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 2025-09-04.

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	18% 13% 4 10% 5% 4 4 3 4 6% 28%	126703
	Night	18% 14% 5% 12% 6% 4 4 3 4 5% 25%	122127
Transparency	Day	58% 5% 10% 17% 9% 1	130272
	Night	61% 5% 10% 16% 7% 1	126645
Seeing	Day	27% 20% 31% 16% 5% 1	18095
	Night	26% 19% 31% 19% 5%	25596

By Month

Percentage Cloud Cover NE (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	14% 26% 9% 18% 11% 4 2 1 2 3 11%	3157
	11th to 20th	Day	13% 22% 10% 27% 5% 4 3 2 3 3 7%	3808
	21st to end	Day	13% 16% 7% 16% 12% 9% 4 2 3 4 14%	4466
February	1st to 10th	Day	17% 17% 7% 14% 11% 7% 4 3 3 4 12%	1800
	11th to 20th	Day	16% 17% 6% 16% 11% 6% 3 2 3 4 16%	1753
	21st to end	Day	11% 16% 7% 11% 8% 5% 4 3 4 4 28%	1530
March	1st to 10th	Day	22% 18% 5% 14% 5% 6% 3 2 3 4 17%	4898
	11th to 20th	Day	14% 18% 4 10% 5% 4 3 3 4 5% 28%	4980
	21st to end	Day	22% 20% 5% 11% 4 4 4 3 3 4 20%	5466
April	1st to 10th	Day	23% 16% 4 7% 4 3 3 3 5% 4 27%	2350
	11th to 20th	Day	20% 14% 3 6% 4 3 3 3 4 6% 35%	2508
	21st to end	Day	21% 12% 3 6% 4 5% 4 3 4 6% 32%	2600
May	1st to 10th	Day	24% 18% 4 8% 4 4 4 3 5% 5% 20%	5904
	11th to 20th	Day	19% 12% 3 8% 5% 5% 5% 3 5% 6% 30%	6318
	21st to end	Day	17% 12% 3 7% 4 4 4 3 5% 8% 32%	6948
June	1st to 10th	Day	24% 10% 3 6% 4 5% 4 4 6% 7% 29%	2860
	11th to 20th	Day	20% 7% 3 6% 5% 4 4 4 5% 7% 36%	3000
	21st to end	Day	23% 10% 3 7% 5% 4 5% 4 5% 6% 29%	3000
July	1st to 10th	Day	21% 9% 3 6% 4 5% 4 4 6% 8% 31%	6820
	11th to 20th	Day	19% 8% 2 6% 4 4 4 3 5% 7% 38%	6526
	21st to end	Day	12% 7% 3 6% 4 5% 5% 4 7% 9% 38%	6568
August	1st to 10th	Day	20% 10% 3 8% 4 4 4 3 5% 6% 31%	4235
	11th to 20th	Day	13% 6% 3 7% 5% 5% 5% 4 6% 7% 39%	4175
	21st to end	Day	11% 5% 2 5% 3 4 4 3 5% 8% 49%	4558
September	1st to 10th	Day	18% 6% 2 4 4 2 3 3 5% 6% 45%	2316
	11th to 20th	Day	16% 7% 3 6% 4 4 3 3 4 5% 46%	2147
	21st to end	Day	19% 7% 2 6% 4 3 3 3 5% 6% 43%	2066
October	1st to 10th	Day	19% 8% 2 6% 4 3 3 3 5% 7% 38%	1900
	11th to 20th	Day	20% 10% 2 7% 5% 3 4 3 4 7% 34%	1900
	21st to end	Day	21% 14% 3 7% 5% 4 3 3 4 5% 30%	2076
November	1st to 10th	Day	21% 14% 4 8% 5% 4 4 4 3 6% 28%	1632
	11th to 20th	Day	19% 20% 8% 9% 6% 4 4 3 3 5% 21%	1600
	21st to end	Day	19% 22% 6% 12% 5% 5% 3 3 3 4 17%	1600
