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 2025-05-11.

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	5% 3 1 3 2 3 3 2 4 6% 68%	118520
	Night	5% 4 1 3 2 3 3 2 4 6% 66%	119367
Transparency	Day	17% 5% 6% 16% 27% 29%	118280
	Night	19% 6% 7% 17% 24% 27%	119196
Seeing	Day	7% 10% 29% 36% 15% 4	16315
	Night	6% 4 18% 37% 30% 5%	26252

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	5% 3 1 3 2 2 3 2 3 5% 71%	3639
	11th to 20th	Day	6% 3 1 3 2 2 3 2 3 6% 68%	4166
	21st to end	Day	11% 4 2 5% 3 4 3 3 5% 9% 52%	5016
February	1st to 10th	Day	11% 4 2 2 3 2 3 3 4 5% 60%	2000
	11th to 20th	Day	11% 4 1 3 2 2 3 3 5% 5% 60%	1950
	21st to end	Day	9% 6% 2 5% 3 4 4 2 4 7% 54%	1650
March	1st to 10th	Day	14% 6% 2 5% 4 3 4 4 5% 6% 47%	4518
	11th to 20th	Day	11% 4 2 4 3 4 4 3 5% 7% 54%	4663
	21st to end	Day	9% 4 1 4 4 4 4 3 6% 9% 52%	5968
April	1st to 10th	Day	6% 4 1 4 3 3 2 4 4 7% 61%	2400
	11th to 20th	Day	5% 5% 2 3 4 4 4 2 5% 6% 62%	2369
	21st to end	Day	5% 4 1 3 3 2 4 2 3 6% 66%	2400
May	1st to 10th	Day	4 4 1 3 2 2 3 2 3 5% 71%	4070
	11th to 20th	Day	3 4 1 3 3 4 3 2 4 8% 64%	4073
	21st to end	Day	3 5% 1 3 3 4 4 3 4 6% 65%	4477
June	1st to 10th	Day	4 3 1 2 3 3 2 2 4 8% 66%	2470
	11th to 20th	Day	2 3 1 2 2 2 2 2 3 4 76%	2627
	21st to end	Day	1 2 1 1 2 2 2 2 3 4 80%	2660
July	1st to 10th	Day	1 1 1 1 2 2 2 2 4 86%	4420
	11th to 20th	Day	1 1 2 2 2 3 3 4 5% 78%	4301
	21st to end	Day	1 1 1 1 1 2 2 3 4 83%	4214
August	1st to 10th	Day	1 1 1 1 1 1 1 3 4 85%	3783
	11th to 20th	Day	1 1 1 1 1 2 4 89%	3724
	21st to end	Day	1 1 1 2 3 2 2 3 85%	4180
September	1st to 10th	Day	2 1 2 1 1 2 2 3 5% 79%	2280
	11th to 20th	Day	2 1 2 1 2 2 1 2 4 82%	2125
	21st to end	Day	2 2 1 1 1 2 2 2 3 4 80%	2090
October	1st to 10th	Day	2 1 1 1 2 2 2 1 2 4 81%	3490
	11th to 20th	Day	2 3 1 3 1 2 2 2 3 6% 74%	3301
	21st to end	Day	3 3 1 3 2 3 3 3 5% 6% 69%	3337
November	1st to 10th	Day	4 3 1 3 2 2 2 2 4 6% 72%	1764
	11th to 20th	Day	5% 2 1 2 2 2 3 3 4 5% 71%	1800
	21st to end	Day	5% 4 1 4 3 4 4 3 4 7% 62%	1800
December	1st to 10th	Day	4 2 1 4 3 3 4 2 5% 7% 66%	2950
	11th to 20th	Day	9% 5% 1 4 3 3 5% 5% 6% 8% 52%	3409
	21st to end	Day	10% 5% 1 5% 5% 4 4 3 5% 7% 51%	4436
January	1st to 10th	Night	5% 3 1 3 2 2 3 2 3 5% 71%	6306
	11th to 20th	Night	4 4 2 4 2 3 4 2 3 6% 66%	5792
	21st to end	Night	13% 5% 1 4 3 3 3 4 4 7% 53%	5984
February	1st to 10th	Night	10% 5% 2 4 3 3 2 2 3 5% 60%	2400
	11th to 20th	Night	9% 4 2 3 3 3 3 2 4 6% 60%	2338
	21st to end	Night	10% 5% 1 3 3 3 4 4 4 5% 57%	1980
March	1st to 10th	Night	13% 9% 2 5% 3 3 4 3 5% 7% 46%	5376
	11th to 20th	Night	8% 4 2 4 4 4 4 3 6% 8% 55%	5320
	21st to end	Night	10% 8% 3 5% 3 4 3 2 4 7% 51%	5006
