South Fork Natural History Museum and Nature Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the South Fork Natural History Museum and Nature Center clear sky chart from 2005-11-06 to 2025-12-18.

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	16% 8% 2 6% 4 4 4 3 5% 7% 41%	126819
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 39%	124200
Transparency	Day	27% 38% 8% 15% 10% 2	130377
	Night	28% 38% 7% 14% 11% 2	128729
Seeing	Day	19% 10% 26% 27% 15% 4	16540
	Night	21% 11% 27% 27% 13% 2	25577

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	13% 7% 2 5% 4 4 4 3 5% 6% 46%	3597
	11th to 20th	Day	16% 8% 2 5% 4 3 4 3 5% 5% 45%	3570
	21st to end	Day	18% 9% 4 7% 4 4 3 2 4 4 39%	4032
February	1st to 10th	Day	17% 7% 2 6% 6% 4 5% 4 6% 7% 36%	1929
	11th to 20th	Day	16% 6% 2 7% 5% 4 4 3 5% 7% 43%	1948
	21st to end	Day	20% 10% 2 5% 3 3 4 2 6% 6% 39%	1650
March	1st to 10th	Day	20% 7% 2 5% 3 3 3 3 4 7% 44%	4476
	11th to 20th	Day	12% 9% 2 5% 4 3 3 3 5% 7% 45%	4675
	21st to end	Day	19% 10% 2 5% 3 4 4 3 5% 5% 39%	5919
April	1st to 10th	Day	19% 8% 2 5% 4 4 4 3 5% 7% 40%	2400
	11th to 20th	Day	18% 7% 2 5% 4 3 3 3 6% 6% 44%	2374
	21st to end	Day	18% 6% 2 5% 4 4 4 4 6% 6% 41%	2530
May	1st to 10th	Day	27% 10% 3 6% 3 3 3 3 5% 8% 29%	5886
	11th to 20th	Day	20% 10% 3 7% 4 3 3 2 4 6% 38%	5950
	21st to end	Day	19% 9% 2 6% 3 4 4 3 6% 8% 36%	6949
June	1st to 10th	Day	17% 5% 2 5% 5% 4 4 3 6% 7% 41%	2800
	11th to 20th	Day	14% 7% 2 5% 4 3 4 4 6% 9% 40%	2800
	21st to end	Day	11% 7% 2 5% 4 4 4 4 8% 9% 43%	2800
July	1st to 10th	Day	14% 9% 3 7% 4 4 4 3 7% 10% 35%	6360
	11th to 20th	Day	12% 8% 3 7% 5% 5% 5% 5% 7% 10% 34%	6360
	21st to end	Day	15% 7% 2 5% 4 4 4 3 6% 8% 43%	6926
August	1st to 10th	Day	17% 7% 2 6% 5% 4 4 4 5% 9% 37%	4287
	11th to 20th	Day	10% 6% 2 6% 5% 4 4 3 6% 8% 45%	3913
	21st to end	Day	7% 3 1 5% 4 4 5% 4 6% 9% 52%	4312
September	1st to 10th	Day	14% 7% 2 6% 3 3 3 3 5% 7% 46%	2400
	11th to 20th	Day	11% 6% 2 4 3 3 3 3 5% 6% 54%	2230
	21st to end	Day	19% 7% 2 6% 4 4 4 4 5% 7% 39%	2200
October	1st to 10th	Day	20% 5% 2 5% 3 3 3 3 4 6% 45%	2200
	11th to 20th	Day	14% 7% 1 5% 4 3 3 4 5% 7% 46%	1980
	21st to end	Day	17% 6% 2 6% 4 4 4 3 5% 7% 42%	1980
November	1st to 10th	Day	15% 6% 2 5% 4 3 4 3 4 7% 48%	1851
	11th to 20th	Day	15% 7% 2 5% 3 4 3 3 4 8% 46%	1890
	21st to end	Day	16% 9% 2 6% 3 3 4 4 5% 6% 42%	1890
December	1st to 10th	Day	14% 8% 3 7% 4 3 4 3 4 7% 43%	2753
	11th to 20th	Day	21% 9% 2 5% 2 2 2 2 3 5% 45%	3042
	21st to end	Day	19% 11% 4 7% 4 4 3 2 4 5% 37%	3960
January	1st to 10th	Night	14% 6% 2 5% 4 3 4 3 5% 7% 48%	6349
	11th to 20th	Night	15% 8% 3 8% 4 4 4 3 4 5% 43%	6315
