Green Ridge Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Green Ridge Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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	14% 9% 4 9% 5% 5% 5% 4 6% 8% 33%	132146
	Night	14% 10% 4 8% 5% 5% 5% 4 5% 7% 34%	130616
Transparency	Day	32% 37% 8% 14% 8% 1	135747
	Night	33% 36% 8% 14% 9% 1	135086
Seeing	Day	17% 10% 29% 26% 14% 5%	17029
	Night	19% 9% 28% 29% 13% 2	26410

By Month

Percentage Cloud Cover SE (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% 14% 5% 16% 7% 6% 4 3 5% 5% 21%	4786
	11th to 20th	Day	12% 18% 8% 16% 8% 7% 4 4 5% 5% 14%	5280
	21st to end	Day	17% 14% 5% 13% 6% 5% 6% 4 5% 6% 19%	5780
February	1st to 10th	Day	14% 15% 5% 12% 7% 6% 4 3 5% 7% 22%	1890
	11th to 20th	Day	14% 12% 5% 11% 7% 6% 5% 5% 5% 6% 24%	1894
	21st to end	Day	16% 12% 4 12% 8% 5% 5% 4 5% 5% 24%	1730
March	1st to 10th	Day	16% 10% 5% 11% 7% 5% 4 3 5% 6% 27%	4861
	11th to 20th	Day	12% 10% 4 10% 5% 5% 5% 4 4 7% 33%	5090
	21st to end	Day	17% 12% 4 10% 6% 5% 5% 3 5% 5% 28%	5586
April	1st to 10th	Day	16% 12% 3 8% 5% 5% 4 4 6% 8% 30%	2365
	11th to 20th	Day	17% 9% 3 7% 5% 5% 5% 4 5% 7% 33%	2702
	21st to end	Day	17% 9% 4 8% 5% 4 4 4 5% 7% 34%	2730
May	1st to 10th	Day	18% 10% 3 8% 5% 4 4 4 5% 6% 32%	5992
	11th to 20th	Day	16% 8% 3 7% 5% 4 5% 4 6% 9% 34%	5939
	21st to end	Day	15% 6% 3 7% 5% 4 4 4 6% 8% 38%	6698
June	1st to 10th	Day	10% 7% 2 6% 5% 4 5% 4 7% 8% 41%	2800
	11th to 20th	Day	9% 6% 2 7% 5% 5% 5% 4 7% 9% 40%	2992
	21st to end	Day	9% 6% 2 5% 5% 4 5% 5% 6% 10% 43%	3000
July	1st to 10th	Day	8% 5% 2 6% 4 4 5% 4 7% 10% 44%	6547
	11th to 20th	Day	7% 5% 2 6% 4 4 5% 4 7% 9% 46%	6048
	21st to end	Day	8% 5% 2 6% 4 5% 5% 4 6% 9% 46%	6424
August	1st to 10th	Day	14% 5% 2 5% 4 4 5% 4 6% 10% 42%	4234
	11th to 20th	Day	8% 5% 2 6% 5% 4 6% 5% 8% 11% 39%	4174
	21st to end	Day	8% 6% 2 6% 5% 5% 5% 4 7% 8% 44%	4648
September	1st to 10th	Day	10% 6% 2 6% 4 4 6% 4 6% 8% 44%	2290
	11th to 20th	Day	9% 5% 2 5% 4 3 4 3 6% 10% 48%	2200
	21st to end	Day	17% 7% 3 6% 4 5% 5% 4 6% 7% 38%	2200
October	1st to 10th	Day	13% 5% 3 6% 3 4 4 4 6% 8% 43%	2073
	11th to 20th	Day	15% 8% 2 7% 5% 5% 6% 4 6% 7% 35%	2000
	21st to end	Day	19% 9% 3 7% 5% 5% 5% 4 6% 7% 30%	2140
November	1st to 10th	Day	13% 8% 3 7% 6% 4 5% 4 6% 8% 34%	1852
	11th to 20th	Day	14% 11% 3 9% 5% 5% 5% 4 5% 8% 31%	1811
	21st to end	Day	17% 13% 5% 10% 6% 5% 5% 4 6% 7% 22%	1680
December	1st to 10th	Day	17% 12% 4 9% 6% 5% 5% 5% 6% 8% 23%	2680
	11th to 20th	Day	22% 16% 5% 10% 7% 5% 5% 4 5% 5% 17%	3070
	21st to end	Day	25% 18% 7% 12% 6% 5% 4 3 4 4 12%	3960
January	1st to 10th	Night	14% 14% 5% 11% 6% 5% 5% 4 5% 6% 25%	8079
	11th to 20th	Night	9% 16% 6% 13% 8% 6% 6% 4 5% 6% 21%	7680
	21st to end	Night	16% 14% 6% 10% 5% 5% 5% 3 4 5% 26%	8401
February	1st to 10th	Night	16% 14% 5% 10% 6% 4 4 4 5% 6% 25%	2730
