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 2026-09-29.

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	2 2 1 3 3 3 4 3 6% 7% 66%	157260
	Night	2 2 1 4 3 3 4 4 5% 7% 65%	151795
Transparency	Day	16% 6% 8% 15% 27% 29%	152211
	Night	18% 6% 7% 14% 27% 28%	147851
Seeing	Day	6% 11% 22% 31% 22% 7%	17177
	Night	4 5% 21% 41% 26% 3	27150

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 4 1 4 2 3 3 3 5% 6% 66%	5318
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 4 84%	5340
	21st to end	Day	4 3 2 5% 3 3 4 3 5% 7% 61%	5844
February	1st to 10th	Day	2 2 1 3 3 2 4 3 4 6% 70%	1890
	11th to 20th	Day	3 4 1 3 2 3 3 3 5% 6% 67%	2023
	21st to end	Day	2 3 1 2 2 3 2 3 4 7% 72%	1903
March	1st to 10th	Day	1 1 1 3 2 3 3 3 5% 6% 72%	5064
	11th to 20th	Day	2 3 1 3 2 2 3 3 5% 7% 70%	5110
	21st to end	Day	1 2 1 3 3 3 4 3 6% 8% 66%	5607
April	1st to 10th	Day	1 2 2 1 2 2 3 5% 7% 74%	2389
	11th to 20th	Day	2 1 1 1 2 2 2 3 5% 80%	2491
	21st to end	Day	1 2 2 1 2 2 2 3 4 81%	2520
May	1st to 10th	Day	1 2 1 2 1 2 2 2 3 4 81%	7057
	11th to 20th	Day	1 1 1 2 2 2 4 5% 80%	7571
	21st to end	Day	1 1 2 1 2 2 2 3 4 83%	8370
June	1st to 10th	Day	1 2 1 2 2 2 3 3 5% 7% 70%	2730
	11th to 20th	Day	1 1 1 2 2 3 2 2 4 6% 75%	2730
	21st to end	Day	3 3 2 4 5% 5% 5% 4 7% 8% 55%	2730
July	1st to 10th	Day	2 2 1 3 3 5% 6% 5% 8% 9% 56%	7649
	11th to 20th	Day	2 2 1 5% 5% 6% 7% 6% 9% 12% 45%	7650
	21st to end	Day	3 3 1 5% 4 5% 7% 6% 10% 12% 43%	8304
August	1st to 10th	Day	2 2 1 4 3 5% 5% 5% 8% 11% 54%	7549
	11th to 20th	Day	2 2 1 4 4 5% 6% 5% 9% 12% 50%	7118
	21st to end	Day	2 2 1 5% 4 5% 6% 5% 8% 10% 51%	7751
September	1st to 10th	Day	5% 4 2 6% 5% 4 6% 5% 8% 8% 47%	2512
	11th to 20th	Day	4 2 2 4 4 4 5% 4 6% 7% 57%	2310
	21st to end	Day	4 3 1 4 2 3 3 3 4 6% 67%	2288
October	1st to 10th	Day	2 1 1 2 1 2 2 2 4 5% 79%	3270
	11th to 20th	Day	2 2 2 2 2 2 1 3 3 81%	3250
	21st to end	Day	1 1 1 1 1 1 1 2 2 88%	3548
November	1st to 10th	Day	1 1 1 2 2 2 2 2 3 6% 78%	2059
	11th to 20th	Day	2 2 1 3 2 2 3 3 5% 5% 74%	2100
	21st to end	Day	4 3 1 3 2 3 4 3 4 7% 65%	2067
December	1st to 10th	Day	3 4 1 3 2 2 3 3 4 4 71%	3040
	11th to 20th	Day	1 3 1 4 3 2 3 2 3 6% 72%	3499
	21st to end	Day	1 2 1 2 2 2 3 2 5% 6% 74%	4609
January	1st to 10th	Night	3 4 2 4 4 3 4 3 5% 6% 62%	7605
	11th to 20th	Night	1 1 1 1 1 1 1 3 4 86%	7650
	21st to end	Night	4 4 2 5% 3 3 4 4 6% 7% 59%	8370
February	1st to 10th	Night	1 1 1 2 2 3 3 3 6% 6% 71%	2730
	11th to 20th	Night	3 3 1 3 3 3 2 3 4 5% 71%	2478
	21st to end	Night	1 2 2 2 2 3 3 5% 7% 73%	1903
March	1st to 10th	Night	1 2 1 2 2 2 3 3 5% 6% 75%	5050
	11th to 20th	Night	2 3 1 2 2 2 3 3 4 6% 73%	5110
	21st to end	Night	1 2 1 3 2 3 4 3 4 6% 70%	5610
