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 2025-03-12.

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 4 2 5% 4 4 5% 4 6% 8% 56%	112942
	Night	3 5% 2 6% 4 5% 5% 4 6% 7% 52%	114928
Transparency	Day	21% 8% 13% 23% 23% 13%	112702
	Night	23% 9% 15% 23% 20% 10%	114758
Seeing	Day	6% 8% 22% 33% 24% 7%	15730
	Night	6% 4 20% 43% 25% 3	26029

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 6% 3 9% 6% 6% 7% 4 7% 9% 40%	3960
	11th to 20th	Day	2 5% 3 8% 5% 6% 7% 6% 6% 7% 47%	4100
	21st to end	Day	2 5% 3 5% 4 4 5% 5% 6% 7% 54%	4510
February	1st to 10th	Day	3 6% 3 6% 5% 5% 5% 4 6% 9% 49%	1800
	11th to 20th	Day	3 7% 2 6% 5% 5% 5% 4 6% 8% 48%	1764
	21st to end	Day	2 7% 3 6% 5% 6% 5% 4 7% 6% 49%	1660
March	1st to 10th	Day	3 7% 3 6% 4 4 6% 5% 7% 7% 49%	3554
	11th to 20th	Day	3 7% 3 7% 5% 5% 5% 4 6% 8% 48%	3502
	21st to end	Day	2 6% 2 8% 6% 6% 6% 6% 7% 8% 42%	3825
April	1st to 10th	Day	3 4 2 5% 4 4 4 4 6% 7% 57%	2273
	11th to 20th	Day	3 5% 3 6% 4 4 5% 5% 6% 7% 51%	2251
	21st to end	Day	3 3 1 5% 3 4 5% 5% 6% 9% 56%	2280
May	1st to 10th	Day	3 4 2 6% 4 5% 5% 5% 8% 8% 51%	4109
	11th to 20th	Day	2 3 1 4 4 4 5% 5% 7% 10% 56%	4124
	21st to end	Day	3 3 1 4 3 3 4 3 6% 8% 61%	4825
June	1st to 10th	Day	2 2 1 3 3 3 3 3 6% 9% 64%	2660
	11th to 20th	Day	2 2 1 3 3 3 4 3 6% 6% 67%	2660
	21st to end	Day	1 2 1 2 3 3 4 3 5% 7% 68%	2660
July	1st to 10th	Day	2 2 1 2 2 3 3 3 4 6% 72%	4415
	11th to 20th	Day	1 2 1 2 3 3 5% 6% 11% 13% 53%	4110
	21st to end	Day	2 2 1 3 3 3 4 4 7% 10% 60%	4454
August	1st to 10th	Day	3 2 1 4 3 5% 6% 6% 10% 14% 46%	4092
	11th to 20th	Day	2 2 1 4 4 4 5% 4 8% 11% 55%	3966
	21st to end	Day	3 2 1 4 4 4 5% 5% 7% 11% 53%	4180
September	1st to 10th	Day	5% 3 1 4 3 3 4 3 5% 7% 63%	2280
	11th to 20th	Day	3 2 1 3 2 3 3 4 5% 8% 66%	2242
	21st to end	Day	4 2 1 3 2 3 3 2 4 6% 71%	2019
October	1st to 10th	Day	4 2 1 2 2 1 2 2 3 5% 77%	3170
	11th to 20th	Day	3 3 1 5% 3 4 5% 3 7% 10% 55%	3150
	21st to end	Day	5% 4 1 4 3 4 4 3 4 6% 62%	3438
November	1st to 10th	Day	1 3 2 4 3 2 3 4 6% 7% 65%	1934
	11th to 20th	Day	3 4 1 4 3 3 4 3 5% 7% 63%	1725
	21st to end	Day	2 5% 2 4 4 3 4 4 6% 6% 61%	1600
December	1st to 10th	Day	2 4 2 5% 4 5% 5% 4 5% 7% 57%	2620
	11th to 20th	Day	2 4 2 6% 5% 6% 8% 5% 8% 8% 47%	3026
	21st to end	Day	3 8% 3 7% 5% 7% 6% 5% 6% 8% 43%	4004
January	1st to 10th	Night	2 8% 3 10% 6% 6% 8% 5% 7% 8% 37%	6034
	11th to 20th	Night	3 9% 2 7% 5% 7% 6% 5% 8% 8% 40%	5880
	21st to end	Night	3 9% 4 7% 4 5% 6% 6% 6% 8% 41%	6468
February	1st to 10th	Night	3 6% 2 7% 4 4 5% 4 6% 7% 51%	2600
	11th to 20th	Night	3 8% 2 8% 5% 4 4 3 6% 8% 48%	2522
	21st to end	Night	4 7% 2 6% 5% 5% 5% 3 6% 7% 52%	1965
March	1st to 10th	Night	3 6% 2 6% 4 5% 6% 5% 7% 7% 50%	3540
	11th to 20th	Night	5% 8% 2 6% 4 4 5% 5% 5% 7% 49%	3506
