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 2024-03-01.

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	6% 3 1 3 3 2 3 3 4 6% 67%	79180
	Night	6% 4 1 3 3 2 3 3 4 6% 64%	80274
Transparency	Day	19% 5% 7% 17% 26% 26%	78903
	Night	20% 6% 8% 17% 24% 24%	80072
Seeing	Day	7% 10% 29% 36% 15% 4	14666
	Night	6% 4 18% 37% 30% 5%	24627

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	7% 3 1 2 3 1 3 3 4 6% 66%	2539
	11th to 20th	Day	9% 4 1 3 3 2 3 4 4 8% 59%	2898
	21st to end	Day	13% 5% 2 4 4 3 3 4 4 7% 50%	3498
February	1st to 10th	Day	10% 4 2 3 3 2 3 3 3 6% 62%	1900
	11th to 20th	Day	10% 3 2 3 3 2 3 4 4 6% 61%	1850
	21st to end	Day	10% 6% 2 4 4 3 4 3 4 7% 54%	1570
March	1st to 10th	Day	9% 5% 2 5% 5% 3 5% 5% 5% 7% 49%	1801
	11th to 20th	Day	8% 4 2 3 4 2 3 3 5% 8% 57%	1838
	21st to end	Day	6% 4 1 3 5% 3 5% 4 6% 8% 56%	2375
April	1st to 10th	Day	7% 4 1 4 4 2 3 5% 4 7% 59%	2160
	11th to 20th	Day	4 4 2 2 4 3 4 3 4 6% 64%	2129
	21st to end	Day	5% 3 1 3 3 2 3 3 3 6% 68%	2160
May	1st to 10th	Day	6% 4 1 4 3 2 3 4 4 7% 61%	2240
	11th to 20th	Day	5% 4 2 3 3 3 4 3 4 7% 63%	2301
	21st to end	Day	6% 4 1 3 4 2 4 4 4 7% 62%	2530
June	1st to 10th	Day	4 2 1 2 4 2 2 3 4 8% 67%	2340
	11th to 20th	Day	2 2 1 2 3 1 2 3 3 4 76%	2488
	21st to end	Day	1 2 1 1 2 2 2 3 3 4 79%	2520
July	1st to 10th	Day	1 1 1 2 2 3 3 3 6% 77%	2520
	11th to 20th	Day	2 1 2 2 1 2 2 3 5% 79%	2453
	21st to end	Day	2 1 1 1 2 1 3 4 5% 7% 72%	2406
August	1st to 10th	Day	1 1 1 1 2 1 1 2 3 4 82%	2149
	11th to 20th	Day	1 1 1 1 1 2 2 3 5% 83%	2122
	21st to end	Day	1 1 1 2 2 2 3 3 5% 80%	2376
September	1st to 10th	Day	2 1 2 2 1 2 3 3 5% 79%	2160
	11th to 20th	Day	2 1 1 1 1 2 2 2 4 84%	2014
	21st to end	Day	2 2 1 1 1 1 1 3 3 5% 79%	1980
October	1st to 10th	Day	5% 2 1 2 3 2 3 3 3 6% 71%	1980
	11th to 20th	Day	4 2 1 2 1 1 2 2 2 6% 76%	1888
	21st to end	Day	4 2 1 2 3 1 2 3 4 4 74%	1922
November	1st to 10th	Day	4 2 1 2 3 1 2 3 4 5% 72%	1674
	11th to 20th	Day	5% 2 1 2 2 2 3 3 4 5% 71%	1710
	21st to end	Day	5% 3 2 3 3 3 3 4 4 7% 63%	1710
December	1st to 10th	Day	8% 3 1 4 3 2 4 3 5% 7% 60%	1710
	11th to 20th	Day	14% 6% 2 5% 4 2 4 4 5% 9% 45%	2169
	21st to end	Day	13% 6% 2 4 6% 3 4 4 4 6% 47%	3100
January	1st to 10th	Night	7% 4 1 3 3 2 3 3 3 5% 65%	4406
	11th to 20th	Night	6% 4 2 4 3 2 3 4 4 8% 59%	4061
	21st to end	Night	14% 6% 2 4 3 3 3 5% 4 7% 50%	4180
February	1st to 10th	Night	10% 4 2 4 3 2 3 3 2 6% 62%	2280
	11th to 20th	Night	9% 4 2 3 3 2 3 3 3 5% 62%	2218
	21st to end	Night	11% 5% 1 3 4 3 4 5% 3 5% 55%	1884
March	1st to 10th	Night	8% 5% 2 4 4 2 4 4 6% 8% 52%	2165
	11th to 20th	Night	8% 4 2 3 4 2 3 3 5% 7% 58%	2118
	21st to end	Night	7% 5% 2 3 4 2 4 4 4 7% 57%	1980