April	1st to 10th	Night	6% 5% 1 3 5% 3 4 4 4 6% 59%	2000
	11th to 20th	Night	5% 6% 2 2 2 4 4 3 4 6% 63%	1979
	21st to end	Night	5% 6% 1 4 3 2 3 3 4 6% 62%	2000
May	1st to 10th	Night	5% 5% 1 3 3 4 2 2 2 4 68%	3125
	11th to 20th	Night	2 4 1 2 3 6% 4 3 4 10% 60%	2838
	21st to end	Night	2 7% 2 3 3 5% 3 2 2 5% 66%	3110
June	1st to 10th	Night	2 3 1 2 5% 5% 3 4 5% 7% 64%	1710
	11th to 20th	Night	1 2 1 2 3 4 4 2 3 5% 74%	1547
	21st to end	Night	1 2 1 1 3 3 3 3 6% 77%	1520
July	1st to 10th	Night	1 1 1 2 2 1 2 4 87%	2540
	11th to 20th	Night	1 1 1 1 2 5% 2 3 6% 78%	2658
	21st to end	Night	1 1 1 2 1 2 2 3 4 81%	3352
August	1st to 10th	Night	1 1 1 1 2 2 2 3 4 83%	3166
	11th to 20th	Night	1 1 1 1 2 1 2 4 87%	3126
	21st to end	Night	1 1 2 1 2 5% 2 3 4 80%	3498
September	1st to 10th	Night	2 1 1 2 1 2 3 3 3 5% 77%	1900
	11th to 20th	Night	1 2 1 2 2 2 2 2 4 81%	2053
	21st to end	Night	1 2 1 1 2 1 2 2 3 5% 80%	2090
October	1st to 10th	Night	2 2 1 2 2 2 2 2 2 3 81%	3490
	11th to 20th	Night	3 6% 3 5% 2 2 3 1 3 5% 66%	3621
	21st to end	Night	3 3 1 3 2 3 3 2 3 4 74%	4227
November	1st to 10th	Night	4 3 1 3 2 2 3 2 3 6% 71%	2535
	11th to 20th	Night	4 2 2 3 2 2 3 2 4 6% 71%	2600
	21st to end	Night	6% 4 1 4 3 4 4 3 6% 7% 59%	2600
December	1st to 10th	Night	5% 3 1 3 3 2 4 3 4 5% 66%	4240
	11th to 20th	Night	9% 5% 2 4 3 4 5% 4 5% 7% 53%	4811
	21st to end	Night	8% 6% 2 5% 3 4 5% 3 6% 11% 47%	6529

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	16% 3 6% 20% 30% 25%	3639
	11th to 20th	Day	18% 3 10% 19% 19% 31%	4166
	21st to end	Day	27% 6% 5% 15% 24% 24%	5016
February	1st to 10th	Day	27% 3 6% 14% 24% 27%	2000
	11th to 20th	Day	24% 4 6% 17% 25% 23%	1960
	21st to end	Day	28% 6% 8% 15% 23% 20%	1650
March	1st to 10th	Day	30% 8% 9% 17% 22% 14%	4518
	11th to 20th	Day	27% 6% 11% 24% 20% 11%	4663
	21st to end	Day	25% 8% 13% 28% 17% 10%	5968
April	1st to 10th	Day	22% 6% 10% 25% 23% 14%	2400
	11th to 20th	Day	21% 6% 7% 20% 28% 17%	2372
	21st to end	Day	18% 6% 8% 24% 32% 12%	2400
May	1st to 10th	Day	15% 6% 8% 17% 37% 16%	4070
	11th to 20th	Day	12% 16% 6% 21% 35% 9%	4076
	21st to end	Day	11% 13% 7% 25% 37% 7%	4477
June	1st to 10th	Day	16% 7% 11% 20% 30% 16%	2470
	11th to 20th	Day	12% 4 5% 13% 32% 34%	2627
	21st to end	Day	11% 3 4 9% 29% 45%	2660
July	1st to 10th	Day	5% 2 3 6% 14% 70%	4351
	11th to 20th	Day	9% 3 3 13% 27% 45%	4226
	21st to end	Day	6% 3 5% 19% 29% 38%	4081
August	1st to 10th	Day	7% 2 2 12% 40% 38%	3770
	11th to 20th	Day	3 2 2 11% 31% 51%	3760
	21st to end	Day	5% 2 3 7% 28% 54%	4179
September	1st to 10th	Day	9% 3 6% 8% 32% 42%	2280
	11th to 20th	Day	8% 3 3 12% 35% 38%	2125
	21st to end	Day	8% 3 5% 15% 32% 38%	2090
October	1st to 10th	Day	10% 2 3 10% 17% 58%	3490
