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 2024-10-25.

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 5% 4 4 4 4 6% 8% 50%	101413
	Night	5% 5% 2 5% 4 4 4 4 6% 8% 53%	97617
Transparency	Day	29% 8% 16% 26% 17% 4	101158
	Night	27% 8% 16% 27% 18% 4	97462
Seeing	Day	15% 20% 35% 22% 7% 2	15895
	Night	13% 19% 34% 26% 7% 1	25830

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 3 2 6% 4 4 4 3 6% 9% 57%	2538
	11th to 20th	Day	1 4 2 6% 5% 5% 5% 4 6% 8% 55%	2540
	21st to end	Day	1 4 2 6% 4 3 4 4 5% 8% 60%	2794
February	1st to 10th	Day	2 8% 4 9% 6% 6% 5% 5% 6% 8% 42%	1705
	11th to 20th	Day	3 8% 4 9% 5% 4 4 4 6% 8% 45%	1800
	21st to end	Day	2 7% 4 9% 5% 3 4 5% 7% 8% 45%	1570
March	1st to 10th	Day	2 6% 3 6% 4 3 4 4 5% 8% 55%	3130
	11th to 20th	Day	5% 7% 2 6% 3 3 3 4 5% 9% 54%	3269
	21st to end	Day	9% 14% 4 8% 5% 5% 5% 4 5% 7% 35%	4165
April	1st to 10th	Day	11% 10% 3 6% 3 4 4 4 5% 9% 41%	2280
	11th to 20th	Day	10% 12% 3 7% 5% 5% 4 4 6% 7% 36%	2251
	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	11% 6% 2 4 3 3 3 4 4 5% 56%	1982
	11th to 20th	Day	8% 7% 2 4 2 3 4 3 5% 7% 55%	1872
	21st to end	Day	5% 6% 2 5% 3 3 3 4 6% 8% 55%	1823
November	1st to 10th	Day	4 4 2 4 3 3 5% 4 7% 9% 56%	1673
	11th to 20th	Day	2 5% 2 5% 4 3 4 4 6% 9% 56%	1696
	21st to end	Day	2 6% 2 5% 2 3 5% 4 8% 10% 53%	1520
December	1st to 10th	Day	1 4 3 6% 4 4 5% 5% 7% 9% 50%	1520
	11th to 20th	Day	2 4 1 4 3 4 4 4 7% 10% 56%	1926
	21st to end	Day	3 8% 4 7% 5% 4 4 4 5% 7% 51%	2794
January	1st to 10th	Night	1 4 2 7% 5% 5% 5% 5% 6% 7% 54%	4417
	11th to 20th	Night	1 4 1 5% 6% 6% 6% 5% 8% 10% 51%	4430
	21st to end	Night	1 4 2 5% 4 3 4 4 6% 8% 59%	4873
February	1st to 10th	Night	3 7% 3 8% 5% 5% 5% 5% 6% 8% 46%	2493
	11th to 20th	Night	2 6% 3 6% 6% 4 5% 4 6% 9% 49%	2267
	21st to end	Night	3 6% 2 7% 4 4 5% 4 7% 8% 49%	1884
March	1st to 10th	Night	3 6% 3 7% 5% 5% 4 4 6% 7% 53%	3734
	11th to 20th	Night	5% 7% 2 6% 4 3 4 4 6% 9% 53%	3700
	21st to end	Night	6% 11% 4 7% 6% 5% 5% 4 6% 10% 36%	3477
April	1st to 10th	Night	11% 7% 2 5% 5% 4 4 3 6% 8% 44%	1900
	11th to 20th	Night	12% 7% 2 5% 4 4 4 4 5% 7% 45%	1877
	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	11% 7% 2 4 3 3 3 3 4 6% 54%	2194
	11th to 20th	Night	8% 5% 1 5% 3 3 3 3 5% 7% 57%	2289
	21st to end	Night	6% 7% 2 5% 3 3 3 3 6% 8% 54%	2639
November	1st to 10th	Night	4 5% 2 6% 3 4 5% 4 6% 9% 52%	2405
	11th to 20th	Night	3 4 2 6% 4 4 4 4 5% 7% 57%	2479
	21st to end	Night	3 5% 2 4 3 4 5% 4 6% 9% 56%	2660
December	1st to 10th	Night	2 5% 2 5% 4 4 3 5% 7% 8% 55%	2660
	11th to 20th	Night	2 4 1 5% 3 3 4 4 8% 10% 56%	3271
	21st to end	Night	2 7% 3 8% 5% 4 4 4 5% 7% 51%	4873

