Greenwood
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Greenwood clear sky chart from 2005-11-06 to 2026-08-15.

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% 6% 2 6% 4 4 5% 4 6% 8% 50%	152803
	Night	5% 5% 2 6% 4 4 4 4 6% 8% 53%	146584
Transparency	Day	29% 8% 15% 26% 18% 5%	147569
	Night	27% 7% 15% 27% 19% 5%	142830
Seeing	Day	16% 20% 34% 21% 7% 1	18426
	Night	13% 19% 34% 26% 7% 1	28320

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	1 5% 3 8% 5% 5% 4 3 7% 10% 50%	4720
	11th to 20th	Day	1 4 3 9% 5% 6% 5% 4 6% 8% 50%	4740
	21st to end	Day	1 3 2 8% 4 4 4 4 6% 9% 56%	5188
February	1st to 10th	Day	2 8% 4 9% 6% 6% 5% 5% 6% 8% 42%	1885
	11th to 20th	Day	2 8% 4 9% 5% 4 5% 4 6% 8% 45%	1996
	21st to end	Day	2 7% 4 8% 5% 3 4 5% 7% 8% 46%	1730
March	1st to 10th	Day	3 7% 3 7% 4 4 4 3 5% 7% 54%	4600
	11th to 20th	Day	5% 7% 2 5% 3 3 3 3 5% 8% 55%	4769
	21st to end	Day	9% 12% 4 7% 5% 5% 5% 4 6% 8% 37%	6101
April	1st to 10th	Day	11% 10% 3 6% 3 3 4 4 5% 8% 41%	2520
	11th to 20th	Day	10% 12% 3 7% 5% 5% 4 4 6% 7% 38%	2491
	21st to end	Day	14% 11% 2 7% 4 4 4 4 6% 7% 37%	2667
May	1st to 10th	Day	8% 8% 3 7% 5% 4 5% 4 5% 7% 43%	7879
	11th to 20th	Day	12% 8% 2 5% 3 4 4 4 6% 8% 43%	8191
	21st to end	Day	13% 6% 2 6% 4 5% 5% 4 7% 8% 41%	9026
June	1st to 10th	Day	9% 5% 2 4 4 4 5% 5% 8% 9% 45%	2940
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 4 7% 11% 46%	2940
	21st to end	Day	6% 3 1 3 3 3 4 5% 6% 8% 57%	2940
July	1st to 10th	Day	4 3 1 5% 4 5% 7% 6% 9% 10% 45%	8249
	11th to 20th	Day	6% 3 1 3 3 3 4 5% 7% 8% 57%	8250
	21st to end	Day	5% 4 1 4 3 4 5% 5% 8% 11% 50%	8957
August	1st to 10th	Day	5% 4 1 5% 4 4 6% 6% 9% 10% 47%	6372
	11th to 20th	Day	4 2 1 4 4 4 5% 5% 8% 10% 52%	5651
	21st to end	Day	8% 4 1 5% 3 3 4 3 6% 7% 55%	5984
September	1st to 10th	Day	7% 4 2 4 2 3 3 3 5% 6% 63%	2400
	11th to 20th	Day	7% 4 1 4 3 3 4 4 4 6% 59%	2322
	21st to end	Day	8% 4 2 3 2 2 3 3 4 7% 61%	2200
October	1st to 10th	Day	7% 4 1 3 2 2 2 3 3 4 69%	3404
	11th to 20th	Day	8% 5% 2 4 2 3 3 2 4 6% 60%	3196
	21st to end	Day	6% 8% 2 5% 3 3 4 4 5% 7% 53%	3190
November	1st to 10th	Day	4 4 2 4 3 3 5% 4 7% 9% 55%	1853
	11th to 20th	Day	2 5% 2 5% 4 3 5% 4 6% 9% 56%	1874
	21st to end	Day	2 6% 2 5% 3 3 5% 4 8% 9% 53%	1680
December	1st to 10th	Day	1 4 3 7% 4 4 4 4 6% 7% 56%	2700
	11th to 20th	Day	1 3 2 6% 3 4 4 4 7% 10% 54%	3106
	21st to end	Day	2 7% 3 8% 5% 4 4 4 5% 7% 50%	4092
January	1st to 10th	Night	2 8% 3 9% 5% 4 5% 4 5% 7% 48%	8201
	11th to 20th	Night	1 3 2 7% 6% 5% 5% 4 7% 10% 51%	8250
	21st to end	Night	1 3 2 6% 5% 4 4 3 5% 7% 62%	9026
February	1st to 10th	Night	2 7% 3 8% 5% 5% 5% 5% 6% 8% 45%	2755
	11th to 20th	Night	2 6% 3 7% 6% 4 5% 4 6% 8% 49%	2511
	21st to end	Night	2 6% 2 7% 4 4 5% 4 7% 8% 51%	2076
March	1st to 10th	Night	3 7% 3 6% 4 4 3 3 5% 6% 56%	5494
