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 2025-04-11.

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	12% 9% 3 8% 5% 5% 5% 4 6% 7% 37%	113759
	Night	11% 9% 4 9% 5% 4 4 4 5% 6% 38%	114747
Transparency	Day	46% 3 12% 25% 12% 1	117394
	Night	46% 3 11% 25% 13% 2	119254
Seeing	Day	16% 10% 31% 26% 13% 3	16482
	Night	16% 9% 33% 31% 10% 1	26555

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	7% 14% 5% 18% 10% 7% 7% 3 5% 6% 18%	3617
	11th to 20th	Day	5% 11% 6% 13% 8% 7% 9% 8% 11% 5% 17%	3758
	21st to end	Day	6% 15% 8% 15% 6% 5% 5% 3 4 5% 28%	4488
February	1st to 10th	Day	9% 11% 5% 11% 7% 7% 5% 5% 5% 6% 28%	1800
	11th to 20th	Day	9% 11% 4 11% 7% 5% 6% 5% 6% 5% 32%	1754
	21st to end	Day	12% 11% 4 8% 5% 4 6% 3 6% 7% 33%	1538
March	1st to 10th	Day	13% 8% 3 6% 4 3 4 3 5% 7% 45%	4887
	11th to 20th	Day	10% 10% 3 6% 3 4 4 4 5% 8% 42%	4980
	21st to end	Day	23% 12% 5% 9% 6% 4 4 3 4 6% 25%	5494
April	1st to 10th	Day	16% 12% 3 7% 4 5% 4 4 5% 7% 33%	2400
	11th to 20th	Day	20% 13% 4 6% 4 4 4 3 5% 5% 33%	2270
	21st to end	Day	20% 13% 4 7% 4 5% 4 3 5% 5% 31%	2470
May	1st to 10th	Day	17% 7% 2 5% 4 4 5% 4 6% 7% 37%	4200
	11th to 20th	Day	12% 7% 2 6% 4 4 4 4 5% 7% 44%	4399
	21st to end	Day	17% 7% 3 7% 5% 4 4 4 6% 7% 36%	4824
June	1st to 10th	Day	11% 6% 2 6% 4 4 4 4 7% 7% 47%	2660
	11th to 20th	Day	14% 7% 3 5% 5% 4 4 4 6% 9% 39%	2660
	21st to end	Day	10% 5% 3 6% 5% 5% 6% 4 7% 9% 41%	2660
July	1st to 10th	Day	9% 6% 2 6% 5% 5% 5% 5% 8% 10% 37%	4440
	11th to 20th	Day	6% 3 1 4 3 3 4 3 6% 9% 58%	4440
	21st to end	Day	6% 4 1 5% 5% 5% 5% 5% 8% 8% 48%	4671
August	1st to 10th	Day	12% 6% 2 6% 5% 4 5% 4 6% 7% 43%	4104
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 7% 8% 42%	4041
	21st to end	Day	8% 6% 2 7% 4 4 4 3 5% 7% 49%	4343
September	1st to 10th	Day	12% 6% 2 5% 3 4 4 3 6% 7% 48%	2280
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 3 5% 7% 49%	2230
	21st to end	Day	14% 6% 3 5% 4 4 4 4 5% 7% 44%	2011
October	1st to 10th	Day	14% 8% 2 6% 4 4 3 3 5% 6% 44%	1900
	11th to 20th	Day	14% 9% 3 5% 4 4 4 3 5% 6% 43%	1900
	21st to end	Day	18% 10% 4 7% 3 3 3 4 5% 5% 37%	2090
November	1st to 10th	Day	12% 10% 3 8% 5% 4 4 3 4 7% 38%	1650
	11th to 20th	Day	11% 13% 6% 11% 5% 4 4 4 5% 6% 31%	1600
	21st to end	Day	12% 10% 5% 9% 6% 4 3 3 6% 7% 35%	1600
December	1st to 10th	Day	7% 10% 4 12% 11% 8% 6% 4 5% 6% 27%	2610
	11th to 20th	Day	10% 15% 5% 11% 8% 6% 6% 4 5% 6% 22%	3008
	21st to end	Day	18% 18% 8% 20% 5% 4 3 2 3 5% 14%	3982
January	1st to 10th	Night	6% 12% 5% 17% 8% 8% 5% 4 5% 5% 25%	6348
	11th to 20th	Night	6% 10% 5% 13% 8% 7% 9% 8% 8% 5% 21%	6208
	21st to end	Night	7% 14% 7% 11% 5% 5% 5% 3 5% 5% 33%	6490
February	1st to 10th	Night	7% 12% 5% 11% 7% 6% 5% 3 5% 6% 34%	2600
	11th to 20th	Night	7% 10% 4 8% 6% 6% 5% 4 5% 7% 39%	2533