	11th to 20th	Day	13% 3 4 13% 34% 33%	3301
	21st to end	Day	16% 3 3 8% 27% 42%	3337
November	1st to 10th	Day	15% 3 6% 15% 27% 34%	1764
	11th to 20th	Day	17% 3 6% 14% 22% 38%	1800
	21st to end	Day	23% 4 8% 12% 21% 32%	1800
December	1st to 10th	Day	19% 3 5% 14% 25% 34%	2950
	11th to 20th	Day	29% 5% 8% 14% 25% 19%	3408
	21st to end	Day	31% 6% 8% 23% 23% 9%	4436
January	1st to 10th	Night	17% 4 5% 14% 33% 26%	6306
	11th to 20th	Night	19% 5% 11% 19% 15% 31%	5792
	21st to end	Night	29% 8% 6% 12% 23% 22%	5984
February	1st to 10th	Night	28% 4 6% 15% 22% 25%	2400
	11th to 20th	Night	24% 5% 8% 17% 23% 23%	2339
	21st to end	Night	29% 5% 7% 15% 21% 23%	1980
March	1st to 10th	Night	32% 8% 11% 19% 18% 13%	5376
	11th to 20th	Night	25% 9% 11% 26% 17% 11%	5320
	21st to end	Night	27% 11% 13% 23% 18% 7%	5006
April	1st to 10th	Night	25% 6% 11% 23% 20% 15%	2000
	11th to 20th	Night	19% 8% 9% 23% 26% 16%	1979
	21st to end	Night	21% 9% 9% 25% 28% 10%	2000
May	1st to 10th	Night	18% 9% 6% 21% 36% 10%	3125
	11th to 20th	Night	14% 20% 7% 26% 26% 7%	2839
	21st to end	Night	10% 15% 8% 32% 29% 5%	3110
June	1st to 10th	Night	15% 10% 13% 20% 26% 16%	1710
	11th to 20th	Night	12% 6% 7% 15% 31% 29%	1547
	21st to end	Night	10% 4 5% 8% 29% 43%	1520
July	1st to 10th	Night	4 2 3 6% 16% 68%	2500
	11th to 20th	Night	8% 3 4 13% 28% 44%	2610
	21st to end	Night	7% 3 5% 22% 25% 38%	3247
August	1st to 10th	Night	7% 3 3 12% 42% 33%	3156
	11th to 20th	Night	3 2 3 15% 35% 42%	3156
	21st to end	Night	7% 3 4 8% 31% 47%	3498
September	1st to 10th	Night	11% 3 6% 9% 29% 41%	1900
	11th to 20th	Night	9% 5% 4 10% 35% 37%	2053
	21st to end	Night	9% 4 5% 14% 33% 35%	2090
October	1st to 10th	Night	10% 3 3 10% 15% 59%	3490
	11th to 20th	Night	21% 4 4 11% 27% 32%	3621
	21st to end	Night	15% 3 3 7% 25% 46%	4227
November	1st to 10th	Night	16% 4 5% 14% 27% 33%	2535
	11th to 20th	Night	17% 3 7% 14% 20% 39%	2600
	21st to end	Night	24% 4 7% 16% 17% 31%	2600
December	1st to 10th	Night	19% 3 6% 13% 22% 36%	4240
	11th to 20th	Night	29% 5% 9% 14% 21% 22%	4811
	21st to end	Night	26% 11% 11% 23% 20% 9%	6529

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% 8% 36% 33% 9% 2	200
	11th to 20th	Day	11% 12% 34% 31% 12% 1	379
	21st to end	Day	10% 16% 37% 27% 9% 1	550
February	1st to 10th	Day	15% 14% 32% 26% 10% 3	500
	11th to 20th	Day	14% 15% 34% 28% 8% 1	495
	21st to end	Day	12% 17% 36% 29% 7%	414
March	1st to 10th	Day	16% 20% 33% 22% 9%	500
	11th to 20th	Day	15% 16% 35% 25% 8% 1	500
	21st to end	Day	12% 21% 37% 23% 7% 1	550
April	1st to 10th	Day	11% 16% 31% 29% 11% 3	500
	11th to 20th	Day	7% 19% 38% 27% 7% 2	492
	21st to end	Day	6% 16% 32% 30% 12% 5%	500
May	1st to 10th	Day	10% 15% 33% 30% 10% 3	503
