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 2026-05-02.

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% 3 1 4 3 3 3 2 4 5% 67%	140332
	Night	8% 4 1 4 3 3 3 3 4 5% 63%	139616
Transparency	Day	21% 4 7% 18% 25% 25%	135295
	Night	24% 5% 8% 19% 23% 21%	135636
Seeing	Day	7% 5% 18% 31% 28% 11%	17172
	Night	9% 3 18% 40% 26% 4	26666

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	16% 8% 2 7% 5% 5% 4 4 6% 8% 35%	4722
	11th to 20th	Day	9% 5% 2 7% 5% 5% 5% 4 4 6% 47%	5318
	21st to end	Day	12% 8% 3 9% 5% 5% 5% 4 6% 7% 37%	5843
February	1st to 10th	Day	15% 7% 2 7% 4 4 4 4 5% 7% 42%	1890
	11th to 20th	Day	13% 5% 2 5% 4 4 5% 4 6% 8% 45%	2021
	21st to end	Day	12% 4 2 6% 4 4 4 4 5% 7% 48%	1730
March	1st to 10th	Day	11% 7% 3 7% 5% 5% 5% 5% 6% 7% 39%	4749
	11th to 20th	Day	12% 4 2 4 3 3 4 3 5% 7% 52%	5110
	21st to end	Day	10% 6% 2 6% 4 5% 5% 4 6% 8% 44%	5831
April	1st to 10th	Day	10% 5% 2 4 3 3 4 4 5% 7% 53%	2520
	11th to 20th	Day	8% 3 1 4 3 3 3 3 6% 6% 60%	2489
	21st to end	Day	5% 4 1 3 2 2 4 2 4 7% 66%	2520
May	1st to 10th	Day	4 2 1 2 1 1 2 2 3 4 79%	5453
	11th to 20th	Day	1 1 1 2 1 1 1 1 3 4 83%	6179
	21st to end	Day	1 1 1 1 1 1 1 2 4 86%	6902
June	1st to 10th	Day	2 3 1 2 2 2 2 2 3 5% 75%	2800
	11th to 20th	Day	1 1 1 1 1 2 2 5% 86%	2800
	21st to end	Day	1 1 1 1 1 1 1 1 2 3 87%	2800
July	1st to 10th	Day	1 1 1 1 95%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 1 2 89%	6239
	21st to end	Day	1 1 1 1 1 1 3 90%	6402
August	1st to 10th	Day	1 1 1 1 1 1 1 2 3 88%	5760
	11th to 20th	Day	1 1 1 2 93%	5364
	21st to end	Day	1 1 1 1 1 1 1 2 4 86%	5984
September	1st to 10th	Day	1 1 1 1 1 1 1 1 3 90%	2400
	11th to 20th	Day	1 1 1 2 2 1 2 2 3 5% 80%	2400
	21st to end	Day	2 1 1 2 1 1 2 2 3 4 82%	2170
October	1st to 10th	Day	2 1 2 2 2 2 2 3 4 80%	3280
	11th to 20th	Day	2 2 1 3 2 2 2 2 2 4 78%	3261
	21st to end	Day	3 2 1 4 2 3 3 2 5% 7% 66%	3558
November	1st to 10th	Day	6% 4 1 4 3 4 3 4 6% 8% 57%	1912
	11th to 20th	Day	9% 7% 2 6% 4 3 4 4 5% 8% 49%	1890
	21st to end	Day	11% 7% 2 6% 3 3 3 3 5% 8% 49%	1816
December	1st to 10th	Day	8% 5% 2 6% 4 5% 6% 5% 7% 9% 44%	2700
	11th to 20th	Day	19% 10% 4 10% 7% 6% 5% 4 5% 6% 23%	3108
	21st to end	Day	17% 8% 3 9% 6% 7% 6% 6% 7% 8% 23%	4092
January	1st to 10th	Night	15% 6% 2 7% 4 4 4 3 6% 8% 41%	8172
	11th to 20th	Night	10% 4 2 5% 3 3 5% 6% 6% 6% 49%	7667
	21st to end	Night	12% 6% 3 8% 4 4 4 4 5% 6% 44%	8371
February	1st to 10th	Night	16% 5% 2 5% 3 3 3 2 4 6% 51%	2730
	11th to 20th	Night	13% 5% 1 4 3 4 3 3 5% 8% 51%	2485
	21st to end	Night	13% 5% 2 5% 3 3 3 3 5% 6% 53%	2076
March	1st to 10th	Night	12% 7% 2 6% 5% 4 5% 4 6% 7% 41%	5361
	11th to 20th	Night	13% 6% 2 5% 4 3 3 3 4 5% 54%	5110