	21st to end	Night	11% 10% 3 9% 6% 5% 5% 3 5% 6% 38%	2091
March	1st to 10th	Night	10% 7% 2 6% 5% 4 3 3 4 6% 49%	5008
	11th to 20th	Night	10% 6% 2 7% 5% 4 5% 4 6% 9% 42%	4980
	21st to end	Night	24% 12% 4 10% 4 4 3 3 4 5% 29%	5434
April	1st to 10th	Night	14% 12% 3 7% 4 4 5% 3 5% 7% 37%	2000
	11th to 20th	Night	19% 10% 4 8% 5% 3 4 3 4 6% 34%	1863
	21st to end	Night	19% 9% 3 7% 5% 3 4 3 5% 6% 36%	1710
May	1st to 10th	Night	14% 8% 2 7% 4 4 4 3 5% 6% 42%	2705
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 6% 8% 50%	2506
	21st to end	Night	14% 7% 3 6% 4 4 4 3 5% 7% 43%	2743
June	1st to 10th	Night	11% 4 2 4 3 3 4 4 6% 8% 50%	1520
	11th to 20th	Night	9% 6% 2 6% 5% 5% 5% 5% 6% 8% 45%	1520
	21st to end	Night	8% 3 1 5% 4 3 4 4 5% 8% 54%	1520
July	1st to 10th	Night	11% 4 2 4 4 4 4 5% 8% 10% 45%	2530
	11th to 20th	Night	3 2 1 4 3 3 4 4 5% 8% 62%	2530
	21st to end	Night	5% 4 1 4 3 4 4 4 6% 9% 57%	2875
August	1st to 10th	Night	11% 7% 2 5% 4 4 4 4 5% 6% 48%	2836
	11th to 20th	Night	13% 4 2 4 3 3 3 3 5% 8% 51%	2798
	21st to end	Night	9% 6% 2 6% 4 4 4 3 5% 7% 50%	3317
September	1st to 10th	Night	10% 4 1 4 3 3 3 2 6% 7% 57%	1900
	11th to 20th	Night	11% 4 1 4 3 3 4 4 5% 8% 53%	1947
	21st to end	Night	11% 6% 2 5% 4 3 3 3 4 6% 53%	2148
October	1st to 10th	Night	14% 6% 2 5% 4 3 3 3 4 6% 51%	2280
	11th to 20th	Night	13% 9% 3 6% 3 3 5% 3 4 7% 42%	2280
	21st to end	Night	17% 11% 3 7% 4 3 3 3 4 7% 38%	2508
November	1st to 10th	Night	13% 9% 3 6% 4 4 4 4 5% 6% 43%	2644
	11th to 20th	Night	11% 13% 4 10% 5% 4 4 3 4 5% 37%	2800
	21st to end	Night	13% 11% 4 8% 6% 4 3 4 4 7% 38%	2800
December	1st to 10th	Night	6% 8% 4 12% 9% 7% 5% 4 6% 8% 32%	4580
	11th to 20th	Night	10% 14% 7% 13% 8% 5% 4 4 4 5% 25%	5199
	21st to end	Night	20% 15% 8% 17% 5% 4 4 3 4 4 16%	6996

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	69% 2 8% 16% 5%	3618
	11th to 20th	Day	61% 3 8% 17% 11%	3758
	21st to end	Day	61% 2 14% 15% 8%	4488
February	1st to 10th	Day	58% 3 11% 18% 9% 1	1800
	11th to 20th	Day	53% 4 11% 24% 7% 1	1763
	21st to end	Day	51% 3 14% 20% 12%	1538
March	1st to 10th	Day	41% 2 16% 29% 11% 2	4887
	11th to 20th	Day	41% 2 13% 31% 12% 1	4980
	21st to end	Day	63% 3 10% 18% 7%	5496
April	1st to 10th	Day	51% 3 11% 23% 11% 1	2400
	11th to 20th	Day	55% 2 10% 20% 12% 1	2270
	21st to end	Day	56% 3 12% 20% 8% 1	2470
May	1st to 10th	Day	46% 3 11% 28% 12% 1	4238
	11th to 20th	Day	40% 3 8% 32% 14% 2	4403
	21st to end	Day	48% 3 10% 28% 10% 1	4837
June	1st to 10th	Day	35% 3 14% 31% 15% 1	2660
	11th to 20th	Day	42% 4 14% 23% 15% 2	2660
	21st to end	Day	38% 4 17% 27% 12% 2	2660
July	1st to 10th	Day	39% 6% 20% 24% 10% 1	4371
	11th to 20th	Day	23% 4 15% 37% 19% 2	4365
	21st to end	Day	30% 6% 14% 32% 16% 1	4524
August	1st to 10th	Day	39% 4 11% 31% 13% 2	4090
	11th to 20th	Day	39% 5% 14% 33% 10%	4080
