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 2024-02-20.

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	13% 9% 4 7% 6% 3 5% 5% 5% 7% 36%	78360
	Night	12% 9% 4 8% 6% 3 4 4 5% 7% 38%	80888
Transparency	Day	48% 3 13% 24% 11% 1	78065
	Night	47% 3 12% 25% 12% 1	80703
Seeing	Day	16% 10% 31% 27% 13% 3	14924
	Night	16% 9% 33% 31% 11% 1	25019

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% 16% 7% 16% 8% 4 4 5% 5% 6% 18%	2527
	11th to 20th	Day	6% 12% 6% 11% 10% 5% 9% 7% 9% 5% 19%	2626
	21st to end	Day	8% 18% 8% 14% 9% 4 5% 4 5% 5% 19%	3135
February	1st to 10th	Day	8% 10% 5% 10% 8% 5% 6% 6% 5% 7% 28%	1710
	11th to 20th	Day	9% 10% 5% 9% 9% 4 7% 6% 5% 5% 32%	1655
	21st to end	Day	13% 11% 5% 7% 7% 3 7% 4 5% 8% 29%	1379
March	1st to 10th	Day	15% 12% 6% 9% 6% 3 4 4 5% 6% 31%	1957
	11th to 20th	Day	14% 9% 4 8% 6% 4 4 5% 5% 7% 34%	1980
	21st to end	Day	20% 11% 5% 7% 6% 3 5% 4 4 7% 30%	2183
April	1st to 10th	Day	16% 11% 4 7% 5% 3 4 4 5% 8% 33%	2160
	11th to 20th	Day	20% 12% 4 5% 5% 3 4 3 4 5% 34%	2149
	21st to end	Day	21% 12% 4 6% 5% 4 4 4 5% 5% 31%	2340
May	1st to 10th	Day	18% 8% 3 6% 5% 4 5% 5% 6% 7% 33%	2402
	11th to 20th	Day	16% 8% 3 6% 4 3 5% 4 4 8% 38%	2479
	21st to end	Day	16% 6% 3 6% 6% 3 5% 5% 6% 8% 36%	2725
June	1st to 10th	Day	12% 5% 2 5% 4 3 4 5% 7% 8% 46%	2520
	11th to 20th	Day	14% 7% 3 4 5% 4 4 5% 6% 9% 40%	2520
	21st to end	Day	9% 5% 3 5% 6% 4 6% 6% 6% 9% 42%	2520
July	1st to 10th	Day	7% 4 2 4 4 4 5% 5% 7% 10% 48%	2520
	11th to 20th	Day	8% 3 2 4 4 3 4 5% 6% 9% 53%	2520
	21st to end	Day	7% 3 2 4 5% 4 5% 6% 8% 8% 49%	2668
August	1st to 10th	Day	9% 4 2 6% 7% 3 5% 6% 6% 8% 42%	2328
	11th to 20th	Day	12% 5% 3 5% 4 3 4 5% 6% 8% 45%	2301
	21st to end	Day	8% 5% 2 5% 4 3 4 4 5% 8% 53%	2469
September	1st to 10th	Day	13% 6% 2 5% 4 3 4 4 6% 7% 46%	2160
	11th to 20th	Day	13% 5% 2 4 5% 3 4 4 5% 7% 49%	2113
	21st to end	Day	15% 6% 3 5% 5% 3 4 6% 5% 7% 42%	1906
October	1st to 10th	Day	15% 8% 3 6% 5% 3 3 4 5% 6% 43%	1800
	11th to 20th	Day	15% 9% 3 5% 4 3 4 3 5% 6% 42%	1800
	21st to end	Day	19% 10% 5% 6% 4 2 3 5% 5% 5% 36%	1980
November	1st to 10th	Day	11% 10% 4 7% 7% 4 4 3 4 7% 40%	1567
	11th to 20th	Day	11% 12% 7% 11% 5% 3 5% 4 5% 7% 30%	1520
	21st to end	Day	13% 10% 5% 7% 6% 2 4 4 6% 7% 37%	1520
December	1st to 10th	Day	11% 12% 5% 9% 10% 5% 6% 5% 4 6% 27%	1520
	11th to 20th	Day	10% 13% 6% 10% 6% 4 5% 5% 5% 7% 28%	1918
	21st to end	Day	19% 13% 7% 17% 6% 3 3 3 4 5% 18%	2783
January	1st to 10th	Night	8% 14% 8% 16% 9% 3 4 4 4 5% 25%	4428
	11th to 20th	Night	7% 10% 6% 13% 10% 5% 8% 8% 8% 5% 21%	4344
	21st to end	Night	10% 18% 8% 12% 7% 4 4 4 5% 5% 25%	4532
February	1st to 10th	Night	7% 12% 5% 9% 8% 4 6% 4 5% 6% 34%	2470
	11th to 20th	Night	8% 9% 4 6% 7% 4 6% 4 5% 8% 40%	2397
	21st to end	Night	12% 9% 4 8% 8% 4 6% 4 4 5% 37%	1876