	21st to end	Night	14% 6% 2 5% 3 4 4 3 5% 7% 47%	5373
April	1st to 10th	Night	9% 5% 2 4 3 3 3 3 5% 7% 56%	2100
	11th to 20th	Night	5% 4 1 4 3 4 4 3 5% 6% 61%	2079
	21st to end	Night	5% 3 1 3 2 3 3 3 4 6% 68%	2100
May	1st to 10th	Night	1 2 1 2 2 2 2 2 3 5% 80%	4570
	11th to 20th	Night	1 1 2 1 2 1 1 3 5% 82%	3695
	21st to end	Night	1 1 1 1 1 1 2 3 3 86%	3972
June	1st to 10th	Night	3 2 1 2 2 2 2 2 3 5% 78%	1600
	11th to 20th	Night	1 1 1 1 1 1 1 3 4 87%	1600
	21st to end	Night	1 2 1 1 1 1 2 2 88%	1600
July	1st to 10th	Night	1 1 1 96%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 2 2 88%	3721
	21st to end	Night	1 1 1 1 1 1 1 2 3 88%	4472
August	1st to 10th	Night	1 1 1 1 1 2 1 2 2 87%	4051
	11th to 20th	Night	1 1 1 1 1 2 93%	4506
	21st to end	Night	2 1 1 1 1 1 1 2 3 86%	5015
September	1st to 10th	Night	1 1 1 1 1 1 2 3 90%	2000
	11th to 20th	Night	2 2 1 2 1 1 1 1 3 4 83%	2000
	21st to end	Night	1 1 1 2 1 1 1 1 2 4 85%	2225
October	1st to 10th	Night	3 1 2 1 2 2 2 3 4 81%	3920
	11th to 20th	Night	2 1 1 2 2 2 2 2 3 4 81%	3897
	21st to end	Night	5% 2 1 3 2 2 3 2 4 6% 71%	4253
November	1st to 10th	Night	7% 3 1 3 3 2 2 3 5% 7% 64%	2605
	11th to 20th	Night	8% 4 1 5% 3 3 3 3 6% 7% 57%	2730
	21st to end	Night	10% 5% 2 4 3 3 3 3 4 7% 56%	2798
December	1st to 10th	Night	8% 4 2 4 3 3 4 4 6% 7% 55%	4700
	11th to 20th	Night	18% 8% 3 6% 5% 5% 6% 4 6% 10% 29%	5316
	21st to end	Night	17% 8% 3 9% 6% 6% 6% 5% 6% 7% 27%	7106

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	44% 10% 16% 21% 7% 2	4723
	11th to 20th	Day	35% 9% 6% 15% 27% 9%	5318
	21st to end	Day	43% 8% 7% 18% 21% 3	5843
February	1st to 10th	Day	40% 6% 11% 21% 15% 6%	1890
	11th to 20th	Day	35% 6% 14% 22% 18% 5%	2031
	21st to end	Day	35% 5% 12% 24% 20% 4	1730
March	1st to 10th	Day	39% 9% 10% 22% 19% 2	4749
	11th to 20th	Day	30% 5% 13% 30% 14% 8%	5110
	21st to end	Day	34% 6% 15% 29% 14% 2	5831
April	1st to 10th	Day	30% 6% 12% 26% 19% 8%	2520
	11th to 20th	Day	23% 5% 12% 28% 24% 7%	2492
	21st to end	Day	20% 3 11% 29% 30% 7%	2520
May	1st to 10th	Day	12% 2 6% 26% 35% 20%	5453
	11th to 20th	Day	8% 2 4 28% 42% 16%	6182
	21st to end	Day	6% 2 6% 22% 48% 16%	6904
June	1st to 10th	Day	13% 4 8% 25% 33% 18%	2800
	11th to 20th	Day	6% 1 5% 19% 35% 34%	2800
	21st to end	Day	7% 1 2 9% 31% 50%	2800
July	1st to 10th	Day	2 1 4 15% 78%	6251
	11th to 20th	Day	5% 1 2 3 11% 78%	6164
	21st to end	Day	4 2 11% 31% 51%	6217
August	1st to 10th	Day	8% 1 2 5% 24% 60%	4189
	11th to 20th	Day	3 1 5% 33% 56%	3880
	21st to end	Day	5% 3 7% 34% 51%	4311
September	1st to 10th	Day	5% 1 3 9% 27% 55%	2400
	11th to 20th	Day	9% 2 5% 19% 40% 25%	2400
	21st to end	Day	9% 1 4 18% 38% 30%	2170
October	1st to 10th	Day	10% 2 4 12% 26% 46%	3280
