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 2024-12-09.

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	3 3 1 3 3 3 4 3 5% 7% 64%	104755
	Night	3 3 1 3 3 3 4 3 6% 8% 64%	103747
Transparency	Day	19% 4 8% 19% 27% 23%	104541
	Night	19% 4 9% 20% 26% 23%	103550
Seeing	Day	4 6% 21% 30% 27% 12%	14619
	Night	4 4 25% 43% 22% 2	24830

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	3 2 1 2 2 2 2 3 3 6% 75%	3167
	11th to 20th	Day	2 2 1 2 2 2 2 2 4 5% 76%	3170
	21st to end	Day	3 4 1 4 3 3 4 3 4 6% 64%	3487
February	1st to 10th	Day	3 2 1 3 3 3 5% 4 6% 6% 64%	1900
	11th to 20th	Day	3 3 1 4 2 3 4 3 5% 7% 65%	1849
	21st to end	Day	3 3 1 2 3 3 3 3 5% 6% 68%	1570
March	1st to 10th	Day	2 4 1 5% 4 4 4 3 5% 8% 60%	3130
	11th to 20th	Day	6% 5% 2 4 3 2 3 3 5% 7% 60%	3413
	21st to end	Day	1 3 1 4 3 4 4 4 6% 7% 63%	4121
April	1st to 10th	Day	3 2 1 3 3 3 3 4 5% 7% 66%	2280
	11th to 20th	Day	2 3 1 3 3 3 4 3 4 6% 68%	2251
	21st to end	Day	3 2 1 3 3 1 2 2 4 6% 73%	2280
May	1st to 10th	Day	2 2 1 2 2 2 2 2 3 4 78%	3800
	11th to 20th	Day	2 2 1 3 2 2 3 2 5% 5% 72%	3765
	21st to end	Day	1 1 2 2 2 2 2 5% 6% 77%	4350
June	1st to 10th	Day	1 2 1 3 3 3 5% 4 6% 9% 64%	2470
	11th to 20th	Day	3 2 1 4 3 3 4 4 6% 8% 63%	2470
	21st to end	Day	3 3 1 4 4 5% 5% 5% 8% 8% 54%	2421
July	1st to 10th	Day	3 3 1 5% 5% 5% 6% 6% 9% 10% 47%	3800
	11th to 20th	Day	3 2 1 3 4 4 6% 5% 8% 9% 55%	3800
	21st to end	Day	3 3 1 5% 4 6% 6% 5% 8% 10% 48%	4118
August	1st to 10th	Day	2 2 1 4 4 5% 5% 5% 8% 10% 55%	3784
	11th to 20th	Day	4 3 1 5% 4 5% 6% 5% 7% 9% 51%	3726
	21st to end	Day	4 3 1 6% 6% 6% 6% 4 6% 8% 50%	4180
September	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 5% 6% 8% 45%	2280
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 5% 6% 8% 46%	2176
	21st to end	Day	6% 5% 2 4 3 4 5% 3 5% 7% 57%	2090
October	1st to 10th	Day	2 1 1 2 2 2 2 2 4 4 78%	3480
	11th to 20th	Day	4 3 1 2 2 2 2 2 3 4 74%	3223
	21st to end	Day	3 2 1 1 1 1 1 2 5% 82%	3438
November	1st to 10th	Day	4 3 1 2 1 2 2 2 4 6% 74%	1924
	11th to 20th	Day	2 4 1 3 2 2 3 3 5% 6% 70%	1800
	21st to end	Day	6% 4 1 4 3 3 3 2 5% 6% 64%	1800
December	1st to 10th	Day	5% 4 2 5% 3 2 3 4 5% 8% 60%	1782
	11th to 20th	Day	1 4 2 4 4 4 4 3 6% 7% 60%	2161
	21st to end	Day	3 3 1 3 2 2 3 3 6% 6% 69%	3299
January	1st to 10th	Night	2 3 1 2 2 2 2 2 4 6% 73%	3788
	11th to 20th	Night	3 2 1 2 2 2 3 2 5% 6% 73%	3800
	21st to end	Night	5% 4 1 4 4 4 4 3 6% 7% 57%	4180
February	1st to 10th	Night	3 3 1 2 3 3 3 3 5% 7% 66%	2280
	11th to 20th	Night	4 4 1 4 3 3 5% 4 6% 7% 59%	2218
	21st to end	Night	4 4 1 3 3 3 4 2 5% 7% 65%	1884
March	1st to 10th	Night	2 3 1 5% 3 4 3 3 6% 8% 61%	3734
