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 2024-03-23.

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% 8% 3 6% 5% 3 5% 5% 5% 7% 47%	78675
	Night	5% 8% 3 6% 5% 3 5% 5% 6% 7% 45%	81238
Transparency	Day	36% 8% 11% 20% 17% 8%	78378
	Night	36% 9% 14% 20% 15% 6%	81056
Seeing	Day	9% 13% 25% 27% 19% 6%	13784
	Night	8% 10% 24% 35% 20% 3	23040

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% 12% 6% 7% 7% 4 7% 6% 8% 9% 28%	2539
	11th to 20th	Day	11% 14% 5% 9% 7% 5% 6% 6% 5% 7% 27%	2540
	21st to end	Day	7% 12% 4 7% 8% 5% 7% 6% 7% 9% 29%	2995
February	1st to 10th	Day	4 11% 5% 8% 7% 4 6% 6% 7% 10% 33%	1710
	11th to 20th	Day	4 10% 5% 7% 7% 3 5% 6% 7% 9% 37%	1694
	21st to end	Day	4 9% 6% 11% 8% 5% 6% 7% 8% 9% 30%	1570
March	1st to 10th	Day	4 10% 6% 8% 7% 5% 8% 6% 8% 9% 30%	1900
	11th to 20th	Day	4 10% 4 8% 6% 3 6% 6% 6% 9% 36%	2090
	21st to end	Day	6% 13% 5% 9% 8% 5% 7% 6% 6% 7% 29%	2309
April	1st to 10th	Day	8% 12% 6% 8% 8% 3 6% 5% 5% 6% 32%	2160
	11th to 20th	Day	8% 12% 6% 8% 6% 3 5% 5% 4 7% 37%	2129
	21st to end	Day	6% 12% 4 5% 4 3 4 4 5% 8% 44%	2184
May	1st to 10th	Day	8% 9% 3 6% 5% 3 4 4 5% 7% 47%	2520
	11th to 20th	Day	10% 9% 3 5% 6% 4 5% 5% 5% 7% 40%	2477
	21st to end	Day	12% 9% 4 8% 6% 3 5% 5% 5% 6% 38%	2723
June	1st to 10th	Day	12% 7% 3 5% 6% 3 5% 5% 5% 7% 40%	2520
	11th to 20th	Day	8% 5% 2 4 5% 3 4 3 5% 7% 54%	2520
	21st to end	Day	4 3 2 3 3 3 4 3 3 5% 65%	2520
July	1st to 10th	Day	2 2 1 2 3 2 3 4 4 5% 72%	2520
	11th to 20th	Day	2 2 1 2 3 2 4 3 3 4 74%	2520
	21st to end	Day	2 1 1 2 3 1 3 3 4 4 76%	2741
August	1st to 10th	Day	3 2 1 3 4 3 4 4 5% 6% 67%	2400
	11th to 20th	Day	3 2 1 2 3 2 2 3 4 5% 74%	2262
	21st to end	Day	2 2 1 2 3 2 2 3 4 5% 76%	2376
September	1st to 10th	Day	2 2 1 2 2 2 3 3 4 5% 75%	2160
	11th to 20th	Day	6% 4 2 4 4 2 5% 4 5% 6% 59%	2160
	21st to end	Day	6% 6% 2 4 4 3 3 3 5% 6% 59%	1936
October	1st to 10th	Day	9% 9% 3 5% 5% 3 3 4 3 5% 51%	1800
	11th to 20th	Day	6% 7% 2 5% 5% 3 5% 4 5% 8% 50%	1800
	21st to end	Day	8% 8% 3 6% 5% 3 5% 5% 5% 7% 46%	1888
November	1st to 10th	Day	8% 9% 3 6% 6% 4 7% 5% 7% 7% 39%	1674
	11th to 20th	Day	6% 9% 3 5% 7% 4 6% 8% 7% 9% 35%	1576
	21st to end	Day	5% 9% 5% 7% 6% 4 5% 5% 6% 9% 38%	1520
December	1st to 10th	Day	4 8% 3 7% 6% 3 5% 7% 8% 9% 40%	1520
	11th to 20th	Day	6% 11% 5% 7% 8% 5% 6% 6% 9% 9% 30%	1928
	21st to end	Day	5% 10% 5% 9% 8% 5% 6% 6% 8% 10% 29%	2794
January	1st to 10th	Night	4 12% 5% 9% 8% 5% 7% 7% 7% 9% 27%	4406
	11th to 20th	Night	6% 12% 5% 8% 7% 5% 6% 6% 7% 8% 30%	4420
	21st to end	Night	6% 14% 4 8% 7% 5% 6% 6% 6% 8% 29%	4665
February	1st to 10th	Night	4 10% 4 9% 7% 3 5% 6% 7% 9% 35%	2470
	11th to 20th	Night	5% 10% 4 7% 6% 3 6% 5% 7% 8% 38%	2372
	21st to end	Night	3 8% 5% 8% 7% 5% 7% 7% 8% 9% 34%	1884
March	1st to 10th	Night	4 10% 4 7% 7% 4 7% 6% 8% 10% 34%	2276
	11th to 20th	Night	3 8% 4 7% 7% 5% 7% 6% 7% 8% 37%	2090
	21st to end	Night	4 10% 4 9% 8% 3 7% 6% 8% 9% 31%	2091
April	1st to 10th	Night	8% 12% 4 6% 6% 4 6% 5% 6% 8% 36%	1800
	11th to 20th	Night	7% 11% 4 7% 6% 3 6% 5% 5% 7% 40%	1779
