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 2026-01-17.

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	16% 7% 2 6% 4 4 5% 4 6% 7% 38%	127449
	Night	17% 8% 3 6% 4 4 4 3 5% 7% 40%	124227
Transparency	Day	44% 5% 11% 23% 14% 2	130950
	Night	43% 5% 12% 23% 15% 2	128813
Seeing	Day	22% 15% 28% 22% 10% 3	16368
	Night	20% 12% 29% 29% 10% 1	25587

By Month

Percentage Cloud Cover NE (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	12% 7% 3 9% 6% 4 5% 4 5% 6% 40%	3677
	11th to 20th	Day	14% 8% 4 9% 6% 5% 5% 4 5% 5% 36%	3618
	21st to end	Day	17% 9% 3 7% 4 4 4 3 5% 6% 37%	4264
February	1st to 10th	Day	15% 9% 3 8% 5% 5% 6% 4 5% 7% 34%	1800
	11th to 20th	Day	14% 8% 3 8% 5% 5% 4 3 5% 6% 39%	1927
	21st to end	Day	18% 9% 3 5% 5% 4 4 4 5% 6% 38%	1650
March	1st to 10th	Day	21% 8% 2 7% 5% 4 3 3 4 5% 38%	4476
	11th to 20th	Day	13% 7% 2 7% 5% 4 4 4 6% 7% 43%	4791
	21st to end	Day	19% 7% 2 6% 5% 4 5% 4 5% 6% 38%	5914
April	1st to 10th	Day	19% 8% 3 6% 4 4 5% 4 5% 6% 37%	2400
	11th to 20th	Day	17% 7% 3 7% 5% 4 4 3 5% 6% 40%	2374
	21st to end	Day	18% 7% 3 7% 4 4 5% 3 5% 5% 39%	2433
May	1st to 10th	Day	29% 7% 2 6% 3 4 4 4 6% 8% 27%	5950
	11th to 20th	Day	20% 8% 2 6% 5% 4 5% 3 6% 7% 34%	6318
	21st to end	Day	20% 7% 2 6% 4 4 5% 5% 7% 9% 32%	6949
June	1st to 10th	Day	16% 5% 2 6% 5% 5% 6% 4 7% 9% 35%	2800
	11th to 20th	Day	15% 6% 2 6% 5% 5% 5% 5% 7% 8% 35%	2800
	21st to end	Day	13% 6% 2 7% 5% 5% 5% 4 7% 8% 39%	2800
July	1st to 10th	Day	10% 8% 3 7% 5% 5% 5% 4 7% 9% 35%	6360
	11th to 20th	Day	9% 6% 2 7% 5% 6% 6% 5% 8% 12% 33%	6360
	21st to end	Day	13% 7% 2 6% 4 4 4 4 6% 8% 41%	6926
August	1st to 10th	Day	19% 6% 2 6% 4 6% 4 4 6% 7% 33%	4383
	11th to 20th	Day	12% 7% 2 6% 4 4 5% 4 7% 9% 40%	3850
	21st to end	Day	12% 5% 2 5% 4 3 5% 4 6% 10% 44%	4312
September	1st to 10th	Day	14% 7% 3 5% 4 5% 4 3 5% 7% 43%	2400
	11th to 20th	Day	13% 5% 1 5% 3 4 4 3 5% 8% 50%	2325
	21st to end	Day	17% 6% 2 6% 5% 4 5% 4 6% 6% 41%	2200
October	1st to 10th	Day	19% 6% 2 5% 4 4 4 3 5% 6% 43%	2069
	11th to 20th	Day	14% 6% 2 6% 3 5% 5% 3 6% 8% 42%	2000
	21st to end	Day	21% 6% 2 6% 4 4 5% 4 5% 6% 37%	2035
November	1st to 10th	Day	13% 6% 2 5% 4 4 3 2 4 6% 51%	1851
	11th to 20th	Day	16% 7% 3 5% 3 4 4 3 5% 7% 43%	1890
	21st to end	Day	16% 8% 3 6% 5% 4 4 5% 5% 6% 38%	1753
