Maury Lewin Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Maury Lewin Astronomical Observatory clear sky chart from 2005-11-06 to 2026-09-14.

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	4 3 1 3 2 3 3 2 4 6% 69%	157364
	Night	5% 4 1 3 2 3 4 2 4 6% 65%	151399
Transparency	Day	16% 4 6% 17% 28% 29%	152451
	Night	19% 6% 7% 17% 25% 26%	147287
Seeing	Day	7% 9% 29% 35% 15% 4	18185
	Night	6% 4 17% 37% 30% 5%	27991

By Month

Percentage Cloud Cover SW (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	11% 3 1 4 3 2 3 2 3 5% 64%	4722
	11th to 20th	Day	5% 2 1 2 1 2 3 2 3 6% 73%	5434
	21st to end	Day	9% 4 2 5% 3 4 4 3 5% 8% 53%	6500
February	1st to 10th	Day	10% 4 2 2 3 2 3 3 4 6% 60%	2100
	11th to 20th	Day	12% 4 2 4 2 2 3 3 5% 6% 58%	2050
	21st to end	Day	9% 6% 2 4 3 4 4 2 4 7% 55%	1730
March	1st to 10th	Day	14% 6% 2 5% 3 3 4 4 5% 6% 47%	4618
	11th to 20th	Day	11% 4 2 4 3 4 4 3 5% 7% 55%	4765
	21st to end	Day	9% 4 1 4 4 4 4 3 6% 9% 52%	6100
April	1st to 10th	Day	6% 4 1 4 4 4 2 4 4 7% 60%	2520
	11th to 20th	Day	5% 5% 2 3 4 4 4 2 5% 6% 62%	2489
	21st to end	Day	5% 4 1 3 3 3 4 2 3 6% 65%	2520
May	1st to 10th	Day	5% 5% 2 4 2 3 4 2 3 5% 65%	7318
	11th to 20th	Day	2 5% 2 4 3 4 4 2 4 7% 63%	7596
	21st to end	Day	2 4 2 4 3 4 4 2 3 6% 66%	8370
June	1st to 10th	Day	4 3 2 3 3 3 3 2 4 7% 66%	2730
	11th to 20th	Day	2 2 1 2 2 2 3 2 3 4 76%	2903
	21st to end	Day	1 2 1 2 2 2 2 2 3 4 79%	2940
July	1st to 10th	Day	1 1 1 1 3 1 2 3 85%	8219
	11th to 20th	Day	1 1 1 2 2 2 4 3 4 5% 76%	7997
	21st to end	Day	1 1 1 1 1 2 2 3 4 84%	7857
August	1st to 10th	Day	1 1 1 1 1 2 4 89%	6986
	11th to 20th	Day	1 1 1 1 3 2 3 5% 83%	7001
	21st to end	Day	1 1 2 1 2 3 2 3 4 80%	7750
September	1st to 10th	Day	3 1 2 1 1 2 2 3 5% 79%	2520
	11th to 20th	Day	2 2 1 2 1 2 2 1 2 4 81%	2273
	21st to end	Day	2 2 1 2 1 2 2 2 4 4 79%	2200
October	1st to 10th	Day	2 2 1 1 2 2 2 2 2 4 80%	3600
	11th to 20th	Day	2 3 1 3 1 2 2 2 3 6% 74%	3406
	21st to end	Day	3 3 1 3 2 3 3 3 5% 6% 69%	3444
November	1st to 10th	Day	3 3 1 3 2 2 2 2 4 6% 72%	1854
	11th to 20th	Day	7% 2 1 3 2 2 3 2 3 6% 68%	1890