	21st to end	Night	17% 9% 3 7% 4 4 4 4 5% 7% 35%	6922
February	1st to 10th	Night	17% 9% 3 7% 5% 3 4 4 5% 6% 37%	2470
	11th to 20th	Night	19% 8% 3 5% 4 4 3 4 6% 7% 37%	2338
	21st to end	Night	20% 7% 2 6% 4 4 4 4 4 6% 38%	1980
March	1st to 10th	Night	21% 9% 2 6% 3 3 3 3 4 6% 40%	5364
	11th to 20th	Night	16% 11% 3 6% 4 3 4 3 5% 5% 41%	5283
	21st to end	Night	20% 9% 2 5% 4 4 4 4 5% 6% 37%	5008
April	1st to 10th	Night	20% 8% 2 6% 4 4 4 3 6% 6% 38%	2000
	11th to 20th	Night	18% 8% 2 6% 3 2 3 3 5% 6% 43%	1975
	21st to end	Night	20% 7% 3 6% 4 3 4 3 4 7% 39%	1870
May	1st to 10th	Night	24% 13% 4 8% 4 3 3 3 5% 6% 27%	4054
	11th to 20th	Night	25% 11% 3 5% 4 3 4 3 5% 7% 31%	3972
	21st to end	Night	21% 10% 3 6% 4 4 4 4 6% 7% 30%	3926
June	1st to 10th	Night	17% 9% 2 6% 5% 3 5% 3 5% 7% 37%	1600
	11th to 20th	Night	18% 6% 2 5% 4 4 3 4 6% 10% 39%	1600
	21st to end	Night	13% 7% 3 6% 5% 4 5% 4 6% 9% 38%	1600
July	1st to 10th	Night	15% 12% 4 8% 4 3 5% 4 5% 8% 32%	3600
	11th to 20th	Night	17% 11% 3 6% 5% 4 4 4 7% 9% 30%	3600
	21st to end	Night	13% 7% 2 7% 5% 4 4 3 6% 8% 41%	3926
August	1st to 10th	Night	21% 8% 3 6% 4 5% 5% 5% 6% 7% 30%	2817
	11th to 20th	Night	9% 6% 2 7% 5% 5% 5% 4 7% 8% 41%	3130
	21st to end	Night	8% 4 2 5% 3 3 3 3 6% 8% 55%	3586
September	1st to 10th	Night	14% 7% 2 6% 4 4 4 4 5% 6% 42%	2000
	11th to 20th	Night	13% 6% 2 4 3 3 3 3 5% 6% 50%	2169
	21st to end	Night	18% 8% 3 6% 6% 4 4 3 5% 6% 38%	2200
October	1st to 10th	Night	18% 8% 3 6% 4 4 4 3 4 5% 42%	2200
	11th to 20th	Night	17% 7% 2 5% 3 3 3 3 4 7% 47%	2414
	21st to end	Night	19% 8% 3 7% 4 4 3 3 5% 5% 40%	2860
November	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 5% 6% 46%	2667
	11th to 20th	Night	17% 7% 2 5% 4 4 4 3 5% 6% 44%	2730
	21st to end	Night	14% 6% 2 6% 4 3 4 4 5% 8% 43%	2730
December	1st to 10th	Night	16% 8% 3 6% 5% 4 4 3 5% 6% 40%	4642
	11th to 20th	Night	24% 8% 2 6% 3 3 3 2 3 5% 40%	5307
	21st to end	Night	18% 9% 3 6% 4 4 3 2 4 6% 41%	6996

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	28% 27% 5% 21% 17% 2	3598
	11th to 20th	Day	26% 29% 8% 17% 14% 7%	3600
	21st to end	Day	37% 29% 6% 15% 11% 1	4032
February	1st to 10th	Day	19% 62% 8% 7% 4	1929
	11th to 20th	Day	19% 60% 2 10% 8% 1	1957
	21st to end	Day	16% 62% 6% 8% 6% 2	1650
March	1st to 10th	Day	31% 29% 8% 11% 11% 9%	4479
	11th to 20th	Day	26% 30% 11% 23% 10% 1	4675
	21st to end	Day	36% 29% 6% 10% 15% 4	5919
April	1st to 10th	Day	20% 62% 4 7% 6% 1	2400
	11th to 20th	Day	15% 62% 4 9% 10%	2374
	21st to end	Day	16% 62% 3 10% 8% 1	2530
May	1st to 10th	Day	42% 32% 13% 9% 4 1	5897
	11th to 20th	Day	37% 30% 9% 14% 9% 1	5954
	21st to end	Day	34% 31% 7% 19% 9%	6949
