3 Mile Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the 3 Mile Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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 4 3 4 4 6% 7% 60%	74732
	Night	4 3 1 3 3 2 4 4 6% 8% 61%	75099
Transparency	Day	22% 4 8% 19% 26% 21%	74482
	Night	21% 4 9% 19% 25% 21%	74871
Seeing	Day	5% 6% 20% 30% 27% 12%	13415
	Night	5% 4 24% 43% 22% 2	23695

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 2 3 2 3 4 4 7% 66%	1897
	11th to 20th	Day	4 3 1 3 3 2 3 3 4 6% 67%	1850
	21st to end	Day	3 3 2 4 3 2 4 4 4 5% 66%	1980
February	1st to 10th	Day	3 2 1 3 3 2 4 5% 6% 7% 64%	1800
	11th to 20th	Day	3 3 1 4 3 2 4 3 5% 7% 64%	1749
	21st to end	Day	3 2 2 3 2 3 4 5% 7% 69%	1480
March	1st to 10th	Day	3 4 1 4 4 3 4 5% 6% 9% 57%	1800
	11th to 20th	Day	4 5% 2 4 3 2 4 4 5% 6% 60%	1931
	21st to end	Day	1 3 1 2 2 2 4 4 5% 7% 68%	2345
April	1st to 10th	Day	3 2 2 2 3 2 4 4 5% 7% 65%	2160
	11th to 20th	Day	2 2 1 3 3 2 4 3 4 5% 70%	2131
	21st to end	Day	3 2 1 2 3 2 2 3 4 6% 73%	2160
May	1st to 10th	Day	3 2 1 3 3 2 4 3 4 5% 70%	2160
	11th to 20th	Day	3 3 1 3 3 2 4 3 6% 6% 65%	2125
	21st to end	Day	3 2 1 2 2 2 3 4 6% 8% 67%	2448
June	1st to 10th	Day	1 1 1 2 3 2 4 5% 6% 9% 65%	2340
	11th to 20th	Day	3 2 1 3 3 2 5% 4 7% 8% 62%	2340
	21st to end	Day	3 3 2 3 4 3 6% 5% 8% 9% 54%	2294
July	1st to 10th	Day	6% 4 2 6% 6% 4 8% 7% 9% 9% 40%	2160
	11th to 20th	Day	5% 3 2 4 5% 3 6% 7% 7% 9% 50%	2160
	21st to end	Day	3 3 2 5% 5% 5% 6% 7% 8% 11% 44%	2349
August	1st to 10th	Day	4 2 1 3 6% 4 7% 7% 9% 10% 47%	2149
	11th to 20th	Day	7% 5% 2 6% 8% 4 8% 7% 9% 9% 34%	2123
	21st to end	Day	6% 4 2 6% 7% 4 6% 6% 7% 9% 43%	2376
September	1st to 10th	Day	8% 4 2 5% 6% 4 6% 6% 6% 8% 45%	2160
	11th to 20th	Day	8% 5% 2 5% 6% 4 6% 6% 6% 9% 45%	2062
	21st to end	Day	6% 5% 2 4 4 3 4 4 5% 8% 56%	1980
October	1st to 10th	Day	3 2 1 3 4 2 4 4 6% 6% 65%	1980
	11th to 20th	Day	6% 4 1 3 2 2 2 4 4 5% 66%	1847
	21st to end	Day	5% 3 1 2 2 1 2 1 3 5% 76%	1980
November	1st to 10th	Day	4 3 1 2 2 1 3 3 4 6% 73%	1826
	11th to 20th	Day	2 4 1 3 3 1 3 3 5% 6% 69%	1710
	21st to end	Day	6% 4 1 3 4 2 3 3 5% 6% 65%	1710
December	1st to 10th	Day	5% 4 2 4 3 1 3 4 5% 7% 61%	1710
	11th to 20th	Day	1 3 2 4 4 3 5% 4 6% 7% 60%	2161
	21st to end	Day	3 2 1 3 3 1 3 3 5% 6% 70%	3299
January	1st to 10th	Night	4 4 2 3 4 2 3 4 4 6% 64%	2268
	11th to 20th	Night	5% 2 1 3 4 2 4 4 5% 6% 64%	2232
	21st to end	Night	4 3 1 3 4 2 3 4 6% 7% 63%	2376
February	1st to 10th	Night	4 3 1 2 3 2 3 3 5% 7% 67%	2160
	11th to 20th	Night	4 4 1 4 3 3 5% 5% 6% 7% 58%	2098
	21st to end	Night	3 3 1 2 4 2 3 4 5% 7% 65%	1776
March	1st to 10th	Night	3 3 2 4 3 3 3 4 6% 10% 59%	2160
	11th to 20th	Night	5% 4 2 4 3 2 4 4 4 6% 62%	2025