	21st to end	Day	5% 4 1 4 3 4 4 3 4 6% 62%	1890
December	1st to 10th	Day	4 2 1 3 3 3 4 2 5% 6% 67%	3040
	11th to 20th	Day	9% 4 1 4 2 3 5% 4 6% 8% 52%	3499
	21st to end	Day	10% 5% 1 5% 5% 4 4 3 5% 7% 50%	4533
January	1st to 10th	Night	12% 4 2 4 2 3 3 2 3 4 61%	8172
	11th to 20th	Night	3 3 1 3 2 2 3 2 3 7% 71%	7523
	21st to end	Night	11% 6% 2 5% 3 3 4 4 5% 6% 52%	7747
February	1st to 10th	Night	10% 5% 2 4 3 3 2 2 3 5% 61%	2520
	11th to 20th	Night	10% 5% 2 4 3 3 3 2 4 6% 59%	2458
	21st to end	Night	10% 5% 1 3 3 3 4 4 4 5% 57%	2076
March	1st to 10th	Night	13% 9% 2 5% 3 3 4 3 5% 7% 47%	5496
	11th to 20th	Night	7% 4 2 4 3 4 4 3 6% 8% 56%	5438
	21st to end	Night	10% 8% 3 5% 3 4 3 3 4 7% 51%	5116
April	1st to 10th	Night	6% 5% 1 3 4 3 4 4 4 6% 59%	2100
	11th to 20th	Night	5% 5% 2 2 2 4 4 3 3 6% 63%	2079
	21st to end	Night	5% 6% 1 4 3 3 3 3 4 6% 62%	2100
May	1st to 10th	Night	8% 6% 3 6% 3 4 4 2 3 4 58%	5645
	11th to 20th	Night	3 6% 2 3 2 5% 4 2 3 7% 61%	5296
	21st to end	Night	3 7% 3 5% 3 4 5% 2 3 4 61%	5837
June	1st to 10th	Night	2 3 1 3 4 4 4 4 5% 7% 63%	1890
	11th to 20th	Night	1 2 1 3 3 3 4 2 3 5% 73%	1709
	21st to end	Night	2 2 2 1 3 3 3 3 6% 75%	1680
July	1st to 10th	Night	1 1 1 2 6% 1 2 4 82%	4740
	11th to 20th	Night	1 2 2 2 7% 3 4 6% 74%	4956
	21st to end	Night	1 1 1 2 1 2 6% 2 3 4 77%	6271
August	1st to 10th	Night	1 1 1 1 1 2 2 3 4 84%	5866
	11th to 20th	Night	1 1 2 1 2 6% 2 3 5% 77%	5886
	21st to end	Night	1 1 1 2 1 2 5% 2 3 4 77%	6499
September	1st to 10th	Night	2 2 1 2 1 2 3 3 3 4 78%	2100
	11th to 20th	Night	1 2 1 1 2 2 3 2 2 4 80%	2192
	21st to end	Night	2 2 1 1 2 1 2 2 3 5% 80%	2200
October	1st to 10th	Night	2 2 1 2 2 2 3 2 2 3 81%	3600
	11th to 20th	Night	3 6% 3 5% 2 2 3 2 3 5% 66%	3736
	21st to end	Night	2 3 1 3 2 3 3 2 3 4 74%	4362
November	1st to 10th	Night	4 3 1 3 2 2 3 2 3 6% 72%	2665
	11th to 20th	Night	6% 3 2 3 2 2 3 2 4 6% 68%	2730
	21st to end	Night	6% 4 1 4 3 4 4 3 6% 7% 60%	2730
December	1st to 10th	Night	5% 3 1 3 3 2 4 3 4 5% 67%	4370
	11th to 20th	Night	9% 5% 1 4 3 4 5% 4 5% 7% 53%	4941
	21st to end	Night	8% 6% 2 5% 3 4 5% 3 6% 10% 46%	6673