	21st to end	Night	6% 11% 3 5% 5% 3 5% 5% 6% 7% 42%	1773
May	1st to 10th	Night	5% 9% 3 4 4 3 4 5% 5% 7% 51%	1440
	11th to 20th	Night	8% 7% 2 4 4 3 5% 6% 6% 9% 47%	1416
	21st to end	Night	9% 9% 2 7% 5% 4 5% 5% 5% 7% 42%	1552
June	1st to 10th	Night	12% 7% 2 5% 4 3 4 4 4 6% 49%	1440
	11th to 20th	Night	6% 5% 2 3 4 2 3 4 5% 7% 59%	1440
	21st to end	Night	4 3 1 3 3 2 3 3 4 5% 70%	1440
July	1st to 10th	Night	2 2 1 2 2 1 4 4 3 4 75%	1440
	11th to 20th	Night	3 1 1 1 2 1 2 2 2 4 80%	1440
	21st to end	Night	2 1 1 2 1 2 2 3 2 4 80%	1576
August	1st to 10th	Night	1 2 1 2 2 2 3 4 5% 6% 73%	1537
	11th to 20th	Night	2 1 2 1 1 3 3 4 5% 77%	1638
	21st to end	Night	2 2 1 2 2 1 3 2 2 4 80%	1980
September	1st to 10th	Night	2 1 1 2 2 1 2 2 3 5% 79%	1800
	11th to 20th	Night	3 4 1 3 4 2 3 4 4 6% 67%	1800
	21st to end	Night	6% 5% 2 4 3 2 3 3 4 5% 62%	2020
October	1st to 10th	Night	5% 8% 3 4 3 2 3 3 5% 5% 58%	2160
	11th to 20th	Night	6% 6% 2 4 4 3 4 4 5% 6% 57%	2160
	21st to end	Night	5% 8% 4 7% 6% 3 5% 5% 5% 6% 47%	2467
November	1st to 10th	Night	8% 10% 3 6% 6% 3 5% 6% 6% 7% 39%	2405
	11th to 20th	Night	6% 9% 3 6% 6% 5% 6% 7% 8% 8% 37%	2603
	21st to end	Night	4 9% 4 6% 5% 4 5% 7% 7% 9% 42%	2660
December	1st to 10th	Night	5% 9% 4 7% 7% 3 5% 6% 7% 9% 40%	2660
	11th to 20th	Night	5% 10% 4 9% 7% 5% 7% 7% 8% 9% 30%	3276
	21st to end	Night	4 9% 4 9% 7% 4 6% 6% 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	47% 12% 15% 16% 9% 1	1900
	11th to 20th	Day	44% 11% 12% 22% 10%	2090
	21st to end	Day	54% 10% 14% 16% 6%	2309
April	1st to 10th	Day	51% 9% 12% 16% 10% 2	2160
	11th to 20th	Day	45% 6% 14% 19% 12% 4	2132
	21st to end	Day	37% 9% 14% 27% 11% 2	2184
May	1st to 10th	Day	35% 7% 12% 28% 16% 1	2520
	11th to 20th	Day	42% 6% 12% 26% 13% 2	2480
	21st to end	Day	44% 8% 13% 23% 11% 1	2724
June	1st to 10th	Day	42% 6% 11% 29% 12%	2520
	11th to 20th	Day	30% 7% 11% 30% 19% 4	2520
	21st to end	Day	21% 4 9% 27% 29% 11%	2520
July	1st to 10th	Day	14% 4 8% 21% 26% 26%	2451
	11th to 20th	Day	14% 3 6% 13% 30% 33%	2445
	21st to end	Day	12% 3 4 12% 36% 33%	2589
August	1st to 10th	Day	17% 5% 7% 23% 30% 19%	2383
	11th to 20th	Day	14% 4 6% 20% 32% 24%	2262
	21st to end	Day	13% 2 6% 23% 38% 17%	2376
September	1st to 10th	Day	12% 4 5% 23% 32% 24%	2160
	11th to 20th	Day	24% 6% 8% 21% 26% 15%	2160
	21st to end	Day	27% 6% 9% 24% 23% 12%	1936
October	1st to 10th	Day	35% 5% 9% 17% 23% 10%	1800
	11th to 20th	Day	33% 7% 11% 23% 20% 7%	1800
	21st to end	Day	36% 9% 11% 21% 21% 2	1888
November	1st to 10th	Day	41% 9% 13% 20% 14% 1	1674
	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	42% 13% 15% 19% 8% 3	2276
	11th to 20th	Night	42% 10% 15% 23% 10% 1	2090
	21st to end	Night	44% 13% 18% 20% 6%	2091
April	1st to 10th	Night	44% 9% 18% 18% 10% 1	1800
	11th to 20th	Night	41% 9% 14% 20% 14% 2	1779
	21st to end	Night	36% 8% 18% 25% 12% 1	1773
May	1st to 10th	Night	28% 8% 16% 34% 14% 1	1440
	11th to 20th	Night	33% 8% 15% 31% 10% 3	1416
	21st to end	Night	38% 9% 20% 24% 9% 1	1552