	21st to end	Night	2 3 1 2 3 2 3 4 4 7% 68%	2009
April	1st to 10th	Night	3 3 2 3 3 3 4 4 5% 7% 63%	1800
	11th to 20th	Night	3 3 1 3 3 2 4 5% 4 6% 66%	1777
	21st to end	Night	2 2 1 2 2 2 4 4 4 5% 73%	1800
May	1st to 10th	Night	3 2 1 3 3 2 3 4 5% 8% 66%	1800
	11th to 20th	Night	4 3 2 3 4 3 4 4 5% 7% 62%	1770
	21st to end	Night	3 2 1 3 4 2 5% 5% 6% 8% 59%	1821
June	1st to 10th	Night	3 2 1 2 3 3 5% 6% 7% 11% 57%	1620
	11th to 20th	Night	3 3 1 4 5% 3 5% 6% 8% 11% 51%	1620
	21st to end	Night	3 2 1 4 3 3 5% 7% 7% 10% 54%	1658
July	1st to 10th	Night	3 3 2 4 4 4 6% 9% 8% 11% 47%	1800
	11th to 20th	Night	3 2 1 3 3 2 4 6% 7% 12% 57%	1800
	21st to end	Night	2 2 2 3 6% 3 5% 5% 8% 12% 52%	1969
August	1st to 10th	Night	3 2 1 3 3 3 5% 6% 10% 14% 50%	1790
	11th to 20th	Night	3 3 2 5% 5% 4 5% 6% 9% 14% 44%	1778
	21st to end	Night	3 3 1 5% 5% 5% 6% 6% 8% 11% 45%	1980
September	1st to 10th	Night	6% 4 2 4 5% 3 5% 6% 7% 10% 49%	1800
	11th to 20th	Night	6% 4 2 4 5% 4 5% 6% 8% 11% 47%	1898
	21st to end	Night	4 2 1 3 5% 3 5% 7% 7% 10% 53%	1980
October	1st to 10th	Night	3 2 1 3 4 2 5% 5% 6% 8% 60%	1980
	11th to 20th	Night	6% 3 2 3 3 2 3 4 4 7% 61%	2108
	21st to end	Night	4 3 1 2 2 1 2 3 3 5% 72%	2376
November	1st to 10th	Night	5% 2 1 2 2 2 2 2 4 5% 74%	2229
	11th to 20th	Night	2 3 1 3 3 2 3 3 5% 7% 67%	2470
	21st to end	Night	5% 2 1 3 3 3 3 3 4 7% 65%	2470
December	1st to 10th	Night	6% 3 1 3 3 3 4 4 4 8% 62%	2470
	11th to 20th	Night	3 4 2 4 4 2 3 3 5% 8% 61%	3036
	21st to end	Night	2 3 1 3 2 2 3 3 5% 6% 72%	4395

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	20% 3 4 18% 33% 22%	1898
	11th to 20th	Day	19% 3 4 15% 37% 22%	1850
	21st to end	Day	21% 3 5% 13% 36% 21%	1980
February	1st to 10th	Day	19% 3 5% 15% 26% 32%	1800
	11th to 20th	Day	21% 2 6% 15% 28% 28%	1759
	21st to end	Day	17% 4 2 12% 31% 34%	1480
March	1st to 10th	Day	24% 4 4 17% 28% 23%	1800
	11th to 20th	Day	24% 3 4 13% 23% 32%	1931
	21st to end	Day	16% 3 4 11% 32% 34%	2345
April	1st to 10th	Day	19% 3 5% 13% 29% 32%	2160
	11th to 20th	Day	17% 3 4 12% 29% 35%	2134
	21st to end	Day	15% 2 3 12% 33% 35%	2160
May	1st to 10th	Day	18% 3 3 11% 27% 38%	2160
	11th to 20th	Day	20% 5% 6% 18% 26% 25%	2128
	21st to end	Day	15% 5% 7% 21% 24% 28%	2448
June	1st to 10th	Day	15% 4 10% 23% 28% 19%	2340
	11th to 20th	Day	18% 5% 7% 19% 29% 22%	2340
	21st to end	Day	25% 7% 11% 22% 22% 14%	2294
July	1st to 10th	Day	35% 9% 15% 24% 13% 3	2101
	11th to 20th	Day	29% 7% 13% 25% 21% 6%	2097
	21st to end	Day	30% 7% 19% 30% 13% 1	2219
August	1st to 10th	Day	26% 8% 14% 31% 19% 2	2136
	11th to 20th	Day	41% 8% 16% 22% 10% 2	2123
	21st to end	Day	35% 7% 18% 20% 15% 5%	2376
September	1st to 10th	Day	35% 6% 14% 20% 16% 9%	2160
	11th to 20th	Day	35% 6% 13% 19% 20% 7%	2062