June	1st to 10th	Day	17% 62% 4 9% 8%	2800
	11th to 20th	Day	16% 63% 5% 9% 7% 1	2800
	21st to end	Day	13% 61% 5% 11% 9% 1	2800
July	1st to 10th	Day	36% 30% 9% 14% 11% 1	6290
	11th to 20th	Day	36% 29% 12% 17% 6%	6285
	21st to end	Day	30% 26% 8% 20% 14% 1	6728
August	1st to 10th	Day	33% 37% 10% 15% 5%	4273
	11th to 20th	Day	23% 38% 10% 24% 5%	3951
	21st to end	Day	15% 38% 11% 23% 13%	4312
September	1st to 10th	Day	16% 58% 6% 12% 8%	2400
	11th to 20th	Day	16% 57% 4 11% 12% 1	2230
	21st to end	Day	23% 58% 4 9% 6%	2200
October	1st to 10th	Day	20% 37% 7% 15% 17% 4	3590
	11th to 20th	Day	14% 36% 6% 16% 26% 2	3209
	21st to end	Day	17% 36% 11% 19% 15% 1	3180
November	1st to 10th	Day	19% 54% 7% 9% 10% 1	1851
	11th to 20th	Day	16% 54% 5% 11% 13% 2	1890
	21st to end	Day	18% 54% 6% 15% 6% 1	1890
December	1st to 10th	Day	25% 35% 10% 21% 8%	2753
	11th to 20th	Day	30% 31% 8% 13% 15% 3	3042
	21st to end	Day	38% 27% 5% 16% 12% 2	3960
January	1st to 10th	Night	26% 27% 6% 23% 16% 2	6349
	11th to 20th	Night	31% 27% 9% 16% 14% 4	6360
	21st to end	Night	38% 29% 7% 13% 10% 2	6922
February	1st to 10th	Night	19% 62% 5% 10% 3 1	2470
	11th to 20th	Night	18% 61% 3 9% 9% 1	2340
	21st to end	Night	19% 62% 3 7% 6% 2	1980
March	1st to 10th	Night	36% 29% 7% 12% 9% 6%	5374
	11th to 20th	Night	32% 30% 11% 17% 8% 2	5284
	21st to end	Night	37% 30% 4 7% 18% 4	5008
April	1st to 10th	Night	20% 62% 3 7% 6% 2	2000
	11th to 20th	Night	15% 63% 5% 8% 8% 1	1975
	21st to end	Night	19% 61% 3 8% 7% 1	1870
May	1st to 10th	Night	44% 31% 11% 9% 5% 1	4060
	11th to 20th	Night	41% 31% 10% 11% 5% 2	3973
	21st to end	Night	39% 31% 9% 15% 6% 1	3927
June	1st to 10th	Night	18% 62% 2 8% 9% 1	1600
	11th to 20th	Night	16% 62% 4 11% 6%	1600
	21st to end	Night	15% 61% 4 11% 8% 1	1600
July	1st to 10th	Night	40% 28% 7% 15% 9%	3560
	11th to 20th	Night	40% 29% 11% 14% 6%	3552
	21st to end	Night	32% 27% 11% 18% 12%	3809
August	1st to 10th	Night	37% 38% 9% 11% 5% 1	2807
	11th to 20th	Night	24% 39% 12% 20% 5%	3158
	21st to end	Night	13% 37% 10% 24% 15% 1	3586
September	1st to 10th	Night	17% 58% 5% 13% 7% 1	2000
	11th to 20th	Night	16% 57% 4 10% 12% 1	2169
	21st to end	Night	24% 57% 4 8% 6%	2200
October	1st to 10th	Night	24% 37% 7% 19% 12% 2	3590
	11th to 20th	Night	13% 36% 5% 16% 27% 3	3924
	21st to end	Night	17% 36% 9% 19% 17% 1	4610
November	1st to 10th	Night	19% 54% 5% 11% 9% 1	2667
	11th to 20th	Night	18% 54% 4 9% 11% 3	2730
	21st to end	Night	18% 55% 5% 15% 6% 1	2730
December	1st to 10th	Night	28% 37% 8% 19% 7% 1	4642
	11th to 20th	Night	35% 31% 7% 15% 12% 2	5307
	21st to end	Night	34% 27% 8% 13% 15% 3	6996

