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 2024-01-17.

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	7% 3 1 3 3 2 3 3 4 6% 65%	74769
	Night	8% 3 2 3 3 2 3 3 4 6% 63%	75068
Transparency	Day	22% 4 9% 18% 25% 22%	74485
	Night	23% 4 9% 19% 23% 20%	74873
Seeing	Day	7% 5% 17% 30% 29% 11%	14146
	Night	9% 3 18% 40% 27% 4	23644

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	17% 10% 2 7% 6% 3 4 4 6% 9% 31%	1519
	11th to 20th	Day	13% 6% 3 7% 5% 3 5% 5% 5% 6% 43%	1666
	21st to end	Day	13% 9% 4 6% 5% 3 6% 5% 5% 8% 36%	1782
February	1st to 10th	Day	15% 5% 3 5% 4 3 4 4 4 7% 45%	1620
	11th to 20th	Day	13% 5% 2 4 4 2 4 4 5% 8% 49%	1725
	21st to end	Day	14% 4 2 5% 5% 2 4 5% 4 6% 49%	1480
March	1st to 10th	Day	13% 6% 3 6% 5% 3 5% 6% 6% 8% 40%	1850
	11th to 20th	Day	15% 5% 2 4 4 3 4 5% 6% 8% 44%	1980
	21st to end	Day	11% 5% 2 4 4 3 5% 4 5% 7% 50%	2268
April	1st to 10th	Day	11% 5% 2 4 4 2 4 4 5% 7% 52%	2160
	11th to 20th	Day	8% 3 2 4 4 2 2 4 5% 6% 61%	2129
	21st to end	Day	6% 3 1 2 3 1 2 3 4 6% 68%	2160
May	1st to 10th	Day	3 3 1 3 2 1 3 3 5% 6% 69%	2160
	11th to 20th	Day	3 2 1 2 3 1 2 3 4 6% 72%	2431
	21st to end	Day	3 2 1 2 3 1 3 3 4 6% 72%	2723
June	1st to 10th	Day	3 3 1 2 3 1 2 3 3 6% 74%	2520
	11th to 20th	Day	1 1 1 1 1 1 2 2 5% 85%	2520
	21st to end	Day	2 1 1 1 1 1 2 1 2 3 86%	2520
July	1st to 10th	Day	1 1 1 2 3 91%	2520
	11th to 20th	Day	1 1 1 1 1 2 2 2 3 86%	2491
	21st to end	Day	1 1 1 1 1 1 2 2 2 4 84%	2546
August	1st to 10th	Day	1 2 1 2 2 1 1 1 2 2 85%	2324
	11th to 20th	Day	1 1 1 1 1 1 2 2 4 87%	2122
	21st to end	Day	1 1 1 1 1 1 1 1 3 89%	2376
September	1st to 10th	Day	1 1 1 1 1 1 1 1 3 89%	2160
	11th to 20th	Day	1 1 1 2 1 2 2 3 5% 81%	2160
	21st to end	Day	2 1 1 1 1 1 2 2 3 4 82%	1954
October	1st to 10th	Day	2 2 1 2 3 1 2 2 3 4 77%	1800
	11th to 20th	Day	3 2 1 3 2 1 3 3 3 6% 73%	1800
	21st to end	Day	5% 2 1 3 3 2 3 3 5% 6% 67%	1980
November	1st to 10th	Day	6% 3 2 4 4 3 4 4 6% 8% 56%	1727
	11th to 20th	Day	7% 6% 2 4 5% 2 4 5% 4 8% 53%	1710
	21st to end	Day	11% 5% 3 4 4 2 3 4 5% 8% 51%	1644
December	1st to 10th	Day	13% 7% 3 6% 5% 3 4 3 4 7% 45%	1520
	11th to 20th	Day	23% 10% 4 8% 6% 3 4 4 4 6% 27%	1928
	21st to end	Day	13% 8% 3 8% 7% 5% 6% 7% 6% 8% 28%	2794
January	1st to 10th	Night	16% 6% 3 5% 5% 2 3 4 6% 9% 41%	2646
	11th to 20th	Night	12% 6% 3 5% 5% 3 5% 5% 5% 7% 46%	2434
	21st to end	Night	16% 5% 2 4 4 3 4 4 5% 7% 46%	2574
February	1st to 10th	Night	15% 4 2 4 3 2 4 2 4 6% 53%	2340
	11th to 20th	Night	12% 4 1 3 4 3 3 4 5% 9% 54%	2121
	21st to end	Night	15% 4 2 4 3 2 3 3 5% 6% 53%	1776
March	1st to 10th	Night	14% 7% 2 5% 5% 3 5% 4 4 6% 45%	2106
	11th to 20th	Night	14% 5% 2 4 5% 2 3 4 4 7% 49%	1980
	21st to end	Night	14% 4 2 3 4 3 4 4 4 7% 53%	2088
