Pine Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Pine Mountain Observatory clear sky chart from 2005-11-06 to 2024-11-04.

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% 7% 3 6% 4 4 4 4 5% 7% 52%	105359
	Night	4 8% 3 6% 4 4 5% 4 6% 7% 49%	102324
Transparency	Day	31% 8% 10% 20% 19% 12%	105102
	Night	32% 9% 13% 22% 17% 8%	102170
Seeing	Day	9% 13% 26% 27% 19% 6%	14678
	Night	8% 10% 24% 36% 20% 3	23737

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	6% 13% 5% 8% 6% 6% 6% 5% 8% 9% 28%	2539
	11th to 20th	Day	11% 14% 4 9% 6% 6% 6% 5% 5% 7% 27%	2540
	21st to end	Day	7% 13% 4 8% 7% 6% 6% 5% 7% 8% 28%	2995
February	1st to 10th	Day	4 11% 5% 9% 6% 5% 5% 5% 7% 9% 33%	1710
	11th to 20th	Day	4 11% 4 8% 6% 5% 5% 5% 7% 9% 36%	1694
	21st to end	Day	4 10% 5% 12% 7% 6% 4 6% 8% 8% 30%	1570
March	1st to 10th	Day	2 9% 4 8% 6% 7% 6% 4 8% 10% 35%	3138
	11th to 20th	Day	3 8% 3 6% 4 4 4 4 5% 7% 53%	3490
	21st to end	Day	4 13% 4 10% 8% 7% 7% 5% 7% 7% 29%	4023
April	1st to 10th	Day	7% 13% 5% 9% 7% 5% 5% 5% 5% 6% 33%	2280
	11th to 20th	Day	7% 13% 4 9% 6% 4 4 4 4 7% 37%	2249
	21st to end	Day	6% 13% 3 6% 4 4 4 4 5% 8% 43%	2307
May	1st to 10th	Day	8% 12% 3 6% 4 3 4 3 4 6% 47%	4420
	11th to 20th	Day	6% 5% 2 4 3 3 3 3 4 5% 62%	4377
	21st to end	Day	8% 9% 3 7% 5% 4 4 3 5% 6% 47%	4813
June	1st to 10th	Day	12% 8% 3 6% 5% 4 4 4 6% 7% 41%	2660
	11th to 20th	Day	8% 5% 2 4 4 4 4 3 6% 7% 54%	2660
	21st to end	Day	4 3 1 4 3 4 4 2 4 5% 66%	2660
July	1st to 10th	Day	1 1 1 1 2 2 2 2 2 3 83%	4420
	11th to 20th	Day	2 2 1 2 2 2 3 2 4 4 77%	4420
	21st to end	Day	2 1 1 2 2 2 2 2 3 4 79%	4790
August	1st to 10th	Day	4 3 1 4 3 3 4 3 4 6% 66%	4202
	11th to 20th	Day	2 2 1 3 3 3 3 3 4 6% 71%	3964
	21st to end	Day	3 3 1 4 2 3 3 2 3 5% 73%	4180
September	1st to 10th	Day	2 2 1 2 2 2 3 3 4 5% 74%	2280
	11th to 20th	Day	5% 5% 2 4 4 4 4 3 5% 6% 58%	2280
	21st to end	Day	6% 6% 2 5% 3 3 3 2 5% 6% 60%	2043
October	1st to 10th	Day	5% 6% 2 3 3 3 2 2 3 4 66%	3180
	11th to 20th	Day	4 6% 2 5% 4 5% 5% 4 6% 9% 50%	3161
	21st to end	Day	6% 10% 4 8% 4 5% 5% 4 6% 7% 41%	3275
November	1st to 10th	Day	7% 11% 2 7% 5% 5% 7% 4 7% 7% 38%	1701
	11th to 20th	Day	6% 9% 3 7% 7% 5% 6% 7% 7% 9% 34%	1576
	21st to end	Day	5% 10% 4 8% 5% 5% 5% 5% 7% 8% 38%	1520
December	1st to 10th	Day	4 8% 3 8% 5% 4 5% 6% 8% 8% 40%	1520
	11th to 20th	Day	6% 12% 4 8% 7% 6% 5% 5% 9% 9% 29%	1928
	21st to end	Day	5% 11% 5% 11% 7% 6% 5% 5% 8% 10% 29%	2794
January	1st to 10th	Night	4 12% 5% 10% 7% 6% 7% 6% 8% 9% 26%	4406
	11th to 20th	Night	6% 13% 5% 9% 6% 6% 6% 5% 7% 8% 30%	4420
	21st to end	Night	6% 14% 4 10% 6% 7% 6% 5% 7% 8% 28%	4665
February	1st to 10th	Night	4 10% 4 10% 6% 6% 4 5% 7% 9% 35%	2470
	11th to 20th	Night	5% 11% 4 8% 5% 4 5% 4 8% 7% 38%	2372
	21st to end	Night	3 9% 4 9% 6% 6% 7% 5% 8% 9% 34%	1884
