day	7% 2 9% 35% 37% 9%	665
	21st to end	Day	5% 1 5% 33% 38% 19%	523
September	1st to 10th	Day	7% 1 7% 26% 42% 17%	300
	11th to 20th	Day	6% 1 7% 35% 36% 15%	300
	21st to end	Day	13% 3 11% 31% 30% 12%	300
October	1st to 10th	Day	4 4 22% 36% 23% 11%	300
	11th to 20th	Day	10% 9% 30% 32% 18% 2	300
	21st to end	Day	20% 9% 26% 28% 14% 2	330
November	1st to 10th	Day	17% 7% 37% 27% 10% 3	300
	11th to 20th	Day	14% 9% 36% 26% 13% 2	300
	21st to end	Day	18% 11% 38% 23% 8% 2	300
December	1st to 10th	Day	13% 12% 34% 31% 9% 2	300
	11th to 20th	Day	18% 13% 37% 26% 6%	300
	21st to end	Day	23% 13% 35% 24% 4	330
January	1st to 10th	Night	14% 11% 38% 30% 7% 1	1000
	11th to 20th	Night	13% 13% 36% 32% 6%	1000
	21st to end	Night	13% 10% 37% 33% 6%	1100
February	1st to 10th	Night	16% 13% 39% 27% 4	905
	11th to 20th	Night	17% 16% 37% 26% 4 1	784
	21st to end	Night	16% 16% 41% 25% 3	660
March	1st to 10th	Night	16% 16% 34% 30% 4	620
	11th to 20th	Night	15% 13% 39% 29% 4	600
	21st to end	Night	13% 12% 44% 26% 4	647
April	1st to 10th	Night	11% 14% 44% 27% 4	570
	11th to 20th	Night	12% 13% 44% 26% 5%	563
	21st to end	Night	13% 9% 44% 26% 7% 1	570
May	1st to 10th	Night	11% 10% 39% 33% 6% 1	570
	11th to 20th	Night	12% 6% 32% 34% 14% 1	561
	21st to end	Night	7% 4 34% 37% 16% 2	617
June	1st to 10th	Night	5% 2 21% 43% 27% 2	570
	11th to 20th	Night	4 1 25% 53% 16%	570
	21st to end	Night	3 1 17% 48% 28% 4	570
July	1st to 10th	Night	5% 1 10% 43% 37% 4	555
	11th to 20th	Night	3 1 9% 45% 40% 3	552
	21st to end	Night	2 1 8% 43% 42% 3	624
August	1st to 10th	Night	2 1 7% 49% 39% 1	567
	11th to 20th	Night	6% 9% 45% 36% 3	564
	21st to end	Night	2 1 6% 44% 42% 6%	627
September	1st to 10th	Night	6% 2 11% 44% 34% 3	570
	11th to 20th	Night	10% 1 13% 41% 31% 3	570
	21st to end	Night	9% 2 15% 43% 28% 3	570
October	1st to 10th	Night	7% 2 24% 39% 25% 2	585
	11th to 20th	Night	11% 7% 31% 38% 13%	760
	21st to end	Night	15% 8% 30% 33% 12% 1	836
November	1st to 10th	Night	12% 5% 34% 36% 12% 1	800
	11th to 20th	Night	8% 12% 34% 35% 11% 1	1000
	21st to end	Night	15% 11% 36% 29% 9%	1000
December	1st to 10th	Night	13% 10% 36% 34% 7%	1000
	11th to 20th	Night	18% 12% 32% 32% 6%	1000
	21st to end	Night	17% 11% 37% 29% 6%	1100

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