December	1st to 10th	Day	18% 19% 9% 22% 5% 3 4 2 3 4 11%	2610
	11th to 20th	Day	22% 18% 7% 16% 8% 4 3 3 3 2 14%	2996
	21st to end	Day	18% 18% 9% 16% 8% 4 3 3 4 4 14%	3632
January	1st to 10th	Night	12% 22% 10% 23% 8% 3 2 2 3 3 12%	6808
	11th to 20th	Night	13% 21% 10% 26% 6% 4 3 2 3 3 11%	6064
	21st to end	Night	12% 13% 6% 18% 15% 6% 4 3 4 4 14%	6490
February	1st to 10th	Night	18% 17% 6% 17% 9% 6% 4 3 3 3 14%	2600
	11th to 20th	Night	17% 15% 6% 15% 10% 5% 4 3 4 4 17%	2533
	21st to end	Night	12% 15% 7% 12% 7% 5% 4 3 4 5% 26%	2095
March	1st to 10th	Night	20% 18% 6% 14% 9% 5% 3 2 3 3 18%	4940
	11th to 20th	Night	17% 17% 7% 9% 5% 4 4 2 4 5% 25%	4979
	21st to end	Night	25% 15% 7% 9% 5% 4 3 2 3 3 24%	5466
April	1st to 10th	Night	24% 16% 3 7% 5% 4 4 3 3 4 27%	2050
	11th to 20th	Night	22% 12% 4 6% 3 4 3 3 4 7% 31%	1838
	21st to end	Night	22% 14% 3 6% 5% 4 4 2 4 6% 30%	1800
May	1st to 10th	Night	24% 19% 4 7% 4 4 4 3 3 4 22%	4037
	11th to 20th	Night	22% 10% 3 7% 5% 4 5% 3 5% 7% 29%	3596
	21st to end	Night	17% 13% 3 8% 4 4 4 3 5% 6% 32%	3948
June	1st to 10th	Night	26% 8% 3 6% 3 4 5% 3 6% 7% 28%	1554
	11th to 20th	Night	21% 8% 3 7% 5% 4 4 3 5% 6% 35%	1400
	21st to end	Night	21% 9% 2 5% 6% 6% 4 4 6% 8% 29%	1400
July	1st to 10th	Night	22% 11% 4 9% 4 5% 5% 4 5% 6% 26%	3160
	11th to 20th	Night	17% 7% 2 6% 4 5% 5% 5% 7% 8% 34%	3404
	21st to end	Night	15% 6% 2 5% 3 5% 4 4 7% 9% 40%	4287
August	1st to 10th	Night	21% 11% 3 6% 4 3 4 3 6% 7% 33%	2915
	11th to 20th	Night	15% 8% 2 7% 5% 5% 5% 5% 6% 9% 34%	2872
	21st to end	Night	12% 6% 2 5% 4 3 4 4 6% 8% 46%	3340
September	1st to 10th	Night	15% 6% 2 4 4 4 4 4 5% 7% 44%	1931
	11th to 20th	Night	19% 7% 2 5% 3 3 3 3 4 7% 45%	2029
	21st to end	Night	17% 8% 2 6% 4 3 4 4 6% 7% 39%	2114
October	1st to 10th	Night	19% 7% 3 6% 4 4 4 3 5% 6% 40%	2280
	11th to 20th	Night	18% 10% 3 6% 4 5% 4 4 4 6% 35%	2280
	21st to end	Night	25% 12% 3 8% 3 3 4 3 4 5% 30%	2518
November	1st to 10th	Night	21% 14% 4 7% 5% 4 3 3 4 5% 30%	2665
	11th to 20th	Night	18% 22% 6% 10% 5% 4 3 3 4 4 21%	2800
	21st to end	Night	20% 21% 6% 10% 5% 4 3 3 4 5% 20%	2800
December	1st to 10th	Night	15% 20% 9% 22% 5% 4 4 3 3 3 11%	4580
	11th to 20th	Night	17% 17% 6% 19% 6% 4 3 2 3 3 19%	5213
	21st to end	Night	21% 20% 6% 14% 7% 3 3 2 3 3 18%	7341