April	1st to 10th	Night	6% 5% 2 3 5% 2 4 5% 4 6% 58%	1800
	11th to 20th	Night	5% 5% 2 2 3 3 4 3 4 6% 65%	1779
	21st to end	Night	5% 4 1 3 3 2 3 3 4 6% 65%	1800
May	1st to 10th	Night	5% 6% 2 4 5% 4 3 3 4 5% 60%	1720
	11th to 20th	Night	3 6% 2 2 4 4 5% 3 3 8% 61%	1594
	21st to end	Night	3 3 1 3 4 3 4 2 4 6% 66%	1746
June	1st to 10th	Night	3 3 1 2 4 3 3 4 5% 8% 66%	1620
	11th to 20th	Night	2 2 1 3 3 4 3 3 5% 75%	1466
	21st to end	Night	2 2 1 1 1 2 3 3 3 7% 76%	1440
July	1st to 10th	Night	1 1 1 1 2 3 3 2 2 6% 79%	1440
	11th to 20th	Night	2 1 1 1 2 2 3 3 4 7% 75%	1502
	21st to end	Night	3 1 1 2 3 2 3 3 5% 7% 71%	1907
August	1st to 10th	Night	2 1 1 1 2 2 2 3 3 5% 79%	1790
	11th to 20th	Night	1 1 1 2 1 1 2 2 3 6% 82%	1780
	21st to end	Night	1 1 1 1 1 2 4 4 6% 79%	1980
September	1st to 10th	Night	2 2 1 1 2 2 3 4 2 4 77%	1800
	11th to 20th	Night	1 1 1 2 2 2 2 2 4 83%	1944
	21st to end	Night	1 2 1 1 2 1 2 2 3 5% 80%	1980
October	1st to 10th	Night	3 2 1 2 3 2 3 4 3 5% 70%	1980
	11th to 20th	Night	4 2 1 2 2 2 2 2 3 4 76%	2069
	21st to end	Night	3 3 1 2 3 2 3 3 3 5% 72%	2433
November	1st to 10th	Night	4 3 1 3 3 2 2 3 3 5% 71%	2405
	11th to 20th	Night	4 2 2 3 2 2 3 2 4 6% 70%	2470
	21st to end	Night	6% 4 1 3 3 3 4 3 6% 7% 60%	2470
December	1st to 10th	Night	8% 4 2 4 4 2 3 3 4 6% 59%	2470
	11th to 20th	Night	15% 6% 2 4 4 3 3 4 5% 7% 47%	3041
	21st to end	Night	11% 7% 2 5% 5% 4 5% 4 6% 8% 45%	4556

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	18% 4 7% 23% 30% 18%	2539
	11th to 20th	Day	25% 5% 14% 22% 19% 15%	2898
	21st to end	Day	31% 7% 6% 17% 21% 17%	3498
February	1st to 10th	Day	26% 3 5% 14% 24% 28%	1900
	11th to 20th	Day	24% 4 6% 16% 25% 24%	1860
	21st to end	Day	29% 6% 9% 16% 24% 17%	1570
March	1st to 10th	Day	32% 6% 9% 18% 17% 20%	1801
	11th to 20th	Day	24% 6% 10% 19% 24% 17%	1838
	21st to end	Day	24% 7% 12% 22% 22% 14%	2375
April	1st to 10th	Day	24% 6% 11% 26% 20% 13%	2160
	11th to 20th	Day	20% 5% 8% 21% 29% 18%	2132
	21st to end	Day	18% 6% 9% 23% 32% 14%	2160
May	1st to 10th	Day	23% 6% 10% 19% 28% 14%	2240
	11th to 20th	Day	20% 6% 8% 21% 31% 13%	2304
	21st to end	Day	20% 7% 10% 25% 29% 9%	2530
June	1st to 10th	Day	17% 6% 12% 19% 31% 17%	2340
	11th to 20th	Day	12% 4 5% 12% 32% 34%	2488
	21st to end	Day	11% 4 4 9% 29% 44%	2520
July	1st to 10th	Day	9% 4 6% 11% 23% 48%	2451
	11th to 20th	Day	11% 4 5% 18% 29% 33%	2378
	21st to end	Day	10% 6% 9% 23% 32% 20%	2272
August	1st to 10th	Day	9% 3 3 13% 31% 42%	2135
	11th to 20th	Day	6% 4 3 13% 35% 39%	2122
	21st to end	Day	8% 3 5% 11% 35% 37%	2376
September	1st to 10th	Day	9% 3 6% 9% 33% 40%	2160
	11th to 20th	Day	7% 3 3 12% 35% 39%	2014
	21st to end	Day	9% 3 5% 15% 33% 36%	1980
October	1st to 10th	Day	16% 4 6% 17% 23% 33%	1980
