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 2025-04-11.

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 4 4 6% 8% 51%	117265
	Night	4 5% 2 6% 4 4 4 4 6% 8% 53%	119854
Transparency	Day	29% 8% 15% 26% 18% 4	117010
	Night	27% 7% 15% 27% 19% 5%	119699
Seeing	Day	15% 20% 35% 22% 7% 2	16421
	Night	13% 19% 34% 26% 7% 1	26577

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 4 3 9% 6% 5% 5% 4 7% 9% 49%	3638
	11th to 20th	Day	1 4 3 8% 5% 6% 5% 4 5% 7% 53%	3640
	21st to end	Day	1 3 1 7% 3 3 4 3 5% 8% 62%	4004
February	1st to 10th	Day	2 8% 4 9% 6% 6% 5% 5% 6% 8% 41%	1795
	11th to 20th	Day	2 8% 4 9% 6% 4 4 4 6% 8% 44%	1898
	21st to end	Day	2 7% 4 9% 5% 3 4 5% 7% 8% 46%	1650
March	1st to 10th	Day	3 6% 3 7% 4 4 4 3 4 7% 54%	4500
	11th to 20th	Day	6% 7% 2 6% 3 3 3 3 5% 8% 55%	4667
	21st to end	Day	9% 12% 4 7% 5% 5% 5% 4 6% 7% 36%	5969
April	1st to 10th	Day	10% 10% 3 6% 3 3 4 4 5% 8% 41%	2400
	11th to 20th	Day	10% 12% 3 7% 5% 5% 4 4 6% 7% 36%	2252
	21st to end	Day	14% 10% 2 7% 4 4 3 4 6% 7% 38%	2413
May	1st to 10th	Day	12% 11% 3 8% 5% 5% 5% 4 6% 6% 34%	4241
	11th to 20th	Day	11% 7% 2 5% 4 4 5% 4 7% 9% 42%	4388
	21st to end	Day	14% 7% 2 5% 4 5% 5% 4 7% 7% 40%	4825
June	1st to 10th	Day	9% 5% 2 5% 4 4 5% 5% 8% 9% 45%	2660
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 4 7% 10% 45%	2660
	21st to end	Day	6% 3 1 3 3 3 4 5% 6% 8% 58%	2660
July	1st to 10th	Day	4 4 1 5% 5% 5% 6% 5% 8% 9% 47%	4430
	11th to 20th	Day	3 2 1 3 3 4 5% 6% 9% 9% 55%	4430
	21st to end	Day	3 2 1 3 3 4 5% 5% 7% 9% 59%	4802
August	1st to 10th	Day	6% 4 2 6% 4 4 5% 5% 8% 9% 47%	4411
	11th to 20th	Day	6% 3 1 5% 4 4 5% 5% 7% 9% 51%	3889
	21st to end	Day	5% 3 1 3 2 3 4 3 6% 7% 65%	4180
September	1st to 10th	Day	7% 4 2 4 2 3 3 3 5% 6% 62%	2280
	11th to 20th	Day	7% 4 1 4 3 3 4 4 4 6% 61%	2206
	21st to end	Day	9% 4 2 3 2 2 3 3 4 7% 61%	2090
October	1st to 10th	Day	7% 4 1 2 2 2 2 3 3 4 70%	3300
	11th to 20th	Day	8% 5% 2 4 2 3 3 2 4 6% 60%	3098
	21st to end	Day	6% 8% 2 5% 3 3 4 4 5% 7% 53%	3091
November	1st to 10th	Day	4 5% 2 4 3 3 5% 4 7% 9% 56%	1763
	11th to 20th	Day	2 5% 2 5% 4 3 5% 4 6% 9% 56%	1785
	21st to end	Day	2 6% 2 5% 3 3 5% 4 8% 9% 53%	1600
December	1st to 10th	Day	1 4 3 7% 4 4 4 4 6% 8% 56%	2620
	11th to 20th	Day	1 3 2 6% 3 4 4 4 7% 10% 54%	3026
	21st to end	Day	2 7% 3 8% 5% 4 4 4 5% 7% 50%	4004
January	1st to 10th	Night	2 7% 3 9% 5% 5% 5% 5% 5% 6% 49%	6327
	11th to 20th	Night	1 3 2 7% 6% 5% 5% 4 6% 8% 51%	6340
	21st to end	Night	1 3 1 5% 4 3 4 4 5% 7% 64%	6974
February	1st to 10th	Night	2 7% 3 8% 5% 5% 6% 5% 6% 8% 44%	2624
	11th to 20th	Night	2 7% 3 7% 6% 4 5% 4 6% 8% 48%	2389
	21st to end	Night	2 6% 2 7% 4 4 5% 4 7% 8% 50%	1980
March	1st to 10th	Night	3 7% 3 6% 4 4 3 3 5% 6% 56%	5374
	11th to 20th	Night	4 6% 2 6% 4 4 4 4 6% 9% 52%	5287