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	80% 5% 10% 5% 1	3158
	11th to 20th	Day	82% 5% 6% 6% 1	3840
	21st to end	Day	73% 4 8% 8% 5% 1	4466
February	1st to 10th	Day	77% 4 7% 8% 4 1	1800
	11th to 20th	Day	73% 6% 8% 9% 4	1761
	21st to end	Day	60% 5% 9% 17% 7% 1	1530
March	1st to 10th	Day	72% 3 6% 11% 7% 1	4902
	11th to 20th	Day	57% 5% 13% 18% 7% 1	4980
	21st to end	Day	71% 3 4 13% 8% 1	5466
April	1st to 10th	Day	62% 5% 7% 13% 11% 2	2350
	11th to 20th	Day	52% 6% 8% 21% 13% 1	2508
	21st to end	Day	54% 5% 10% 17% 11% 3	2600
May	1st to 10th	Day	65% 5% 10% 14% 6% 1	5915
	11th to 20th	Day	54% 7% 12% 19% 6% 1	6322
	21st to end	Day	51% 6% 13% 17% 12%	6948
June	1st to 10th	Day	55% 7% 10% 20% 9%	2860
	11th to 20th	Day	48% 6% 13% 20% 12% 1	3000
	21st to end	Day	58% 6% 8% 16% 10% 1	3000
July	1st to 10th	Day	53% 4 15% 17% 10% 1	6745
	11th to 20th	Day	46% 5% 13% 26% 10%	6446
	21st to end	Day	43% 5% 16% 26% 10%	6381
August	1st to 10th	Day	55% 6% 10% 19% 10%	4222
	11th to 20th	Day	44% 6% 14% 24% 11%	4214
	21st to end	Day	36% 4 13% 28% 18% 1	4558
September	1st to 10th	Day	40% 6% 12% 26% 14% 2	2316
	11th to 20th	Day	42% 5% 11% 24% 17% 1	2147
	21st to end	Day	43% 4 10% 26% 16% 1	2066
October	1st to 10th	Day	47% 4 11% 26% 12%	3160
	11th to 20th	Day	47% 4 9% 26% 14%	3140
	21st to end	Day	49% 3 13% 22% 13% 1	3402
November	1st to 10th	Day	60% 5% 10% 16% 8% 1	1632
	11th to 20th	Day	67% 6% 11% 13% 4 1	1600
	21st to end	Day	72% 6% 9% 10% 2	1600
December	1st to 10th	Day	80% 3 6% 10% 1	2610
	11th to 20th	Day	75% 4 6% 10% 4	2995
	21st to end	Day	74% 5% 10% 7% 5%	3632
January	1st to 10th	Night	78% 5% 7% 9% 1	6808
	11th to 20th	Night	79% 5% 7% 7% 2	6107
	21st to end	Night	73% 5% 8% 11% 3	6490
February	1st to 10th	Night	76% 5% 8% 7% 4 1	2600
	11th to 20th	Night	72% 5% 8% 10% 4 1	2536
	21st to end	Night	61% 6% 10% 14% 8% 1	2095
March	1st to 10th	Night	73% 3 5% 9% 8% 1	4949
	11th to 20th	Night	59% 5% 14% 16% 6% 1	4980
	21st to end	Night	67% 3 6% 15% 8% 1	5466
April	1st to 10th	Night	62% 4 9% 12% 11% 3	2050
	11th to 20th	Night	53% 5% 9% 19% 13% 1	1838
	21st to end	Night	57% 5% 9% 15% 12% 2	1800
May	1st to 10th	Night	65% 5% 9% 13% 7%	4043
	11th to 20th	Night	54% 7% 11% 21% 5% 2	3597
	21st to end	Night	51% 7% 15% 19% 7% 1	3949
June	1st to 10th	Night	53% 6% 13% 21% 7%	1554
	11th to 20th	Night	49% 6% 11% 23% 9% 1	1400
	21st to end	Night	54% 8% 11% 18% 9% 1	1400
July	1st to 10th	Night	57% 5% 13% 17% 8%	3125
	11th to 20th	Night	46% 4 17% 19% 13% 1	3361
	21st to end	Night	40% 5% 17% 25% 12%	4160
August	1st to 10th	Night	50% 7% 13% 23% 7%	2903
	11th to 20th	Night	46% 6% 17% 24% 7%	2899
	21st to end	Night	36% 4 16% 28% 16% 1	3339
September	1st to 10th	Night	39% 8% 10% 28% 14% 2	1931
	11th to 20th	Night	41% 4 12% 28% 15% 1	2029
	21st to end	Night	45% 7% 9% 23% 14% 1	2114
October	1st to 10th	Night	49% 4 11% 24% 11% 1	3800
	11th to 20th	Night	44% 5% 12% 24% 15%	3779
	21st to end	Night	49% 4 11% 23% 13% 1	4142
November	1st to 10th	Night	56% 6% 12% 17% 8%	2665
	11th to 20th	Night	67% 6% 10% 13% 5% 1	2800
	21st to end	Night	67% 6% 14% 11% 2	2800
December	1st to 10th	Night	76% 4 7% 11% 1	4580
	11th to 20th	Night	71% 4 9% 14% 3	5215
	21st to end	Night	71% 4 7% 12% 5%	7341