April	1st to 10th	Night	1 2 1 1 2 2 3 3 4 6% 75%	2230
	11th to 20th	Night	1 2 1 2 2 2 2 3 3 5% 77%	2077
	21st to end	Night	1 1 1 1 2 2 2 2 3 5% 81%	2100
May	1st to 10th	Night	2 1 1 2 2 2 2 2 3 4 79%	5890
	11th to 20th	Night	1 1 1 3 2 2 2 2 3 5% 79%	5305
	21st to end	Night	1 1 2 1 2 2 1 3 4 83%	5837
June	1st to 10th	Night	3 2 1 3 2 2 3 2 4 5% 73%	1890
	11th to 20th	Night	2 2 1 2 2 3 2 2 4 5% 75%	1890
	21st to end	Night	3 3 2 4 4 4 5% 5% 8% 9% 53%	1890
July	1st to 10th	Night	2 3 1 6% 5% 5% 6% 6% 8% 11% 46%	5340
	11th to 20th	Night	3 5% 2 8% 8% 8% 8% 8% 9% 12% 29%	5340
	21st to end	Night	3 4 2 7% 6% 7% 8% 8% 11% 14% 29%	5805
August	1st to 10th	Night	3 4 2 8% 7% 7% 9% 7% 9% 12% 33%	5273
	11th to 20th	Night	2 3 2 7% 6% 7% 7% 7% 10% 14% 35%	5735
	21st to end	Night	2 3 2 6% 6% 6% 7% 6% 11% 12% 40%	6465
September	1st to 10th	Night	6% 5% 2 7% 4 6% 6% 5% 7% 9% 43%	2102
	11th to 20th	Night	5% 3 1 3 4 4 4 4 6% 7% 59%	2310
	21st to end	Night	2 2 1 4 3 3 3 3 3 6% 68%	2288
October	1st to 10th	Night	2 1 2 2 2 3 2 3 4 80%	3920
	11th to 20th	Night	2 1 1 2 1 2 3 2 4 5% 78%	3898
	21st to end	Night	1 1 1 1 1 2 3 4 87%	4252
November	1st to 10th	Night	2 1 1 1 1 2 2 2 3 6% 80%	2460
	11th to 20th	Night	2 3 1 2 2 2 2 2 4 5% 74%	2520
	21st to end	Night	3 2 1 3 3 3 3 2 4 7% 68%	2550
December	1st to 10th	Night	3 3 1 3 1 2 2 2 3 4 76%	4370
	11th to 20th	Night	1 2 1 2 3 3 4 3 5% 5% 71%	4941
	21st to end	Night	2 2 1 2 2 2 3 3 5% 6% 73%	6611

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	20% 4 5% 16% 35% 20%	5321
	11th to 20th	Day	7% 3 3 9% 43% 35%	5340
	21st to end	Day	22% 6% 5% 14% 28% 25%	5844
February	1st to 10th	Day	17% 4 4 13% 29% 33%	1890
	11th to 20th	Day	19% 4 6% 14% 30% 26%	2032
	21st to end	Day	16% 3 4 12% 28% 38%	1903
March	1st to 10th	Day	12% 4 5% 7% 33% 38%	5064
	11th to 20th	Day	11% 5% 6% 15% 33% 29%	5110
	21st to end	Day	14% 6% 8% 11% 34% 28%	5607
April	1st to 10th	Day	11% 3 4 10% 37% 34%	2389
	11th to 20th	Day	10% 2 3 8% 29% 48%	2494
	21st to end	Day	10% 2 3 9% 30% 46%	2520
May	1st to 10th	Day	8% 3 4 9% 27% 50%	7057
	11th to 20th	Day	7% 2 4 10% 27% 48%	7574
	21st to end	Day	7% 2 3 5% 17% 66%	8371
June	1st to 10th	Day	13% 4 4 7% 27% 44%	2730
	11th to 20th	Day	11% 3 5% 6% 20% 55%	2730
	21st to end	Day	24% 7% 8% 12% 19% 29%	2730
July	1st to 10th	Day	19% 8% 10% 16% 26% 20%	7586
	11th to 20th	Day	25% 13% 20% 22% 16% 4	7581
	21st to end	Day	26% 12% 18% 24% 15% 5%	8120
August	1st to 10th	Day	23% 10% 14% 30% 19% 4	5978
	11th to 20th	Day	22% 9% 13% 27% 23% 5%	5611
	21st to end	Day	24% 10% 13% 21% 22% 10%	6078
September	1st to 10th	Day	31% 9% 11% 15% 19% 14%	2512
	11th to 20th	Day	24% 7% 9% 17% 24% 19%	2310
	21st to end	Day	17% 5% 7% 19% 31% 21%	2288
October	1st to 10th	Day	9% 3 5% 10% 22% 50%	3270
