Pool Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Pool Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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 4 2 4 4 4 5% 4 6% 8% 57%	146935
	Night	3 5% 2 6% 4 5% 5% 4 6% 7% 52%	143308
Transparency	Day	20% 7% 13% 23% 24% 13%	141810
	Night	24% 8% 15% 24% 20% 9%	139442
Seeing	Day	5% 8% 21% 33% 25% 8%	17644
	Night	6% 4 19% 43% 25% 3	27769

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 6% 3 9% 6% 5% 7% 5% 8% 9% 40%	5140
	11th to 20th	Day	2 4 2 6% 5% 6% 7% 5% 6% 6% 51%	5340
	21st to end	Day	2 5% 2 5% 4 4 6% 5% 7% 8% 53%	5844
February	1st to 10th	Day	2 6% 3 6% 5% 5% 5% 4 6% 9% 50%	1890
	11th to 20th	Day	3 7% 2 7% 5% 5% 6% 4 6% 8% 46%	1855
	21st to end	Day	3 7% 3 6% 5% 6% 5% 4 7% 6% 50%	1740
March	1st to 10th	Day	2 6% 3 5% 4 4 6% 4 6% 7% 53%	5064
	11th to 20th	Day	2 8% 3 7% 4 4 5% 4 6% 8% 48%	5110
	21st to end	Day	3 6% 2 7% 6% 6% 6% 5% 7% 8% 45%	5607
April	1st to 10th	Day	3 4 2 5% 4 4 4 4 6% 7% 58%	2513
	11th to 20th	Day	3 5% 3 6% 4 4 5% 5% 6% 7% 53%	2491
	21st to end	Day	2 3 1 5% 3 4 5% 5% 6% 9% 56%	2520
May	1st to 10th	Day	3 3 1 5% 3 4 5% 4 7% 9% 55%	7625
	11th to 20th	Day	2 3 1 3 3 4 5% 4 6% 9% 59%	7691
	21st to end	Day	2 3 1 4 3 3 3 3 6% 9% 64%	9026
June	1st to 10th	Day	2 2 1 3 3 3 3 3 6% 9% 64%	2940
	11th to 20th	Day	2 2 1 3 3 3 3 3 5% 6% 69%	2940
	21st to end	Day	1 2 1 2 3 3 4 3 5% 7% 70%	2940
July	1st to 10th	Day	2 1 1 3 2 3 4 3 4 5% 73%	6464
	11th to 20th	Day	1 2 2 3 3 5% 6% 10% 12% 57%	5946
	21st to end	Day	2 2 1 3 3 3 4 4 7% 10% 61%	6401
August	1st to 10th	Day	2 2 1 3 3 4 5% 5% 9% 12% 54%	5779
	11th to 20th	Day	2 2 1 3 3 4 4 4 7% 10% 61%	5710
	21st to end	Day	4 3 1 5% 4 4 6% 5% 8% 11% 49%	5984
September	1st to 10th	Day	4 3 1 4 3 3 3 4 5% 7% 63%	2400
	11th to 20th	Day	3 2 1 3 2 3 4 4 5% 8% 65%	2360
	21st to end	Day	4 3 1 3 2 3 3 2 4 6% 70%	2125
October	1st to 10th	Day	4 2 1 2 1 1 2 2 3 5% 77%	3270
	11th to 20th	Day	4 3 1 5% 3 4 5% 3 7% 10% 55%	3250
	21st to end	Day	5% 3 1 5% 3 4 4 3 4 6% 63%	3548
November	1st to 10th	Day	1 3 2 3 3 2 3 4 6% 7% 66%	2033
	11th to 20th	Day	3 4 1 4 3 3 4 3 5% 7% 63%	1811
	21st to end	Day	2 5% 2 5% 4 3 4 4 5% 6% 61%	1680
December	1st to 10th	Day	2 5% 2 5% 4 5% 5% 4 5% 6% 56%	2700
	11th to 20th	Day	2 4 2 6% 5% 6% 8% 5% 8% 8% 47%	3106
	21st to end	Day	3 8% 3 7% 5% 7% 6% 5% 6% 8% 43%	4092
January	1st to 10th	Night	3 9% 4 11% 7% 7% 8% 5% 7% 7% 33%	7824
	11th to 20th	Night	3 7% 2 6% 5% 7% 8% 5% 8% 8% 41%	7650
	21st to end	Night	3 8% 3 7% 4 5% 7% 7% 8% 8% 41%	8370
February	1st to 10th	Night	3 5% 2 7% 4 4 6% 4 6% 7% 51%	2730
	11th to 20th	Night	4 9% 3 8% 5% 4 4 3 6% 8% 47%	2651
	21st to end	Night	3 6% 2 5% 5% 5% 5% 3 5% 7% 52%	2060
March	1st to 10th	Night	3 7% 2 6% 4 6% 7% 5% 6% 7% 47%	5050
	11th to 20th	Night	4 8% 2 7% 4 5% 5% 4 5% 6% 50%	5110