	11th to 20th	Night	4 6% 2 6% 4 4 4 4 6% 9% 52%	5405
	21st to end	Night	7% 10% 3 8% 6% 5% 5% 4 7% 10% 36%	5094
April	1st to 10th	Night	11% 8% 3 5% 5% 4 4 3 6% 8% 43%	2100
	11th to 20th	Night	11% 6% 2 5% 4 4 4 4 5% 8% 47%	2077
	21st to end	Night	11% 8% 2 5% 3 3 4 4 6% 8% 46%	1951
May	1st to 10th	Night	7% 6% 2 5% 3 4 4 3 6% 8% 51%	5065
	11th to 20th	Night	10% 8% 2 6% 4 4 4 3 5% 8% 47%	4701
	21st to end	Night	12% 9% 2 6% 3 3 3 3 5% 8% 46%	5181
June	1st to 10th	Night	8% 5% 1 4 4 4 5% 4 6% 9% 50%	1680
	11th to 20th	Night	9% 5% 1 4 4 4 5% 4 8% 9% 48%	1680
	21st to end	Night	6% 4 1 4 4 3 4 3 6% 8% 57%	1680
July	1st to 10th	Night	5% 4 2 4 3 4 4 4 7% 10% 54%	4740
	11th to 20th	Night	5% 3 1 3 2 3 4 4 6% 9% 60%	4740
	21st to end	Night	8% 5% 2 5% 3 4 5% 4 8% 9% 48%	5152
August	1st to 10th	Night	6% 4 2 5% 4 4 4 4 6% 9% 53%	3660
	11th to 20th	Night	4 3 1 4 3 3 4 4 6% 10% 58%	4282
	21st to end	Night	6% 4 2 6% 4 5% 4 3 5% 7% 54%	4993
September	1st to 10th	Night	6% 3 1 3 3 3 3 3 5% 8% 63%	2000
	11th to 20th	Night	7% 4 1 4 3 3 4 3 5% 7% 59%	2077
	21st to end	Night	10% 4 1 3 3 3 3 2 4 5% 62%	2200
October	1st to 10th	Night	7% 4 1 3 2 2 2 2 3 4 71%	3778
	11th to 20th	Night	7% 5% 1 4 3 3 3 3 5% 8% 59%	3920
	21st to end	Night	6% 7% 2 5% 3 3 3 3 6% 9% 54%	4610
November	1st to 10th	Night	4 5% 3 6% 4 4 4 4 6% 8% 52%	2665
	11th to 20th	Night	3 4 2 6% 4 4 4 4 6% 7% 57%	2741
	21st to end	Night	3 5% 2 5% 3 4 5% 4 6% 8% 56%	2940
December	1st to 10th	Night	1 4 2 5% 4 4 4 4 7% 8% 57%	4710
	11th to 20th	Night	1 4 2 6% 4 4 5% 5% 8% 10% 51%	5321
	21st to end	Night	2 7% 3 8% 5% 4 5% 4 6% 8% 49%	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	29% 5% 19% 28% 16% 2	4723
	11th to 20th	Day	29% 8% 19% 24% 14% 6%	4740
	21st to end	Day	22% 7% 17% 30% 19% 5%	5188
February	1st to 10th	Day	38% 8% 16% 22% 12% 3	1885
	11th to 20th	Day	36% 7% 17% 21% 16% 4	2006
	21st to end	Day	32% 9% 16% 24% 15% 5%	1730
March	1st to 10th	Day	28% 6% 9% 28% 24% 5%	4600
	11th to 20th	Day	27% 5% 16% 27% 23% 1	4769
	21st to end	Day	43% 10% 14% 23% 9% 1	6101
April	1st to 10th	Day	40% 8% 17% 23% 11% 3	2520
	11th to 20th	Day	42% 8% 13% 21% 14% 2	2494
	21st to end	Day	44% 8% 16% 23% 8% 1	2667
May	1st to 10th	Day	37% 8% 14% 27% 14% 1	7879
	11th to 20th	Day	35% 8% 12% 24% 16% 5%	8195
	21st to end	Day	36% 9% 15% 26% 14% 1	9027
June	1st to 10th	Day	31% 11% 20% 25% 12% 2	2940
	11th to 20th	Day	30% 8% 18% 26% 14% 3	2940
	21st to end	Day	22% 8% 16% 28% 20% 5%	2940
July	1st to 10th	Day	27% 10% 14% 28% 19% 2	8181
	11th to 20th	Day	21% 9% 12% 27% 18% 13%	8175
	21st to end	Day	24% 9% 15% 26% 18% 8%	8759
August	1st to 10th	Day	26% 10% 16% 21% 19% 7%	4676
	11th to 20th	Day	26% 9% 16% 28% 18% 4	4108
	21st to end	Day	19% 5% 11% 25% 30% 10%	4311
September	1st to 10th	Day	22% 4 11% 29% 25% 9%	2400
