Twelve Mile Post
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Twelve Mile Post clear sky chart from 2005-11-06 to 2024-03-06.

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	14% 5% 2 4 5% 3 5% 5% 6% 8% 44%	78916
	Night	15% 5% 2 4 4 3 4 4 5% 7% 46%	80613
Transparency	Day	38% 7% 12% 20% 18% 5%	78639
	Night	38% 6% 11% 19% 20% 5%	80409
Seeing	Day	18% 8% 21% 25% 18% 10%	14140
	Night	17% 5% 21% 32% 21% 4	24320

By Month

Percentage Cloud Cover SE (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	16% 3 1 3 3 1 3 3 4 7% 56%	2836
	11th to 20th	Day	13% 5% 2 4 4 2 3 4 4 4 54%	2840
	21st to end	Day	24% 4 1 3 3 2 3 3 4 5% 48%	3124
February	1st to 10th	Day	14% 6% 2 5% 4 3 4 5% 4 7% 47%	1767
	11th to 20th	Day	17% 5% 2 5% 5% 2 4 4 5% 6% 46%	1848
	21st to end	Day	16% 6% 2 5% 5% 4 4 4 5% 7% 42%	1570
March	1st to 10th	Day	14% 4 2 3 4 2 3 4 4 7% 52%	2011
	11th to 20th	Day	18% 5% 3 4 5% 2 4 4 5% 6% 42%	1980
	21st to end	Day	17% 6% 2 4 5% 2 4 5% 6% 7% 42%	2178
April	1st to 10th	Day	11% 3 1 5% 5% 3 4 5% 6% 8% 49%	1980
	11th to 20th	Day	14% 4 2 4 5% 3 5% 4 6% 6% 47%	2130
	21st to end	Day	13% 4 2 5% 5% 3 4 5% 6% 7% 48%	2340
May	1st to 10th	Day	10% 4 2 5% 4 3 6% 6% 6% 9% 45%	2340
	11th to 20th	Day	15% 5% 2 5% 6% 4 6% 6% 6% 8% 37%	2301
	21st to end	Day	13% 5% 2 4 6% 3 5% 6% 8% 10% 38%	2531
June	1st to 10th	Day	10% 6% 3 6% 5% 4 6% 6% 7% 12% 35%	2340
	11th to 20th	Day	7% 4 2 6% 5% 4 6% 7% 9% 11% 38%	2340
	21st to end	Day	9% 4 2 5% 7% 4 6% 7% 8% 11% 36%	2340
July	1st to 10th	Day	9% 6% 3 6% 7% 4 8% 7% 9% 11% 30%	2340
	11th to 20th	Day	10% 4 3 6% 7% 5% 8% 7% 9% 11% 30%	2340
	21st to end	Day	8% 4 3 5% 6% 5% 8% 8% 9% 11% 34%	2547
August	1st to 10th	Day	14% 6% 2 6% 7% 5% 7% 7% 7% 10% 29%	2327
	11th to 20th	Day	13% 6% 2 6% 6% 4 6% 7% 9% 10% 31%	2303
	21st to end	Day	12% 5% 2 6% 6% 4 6% 6% 8% 10% 35%	2401
September	1st to 10th	Day	11% 4 2 5% 5% 4 6% 6% 5% 9% 42%	1980
	11th to 20th	Day	17% 5% 2 5% 5% 3 5% 6% 5% 7% 41%	1980
	21st to end	Day	17% 5% 2 5% 4 3 5% 4 5% 6% 43%	1980
October	1st to 10th	Day	16% 4 1 4 4 2 3 3 4 6% 53%	1929
	11th to 20th	Day	13% 4 2 2 3 2 3 3 4 6% 58%	1800
	21st to end	Day	15% 4 1 3 3 2 3 3 4 5% 58%	1980
November	1st to 10th	Day	12% 4 2 4 3 2 3 4 3 5% 60%	1806
	11th to 20th	Day	13% 4 2 3 3 2 3 3 5% 5% 56%	1710