March	1st to 10th	Night	13% 9% 4 6% 7% 3 4 5% 4 7% 38%	2017
	11th to 20th	Night	13% 10% 3 8% 6% 3 5% 4 4 6% 37%	1980
	21st to end	Night	18% 10% 4 6% 5% 3 4 4 4 6% 33%	2172
April	1st to 10th	Night	14% 11% 4 6% 4 3 5% 4 5% 8% 37%	1800
	11th to 20th	Night	20% 10% 4 7% 6% 3 3 4 4 6% 33%	1758
	21st to end	Night	19% 9% 4 6% 6% 3 3 4 5% 7% 35%	1620
May	1st to 10th	Night	16% 8% 2 5% 5% 3 5% 4 4 7% 40%	1558
	11th to 20th	Night	15% 5% 3 5% 4 3 3 4 6% 8% 44%	1416
	21st to end	Night	14% 5% 3 5% 6% 3 5% 4 6% 8% 42%	1551
June	1st to 10th	Night	10% 3 3 4 3 2 4 5% 6% 9% 50%	1440
	11th to 20th	Night	9% 5% 2 5% 5% 4 5% 5% 5% 8% 46%	1440
	21st to end	Night	8% 3 2 4 5% 2 4 5% 5% 8% 55%	1440
July	1st to 10th	Night	6% 2 2 3 3 2 4 4 7% 11% 57%	1440
	11th to 20th	Night	5% 2 2 4 3 3 4 4 5% 8% 60%	1440
	21st to end	Night	5% 3 1 3 3 2 3 5% 5% 10% 60%	1647
August	1st to 10th	Night	6% 5% 2 4 5% 3 6% 6% 6% 8% 50%	1611
	11th to 20th	Night	9% 4 2 3 4 2 3 3 6% 9% 54%	1600
	21st to end	Night	7% 5% 2 4 4 2 3 4 5% 6% 58%	1886
September	1st to 10th	Night	11% 4 1 4 4 2 3 3 6% 7% 57%	1800
	11th to 20th	Night	11% 4 1 3 4 2 4 5% 5% 8% 53%	1844
	21st to end	Night	11% 6% 2 4 5% 2 4 4 4 6% 51%	2034
October	1st to 10th	Night	14% 6% 2 5% 4 2 3 4 4 6% 50%	2160
	11th to 20th	Night	14% 9% 3 5% 4 2 4 4 4 7% 41%	2160
	21st to end	Night	18% 11% 3 6% 4 3 3 4 4 6% 39%	2376
November	1st to 10th	Night	13% 8% 3 5% 4 3 4 5% 5% 6% 44%	2508
	11th to 20th	Night	12% 11% 5% 9% 6% 3 4 4 4 5% 36%	2660
	21st to end	Night	13% 9% 4 6% 7% 3 4 4 4 7% 39%	2660
December	1st to 10th	Night	10% 10% 5% 9% 9% 4 5% 5% 5% 7% 31%	2660
	11th to 20th	Night	11% 11% 5% 8% 9% 5% 5% 5% 5% 7% 28%	3279
	21st to end	Night	21% 13% 6% 12% 7% 4 4 4 4 5% 20%	4884

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 10% 15% 4	2528
	11th to 20th	Day	63% 2 10% 18% 7%	2626
	21st to end	Day	69% 2 11% 13% 4	3135
February	1st to 10th	Day	57% 3 11% 19% 9% 1	1710
	11th to 20th	Day	53% 4 11% 23% 7% 1	1664
	21st to end	Day	54% 4 12% 18% 12%	1379
March	1st to 10th	Day	56% 3 14% 21% 7% 1	1957
	11th to 20th	Day	51% 3 13% 24% 8% 1	1980
	21st to end	Day	56% 3 11% 22% 6% 1	2183
April	1st to 10th	Day	51% 3 12% 23% 10% 1	2160
	11th to 20th	Day	54% 2 11% 20% 11% 1	2149
	21st to end	Day	56% 3 12% 20% 8% 1	2340
May	1st to 10th	Day	51% 4 12% 24% 8% 1	2402
	11th to 20th	Day	46% 5% 11% 27% 11% 1	2483
	21st to end	Day	46% 5% 12% 24% 12% 1	2725
June	1st to 10th	Day	36% 3 15% 30% 14% 1	2520
	11th to 20th	Day	41% 4 14% 24% 16% 1	2520
	21st to end	Day	37% 4 17% 28% 12% 2	2520
July	1st to 10th	Day	30% 4 17% 34% 14% 1	2451
	11th to 20th	Day	28% 4 15% 30% 19% 4	2445
	21st to end	Day	30% 4 16% 33% 15% 3	2520
August	1st to 10th	Day	38% 4 14% 32% 10% 3	2314
	11th to 20th	Day	38% 4 15% 31% 12% 1	2301