	11th to 20th	Night	6% 6% 2 4 2 3 4 3 4 6% 60%	3561
	21st to end	Night	2 4 1 4 3 4 4 3 5% 6% 64%	3520
April	1st to 10th	Night	3 4 1 4 3 3 4 3 5% 7% 64%	1900
	11th to 20th	Night	4 3 1 3 3 3 4 4 4 6% 65%	1877
	21st to end	Night	2 2 1 3 2 3 3 3 4 5% 73%	1900
May	1st to 10th	Night	2 2 1 2 2 2 2 2 3 5% 76%	3170
	11th to 20th	Night	3 3 1 4 3 3 3 3 4 6% 67%	3140
	21st to end	Night	2 2 1 2 2 3 4 4 5% 7% 68%	3230
June	1st to 10th	Night	3 2 1 3 3 4 5% 5% 8% 11% 57%	1710
	11th to 20th	Night	3 3 1 4 4 4 5% 5% 8% 10% 53%	1710
	21st to end	Night	4 3 1 4 3 4 5% 6% 8% 9% 54%	1751
July	1st to 10th	Night	2 3 1 4 3 5% 6% 6% 9% 12% 50%	3170
	11th to 20th	Night	2 1 2 2 2 3 4 6% 12% 66%	3170
	21st to end	Night	2 2 2 4 5% 4 6% 5% 8% 12% 51%	3445
August	1st to 10th	Night	2 2 1 3 2 4 5% 5% 9% 12% 56%	3155
	11th to 20th	Night	2 2 1 4 4 4 4 4 8% 13% 55%	3113
	21st to end	Night	2 2 1 4 3 4 5% 4 7% 12% 55%	3487
September	1st to 10th	Night	6% 4 2 4 5% 4 6% 4 7% 9% 49%	1900
	11th to 20th	Night	5% 4 2 4 4 5% 4 5% 8% 10% 49%	2004
	21st to end	Night	4 2 1 4 4 4 5% 5% 7% 10% 54%	2090
October	1st to 10th	Night	2 1 1 2 2 2 3 3 4 5% 75%	3480
	11th to 20th	Night	4 2 1 3 2 2 2 2 4 6% 70%	3698
	21st to end	Night	2 2 1 2 1 1 1 2 3 4 80%	4120
November	1st to 10th	Night	5% 2 1 2 2 2 1 2 4 5% 74%	2350
	11th to 20th	Night	2 3 1 4 2 2 3 3 5% 7% 68%	2600
	21st to end	Night	4 3 1 3 3 4 3 2 5% 7% 65%	2600
December	1st to 10th	Night	6% 3 1 4 2 4 4 3 5% 8% 60%	2581
	11th to 20th	Night	3 4 2 4 4 3 3 2 5% 8% 61%	3036
	21st to end	Night	2 3 1 3 2 2 3 2 5% 6% 71%	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	14% 3 2 13% 30% 38%	3168
	11th to 20th	Day	13% 2 4 14% 44% 23%	3170
	21st to end	Day	22% 3 5% 20% 36% 14%	3487
February	1st to 10th	Day	20% 3 5% 14% 26% 33%	1900
	11th to 20th	Day	20% 2 5% 15% 28% 29%	1859
	21st to end	Day	17% 4 3 12% 31% 34%	1570
March	1st to 10th	Day	22% 4 3 11% 29% 31%	3130
	11th to 20th	Day	24% 3 5% 15% 26% 26%	3413
	21st to end	Day	18% 4 3 9% 32% 33%	4121
April	1st to 10th	Day	19% 3 5% 12% 28% 35%	2280
	11th to 20th	Day	18% 4 4 12% 28% 35%	2254
	21st to end	Day	15% 2 4 12% 33% 35%	2280
May	1st to 10th	Day	11% 2 3 11% 23% 50%	3800
	11th to 20th	Day	14% 4 6% 12% 24% 40%	3768
	21st to end	Day	9% 3 5% 15% 21% 46%	4350
June	1st to 10th	Day	15% 5% 10% 23% 28% 19%	2470
	11th to 20th	Day	18% 4 7% 18% 30% 23%	2470
	21st to end	Day	25% 7% 10% 23% 22% 13%	2421
July	1st to 10th	Day	26% 9% 17% 31% 15% 2	3741
	11th to 20th	Day	21% 7% 15% 33% 20% 4	3737
	21st to end	Day	27% 7% 15% 27% 19% 5%	3988
August	1st to 10th	Day	21% 7% 13% 29% 27% 4	3772
	11th to 20th	Day	26% 7% 15% 29% 19% 4	3762