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	23% 7% 24% 29% 15% 1	2539
	11th to 20th	Day	26% 8% 20% 27% 14% 5%	2540
	21st to end	Day	22% 8% 21% 33% 14% 2	2794
February	1st to 10th	Day	39% 9% 16% 22% 11% 3	1705
	11th to 20th	Day	36% 7% 18% 22% 14% 4	1810
	21st to end	Day	33% 9% 16% 22% 15% 5%	1570
March	1st to 10th	Day	25% 7% 12% 27% 23% 5%	3130
	11th to 20th	Day	26% 6% 22% 31% 14% 2	3269
	21st to end	Day	46% 10% 17% 22% 5%	4165
April	1st to 10th	Day	39% 8% 17% 22% 11% 3	2280
	11th to 20th	Day	44% 8% 14% 21% 11% 2	2254
	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	30% 5% 14% 23% 23% 5%	1982
	11th to 20th	Day	29% 5% 12% 30% 18% 6%	1872
	21st to end	Day	27% 5% 21% 25% 17% 4	1823
November	1st to 10th	Day	22% 10% 16% 25% 22% 4	1673
	11th to 20th	Day	25% 7% 15% 29% 19% 5%	1696
	21st to end	Day	24% 7% 17% 30% 17% 4	1520
December	1st to 10th	Day	27% 11% 17% 25% 18% 2	1520
	11th to 20th	Day	21% 7% 21% 34% 14% 3	1924
	21st to end	Day	32% 7% 10% 25% 22% 4	2794
January	1st to 10th	Night	27% 9% 13% 32% 16% 3	4417
	11th to 20th	Night	25% 13% 19% 24% 15% 4	4430
	21st to end	Night	22% 9% 20% 32% 15% 3	4873
February	1st to 10th	Night	35% 9% 19% 22% 13% 3	2493
	11th to 20th	Night	31% 8% 18% 25% 15% 4	2267
	21st to end	Night	30% 8% 15% 24% 16% 6%	1884
March	1st to 10th	Night	26% 8% 13% 25% 24% 3	3734
	11th to 20th	Night	27% 6% 23% 23% 18% 2	3700
	21st to end	Night	39% 11% 21% 22% 6% 1	3477
April	1st to 10th	Night	36% 7% 16% 25% 13% 2	1900
	11th to 20th	Night	36% 7% 16% 25% 14% 2	1877
	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	32% 5% 13% 23% 23% 5%	2194
	11th to 20th	Night	27% 5% 13% 29% 21% 7%	2289
	21st to end	Night	28% 6% 18% 24% 19% 6%	2639
November	1st to 10th	Night	28% 7% 17% 22% 21% 6%	2405
	11th to 20th	Night	25% 6% 17% 27% 20% 5%	2479
	21st to end	Night	25% 6% 18% 27% 19% 5%	2660
December	1st to 10th	Night	25% 8% 19% 26% 18% 4	2660
	11th to 20th	Night	20% 10% 17% 37% 15% 2	3272
	21st to end	Night	31% 8% 12% 28% 18% 4	4873

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% 32% 24% 10% 3	270
	11th to 20th	Day	17% 22% 32% 19% 7% 2	270
	21st to end	Day	18% 33% 31% 13% 5% 1	297
February	1st to 10th	Day	17% 28% 41% 11% 3	270
	11th to 20th	Day	14% 30% 40% 13% 2	270
	21st to end	Day	16% 29% 40% 14% 1	225
March	1st to 10th	Day	12% 31% 39% 12% 5% 1	270
	11th to 20th	Day	16% 24% 38% 15% 6% 1	313
	21st to end	Day	23% 26% 32% 14% 5% 1	506
April	1st to 10th	Day	22% 31% 33% 12% 2	460
	11th to 20th	Day	27% 32% 32% 7% 1	452
	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	311
November	1st to 10th	Day	15% 26% 33% 20% 4 2	270
	11th to 20th	Day	18% 25% 36% 16% 5%	270
	21st to end	Day	14% 24% 34% 21% 6% 1	270
December	1st to 10th	Day	17% 28% 32% 17% 4 1	270
	11th to 20th	Day	16% 26% 40% 15% 2	270
	21st to end	Day	18% 26% 33% 19% 4 1	297
January	1st to 10th	Night	13% 26% 33% 20% 7% 1	950
	11th to 20th	Night	15% 29% 32% 16% 7% 1	950
	21st to end	Night	13% 29% 36% 17% 4	1045
February	1st to 10th	Night	13% 29% 34% 18% 5%	783
	11th to 20th	Night	12% 26% 35% 20% 6% 1	744
	21st to end	Night	10% 23% 40% 20% 6%	628
March	1st to 10th	Night	11% 25% 36% 22% 4 1	760
	11th to 20th	Night	12% 17% 41% 23% 6% 1	755
	21st to end	Night	16% 18% 33% 26% 7%	627
April	1st to 10th	Night	18% 16% 38% 23% 4	570
	11th to 20th	Night	22% 20% 34% 20% 2 1	563
	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	17% 20% 34% 22% 6% 1	1015
November	1st to 10th	Night	10% 24% 38% 22% 5% 2	925
	11th to 20th	Night	13% 25% 37% 19% 5% 1	950
	21st to end	Night	11% 29% 36% 21% 3	950
December	1st to 10th	Night	14% 28% 35% 17% 5% 1	950
	11th to 20th	Night	12% 25% 37% 21% 5% 1	950
	21st to end	Night	16% 28% 31% 19% 5%	1045

Page generated at 2024-10-22 18:21:03. Version ac07

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