April	1st to 10th	Night	10% 4 2 4 4 2 3 4 4 7% 55%	1800
	11th to 20th	Night	6% 3 2 3 3 2 4 4 4 6% 62%	1779
	21st to end	Night	4 2 1 2 2 1 3 4 3 7% 69%	1800
May	1st to 10th	Night	3 2 1 3 3 2 2 3 4 6% 71%	1800
	11th to 20th	Night	3 2 1 2 2 2 2 2 4 7% 74%	1453
	21st to end	Night	2 2 1 1 2 1 2 3 4 6% 76%	1552
June	1st to 10th	Night	3 2 1 2 2 2 2 3 2 4 77%	1440
	11th to 20th	Night	1 1 1 2 1 3 5% 86%	1440
	21st to end	Night	1 2 1 2 1 2 3 87%	1440
July	1st to 10th	Night	1 1 1 1 2 2 90%	1440
	11th to 20th	Night	1 1 1 1 1 1 1 2 2 3 86%	1465
	21st to end	Night	1 1 1 2 2 1 2 2 3 4 80%	1773
August	1st to 10th	Night	1 2 1 1 1 1 1 2 2 2 85%	1613
	11th to 20th	Night	1 1 1 1 2 1 2 3 3 3 83%	1780
	21st to end	Night	1 1 1 1 1 1 2 3 89%	1980
September	1st to 10th	Night	1 1 2 1 1 1 2 2 90%	1800
	11th to 20th	Night	2 1 1 1 1 1 1 2 4 85%	1800
	21st to end	Night	1 1 1 1 2 1 2 1 2 3 85%	2002
October	1st to 10th	Night	5% 2 1 2 2 2 3 3 3 4 75%	2160
	11th to 20th	Night	3 1 1 2 3 2 2 2 3 4 76%	2160
	21st to end	Night	5% 1 1 2 1 1 2 3 4 6% 75%	2376
November	1st to 10th	Night	8% 3 2 2 3 2 3 3 4 7% 64%	2351
	11th to 20th	Night	7% 4 1 4 3 2 3 4 6% 6% 60%	2470
	21st to end	Night	9% 4 2 3 4 2 3 4 4 6% 59%	2531
December	1st to 10th	Night	14% 5% 2 4 4 2 4 4 5% 6% 50%	2660
	11th to 20th	Night	20% 7% 3 5% 5% 3 4 5% 5% 7% 35%	3276
	21st to end	Night	16% 6% 3 8% 8% 4 5% 6% 6% 7% 31%	4862

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% 13% 16% 10% 1	1519
	11th to 20th	Day	42% 7% 11% 17% 16% 6%	1666
	21st to end	Day	47% 6% 12% 16% 16% 3	1782
February	1st to 10th	Day	38% 6% 11% 23% 15% 6%	1620
	11th to 20th	Day	33% 5% 13% 23% 20% 6%	1735
	21st to end	Day	35% 5% 11% 24% 21% 4	1480
March	1st to 10th	Day	39% 7% 12% 23% 16% 3	1850
	11th to 20th	Day	36% 6% 12% 25% 17% 4	1980
	21st to end	Day	33% 6% 17% 21% 19% 5%	2268
April	1st to 10th	Day	31% 6% 13% 24% 17% 9%	2160
	11th to 20th	Day	23% 5% 11% 29% 24% 8%	2132
	21st to end	Day	19% 3 11% 29% 30% 7%	2160
May	1st to 10th	Day	15% 4 11% 25% 34% 11%	2160
	11th to 20th	Day	15% 4 7% 26% 33% 15%	2434
	21st to end	Day	15% 5% 12% 28% 29% 11%	2724
June	1st to 10th	Day	14% 4 8% 24% 32% 18%	2520
	11th to 20th	Day	6% 1 6% 20% 33% 33%	2520
	21st to end	Day	7% 1 2 10% 32% 48%	2520
July	1st to 10th	Day	3 1 3 7% 22% 64%	2451
	11th to 20th	Day	7% 2 4 8% 23% 57%	2416
	21st to end	Day	6% 1 4 6% 26% 57%	2405
August	1st to 10th	Day	9% 1 3 8% 26% 53%	2308
	11th to 20th	Day	5% 1 2 8% 32% 52%	2122
	21st to end	Day	5% 1 3 9% 33% 50%	2376
September	1st to 10th	Day	5% 1 3 7% 27% 57%	2160
	11th to 20th	Day	8% 2 5% 18% 40% 27%	2160
	21st to end	Day	9% 1 4 18% 38% 30%	1954
October	1st to 10th	Day	13% 3 7% 19% 28% 31%	1800
