A-T-P
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the A-T-P clear sky chart from 2005-11-06 to 2024-03-08.

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 2 1 3 3 2 4 4 5% 7% 64%	79117
	Night	3 2 1 3 3 2 4 4 5% 7% 66%	80516
Transparency	Day	19% 6% 7% 14% 27% 26%	78843
	Night	19% 6% 7% 15% 29% 25%	80311
Seeing	Day	6% 11% 22% 31% 23% 7%	13715
	Night	4 5% 20% 40% 27% 4	23882

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	2 2 1 3 3 2 3 3 5% 7% 70%	2858
	11th to 20th	Day	1 2 1 3 3 2 2 3 5% 7% 72%	2860
	21st to end	Day	4 4 2 5% 5% 2 4 4 5% 8% 55%	3146
February	1st to 10th	Day	2 2 1 3 3 2 4 4 4 6% 70%	1710
	11th to 20th	Day	3 4 1 3 2 2 3 4 5% 6% 67%	1824
	21st to end	Day	2 3 1 2 3 2 3 3 5% 7% 71%	1727
March	1st to 10th	Day	2 2 1 3 3 3 4 4 5% 8% 66%	2058
	11th to 20th	Day	2 2 1 3 2 1 3 3 4 6% 72%	1980
	21st to end	Day	1 2 1 2 2 2 3 3 5% 7% 73%	2178
April	1st to 10th	Day	1 1 1 2 2 1 2 3 5% 7% 75%	2047
	11th to 20th	Day	2 2 1 1 2 1 2 2 3 4 81%	2131
	21st to end	Day	1 1 1 2 1 2 2 3 5% 82%	2160
May	1st to 10th	Day	1 1 1 2 2 2 3 2 3 5% 77%	2160
	11th to 20th	Day	1 2 1 1 3 2 3 3 4 5% 75%	2288
	21st to end	Day	1 1 1 2 1 2 3 3 5% 81%	2530
June	1st to 10th	Day	1 2 1 2 3 2 3 4 5% 7% 71%	2340
	11th to 20th	Day	2 1 1 2 2 2 3 3 5% 6% 75%	2340
	21st to end	Day	4 3 2 3 6% 3 5% 5% 6% 8% 55%	2340
July	1st to 10th	Day	5% 3 2 4 5% 5% 6% 7% 9% 11% 44%	2340
	11th to 20th	Day	4 3 2 5% 7% 5% 8% 9% 9% 11% 37%	2340
	21st to end	Day	8% 5% 2 7% 7% 5% 8% 9% 10% 10% 30%	2545
August	1st to 10th	Day	5% 3 2 5% 5% 6% 7% 8% 10% 11% 37%	2328
	11th to 20th	Day	4 5% 2 6% 6% 5% 8% 7% 8% 11% 37%	2162
	21st to end	Day	6% 4 2 5% 5% 4 6% 8% 9% 11% 40%	2376
September	1st to 10th	Day	6% 4 3 5% 6% 3 6% 6% 7% 7% 47%	2154
	11th to 20th	Day	4 2 2 3 5% 3 4 6% 6% 8% 58%	1980
	21st to end	Day	4 3 1 3 3 2 3 4 5% 6% 66%	1980
October	1st to 10th	Day	3 2 1 2 3 2 3 3 5% 7% 68%	1800
	11th to 20th	Day	4 2 1 2 3 2 3 3 3 4 74%	1800
	21st to end	Day	1 2 1 2 1 2 3 4 4 80%	1980
November	1st to 10th	Day	1 1 1 2 3 1 2 3 3 6% 78%	1859
	11th to 20th	Day	2 2 1 2 3 2 2 3 5% 5% 73%	1900
	21st to end	Day	4 3 1 2 3 2 4 4 4 6% 65%	1871
December	1st to 10th	Day	4 2 1 3 3 2 4 5% 5% 7% 65%	1710
	11th to 20th	Day	2 4 2 4 5% 2 5% 4 4 7% 61%	2169
	21st to end	Day	2 2 1 3 3 2 4 4 5% 7% 69%	3146
January	1st to 10th	Night	2 3 1 3 3 2 3 4 5% 6% 69%	4097
	11th to 20th	Night	1 1 1 2 2 1 2 2 4 5% 79%	4110
	21st to end	Night	6% 4 2 4 5% 2 4 5% 6% 8% 53%	4521
February	1st to 10th	Night	1 1 1 2 3 2 4 3 6% 6% 71%	2470
	11th to 20th	Night	3 2 1 3 3 1 3 3 4 5% 72%	2238
	21st to end	Night	1 2 1 3 1 4 4 5% 8% 72%	1727
March	1st to 10th	Night	2 2 1 2 3 2 3 4 6% 7% 69%	2062
	11th to 20th	Night	2 2 1 2 2 2 3 4 4 6% 73%	1980
	21st to end	Night	2 1 1 2 2 2 3 4 4 6% 74%	2178
