Green Acres Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Green Acres Observatory clear sky chart from 2005-11-06 to 2025-01-07.

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	6% 4 1 4 3 3 3 2 4 5% 66%	108706
	Night	8% 4 1 4 3 3 3 3 4 5% 62%	109050
Transparency	Day	21% 4 8% 17% 24% 26%	108459
	Night	24% 5% 9% 19% 22% 22%	108886
Seeing	Day	7% 5% 17% 31% 29% 11%	15436
	Night	9% 3 18% 40% 26% 4	24905

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	18% 11% 3 9% 5% 4 4 3 5% 7% 30%	2587
	11th to 20th	Day	17% 9% 4 12% 6% 5% 4 3 4 5% 30%	2838
	21st to end	Day	19% 11% 4 8% 5% 5% 5% 4 6% 7% 26%	3146
February	1st to 10th	Day	15% 7% 2 6% 4 3 4 4 5% 7% 43%	1710
	11th to 20th	Day	13% 5% 2 5% 4 4 4 3 5% 8% 48%	1823
	21st to end	Day	13% 4 2 6% 4 4 4 4 4 6% 49%	1570
March	1st to 10th	Day	10% 8% 3 8% 5% 5% 6% 6% 7% 7% 33%	3227
	11th to 20th	Day	10% 4 2 3 3 3 3 3 4 7% 60%	3490
	21st to end	Day	10% 7% 3 6% 5% 5% 5% 4 6% 8% 41%	3979
April	1st to 10th	Day	10% 5% 2 4 3 3 4 3 5% 7% 53%	2280
	11th to 20th	Day	8% 3 1 4 3 2 3 3 6% 6% 61%	2249
	21st to end	Day	6% 3 1 3 2 2 3 2 4 6% 68%	2280
May	1st to 10th	Day	5% 2 1 2 2 2 2 2 3 4 76%	3800
	11th to 20th	Day	2 1 1 2 1 1 1 1 3 4 83%	4305
	21st to end	Day	2 1 1 1 1 1 1 2 3 4 82%	4813
June	1st to 10th	Day	3 3 1 2 2 2 2 2 3 6% 73%	2660
	11th to 20th	Day	1 1 1 1 1 2 2 5% 85%	2660
	21st to end	Day	1 1 1 1 1 1 1 1 2 3 86%	2660
July	1st to 10th	Day	1 1 1 1 95%	4420
	11th to 20th	Day	1 1 2 1 2 2 1 2 3 85%	4365
	21st to end	Day	1 1 1 1 1 1 1 2 90%	4455
August	1st to 10th	Day	1 1 2 1 1 1 2 3 4 83%	4074
	11th to 20th	Day	1 1 1 1 1 1 2 3 91%	3724
	21st to end	Day	1 1 1 1 1 1 1 1 3 90%	4180
September	1st to 10th	Day	1 1 1 1 1 1 1 1 3 90%	2280
	11th to 20th	Day	1 1 1 1 2 1 2 2 3 5% 80%	2280
	21st to end	Day	2 1 1 2 1 1 2 1 3 4 83%	2061
October	1st to 10th	Day	1 1 2 2 2 2 1 3 4 80%	3180
	11th to 20th	Day	2 2 1 3 2 2 2 2 2 4 78%	3161
	21st to end	Day	3 2 1 4 2 3 3 2 5% 7% 67%	3448
November	1st to 10th	Day	6% 4 1 5% 3 4 3 4 6% 8% 57%	1819
	11th to 20th	Day	8% 7% 2 6% 3 3 4 4 5% 8% 52%	1800
	21st to end	Day	11% 6% 2 6% 3 3 3 3 5% 8% 50%	1730
December	1st to 10th	Day	8% 5% 2 5% 4 4 6% 5% 7% 10% 45%	2620
	11th to 20th	Day	20% 10% 4 10% 7% 5% 5% 4 5% 6% 23%	3028
	21st to end	Day	17% 8% 3 9% 6% 7% 6% 6% 7% 8% 23%	4004
January	1st to 10th	Night	14% 7% 3 8% 5% 3 4 3 6% 7% 41%	4490
	11th to 20th	Night	19% 8% 3 7% 4 4 4 4 6% 6% 33%	4127
	21st to end	Night	21% 8% 3 7% 5% 4 4 4 5% 6% 34%	4521
February	1st to 10th	Night	16% 5% 2 5% 3 3 3 2 4 6% 51%	2470
	11th to 20th	Night	12% 4 1 4 3 4 3 3 5% 9% 53%	2243
	21st to end	Night	14% 4 2 4 3 3 3 3 5% 6% 54%	1884
March	1st to 10th	Night	13% 8% 2 7% 6% 4 6% 4 6% 6% 38%	3643