	21st to end	Day	30% 4 14% 32% 19% 2	2469
September	1st to 10th	Day	37% 3 12% 31% 16%	2160
	11th to 20th	Day	36% 3 16% 33% 12% 1	2113
	21st to end	Day	42% 3 12% 33% 10% 1	1906
October	1st to 10th	Day	43% 2 10% 31% 14% 1	1800
	11th to 20th	Day	44% 2 13% 26% 14% 2	1800
	21st to end	Day	50% 3 12% 24% 11% 1	1980
November	1st to 10th	Day	46% 3 12% 21% 15% 3	1567
	11th to 20th	Day	54% 4 11% 20% 10% 1	1520
	21st to end	Day	48% 2 19% 23% 7% 1	1520
December	1st to 10th	Day	59% 3 15% 15% 7% 1	1520
	11th to 20th	Day	56% 3 20% 15% 6%	1915
	21st to end	Day	71% 2 9% 14% 4	2783
January	1st to 10th	Night	64% 2 9% 19% 6%	4428
	11th to 20th	Night	63% 2 10% 19% 6%	4344
	21st to end	Night	64% 2 9% 18% 6%	4532
February	1st to 10th	Night	53% 2 11% 23% 10% 1	2470
	11th to 20th	Night	46% 3 13% 26% 10% 2	2398
	21st to end	Night	50% 3 10% 23% 13% 2	1876
March	1st to 10th	Night	47% 3 15% 25% 8% 1	2017
	11th to 20th	Night	49% 2 11% 23% 13% 1	1980
	21st to end	Night	52% 4 11% 24% 8% 1	2172
April	1st to 10th	Night	48% 2 11% 24% 14% 1	1800
	11th to 20th	Night	53% 3 8% 17% 16% 3	1758
	21st to end	Night	50% 3 7% 24% 14% 2	1620
May	1st to 10th	Night	45% 3 13% 23% 15% 1	1558
	11th to 20th	Night	40% 4 14% 25% 15% 3	1417
	21st to end	Night	42% 3 12% 24% 16% 2	1552
June	1st to 10th	Night	33% 3 12% 34% 16% 2	1440
	11th to 20th	Night	37% 4 14% 24% 18% 3	1440
	21st to end	Night	29% 2 15% 33% 18% 3	1440
July	1st to 10th	Night	23% 3 15% 37% 19% 3	1400
	11th to 20th	Night	23% 3 14% 36% 19% 4	1392
	21st to end	Night	21% 3 15% 38% 18% 4	1555
August	1st to 10th	Night	31% 3 14% 32% 17% 3	1601
	11th to 20th	Night	30% 3 16% 31% 19% 1	1600
	21st to end	Night	28% 4 13% 32% 20% 4	1886
September	1st to 10th	Night	29% 2 13% 34% 21% 1	1800
	11th to 20th	Night	31% 3 15% 33% 17% 1	1844
	21st to end	Night	35% 2 11% 33% 18% 1	2034
October	1st to 10th	Night	38% 2 9% 32% 16% 3	2160
	11th to 20th	Night	43% 3 12% 25% 16% 2	2160
	21st to end	Night	47% 2 12% 25% 13% 1	2376
November	1st to 10th	Night	41% 3 13% 26% 16% 2	2508
	11th to 20th	Night	51% 3 11% 21% 13% 1	2660
	21st to end	Night	47% 3 15% 25% 9% 1	2660
December	1st to 10th	Night	52% 3 16% 21% 7% 1	2660
	11th to 20th	Night	57% 3 14% 19% 6% 1	3281
	21st to end	Night	69% 2 8% 17% 5%	4884

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	13% 14% 39% 23% 9% 2	270
	11th to 20th	Day	13% 15% 41% 25% 5% 1	270
	21st to end	Day	15% 9% 41% 24% 9% 2	297
February	1st to 10th	Day	12% 16% 47% 22% 3	270
	11th to 20th	Day	15% 17% 39% 22% 5% 1	267
	21st to end	Day	22% 13% 40% 22% 3	198
March	1st to 10th	Day	19% 18% 38% 19% 6%	240
	11th to 20th	Day	23% 13% 33% 24% 7% 1	240
	21st to end	Day	29% 13% 28% 21% 7% 3	269
April	1st to 10th	Day	24% 13% 38% 19% 6%	420
	11th to 20th	Day	27% 15% 24% 23% 9% 2	427
	21st to end	Day	25% 13% 30% 22% 9% 2	600