	11th to 20th	Night	16% 12% 5% 9% 6% 5% 5% 4 6% 6% 26%	2610
	21st to end	Night	18% 11% 4 9% 6% 4 5% 4 5% 6% 27%	2076
March	1st to 10th	Night	16% 11% 3 8% 6% 5% 5% 3 5% 6% 33%	5211
	11th to 20th	Night	10% 7% 3 7% 5% 5% 5% 4 6% 8% 40%	5090
	21st to end	Night	15% 10% 4 8% 5% 4 4 3 5% 5% 36%	5588
April	1st to 10th	Night	17% 12% 3 6% 6% 4 4 4 5% 5% 34%	2268
	11th to 20th	Night	16% 7% 2 8% 5% 4 3 4 6% 8% 36%	1870
	21st to end	Night	15% 8% 3 7% 5% 5% 4 3 6% 7% 38%	1890
May	1st to 10th	Night	20% 9% 3 8% 5% 4 4 3 6% 6% 32%	4123
	11th to 20th	Night	13% 9% 3 8% 5% 5% 4 4 5% 7% 36%	4086
	21st to end	Night	15% 7% 2 6% 4 4 4 4 5% 7% 42%	4152
June	1st to 10th	Night	10% 6% 2 6% 4 5% 4 3 6% 9% 45%	1600
	11th to 20th	Night	9% 5% 2 5% 4 5% 5% 4 8% 10% 43%	1402
	21st to end	Night	9% 5% 2 6% 4 5% 4 4 6% 8% 48%	1400
July	1st to 10th	Night	6% 4 2 6% 5% 5% 5% 5% 6% 10% 47%	3425
	11th to 20th	Night	7% 4 2 5% 4 4 4 4 6% 9% 49%	3898
	21st to end	Night	7% 5% 2 5% 4 4 4 4 6% 7% 51%	4427
August	1st to 10th	Night	11% 6% 2 5% 4 4 4 4 6% 9% 46%	2915
	11th to 20th	Night	7% 6% 2 6% 5% 4 4 4 5% 8% 50%	2875
	21st to end	Night	8% 5% 2 6% 4 4 5% 4 7% 8% 48%	3223
September	1st to 10th	Night	9% 6% 3 6% 4 4 4 4 6% 8% 47%	2099
	11th to 20th	Night	8% 4 2 5% 3 3 4 3 4 6% 57%	2200
	21st to end	Night	15% 7% 3 7% 4 4 3 4 5% 6% 42%	2200
October	1st to 10th	Night	12% 6% 2 6% 4 3 4 3 5% 7% 48%	2326
	11th to 20th	Night	17% 8% 3 7% 4 4 4 4 5% 6% 38%	2400
	21st to end	Night	20% 8% 3 7% 5% 4 4 3 5% 7% 33%	2695
November	1st to 10th	Night	13% 8% 3 6% 4 4 4 3 6% 7% 42%	2666
	11th to 20th	Night	16% 10% 3 8% 5% 5% 4 4 5% 7% 33%	2806
	21st to end	Night	20% 11% 3 9% 6% 5% 4 3 5% 6% 28%	2940
December	1st to 10th	Night	14% 14% 4 9% 6% 5% 5% 4 6% 7% 24%	4720
	11th to 20th	Night	17% 16% 5% 10% 5% 6% 5% 4 5% 5% 22%	5345
	21st to end	Night	24% 17% 6% 11% 6% 5% 5% 3 4 5% 13%	7200

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	52% 23% 8% 11% 6% 1	4787
	11th to 20th	Day	58% 24% 4 6% 7%	5280
	21st to end	Day	53% 22% 8% 10% 7%	5780
February	1st to 10th	Day	27% 60% 3 7% 3	1890
	11th to 20th	Day	25% 59% 4 8% 3	1903
	21st to end	Day	23% 60% 4 8% 4 1	1730
March	1st to 10th	Day	44% 29% 9% 13% 5%	4861
	11th to 20th	Day	36% 30% 14% 15% 5%	5090
	21st to end	Day	43% 30% 8% 12% 7% 1	5586
April	1st to 10th	Day	26% 59% 5% 6% 3	2365
	11th to 20th	Day	21% 60% 4 9% 5% 1	2702
	21st to end	Day	20% 60% 6% 9% 5% 1	2730
May	1st to 10th	Day	38% 30% 7% 14% 8% 3	6027
	11th to 20th	Day	34% 31% 11% 17% 6% 1	5943
	21st to end	Day	34% 30% 6% 14% 11% 5%	6713
June	1st to 10th	Day	16% 63% 4 11% 6% 1	2800
	11th to 20th	Day	18% 63% 5% 7% 6% 1	2992
	21st to end	Day	14% 61% 6% 10% 8% 1	3000
July	1st to 10th	Day	28% 26% 10% 23% 12% 1	6478
	11th to 20th	Day	26% 26% 10% 24% 13% 1	5975
	21st to end	Day	26% 26% 12% 19% 17% 1	6240
August	1st to 10th	Day	26% 36% 10% 16% 11%	4221