	11th to 20th	Day	13% 4 4 14% 28% 37%	1888
	21st to end	Day	14% 4 5% 10% 22% 44%	1922
November	1st to 10th	Day	15% 3 6% 16% 26% 33%	1674
	11th to 20th	Day	17% 3 6% 14% 21% 38%	1710
	21st to end	Day	22% 4 8% 12% 21% 33%	1710
December	1st to 10th	Day	25% 5% 7% 18% 19% 26%	1710
	11th to 20th	Day	36% 6% 9% 14% 18% 17%	2168
	21st to end	Day	36% 7% 8% 21% 16% 13%	3100
January	1st to 10th	Night	22% 5% 6% 16% 31% 20%	4406
	11th to 20th	Night	23% 7% 15% 23% 15% 16%	4061
	21st to end	Night	31% 11% 7% 13% 22% 16%	4180
February	1st to 10th	Night	26% 4 6% 15% 22% 26%	2280
	11th to 20th	Night	24% 5% 7% 16% 23% 24%	2219
	21st to end	Night	31% 5% 7% 16% 21% 21%	1884
March	1st to 10th	Night	30% 6% 8% 20% 18% 17%	2165
	11th to 20th	Night	25% 6% 10% 17% 24% 17%	2118
	21st to end	Night	26% 6% 9% 23% 22% 13%	1980
April	1st to 10th	Night	27% 6% 11% 24% 18% 14%	1800
	11th to 20th	Night	18% 7% 9% 23% 25% 17%	1779
	21st to end	Night	20% 8% 9% 24% 29% 11%	1800
May	1st to 10th	Night	24% 8% 9% 21% 24% 14%	1720
	11th to 20th	Night	23% 8% 9% 24% 25% 11%	1595
	21st to end	Night	16% 8% 12% 27% 28% 8%	1746
June	1st to 10th	Night	15% 8% 13% 20% 27% 17%	1620
	11th to 20th	Night	12% 6% 7% 15% 31% 28%	1466
	21st to end	Night	10% 4 6% 8% 30% 42%	1440
July	1st to 10th	Night	8% 4 6% 11% 26% 45%	1400
	11th to 20th	Night	11% 5% 8% 20% 25% 31%	1454
	21st to end	Night	13% 6% 8% 25% 28% 20%	1801
August	1st to 10th	Night	10% 4 5% 11% 34% 37%	1780
	11th to 20th	Night	5% 4 5% 14% 38% 34%	1780
	21st to end	Night	7% 5% 6% 11% 34% 36%	1980
September	1st to 10th	Night	11% 3 7% 9% 29% 40%	1800
	11th to 20th	Night	8% 5% 4 11% 35% 38%	1944
	21st to end	Night	9% 4 5% 14% 33% 34%	1980
October	1st to 10th	Night	17% 5% 5% 17% 24% 32%	1980
	11th to 20th	Night	14% 5% 5% 14% 29% 34%	2069
	21st to end	Night	16% 5% 5% 8% 23% 44%	2433
November	1st to 10th	Night	17% 4 6% 14% 27% 33%	2405
	11th to 20th	Night	17% 3 7% 14% 19% 39%	2470
	21st to end	Night	24% 5% 7% 16% 17% 31%	2470
December	1st to 10th	Night	26% 5% 8% 15% 18% 28%	2470
	11th to 20th	Night	36% 6% 8% 14% 20% 16%	3041
	21st to end	Night	29% 15% 10% 19% 16% 12%	4556

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	14% 9% 34% 34% 8% 1	180
	11th to 20th	Day	12% 12% 33% 30% 12% 1	347
	21st to end	Day	11% 17% 36% 27% 8% 1	506
February	1st to 10th	Day	14% 15% 32% 26% 10% 3	460
	11th to 20th	Day	12% 16% 35% 28% 8% 1	455
	21st to end	Day	13% 18% 35% 28% 6%	382
March	1st to 10th	Day	15% 19% 34% 22% 9%	421
	11th to 20th	Day	14% 15% 35% 26% 9% 1	420
	21st to end	Day	12% 19% 39% 23% 7% 1	462
April	1st to 10th	Day	11% 15% 32% 29% 12% 2	420
	11th to 20th	Day	6% 21% 39% 26% 6% 2	412
	21st to end	Day	5% 18% 34% 28% 11% 4	420