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	24% 3 7% 17% 26% 23%	4723
	11th to 20th	Day	14% 3 8% 15% 18% 42%	5434
	21st to end	Day	28% 5% 4 14% 22% 27%	6500
February	1st to 10th	Day	26% 3 5% 14% 24% 27%	2100
	11th to 20th	Day	26% 4 6% 17% 24% 22%	2060
	21st to end	Day	27% 6% 8% 16% 24% 20%	1730
March	1st to 10th	Day	30% 7% 9% 17% 22% 15%	4618
	11th to 20th	Day	27% 6% 11% 24% 20% 12%	4765
	21st to end	Day	25% 7% 13% 28% 17% 10%	6100
April	1st to 10th	Day	23% 6% 10% 24% 23% 14%	2520
	11th to 20th	Day	22% 5% 7% 21% 28% 17%	2492
	21st to end	Day	18% 6% 8% 24% 31% 12%	2520
May	1st to 10th	Day	20% 4 7% 19% 34% 16%	7318
	11th to 20th	Day	16% 11% 5% 23% 31% 14%	7599
	21st to end	Day	13% 8% 7% 28% 34% 10%	8371
June	1st to 10th	Day	16% 6% 10% 20% 31% 16%	2730
	11th to 20th	Day	12% 4 5% 13% 32% 35%	2903
	21st to end	Day	11% 3 3 8% 29% 45%	2940
July	1st to 10th	Day	6% 1 2 6% 21% 64%	8151
	11th to 20th	Day	9% 2 5% 16% 35% 33%	7922
	21st to end	Day	6% 2 3 19% 34% 36%	7683
August	1st to 10th	Day	5% 1 1 16% 41% 35%	5530
	11th to 20th	Day	6% 2 4 18% 35% 36%	5517
	21st to end	Day	8% 2 3 12% 32% 43%	6077
September	1st to 10th	Day	9% 3 6% 9% 32% 41%	2520
	11th to 20th	Day	9% 3 3 12% 35% 37%	2273
	21st to end	Day	9% 3 5% 15% 31% 38%	2200
October	1st to 10th	Day	10% 2 3 10% 17% 57%	3600
	11th to 20th	Day	13% 2 4 13% 34% 34%	3406
	21st to end	Day	16% 3 3 8% 27% 42%	3444
November	1st to 10th	Day	14% 3 6% 15% 27% 35%	1854
	11th to 20th	Day	19% 3 6% 14% 22% 36%	1890
	21st to end	Day	23% 4 8% 12% 21% 32%	1890
December	1st to 10th	Day	19% 3 5% 14% 26% 34%	3040
	11th to 20th	Day	28% 5% 7% 14% 26% 20%	3498
	21st to end	Day	31% 6% 8% 23% 22% 9%	4533
January	1st to 10th	Night	27% 4 5% 13% 28% 23%	8172
	11th to 20th	Night	15% 4 9% 15% 14% 44%	7523
	21st to end	Night	30% 6% 5% 11% 22% 26%	7747
February	1st to 10th	Night	28% 3 6% 15% 22% 26%	2520
	11th to 20th	Night	26% 5% 8% 17% 22% 22%	2459
	21st to end	Night	28% 5% 6% 16% 21% 23%	2076
March	1st to 10th	Night	31% 7% 11% 19% 18% 14%	5496
	11th to 20th	Night	24% 9% 11% 26% 17% 13%	5438
	21st to end	Night	27% 11% 13% 23% 18% 7%	5116
April	1st to 10th	Night	25% 6% 10% 24% 20% 15%	2100
	11th to 20th	Night	19% 8% 8% 24% 25% 15%	2079
	21st to end	Night	21% 9% 9% 25% 28% 9%	2100
May	1st to 10th	Night	26% 8% 6% 22% 30% 8%	5643
	11th to 20th	Night	19% 13% 6% 26% 26% 11%	5297
	21st to end	Night	18% 11% 7% 29% 27% 7%	5838
June	1st to 10th	Night	16% 10% 12% 20% 27% 16%	1890
	11th to 20th	Night	13% 6% 6% 15% 31% 29%	1709
	21st to end	Night	11% 4 5% 8% 30% 41%	1680
July	1st to 10th	Night	5% 2 3 7% 23% 60%	4700
	11th to 20th	Night	8% 2 7% 21% 33% 29%	4908
	21st to end	Night	8% 2 5% 24% 31% 31%	6133
August	1st to 10th	Night	6% 2 3 18% 41% 31%	4636
	11th to 20th	Night	6% 3 5% 23% 33% 29%	4636
	21st to end	Night	9% 3 3 16% 32% 37%	5092
September	1st to 10th	Night	11% 3 6% 10% 29% 41%	2100
	11th to 20th	Night	9% 4 4 11% 36% 35%	2192
	21st to end	Night	10% 4 5% 14% 32% 35%	2200
October	1st to 10th	Night	10% 3 3 10% 16% 57%	3600
	11th to 20th	Night	21% 4 3 11% 28% 33%	3736
	21st to end	Night	15% 3 3 8% 25% 46%	4362
November	1st to 10th	Night	16% 4 5% 14% 27% 34%	2665
	11th to 20th	Night	19% 3 7% 14% 20% 37%	2730
	21st to end	Night	24% 4 7% 16% 18% 31%	2730
December	1st to 10th	Night	19% 3 6% 12% 23% 36%	4370
	11th to 20th	Night	28% 4 9% 15% 21% 23%	4941
	21st to end	Night	27% 11% 11% 23% 20% 9%	6673