	21st to end	Night	3 8% 3 8% 6% 6% 6% 5% 7% 7% 41%	3828
April	1st to 10th	Night	5% 5% 2 6% 4 4 4 6% 6% 6% 52%	1906
	11th to 20th	Night	5% 6% 3 6% 4 4 5% 4 5% 6% 51%	1877
	21st to end	Night	5% 5% 2 5% 4 5% 4 3 6% 7% 54%	1897
May	1st to 10th	Night	5% 6% 2 6% 4 5% 4 5% 7% 8% 48%	2860
	11th to 20th	Night	4 3 1 4 3 3 5% 4 8% 10% 54%	2779
	21st to end	Night	5% 4 1 4 3 3 4 3 5% 7% 62%	2762
June	1st to 10th	Night	2 3 1 2 3 3 3 4 5% 7% 67%	1520
	11th to 20th	Night	3 2 1 4 2 2 3 3 4 7% 69%	1520
	21st to end	Night	1 1 1 3 2 4 3 3 5% 9% 69%	1520
July	1st to 10th	Night	2 1 1 3 3 4 4 4 5% 5% 68%	2540
	11th to 20th	Night	1 1 3 3 3 4 4 7% 12% 61%	2857
	21st to end	Night	3 2 1 4 4 4 4 4 6% 9% 60%	3109
August	1st to 10th	Night	3 2 2 6% 4 5% 6% 6% 10% 12% 43%	2847
	11th to 20th	Night	3 2 2 7% 5% 5% 4 4 7% 9% 53%	2873
	21st to end	Night	3 3 2 7% 5% 5% 5% 4 7% 10% 48%	3487
September	1st to 10th	Night	3 2 1 3 3 3 4 3 6% 8% 64%	1900
	11th to 20th	Night	4 2 1 3 2 3 3 3 5% 8% 66%	1933
	21st to end	Night	5% 2 1 3 2 2 3 2 3 5% 71%	2161
October	1st to 10th	Night	2 2 1 2 2 2 2 2 3 5% 76%	3800
	11th to 20th	Night	4 5% 2 5% 3 3 4 3 5% 8% 59%	3778
	21st to end	Night	6% 6% 1 8% 7% 7% 5% 3 3 4 51%	4120
November	1st to 10th	Night	3 4 1 4 3 3 2 3 5% 6% 65%	2342
	11th to 20th	Night	3 4 1 4 3 3 2 3 5% 6% 66%	2672
	21st to end	Night	4 6% 3 5% 3 3 3 4 5% 6% 58%	2800
December	1st to 10th	Night	2 4 2 5% 5% 9% 9% 5% 5% 6% 48%	4570
	11th to 20th	Night	3 6% 2 5% 5% 7% 9% 8% 8% 9% 37%	5181
	21st to end	Night	4 10% 3 9% 6% 6% 8% 5% 5% 6% 38%	6974

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% 10% 19% 30% 9% 1	3961
	11th to 20th	Day	25% 12% 20% 22% 17% 5%	4100
	21st to end	Day	23% 6% 16% 31% 19% 5%	4510
February	1st to 10th	Day	30% 8% 19% 25% 16% 4	1800
	11th to 20th	Day	31% 8% 13% 26% 18% 3	1773
	21st to end	Day	29% 9% 11% 24% 22% 5%	1660
March	1st to 10th	Day	22% 13% 13% 26% 22% 4	3554
	11th to 20th	Day	27% 11% 14% 33% 13% 2	3502
	21st to end	Day	25% 16% 15% 23% 17% 3	3825
April	1st to 10th	Day	24% 7% 11% 24% 23% 11%	2273
	11th to 20th	Day	27% 8% 12% 21% 23% 8%	2254
	21st to end	Day	22% 8% 11% 26% 26% 7%	2280
May	1st to 10th	Day	20% 10% 14% 18% 28% 10%	4109
	11th to 20th	Day	17% 8% 12% 25% 28% 10%	4127
	21st to end	Day	16% 7% 9% 17% 39% 13%	4825
June	1st to 10th	Day	15% 5% 8% 19% 31% 23%	2660
	11th to 20th	Day	15% 5% 7% 10% 23% 38%	2660
	21st to end	Day	14% 5% 7% 14% 26% 35%	2660
July	1st to 10th	Day	13% 5% 8% 9% 18% 47%	4346
	11th to 20th	Day	15% 6% 11% 24% 26% 18%	4041
	21st to end	Day	16% 8% 11% 19% 16% 29%	4314
August	1st to 10th	Day	22% 10% 18% 23% 13% 14%	4078
	11th to 20th	Day	20% 9% 14% 18% 23% 17%	4005
	21st to end	Day	19% 8% 15% 18% 23% 16%	4179
September	1st to 10th	Day	20% 6% 8% 14% 25% 26%	2280
	11th to 20th	Day	15% 8% 11% 18% 27% 22%	2242