	11th to 20th	Day	14% 3 8% 21% 27% 26%	1800
	21st to end	Day	18% 4 8% 21% 29% 20%	1980
November	1st to 10th	Day	25% 5% 11% 24% 25% 9%	1727
	11th to 20th	Day	28% 6% 15% 26% 22% 3	1710
	21st to end	Day	32% 5% 14% 24% 19% 6%	1644
December	1st to 10th	Day	39% 7% 14% 18% 17% 6%	1520
	11th to 20th	Day	57% 6% 9% 14% 11% 3	1928
	21st to end	Day	47% 13% 17% 14% 8% 2	2794
January	1st to 10th	Night	39% 7% 15% 23% 12% 4	2646
	11th to 20th	Night	37% 8% 11% 20% 13% 11%	2434
	21st to end	Night	37% 5% 16% 16% 21% 6%	2574
February	1st to 10th	Night	33% 3 9% 25% 21% 8%	2340
	11th to 20th	Night	28% 5% 11% 23% 25% 7%	2122
	21st to end	Night	33% 4 10% 27% 21% 5%	1776
March	1st to 10th	Night	39% 6% 12% 18% 18% 6%	2106
	11th to 20th	Night	34% 6% 9% 23% 22% 7%	1980
	21st to end	Night	32% 6% 12% 24% 17% 9%	2088
April	1st to 10th	Night	29% 6% 13% 23% 21% 8%	1800
	11th to 20th	Night	23% 5% 13% 26% 22% 12%	1779
	21st to end	Night	17% 5% 13% 26% 32% 7%	1800
May	1st to 10th	Night	15% 4 10% 23% 35% 15%	1800
	11th to 20th	Night	13% 2 8% 29% 32% 15%	1453
	21st to end	Night	11% 4 12% 33% 29% 11%	1552
June	1st to 10th	Night	12% 4 9% 23% 34% 17%	1440
	11th to 20th	Night	5% 2 6% 18% 34% 35%	1440
	21st to end	Night	6% 1 2 13% 29% 49%	1440
July	1st to 10th	Night	4 1 2 8% 20% 65%	1400
	11th to 20th	Night	6% 2 4 7% 22% 59%	1417
	21st to end	Night	9% 2 4 5% 22% 57%	1674
August	1st to 10th	Night	8% 1 2 11% 26% 52%	1604
	11th to 20th	Night	7% 1 4 8% 27% 53%	1780
	21st to end	Night	5% 1 2 9% 30% 54%	1980
September	1st to 10th	Night	5% 1 3 6% 27% 59%	1800
	11th to 20th	Night	8% 1 5% 19% 38% 30%	1800
	21st to end	Night	8% 1 4 17% 38% 33%	2002
October	1st to 10th	Night	14% 3 6% 18% 26% 33%	2160
	11th to 20th	Night	13% 3 7% 19% 28% 30%	2160
	21st to end	Night	12% 3 6% 20% 32% 26%	2376
November	1st to 10th	Night	21% 4 10% 25% 24% 16%	2351
	11th to 20th	Night	24% 4 11% 25% 27% 9%	2470
	21st to end	Night	26% 4 10% 24% 27% 9%	2531
December	1st to 10th	Night	33% 6% 12% 21% 18% 9%	2660
	11th to 20th	Night	48% 6% 12% 16% 14% 4	3276
	21st to end	Night	45% 12% 14% 17% 8% 3	4862

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	29% 7% 24% 19% 17% 4	180
	11th to 20th	Day	16% 5% 21% 23% 21% 14%	350
	21st to end	Day	16% 5% 16% 25% 25% 13%	374
February	1st to 10th	Day	18% 8% 14% 20% 23% 17%	340
	11th to 20th	Day	15% 6% 25% 24% 20% 11%	405
	21st to end	Day	13% 9% 23% 27% 23% 5%	346
March	1st to 10th	Day	18% 8% 20% 31% 18% 5%	420
	11th to 20th	Day	16% 10% 20% 26% 16% 12%	420
	21st to end	Day	15% 8% 22% 28% 20% 7%	462
April	1st to 10th	Day	15% 9% 26% 24% 18% 7%	420
	11th to 20th	Day	9% 10% 27% 29% 17% 9%	412
	21st to end	Day	4 8% 25% 34% 18% 11%	420
May	1st to 10th	Day	4 9% 28% 31% 19% 9%	423
	11th to 20th	Day	6% 7% 23% 31% 26% 7%	448