	21st to end	Night	7% 10% 3 8% 6% 5% 4 4 7% 10% 36%	4984
April	1st to 10th	Night	11% 8% 2 5% 5% 4 4 3 6% 8% 44%	2000
	11th to 20th	Night	12% 7% 2 5% 4 4 4 4 5% 7% 46%	1882
	21st to end	Night	11% 8% 2 5% 3 3 4 4 6% 8% 46%	1765
May	1st to 10th	Night	11% 8% 2 7% 4 5% 4 3 6% 7% 44%	2726
	11th to 20th	Night	7% 5% 1 4 3 4 4 4 6% 10% 51%	2517
	21st to end	Night	13% 6% 2 6% 4 3 4 4 6% 9% 44%	2762
June	1st to 10th	Night	9% 5% 1 4 4 3 5% 3 7% 9% 50%	1520
	11th to 20th	Night	10% 5% 1 4 4 4 4 4 8% 9% 48%	1520
	21st to end	Night	6% 4 1 3 4 3 4 3 7% 8% 58%	1520
July	1st to 10th	Night	5% 3 2 4 3 3 4 4 6% 8% 59%	2540
	11th to 20th	Night	3 2 1 4 2 3 4 5% 7% 10% 59%	2540
	21st to end	Night	3 3 1 3 3 3 4 4 7% 9% 60%	2761
August	1st to 10th	Night	6% 4 2 6% 4 4 5% 5% 7% 9% 49%	2528
	11th to 20th	Night	5% 3 1 4 3 3 4 4 7% 10% 56%	2946
	21st to end	Night	3 2 1 5% 3 4 4 3 5% 7% 63%	3487
September	1st to 10th	Night	6% 3 1 3 3 3 3 3 5% 8% 63%	1900
	11th to 20th	Night	7% 4 1 3 3 3 4 3 5% 7% 61%	1973
	21st to end	Night	10% 4 1 3 3 3 3 2 4 5% 61%	2090
October	1st to 10th	Night	7% 4 1 3 2 2 2 2 3 4 72%	3662
	11th to 20th	Night	7% 5% 1 4 3 3 3 3 5% 8% 59%	3799
	21st to end	Night	6% 7% 2 5% 3 3 3 3 6% 9% 53%	4467
November	1st to 10th	Night	4 5% 3 6% 4 4 5% 4 6% 8% 51%	2535
	11th to 20th	Night	3 4 2 6% 4 4 4 4 5% 7% 57%	2610
	21st to end	Night	3 5% 2 4 3 4 5% 4 6% 9% 56%	2800
December	1st to 10th	Night	1 4 2 5% 4 4 4 4 7% 8% 57%	4570
	11th to 20th	Night	1 4 2 6% 4 4 5% 5% 8% 10% 51%	5181
	21st to end	Night	2 7% 3 8% 5% 4 5% 4 6% 8% 49%	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	30% 6% 19% 28% 14% 3	3639
	11th to 20th	Day	29% 7% 18% 24% 15% 8%	3640
	21st to end	Day	19% 7% 17% 31% 20% 6%	4004
February	1st to 10th	Day	39% 9% 16% 22% 12% 3	1795
	11th to 20th	Day	37% 7% 17% 21% 15% 4	1908
	21st to end	Day	33% 9% 16% 23% 15% 5%	1650
March	1st to 10th	Day	28% 6% 9% 28% 24% 5%	4500
	11th to 20th	Day	27% 5% 16% 27% 23% 1	4667
	21st to end	Day	43% 10% 14% 23% 9% 1	5969
April	1st to 10th	Day	39% 8% 17% 22% 11% 3	2400
	11th to 20th	Day	44% 8% 14% 21% 11% 2	2255
	21st to end	Day	43% 7% 17% 24% 8% 1	2413
May	1st to 10th	Day	46% 8% 16% 20% 8% 2	4241
	11th to 20th	Day	33% 10% 16% 29% 9% 4	4392
	21st to end	Day	37% 10% 17% 24% 10% 1	4825
June	1st to 10th	Day	32% 11% 20% 25% 10% 2	2660
	11th to 20th	Day	31% 8% 18% 26% 13% 3	2660
	21st to end	Day	22% 8% 16% 30% 21% 5%	2660
July	1st to 10th	Day	28% 8% 13% 29% 21% 2	4361
	11th to 20th	Day	20% 10% 13% 26% 25% 7%	4355
	21st to end	Day	17% 7% 13% 26% 25% 12%	4651
August	1st to 10th	Day	27% 10% 17% 21% 18% 7%	4396
	11th to 20th	Day	25% 9% 17% 28% 18% 4	3929
	21st to end	Day	18% 5% 11% 25% 31% 10%	4179
September	1st to 10th	Day	22% 4 11% 28% 26% 9%	2280
	11th to 20th	Day	23% 5% 12% 27% 25% 7%	2206