	21st to end	Day	15% 5% 7% 16% 39% 18%	2019
October	1st to 10th	Day	12% 3 6% 17% 36% 26%	3170
	11th to 20th	Day	22% 4 8% 29% 31% 7%	3150
	21st to end	Day	18% 3 13% 28% 32% 6%	3438
November	1st to 10th	Day	15% 6% 13% 27% 33% 6%	1934
	11th to 20th	Day	19% 5% 12% 34% 25% 4	1725
	21st to end	Day	23% 5% 13% 32% 24% 3	1600
December	1st to 10th	Day	21% 8% 17% 37% 16% 1	2620
	11th to 20th	Day	23% 11% 20% 31% 15%	3024
	21st to end	Day	31% 9% 18% 22% 19% 1	4004
January	1st to 10th	Night	33% 12% 18% 26% 12%	6034
	11th to 20th	Night	24% 15% 21% 25% 14% 1	5880
	21st to end	Night	29% 7% 21% 25% 16% 2	6468
February	1st to 10th	Night	30% 8% 17% 27% 15% 3	2600
	11th to 20th	Night	30% 8% 13% 28% 18% 3	2523
	21st to end	Night	27% 8% 10% 28% 23% 4	1965
March	1st to 10th	Night	23% 13% 13% 26% 24% 2	3540
	11th to 20th	Night	28% 10% 12% 29% 17% 3	3506
	21st to end	Night	29% 16% 12% 24% 17% 2	3828
April	1st to 10th	Night	27% 7% 13% 20% 23% 10%	1906
	11th to 20th	Night	29% 9% 10% 19% 21% 13%	1877
	21st to end	Night	24% 9% 9% 22% 27% 8%	1897
May	1st to 10th	Night	23% 11% 10% 19% 30% 8%	2860
	11th to 20th	Night	17% 10% 13% 28% 25% 8%	2780
	21st to end	Night	18% 7% 9% 16% 32% 18%	2762
June	1st to 10th	Night	15% 5% 8% 18% 28% 26%	1520
	11th to 20th	Night	15% 5% 8% 14% 19% 40%	1520
	21st to end	Night	12% 4 8% 16% 22% 37%	1520
July	1st to 10th	Night	15% 6% 9% 9% 17% 46%	2500
	11th to 20th	Night	14% 6% 11% 31% 19% 18%	2804
	21st to end	Night	18% 7% 13% 20% 17% 24%	3011
August	1st to 10th	Night	26% 10% 22% 14% 14% 13%	2838
	11th to 20th	Night	22% 9% 17% 14% 18% 19%	2900
	21st to end	Night	26% 8% 14% 15% 21% 16%	3487
September	1st to 10th	Night	18% 5% 10% 15% 22% 29%	1900
	11th to 20th	Night	16% 6% 12% 18% 23% 25%	1933
	21st to end	Night	17% 4 6% 16% 37% 20%	2161
October	1st to 10th	Night	12% 3 5% 18% 39% 23%	3800
	11th to 20th	Night	21% 5% 9% 28% 26% 9%	3778
	21st to end	Night	22% 4 22% 25% 23% 4	4120
November	1st to 10th	Night	18% 5% 12% 28% 30% 7%	2342
	11th to 20th	Night	18% 5% 11% 34% 28% 5%	2672
	21st to end	Night	24% 6% 12% 27% 27% 4	2800
December	1st to 10th	Night	18% 12% 24% 29% 15% 2	4570
	11th to 20th	Night	24% 14% 22% 30% 9% 1	5182
	21st to end	Night	36% 11% 16% 22% 15% 1	6974

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	9% 7% 20% 37% 20% 7%	300
	11th to 20th	Day	6% 6% 26% 39% 20% 3	300
	21st to end	Day	5% 5% 19% 48% 18% 6%	330
February	1st to 10th	Day	7% 9% 31% 41% 11% 2	300
	11th to 20th	Day	9% 9% 33% 36% 12% 1	300
	21st to end	Day	8% 12% 27% 35% 15% 3	259
March	1st to 10th	Day	11% 15% 29% 29% 13% 3	500
	11th to 20th	Day	7% 12% 30% 30% 18% 3	465
	21st to end	Day	10% 20% 33% 26% 9% 1	506
April	1st to 10th	Day	9% 17% 32% 26% 13% 3	460
	11th to 20th	Day	11% 20% 34% 23% 9% 2	452
	21st to end	Day	5% 18% 35% 27% 14% 1	460