	11th to 20th	Day	8% 10% 34% 31% 14% 2	489
	21st to end	Day	8% 11% 35% 31% 13% 2	529
June	1st to 10th	Day	2 8% 30% 36% 20% 4	490
	11th to 20th	Day	2 6% 30% 42% 15% 6%	490
	21st to end	Day	2 3 28% 44% 16% 6%	490
July	1st to 10th	Day	1 1 18% 50% 22% 8%	486
	11th to 20th	Day	1 1 14% 49% 22% 13%	484
	21st to end	Day	2 1 16% 48% 20% 13%	531
August	1st to 10th	Day	2 2 22% 47% 18% 9%	487
	11th to 20th	Day	1 2 19% 47% 21% 10%	477
	21st to end	Day	1 1 20% 48% 22% 7%	539
September	1st to 10th	Day	1 3 28% 42% 19% 7%	490
	11th to 20th	Day	3 6% 30% 41% 16% 4	490
	21st to end	Day	3 6% 31% 36% 18% 6%	490
October	1st to 10th	Day	4 10% 30% 34% 19% 3	490
	11th to 20th	Day	3 9% 30% 38% 17% 2	490
	21st to end	Day	6% 9% 28% 34% 18% 5%	477
November	1st to 10th	Day	4 9% 23% 41% 21% 1	300
	11th to 20th	Day	6% 7% 26% 40% 19% 2	300
	21st to end	Day	7% 7% 31% 35% 18% 2	300
December	1st to 10th	Day	9% 6% 24% 41% 16% 3	300
	11th to 20th	Day	6% 8% 28% 36% 20% 2	300
	21st to end	Day	15% 12% 29% 35% 8% 2	313
January	1st to 10th	Night	11% 6% 24% 39% 17% 3	800
	11th to 20th	Night	8% 4 26% 41% 19% 2	800
	21st to end	Night	13% 6% 26% 38% 15% 2	880
February	1st to 10th	Night	12% 6% 22% 38% 20% 3	800
	11th to 20th	Night	10% 5% 27% 37% 17% 3	784
	21st to end	Night	12% 8% 25% 37% 16% 2	660
March	1st to 10th	Night	10% 5% 27% 40% 17% 1	800
	11th to 20th	Night	9% 6% 23% 37% 21% 5%	800
	21st to end	Night	9% 7% 24% 40% 19% 2	880
April	1st to 10th	Night	8% 5% 22% 40% 23% 3	800
	11th to 20th	Night	6% 6% 24% 39% 23% 2	791
	21st to end	Night	5% 6% 22% 38% 24% 5%	800
May	1st to 10th	Night	8% 6% 21% 38% 24% 3	710
	11th to 20th	Night	6% 4 18% 37% 33% 2	563
	21st to end	Night	3 5% 21% 41% 27% 4	617
June	1st to 10th	Night	3 2 19% 41% 33% 2	570
	11th to 20th	Night	1 2 6% 44% 43% 5%	407
	21st to end	Night	2 1 2 38% 53% 5%	380
July	1st to 10th	Night	1 2 19% 67% 11%	370
	11th to 20th	Night	2 2 23% 65% 9%	434
	21st to end	Night	1 1 3 28% 55% 11%	766
August	1st to 10th	Night	1 1 4 33% 49% 12%	756
	11th to 20th	Night	1 3 26% 56% 14%	752
	21st to end	Night	2 3 28% 54% 13%	836
September	1st to 10th	Night	2 1 8% 29% 48% 11%	760
	11th to 20th	Night	2 2 7% 36% 44% 10%	760
	21st to end	Night	2 1 11% 35% 43% 8%	760
October	1st to 10th	Night	3 2 13% 45% 34% 3	760
	11th to 20th	Night	5% 3 15% 39% 33% 6%	760
	21st to end	Night	4 3 20% 38% 27% 9%	836
November	1st to 10th	Night	5% 2 16% 42% 29% 5%	780
	11th to 20th	Night	5% 2 18% 44% 24% 7%	800
	21st to end	Night	7% 3 23% 41% 22% 4	800
December	1st to 10th	Night	9% 4 22% 41% 20% 3	800
	11th to 20th	Night	12% 4 24% 39% 18% 3	800
	21st to end	Night	9% 5% 30% 38% 15% 2	880

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