	11th to 20th	Day	12% 2 5% 24% 37% 20%	3261
	21st to end	Day	18% 3 5% 21% 39% 14%	3558
November	1st to 10th	Day	24% 5% 12% 26% 25% 8%	1912
	11th to 20th	Day	32% 6% 14% 25% 20% 3	1890
	21st to end	Day	34% 5% 13% 24% 19% 6%	1816
December	1st to 10th	Day	32% 10% 12% 27% 16% 4	2700
	11th to 20th	Day	58% 8% 8% 14% 10% 2	3108
	21st to end	Day	53% 12% 15% 13% 6% 1	4092
January	1st to 10th	Night	38% 10% 16% 25% 8% 3	8172
	11th to 20th	Night	30% 11% 6% 18% 26% 8%	7667
	21st to end	Night	39% 7% 8% 19% 21% 6%	8371
February	1st to 10th	Night	36% 4 9% 23% 20% 8%	2730
	11th to 20th	Night	32% 5% 12% 23% 22% 6%	2486
	21st to end	Night	32% 5% 11% 28% 20% 5%	2076
March	1st to 10th	Night	39% 9% 10% 18% 20% 4	5361
	11th to 20th	Night	33% 5% 12% 25% 16% 9%	5110
	21st to end	Night	35% 7% 14% 26% 14% 4	5373
April	1st to 10th	Night	27% 6% 12% 25% 23% 7%	2100
	11th to 20th	Night	23% 5% 13% 27% 21% 11%	2079
	21st to end	Night	18% 5% 13% 27% 31% 7%	2100
May	1st to 10th	Night	9% 2 9% 25% 34% 20%	4567
	11th to 20th	Night	9% 1 5% 28% 42% 15%	3695
	21st to end	Night	5% 2 6% 26% 48% 13%	3972
June	1st to 10th	Night	11% 3 9% 24% 35% 17%	1600
	11th to 20th	Night	4 1 6% 17% 35% 37%	1600
	21st to end	Night	6% 1 2 12% 28% 52%	1600
July	1st to 10th	Night	2 1 4 13% 80%	3600
	11th to 20th	Night	6% 1 2 3 12% 77%	3673
	21st to end	Night	5% 1 2 12% 29% 51%	4345
August	1st to 10th	Night	8% 2 1 6% 21% 62%	2945
	11th to 20th	Night	4 1 2 5% 28% 61%	3256
	21st to end	Night	6% 2 8% 29% 55%	3608
September	1st to 10th	Night	4 1 3 8% 27% 58%	2000
	11th to 20th	Night	9% 1 5% 20% 38% 29%	2000
	21st to end	Night	8% 1 4 17% 37% 34%	2225
October	1st to 10th	Night	10% 2 4 12% 19% 53%	3920
	11th to 20th	Night	10% 2 5% 26% 32% 24%	3897
	21st to end	Night	17% 3 5% 20% 35% 21%	4253
November	1st to 10th	Night	20% 4 10% 26% 25% 15%	2605
	11th to 20th	Night	26% 4 11% 26% 26% 8%	2730
	21st to end	Night	29% 4 10% 24% 25% 8%	2798
December	1st to 10th	Night	26% 8% 9% 27% 23% 7%	4700
	11th to 20th	Night	49% 8% 12% 17% 12% 2	5316
	21st to end	Night	50% 10% 14% 17% 6% 2	7106

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	27% 9% 22% 21% 16% 5%	220
	11th to 20th	Day	14% 5% 20% 22% 23% 16%	421
	21st to end	Day	15% 5% 16% 28% 24% 12%	473
February	1st to 10th	Day	18% 8% 17% 20% 22% 14%	430
	11th to 20th	Day	15% 8% 26% 24% 18% 9%	521
	21st to end	Day	10% 8% 22% 30% 23% 5%	446
March	1st to 10th	Day	16% 10% 21% 31% 18% 5%	540
	11th to 20th	Day	15% 9% 20% 26% 18% 11%	540
	21st to end	Day	14% 8% 22% 29% 19% 7%	594
April	1st to 10th	Day	13% 9% 25% 24% 20% 10%	540
	11th to 20th	Day	8% 9% 26% 28% 19% 10%	532
	21st to end	Day	4 7% 25% 33% 21% 10%	540
May	1st to 10th	Day	4 8% 29% 30% 20% 9%	508
	11th to 20th	Day	5% 6% 23% 32% 26% 8%	528