	11th to 20th	Night	11% 5% 1 3 3 2 2 2 4 4 61%	3490
	21st to end	Night	17% 6% 2 5% 4 4 4 3 5% 7% 43%	3661
April	1st to 10th	Night	10% 5% 2 4 3 3 3 3 5% 7% 55%	1900
	11th to 20th	Night	6% 4 1 4 3 4 3 3 5% 6% 62%	1879
	21st to end	Night	4 2 1 2 2 3 3 3 3 6% 69%	1900
May	1st to 10th	Night	2 2 1 2 2 2 2 2 3 5% 79%	3180
	11th to 20th	Night	2 1 1 1 1 1 1 3 4 84%	2567
	21st to end	Night	1 1 1 1 1 1 2 3 4 84%	2762
June	1st to 10th	Night	3 2 1 3 2 2 2 2 3 5% 77%	1520
	11th to 20th	Night	1 1 1 1 1 1 1 3 4 87%	1520
	21st to end	Night	1 2 1 1 1 1 2 2 88%	1520
July	1st to 10th	Night	1 1 1 1 1 94%	2540
	11th to 20th	Night	1 2 2 1 2 1 2 2 3 83%	2593
	21st to end	Night	1 1 1 1 1 1 1 1 2 3 87%	3109
August	1st to 10th	Night	2 1 1 1 1 1 2 2 3 3 82%	2864
	11th to 20th	Night	1 1 1 1 1 2 2 90%	3126
	21st to end	Night	2 1 1 1 1 1 1 2 90%	3498
September	1st to 10th	Night	1 1 1 1 1 1 2 2 90%	1900
	11th to 20th	Night	2 1 1 2 1 1 1 1 3 4 83%	1900
	21st to end	Night	1 1 1 1 1 1 1 1 2 3 86%	2114
October	1st to 10th	Night	3 1 2 1 1 2 2 3 4 81%	3800
	11th to 20th	Night	2 1 1 2 2 2 2 2 3 4 81%	3777
	21st to end	Night	5% 2 1 3 2 2 2 2 4 6% 71%	4121
November	1st to 10th	Night	8% 3 1 3 3 2 2 3 5% 7% 64%	2478
	11th to 20th	Night	7% 4 1 5% 3 3 3 3 6% 7% 59%	2600
	21st to end	Night	10% 4 2 4 3 3 3 3 4 7% 57%	2665
December	1st to 10th	Night	8% 4 1 4 3 3 4 4 6% 7% 56%	4560
	11th to 20th	Night	18% 8% 3 6% 5% 5% 5% 4 6% 10% 29%	5176
	21st to end	Night	17% 7% 3 9% 6% 6% 6% 5% 6% 7% 27%	6952

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	49% 10% 15% 18% 6% 1	2587
	11th to 20th	Day	55% 7% 10% 14% 11% 4	2838
	21st to end	Day	53% 9% 10% 17% 9% 2	3146
February	1st to 10th	Day	40% 6% 11% 22% 15% 6%	1710
	11th to 20th	Day	34% 6% 13% 22% 19% 6%	1833
	21st to end	Day	35% 5% 12% 24% 21% 4	1570
March	1st to 10th	Day	42% 11% 13% 18% 13% 2	3227
	11th to 20th	Day	24% 6% 12% 31% 17% 11%	3490
	21st to end	Day	37% 7% 16% 24% 14% 3	3979
April	1st to 10th	Day	31% 6% 12% 25% 18% 8%	2280
	11th to 20th	Day	23% 5% 12% 29% 23% 8%	2252
	21st to end	Day	19% 3 11% 29% 31% 7%	2280
May	1st to 10th	Day	13% 3 8% 27% 30% 19%	3800
	11th to 20th	Day	8% 2 4 25% 42% 18%	4308
	21st to end	Day	9% 3 8% 26% 40% 14%	4814
June	1st to 10th	Day	14% 4 8% 24% 32% 18%	2660
	11th to 20th	Day	6% 1 6% 20% 35% 33%	2660
	21st to end	Day	7% 1 2 9% 31% 50%	2660
July	1st to 10th	Day	2 2 4 12% 80%	4351
	11th to 20th	Day	8% 2 2 5% 14% 70%	4290
	21st to end	Day	4 2 4 24% 66%	4315
August	1st to 10th	Day	8% 1 2 5% 24% 60%	4059
	11th to 20th	Day	3 1 5% 33% 56%	3760
	21st to end	Day	5% 3 7% 33% 51%	4179
September	1st to 10th	Day	5% 1 3 7% 27% 58%	2280
	11th to 20th	Day	9% 2 5% 18% 40% 26%	2280
	21st to end	Day	8% 1 4 17% 39% 31%	2061
October	1st to 10th	Day	10% 2 4 12% 26% 47%	3180
