Montgomery
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Montgomery clear sky chart from 2005-11-06 to 2025-10-19.

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% 6% 2 6% 4 5% 5% 4 6% 8% 43%	126233
	Night	11% 6% 2 6% 4 4 4 4 6% 8% 44%	123498
Transparency	Day	36% 8% 12% 20% 18% 6%	130064
	Night	36% 7% 11% 19% 19% 8%	127739
Seeing	Day	12% 7% 19% 24% 23% 15%	15904
	Night	13% 5% 20% 33% 25% 5%	25828

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% 9% 3 5% 4 3 3 2 4 5% 49%	4056
	11th to 20th	Day	10% 8% 4 5% 3 3 4 3 5% 7% 49%	4289
	21st to end	Day	20% 9% 3 7% 3 3 3 3 5% 6% 38%	4972
February	1st to 10th	Day	14% 9% 3 7% 4 5% 4 4 5% 7% 38%	2000
	11th to 20th	Day	19% 8% 2 5% 4 4 3 3 5% 6% 42%	1948
	21st to end	Day	15% 7% 2 8% 4 4 4 3 5% 8% 40%	1650
March	1st to 10th	Day	18% 7% 3 6% 5% 4 4 4 5% 6% 37%	4480
	11th to 20th	Day	9% 5% 2 6% 3 4 4 3 6% 7% 51%	4842
	21st to end	Day	14% 6% 2 7% 4 4 5% 4 5% 7% 43%	5594
April	1st to 10th	Day	11% 5% 2 6% 5% 4 4 4 5% 7% 47%	2400
	11th to 20th	Day	11% 5% 2 6% 4 4 3 4 6% 8% 47%	2374
	21st to end	Day	7% 4 1 4 5% 5% 4 4 6% 7% 53%	2400
May	1st to 10th	Day	11% 7% 3 8% 5% 5% 5% 4 6% 9% 39%	5769
	11th to 20th	Day	12% 7% 2 6% 4 5% 5% 5% 7% 11% 35%	5861
	21st to end	Day	8% 5% 2 6% 4 5% 5% 4 7% 9% 44%	6433
June	1st to 10th	Day	7% 6% 3 8% 6% 5% 6% 5% 7% 10% 36%	2600
	11th to 20th	Day	6% 4 2 5% 5% 5% 7% 5% 8% 12% 42%	2600
	21st to end	Day	6% 5% 3 6% 5% 6% 7% 6% 8% 11% 37%	2600
July	1st to 10th	Day	6% 5% 2 7% 6% 7% 7% 7% 10% 12% 30%	5900
	11th to 20th	Day	4 3 2 6% 6% 7% 8% 6% 9% 13% 36%	5900
	21st to end	Day	5% 5% 2 8% 7% 7% 8% 7% 11% 14% 26%	6246
August	1st to 10th	Day	4 3 1 5% 4 5% 6% 5% 10% 13% 44%	3905
	11th to 20th	Day	6% 5% 2 6% 5% 5% 5% 5% 6% 8% 48%	3849
	21st to end	Day	6% 4 2 5% 5% 5% 5% 5% 8% 9% 46%	4312
September	1st to 10th	Day	9% 6% 3 8% 5% 5% 6% 4 6% 9% 39%	2311
	11th to 20th	Day	12% 6% 3 6% 5% 4 4 3 6% 8% 43%	2200
	21st to end	Day	9% 4 1 7% 4 4 5% 3 5% 8% 50%	2200
October	1st to 10th	Day	10% 4 1 6% 4 4 3 4 5% 5% 53%	2200
	11th to 20th	Day	10% 5% 1 4 3 3 3 3 5% 6% 58%	2178
	21st to end	Day	10% 5% 1 4 4 3 3 3 4 6% 56%	2033
November	1st to 10th	Day	10% 6% 3 5% 4 3 3 4 4 4 54%	1762
	11th to 20th	Day	11% 6% 3 5% 3 3 3 3 4 7% 53%	1800
	21st to end	Day	13% 7% 2 6% 3 4 4 3 5% 7% 47%	1800
December	1st to 10th	Day	15% 6% 2 5% 4 3 4 2 4 6% 49%	2930
	11th to 20th	Day	12% 8% 2 5% 4 4 4 3 5% 5% 47%	3373
	21st to end	Day	19% 7% 2 6% 4 4 4 3 4 5% 43%	4466
January	1st to 10th	Night	15% 6% 4 5% 4 3 4 3 4 5% 47%	5889
	11th to 20th	Night	14% 7% 3 7% 4 3 4 3 5% 6% 45%	5655
	21st to end	Night	18% 10% 3 6% 4 3 4 3 4 6% 40%	5984
February	1st to 10th	Night	14% 7% 2 7% 5% 4 4 4 4 6% 43%	2400
	11th to 20th	Night	19% 8% 2 5% 3 3 3 3 4 4 45%	2338
	21st to end	Night	15% 7% 3 5% 4 3 4 3 5% 8% 44%	1980