June	1st to 10th	Night	36% 7% 14% 31% 12%	1440
	11th to 20th	Night	23% 7% 14% 35% 18% 3	1440
	21st to end	Night	17% 4 12% 30% 29% 8%	1440
July	1st to 10th	Night	12% 5% 10% 25% 27% 22%	1400
	11th to 20th	Night	9% 4 6% 20% 31% 30%	1392
	21st to end	Night	12% 2 4 15% 42% 26%	1488
August	1st to 10th	Night	13% 3 7% 32% 31% 14%	1529
	11th to 20th	Night	9% 4 8% 26% 34% 19%	1638
	21st to end	Night	10% 3 6% 25% 37% 19%	1980
September	1st to 10th	Night	10% 3 5% 25% 36% 21%	1800
	11th to 20th	Night	17% 5% 10% 24% 30% 14%	1800
	21st to end	Night	24% 4 10% 25% 22% 16%	2020
October	1st to 10th	Night	27% 5% 13% 21% 25% 9%	2160
	11th to 20th	Night	27% 6% 15% 24% 20% 8%	2160
	21st to end	Night	35% 8% 15% 17% 20% 4	2467
November	1st to 10th	Night	43% 9% 13% 16% 15% 4	2405
	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	14% 21% 28% 26% 10% 1	471
April	1st to 10th	Day	16% 18% 30% 20% 13% 3	420
	11th to 20th	Day	12% 23% 31% 21% 9% 5%	412
	21st to end	Day	13% 19% 27% 22% 16% 4	420
May	1st to 10th	Day	10% 18% 26% 27% 14% 5%	423
	11th to 20th	Day	18% 15% 23% 21% 15% 7%	448
	21st to end	Day	11% 12% 23% 27% 19% 7%	485
June	1st to 10th	Day	10% 11% 26% 25% 20% 7%	450
	11th to 20th	Day	9% 16% 28% 27% 16% 3	450
	21st to end	Day	5% 6% 24% 29% 26% 10%	450
July	1st to 10th	Day	3 4 24% 36% 26% 7%	446
	11th to 20th	Day	1 5% 27% 32% 26% 9%	444
	21st to end	Day	2 3 25% 32% 26% 12%	487
August	1st to 10th	Day	3 4 22% 32% 28% 12%	447
	11th to 20th	Day	3 2 21% 30% 27% 17%	437
	21st to end	Day	4 7% 19% 32% 27% 11%	495
September	1st to 10th	Day	3 5% 19% 33% 30% 10%	450
	11th to 20th	Day	8% 8% 17% 32% 26% 9%	450
	21st to end	Day	9% 10% 21% 28% 28% 5%	450
October	1st to 10th	Day	13% 11% 21% 27% 20% 8%	450
	11th to 20th	Day	10% 15% 22% 24% 23% 6%	450
	21st to end	Day	14% 13% 29% 21% 17% 7%	403
November	1st to 10th	Day	10% 16% 29% 27% 14% 4	270
	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% 13% 30% 32% 13% 1	800
April	1st to 10th	Night	14% 13% 25% 31% 16% 2	720
	11th to 20th	Night	10% 12% 29% 29% 16% 4	711
	21st to end	Night	12% 11% 25% 34% 16% 1	693
May	1st to 10th	Night	8% 8% 24% 42% 18% 1	360
	11th to 20th	Night	10% 7% 18% 39% 25% 2	354
	21st to end	Night	12% 8% 17% 38% 23% 2	389
June	1st to 10th	Night	13% 6% 20% 42% 19% 1	360
	11th to 20th	Night	9% 4 27% 44% 16%	360
	21st to end	Night	5% 2 18% 55% 20% 1	360
July	1st to 10th	Night	1 2 15% 58% 23% 1	350
	11th to 20th	Night	3 1 22% 55% 17% 1	348
	21st to end	Night	1 20% 53% 25% 1	394
August	1st to 10th	Night	2 1 11% 50% 34% 3	463
	11th to 20th	Night	2 1 9% 43% 40% 6%	570
	21st to end	Night	3 2 17% 45% 31% 3	792
September	1st to 10th	Night	2 1 17% 44% 33% 4	720
	11th to 20th	Night	5% 4 13% 39% 32% 7%	720
	21st to end	Night	8% 6% 14% 34% 33% 5%	720
October	1st to 10th	Night	9% 5% 20% 37% 24% 6%	720
	11th to 20th	Night	8% 7% 21% 33% 26% 4	720
	21st to end	Night	8% 11% 23% 33% 18% 7%	792
November	1st to 10th	Night	13% 11% 27% 31% 17% 2	740
	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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