	11th to 20th	Day	12% 2 5% 24% 37% 20%	3161
	21st to end	Day	18% 3 5% 21% 39% 14%	3448
November	1st to 10th	Day	25% 5% 11% 25% 26% 8%	1819
	11th to 20th	Day	29% 6% 14% 26% 21% 3	1800
	21st to end	Day	33% 5% 13% 24% 19% 6%	1730
December	1st to 10th	Day	31% 10% 12% 27% 16% 4	2620
	11th to 20th	Day	57% 8% 8% 15% 10% 2	3028
	21st to end	Day	52% 12% 16% 13% 6% 1	4004
January	1st to 10th	Night	39% 11% 17% 23% 8% 2	4490
	11th to 20th	Night	48% 9% 11% 16% 9% 7%	4127
	21st to end	Night	48% 7% 13% 16% 12% 3	4521
February	1st to 10th	Night	35% 4 10% 24% 20% 8%	2470
	11th to 20th	Night	29% 6% 11% 23% 24% 7%	2244
	21st to end	Night	32% 4 10% 28% 21% 5%	1884
March	1st to 10th	Night	41% 12% 13% 15% 15% 4	3643
	11th to 20th	Night	27% 5% 10% 26% 20% 12%	3490
	21st to end	Night	39% 8% 13% 25% 11% 5%	3661
April	1st to 10th	Night	28% 6% 12% 24% 22% 8%	1900
	11th to 20th	Night	23% 5% 13% 28% 20% 11%	1879
	21st to end	Night	16% 5% 14% 27% 32% 7%	1900
May	1st to 10th	Night	9% 3 12% 25% 33% 18%	3180
	11th to 20th	Night	7% 2 5% 24% 42% 20%	2567
	21st to end	Night	7% 3 8% 31% 41% 11%	2762
June	1st to 10th	Night	12% 4 10% 23% 34% 17%	1520
	11th to 20th	Night	4 2 6% 18% 35% 35%	1520
	21st to end	Night	6% 1 2 12% 28% 52%	1520
July	1st to 10th	Night	2 1 5% 11% 80%	2500
	11th to 20th	Night	9% 2 2 4 15% 69%	2545
	21st to end	Night	6% 1 2 4 22% 65%	3011
August	1st to 10th	Night	8% 2 1 7% 20% 62%	2855
	11th to 20th	Night	4 1 2 5% 28% 61%	3156
	21st to end	Night	6% 2 8% 29% 55%	3498
September	1st to 10th	Night	4 1 3 6% 26% 60%	1900
	11th to 20th	Night	9% 1 4 19% 38% 29%	1900
	21st to end	Night	7% 1 4 16% 38% 34%	2114
October	1st to 10th	Night	10% 2 4 11% 19% 55%	3800
	11th to 20th	Night	10% 2 6% 25% 32% 25%	3777
	21st to end	Night	17% 3 4 20% 35% 22%	4121
November	1st to 10th	Night	21% 4 9% 26% 25% 15%	2478
	11th to 20th	Night	24% 4 11% 26% 27% 8%	2600
	21st to end	Night	28% 4 10% 23% 26% 9%	2665
December	1st to 10th	Night	26% 8% 9% 27% 23% 7%	4560
	11th to 20th	Night	49% 8% 12% 17% 12% 2	5176
	21st to end	Night	50% 10% 14% 17% 6% 3	6952

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	28% 6% 23% 20% 17% 5%	192
	11th to 20th	Day	16% 6% 21% 24% 20% 14%	361
	21st to end	Day	16% 5% 17% 25% 24% 13%	407
February	1st to 10th	Day	19% 8% 15% 20% 22% 16%	370
	11th to 20th	Day	15% 7% 25% 23% 19% 10%	443
	21st to end	Day	12% 9% 22% 29% 24% 6%	382
March	1st to 10th	Day	17% 9% 20% 31% 18% 5%	460
	11th to 20th	Day	15% 10% 20% 26% 17% 12%	460
	21st to end	Day	15% 8% 22% 29% 18% 6%	506
April	1st to 10th	Day	14% 9% 26% 23% 19% 8%	460
	11th to 20th	Day	9% 10% 25% 29% 18% 9%	452
	21st to end	Day	4 8% 25% 33% 19% 11%	460
May	1st to 10th	Day	5% 8% 29% 31% 19% 8%	463