March	1st to 10th	Night	20% 9% 3 6% 4 4 4 3 5% 6% 36%	5373
	11th to 20th	Night	11% 6% 2 5% 4 3 4 4 6% 7% 49%	5103
	21st to end	Night	10% 6% 3 7% 4 4 4 3 5% 6% 47%	5376
April	1st to 10th	Night	9% 5% 2 5% 5% 5% 4 4 7% 7% 46%	2000
	11th to 20th	Night	11% 4 2 6% 3 3 3 3 7% 8% 49%	1975
	21st to end	Night	6% 5% 2 5% 4 4 4 3 6% 8% 53%	2000
May	1st to 10th	Night	11% 5% 2 7% 5% 4 4 4 7% 10% 40%	4124
	11th to 20th	Night	16% 6% 2 7% 5% 4 5% 4 6% 8% 37%	4032
	21st to end	Night	6% 5% 2 6% 5% 5% 5% 5% 8% 11% 43%	4421
June	1st to 10th	Night	7% 6% 3 8% 6% 7% 6% 6% 9% 11% 34%	1800
	11th to 20th	Night	5% 5% 2 5% 5% 4 5% 4 8% 13% 44%	1800
	21st to end	Night	4 5% 2 7% 6% 6% 6% 6% 9% 12% 38%	1800
July	1st to 10th	Night	6% 7% 3 8% 7% 6% 7% 7% 9% 12% 28%	4058
	11th to 20th	Night	3 4 2 7% 6% 6% 7% 7% 10% 15% 34%	4057
	21st to end	Night	6% 6% 3 9% 7% 7% 7% 6% 9% 12% 28%	4600
August	1st to 10th	Night	4 4 2 6% 5% 5% 5% 6% 9% 13% 44%	3244
	11th to 20th	Night	5% 4 2 5% 4 4 4 4 7% 11% 49%	3200
	21st to end	Night	6% 4 2 5% 5% 5% 4 5% 7% 10% 47%	3586
September	1st to 10th	Night	7% 6% 2 6% 4 7% 6% 5% 8% 9% 39%	2067
	11th to 20th	Night	11% 7% 2 6% 4 4 5% 4 6% 8% 43%	2200
	21st to end	Night	7% 6% 2 6% 5% 5% 4 5% 6% 8% 48%	2200
October	1st to 10th	Night	8% 4 2 6% 3 3 4 3 5% 6% 56%	2200
	11th to 20th	Night	9% 3 2 4 3 3 3 2 5% 6% 59%	2179
	21st to end	Night	12% 4 2 5% 3 3 4 2 4 5% 56%	2559
November	1st to 10th	Night	9% 4 2 5% 3 3 4 3 4 5% 56%	2536
	11th to 20th	Night	12% 4 2 4 4 3 3 2 5% 6% 56%	2600
	21st to end	Night	12% 5% 2 5% 4 4 4 3 4 6% 50%	2600
December	1st to 10th	Night	14% 6% 2 5% 3 3 3 3 4 6% 52%	4250
	11th to 20th	Night	14% 6% 2 5% 4 3 3 3 4 6% 50%	4822
	21st to end	Night	17% 7% 2 6% 4 4 4 3 4 6% 43%	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	39% 3 4 24% 26% 4	4057
	11th to 20th	Day	36% 4 5% 21% 27% 7%	4289
	21st to end	Day	47% 4 8% 14% 16% 11%	4972
February	1st to 10th	Day	45% 4 8% 17% 19% 7%	2000
	11th to 20th	Day	43% 5% 6% 16% 22% 8%	1958
	21st to end	Day	43% 5% 8% 16% 18% 10%	1650
March	1st to 10th	Day	45% 4 9% 14% 20% 7%	4480
	11th to 20th	Day	32% 5% 7% 16% 22% 18%	4842
	21st to end	Day	41% 4 9% 18% 19% 9%	5596
April	1st to 10th	Day	36% 6% 10% 21% 17% 9%	2400
	11th to 20th	Day	34% 5% 9% 21% 22% 9%	2374
	21st to end	Day	29% 6% 10% 25% 26% 5%	2400
May	1st to 10th	Day	39% 8% 12% 23% 16% 2	5805
	11th to 20th	Day	40% 9% 18% 22% 10% 1	5865
	21st to end	Day	33% 8% 14% 24% 20% 1	6447
June	1st to 10th	Day	40% 10% 15% 22% 12% 2	2600
	11th to 20th	Day	32% 10% 18% 24% 15% 2	2600
	21st to end	Day	35% 13% 18% 19% 11% 3	2600
July	1st to 10th	Day	37% 16% 24% 17% 5% 1	5835
	11th to 20th	Day	32% 16% 27% 20% 3	5831
	21st to end	Day	40% 17% 21% 20% 3	6065
August	1st to 10th	Day	26% 9% 16% 30% 18% 1	3893
	11th to 20th	Day	31% 10% 14% 27% 17% 2	3885
	21st to end	Day	29% 11% 17% 24% 16% 3	4311