March	1st to 10th	Night	2 9% 3 6% 6% 5% 6% 5% 10% 11% 36%	3720
	11th to 20th	Night	2 7% 3 6% 5% 5% 4 3 5% 6% 55%	3490
	21st to end	Night	4 12% 4 10% 6% 5% 7% 5% 8% 9% 30%	3629
April	1st to 10th	Night	8% 12% 4 7% 5% 5% 5% 4 6% 8% 36%	1900
	11th to 20th	Night	6% 12% 3 7% 5% 4 5% 4 5% 7% 40%	1879
	21st to end	Night	6% 12% 3 6% 4 4 4 5% 7% 6% 42%	1870
May	1st to 10th	Night	6% 10% 5% 6% 4 3 4 3 5% 7% 47%	2540
	11th to 20th	Night	4 6% 2 3 2 3 3 3 4 6% 64%	2516
	21st to end	Night	6% 9% 2 5% 3 4 3 4 4 6% 55%	2762
June	1st to 10th	Night	11% 7% 2 5% 4 4 5% 3 4 6% 48%	1520
	11th to 20th	Night	5% 5% 2 4 3 3 3 3 5% 7% 59%	1520
	21st to end	Night	4 3 1 3 2 3 3 2 4 5% 71%	1520
July	1st to 10th	Night	1 1 1 1 2 2 2 2 2 85%	2540
	11th to 20th	Night	3 2 1 2 3 2 2 1 3 3 78%	2540
	21st to end	Night	1 1 1 2 1 2 2 2 2 3 82%	2761
August	1st to 10th	Night	2 3 1 4 2 3 3 3 4 4 70%	2718
	11th to 20th	Night	2 2 1 2 1 2 3 2 4 5% 76%	2876
	21st to end	Night	2 2 1 3 2 2 3 2 3 4 77%	3498
September	1st to 10th	Night	2 1 1 2 2 2 3 2 3 5% 78%	1900
	11th to 20th	Night	3 4 1 4 3 3 3 3 4 6% 66%	1900
	21st to end	Night	6% 5% 2 5% 2 3 3 2 4 6% 63%	2133
October	1st to 10th	Night	3 5% 2 3 2 2 3 2 3 4 71%	3800
	11th to 20th	Night	5% 6% 2 5% 3 4 4 4 5% 8% 53%	3777
	21st to end	Night	4 8% 4 9% 5% 5% 5% 3 6% 7% 45%	4287
November	1st to 10th	Night	7% 11% 3 7% 6% 4 5% 4 6% 6% 39%	2450
	11th to 20th	Night	5% 10% 2 7% 6% 6% 6% 5% 8% 8% 37%	2603
	21st to end	Night	4 10% 3 6% 4 5% 5% 5% 7% 9% 42%	2660
December	1st to 10th	Night	4 10% 4 8% 6% 5% 4 5% 7% 9% 39%	2660
	11th to 20th	Night	4 10% 4 10% 6% 7% 6% 6% 8% 9% 29%	3276
	21st to end	Night	4 10% 3 10% 6% 6% 6% 5% 7% 9% 35%	4862

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	48% 15% 15% 15% 6% 1	2539
	11th to 20th	Day	57% 9% 12% 15% 4 3	2540
	21st to end	Day	50% 15% 13% 14% 6% 2	2995
February	1st to 10th	Day	45% 12% 16% 19% 9%	1710
	11th to 20th	Day	40% 13% 16% 17% 11% 3	1703
	21st to end	Day	48% 12% 14% 15% 9% 2	1570
March	1st to 10th	Day	39% 14% 12% 20% 13% 1	3138
	11th to 20th	Day	30% 10% 12% 30% 18%	3490
	21st to end	Day	49% 12% 13% 20% 6%	4023
April	1st to 10th	Day	50% 9% 13% 16% 9% 2	2280
	11th to 20th	Day	45% 6% 14% 19% 12% 4	2252
	21st to end	Day	36% 10% 14% 27% 11% 2	2307
May	1st to 10th	Day	34% 10% 11% 25% 18% 1	4420
	11th to 20th	Day	24% 4 7% 25% 34% 6%	4380
	21st to end	Day	34% 7% 16% 31% 12% 1	4814
June	1st to 10th	Day	41% 6% 11% 29% 13%	2660
	11th to 20th	Day	29% 6% 10% 30% 20% 4	2660
	21st to end	Day	21% 4 9% 27% 29% 11%	2660
July	1st to 10th	Day	8% 3 5% 12% 18% 54%	4351
	11th to 20th	Day	12% 3 4 8% 21% 52%	4345
	21st to end	Day	10% 3 3 14% 37% 33%	4639
August	1st to 10th	Day	18% 5% 4 14% 29% 30%	4186
	11th to 20th	Day	15% 3 4 21% 36% 20%	4003
	21st to end	Day	16% 2 6% 27% 25% 23%	4179
September	1st to 10th	Day	12% 4 5% 23% 33% 24%	2280