	21st to end	Day	4 6% 17% 30% 32% 11%	573
June	1st to 10th	Day	3 6% 18% 35% 30% 8%	530
	11th to 20th	Day	1 5% 22% 39% 28% 5%	530
	21st to end	Day	1 3 17% 37% 31% 11%	530
July	1st to 10th	Day	12% 38% 40% 10%	526
	11th to 20th	Day	1 6% 38% 47% 9%	524
	21st to end	Day	7% 37% 43% 12%	575
August	1st to 10th	Day	3 1 12% 35% 39% 10%	527
	11th to 20th	Day	1 1 14% 38% 35% 11%	517
	21st to end	Day	1 1 9% 38% 40% 11%	583
September	1st to 10th	Day	1 1 11% 38% 38% 12%	530
	11th to 20th	Day	2 3 17% 35% 30% 14%	530
	21st to end	Day	2 2 13% 34% 34% 15%	530
October	1st to 10th	Day	3 5% 16% 25% 31% 20%	530
	11th to 20th	Day	5% 4 16% 24% 31% 20%	530
	21st to end	Day	5% 5% 15% 26% 31% 18%	583
November	1st to 10th	Day	6% 4 15% 24% 27% 24%	388
	11th to 20th	Day	9% 6% 16% 30% 25% 14%	330
	21st to end	Day	11% 2 12% 33% 33% 10%	291
December	1st to 10th	Day	15% 3 18% 25% 25% 15%	220
	11th to 20th	Day	14% 5% 20% 24% 24% 14%	220
	21st to end	Day	19% 8% 21% 24% 23% 5%	242
January	1st to 10th	Night	18% 4 17% 34% 23% 5%	840
	11th to 20th	Night	13% 4 12% 36% 28% 6%	840
	21st to end	Night	17% 3 13% 35% 27% 5%	924
February	1st to 10th	Night	17% 3 16% 37% 23% 4	840
	11th to 20th	Night	13% 7% 19% 31% 25% 4	824
	21st to end	Night	15% 4 19% 41% 18% 2	692
March	1st to 10th	Night	16% 6% 18% 37% 20% 2	840
	11th to 20th	Night	14% 5% 20% 33% 24% 4	840
	21st to end	Night	15% 3 24% 37% 19% 1	924
April	1st to 10th	Night	12% 4 25% 36% 21% 2	840
	11th to 20th	Night	6% 6% 29% 37% 19% 3	831
	21st to end	Night	6% 4 29% 37% 22% 3	840
May	1st to 10th	Night	3 4 29% 43% 18% 3	804
	11th to 20th	Night	5% 3 28% 44% 19% 1	434
	21st to end	Night	3 3 29% 46% 18%	433
June	1st to 10th	Night	3 3 30% 46% 17% 1	400
	11th to 20th	Night	1 3 27% 51% 19% 1	400
	21st to end	Night	1 2 24% 50% 23% 1	400
July	1st to 10th	Night	1 18% 56% 24%	390
	11th to 20th	Night	1 1 11% 66% 20% 1	417
	21st to end	Night	2 14% 49% 32% 2	657
August	1st to 10th	Night	2 1 16% 46% 33% 3	600
	11th to 20th	Night	1 1 20% 51% 26% 2	792
	21st to end	Night	1 1 14% 51% 32% 2	880
September	1st to 10th	Night	1 1 15% 51% 31% 2	800
	11th to 20th	Night	3 2 17% 45% 29% 4	800
	21st to end	Night	2 1 16% 41% 34% 6%	800
October	1st to 10th	Night	5% 2 16% 38% 34% 6%	800
	11th to 20th	Night	5% 3 15% 36% 36% 6%	800
	21st to end	Night	4 2 14% 38% 32% 10%	880
November	1st to 10th	Night	8% 2 14% 34% 35% 8%	820
	11th to 20th	Night	10% 2 15% 38% 30% 5%	840
	21st to end	Night	12% 2 13% 38% 30% 5%	840
December	1st to 10th	Night	14% 2 13% 38% 26% 7%	840
	11th to 20th	Night	20% 2 12% 35% 26% 4	840
	21st to end	Night	19% 6% 18% 36% 19% 2	924

Page generated at 2026-04-28 23:21:45. Version ac07

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