May	1st to 10th	Day	11% 15% 33% 29% 10% 3	423
	11th to 20th	Day	9% 10% 34% 31% 13% 3	448
	21st to end	Day	8% 11% 36% 29% 13% 2	485
June	1st to 10th	Day	3 9% 31% 35% 18% 4	450
	11th to 20th	Day	2 6% 29% 41% 15% 6%	450
	21st to end	Day	3 4 29% 44% 15% 6%	450
July	1st to 10th	Day	1 1 19% 51% 21% 7%	446
	11th to 20th	Day	1 1 14% 49% 23% 13%	444
	21st to end	Day	2 1 16% 49% 19% 13%	487
August	1st to 10th	Day	2 2 23% 47% 19% 8%	447
	11th to 20th	Day	2 2 17% 48% 21% 10%	437
	21st to end	Day	1 1 20% 48% 22% 7%	495
September	1st to 10th	Day	2 3 29% 42% 18% 6%	450
	11th to 20th	Day	3 5% 30% 41% 17% 4	450
	21st to end	Day	4 6% 30% 35% 19% 5%	450
October	1st to 10th	Day	5% 11% 31% 34% 17% 2	450
	11th to 20th	Day	4 9% 32% 37% 17% 2	450
	21st to end	Day	5% 10% 27% 35% 19% 4	437
November	1st to 10th	Day	5% 9% 22% 41% 21% 1	270
	11th to 20th	Day	6% 7% 23% 42% 19% 2	270
	21st to end	Day	6% 8% 31% 34% 19% 2	270
December	1st to 10th	Day	10% 7% 26% 40% 16% 2	270
	11th to 20th	Day	6% 9% 28% 38% 17% 2	270
	21st to end	Day	16% 12% 29% 33% 8% 2	282
January	1st to 10th	Night	11% 6% 23% 39% 17% 3	760
	11th to 20th	Night	8% 4 26% 41% 19% 2	760
	21st to end	Night	13% 6% 26% 38% 15% 2	836
February	1st to 10th	Night	11% 6% 22% 38% 21% 3	760
	11th to 20th	Night	9% 6% 27% 37% 17% 4	744
	21st to end	Night	13% 8% 25% 36% 16% 2	628
March	1st to 10th	Night	9% 4 28% 40% 18% 1	722
	11th to 20th	Night	9% 5% 21% 36% 23% 5%	720
	21st to end	Night	8% 6% 25% 40% 19% 2	792
April	1st to 10th	Night	9% 4 20% 41% 23% 3	720
	11th to 20th	Night	6% 7% 25% 37% 24% 2	711
	21st to end	Night	5% 6% 22% 38% 24% 5%	720
May	1st to 10th	Night	7% 6% 22% 39% 23% 3	640
	11th to 20th	Night	6% 3 18% 37% 34% 2	531
	21st to end	Night	3 5% 22% 40% 26% 4	584
June	1st to 10th	Night	3 2 19% 41% 34% 2	540
	11th to 20th	Night	1 2 6% 43% 44% 5%	386
	21st to end	Night	2 1 2 38% 52% 4	360
July	1st to 10th	Night	1 2 20% 67% 10%	350
	11th to 20th	Night	2 1 23% 65% 8%	410
	21st to end	Night	2 1 3 28% 56% 10%	725
August	1st to 10th	Night	2 1 5% 34% 49% 11%	716
	11th to 20th	Night	1 3 25% 57% 14%	712
	21st to end	Night	2 3 27% 55% 13%	792
September	1st to 10th	Night	2 1 8% 29% 47% 12%	720
	11th to 20th	Night	1 2 6% 35% 45% 11%	720
	21st to end	Night	2 1 11% 35% 43% 8%	720
October	1st to 10th	Night	4 2 14% 45% 34% 2	720
	11th to 20th	Night	4 3 15% 39% 33% 6%	720
	21st to end	Night	4 3 19% 38% 27% 9%	792
November	1st to 10th	Night	5% 3 16% 41% 30% 6%	740
	11th to 20th	Night	5% 2 18% 45% 24% 7%	760
	21st to end	Night	6% 3 24% 40% 23% 4	760
December	1st to 10th	Night	9% 4 22% 41% 20% 3	760
	11th to 20th	Night	13% 4 24% 39% 17% 3	760
	21st to end	Night	10% 5% 30% 38% 15% 3	836

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