	11th to 20th	Day	12% 2 3 11% 30% 42%	3250
	21st to end	Day	6% 1 3 10% 34% 46%	3548
November	1st to 10th	Day	10% 3 5% 17% 36% 29%	2059
	11th to 20th	Day	13% 4 5% 17% 33% 27%	2100
	21st to end	Day	20% 4 5% 19% 30% 22%	2067
December	1st to 10th	Day	17% 4 5% 16% 34% 23%	3040
	11th to 20th	Day	18% 3 4 12% 36% 26%	3498
	21st to end	Day	13% 3 6% 16% 41% 20%	4609
January	1st to 10th	Night	20% 6% 6% 16% 36% 15%	7605
	11th to 20th	Night	6% 2 2 12% 40% 37%	7650
	21st to end	Night	25% 6% 5% 14% 24% 27%	8370
February	1st to 10th	Night	15% 4 6% 14% 26% 35%	2730
	11th to 20th	Night	18% 4 5% 16% 31% 26%	2479
	21st to end	Night	13% 4 6% 11% 30% 36%	1903
March	1st to 10th	Night	11% 4 4 9% 33% 39%	5050
	11th to 20th	Night	9% 4 6% 18% 36% 26%	5110
	21st to end	Night	14% 4 7% 16% 32% 26%	5610
April	1st to 10th	Night	12% 3 5% 13% 35% 32%	2230
	11th to 20th	Night	12% 2 4 10% 30% 43%	2077
	21st to end	Night	10% 2 3 10% 32% 43%	2100
May	1st to 10th	Night	10% 4 4 9% 29% 44%	5888
	11th to 20th	Night	9% 3 4 10% 25% 49%	5306
	21st to end	Night	8% 2 2 5% 19% 63%	5838
June	1st to 10th	Night	14% 4 4 8% 27% 43%	1890
	11th to 20th	Night	13% 3 5% 6% 20% 54%	1890
	21st to end	Night	24% 7% 9% 12% 18% 29%	1890
July	1st to 10th	Night	27% 9% 12% 16% 20% 16%	5295
	11th to 20th	Night	41% 14% 15% 17% 10% 4	5286
	21st to end	Night	38% 14% 20% 15% 9% 4	5677
August	1st to 10th	Night	41% 12% 14% 19% 11% 4	4168
	11th to 20th	Night	29% 12% 16% 23% 17% 3	4518
	21st to end	Night	31% 11% 16% 17% 16% 9%	5069
September	1st to 10th	Night	32% 11% 13% 13% 17% 13%	2102
	11th to 20th	Night	23% 7% 9% 18% 24% 18%	2310
	21st to end	Night	19% 5% 7% 16% 34% 20%	2288
October	1st to 10th	Night	11% 3 6% 9% 22% 49%	3920
	11th to 20th	Night	11% 4 4 13% 23% 44%	3898
	21st to end	Night	5% 2 3 9% 31% 50%	4252
November	1st to 10th	Night	8% 3 5% 17% 37% 30%	2460
	11th to 20th	Night	14% 3 5% 18% 33% 28%	2520
	21st to end	Night	19% 5% 6% 18% 30% 23%	2550
December	1st to 10th	Night	14% 4 4 16% 40% 21%	4370
	11th to 20th	Night	16% 5% 5% 12% 35% 27%	4941
	21st to end	Night	15% 4 5% 15% 38% 24%	6611

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% 28% 33% 12% 2	330
	11th to 20th	Day	6% 8% 28% 39% 15% 3	330
	21st to end	Day	10% 13% 28% 35% 12% 2	363
February	1st to 10th	Day	5% 16% 26% 30% 19% 3	330
	11th to 20th	Day	17% 19% 30% 24% 10%	416
	21st to end	Day	11% 26% 34% 20% 8%	446
March	1st to 10th	Day	9% 25% 34% 24% 7% 2	540
	11th to 20th	Day	10% 21% 29% 26% 11% 3	540
	21st to end	Day	9% 25% 30% 24% 10% 3	594
April	1st to 10th	Day	6% 19% 33% 25% 14% 3	540
	11th to 20th	Day	6% 26% 32% 25% 10% 2	532
	21st to end	Day	6% 24% 32% 26% 9% 3	540
May	1st to 10th	Day	7% 20% 33% 25% 12% 3	543
	11th to 20th	Day	6% 17% 30% 29% 15% 3	568