	21st to end	Day	15% 3 1 3 3 3 4 3 5% 7% 54%	1710
December	1st to 10th	Day	17% 7% 2 4 5% 3 4 5% 5% 6% 42%	1710
	11th to 20th	Day	16% 3 1 3 3 2 3 4 5% 7% 54%	2153
	21st to end	Day	20% 4 2 3 3 2 4 4 5% 7% 46%	3124
January	1st to 10th	Night	16% 4 1 3 3 2 4 3 5% 6% 54%	4109
	11th to 20th	Night	12% 5% 3 4 4 2 4 4 4 5% 53%	4120
	21st to end	Night	23% 5% 1 4 4 3 4 3 3 6% 44%	4532
February	1st to 10th	Night	14% 4 2 4 4 3 4 4 4 6% 51%	2409
	11th to 20th	Night	18% 5% 2 5% 4 2 4 4 4 6% 47%	2218
	21st to end	Night	15% 6% 3 5% 4 3 4 5% 5% 6% 43%	1884
March	1st to 10th	Night	13% 6% 2 4 3 2 4 4 4 6% 50%	2066
	11th to 20th	Night	16% 7% 2 5% 4 2 4 4 4 8% 44%	1980
	21st to end	Night	16% 5% 3 5% 4 3 5% 4 5% 6% 44%	2178
April	1st to 10th	Night	12% 5% 2 4 5% 2 3 4 5% 7% 51%	1980
	11th to 20th	Night	14% 5% 2 4 3 2 3 3 6% 7% 50%	1791
	21st to end	Night	13% 4 2 4 4 2 4 4 5% 6% 52%	1620
May	1st to 10th	Night	13% 4 2 4 5% 3 4 4 6% 7% 46%	1620
	11th to 20th	Night	15% 5% 2 5% 7% 4 5% 6% 5% 7% 39%	1592
	21st to end	Night	14% 5% 2 5% 6% 3 4 5% 6% 10% 40%	1744
June	1st to 10th	Night	10% 5% 2 6% 6% 4 6% 6% 8% 14% 34%	1620
	11th to 20th	Night	11% 5% 2 5% 5% 3 6% 5% 6% 10% 41%	1620
	21st to end	Night	9% 4 2 5% 5% 4 6% 7% 8% 11% 40%	1620
July	1st to 10th	Night	10% 5% 2 7% 7% 5% 8% 7% 8% 10% 32%	1620
	11th to 20th	Night	7% 5% 2 6% 7% 4 5% 7% 9% 11% 35%	1620
	21st to end	Night	6% 5% 3 5% 7% 5% 6% 7% 11% 13% 32%	1771
August	1st to 10th	Night	13% 5% 2 6% 6% 4 6% 7% 7% 10% 34%	1611
	11th to 20th	Night	12% 7% 3 6% 6% 4 7% 6% 7% 10% 34%	1598
	21st to end	Night	12% 6% 2 6% 6% 4 6% 7% 7% 9% 35%	1933
September	1st to 10th	Night	12% 5% 2 5% 6% 4 5% 6% 7% 7% 41%	1980
	11th to 20th	Night	17% 6% 2 7% 6% 3 5% 4 6% 8% 37%	1980
	21st to end	Night	19% 5% 2 4 4 3 4 4 5% 6% 45%	1980
October	1st to 10th	Night	19% 5% 1 4 4 2 3 3 4 5% 50%	2029
	11th to 20th	Night	16% 5% 1 4 3 2 3 4 5% 6% 52%	2160
	21st to end	Night	14% 3 2 3 3 2 3 3 3 5% 59%	2376
November	1st to 10th	Night	15% 4 1 4 4 2 3 3 4 4 55%	2268
	11th to 20th	Night	15% 4 2 3 3 2 4 3 4 6% 55%	2470
	21st to end	Night	15% 4 1 4 3 2 4 5% 5% 7% 51%	2470
December	1st to 10th	Night	17% 7% 2 4 4 3 3 4 5% 6% 44%	2470
	11th to 20th	Night	15% 5% 2 3 3 2 4 3 4 6% 54%	3042
	21st to end	Night	22% 4 2 4 4 2 4 4 5% 7% 42%	4532