December	1st to 10th	Day	17% 7% 2 7% 4 5% 5% 4 6% 6% 38%	2680
	11th to 20th	Day	21% 7% 2 5% 3 4 3 3 5% 5% 42%	3066
	21st to end	Day	17% 9% 3 7% 5% 5% 4 4 6% 7% 32%	4048
January	1st to 10th	Night	14% 6% 3 9% 5% 5% 4 4 5% 6% 40%	6489
	11th to 20th	Night	12% 8% 4 9% 5% 4 4 4 5% 6% 39%	6400
	21st to end	Night	16% 9% 2 6% 4 4 5% 4 5% 7% 39%	6676
February	1st to 10th	Night	15% 10% 2 7% 4 4 5% 3 5% 6% 39%	2600
	11th to 20th	Night	15% 8% 3 7% 5% 4 5% 4 6% 5% 39%	2358
	21st to end	Night	17% 8% 2 7% 5% 4 4 3 4 5% 40%	1980
March	1st to 10th	Night	19% 8% 3 6% 3 3 3 3 5% 6% 40%	5364
	11th to 20th	Night	15% 9% 3 5% 4 4 4 3 6% 7% 42%	5163
	21st to end	Night	18% 9% 3 7% 5% 4 5% 3 4 5% 38%	5017
April	1st to 10th	Night	20% 7% 2 7% 5% 5% 4 4 5% 6% 35%	2000
	11th to 20th	Night	20% 6% 2 7% 4 4 3 3 5% 7% 38%	1975
	21st to end	Night	20% 7% 3 6% 3 4 3 3 5% 6% 41%	1965
May	1st to 10th	Night	32% 7% 2 5% 3 3 3 3 5% 7% 28%	4018
	11th to 20th	Night	22% 8% 3 7% 3 3 4 3 5% 7% 34%	3575
	21st to end	Night	19% 8% 3 7% 4 5% 4 3 6% 7% 36%	3926
June	1st to 10th	Night	15% 6% 2 6% 4 4 5% 4 5% 8% 41%	1600
	11th to 20th	Night	14% 7% 2 5% 4 5% 4 4 5% 10% 40%	1600
	21st to end	Night	11% 5% 2 5% 3 4 4 4 7% 9% 45%	1600
July	1st to 10th	Night	11% 7% 3 6% 5% 4 5% 4 6% 10% 39%	3600
	11th to 20th	Night	12% 7% 2 8% 5% 6% 5% 4 6% 10% 35%	3600
	21st to end	Night	12% 6% 2 6% 3 4 4 4 6% 8% 44%	3926
August	1st to 10th	Night	20% 6% 2 7% 4 5% 5% 4 6% 7% 34%	2737
	11th to 20th	Night	14% 7% 3 7% 5% 4 5% 3 5% 6% 41%	3189
	21st to end	Night	11% 5% 2 5% 4 3 4 3 5% 8% 50%	3586
September	1st to 10th	Night	14% 7% 2 5% 4 3 3 3 5% 7% 47%	2000
	11th to 20th	Night	13% 5% 1 4 3 4 3 3 4 7% 53%	2075
	21st to end	Night	17% 6% 2 5% 3 3 3 3 5% 6% 45%	2200
October	1st to 10th	Night	19% 6% 2 6% 4 4 3 3 4 6% 42%	2329
	11th to 20th	Night	16% 6% 2 5% 3 4 3 3 4 6% 48%	2400
	21st to end	Night	22% 7% 2 5% 3 3 4 4 5% 6% 40%	2803
November	1st to 10th	Night	12% 6% 2 6% 4 4 4 3 5% 7% 47%	2667
	11th to 20th	Night	17% 7% 2 5% 4 3 4 3 5% 6% 44%	2730
	21st to end	Night	16% 6% 2 5% 4 4 4 4 5% 7% 43%	2861
December	1st to 10th	Night	15% 10% 3 6% 4 3 3 3 5% 7% 43%	4720
	11th to 20th	Night	25% 8% 3 4 3 3 3 3 4 5% 38%	5348
	21st to end	Night	19% 10% 3 7% 4 4 4 3 5% 6% 35%	7150