	21st to end	Night	4 8% 3 7% 5% 5% 6% 5% 6% 7% 44%	5610
April	1st to 10th	Night	5% 5% 2 6% 4 4 4 6% 6% 6% 53%	2106
	11th to 20th	Night	5% 6% 2 7% 4 4 5% 4 6% 6% 51%	2077
	21st to end	Night	5% 5% 2 5% 3 5% 4 3 6% 7% 54%	2097
May	1st to 10th	Night	5% 5% 2 6% 4 5% 5% 4 6% 8% 49%	5321
	11th to 20th	Night	4 5% 2 5% 3 3 4 4 6% 8% 56%	5185
	21st to end	Night	4 3 1 4 3 3 4 3 5% 6% 64%	5181
June	1st to 10th	Night	2 2 1 2 3 3 4 4 6% 7% 67%	1680
	11th to 20th	Night	3 2 1 4 2 2 4 3 4 6% 70%	1680
	21st to end	Night	1 1 1 3 2 3 3 3 5% 8% 70%	1680
July	1st to 10th	Night	2 2 1 3 3 4 5% 4 5% 6% 65%	3720
	11th to 20th	Night	1 1 1 3 3 4 5% 4 7% 12% 59%	4142
	21st to end	Night	2 2 1 4 4 4 4 4 6% 9% 59%	4472
August	1st to 10th	Night	3 2 1 5% 4 4 5% 4 8% 10% 53%	4033
	11th to 20th	Night	2 2 1 5% 4 4 4 4 6% 9% 59%	4139
	21st to end	Night	4 3 3 8% 5% 5% 6% 5% 8% 10% 43%	4993
September	1st to 10th	Night	4 2 1 3 3 2 4 3 6% 8% 65%	2000
	11th to 20th	Night	4 2 1 3 3 3 3 3 5% 8% 65%	2035
	21st to end	Night	4 2 1 3 3 2 4 3 3 5% 69%	2275
October	1st to 10th	Night	2 2 1 2 2 2 2 2 3 5% 76%	3920
	11th to 20th	Night	4 5% 2 5% 3 3 4 3 5% 8% 59%	3898
	21st to end	Night	6% 6% 2 8% 7% 7% 5% 3 3 5% 51%	4252
November	1st to 10th	Night	3 4 1 3 3 3 3 3 5% 6% 65%	2462
	11th to 20th	Night	3 4 1 4 3 3 3 3 5% 6% 64%	2806
	21st to end	Night	4 7% 3 6% 3 3 3 4 5% 6% 56%	2940
December	1st to 10th	Night	3 4 2 5% 5% 9% 9% 5% 5% 6% 47%	4710
	11th to 20th	Night	3 6% 2 5% 5% 7% 9% 8% 8% 9% 37%	5321
	21st to end	Night	4 10% 3 9% 6% 6% 8% 5% 5% 6% 38%	7128

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	30% 10% 21% 29% 9%	5143
	11th to 20th	Day	21% 10% 21% 26% 18% 4	5340
	21st to end	Day	23% 5% 16% 32% 19% 4	5844
February	1st to 10th	Day	29% 8% 18% 26% 16% 3	1890
	11th to 20th	Day	32% 8% 13% 25% 18% 4	1864
	21st to end	Day	28% 9% 11% 25% 22% 5%	1740
March	1st to 10th	Day	22% 9% 12% 23% 26% 7%	5064
	11th to 20th	Day	29% 8% 13% 28% 18% 4	5110
	21st to end	Day	26% 12% 15% 27% 17% 3	5607
April	1st to 10th	Day	23% 7% 11% 24% 24% 11%	2513
	11th to 20th	Day	26% 7% 11% 22% 24% 8%	2494
	21st to end	Day	22% 7% 10% 26% 27% 8%	2520
May	1st to 10th	Day	18% 7% 14% 26% 28% 7%	7625
	11th to 20th	Day	17% 6% 10% 22% 32% 14%	7694
	21st to end	Day	14% 5% 9% 23% 38% 11%	9027
June	1st to 10th	Day	15% 5% 8% 18% 31% 22%	2940
	11th to 20th	Day	14% 5% 7% 10% 24% 41%	2940
	21st to end	Day	13% 4 6% 13% 25% 38%	2940
July	1st to 10th	Day	14% 4 7% 8% 17% 50%	6396
	11th to 20th	Day	14% 5% 10% 24% 27% 19%	5877
	21st to end	Day	16% 7% 11% 19% 23% 25%	6217
August	1st to 10th	Day	22% 9% 17% 23% 14% 15%	4208
	11th to 20th	Day	19% 8% 14% 18% 23% 18%	4133
	21st to end	Day	20% 8% 15% 18% 23% 16%	4311
September	1st to 10th	Day	19% 6% 9% 15% 25% 26%	2400
	11th to 20th	Day	16% 8% 10% 18% 27% 21%	2360