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	31% 3 6% 25% 26% 8%	2837
	11th to 20th	Day	35% 3 7% 15% 32% 6%	2840
	21st to end	Day	41% 2 6% 18% 27% 6%	3124
February	1st to 10th	Day	38% 4 10% 22% 19% 7%	1767
	11th to 20th	Day	39% 5% 9% 19% 21% 7%	1858
	21st to end	Day	43% 6% 7% 16% 19% 9%	1570
March	1st to 10th	Day	34% 5% 8% 21% 24% 9%	2011
	11th to 20th	Day	44% 5% 10% 17% 17% 7%	1980
	21st to end	Day	40% 7% 12% 21% 17% 3	2178
April	1st to 10th	Day	33% 5% 11% 21% 23% 6%	1980
	11th to 20th	Day	37% 6% 8% 19% 21% 9%	2130
	21st to end	Day	36% 6% 11% 26% 18% 4	2340
May	1st to 10th	Day	35% 8% 16% 21% 17% 4	2340
	11th to 20th	Day	43% 10% 15% 18% 12% 2	2305
	21st to end	Day	38% 8% 15% 24% 14% 1	2531
June	1st to 10th	Day	41% 9% 17% 22% 9% 1	2340
	11th to 20th	Day	37% 11% 19% 21% 12% 2	2340
	21st to end	Day	39% 10% 16% 19% 13% 2	2340
July	1st to 10th	Day	43% 12% 17% 15% 11% 3	2275
	11th to 20th	Day	42% 12% 20% 15% 9% 2	2271
	21st to end	Day	38% 13% 19% 19% 9% 2	2405
August	1st to 10th	Day	48% 10% 16% 20% 6%	2314
	11th to 20th	Day	44% 12% 17% 17% 9% 1	2303
	21st to end	Day	40% 10% 17% 21% 8% 3	2401
September	1st to 10th	Day	38% 9% 13% 20% 15% 4	1980
	11th to 20th	Day	42% 7% 11% 17% 20% 3	1980
	21st to end	Day	41% 6% 9% 21% 18% 6%	1980
October	1st to 10th	Day	33% 4 11% 19% 27% 6%	1929
	11th to 20th	Day	29% 3 9% 26% 24% 9%	1800
	21st to end	Day	31% 4 6% 22% 27% 11%	1980
November	1st to 10th	Day	30% 3 7% 20% 29% 10%	1806
	11th to 20th	Day	31% 4 7% 23% 26% 9%	1710
	21st to end	Day	33% 4 8% 22% 26% 7%	1710
December	1st to 10th	Day	43% 6% 7% 21% 17% 6%	1710
	11th to 20th	Day	32% 3 8% 22% 26% 10%	2150
	21st to end	Day	38% 4 9% 23% 21% 4	3124
January	1st to 10th	Night	33% 3 9% 18% 29% 7%	4109
	11th to 20th	Night	34% 4 7% 17% 30% 8%	4120
	21st to end	Night	44% 3 7% 16% 24% 7%	4532
February	1st to 10th	Night	35% 5% 10% 21% 23% 6%	2409
	11th to 20th	Night	41% 5% 7% 18% 23% 6%	2219
	21st to end	Night	42% 4 11% 16% 20% 7%	1884
March	1st to 10th	Night	37% 4 7% 18% 24% 10%	2066
	11th to 20th	Night	41% 5% 9% 19% 19% 6%	1980
	21st to end	Night	42% 6% 10% 21% 18% 3	2178
April	1st to 10th	Night	33% 6% 10% 21% 25% 5%	1980
	11th to 20th	Night	34% 5% 8% 23% 22% 8%	1791
	21st to end	Night	33% 6% 11% 24% 22% 4	1620
May	1st to 10th	Night	36% 6% 15% 23% 17% 3	1620
	11th to 20th	Night	44% 9% 11% 18% 15% 1	1593
	21st to end	Night	39% 9% 13% 26% 11% 2	1745
June	1st to 10th	Night	40% 10% 18% 20% 9% 1	1620
	11th to 20th	Night	37% 10% 20% 21% 11% 1	1620
	21st to end	Night	37% 11% 18% 16% 13% 4	1620
July	1st to 10th	Night	44% 12% 18% 14% 9% 2	1575
	11th to 20th	Night	40% 11% 22% 17% 7% 2	1566
	21st to end	Night	37% 13% 22% 20% 7% 1	1672
August	1st to 10th	Night	43% 11% 21% 17% 8%	1600
	11th to 20th	Night	43% 12% 19% 14% 11% 1	1598
	21st to end	Night	44% 10% 16% 20% 8% 2	1933
September	1st to 10th	Night	40% 9% 13% 20% 14% 4	1980
	11th to 20th	Night	46% 8% 11% 15% 18% 2	1980
	21st to end	Night	41% 5% 11% 20% 19% 5%	1980
October	1st to 10th	Night	38% 4 10% 18% 25% 5%	2029
	11th to 20th	Night	34% 4 8% 24% 22% 8%	2160
	21st to end	Night	30% 3 5% 22% 29% 11%	2376
November	1st to 10th	Night	34% 4 7% 19% 25% 11%	2268
	11th to 20th	Night	33% 4 6% 18% 27% 11%	2470
	21st to end	Night	34% 4 6% 22% 28% 6%	2470
December	1st to 10th	Night	43% 5% 9% 18% 20% 6%	2470
	11th to 20th	Night	33% 4 10% 18% 28% 8%	3044
	21st to end	Night	42% 5% 9% 21% 19% 4	4532