	21st to end	Day	28% 5% 8% 22% 24% 12%	1980
October	1st to 10th	Day	19% 4 10% 26% 22% 19%	1980
	11th to 20th	Day	22% 2 6% 21% 24% 25%	1847
	21st to end	Day	15% 2 3 16% 34% 30%	1980
November	1st to 10th	Day	15% 2 6% 20% 31% 25%	1826
	11th to 20th	Day	17% 2 6% 22% 30% 22%	1710
	21st to end	Day	22% 2 6% 18% 30% 21%	1710
December	1st to 10th	Day	24% 3 6% 21% 30% 17%	1710
	11th to 20th	Day	22% 4 5% 19% 33% 18%	2159
	21st to end	Day	15% 3 3 15% 35% 30%	3299
January	1st to 10th	Night	22% 3 4 16% 31% 24%	2268
	11th to 20th	Night	22% 3 4 15% 33% 23%	2232
	21st to end	Night	21% 3 5% 16% 35% 20%	2376
February	1st to 10th	Night	19% 2 5% 16% 26% 32%	2160
	11th to 20th	Night	24% 4 7% 12% 24% 29%	2098
	21st to end	Night	20% 3 4 8% 28% 37%	1776
March	1st to 10th	Night	22% 3 3 18% 28% 25%	2160
	11th to 20th	Night	25% 3 4 13% 24% 30%	2025
	21st to end	Night	18% 3 5% 14% 28% 33%	2009
April	1st to 10th	Night	21% 4 4 13% 29% 29%	1800
	11th to 20th	Night	20% 3 8% 13% 24% 32%	1777
	21st to end	Night	15% 2 6% 14% 26% 36%	1800
May	1st to 10th	Night	18% 4 4 14% 25% 35%	1800
	11th to 20th	Night	22% 5% 9% 20% 22% 22%	1771
	21st to end	Night	22% 5% 11% 22% 17% 22%	1822
June	1st to 10th	Night	19% 6% 12% 28% 22% 13%	1620
	11th to 20th	Night	24% 6% 9% 20% 24% 17%	1620
	21st to end	Night	23% 6% 13% 23% 21% 13%	1658
July	1st to 10th	Night	26% 8% 22% 28% 13% 3	1750
	11th to 20th	Night	20% 6% 16% 30% 24% 5%	1740
	21st to end	Night	22% 6% 23% 34% 14% 1	1859
August	1st to 10th	Night	18% 7% 20% 34% 19% 2	1779
	11th to 20th	Night	27% 7% 24% 29% 11% 2	1778
	21st to end	Night	30% 8% 18% 22% 16% 6%	1980
September	1st to 10th	Night	29% 7% 17% 23% 15% 9%	1800
	11th to 20th	Night	29% 8% 19% 20% 16% 8%	1898
	21st to end	Night	24% 5% 15% 22% 21% 11%	1980
October	1st to 10th	Night	20% 5% 16% 21% 19% 18%	1980
	11th to 20th	Night	23% 3 8% 21% 21% 24%	2108
	21st to end	Night	16% 2 3 15% 32% 31%	2376
November	1st to 10th	Night	16% 2 6% 18% 35% 25%	2229
	11th to 20th	Night	17% 3 6% 21% 29% 24%	2470
	21st to end	Night	20% 3 6% 18% 31% 23%	2470
December	1st to 10th	Night	22% 3 5% 22% 27% 20%	2470
	11th to 20th	Night	23% 3 4 19% 27% 23%	3037
	21st to end	Night	16% 2 3 17% 39% 24%	4395

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	6% 12% 35% 33% 14% 1	270
	11th to 20th	Day	4 12% 27% 35% 18% 2	255
	21st to end	Day	2 14% 38% 30% 11% 3	264
February	1st to 10th	Day	6% 7% 32% 28% 24% 3	240
	11th to 20th	Day	6% 14% 36% 27% 15% 3	240
	21st to end	Day	6% 16% 38% 32% 8% 1	198
March	1st to 10th	Day	6% 10% 32% 39% 11% 2	240
	11th to 20th	Day	9% 13% 38% 23% 14% 4	371
	21st to end	Day	5% 16% 39% 28% 10% 1	462
April	1st to 10th	Day	4 10% 40% 31% 13% 2	420
	11th to 20th	Day	3 16% 42% 27% 9% 3	412