	11th to 20th	Day	25% 6% 9% 23% 24% 14%	2280
	21st to end	Day	26% 5% 9% 25% 23% 12%	2043
October	1st to 10th	Day	23% 4 9% 19% 28% 18%	3180
	11th to 20th	Day	31% 5% 13% 27% 20% 4	3161
	21st to end	Day	37% 8% 8% 23% 22% 2	3275
November	1st to 10th	Day	42% 9% 13% 20% 14% 1	1701
	11th to 20th	Day	43% 12% 20% 14% 10% 1	1576
	21st to end	Day	41% 11% 15% 15% 15% 3	1520
December	1st to 10th	Day	41% 14% 14% 21% 10% 1	1520
	11th to 20th	Day	51% 10% 15% 15% 8% 1	1928
	21st to end	Day	52% 15% 14% 12% 6% 1	2794
January	1st to 10th	Night	49% 14% 19% 13% 5% 1	4406
	11th to 20th	Night	50% 13% 12% 16% 6% 4	4420
	21st to end	Night	51% 11% 15% 15% 7% 1	4665
February	1st to 10th	Night	42% 12% 18% 16% 10% 2	2470
	11th to 20th	Night	40% 10% 17% 19% 10% 3	2374
	21st to end	Night	41% 13% 15% 21% 9% 1	1884
March	1st to 10th	Night	34% 16% 13% 24% 11% 2	3720
	11th to 20th	Night	29% 9% 13% 32% 17%	3490
	21st to end	Night	43% 14% 18% 19% 5% 1	3629
April	1st to 10th	Night	43% 9% 19% 19% 10% 1	1900
	11th to 20th	Night	40% 9% 15% 21% 14% 2	1879
	21st to end	Night	36% 8% 18% 25% 12% 1	1870
May	1st to 10th	Night	31% 9% 14% 32% 13% 1	2540
	11th to 20th	Night	19% 6% 9% 33% 29% 3	2516
	21st to end	Night	27% 7% 27% 32% 7%	2762
June	1st to 10th	Night	36% 7% 13% 30% 13%	1520
	11th to 20th	Night	23% 7% 14% 34% 19% 3	1520
	21st to end	Night	17% 4 11% 30% 29% 8%	1520
July	1st to 10th	Night	7% 3 6% 14% 20% 51%	2500
	11th to 20th	Night	12% 3 4 13% 23% 45%	2492
	21st to end	Night	11% 2 3 16% 37% 31%	2674
August	1st to 10th	Night	18% 3 4 20% 34% 22%	2710
	11th to 20th	Night	12% 3 5% 30% 38% 12%	2903
	21st to end	Night	13% 3 7% 29% 27% 21%	3498
September	1st to 10th	Night	10% 3 5% 25% 36% 21%	1900
	11th to 20th	Night	18% 5% 10% 25% 29% 13%	1900
	21st to end	Night	23% 4 10% 25% 23% 15%	2133
October	1st to 10th	Night	19% 3 9% 21% 29% 19%	3800
	11th to 20th	Night	28% 5% 17% 26% 20% 5%	3777
	21st to end	Night	34% 7% 10% 28% 18% 2	4287
November	1st to 10th	Night	43% 9% 13% 16% 15% 3	2450
	11th to 20th	Night	42% 12% 15% 19% 11% 1	2603
	21st to end	Night	37% 11% 14% 18% 16% 5%	2660
December	1st to 10th	Night	41% 13% 14% 19% 10% 3	2660
	11th to 20th	Night	49% 11% 13% 16% 10% 1	3276
	21st to end	Night	46% 17% 13% 13% 10% 1	4862

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	17% 20% 30% 20% 12% 1	180
	11th to 20th	Day	16% 17% 29% 22% 14% 3	180
	21st to end	Day	8% 17% 29% 30% 16% 1	321
February	1st to 10th	Day	10% 21% 28% 29% 11% 1	370
	11th to 20th	Day	10% 21% 35% 22% 10% 1	379
	21st to end	Day	9% 21% 29% 29% 12% 1	382
March	1st to 10th	Day	10% 19% 34% 27% 9% 1	460
	11th to 20th	Day	11% 20% 24% 27% 15% 3	460
	21st to end	Day	13% 22% 29% 26% 10% 1	506
April	1st to 10th	Day	16% 18% 29% 23% 13% 3	460
	11th to 20th	Day	11% 21% 32% 21% 10% 4	452
	21st to end	Day	12% 18% 28% 22% 16% 4	460