September	1st to 10th	Day	40% 9% 14% 17% 16% 5%	2311
	11th to 20th	Day	39% 7% 12% 19% 19% 4	2200
	21st to end	Day	32% 6% 10% 21% 24% 7%	2200
October	1st to 10th	Day	28% 3 8% 12% 26% 23%	3590
	11th to 20th	Day	16% 3 4 16% 26% 34%	3546
	21st to end	Day	20% 3 6% 17% 40% 13%	3334
November	1st to 10th	Day	34% 4 7% 16% 27% 12%	1762
	11th to 20th	Day	32% 4 7% 17% 27% 13%	1800
	21st to end	Day	38% 5% 9% 17% 23% 9%	1800
December	1st to 10th	Day	38% 4 6% 19% 28% 6%	2930
	11th to 20th	Day	38% 4 9% 20% 22% 5%	3370
	21st to end	Day	44% 4 6% 23% 22% 3	4466
January	1st to 10th	Night	39% 5% 4 20% 27% 5%	5889
	11th to 20th	Night	40% 4 5% 19% 23% 9%	5655
	21st to end	Night	46% 4 6% 15% 22% 8%	5984
February	1st to 10th	Night	42% 4 7% 17% 19% 11%	2400
	11th to 20th	Night	42% 5% 6% 15% 21% 11%	2339
	21st to end	Night	38% 6% 9% 17% 16% 13%	1980
March	1st to 10th	Night	48% 3 7% 14% 17% 10%	5373
	11th to 20th	Night	33% 6% 9% 14% 20% 18%	5103
	21st to end	Night	37% 5% 7% 21% 20% 11%	5376
April	1st to 10th	Night	35% 6% 11% 17% 20% 11%	2000
	11th to 20th	Night	31% 6% 7% 19% 25% 11%	1975
	21st to end	Night	28% 5% 11% 26% 22% 8%	2000
May	1st to 10th	Night	36% 9% 15% 20% 17% 3	4152
	11th to 20th	Night	42% 7% 16% 24% 9% 2	4033
	21st to end	Night	31% 10% 15% 25% 17% 2	4429
June	1st to 10th	Night	39% 12% 16% 17% 14% 1	1800
	11th to 20th	Night	29% 10% 20% 25% 14% 2	1800
	21st to end	Night	34% 11% 17% 21% 12% 4	1800
July	1st to 10th	Night	43% 15% 22% 16% 4 1	4015
	11th to 20th	Night	33% 14% 29% 20% 3	4006
	21st to end	Night	45% 13% 20% 18% 4	4467
August	1st to 10th	Night	28% 10% 17% 31% 12% 2	3232
	11th to 20th	Night	28% 9% 18% 27% 15% 3	3230
	21st to end	Night	30% 9% 17% 24% 18% 3	3586
September	1st to 10th	Night	36% 13% 15% 15% 15% 6%	2067
	11th to 20th	Night	37% 9% 12% 19% 17% 6%	2200
	21st to end	Night	32% 7% 11% 19% 22% 8%	2200
October	1st to 10th	Night	30% 4 8% 12% 23% 23%	3590
	11th to 20th	Night	16% 3 4 14% 31% 33%	3550
	21st to end	Night	21% 3 4 18% 34% 19%	4208
November	1st to 10th	Night	29% 5% 8% 17% 25% 16%	2536
	11th to 20th	Night	30% 4 5% 19% 28% 14%	2600
	21st to end	Night	35% 5% 7% 19% 21% 12%	2600
December	1st to 10th	Night	34% 4 7% 22% 27% 7%	4250
	11th to 20th	Night	38% 3 8% 21% 25% 6%	4824
	21st to end	Night	41% 5% 7% 21% 24% 2	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% 19% 31% 26% 7% 1	300
	11th to 20th	Day	17% 17% 30% 21% 12% 3	300
	21st to end	Day	18% 18% 35% 22% 5% 1	330
February	1st to 10th	Day	18% 14% 36% 22% 9%	300
	11th to 20th	Day	29% 15% 32% 20% 5%	300
	21st to end	Day	14% 22% 38% 22% 4	249
March	1st to 10th	Day	20% 15% 32% 23% 8% 1	300
	11th to 20th	Day	15% 12% 33% 27% 11% 1	440
	21st to end	Day	17% 10% 32% 30% 10% 1	550
April	1st to 10th	Day	15% 13% 27% 27% 14% 3	500
	11th to 20th	Day	11% 13% 27% 28% 16% 5%	492