	11th to 20th	Day	6% 6% 22% 31% 27% 8%	488
	21st to end	Day	4 6% 17% 29% 33% 12%	529
June	1st to 10th	Day	3 6% 20% 34% 29% 8%	490
	11th to 20th	Day	1 5% 20% 39% 29% 6%	490
	21st to end	Day	1 2 17% 38% 31% 11%	490
July	1st to 10th	Day	11% 37% 42% 10%	486
	11th to 20th	Day	1 6% 37% 47% 9%	484
	21st to end	Day	7% 37% 44% 12%	531
August	1st to 10th	Day	3 1 11% 35% 39% 10%	487
	11th to 20th	Day	1 1 14% 38% 35% 10%	477
	21st to end	Day	1 1 9% 38% 40% 12%	539
September	1st to 10th	Day	1 1 10% 38% 38% 12%	490
	11th to 20th	Day	2 3 17% 36% 29% 13%	490
	21st to end	Day	2 2 13% 32% 36% 16%	490
October	1st to 10th	Day	3 5% 17% 25% 31% 19%	490
	11th to 20th	Day	5% 4 15% 25% 31% 20%	490
	21st to end	Day	4 5% 16% 27% 31% 17%	539
November	1st to 10th	Day	6% 4 16% 24% 28% 22%	355
	11th to 20th	Day	6% 6% 16% 30% 27% 15%	300
	21st to end	Day	11% 2 13% 33% 33% 9%	265
December	1st to 10th	Day	16% 3 20% 24% 24% 13%	200
	11th to 20th	Day	14% 5% 21% 25% 24% 12%	200
	21st to end	Day	17% 7% 22% 25% 24% 5%	220
January	1st to 10th	Night	18% 3 16% 33% 25% 5%	784
	11th to 20th	Night	14% 5% 12% 36% 27% 7%	760
	21st to end	Night	19% 3 14% 33% 27% 5%	836
February	1st to 10th	Night	17% 3 15% 36% 24% 4	760
	11th to 20th	Night	12% 6% 19% 31% 27% 4	744
	21st to end	Night	17% 5% 19% 39% 19% 2	628
March	1st to 10th	Night	17% 6% 17% 37% 21% 2	760
	11th to 20th	Night	15% 5% 19% 33% 24% 4	760
	21st to end	Night	16% 3 25% 36% 19% 1	836
April	1st to 10th	Night	13% 4 26% 35% 22% 2	760
	11th to 20th	Night	6% 6% 29% 36% 20% 3	751
	21st to end	Night	5% 4 29% 37% 23% 3	760
May	1st to 10th	Night	3 4 29% 44% 18% 3	760
	11th to 20th	Night	5% 2 27% 45% 20% 1	412
	21st to end	Night	3 3 27% 47% 19%	411
June	1st to 10th	Night	3 3 30% 46% 17% 1	380
	11th to 20th	Night	1 3 26% 50% 20% 1	380
	21st to end	Night	1 2 23% 51% 23% 1	380
July	1st to 10th	Night	1 16% 57% 25%	370
	11th to 20th	Night	1 1 11% 65% 21% 1	395
	21st to end	Night	2 14% 48% 33% 3	624
August	1st to 10th	Night	2 1 15% 46% 33% 3	570
	11th to 20th	Night	1 1 20% 50% 26% 2	752
	21st to end	Night	1 1 13% 50% 33% 2	836
September	1st to 10th	Night	1 1 13% 51% 32% 2	760
	11th to 20th	Night	3 2 17% 45% 30% 3	760
	21st to end	Night	2 1 16% 40% 35% 6%	760
October	1st to 10th	Night	5% 2 16% 37% 34% 6%	760
	11th to 20th	Night	4 3 15% 36% 37% 6%	760
	21st to end	Night	5% 2 13% 38% 32% 11%	836
November	1st to 10th	Night	8% 2 13% 34% 34% 8%	780
	11th to 20th	Night	9% 1 15% 38% 32% 5%	800
	21st to end	Night	12% 2 14% 38% 30% 5%	800
December	1st to 10th	Night	15% 2 13% 38% 26% 7%	800
	11th to 20th	Night	21% 2 13% 36% 25% 3	800
	21st to end	Night	18% 6% 18% 37% 19% 3	880

Page generated at 2025-01-04 00:24:01. Version ac07

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