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	15% 8% 35% 32% 9% 1	220
	11th to 20th	Day	10% 11% 32% 31% 15% 1	411
	21st to end	Day	10% 15% 35% 28% 10% 2	594
February	1st to 10th	Day	14% 13% 32% 27% 11% 3	540
	11th to 20th	Day	14% 15% 35% 27% 8% 1	535
	21st to end	Day	11% 15% 35% 30% 9%	446
March	1st to 10th	Day	15% 20% 34% 22% 9%	540
	11th to 20th	Day	14% 15% 34% 26% 9% 2	540
	21st to end	Day	11% 20% 35% 24% 8% 2	594
April	1st to 10th	Day	11% 16% 31% 29% 11% 3	540
	11th to 20th	Day	8% 18% 37% 27% 7% 3	532
	21st to end	Day	7% 17% 33% 28% 11% 4	540
May	1st to 10th	Day	10% 15% 32% 29% 10% 3	543
	11th to 20th	Day	9% 12% 33% 30% 14% 2	568
	21st to end	Day	7% 10% 36% 32% 13% 2	617
June	1st to 10th	Day	2 8% 30% 37% 20% 4	570
	11th to 20th	Day	2 5% 31% 41% 15% 5%	570
	21st to end	Day	2 4 30% 43% 16% 5%	570
July	1st to 10th	Day	1 2 20% 49% 21% 7%	566
	11th to 20th	Day	1 1 15% 48% 23% 13%	564
	21st to end	Day	1 1 18% 47% 21% 12%	619
August	1st to 10th	Day	1 2 21% 46% 19% 10%	567
	11th to 20th	Day	1 2 19% 46% 20% 11%	557
	21st to end	Day	1 3 22% 45% 20% 8%	627
September	1st to 10th	Day	3 3 28% 41% 18% 8%	570
	11th to 20th	Day	4 6% 29% 40% 16% 5%	543
	21st to end	Day	4 7% 31% 35% 18% 6%	530
October	1st to 10th	Day	5% 10% 30% 34% 18% 3	530
	11th to 20th	Day	4 9% 31% 36% 17% 3	530
	21st to end	Day	5% 9% 27% 34% 19% 5%	518
November	1st to 10th	Day	4 8% 22% 39% 24% 2	330
	11th to 20th	Day	9% 7% 26% 39% 18% 2	330
	21st to end	Day	7% 7% 28% 36% 19% 2	330
December	1st to 10th	Day	8% 6% 24% 40% 17% 5%	330
	11th to 20th	Day	5% 8% 26% 36% 22% 3	330
	21st to end	Day	17% 11% 28% 35% 8% 1	344
January	1st to 10th	Night	12% 6% 25% 38% 16% 3	840
	11th to 20th	Night	7% 4 25% 43% 19% 2	840
	21st to end	Night	13% 6% 26% 38% 15% 2	924
February	1st to 10th	Night	12% 6% 22% 39% 20% 3	840
	11th to 20th	Night	11% 5% 28% 37% 16% 3	824
	21st to end	Night	11% 7% 25% 39% 16% 2	692
March	1st to 10th	Night	9% 5% 28% 40% 17% 1	840
	11th to 20th	Night	9% 5% 22% 38% 22% 5%	840
	21st to end	Night	8% 6% 23% 40% 19% 2	924
April	1st to 10th	Night	8% 5% 21% 41% 22% 3	840
	11th to 20th	Night	7% 6% 24% 38% 23% 2	831
	21st to end	Night	5% 6% 22% 38% 23% 5%	840
May	1st to 10th	Night	8% 6% 21% 39% 23% 3	746
	11th to 20th	Night	6% 4 19% 37% 32% 3	621
	21st to end	Night	4 5% 20% 40% 28% 4	683
June	1st to 10th	Night	3 2 19% 41% 33% 2	630
	11th to 20th	Night	1 2 6% 43% 44% 5%	449
	21st to end	Night	3 1 2 38% 52% 5%	420
July	1st to 10th	Night	1 2 20% 68% 10%	410
	11th to 20th	Night	2 2 23% 63% 10%	481
	21st to end	Night	2 1 3 29% 54% 11%	848
August	1st to 10th	Night	1 1 4 33% 48% 13%	836
	11th to 20th	Night	1 3 26% 55% 15%	832
	21st to end	Night	2 3 29% 53% 12%	924
September	1st to 10th	Night	3 1 8% 31% 46% 11%	840
	11th to 20th	Night	2 2 7% 36% 43% 10%	812
	21st to end	Night	2 1 12% 36% 42% 8%	800
October	1st to 10th	Night	3 2 13% 45% 34% 3	800
	11th to 20th	Night	5% 3 15% 40% 32% 6%	800
	21st to end	Night	4 3 19% 39% 28% 8%	880
November	1st to 10th	Night	5% 2 16% 41% 31% 5%	820
	11th to 20th	Night	7% 2 18% 43% 23% 6%	840
	21st to end	Night	7% 3 23% 42% 22% 4	840
December	1st to 10th	Night	9% 4 23% 41% 20% 3	840
	11th to 20th	Night	11% 4 24% 39% 19% 2	840
	21st to end	Night	10% 5% 30% 38% 15% 2	924

Page generated at 2026-09-11 01:11:53. Version ac07

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