April	1st to 10th	Night	1 2 1 1 2 1 3 3 4 7% 76%	1912
	11th to 20th	Night	1 2 1 2 2 1 2 3 3 6% 76%	1777
	21st to end	Night	1 1 1 1 2 2 2 2 3 5% 81%	1800
May	1st to 10th	Night	1 1 1 1 3 1 3 3 4 5% 78%	1800
	11th to 20th	Night	3 1 2 2 1 2 3 4 6% 75%	1603
	21st to end	Night	1 1 2 1 1 2 2 3 4 83%	1746
June	1st to 10th	Night	3 2 1 3 2 1 2 4 4 5% 73%	1620
	11th to 20th	Night	1 1 1 2 2 2 3 3 4 5% 76%	1620
	21st to end	Night	3 3 2 3 4 3 5% 7% 8% 8% 54%	1620
July	1st to 10th	Night	4 4 1 5% 5% 4 8% 7% 8% 11% 44%	1620
	11th to 20th	Night	3 4 3 6% 8% 6% 8% 10% 10% 13% 28%	1620
	21st to end	Night	4 5% 2 7% 8% 6% 9% 11% 10% 14% 25%	1773
August	1st to 10th	Night	3 4 2 6% 6% 4 8% 8% 9% 13% 36%	1611
	11th to 20th	Night	4 4 2 5% 6% 4 6% 8% 9% 11% 41%	1736
	21st to end	Night	5% 4 2 5% 6% 6% 6% 7% 10% 9% 39%	1980
September	1st to 10th	Night	6% 5% 2 5% 5% 4 5% 7% 7% 10% 43%	1801
	11th to 20th	Night	5% 3 2 2 3 3 4 5% 6% 7% 60%	1980
	21st to end	Night	2 3 1 3 3 2 3 4 4 6% 68%	1980
October	1st to 10th	Night	3 2 1 3 3 2 3 4 4 6% 70%	2160
	11th to 20th	Night	3 1 1 2 2 1 3 4 4 5% 76%	2160
	21st to end	Night	1 1 1 2 2 1 2 3 3 4 82%	2376
November	1st to 10th	Night	2 1 1 1 2 1 2 3 4 6% 79%	2220
	11th to 20th	Night	2 2 1 2 2 2 3 3 3 5% 75%	2280
	21st to end	Night	3 2 1 3 3 2 3 3 4 7% 68%	2306
December	1st to 10th	Night	3 2 2 3 2 2 3 5% 5% 6% 67%	2470
	11th to 20th	Night	2 2 1 3 4 3 5% 5% 6% 7% 63%	3041
	21st to end	Night	3 3 1 3 3 2 3 4 6% 6% 67%	4521

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	16% 5% 6% 12% 39% 22%	2859
	11th to 20th	Day	13% 6% 5% 15% 43% 18%	2860
	21st to end	Day	25% 8% 6% 21% 27% 13%	3146
February	1st to 10th	Day	17% 4 4 14% 28% 32%	1710
	11th to 20th	Day	19% 4 6% 13% 29% 28%	1833
	21st to end	Day	17% 3 4 12% 27% 37%	1727
March	1st to 10th	Day	19% 4 5% 12% 32% 29%	2058
	11th to 20th	Day	16% 3 5% 11% 32% 33%	1980
	21st to end	Day	13% 3 7% 11% 35% 30%	2178
April	1st to 10th	Day	11% 2 4 10% 39% 34%	2047
	11th to 20th	Day	9% 2 3 8% 30% 48%	2134
	21st to end	Day	9% 2 3 10% 31% 44%	2160
May	1st to 10th	Day	12% 2 4 8% 23% 50%	2160
	11th to 20th	Day	13% 3 5% 10% 29% 39%	2291
	21st to end	Day	9% 2 4 6% 27% 51%	2530
June	1st to 10th	Day	13% 4 4 7% 29% 43%	2340
	11th to 20th	Day	12% 3 5% 6% 21% 54%	2340
	21st to end	Day	24% 6% 8% 12% 20% 30%	2340
July	1st to 10th	Day	29% 10% 12% 17% 17% 14%	2276
	11th to 20th	Day	32% 14% 19% 21% 11% 4	2271
	21st to end	Day	39% 15% 19% 17% 10% 1	2404
August	1st to 10th	Day	33% 13% 16% 20% 13% 4	2313
	11th to 20th	Day	36% 12% 15% 17% 15% 6%	2162
	21st to end	Day	33% 11% 12% 20% 15% 8%	2376
September	1st to 10th	Day	33% 9% 11% 14% 19% 14%	2154
	11th to 20th	Day	24% 6% 8% 18% 25% 19%	1980
	21st to end	Day	18% 5% 7% 18% 32% 20%	1980