	21st to end	Day	29% 8% 16% 26% 18% 3	4179
September	1st to 10th	Day	35% 6% 13% 20% 16% 10%	2280
	11th to 20th	Day	34% 6% 13% 19% 20% 8%	2176
	21st to end	Day	27% 5% 8% 22% 24% 14%	2090
October	1st to 10th	Day	12% 3 6% 15% 24% 40%	3480
	11th to 20th	Day	16% 3 6% 22% 21% 33%	3223
	21st to end	Day	9% 1 2 13% 47% 28%	3438
November	1st to 10th	Day	14% 2 6% 19% 31% 27%	1924
	11th to 20th	Day	17% 2 6% 21% 30% 23%	1800
	21st to end	Day	22% 2 6% 18% 30% 22%	1800
December	1st to 10th	Day	24% 3 5% 21% 30% 17%	1782
	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	14% 3 2 11% 37% 33%	3788
	11th to 20th	Night	13% 3 4 15% 36% 29%	3800
	21st to end	Night	25% 4 5% 18% 34% 13%	4180
February	1st to 10th	Night	19% 2 5% 16% 26% 33%	2280
	11th to 20th	Night	24% 4 7% 11% 25% 29%	2218
	21st to end	Night	20% 3 4 9% 29% 36%	1884
March	1st to 10th	Night	20% 4 2 12% 27% 34%	3734
	11th to 20th	Night	24% 6% 8% 16% 23% 23%	3561
	21st to end	Night	20% 4 4 10% 32% 31%	3520
April	1st to 10th	Night	21% 4 4 13% 27% 31%	1900
	11th to 20th	Night	20% 3 7% 13% 24% 33%	1877
	21st to end	Night	15% 2 6% 14% 26% 36%	1900
May	1st to 10th	Night	12% 3 3 11% 22% 50%	3170
	11th to 20th	Night	18% 5% 8% 14% 18% 37%	3141
	21st to end	Night	16% 4 7% 16% 19% 39%	3231
June	1st to 10th	Night	19% 5% 12% 28% 23% 14%	1710
	11th to 20th	Night	23% 6% 9% 19% 25% 19%	1710
	21st to end	Night	23% 6% 13% 25% 21% 13%	1751
July	1st to 10th	Night	24% 6% 21% 35% 12% 2	3120
	11th to 20th	Night	12% 4 17% 44% 20% 3	3110
	21st to end	Night	22% 7% 19% 33% 17% 2	3336
August	1st to 10th	Night	15% 6% 15% 34% 25% 4	3144
	11th to 20th	Night	19% 6% 22% 33% 19% 2	3143
	21st to end	Night	20% 7% 20% 32% 17% 3	3487
September	1st to 10th	Night	29% 7% 16% 22% 16% 10%	1900
	11th to 20th	Night	29% 7% 18% 21% 16% 9%	2004
	21st to end	Night	24% 5% 15% 22% 21% 13%	2090
October	1st to 10th	Night	12% 3 10% 12% 22% 41%	3480
	11th to 20th	Night	16% 3 10% 20% 18% 33%	3698
	21st to end	Night	11% 2 3 14% 42% 28%	4120
November	1st to 10th	Night	15% 2 6% 17% 34% 26%	2350
	11th to 20th	Night	17% 3 5% 21% 29% 26%	2600
	21st to end	Night	20% 3 6% 17% 30% 23%	2600
December	1st to 10th	Night	23% 4 5% 21% 27% 19%	2581
	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% 29% 35% 18% 2	270
	21st to end	Day	2 14% 38% 30% 13% 3	297
February	1st to 10th	Day	8% 9% 32% 27% 22% 3	270
	11th to 20th	Day	6% 14% 35% 29% 15% 2	270
	21st to end	Day	6% 16% 38% 31% 8% 1	225
March	1st to 10th	Day	5% 10% 34% 38% 11% 2	270
	11th to 20th	Day	9% 13% 38% 23% 13% 4	409
	21st to end	Day	6% 18% 40% 27% 9% 1	506
April	1st to 10th	Day	4 12% 40% 30% 13% 2	460