May	1st to 10th	Day	10% 19% 26% 26% 14% 5%	463
	11th to 20th	Day	17% 15% 23% 22% 16% 8%	488
	21st to end	Day	11% 12% 24% 27% 20% 7%	529
June	1st to 10th	Day	10% 11% 27% 25% 20% 7%	490
	11th to 20th	Day	9% 17% 28% 27% 16% 3	490
	21st to end	Day	5% 8% 24% 29% 25% 9%	490
July	1st to 10th	Day	3 5% 24% 36% 26% 7%	486
	11th to 20th	Day	1 6% 27% 31% 26% 8%	484
	21st to end	Day	2 4 26% 31% 25% 12%	531
August	1st to 10th	Day	4 4 22% 32% 27% 11%	487
	11th to 20th	Day	3 4 21% 30% 26% 16%	477
	21st to end	Day	5% 8% 18% 31% 27% 11%	539
September	1st to 10th	Day	3 5% 20% 33% 30% 9%	490
	11th to 20th	Day	8% 8% 17% 31% 26% 9%	490
	21st to end	Day	9% 10% 21% 28% 28% 5%	490
October	1st to 10th	Day	12% 11% 21% 28% 20% 8%	490
	11th to 20th	Day	10% 16% 23% 24% 21% 6%	490
	21st to end	Day	16% 14% 28% 21% 16% 6%	441
November	1st to 10th	Day	12% 16% 28% 27% 13% 4	279
	11th to 20th	Day	11% 17% 24% 28% 17% 3	206
	21st to end	Day	11% 22% 20% 28% 13% 6%	180
December	1st to 10th	Day	9% 16% 28% 26% 16% 6%	180
	11th to 20th	Day	12% 18% 32% 29% 8% 1	180
	21st to end	Day	11% 18% 31% 29% 11% 2	198
January	1st to 10th	Night	10% 17% 31% 24% 14% 3	760
	11th to 20th	Night	8% 18% 27% 25% 19% 4	760
	21st to end	Night	10% 12% 23% 35% 17% 3	836
February	1st to 10th	Night	9% 14% 31% 30% 16% 1	760
	11th to 20th	Night	10% 12% 29% 31% 16% 2	744
	21st to end	Night	7% 14% 32% 34% 12% 1	628
March	1st to 10th	Night	8% 16% 29% 32% 13% 2	760
	11th to 20th	Night	8% 12% 29% 33% 17% 2	760
	21st to end	Night	10% 14% 31% 32% 13% 1	836
April	1st to 10th	Night	13% 12% 26% 31% 16% 2	760
	11th to 20th	Night	10% 12% 29% 30% 16% 4	751
	21st to end	Night	12% 11% 26% 35% 15% 1	730
May	1st to 10th	Night	8% 8% 25% 41% 17% 1	380
	11th to 20th	Night	10% 7% 18% 39% 24% 2	374
	21st to end	Night	12% 8% 17% 38% 22% 2	411
June	1st to 10th	Night	13% 6% 20% 43% 18% 1	380
	11th to 20th	Night	9% 4 28% 44% 16%	380
	21st to end	Night	5% 2 18% 56% 19% 1	380
July	1st to 10th	Night	1 2 16% 57% 23% 1	370
	11th to 20th	Night	4 1 22% 56% 16% 1	368
	21st to end	Night	1 21% 53% 24% 1	416
August	1st to 10th	Night	2 1 12% 49% 33% 3	489
	11th to 20th	Night	2 1 10% 43% 38% 6%	602
	21st to end	Night	3 2 17% 44% 31% 3	836
September	1st to 10th	Night	2 1 16% 44% 33% 3	760
	11th to 20th	Night	5% 4 14% 39% 31% 7%	760
	21st to end	Night	8% 6% 15% 34% 33% 5%	760
October	1st to 10th	Night	8% 5% 20% 38% 24% 6%	760
	11th to 20th	Night	9% 7% 20% 35% 25% 4	760
	21st to end	Night	8% 11% 24% 33% 17% 6%	836
November	1st to 10th	Night	14% 11% 27% 31% 16% 2	754
	11th to 20th	Night	10% 18% 24% 30% 15% 3	760
	21st to end	Night	9% 13% 23% 32% 19% 4	760
December	1st to 10th	Night	10% 13% 24% 30% 20% 3	760
	11th to 20th	Night	10% 15% 27% 32% 15% 1	760
	21st to end	Night	9% 21% 31% 27% 11% 1	836

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