October	1st to 10th	Day	15% 5% 8% 17% 27% 27%	1800
	11th to 20th	Day	17% 3 4 14% 27% 35%	1800
	21st to end	Day	10% 2 4 17% 34% 32%	1980
November	1st to 10th	Day	11% 3 5% 18% 37% 27%	1859
	11th to 20th	Day	13% 4 5% 18% 33% 26%	1900
	21st to end	Day	21% 4 4 18% 29% 23%	1871
December	1st to 10th	Day	19% 4 7% 22% 28% 19%	1710
	11th to 20th	Day	26% 5% 6% 19% 29% 15%	2168
	21st to end	Day	16% 4 8% 16% 41% 15%	3146
January	1st to 10th	Night	17% 5% 7% 13% 41% 18%	4097
	11th to 20th	Night	11% 4 3 20% 47% 16%	4110
	21st to end	Night	30% 7% 7% 20% 25% 12%	4521
February	1st to 10th	Night	15% 4 6% 15% 27% 32%	2470
	11th to 20th	Night	17% 4 4 16% 30% 28%	2239
	21st to end	Night	14% 5% 6% 11% 28% 35%	1727
March	1st to 10th	Night	16% 3 5% 14% 32% 29%	2062
	11th to 20th	Night	14% 4 5% 12% 32% 33%	1980
	21st to end	Night	14% 3 6% 11% 38% 28%	2178
April	1st to 10th	Night	11% 2 4 13% 36% 33%	1912
	11th to 20th	Night	12% 2 4 11% 30% 41%	1777
	21st to end	Night	10% 2 4 10% 34% 40%	1800
May	1st to 10th	Night	11% 3 3 8% 27% 47%	1800
	11th to 20th	Night	13% 4 5% 12% 26% 39%	1604
	21st to end	Night	9% 2 3 8% 29% 49%	1746
June	1st to 10th	Night	14% 3 4 8% 29% 42%	1620
	11th to 20th	Night	12% 3 5% 6% 21% 54%	1620
	21st to end	Night	24% 8% 9% 12% 18% 30%	1620
July	1st to 10th	Night	29% 10% 14% 19% 14% 13%	1575
	11th to 20th	Night	38% 16% 18% 18% 7% 3	1566
	21st to end	Night	41% 17% 22% 13% 7% 1	1674
August	1st to 10th	Night	33% 14% 18% 20% 11% 3	1602
	11th to 20th	Night	34% 12% 16% 17% 15% 5%	1736
	21st to end	Night	34% 12% 17% 18% 12% 7%	1980
September	1st to 10th	Night	32% 12% 13% 13% 17% 12%	1801
	11th to 20th	Night	23% 7% 9% 18% 25% 19%	1980
	21st to end	Night	18% 5% 7% 17% 35% 18%	1980
October	1st to 10th	Night	17% 5% 9% 15% 26% 27%	2160
	11th to 20th	Night	13% 4 5% 16% 25% 38%	2160
	21st to end	Night	9% 3 4 15% 35% 35%	2376
November	1st to 10th	Night	9% 4 5% 18% 37% 27%	2220
	11th to 20th	Night	14% 3 5% 18% 33% 27%	2280
	21st to end	Night	19% 5% 6% 17% 29% 24%	2306
December	1st to 10th	Night	20% 5% 6% 21% 28% 20%	2470
	11th to 20th	Night	21% 7% 8% 19% 30% 15%	3041
	21st to end	Night	19% 4 6% 16% 35% 21%	4521

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	10% 15% 26% 35% 12% 1	270
	11th to 20th	Day	8% 7% 27% 39% 16% 3	270
	21st to end	Day	7% 13% 26% 38% 14% 2	297
February	1st to 10th	Day	6% 16% 24% 30% 20% 3	270
	11th to 20th	Day	15% 18% 31% 24% 11%	347
	21st to end	Day	13% 29% 33% 20% 6%	382
March	1st to 10th	Day	7% 26% 33% 25% 7% 2	449
	11th to 20th	Day	8% 22% 29% 27% 11% 3	420
	21st to end	Day	8% 28% 31% 21% 9% 2	462
April	1st to 10th	Day	4 18% 36% 25% 13% 3	420
	11th to 20th	Day	5% 25% 33% 26% 10% 2	412
	21st to end	Day	4 21% 35% 27% 10% 3	420
May	1st to 10th	Day	7% 20% 31% 26% 13% 3	423