	21st to end	Day	16% 5% 7% 17% 38% 18%	2125
October	1st to 10th	Day	12% 3 6% 17% 36% 26%	3270
	11th to 20th	Day	22% 4 8% 28% 31% 7%	3250
	21st to end	Day	18% 3 13% 28% 32% 6%	3548
November	1st to 10th	Day	15% 6% 13% 28% 33% 6%	2033
	11th to 20th	Day	19% 5% 12% 35% 24% 4	1811
	21st to end	Day	23% 6% 14% 32% 23% 3	1680
December	1st to 10th	Day	21% 8% 17% 36% 16% 1	2700
	11th to 20th	Day	22% 11% 20% 32% 15%	3104
	21st to end	Day	31% 9% 18% 22% 19% 1	4092
January	1st to 10th	Night	37% 13% 19% 23% 9%	7825
	11th to 20th	Night	20% 13% 24% 29% 13% 1	7650
	21st to end	Night	27% 7% 22% 25% 16% 3	8370
February	1st to 10th	Night	29% 8% 18% 28% 15% 3	2730
	11th to 20th	Night	32% 7% 13% 27% 18% 3	2652
	21st to end	Night	26% 9% 11% 28% 22% 4	2060
March	1st to 10th	Night	25% 9% 14% 25% 23% 3	5050
	11th to 20th	Night	29% 8% 11% 27% 22% 4	5110
	21st to end	Night	28% 12% 16% 28% 14% 2	5610
April	1st to 10th	Night	27% 7% 12% 21% 24% 9%	2106
	11th to 20th	Night	29% 8% 9% 20% 22% 12%	2077
	21st to end	Night	25% 8% 9% 22% 28% 8%	2097
May	1st to 10th	Night	25% 9% 13% 25% 24% 5%	5319
	11th to 20th	Night	22% 7% 10% 24% 27% 10%	5186
	21st to end	Night	18% 5% 9% 22% 33% 13%	5182
June	1st to 10th	Night	15% 5% 8% 18% 29% 25%	1680
	11th to 20th	Night	15% 4 7% 13% 19% 42%	1680
	21st to end	Night	12% 4 8% 15% 21% 40%	1680
July	1st to 10th	Night	18% 4 8% 9% 14% 46%	3680
	11th to 20th	Night	16% 6% 11% 28% 21% 17%	4089
	21st to end	Night	19% 6% 13% 22% 21% 20%	4344
August	1st to 10th	Night	26% 10% 22% 14% 15% 13%	2928
	11th to 20th	Night	22% 9% 17% 14% 19% 20%	2992
	21st to end	Night	26% 8% 14% 14% 21% 16%	3597
September	1st to 10th	Night	18% 5% 10% 16% 22% 29%	2000
	11th to 20th	Night	16% 6% 12% 18% 24% 24%	2035
	21st to end	Night	17% 4 7% 16% 36% 20%	2275
October	1st to 10th	Night	12% 3 5% 18% 39% 23%	3920
	11th to 20th	Night	22% 5% 10% 28% 26% 9%	3898
	21st to end	Night	22% 4 22% 25% 23% 4	4252
November	1st to 10th	Night	17% 5% 13% 29% 29% 7%	2462
	11th to 20th	Night	19% 5% 12% 33% 27% 4	2806
	21st to end	Night	24% 7% 12% 27% 26% 4	2940
December	1st to 10th	Night	19% 13% 24% 28% 15% 2	4710
	11th to 20th	Night	23% 14% 23% 30% 10% 1	5322
	21st to end	Night	36% 11% 16% 22% 15% 1	7128

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	8% 8% 19% 39% 19% 7%	330
	11th to 20th	Day	5% 6% 24% 41% 21% 4	330
	21st to end	Day	5% 4 18% 50% 17% 5%	363
February	1st to 10th	Day	6% 8% 28% 41% 14% 3	330
	11th to 20th	Day	10% 10% 31% 36% 13% 1	330
	21st to end	Day	8% 11% 25% 37% 16% 3	283
March	1st to 10th	Day	10% 14% 28% 29% 15% 4	540
	11th to 20th	Day	7% 14% 29% 30% 18% 3	540
	21st to end	Day	10% 18% 31% 27% 12% 3	594
April	1st to 10th	Day	8% 16% 30% 26% 16% 5%	540
	11th to 20th	Day	11% 20% 33% 22% 12% 3	532
	21st to end	Day	5% 18% 34% 26% 15% 2	540