	21st to end	Day	35% 5% 16% 30% 15% 1	4342
September	1st to 10th	Day	36% 3 13% 32% 16%	2280
	11th to 20th	Day	36% 3 16% 32% 12% 1	2230
	21st to end	Day	40% 3 12% 33% 13% 1	2011
October	1st to 10th	Day	30% 2 6% 38% 24% 1	3160
	11th to 20th	Day	39% 2 11% 25% 21% 1	3140
	21st to end	Day	43% 3 13% 30% 10%	3428
November	1st to 10th	Day	46% 3 12% 21% 14% 3	1650
	11th to 20th	Day	54% 4 11% 21% 10% 1	1600
	21st to end	Day	49% 2 18% 22% 7% 1	1600
December	1st to 10th	Day	58% 3 15% 17% 6% 1	2610
	11th to 20th	Day	64% 3 16% 13% 4	3005
	21st to end	Day	76% 2 8% 10% 4	3982
January	1st to 10th	Night	61% 3 9% 20% 7%	6348
	11th to 20th	Night	61% 3 8% 16% 11% 1	6208
	21st to end	Night	54% 2 11% 23% 9%	6490
February	1st to 10th	Night	53% 2 11% 22% 10% 1	2600
	11th to 20th	Night	46% 3 13% 27% 10% 2	2534
	21st to end	Night	48% 3 11% 23% 13% 2	2091
March	1st to 10th	Night	38% 2 11% 31% 16% 2	5008
	11th to 20th	Night	39% 2 11% 31% 15% 1	4980
	21st to end	Night	59% 3 8% 18% 10% 1	5434
April	1st to 10th	Night	48% 2 10% 25% 14% 1	2000
	11th to 20th	Night	52% 3 8% 18% 17% 3	1863
	21st to end	Night	50% 3 7% 24% 15% 2	1710
May	1st to 10th	Night	44% 2 9% 22% 22% 1	2732
	11th to 20th	Night	35% 2 10% 28% 20% 5%	2507
	21st to end	Night	42% 3 10% 28% 16% 1	2751
June	1st to 10th	Night	33% 3 12% 34% 16% 2	1520
	11th to 20th	Night	37% 4 14% 24% 19% 3	1520
	21st to end	Night	30% 3 14% 33% 18% 3	1520
July	1st to 10th	Night	32% 6% 20% 28% 13% 2	2490
	11th to 20th	Night	20% 4 14% 40% 20% 2	2482
	21st to end	Night	24% 5% 16% 33% 20% 2	2783
August	1st to 10th	Night	37% 3 12% 30% 15% 4	2826
	11th to 20th	Night	32% 4 14% 33% 16% 1	2825
	21st to end	Night	33% 4 16% 28% 16% 2	3317
September	1st to 10th	Night	28% 2 13% 35% 21% 1	1900
	11th to 20th	Night	30% 3 15% 33% 17% 1	1947
	21st to end	Night	34% 2 11% 33% 19% 1	2148
October	1st to 10th	Night	25% 1 5% 36% 29% 4	3800
	11th to 20th	Night	31% 3 12% 27% 23% 4	3779
	21st to end	Night	43% 3 11% 30% 13% 1	4120
November	1st to 10th	Night	42% 2 13% 26% 15% 2	2644
	11th to 20th	Night	50% 3 11% 21% 13% 1	2800
	21st to end	Night	48% 3 15% 25% 8% 1	2800
December	1st to 10th	Night	51% 3 19% 21% 6%	4580
	11th to 20th	Night	62% 3 11% 18% 5% 1	5201
	21st to end	Night	73% 2 7% 14% 4	6996

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	12% 14% 39% 23% 10% 2	300
	11th to 20th	Day	13% 15% 41% 25% 4 1	300
	21st to end	Day	15% 10% 42% 23% 8% 2	330
February	1st to 10th	Day	14% 16% 46% 21% 2	300
	11th to 20th	Day	14% 17% 39% 24% 5% 1	300
	21st to end	Day	19% 13% 44% 22% 2	249
March	1st to 10th	Day	19% 16% 41% 19% 5%	300
	11th to 20th	Day	23% 14% 32% 23% 7% 1	300
	21st to end	Day	29% 13% 31% 20% 6% 2	337
April	1st to 10th	Day	23% 15% 37% 20% 5%	500
	11th to 20th	Day	27% 16% 25% 22% 8% 1	468
	21st to end	Day	25% 13% 30% 22% 9% 2	650
May	1st to 10th	Day	20% 13% 34% 24% 8% 1	653