	11th to 20th	Day	4 13% 31% 32% 17% 3	448
	21st to end	Day	3 11% 35% 35% 14% 2	485
June	1st to 10th	Day	5% 7% 26% 37% 22% 5%	450
	11th to 20th	Day	2 5% 22% 40% 26% 5%	450
	21st to end	Day	6% 3 13% 41% 31% 6%	450
July	1st to 10th	Day	5% 2 8% 34% 42% 9%	446
	11th to 20th	Day	6% 1 5% 25% 47% 16%	444
	21st to end	Day	7% 1 6% 25% 41% 21%	487
August	1st to 10th	Day	5% 1 5% 22% 41% 25%	447
	11th to 20th	Day	5% 2 6% 32% 41% 14%	437
	21st to end	Day	6% 6% 25% 39% 23%	495
September	1st to 10th	Day	8% 3 11% 34% 34% 10%	450
	11th to 20th	Day	4 2 11% 36% 36% 11%	450
	21st to end	Day	5% 7% 18% 36% 24% 10%	450
October	1st to 10th	Day	4 6% 20% 33% 26% 11%	270
	11th to 20th	Day	9% 6% 20% 34% 22% 9%	270
	21st to end	Day	4 8% 21% 31% 26% 9%	297
November	1st to 10th	Day	5% 10% 27% 35% 18% 6%	270
	11th to 20th	Day	2 6% 25% 36% 27% 4	270
	21st to end	Day	8% 19% 26% 31% 15% 1	270
December	1st to 10th	Day	8% 11% 24% 33% 20% 3	270
	11th to 20th	Day	6% 14% 29% 39% 11% 1	270
	21st to end	Day	10% 14% 28% 35% 9% 3	297
January	1st to 10th	Night	5% 5% 24% 44% 19% 3	760
	11th to 20th	Night	5% 4 22% 45% 21% 3	760
	21st to end	Night	7% 7% 25% 42% 16% 2	836
February	1st to 10th	Night	3 9% 26% 39% 21% 3	760
	11th to 20th	Night	6% 10% 28% 42% 14% 1	744
	21st to end	Night	3 12% 35% 38% 11% 1	628
March	1st to 10th	Night	4 11% 31% 41% 12% 1	750
	11th to 20th	Night	5% 8% 28% 37% 19% 3	720
	21st to end	Night	3 10% 27% 42% 16% 2	792
April	1st to 10th	Night	2 6% 33% 43% 14% 2	652
	11th to 20th	Night	3 10% 32% 39% 16% 1	533
	21st to end	Night	1 10% 31% 44% 12% 2	540
May	1st to 10th	Night	2 7% 30% 45% 16% 1	540
	11th to 20th	Night	3 5% 25% 47% 19% 1	531
	21st to end	Night	3 4 24% 49% 20%	584
June	1st to 10th	Night	6% 1 14% 54% 23% 2	540
	11th to 20th	Night	2 2 17% 52% 26% 1	540
	21st to end	Night	4 2 9% 44% 40% 1	540
July	1st to 10th	Night	6% 1 6% 40% 42% 5%	525
	11th to 20th	Night	5% 1 6% 32% 50% 6%	522
	21st to end	Night	5% 1 4 28% 53% 8%	591
August	1st to 10th	Night	5% 1 5% 22% 55% 12%	537
	11th to 20th	Night	7% 1 5% 24% 55% 8%	534
	21st to end	Night	6% 1 7% 24% 54% 9%	594
September	1st to 10th	Night	8% 2 6% 33% 48% 4	541
	11th to 20th	Night	6% 1 7% 33% 47% 6%	720
	21st to end	Night	4 2 13% 41% 36% 5%	720
October	1st to 10th	Night	3 2 17% 43% 31% 4	720
	11th to 20th	Night	4 3 16% 43% 29% 4	720
	21st to end	Night	3 4 13% 41% 32% 6%	792
November	1st to 10th	Night	2 4 18% 43% 27% 6%	740
	11th to 20th	Night	3 4 18% 44% 27% 3	760
	21st to end	Night	6% 6% 26% 39% 20% 4	760
December	1st to 10th	Night	6% 4 22% 43% 21% 3	760
	11th to 20th	Night	3 7% 25% 43% 19% 2	760
	21st to end	Night	6% 8% 25% 40% 16% 4	836

Page generated at 2024-03-05 05:24:03. Version ac07

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