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	42% 5% 9% 28% 13% 3	3678
	11th to 20th	Day	48% 4 9% 22% 14% 4	3648
	21st to end	Day	47% 3 10% 25% 14% 1	4264
February	1st to 10th	Day	48% 4 12% 20% 15% 1	1800
	11th to 20th	Day	46% 4 8% 22% 16% 3	1936
	21st to end	Day	47% 3 11% 23% 14% 2	1650
March	1st to 10th	Day	48% 3 10% 19% 11% 8%	4479
	11th to 20th	Day	40% 4 13% 30% 9% 4	4791
	21st to end	Day	46% 5% 8% 13% 23% 5%	5914
April	1st to 10th	Day	49% 5% 8% 21% 14% 4	2400
	11th to 20th	Day	45% 4 10% 21% 17% 3	2374
	21st to end	Day	46% 4 8% 21% 16% 4	2433
May	1st to 10th	Day	55% 5% 13% 15% 11% 1	5961
	11th to 20th	Day	50% 4 11% 21% 13% 1	6322
	21st to end	Day	45% 6% 12% 23% 12% 1	6949
June	1st to 10th	Day	43% 6% 13% 22% 14% 1	2800
	11th to 20th	Day	43% 6% 14% 24% 11% 1	2800
	21st to end	Day	41% 7% 11% 26% 14% 1	2800
July	1st to 10th	Day	44% 8% 12% 21% 14% 1	6290
	11th to 20th	Day	41% 7% 16% 25% 11%	6285
	21st to end	Day	41% 5% 11% 24% 18% 1	6728
August	1st to 10th	Day	49% 5% 13% 22% 10% 1	4369
	11th to 20th	Day	39% 6% 16% 29% 10%	3886
	21st to end	Day	35% 5% 11% 32% 15% 1	4312
September	1st to 10th	Day	40% 5% 11% 25% 17% 2	2400
	11th to 20th	Day	34% 4 9% 27% 24% 2	2325
	21st to end	Day	43% 5% 11% 22% 17% 3	2200
October	1st to 10th	Day	40% 5% 8% 21% 23% 4	3368
	11th to 20th	Day	32% 3 9% 29% 23% 3	3240
	21st to end	Day	31% 4 15% 28% 19% 4	3260
November	1st to 10th	Day	36% 4 12% 26% 19% 2	1851
	11th to 20th	Day	41% 3 10% 24% 19% 3	1890
	21st to end	Day	45% 4 13% 27% 9% 2	1753
December	1st to 10th	Day	45% 5% 13% 26% 10% 1	2680
	11th to 20th	Day	44% 3 14% 22% 14% 2	3066
	21st to end	Day	50% 5% 10% 21% 14% 1	4048
January	1st to 10th	Night	43% 4 9% 23% 17% 3	6489
	11th to 20th	Night	44% 5% 13% 20% 14% 4	6445
	21st to end	Night	46% 4 11% 19% 17% 4	6676
February	1st to 10th	Night	47% 5% 13% 22% 12% 1	2600
	11th to 20th	Night	46% 4 10% 19% 17% 4	2360
	21st to end	Night	46% 3 7% 23% 16% 4	1980
March	1st to 10th	Night	45% 4 12% 19% 15% 6%	5374
	11th to 20th	Night	42% 5% 15% 24% 11% 3	5164
	21st to end	Night	48% 4 6% 14% 23% 4	5017
April	1st to 10th	Night	51% 4 9% 19% 14% 3	2000
	11th to 20th	Night	45% 6% 9% 20% 18% 1	1975
	21st to end	Night	44% 4 8% 23% 16% 4	1965
May	1st to 10th	Night	56% 4 12% 16% 11% 1	4024
	11th to 20th	Night	50% 5% 12% 16% 15% 1	3576
	21st to end	Night	47% 6% 13% 24% 10% 1	3927
June	1st to 10th	Night	41% 6% 11% 23% 18% 1	1600
	11th to 20th	Night	40% 7% 15% 27% 10% 1	1600
	21st to end	Night	35% 7% 15% 27% 15% 2	1600
July	1st to 10th	Night	40% 8% 14% 21% 15% 2	3560
	11th to 20th	Night	44% 7% 14% 23% 11% 1	3552
	21st to end	Night	36% 6% 12% 25% 19% 2	3809
August	1st to 10th	Night	49% 7% 15% 19% 9% 1	2727
	11th to 20th	Night	43% 6% 14% 28% 9%	3219
	21st to end	Night	33% 5% 10% 35% 16% 1	3586
September	1st to 10th	Night	38% 6% 10% 28% 18% 1	2000
	11th to 20th	Night	32% 3 11% 26% 25% 2	2075
	21st to end	Night	40% 5% 9% 23% 22% 1	2200
October	1st to 10th	Night	41% 5% 10% 22% 19% 1	3810
	11th to 20th	Night	29% 3 9% 31% 25% 3	3899
	21st to end	Night	33% 4 13% 31% 18% 1	4528
November	1st to 10th	Night	38% 3 12% 24% 21% 3	2667
	11th to 20th	Night	42% 3 9% 24% 19% 4	2730
	21st to end	Night	40% 3 13% 30% 11% 1	2861
December	1st to 10th	Night	43% 4 12% 32% 9% 1	4720
	11th to 20th	Night	48% 4 12% 18% 15% 3	5348
	21st to end	Night	50% 4 11% 20% 13% 3	7150