May	1st to 10th	Day	21% 13% 33% 25% 8% 1	603
	11th to 20th	Day	18% 15% 27% 25% 12% 3	625
	21st to end	Day	18% 9% 27% 29% 15% 2	679
June	1st to 10th	Day	10% 8% 24% 33% 19% 6%	630
	11th to 20th	Day	16% 6% 33% 30% 11% 3	630
	21st to end	Day	10% 8% 31% 30% 17% 3	630
July	1st to 10th	Day	8% 4 24% 32% 24% 9%	621
	11th to 20th	Day	9% 5% 27% 31% 20% 7%	618
	21st to end	Day	7% 4 23% 39% 21% 6%	684
August	1st to 10th	Day	12% 2 19% 37% 25% 5%	626
	11th to 20th	Day	13% 4 20% 33% 24% 6%	615
	21st to end	Day	11% 3 27% 36% 18% 6%	588
September	1st to 10th	Day	17% 7% 26% 29% 17% 4	450
	11th to 20th	Day	16% 5% 29% 29% 16% 5%	403
	21st to end	Day	18% 12% 31% 23% 13% 4	270
October	1st to 10th	Day	17% 10% 26% 27% 15% 5%	270
	11th to 20th	Day	15% 18% 39% 18% 9% 1	270
	21st to end	Day	26% 16% 34% 15% 8%	297
November	1st to 10th	Day	19% 14% 36% 20% 11%	270
	11th to 20th	Day	16% 19% 41% 20% 4	270
	21st to end	Day	20% 13% 44% 19% 4	270
December	1st to 10th	Day	14% 13% 42% 24% 6% 1	270
	11th to 20th	Day	19% 15% 38% 23% 5%	270
	21st to end	Day	24% 16% 41% 15% 3	297
January	1st to 10th	Night	12% 13% 38% 28% 8% 1	950
	11th to 20th	Night	16% 10% 39% 28% 6% 1	950
	21st to end	Night	18% 11% 35% 29% 6%	1045
February	1st to 10th	Night	13% 12% 41% 28% 6% 1	950
	11th to 20th	Night	13% 13% 42% 25% 6% 1	927
	21st to end	Night	17% 12% 37% 26% 8% 1	692
March	1st to 10th	Night	18% 11% 35% 25% 10% 1	577
	11th to 20th	Night	18% 10% 37% 27% 6% 1	540
	21st to end	Night	21% 10% 31% 25% 12% 1	594
April	1st to 10th	Night	19% 13% 36% 26% 6%	540
	11th to 20th	Night	24% 11% 28% 29% 8%	533
	21st to end	Night	22% 10% 31% 28% 8% 1	540
May	1st to 10th	Night	19% 9% 31% 33% 8% 1	540
	11th to 20th	Night	18% 8% 30% 32% 11% 1	531
	21st to end	Night	16% 8% 27% 34% 14% 1	584
June	1st to 10th	Night	12% 3 30% 40% 15% 1	540
	11th to 20th	Night	12% 5% 31% 42% 10%	540
	21st to end	Night	8% 5% 23% 48% 15% 1	540
July	1st to 10th	Night	6% 1 21% 42% 28% 2	525
	11th to 20th	Night	4 4 23% 47% 20% 2	522
	21st to end	Night	6% 2 19% 53% 17% 2	591
August	1st to 10th	Night	8% 3 17% 48% 23% 2	537
	11th to 20th	Night	11% 3 19% 37% 26% 4	534
	21st to end	Night	8% 3 24% 39% 23% 2	594
September	1st to 10th	Night	12% 4 21% 41% 19% 3	540
	11th to 20th	Night	13% 6% 30% 32% 17% 2	540
	21st to end	Night	15% 8% 32% 31% 13% 1	594
October	1st to 10th	Night	18% 9% 28% 29% 16%	720
	11th to 20th	Night	19% 9% 36% 27% 7% 1	720
	21st to end	Night	25% 12% 28% 28% 7%	792
November	1st to 10th	Night	16% 12% 37% 26% 8% 1	852
	11th to 20th	Night	17% 13% 39% 25% 6%	950
	21st to end	Night	18% 13% 39% 24% 7%	950
December	1st to 10th	Night	17% 10% 38% 27% 8% 1	950
	11th to 20th	Night	20% 9% 39% 26% 6% 1	950
	21st to end	Night	24% 11% 35% 25% 5%	1045

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