	11th to 20th	Day	3 16% 42% 28% 9% 2	452
	21st to end	Day	4 13% 33% 37% 12%	460
May	1st to 10th	Day	3 8% 36% 32% 18% 3	463
	11th to 20th	Day	2 6% 31% 38% 19% 3	488
	21st to end	Day	2 2 26% 43% 25% 2	647
June	1st to 10th	Day	1 1 13% 45% 32% 7%	680
	11th to 20th	Day	3 1 8% 37% 45% 7%	680
	21st to end	Day	3 7% 29% 46% 15%	631
July	1st to 10th	Day	4 1 4 24% 37% 29%	486
	11th to 20th	Day	4 3 19% 42% 32%	484
	21st to end	Day	2 1 4 26% 42% 26%	531
August	1st to 10th	Day	4 3 16% 42% 35%	487
	11th to 20th	Day	6% 1 5% 21% 37% 29%	477
	21st to end	Day	5% 1 4 20% 39% 31%	539
September	1st to 10th	Day	6% 2 7% 25% 41% 19%	490
	11th to 20th	Day	7% 2 7% 22% 35% 26%	386
	21st to end	Day	7% 1 9% 24% 34% 25%	300
October	1st to 10th	Day	7% 1 9% 25% 38% 20%	300
	11th to 20th	Day	6% 3 17% 31% 29% 13%	300
	21st to end	Day	6% 6% 19% 30% 26% 13%	330
November	1st to 10th	Day	6% 4 23% 32% 27% 8%	300
	11th to 20th	Day	4 11% 19% 29% 27% 9%	300
	21st to end	Day	7% 13% 29% 28% 19% 4	300
December	1st to 10th	Day	7% 7% 20% 39% 23% 3	294
	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% 27% 44% 16% 1	760
	21st to end	Night	5% 6% 35% 40% 14% 1	836
February	1st to 10th	Night	5% 7% 33% 38% 16% 1	760
	11th to 20th	Night	6% 6% 38% 40% 10% 1	744
	21st to end	Night	4 9% 44% 38% 5%	628
March	1st to 10th	Night	5% 8% 37% 40% 9% 2	760
	11th to 20th	Night	6% 8% 36% 36% 13% 1	760
	21st to end	Night	4 7% 44% 37% 9% 1	657
April	1st to 10th	Night	4 7% 41% 40% 8%	570
	11th to 20th	Night	5% 9% 42% 39% 5%	563
	21st to end	Night	2 7% 40% 43% 7%	570
May	1st to 10th	Night	4 5% 37% 43% 11%	570
	11th to 20th	Night	4 5% 33% 47% 11%	561
	21st to end	Night	3 3 33% 47% 14%	617
June	1st to 10th	Night	4 1 19% 59% 17%	570
	11th to 20th	Night	3 1 16% 58% 21%	570
	21st to end	Night	4 2 11% 55% 28% 1	570
July	1st to 10th	Night	3 8% 45% 41% 3	555
	11th to 20th	Night	3 5% 49% 41% 2	552
	21st to end	Night	2 6% 48% 42% 3	624
August	1st to 10th	Night	3 3 38% 52% 3	567
	11th to 20th	Night	1 1 4 32% 55% 7%	564
	21st to end	Night	5% 6% 36% 51% 3	627
September	1st to 10th	Night	6% 1 6% 40% 43% 3	570
	11th to 20th	Night	5% 1 7% 46% 39% 2	570
	21st to end	Night	4 1 11% 46% 36% 2	570
October	1st to 10th	Night	3 1 15% 51% 28% 2	570
	11th to 20th	Night	8% 2 18% 45% 24% 3	706
	21st to end	Night	4 4 19% 40% 26% 7%	836
November	1st to 10th	Night	5% 3 19% 44% 25% 5%	790
	11th to 20th	Night	3 4 23% 46% 23% 2	1000
	21st to end	Night	5% 5% 30% 39% 19% 3	1000
December	1st to 10th	Night	7% 4 23% 45% 19% 2	993
	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 2024-12-06 02:54:48. Version ac07

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