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	27% 23% 28% 18% 4	220
	11th to 20th	Day	33% 23% 29% 11% 3	212
	21st to end	Day	24% 25% 30% 16% 3 2	287
February	1st to 10th	Day	32% 23% 32% 10% 2	300
	11th to 20th	Day	27% 24% 33% 13% 2	475
	21st to end	Day	29% 21% 31% 15% 3	414
March	1st to 10th	Day	27% 20% 31% 17% 5%	500
	11th to 20th	Day	24% 24% 29% 18% 4 1	500
	21st to end	Day	31% 18% 29% 17% 5% 2	550
April	1st to 10th	Day	32% 21% 24% 15% 7% 1	500
	11th to 20th	Day	27% 25% 27% 12% 7% 2	492
	21st to end	Day	28% 19% 28% 18% 6% 1	500
May	1st to 10th	Day	35% 15% 26% 17% 7% 1	503
	11th to 20th	Day	20% 16% 34% 18% 9% 2	527
	21st to end	Day	21% 9% 27% 26% 14% 2	573
June	1st to 10th	Day	19% 11% 29% 25% 13% 2	530
	11th to 20th	Day	17% 12% 28% 29% 12% 2	530
	21st to end	Day	16% 13% 26% 30% 12% 3	530
July	1st to 10th	Day	14% 8% 27% 30% 17% 4	525
	11th to 20th	Day	13% 4 27% 32% 18% 6%	524
	21st to end	Day	13% 7% 30% 34% 14% 3	576
August	1st to 10th	Day	12% 7% 25% 30% 19% 7%	526
	11th to 20th	Day	14% 6% 24% 31% 18% 7%	519
	21st to end	Day	12% 10% 26% 27% 17% 8%	583
September	1st to 10th	Day	18% 10% 22% 24% 17% 9%	530
	11th to 20th	Day	13% 8% 24% 23% 19% 13%	530
	21st to end	Day	20% 9% 26% 26% 13% 5%	530
October	1st to 10th	Day	25% 11% 20% 23% 14% 7%	530
	11th to 20th	Day	17% 15% 28% 24% 13% 3	530
	21st to end	Day	24% 13% 31% 20% 10% 1	492
November	1st to 10th	Day	19% 15% 29% 25% 9% 2	425
	11th to 20th	Day	23% 20% 26% 23% 7% 1	430
	21st to end	Day	26% 17% 31% 19% 6% 1	293
December	1st to 10th	Day	24% 21% 35% 15% 6%	220
	11th to 20th	Day	33% 22% 26% 15% 4	220
	21st to end	Day	25% 22% 31% 17% 4 1	242
January	1st to 10th	Night	20% 18% 36% 22% 4	840
	11th to 20th	Night	24% 17% 31% 25% 3	825
	21st to end	Night	18% 17% 37% 24% 3	880
February	1st to 10th	Night	22% 16% 37% 23% 2	800
	11th to 20th	Night	23% 18% 39% 18% 1	783
	21st to end	Night	23% 18% 36% 19% 4	660
March	1st to 10th	Night	24% 18% 32% 23% 4	800
	11th to 20th	Night	22% 21% 31% 22% 4	800
	21st to end	Night	20% 18% 34% 22% 5%	880
April	1st to 10th	Night	26% 15% 34% 21% 5%	800
	11th to 20th	Night	24% 14% 32% 25% 5%	791
	21st to end	Night	24% 14% 29% 26% 7% 1	765
May	1st to 10th	Night	28% 11% 29% 25% 7%	590
	11th to 20th	Night	22% 11% 29% 32% 6% 1	394
	21st to end	Night	15% 7% 21% 42% 13% 1	433
June	1st to 10th	Night	16% 6% 28% 36% 15% 1	400
	11th to 20th	Night	16% 6% 24% 39% 16%	400
	21st to end	Night	13% 4 23% 46% 15% 1	400
July	1st to 10th	Night	14% 3 17% 44% 21% 1	390
	11th to 20th	Night	12% 3 19% 42% 23% 1	388
	21st to end	Night	11% 5% 20% 46% 17% 1	438
August	1st to 10th	Night	13% 2 17% 44% 22% 2	484
	11th to 20th	Night	13% 4 18% 40% 24% 2	782
	21st to end	Night	15% 4 17% 37% 24% 2	880
September	1st to 10th	Night	16% 5% 21% 36% 21% 2	800
	11th to 20th	Night	16% 5% 20% 33% 23% 5%	800
	21st to end	Night	20% 6% 20% 35% 17% 2	800
October	1st to 10th	Night	23% 8% 24% 25% 17% 2	800
	11th to 20th	Night	20% 11% 27% 31% 11% 1	800
	21st to end	Night	25% 11% 31% 25% 7%	880
November	1st to 10th	Night	17% 12% 32% 28% 10% 1	820
	11th to 20th	Night	23% 16% 29% 24% 7% 1	840
	21st to end	Night	20% 15% 32% 27% 6%	840
December	1st to 10th	Night	25% 15% 30% 25% 5%	840
	11th to 20th	Night	25% 15% 30% 26% 3	840
	21st to end	Night	26% 14% 34% 23% 3	924

Page generated at 2026-01-14 01:14:28. Version ac07

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