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	34% 31% 26% 9%	200
	11th to 20th	Day	35% 25% 25% 12% 3	386
	21st to end	Day	25% 28% 36% 10% 1	440
February	1st to 10th	Day	31% 27% 29% 12% 1 1	400
	11th to 20th	Day	32% 32% 26% 9% 2	396
	21st to end	Day	24% 29% 32% 12% 3	354
March	1st to 10th	Day	32% 26% 31% 9% 2	500
	11th to 20th	Day	27% 30% 29% 10% 3	500
	21st to end	Day	32% 29% 25% 10% 3	550
April	1st to 10th	Day	39% 28% 24% 6% 3	500
	11th to 20th	Day	33% 29% 26% 10% 2	492
	21st to end	Day	30% 22% 29% 14% 5%	500
May	1st to 10th	Day	31% 24% 30% 11% 3	508
	11th to 20th	Day	27% 19% 29% 19% 6% 1	724
	21st to end	Day	29% 18% 35% 15% 3 1	790
June	1st to 10th	Day	32% 16% 30% 17% 5%	730
	11th to 20th	Day	23% 16% 34% 20% 6% 1	730
	21st to end	Day	26% 16% 32% 19% 5% 1	730
July	1st to 10th	Day	22% 10% 38% 21% 8% 1	720
	11th to 20th	Day	21% 8% 36% 24% 9%	718
	21st to end	Day	20% 11% 38% 25% 6% 1	641
August	1st to 10th	Day	16% 9% 40% 27% 8%	526
	11th to 20th	Day	19% 11% 32% 25% 10% 2	519
	21st to end	Day	18% 15% 31% 24% 9% 3	583
September	1st to 10th	Day	19% 13% 34% 24% 8% 1	502
	11th to 20th	Day	23% 14% 32% 20% 9% 2	490
	21st to end	Day	20% 19% 29% 21% 9% 1	477
October	1st to 10th	Day	23% 16% 28% 22% 8% 3	390
	11th to 20th	Day	28% 20% 28% 16% 6% 2	390
	21st to end	Day	28% 26% 26% 16% 3 1	429
November	1st to 10th	Day	28% 23% 31% 14% 3 1	395
	11th to 20th	Day	38% 22% 24% 11% 4 1	400
	21st to end	Day	34% 26% 32% 7% 2	400
December	1st to 10th	Day	34% 27% 28% 10% 2	400
	11th to 20th	Day	39% 30% 21% 9% 1	400
	21st to end	Day	31% 30% 30% 6% 1 1	285
January	1st to 10th	Night	29% 26% 31% 12% 1	900
	11th to 20th	Night	30% 24% 29% 14% 3	840
	21st to end	Night	24% 27% 32% 14% 3	880
February	1st to 10th	Night	32% 26% 28% 14% 1	800
	11th to 20th	Night	33% 28% 26% 12% 2	783
	21st to end	Night	21% 25% 34% 17% 2	660
March	1st to 10th	Night	28% 24% 32% 14% 2	800
	11th to 20th	Night	25% 23% 32% 17% 4	800
	21st to end	Night	27% 24% 32% 14% 4	880
April	1st to 10th	Night	36% 21% 27% 14% 2	650
	11th to 20th	Night	27% 23% 30% 17% 3	594
	21st to end	Night	27% 20% 30% 20% 3	600
May	1st to 10th	Night	29% 14% 33% 19% 4 1	600
	11th to 20th	Night	27% 14% 31% 24% 3	592
	21st to end	Night	22% 13% 42% 18% 4	649
June	1st to 10th	Night	29% 12% 32% 22% 5%	554
	11th to 20th	Night	23% 10% 35% 27% 5% 1	400
	21st to end	Night	24% 8% 36% 26% 6% 1	400
July	1st to 10th	Night	19% 6% 42% 28% 6%	390
	11th to 20th	Night	16% 10% 36% 27% 11%	488
	21st to end	Night	18% 8% 31% 38% 6%	656
August	1st to 10th	Night	19% 7% 30% 35% 7%	598
	11th to 20th	Night	17% 8% 29% 33% 12% 1	592
	21st to end	Night	17% 7% 32% 32% 11% 1	660
September	1st to 10th	Night	19% 9% 32% 30% 9% 1	580
	11th to 20th	Night	21% 11% 28% 28% 10% 1	699
	21st to end	Night	21% 14% 32% 24% 9% 1	760
October	1st to 10th	Night	21% 11% 32% 26% 8% 1	760
	11th to 20th	Night	24% 17% 33% 19% 6% 1	760
	21st to end	Night	33% 21% 25% 17% 3	836
November	1st to 10th	Night	32% 17% 29% 17% 5% 1	845
	11th to 20th	Night	30% 26% 25% 14% 4	900
	21st to end	Night	33% 23% 27% 15% 2	900
December	1st to 10th	Night	32% 22% 31% 13% 2	900
	11th to 20th	Night	32% 29% 27% 10% 2	900
	21st to end	Night	33% 25% 27% 14% 2	990

Page generated at 2025-09-01 02:51:08. Version ac07

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