	21st to end	Day	3 12% 36% 34% 13% 2	617
June	1st to 10th	Day	4 7% 26% 38% 20% 4	570
	11th to 20th	Day	2 6% 23% 41% 25% 5%	570
	21st to end	Day	5% 4 15% 39% 31% 6%	570
July	1st to 10th	Day	4 2 10% 34% 40% 10%	566
	11th to 20th	Day	5% 1 6% 28% 46% 14%	564
	21st to end	Day	6% 1 6% 26% 42% 18%	619
August	1st to 10th	Day	4 1 5% 25% 42% 23%	567
	11th to 20th	Day	4 2 6% 34% 41% 14%	557
	21st to end	Day	5% 1 6% 29% 38% 21%	627
September	1st to 10th	Day	7% 2 12% 35% 34% 9%	570
	11th to 20th	Day	4 4 13% 35% 35% 9%	570
	21st to end	Day	5% 7% 19% 37% 22% 10%	562
October	1st to 10th	Day	3 6% 21% 32% 26% 12%	330
	11th to 20th	Day	9% 8% 21% 33% 20% 9%	330
	21st to end	Day	4 7% 21% 32% 26% 10%	363
November	1st to 10th	Day	5% 10% 26% 34% 19% 6%	330
	11th to 20th	Day	4 8% 25% 33% 26% 5%	330
	21st to end	Day	7% 18% 26% 33% 15% 2	330
December	1st to 10th	Day	7% 10% 25% 35% 20% 3	330
	11th to 20th	Day	5% 12% 28% 37% 16% 3	330
	21st to end	Day	10% 13% 27% 35% 11% 4	363
January	1st to 10th	Night	5% 5% 26% 43% 19% 3	840
	11th to 20th	Night	4 4 23% 46% 20% 3	840
	21st to end	Night	7% 8% 26% 42% 15% 2	924
February	1st to 10th	Night	3 8% 25% 42% 20% 3	840
	11th to 20th	Night	6% 11% 28% 41% 13% 1	824
	21st to end	Night	3 12% 34% 40% 11% 1	692
March	1st to 10th	Night	5% 12% 30% 41% 11% 1	840
	11th to 20th	Night	5% 8% 30% 37% 18% 2	840
	21st to end	Night	3 10% 27% 42% 17% 2	924
April	1st to 10th	Night	3 7% 34% 42% 14% 1	760
	11th to 20th	Night	3 11% 33% 39% 14% 1	623
	21st to end	Night	1 10% 34% 42% 11% 2	630
May	1st to 10th	Night	2 8% 31% 44% 14% 1	630
	11th to 20th	Night	3 6% 27% 47% 17% 1	621
	21st to end	Night	2 5% 27% 48% 18%	683
June	1st to 10th	Night	5% 2 16% 55% 20% 2	630
	11th to 20th	Night	2 2 19% 52% 24% 1	630
	21st to end	Night	4 2 11% 45% 37% 1	630
July	1st to 10th	Night	5% 2 7% 42% 39% 4	615
	11th to 20th	Night	5% 1 9% 33% 46% 5%	612
	21st to end	Night	5% 2 6% 31% 49% 7%	690
August	1st to 10th	Night	5% 2 7% 25% 51% 11%	627
	11th to 20th	Night	6% 2 6% 27% 52% 7%	624
	21st to end	Night	5% 1 7% 27% 53% 8%	693
September	1st to 10th	Night	7% 2 8% 36% 44% 3	632
	11th to 20th	Night	6% 2 10% 34% 43% 5%	840
	21st to end	Night	4 2 13% 42% 34% 5%	832
October	1st to 10th	Night	3 2 16% 44% 31% 3	800
	11th to 20th	Night	5% 4 18% 43% 28% 4	800
	21st to end	Night	3 4 13% 43% 32% 6%	880
November	1st to 10th	Night	2 4 18% 43% 27% 6%	820
	11th to 20th	Night	4 5% 19% 43% 26% 3	840
	21st to end	Night	5% 6% 27% 40% 20% 3	840
December	1st to 10th	Night	6% 5% 23% 44% 20% 3	840
	11th to 20th	Night	3 6% 24% 44% 21% 2	840
	21st to end	Night	6% 8% 25% 41% 16% 4	924

Page generated at 2026-09-26 02:18:45. Version ac07

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