May	1st to 10th	Day	8% 15% 28% 32% 12% 4	543
	11th to 20th	Day	6% 10% 27% 31% 22% 5%	598
	21st to end	Day	5% 10% 23% 32% 24% 6%	844
June	1st to 10th	Day	4 6% 20% 35% 28% 7%	780
	11th to 20th	Day	3 7% 25% 33% 26% 6%	780
	21st to end	Day	2 7% 19% 37% 30% 6%	780
July	1st to 10th	Day	1 2 11% 34% 40% 11%	769
	11th to 20th	Day	3 1 8% 32% 41% 14%	545
	21st to end	Day	5% 1 10% 33% 39% 12%	575
August	1st to 10th	Day	4 2 13% 33% 36% 11%	527
	11th to 20th	Day	1 3 12% 36% 35% 13%	517
	21st to end	Day	2 2 10% 37% 37% 12%	583
September	1st to 10th	Day	5% 2 14% 31% 32% 15%	530
	11th to 20th	Day	5% 5% 20% 37% 28% 6%	530
	21st to end	Day	4 8% 18% 24% 33% 14%	455
October	1st to 10th	Day	8% 6% 17% 22% 28% 18%	330
	11th to 20th	Day	3 8% 19% 34% 25% 11%	330
	21st to end	Day	8% 5% 18% 31% 29% 9%	363
November	1st to 10th	Day	4 7% 25% 32% 22% 10%	330
	11th to 20th	Day	2 7% 22% 37% 23% 10%	330
	21st to end	Day	6% 5% 24% 35% 21% 8%	330
December	1st to 10th	Day	6% 6% 23% 38% 18% 8%	330
	11th to 20th	Day	5% 4 20% 40% 21% 9%	330
	21st to end	Day	8% 8% 21% 44% 13% 6%	363
January	1st to 10th	Night	7% 4 19% 47% 20% 3	920
	11th to 20th	Night	7% 3 20% 44% 23% 3	840
	21st to end	Night	7% 5% 19% 49% 19% 2	924
February	1st to 10th	Night	7% 3 27% 47% 14% 2	840
	11th to 20th	Night	6% 6% 29% 46% 12% 1	824
	21st to end	Night	7% 4 31% 48% 10%	692
March	1st to 10th	Night	6% 6% 30% 42% 15% 2	840
	11th to 20th	Night	5% 4 26% 44% 21% 1	840
	21st to end	Night	6% 7% 27% 45% 14% 1	924
April	1st to 10th	Night	7% 8% 31% 43% 12%	636
	11th to 20th	Night	9% 7% 27% 43% 13% 1	623
	21st to end	Night	7% 6% 29% 41% 16% 2	630
May	1st to 10th	Night	8% 8% 24% 43% 16% 1	630
	11th to 20th	Night	6% 5% 16% 45% 24% 4	621
	21st to end	Night	5% 6% 23% 39% 25% 3	683
June	1st to 10th	Night	2 2 17% 45% 30% 4	630
	11th to 20th	Night	2 5% 19% 47% 24% 3	630
	21st to end	Night	2 2 14% 44% 35% 3	630
July	1st to 10th	Night	3 8% 39% 45% 4	615
	11th to 20th	Night	2 4 37% 50% 6%	597
	21st to end	Night	4 1 4 35% 51% 5%	657
August	1st to 10th	Night	4 2 8% 36% 48% 3	597
	11th to 20th	Night	4 1 8% 38% 45% 5%	594
	21st to end	Night	3 2 7% 40% 41% 7%	660
September	1st to 10th	Night	4 1 8% 43% 40% 5%	600
	11th to 20th	Night	5% 2 12% 43% 34% 4	635
	21st to end	Night	6% 2 14% 36% 36% 7%	800
October	1st to 10th	Night	5% 4 16% 41% 31% 2	800
	11th to 20th	Night	4 3 19% 43% 27% 4	800
	21st to end	Night	8% 3 17% 41% 27% 4	880
November	1st to 10th	Night	4 4 18% 45% 26% 3	822
	11th to 20th	Night	5% 4 21% 47% 20% 3	1050
	21st to end	Night	7% 5% 21% 41% 23% 3	1050
December	1st to 10th	Night	7% 4 20% 47% 20% 2	1050
	11th to 20th	Night	8% 5% 23% 41% 22% 2	1050
	21st to end	Night	10% 6% 22% 46% 13% 2	1155

Page generated at 2026-07-13 01:14:48. Version ac07

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