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	22% 18% 35% 19% 7%	270
	11th to 20th	Day	25% 20% 31% 18% 6%	270
	21st to end	Day	23% 16% 41% 17% 4	297
February	1st to 10th	Day	22% 16% 32% 23% 6% 1	270
	11th to 20th	Day	35% 23% 29% 13% 1	270
	21st to end	Day	31% 24% 31% 12% 3	225
March	1st to 10th	Day	21% 15% 29% 25% 9% 1	416
	11th to 20th	Day	25% 12% 30% 26% 6% 1	420
	21st to end	Day	23% 14% 28% 27% 8%	462
April	1st to 10th	Day	20% 14% 28% 24% 13% 2	420
	11th to 20th	Day	16% 16% 26% 23% 17% 3	412
	21st to end	Day	20% 8% 25% 25% 14% 7%	420
May	1st to 10th	Day	13% 13% 28% 26% 14% 6%	423
	11th to 20th	Day	18% 6% 17% 32% 18% 10%	447
	21st to end	Day	17% 4 14% 27% 22% 15%	485
June	1st to 10th	Day	11% 5% 15% 29% 25% 14%	450
	11th to 20th	Day	12% 4 17% 23% 24% 20%	450
	21st to end	Day	11% 2 17% 27% 26% 16%	450
July	1st to 10th	Day	10% 2 8% 22% 31% 26%	445
	11th to 20th	Day	9% 2 7% 25% 32% 25%	444
	21st to end	Day	7% 2 7% 25% 32% 27%	488
August	1st to 10th	Day	17% 2 12% 22% 25% 22%	446
	11th to 20th	Day	12% 2 10% 26% 28% 21%	439
	21st to end	Day	14% 2 10% 22% 28% 23%	495
September	1st to 10th	Day	13% 2 10% 24% 28% 23%	450
	11th to 20th	Day	18% 3 12% 19% 31% 18%	450
	21st to end	Day	20% 4 16% 26% 26% 9%	450
October	1st to 10th	Day	22% 4 15% 26% 22% 12%	450
	11th to 20th	Day	15% 9% 22% 29% 20% 6%	450
	21st to end	Day	20% 6% 18% 36% 17% 4	495
November	1st to 10th	Day	20% 9% 22% 28% 16% 5%	404
	11th to 20th	Day	19% 13% 30% 28% 8% 3	270
	21st to end	Day	11% 13% 33% 28% 13% 2	270
December	1st to 10th	Day	27% 14% 26% 26% 7%	270
	11th to 20th	Day	21% 11% 33% 26% 8% 1	270
	21st to end	Day	25% 11% 36% 20% 7% 1	297
January	1st to 10th	Night	20% 9% 34% 30% 7%	760
	11th to 20th	Night	19% 7% 34% 34% 6%	760
	21st to end	Night	19% 8% 38% 32% 4	836
February	1st to 10th	Night	17% 8% 33% 39% 4	760
	11th to 20th	Night	23% 10% 36% 29% 2	743
	21st to end	Night	18% 11% 38% 30% 3	628
March	1st to 10th	Night	18% 9% 30% 35% 7%	743
	11th to 20th	Night	21% 10% 29% 31% 8% 1	720
	21st to end	Night	20% 7% 31% 33% 9% 1	792
April	1st to 10th	Night	17% 11% 29% 32% 11%	720
	11th to 20th	Night	18% 8% 27% 30% 16% 1	660
	21st to end	Night	16% 5% 23% 35% 20% 1	540
May	1st to 10th	Night	14% 4 22% 38% 21% 1	540
	11th to 20th	Night	18% 3 17% 33% 25% 3	531
	21st to end	Night	18% 1 6% 31% 37% 7%	584
June	1st to 10th	Night	12% 1 9% 33% 38% 6%	540
	11th to 20th	Night	14% 1 9% 29% 34% 14%	540
	21st to end	Night	11% 1 7% 29% 42% 10%	540
July	1st to 10th	Night	12% 2 5% 18% 51% 11%	525
	11th to 20th	Night	10% 1 4 20% 50% 15%	522
	21st to end	Night	8% 1 5% 28% 46% 13%	591
August	1st to 10th	Night	16% 1 6% 24% 41% 12%	537
	11th to 20th	Night	14% 1 5% 27% 41% 12%	534
	21st to end	Night	15% 1 6% 24% 45% 11%	666
September	1st to 10th	Night	13% 2 7% 27% 41% 10%	720
	11th to 20th	Night	17% 4 8% 25% 38% 8%	720
	21st to end	Night	20% 3 11% 33% 29% 3	720
October	1st to 10th	Night	19% 4 14% 26% 28% 8%	720
	11th to 20th	Night	19% 4 21% 33% 20% 2	720
	21st to end	Night	16% 4 20% 39% 18% 3	792
November	1st to 10th	Night	18% 5% 19% 37% 19% 3	740
	11th to 20th	Night	16% 6% 27% 37% 12% 1	760
	21st to end	Night	17% 6% 25% 36% 15% 1	760
December	1st to 10th	Night	22% 8% 26% 36% 8%	760
	11th to 20th	Night	18% 5% 29% 40% 8% 1	760
	21st to end	Night	26% 7% 26% 34% 7%	836

Page generated at 2024-03-03 06:28:58. Version ac07

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