	21st to end	Day	25% 4 11% 34% 23% 3	2090
October	1st to 10th	Day	19% 3 11% 20% 34% 13%	3300
	11th to 20th	Day	26% 4 11% 32% 23% 4	3098
	21st to end	Day	29% 5% 16% 29% 17% 4	3091
November	1st to 10th	Day	23% 10% 16% 25% 22% 5%	1763
	11th to 20th	Day	24% 7% 15% 28% 20% 6%	1785
	21st to end	Day	24% 7% 17% 31% 17% 4	1600
December	1st to 10th	Day	24% 8% 18% 30% 18% 1	2620
	11th to 20th	Day	21% 8% 20% 35% 15% 2	3024
	21st to end	Day	32% 7% 10% 29% 20% 3	4004
January	1st to 10th	Night	34% 7% 11% 29% 14% 4	6327
	11th to 20th	Night	27% 11% 18% 22% 15% 7%	6340
	21st to end	Night	18% 7% 16% 31% 22% 6%	6974
February	1st to 10th	Night	35% 9% 18% 21% 13% 3	2624
	11th to 20th	Night	32% 7% 18% 24% 16% 4	2389
	21st to end	Night	29% 8% 16% 25% 16% 6%	1980
March	1st to 10th	Night	26% 6% 11% 25% 23% 8%	5374
	11th to 20th	Night	26% 6% 20% 23% 22% 2	5287
	21st to end	Night	39% 10% 21% 21% 10% 1	4984
April	1st to 10th	Night	37% 7% 15% 25% 13% 2	2000
	11th to 20th	Night	36% 7% 16% 25% 14% 2	1882
	21st to end	Night	35% 7% 18% 27% 12% 1	1765
May	1st to 10th	Night	39% 7% 14% 24% 15% 2	2726
	11th to 20th	Night	26% 8% 17% 37% 10% 1	2518
	21st to end	Night	35% 9% 16% 29% 11% 1	2762
June	1st to 10th	Night	29% 8% 22% 27% 12% 1	1520
	11th to 20th	Night	30% 9% 23% 21% 14% 2	1520
	21st to end	Night	22% 7% 18% 28% 21% 4	1520
July	1st to 10th	Night	22% 6% 13% 33% 24% 3	2500
	11th to 20th	Night	19% 7% 14% 32% 23% 5%	2492
	21st to end	Night	18% 5% 16% 31% 23% 7%	2674
August	1st to 10th	Night	28% 9% 15% 22% 19% 7%	2520
	11th to 20th	Night	23% 7% 20% 26% 20% 4	2972
	21st to end	Night	20% 6% 12% 24% 32% 7%	3487
September	1st to 10th	Night	20% 5% 9% 31% 26% 9%	1900
	11th to 20th	Night	23% 5% 12% 27% 25% 8%	1973
	21st to end	Night	26% 4 9% 30% 25% 6%	2090
October	1st to 10th	Night	19% 3 9% 22% 33% 14%	3662
	11th to 20th	Night	24% 4 11% 30% 25% 5%	3799
	21st to end	Night	27% 6% 14% 29% 17% 7%	4467
November	1st to 10th	Night	28% 6% 16% 21% 21% 6%	2535
	11th to 20th	Night	25% 6% 16% 27% 20% 5%	2610
	21st to end	Night	24% 6% 18% 28% 19% 4	2800
December	1st to 10th	Night	22% 7% 18% 31% 18% 3	4570
	11th to 20th	Night	23% 10% 17% 35% 14% 1	5182
	21st to end	Night	32% 7% 10% 30% 19% 3	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	11% 20% 34% 23% 9% 3	300
	11th to 20th	Day	18% 25% 31% 17% 7% 2	300
	21st to end	Day	19% 33% 30% 12% 5% 1	330
February	1st to 10th	Day	17% 29% 40% 10% 3	300
	11th to 20th	Day	13% 30% 41% 14% 2	300
	21st to end	Day	18% 31% 36% 13% 1	249
March	1st to 10th	Day	13% 31% 38% 12% 5% 1	300
	11th to 20th	Day	18% 25% 37% 14% 6% 1	345
	21st to end	Day	26% 25% 30% 14% 4 1	550
April	1st to 10th	Day	23% 30% 34% 11% 2	500
	11th to 20th	Day	27% 32% 32% 7% 1	453
	21st to end	Day	21% 22% 33% 18% 5% 1	593
May	1st to 10th	Day	21% 24% 28% 18% 7% 2	653