	11th to 20th	Day	17% 15% 28% 26% 12% 3	675
	21st to end	Day	18% 9% 27% 28% 15% 2	734
June	1st to 10th	Day	11% 9% 25% 32% 18% 5%	680
	11th to 20th	Day	17% 7% 33% 29% 10% 3	680
	21st to end	Day	12% 8% 31% 29% 16% 3	680
July	1st to 10th	Day	8% 5% 24% 31% 24% 8%	671
	11th to 20th	Day	9% 6% 27% 30% 21% 7%	668
	21st to end	Day	6% 4 23% 40% 21% 6%	739
August	1st to 10th	Day	12% 3 21% 35% 24% 5%	676
	11th to 20th	Day	13% 4 21% 33% 23% 6%	665
	21st to end	Day	11% 4 28% 34% 18% 6%	637
September	1st to 10th	Day	16% 7% 27% 29% 18% 4	490
	11th to 20th	Day	15% 5% 29% 30% 17% 5%	440
	21st to end	Day	16% 11% 31% 22% 16% 4	300
October	1st to 10th	Day	16% 11% 26% 27% 15% 5%	300
	11th to 20th	Day	15% 17% 41% 17% 8% 1	300
	21st to end	Day	25% 15% 36% 16% 7%	330
November	1st to 10th	Day	21% 13% 36% 20% 10%	300
	11th to 20th	Day	17% 19% 41% 19% 5%	300
	21st to end	Day	19% 14% 44% 18% 4	300
December	1st to 10th	Day	13% 13% 45% 21% 6% 1	300
	11th to 20th	Day	19% 15% 39% 21% 5%	300
	21st to end	Day	24% 16% 40% 15% 4	330
January	1st to 10th	Night	12% 13% 38% 29% 8% 1	1000
	11th to 20th	Night	16% 11% 39% 27% 6% 1	1000
	21st to end	Night	17% 12% 36% 29% 6%	1100
February	1st to 10th	Night	14% 13% 41% 27% 6% 1	1000
	11th to 20th	Night	13% 13% 43% 25% 6% 1	979
	21st to end	Night	16% 12% 38% 27% 7% 1	771
March	1st to 10th	Night	17% 11% 37% 25% 9% 1	641
	11th to 20th	Night	17% 12% 38% 27% 6% 1	600
	21st to end	Night	22% 10% 32% 24% 11% 1	660
April	1st to 10th	Night	19% 14% 35% 27% 6%	600
	11th to 20th	Night	23% 12% 28% 29% 8%	565
	21st to end	Night	22% 10% 32% 27% 8% 1	570
May	1st to 10th	Night	18% 9% 32% 32% 8% 1	570
	11th to 20th	Night	18% 8% 30% 32% 11% 1	561
	21st to end	Night	16% 8% 28% 34% 14% 1	617
June	1st to 10th	Night	12% 3 30% 39% 15% 1	570
	11th to 20th	Night	12% 5% 31% 42% 10%	570
	21st to end	Night	9% 5% 23% 47% 15% 1	570
July	1st to 10th	Night	7% 1 21% 42% 27% 2	555
	11th to 20th	Night	4 4 23% 46% 21% 2	552
	21st to end	Night	6% 2 18% 53% 18% 2	624
August	1st to 10th	Night	8% 4 18% 47% 22% 2	567
	11th to 20th	Night	12% 3 18% 39% 25% 4	564
	21st to end	Night	9% 4 24% 39% 22% 2	627
September	1st to 10th	Night	12% 4 22% 42% 18% 3	570
	11th to 20th	Night	13% 6% 30% 33% 16% 2	570
	21st to end	Night	14% 8% 32% 32% 14% 1	628
October	1st to 10th	Night	17% 9% 28% 30% 15%	760
	11th to 20th	Night	18% 9% 37% 28% 7% 1	760
	21st to end	Night	25% 12% 29% 27% 6%	836
November	1st to 10th	Night	16% 12% 37% 26% 8% 1	898
	11th to 20th	Night	17% 13% 39% 24% 6%	1000
	21st to end	Night	18% 13% 38% 24% 7%	1000
December	1st to 10th	Night	16% 10% 39% 27% 8% 1	1000
	11th to 20th	Night	19% 10% 39% 25% 6% 1	1000
	21st to end	Night	24% 10% 34% 25% 5%	1100

Page generated at 2025-04-08 08:17:29. Version ac07

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