	11th to 20th	Day	25% 37% 9% 26% 3	4213
	21st to end	Day	25% 37% 9% 23% 6%	4648
September	1st to 10th	Day	15% 58% 7% 13% 6%	2290
	11th to 20th	Day	13% 58% 7% 14% 8%	2200
	21st to end	Day	19% 57% 5% 11% 7%	2200
October	1st to 10th	Day	18% 36% 10% 19% 16% 1	3372
	11th to 20th	Day	25% 36% 6% 18% 15%	3240
	21st to end	Day	26% 36% 11% 19% 7%	3442
November	1st to 10th	Day	26% 54% 5% 10% 4 1	1852
	11th to 20th	Day	24% 55% 7% 9% 5%	1811
	21st to end	Day	29% 56% 6% 6% 2	1680
December	1st to 10th	Day	39% 36% 12% 10% 4	2680
	11th to 20th	Day	49% 32% 7% 7% 5%	3066
	21st to end	Day	57% 28% 5% 7% 2	3960
January	1st to 10th	Night	46% 23% 9% 15% 6% 1	8080
	11th to 20th	Night	52% 23% 7% 11% 6% 1	7680
	21st to end	Night	48% 22% 6% 14% 9%	8401
February	1st to 10th	Night	26% 59% 3 8% 3 1	2730
	11th to 20th	Night	23% 58% 5% 8% 5%	2612
	21st to end	Night	24% 59% 4 8% 4	2076
March	1st to 10th	Night	39% 28% 10% 14% 8% 1	5211
	11th to 20th	Night	29% 30% 12% 18% 10% 1	5090
	21st to end	Night	36% 29% 8% 12% 14% 1	5588
April	1st to 10th	Night	25% 60% 4 7% 4 1	2268
	11th to 20th	Night	21% 60% 4 8% 6% 1	1870
	21st to end	Night	18% 59% 6% 11% 6% 1	1890
May	1st to 10th	Night	40% 29% 7% 9% 10% 6%	4150
	11th to 20th	Night	36% 31% 8% 18% 6% 1	4087
	21st to end	Night	33% 30% 8% 14% 13% 2	4159
June	1st to 10th	Night	14% 62% 4 11% 8% 1	1600
	11th to 20th	Night	16% 62% 4 11% 5% 1	1402
	21st to end	Night	15% 60% 5% 10% 10%	1400
July	1st to 10th	Night	25% 26% 13% 21% 12% 3	3387
	11th to 20th	Night	25% 26% 10% 22% 16% 1	3851
	21st to end	Night	25% 26% 9% 16% 22% 2	4297
August	1st to 10th	Night	24% 36% 9% 16% 13% 2	2904
	11th to 20th	Night	24% 38% 9% 20% 10%	2902
	21st to end	Night	24% 36% 8% 22% 10%	3223
September	1st to 10th	Night	13% 58% 6% 14% 8% 1	2099
	11th to 20th	Night	11% 57% 5% 14% 11% 1	2200
	21st to end	Night	20% 57% 4 12% 7% 1	2200
October	1st to 10th	Night	14% 36% 4 23% 22% 1	3807
	11th to 20th	Night	29% 36% 5% 14% 16%	3899
	21st to end	Night	23% 35% 10% 21% 10%	4343
November	1st to 10th	Night	24% 54% 6% 9% 7% 1	2666
	11th to 20th	Night	21% 55% 6% 11% 6%	2806
	21st to end	Night	28% 55% 6% 6% 4	2940
December	1st to 10th	Night	39% 35% 12% 12% 2	4720
	11th to 20th	Night	45% 32% 7% 10% 6%	5348
	21st to end	Night	56% 28% 8% 7% 2	7200

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	22% 16% 39% 16% 6% 1	330
	11th to 20th	Day	24% 20% 41% 11% 4 1	330
	21st to end	Day	25% 15% 34% 20% 4 1	363
February	1st to 10th	Day	19% 20% 37% 20% 4 1	330
	11th to 20th	Day	24% 18% 35% 18% 4	330
	21st to end	Day	26% 19% 42% 12% 1	273
March	1st to 10th	Day	22% 17% 36% 20% 5%	455
	11th to 20th	Day	21% 14% 36% 22% 6% 1	540
	21st to end	Day	26% 15% 32% 22% 4 1	594
April	1st to 10th	Day	20% 13% 39% 21% 7% 1	540
	11th to 20th	Day	25% 15% 29% 20% 8% 3	532
	21st to end	Day	23% 10% 33% 25% 8% 1	540