	11th to 20th	Day	24% 14% 31% 20% 8% 2	675
	21st to end	Day	21% 19% 32% 19% 7% 2	734
June	1st to 10th	Day	10% 15% 32% 30% 11% 2	680
	11th to 20th	Day	16% 21% 37% 19% 6% 1	680
	21st to end	Day	10% 17% 36% 26% 9% 2	680
July	1st to 10th	Day	7% 11% 37% 33% 11% 1	671
	11th to 20th	Day	5% 12% 40% 30% 12% 1	668
	21st to end	Day	5% 9% 36% 35% 13% 2	739
August	1st to 10th	Day	6% 10% 38% 33% 12% 1	676
	11th to 20th	Day	7% 10% 39% 31% 9% 4	540
	21st to end	Day	6% 16% 36% 31% 9% 3	539
September	1st to 10th	Day	8% 16% 38% 28% 7% 4	490
	11th to 20th	Day	12% 14% 32% 29% 10% 2	416
	21st to end	Day	19% 21% 30% 16% 11% 2	300
October	1st to 10th	Day	21% 16% 29% 21% 10% 3	300
	11th to 20th	Day	21% 27% 31% 14% 5% 2	300
	21st to end	Day	17% 26% 35% 17% 4 1	330
November	1st to 10th	Day	17% 25% 34% 19% 4 2	300
	11th to 20th	Day	20% 26% 34% 15% 5%	300
	21st to end	Day	14% 25% 33% 21% 6% 1	300
December	1st to 10th	Day	18% 28% 33% 17% 3 1	300
	11th to 20th	Day	18% 25% 41% 14% 3	300
	21st to end	Day	18% 25% 34% 17% 4 1	330
January	1st to 10th	Night	14% 25% 34% 20% 7% 1	1000
	11th to 20th	Night	16% 30% 32% 16% 6% 1	1000
	21st to end	Night	14% 29% 37% 17% 4	1100
February	1st to 10th	Night	14% 30% 34% 17% 5%	824
	11th to 20th	Night	12% 25% 37% 20% 5% 1	784
	21st to end	Night	11% 23% 40% 19% 6%	660
March	1st to 10th	Night	10% 25% 37% 23% 4 1	800
	11th to 20th	Night	12% 17% 41% 22% 6% 1	794
	21st to end	Night	16% 19% 33% 25% 7%	660
April	1st to 10th	Night	18% 17% 37% 24% 4	600
	11th to 20th	Night	22% 20% 34% 21% 2 1	565
	21st to end	Night	19% 16% 27% 29% 7% 1	570
May	1st to 10th	Night	15% 14% 32% 27% 11% 1	570
	11th to 20th	Night	17% 14% 29% 29% 11% 1	561
	21st to end	Night	20% 15% 36% 21% 7%	617
June	1st to 10th	Night	11% 14% 33% 31% 11% 1	570
	11th to 20th	Night	12% 15% 40% 27% 6%	570
	21st to end	Night	8% 12% 36% 36% 9% 1	570
July	1st to 10th	Night	7% 7% 31% 39% 14% 1	555
	11th to 20th	Night	5% 5% 35% 44% 10% 1	552
	21st to end	Night	3 4 32% 47% 13%	624
August	1st to 10th	Night	6% 7% 29% 45% 12% 1	567
	11th to 20th	Night	5% 5% 32% 41% 16% 1	564
	21st to end	Night	6% 9% 27% 40% 16% 1	627
September	1st to 10th	Night	8% 8% 29% 41% 12% 2	570
	11th to 20th	Night	11% 10% 27% 40% 11% 1	570
	21st to end	Night	13% 17% 30% 30% 8% 1	570
October	1st to 10th	Night	18% 14% 30% 28% 9% 1	674
	11th to 20th	Night	16% 17% 34% 25% 7% 1	769
	21st to end	Night	16% 20% 34% 22% 6% 1	1045
November	1st to 10th	Night	10% 23% 39% 21% 5% 2	975
	11th to 20th	Night	14% 24% 37% 18% 5% 1	1000
	21st to end	Night	11% 28% 36% 21% 3	1000
December	1st to 10th	Night	14% 28% 36% 17% 5% 1	1000
	11th to 20th	Night	13% 24% 37% 21% 5% 1	1000
	21st to end	Night	16% 28% 32% 19% 5%	1100

Page generated at 2025-04-08 07:17:27. Version ac07

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