	21st to end	Day	9% 7% 24% 30% 24% 6%	500
May	1st to 10th	Day	9% 7% 22% 29% 26% 8%	503
	11th to 20th	Day	8% 4 20% 27% 26% 15%	527
	21st to end	Day	10% 5% 15% 26% 26% 18%	573
June	1st to 10th	Day	9% 3 10% 26% 29% 22%	530
	11th to 20th	Day	8% 1 12% 19% 31% 29%	530
	21st to end	Day	7% 2 8% 22% 28% 33%	530
July	1st to 10th	Day	6% 1 5% 17% 33% 38%	525
	11th to 20th	Day	6% 2 3 16% 32% 42%	524
	21st to end	Day	4 1 5% 15% 38% 36%	576
August	1st to 10th	Day	10% 1 7% 20% 30% 32%	526
	11th to 20th	Day	9% 2 6% 16% 36% 30%	519
	21st to end	Day	12% 2 9% 19% 33% 25%	583
September	1st to 10th	Day	8% 4 8% 24% 32% 23%	530
	11th to 20th	Day	14% 4 8% 21% 37% 16%	530
	21st to end	Day	11% 4 12% 25% 33% 16%	530
October	1st to 10th	Day	10% 4 21% 33% 24% 8%	530
	11th to 20th	Day	10% 7% 16% 29% 30% 8%	522
	21st to end	Day	12% 9% 20% 28% 25% 6%	425
November	1st to 10th	Day	17% 5% 22% 32% 17% 7%	300
	11th to 20th	Day	16% 7% 32% 28% 13% 4	300
	21st to end	Day	12% 13% 28% 28% 17% 2	300
December	1st to 10th	Day	20% 11% 35% 21% 12% 2	300
	11th to 20th	Day	22% 13% 27% 28% 8%	300
	21st to end	Day	24% 12% 25% 27% 11%	330
January	1st to 10th	Night	19% 10% 33% 31% 7%	800
	11th to 20th	Night	19% 8% 28% 35% 9%	800
	21st to end	Night	18% 9% 34% 34% 4	880
February	1st to 10th	Night	17% 8% 29% 39% 7%	800
	11th to 20th	Night	23% 8% 32% 31% 6%	783
	21st to end	Night	17% 11% 33% 35% 3	660
March	1st to 10th	Night	14% 10% 31% 37% 8%	800
	11th to 20th	Night	14% 7% 33% 33% 12%	800
	21st to end	Night	13% 8% 31% 36% 11%	840
April	1st to 10th	Night	10% 10% 29% 36% 15%	600
	11th to 20th	Night	13% 6% 28% 34% 18% 1	593
	21st to end	Night	9% 4 21% 41% 24% 1	600
May	1st to 10th	Night	9% 4 21% 40% 24% 3	600
	11th to 20th	Night	9% 3 15% 39% 28% 6%	591
	21st to end	Night	8% 3 9% 36% 36% 8%	650
June	1st to 10th	Night	8% 2 9% 33% 42% 6%	600
	11th to 20th	Night	6% 2 8% 24% 50% 11%	600
	21st to end	Night	5% 6% 28% 46% 15%	600
July	1st to 10th	Night	8% 4 21% 53% 14%	585
	11th to 20th	Night	5% 1 2 20% 49% 23%	582
	21st to end	Night	6% 1 4 21% 53% 15%	657
August	1st to 10th	Night	7% 1 3 24% 47% 18%	597
	11th to 20th	Night	9% 1 6% 19% 51% 14%	594
	21st to end	Night	10% 2 7% 25% 42% 14%	660
September	1st to 10th	Night	9% 1 9% 25% 44% 11%	667
	11th to 20th	Night	15% 1 8% 30% 39% 7%	800
	21st to end	Night	8% 2 11% 35% 40% 4	800
October	1st to 10th	Night	10% 2 14% 40% 31% 3	800
	11th to 20th	Night	10% 3 17% 37% 29% 3	793
	21st to end	Night	13% 4 17% 39% 25% 2	836
November	1st to 10th	Night	11% 5% 18% 42% 22% 2	780
	11th to 20th	Night	12% 5% 27% 39% 16% 1	800
	21st to end	Night	14% 7% 25% 38% 14% 1	800
December	1st to 10th	Night	17% 7% 27% 37% 13% 1	800
	11th to 20th	Night	22% 5% 28% 35% 9% 1	800
	21st to end	Night	22% 6% 27% 35% 9%	880

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