	11th to 20th	Day	24% 6% 12% 27% 25% 6%	2322
	21st to end	Day	25% 4 11% 33% 24% 3	2200
October	1st to 10th	Day	19% 3 11% 20% 34% 13%	3404
	11th to 20th	Day	26% 4 11% 32% 23% 4	3196
	21st to end	Day	29% 5% 16% 29% 18% 4	3190
November	1st to 10th	Day	23% 9% 16% 26% 22% 4	1853
	11th to 20th	Day	24% 7% 15% 28% 20% 6%	1874
	21st to end	Day	24% 7% 17% 31% 17% 3	1680
December	1st to 10th	Day	25% 8% 18% 30% 18% 1	2700
	11th to 20th	Day	21% 8% 20% 34% 15% 2	3104
	21st to end	Day	31% 7% 10% 30% 20% 3	4092
January	1st to 10th	Night	34% 7% 12% 31% 12% 3	8202
	11th to 20th	Night	24% 10% 21% 22% 15% 6%	8250
	21st to end	Night	21% 7% 17% 28% 21% 6%	9026
February	1st to 10th	Night	34% 9% 18% 22% 14% 3	2755
	11th to 20th	Night	31% 7% 17% 24% 17% 4	2511
	21st to end	Night	29% 8% 15% 25% 17% 6%	2076
March	1st to 10th	Night	26% 6% 11% 25% 23% 8%	5494
	11th to 20th	Night	27% 6% 20% 23% 22% 2	5405
	21st to end	Night	38% 10% 20% 21% 10% 1	5094
April	1st to 10th	Night	37% 7% 15% 26% 12% 2	2100
	11th to 20th	Night	34% 7% 15% 25% 17% 2	2077
	21st to end	Night	35% 7% 17% 27% 12% 1	1951
May	1st to 10th	Night	30% 7% 13% 30% 20% 1	5066
	11th to 20th	Night	34% 7% 13% 30% 15% 1	4702
	21st to end	Night	35% 7% 13% 28% 16%	5182
June	1st to 10th	Night	29% 8% 22% 26% 13% 1	1680
	11th to 20th	Night	30% 9% 22% 21% 16% 2	1680
	21st to end	Night	23% 7% 18% 28% 20% 4	1680
July	1st to 10th	Night	24% 6% 15% 31% 22% 2	4700
	11th to 20th	Night	20% 6% 13% 28% 21% 13%	4692
	21st to end	Night	30% 7% 15% 27% 16% 5%	5038
August	1st to 10th	Night	27% 8% 15% 22% 20% 7%	2680
	11th to 20th	Night	23% 7% 20% 26% 20% 4	3101
	21st to end	Night	21% 6% 12% 24% 31% 7%	3597
September	1st to 10th	Night	20% 5% 9% 32% 25% 9%	2000
	11th to 20th	Night	24% 5% 13% 27% 24% 7%	2077
	21st to end	Night	25% 4 9% 30% 25% 6%	2200
October	1st to 10th	Night	19% 3 9% 23% 32% 14%	3778
	11th to 20th	Night	25% 4 11% 30% 25% 5%	3920
	21st to end	Night	27% 6% 14% 29% 18% 7%	4610
November	1st to 10th	Night	28% 6% 16% 22% 21% 6%	2665
	11th to 20th	Night	25% 6% 17% 27% 20% 5%	2741
	21st to end	Night	24% 6% 18% 28% 19% 4	2940
December	1st to 10th	Night	23% 7% 18% 31% 18% 3	4710
	11th to 20th	Night	23% 10% 17% 34% 14% 2	5322
	21st to end	Night	31% 7% 11% 30% 19% 3	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	12% 21% 34% 22% 8% 2	330
	11th to 20th	Day	22% 25% 29% 16% 6% 2	330
	21st to end	Day	19% 33% 31% 12% 4 1	363
February	1st to 10th	Day	17% 30% 40% 10% 3	330
	11th to 20th	Day	13% 28% 41% 14% 3	330
	21st to end	Day	19% 33% 34% 13% 1	273
March	1st to 10th	Day	14% 31% 38% 12% 5% 2	330
	11th to 20th	Day	20% 25% 35% 14% 5% 1	377
	21st to end	Day	25% 26% 31% 13% 4 1	594
April	1st to 10th	Day	24% 30% 34% 11% 1	540
	11th to 20th	Day	28% 31% 32% 8% 1	532
	21st to end	Day	23% 23% 32% 17% 5% 1	687
May	1st to 10th	Day	20% 24% 30% 18% 7% 1	753