	21st to end	Day	5% 13% 33% 38% 11%	420
May	1st to 10th	Day	3 9% 35% 31% 19% 3	423
	11th to 20th	Day	2 5% 30% 40% 19% 3	448
	21st to end	Day	3 3 24% 44% 25% 2	596
June	1st to 10th	Day	1 1 13% 45% 33% 7%	630
	11th to 20th	Day	3 1 7% 37% 45% 7%	630
	21st to end	Day	3 7% 29% 47% 14%	584
July	1st to 10th	Day	4 1 4 24% 37% 30%	446
	11th to 20th	Day	4 3 19% 41% 33%	444
	21st to end	Day	2 1 4 25% 42% 26%	487
August	1st to 10th	Day	4 3 15% 42% 36%	447
	11th to 20th	Day	7% 1 5% 21% 36% 29%	437
	21st to end	Day	5% 1 4 19% 39% 32%	495
September	1st to 10th	Day	6% 1 6% 25% 42% 20%	450
	11th to 20th	Day	7% 2 8% 22% 34% 27%	352
	21st to end	Day	6% 9% 21% 37% 26%	270
October	1st to 10th	Day	7% 1 10% 27% 37% 19%	270
	11th to 20th	Day	6% 3 16% 32% 30% 13%	270
	21st to end	Day	6% 6% 18% 31% 25% 13%	297
November	1st to 10th	Day	6% 3 21% 32% 29% 9%	270
	11th to 20th	Day	4 11% 18% 30% 27% 10%	270
	21st to end	Day	7% 14% 29% 29% 17% 4	270
December	1st to 10th	Day	7% 7% 21% 38% 23% 4	270
	11th to 20th	Day	2 12% 32% 38% 16% 1	270
	21st to end	Day	4 9% 35% 37% 13% 2	297
January	1st to 10th	Night	6% 4 31% 43% 14% 1	760
	11th to 20th	Night	7% 5% 26% 44% 17% 1	744
	21st to end	Night	5% 6% 35% 39% 14% 1	792
February	1st to 10th	Night	5% 6% 32% 40% 17% 1	720
	11th to 20th	Night	7% 6% 37% 40% 10% 1	704
	21st to end	Night	4 9% 44% 38% 6%	592
March	1st to 10th	Night	5% 8% 36% 40% 10% 2	720
	11th to 20th	Night	6% 8% 35% 36% 13% 2	720
	21st to end	Night	4 6% 44% 37% 9% 1	623
April	1st to 10th	Night	4 6% 40% 42% 8%	540
	11th to 20th	Night	5% 8% 41% 40% 5%	533
	21st to end	Night	2 7% 41% 42% 7%	540
May	1st to 10th	Night	4 6% 36% 43% 11%	540
	11th to 20th	Night	4 5% 32% 48% 12%	531
	21st to end	Night	3 3 32% 47% 15%	584
June	1st to 10th	Night	4 1 19% 58% 18%	540
	11th to 20th	Night	3 1 16% 58% 22%	540
	21st to end	Night	4 1 11% 55% 28%	540
July	1st to 10th	Night	4 8% 45% 41% 3	525
	11th to 20th	Night	3 5% 49% 41% 2	522
	21st to end	Night	2 6% 48% 42% 3	591
August	1st to 10th	Night	4 3 37% 53% 4	537
	11th to 20th	Night	1 1 4 30% 56% 8%	534
	21st to end	Night	5% 5% 35% 52% 3	594
September	1st to 10th	Night	6% 1 6% 40% 44% 3	540
	11th to 20th	Night	5% 1 7% 46% 40% 2	540
	21st to end	Night	4 1 11% 45% 37% 2	540
October	1st to 10th	Night	3 1 16% 51% 27% 2	540
	11th to 20th	Night	9% 2 18% 45% 23% 3	668
	21st to end	Night	5% 4 19% 40% 25% 7%	792
November	1st to 10th	Night	5% 3 17% 44% 27% 5%	749
	11th to 20th	Night	3 3 23% 46% 24% 2	950
	21st to end	Night	5% 5% 30% 38% 19% 3	950
December	1st to 10th	Night	7% 4 23% 45% 19% 2	950
	11th to 20th	Night	5% 5% 31% 44% 13% 2	950
	21st to end	Night	4 7% 30% 44% 13% 2	960

Page generated at 2024-01-13 08:37:56. Version ac07

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