May	1st to 10th	Day	22% 12% 29% 26% 8% 2	543
	11th to 20th	Day	17% 9% 28% 27% 15% 3	551
	21st to end	Day	13% 6% 26% 29% 21% 5%	573
June	1st to 10th	Day	13% 8% 28% 34% 14% 3	530
	11th to 20th	Day	11% 6% 30% 35% 16% 2	722
	21st to end	Day	12% 6% 28% 36% 15% 3	730
July	1st to 10th	Day	9% 3 24% 32% 24% 8%	721
	11th to 20th	Day	12% 4 24% 30% 21% 10%	590
	21st to end	Day	9% 2 20% 33% 26% 10%	576
August	1st to 10th	Day	10% 4 17% 32% 25% 12%	526
	11th to 20th	Day	12% 5% 15% 34% 21% 13%	519
	21st to end	Day	10% 4 18% 32% 26% 10%	583
September	1st to 10th	Day	12% 4 22% 28% 23% 12%	530
	11th to 20th	Day	7% 3 17% 27% 32% 13%	530
	21st to end	Day	20% 5% 22% 29% 16% 8%	530
October	1st to 10th	Day	17% 7% 24% 27% 19% 6%	530
	11th to 20th	Day	17% 13% 34% 24% 9% 3	530
	21st to end	Day	24% 12% 36% 20% 7% 1	523
November	1st to 10th	Day	16% 18% 35% 22% 7% 2	330
	11th to 20th	Day	23% 20% 29% 19% 7% 2	289
	21st to end	Day	23% 14% 38% 20% 5%	220
December	1st to 10th	Day	23% 18% 32% 19% 8% 1	220
	11th to 20th	Day	30% 16% 32% 19% 3	220
	21st to end	Day	31% 18% 32% 16% 4	256
January	1st to 10th	Night	25% 14% 35% 19% 6% 1	1001
	11th to 20th	Night	27% 14% 34% 21% 3	840
	21st to end	Night	21% 12% 35% 25% 6%	924
February	1st to 10th	Night	23% 13% 36% 25% 4	840
	11th to 20th	Night	22% 14% 33% 25% 6%	823
	21st to end	Night	24% 12% 40% 22% 2	692
March	1st to 10th	Night	21% 11% 34% 29% 5%	840
	11th to 20th	Night	19% 11% 37% 24% 7% 1	840
	21st to end	Night	21% 12% 33% 26% 7% 1	924
April	1st to 10th	Night	22% 12% 33% 28% 5% 1	840
	11th to 20th	Night	19% 10% 33% 28% 9% 1	626
	21st to end	Night	19% 8% 32% 32% 8%	630
May	1st to 10th	Night	19% 5% 30% 32% 13% 1	630
	11th to 20th	Night	14% 6% 27% 37% 14% 2	609
	21st to end	Night	12% 3 24% 39% 21% 1	536
June	1st to 10th	Night	11% 5% 28% 42% 14%	400
	11th to 20th	Night	12% 4 22% 47% 16% 1	400
	21st to end	Night	11% 3 25% 41% 19% 1	400
July	1st to 10th	Night	7% 2 14% 44% 31% 3	496
	11th to 20th	Night	10% 3 16% 38% 29% 3	582
	21st to end	Night	9% 2 13% 45% 28% 3	657
August	1st to 10th	Night	9% 2 12% 40% 32% 4	597
	11th to 20th	Night	8% 3 12% 37% 34% 6%	594
	21st to end	Night	11% 2 16% 38% 30% 4	660
September	1st to 10th	Night	12% 4 17% 34% 28% 6%	792
	11th to 20th	Night	9% 4 14% 34% 32% 6%	800
	21st to end	Night	19% 5% 20% 32% 19% 6%	800
October	1st to 10th	Night	14% 6% 25% 33% 19% 3	800
	11th to 20th	Night	20% 11% 29% 27% 12% 2	800
	21st to end	Night	24% 10% 31% 28% 6% 1	880
November	1st to 10th	Night	17% 12% 30% 28% 12% 2	820
	11th to 20th	Night	22% 15% 33% 23% 7% 1	840
	21st to end	Night	27% 13% 31% 22% 7%	840
December	1st to 10th	Night	25% 15% 33% 21% 6%	840
	11th to 20th	Night	23% 16% 36% 22% 3	840
	21st to end	Night	29% 14% 32% 23% 3	977

Page generated at 2026-05-14 01:58:53. Version ac07

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