	11th to 20th	Day	25% 16% 30% 19% 7% 2	775
	21st to end	Day	20% 20% 32% 19% 7% 2	844
June	1st to 10th	Day	11% 17% 33% 28% 10% 2	780
	11th to 20th	Day	17% 22% 36% 18% 5% 1	780
	21st to end	Day	12% 18% 35% 25% 9% 2	780
July	1st to 10th	Day	6% 11% 39% 32% 11% 1	771
	11th to 20th	Day	6% 11% 40% 30% 11% 1	768
	21st to end	Day	6% 10% 37% 33% 12% 2	849
August	1st to 10th	Day	6% 11% 38% 33% 11% 1	776
	11th to 20th	Day	7% 10% 40% 30% 10% 3	603
	21st to end	Day	6% 16% 37% 30% 8% 3	583
September	1st to 10th	Day	8% 17% 37% 27% 7% 4	530
	11th to 20th	Day	12% 14% 33% 29% 10% 2	452
	21st to end	Day	17% 21% 30% 18% 11% 2	330
October	1st to 10th	Day	21% 17% 29% 20% 10% 3	330
	11th to 20th	Day	24% 27% 29% 13% 5% 2	330
	21st to end	Day	18% 26% 33% 17% 4 1	363
November	1st to 10th	Day	18% 24% 35% 19% 3 2	330
	11th to 20th	Day	19% 26% 34% 15% 5% 1	330
	21st to end	Day	15% 24% 33% 22% 6% 1	330
December	1st to 10th	Day	21% 28% 32% 15% 3 1	330
	11th to 20th	Day	21% 25% 39% 13% 2	330
	21st to end	Day	21% 26% 32% 16% 4 1	363
January	1st to 10th	Night	14% 25% 33% 19% 7% 1	1050
	11th to 20th	Night	18% 30% 30% 15% 6% 1	1050
	21st to end	Night	14% 29% 37% 16% 4	1155
February	1st to 10th	Night	14% 30% 34% 17% 5%	865
	11th to 20th	Night	12% 25% 37% 21% 5% 1	824
	21st to end	Night	11% 23% 41% 18% 6%	692
March	1st to 10th	Night	10% 26% 37% 23% 4 1	840
	11th to 20th	Night	13% 18% 42% 21% 6% 1	834
	21st to end	Night	16% 18% 33% 25% 6%	693
April	1st to 10th	Night	18% 17% 37% 24% 4	630
	11th to 20th	Night	22% 20% 35% 21% 2 1	623
	21st to end	Night	20% 16% 28% 30% 7% 1	630
May	1st to 10th	Night	14% 14% 33% 27% 11% 1	630
	11th to 20th	Night	18% 15% 29% 28% 10%	621
	21st to end	Night	19% 16% 36% 22% 7%	683
June	1st to 10th	Night	11% 15% 31% 32% 10%	630
	11th to 20th	Night	12% 16% 40% 27% 5%	630
	21st to end	Night	8% 13% 35% 35% 8%	630
July	1st to 10th	Night	7% 7% 31% 41% 13% 1	615
	11th to 20th	Night	6% 5% 36% 42% 10% 1	612
	21st to end	Night	4 5% 33% 46% 12%	690
August	1st to 10th	Night	5% 7% 30% 45% 12% 1	627
	11th to 20th	Night	5% 5% 32% 42% 16% 1	606
	21st to end	Night	6% 9% 28% 40% 15% 1	660
September	1st to 10th	Night	8% 8% 29% 41% 12% 2	600
	11th to 20th	Night	11% 11% 27% 40% 11% 1	600
	21st to end	Night	13% 16% 30% 32% 8% 1	600
October	1st to 10th	Night	18% 14% 29% 30% 9% 1	710
	11th to 20th	Night	17% 18% 34% 24% 7% 1	810
	21st to end	Night	17% 21% 33% 22% 6% 1	1100
November	1st to 10th	Night	11% 23% 39% 20% 4 1	1025
	11th to 20th	Night	14% 24% 37% 19% 5% 1	1050
	21st to end	Night	11% 28% 36% 22% 3	1050
December	1st to 10th	Night	15% 28% 36% 17% 4	1050
	11th to 20th	Night	14% 24% 37% 20% 4	1050
	21st to end	Night	16% 28% 33% 19% 4	1155

Page generated at 2026-08-12 01:37:14. Version ac07

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