	21st to end	Day	5% 7% 17% 28% 32% 11%	485
June	1st to 10th	Day	3 7% 20% 32% 29% 8%	450
	11th to 20th	Day	1 5% 19% 39% 29% 6%	450
	21st to end	Day	1 2 17% 38% 30% 11%	450
July	1st to 10th	Day	11% 37% 41% 10%	446
	11th to 20th	Day	1 6% 36% 48% 9%	444
	21st to end	Day	6% 36% 44% 13%	487
August	1st to 10th	Day	3 1 11% 36% 39% 10%	447
	11th to 20th	Day	1 1 12% 38% 38% 11%	437
	21st to end	Day	1 9% 38% 41% 11%	495
September	1st to 10th	Day	1 1 9% 38% 38% 13%	450
	11th to 20th	Day	2 2 17% 36% 30% 13%	450
	21st to end	Day	2 2 13% 31% 36% 15%	450
October	1st to 10th	Day	3 5% 18% 24% 31% 18%	450
	11th to 20th	Day	4 4 15% 25% 31% 21%	450
	21st to end	Day	5% 5% 16% 25% 31% 19%	495
November	1st to 10th	Day	7% 4 16% 23% 30% 20%	323
	11th to 20th	Day	4 6% 14% 30% 29% 16%	270
	21st to end	Day	10% 1 13% 33% 34% 10%	239
December	1st to 10th	Day	18% 3 22% 23% 23% 11%	180
	11th to 20th	Day	14% 4 21% 26% 23% 11%	180
	21st to end	Day	16% 8% 20% 25% 25% 6%	198
January	1st to 10th	Night	18% 3 16% 33% 24% 5%	760
	11th to 20th	Night	14% 4 12% 36% 27% 7%	748
	21st to end	Night	19% 2 14% 33% 27% 5%	792
February	1st to 10th	Night	16% 2 15% 37% 25% 4	720
	11th to 20th	Night	12% 6% 19% 32% 27% 4	704
	21st to end	Night	17% 5% 19% 38% 19% 2	592
March	1st to 10th	Night	18% 6% 17% 36% 22% 2	720
	11th to 20th	Night	15% 5% 18% 33% 24% 4	720
	21st to end	Night	16% 3 25% 35% 20% 1	792
April	1st to 10th	Night	13% 3 26% 35% 21% 2	720
	11th to 20th	Night	6% 6% 30% 36% 19% 3	711
	21st to end	Night	5% 4 28% 36% 23% 3	720
May	1st to 10th	Night	3 4 28% 43% 19% 3	720
	11th to 20th	Night	5% 2 28% 43% 21% 1	391
	21st to end	Night	3 3 28% 46% 19% 1	389
June	1st to 10th	Night	3 3 30% 45% 18% 1	360
	11th to 20th	Night	1 3 25% 51% 21% 1	360
	21st to end	Night	1 2 23% 50% 24% 1	360
July	1st to 10th	Night	1 17% 56% 26%	350
	11th to 20th	Night	1 1 11% 65% 21% 1	373
	21st to end	Night	2 1 13% 47% 35% 3	591
August	1st to 10th	Night	2 1 15% 45% 34% 3	539
	11th to 20th	Night	1 1 18% 50% 28% 2	712
	21st to end	Night	1 1 13% 51% 34% 2	792
September	1st to 10th	Night	1 1 12% 51% 33% 2	720
	11th to 20th	Night	3 2 16% 45% 30% 4	720
	21st to end	Night	2 1 16% 39% 35% 6%	720
October	1st to 10th	Night	5% 2 16% 36% 34% 6%	720
	11th to 20th	Night	4 2 14% 36% 38% 6%	720
	21st to end	Night	5% 2 13% 37% 32% 11%	792
November	1st to 10th	Night	8% 1 13% 35% 34% 8%	740
	11th to 20th	Night	9% 1 14% 38% 33% 5%	760
	21st to end	Night	11% 2 13% 40% 30% 5%	760
December	1st to 10th	Night	15% 2 13% 39% 25% 6%	760
	11th to 20th	Night	21% 2 13% 36% 25% 3	760
	21st to end	Night	18% 6% 17% 36% 20% 3	836

Page generated at 2024-01-14 06:54:59. Version ac07

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