May	1st to 10th	Day	9% 17% 32% 30% 10% 2	463
	11th to 20th	Day	6% 10% 27% 31% 22% 4	515
	21st to end	Day	5% 11% 24% 32% 23% 5%	734
June	1st to 10th	Day	4 7% 22% 34% 28% 6%	680
	11th to 20th	Day	4 7% 26% 33% 25% 5%	680
	21st to end	Day	2 5% 19% 39% 31% 4	680
July	1st to 10th	Day	2 2 12% 36% 40% 9%	669
	11th to 20th	Day	4 1 8% 34% 40% 13%	484
	21st to end	Day	5% 1 9% 34% 38% 12%	531
August	1st to 10th	Day	4 2 12% 33% 36% 12%	487
	11th to 20th	Day	1 3 12% 39% 36% 10%	477
	21st to end	Day	2 2 11% 37% 37% 12%	539
September	1st to 10th	Day	6% 2 15% 32% 31% 14%	490
	11th to 20th	Day	4 5% 20% 37% 28% 6%	490
	21st to end	Day	4 9% 19% 24% 32% 13%	419
October	1st to 10th	Day	8% 6% 18% 22% 27% 19%	300
	11th to 20th	Day	3 8% 18% 32% 26% 12%	300
	21st to end	Day	8% 5% 19% 31% 30% 6%	330
November	1st to 10th	Day	5% 7% 27% 32% 20% 9%	300
	11th to 20th	Day	2 7% 24% 38% 20% 9%	300
	21st to end	Day	6% 5% 25% 34% 21% 8%	300
December	1st to 10th	Day	7% 6% 24% 37% 17% 9%	300
	11th to 20th	Day	6% 4 22% 40% 20% 7%	300
	21st to end	Day	8% 9% 22% 43% 13% 5%	330
January	1st to 10th	Night	6% 4 20% 46% 21% 3	876
	11th to 20th	Night	7% 4 21% 43% 23% 3	800
	21st to end	Night	7% 5% 19% 48% 19% 2	880
February	1st to 10th	Night	7% 3 28% 47% 13% 2	800
	11th to 20th	Night	5% 6% 29% 45% 13% 1	784
	21st to end	Night	7% 4 32% 47% 9%	660
March	1st to 10th	Night	6% 6% 31% 42% 14% 2	800
	11th to 20th	Night	5% 4 25% 44% 22% 1	766
	21st to end	Night	6% 8% 28% 43% 14% 1	836
April	1st to 10th	Night	6% 7% 33% 42% 11%	576
	11th to 20th	Night	9% 8% 26% 42% 13% 2	563
	21st to end	Night	7% 6% 29% 40% 16% 2	570
May	1st to 10th	Night	7% 8% 26% 43% 15% 1	570
	11th to 20th	Night	6% 5% 15% 46% 24% 4	561
	21st to end	Night	5% 7% 24% 38% 23% 3	617
June	1st to 10th	Night	3 2 18% 45% 30% 3	570
	11th to 20th	Night	2 5% 20% 46% 24% 3	570
	21st to end	Night	2 2 12% 44% 37% 3	570
July	1st to 10th	Night	3 8% 38% 46% 5%	555
	11th to 20th	Night	2 4 37% 51% 6%	552
	21st to end	Night	4 1 4 34% 52% 5%	624
August	1st to 10th	Night	4 1 8% 35% 50% 3	567
	11th to 20th	Night	4 1 8% 38% 44% 5%	564
	21st to end	Night	3 2 8% 41% 41% 7%	627
September	1st to 10th	Night	4 1 8% 43% 39% 5%	570
	11th to 20th	Night	5% 2 12% 43% 35% 4	603
	21st to end	Night	6% 2 14% 36% 35% 6%	760
October	1st to 10th	Night	5% 4 17% 41% 31% 2	760
	11th to 20th	Night	4 3 18% 44% 28% 4	760
	21st to end	Night	8% 3 17% 41% 27% 4	836
November	1st to 10th	Night	4 4 19% 45% 25% 3	782
	11th to 20th	Night	4 4 22% 46% 20% 4	1000
	21st to end	Night	7% 5% 22% 41% 23% 3	1000
December	1st to 10th	Night	7% 4 20% 47% 20% 2	1000
	11th to 20th	Night	8% 5% 23% 41% 21% 2	1000
	21st to end	Night	10% 6% 22% 46% 13% 2	1100

Page generated at 2025-03-09 06:08:44. Version ac07

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