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	23% 22% 31% 19% 5% 1	200
	11th to 20th	Day	21% 25% 38% 14% 4	200
	21st to end	Day	24% 15% 35% 17% 8% 1	237
February	1st to 10th	Day	25% 15% 35% 21% 4	429
	11th to 20th	Day	24% 18% 36% 19% 2	496
	21st to end	Day	22% 17% 34% 20% 6%	414
March	1st to 10th	Day	21% 15% 36% 23% 5%	500
	11th to 20th	Day	18% 14% 40% 21% 6% 1	500
	21st to end	Day	23% 15% 35% 20% 7% 1	550
April	1st to 10th	Day	25% 13% 33% 21% 8% 1	500
	11th to 20th	Day	23% 11% 28% 28% 8% 1	492
	21st to end	Day	23% 10% 28% 29% 8% 2	500
May	1st to 10th	Day	25% 9% 30% 25% 9% 1	503
	11th to 20th	Day	18% 7% 29% 32% 13% 2	527
	21st to end	Day	19% 4 17% 31% 22% 6%	573
June	1st to 10th	Day	16% 4 20% 34% 22% 4	530
	11th to 20th	Day	16% 3 23% 34% 19% 4	530
	21st to end	Day	16% 3 18% 34% 23% 5%	530
July	1st to 10th	Day	15% 3 14% 36% 25% 7%	525
	11th to 20th	Day	11% 3 15% 34% 28% 9%	524
	21st to end	Day	13% 3 20% 38% 24% 3	576
August	1st to 10th	Day	13% 5% 15% 32% 27% 8%	526
	11th to 20th	Day	15% 2 16% 33% 27% 8%	519
	21st to end	Day	10% 4 21% 32% 23% 10%	583
September	1st to 10th	Day	18% 5% 20% 28% 21% 9%	530
	11th to 20th	Day	15% 4 24% 27% 21% 10%	530
	21st to end	Day	23% 5% 24% 29% 15% 4	530
October	1st to 10th	Day	24% 8% 20% 23% 18% 7%	530
	11th to 20th	Day	17% 10% 28% 25% 16% 3	508
	21st to end	Day	23% 11% 30% 23% 10% 2	462
November	1st to 10th	Day	16% 13% 28% 26% 14% 2	425
	11th to 20th	Day	21% 18% 32% 20% 8%	430
	21st to end	Day	22% 18% 28% 21% 9% 1	430
December	1st to 10th	Day	21% 21% 36% 14% 8%	267
	11th to 20th	Day	28% 23% 27% 20% 3	214
	21st to end	Day	23% 16% 33% 20% 7% 1	220
January	1st to 10th	Night	21% 20% 36% 19% 4	800
	11th to 20th	Night	27% 17% 33% 19% 4	800
	21st to end	Night	21% 19% 36% 21% 3	880
February	1st to 10th	Night	23% 14% 39% 21% 3	800
	11th to 20th	Night	25% 18% 39% 15% 2	783
	21st to end	Night	23% 18% 34% 20% 5%	660
March	1st to 10th	Night	23% 15% 32% 25% 5%	800
	11th to 20th	Night	21% 11% 34% 27% 6% 1	800
	21st to end	Night	22% 10% 36% 23% 8% 1	880
April	1st to 10th	Night	24% 9% 32% 28% 7% 1	800
	11th to 20th	Night	20% 9% 30% 30% 10% 1	791
	21st to end	Night	22% 7% 30% 29% 11% 1	670
May	1st to 10th	Night	23% 6% 26% 29% 13% 3	600
	11th to 20th	Night	23% 6% 28% 33% 10% 1	570
	21st to end	Night	22% 3 17% 31% 24% 4	433
June	1st to 10th	Night	20% 3 20% 31% 24% 2	400
	11th to 20th	Night	20% 2 18% 37% 22% 2	400
	21st to end	Night	16% 3 14% 41% 25% 2	400
July	1st to 10th	Night	14% 2 14% 39% 27% 3	390
	11th to 20th	Night	14% 3 14% 36% 28% 5%	388
	21st to end	Night	14% 3 13% 41% 26% 3	438
August	1st to 10th	Night	11% 3 12% 43% 28% 4	538
	11th to 20th	Night	13% 3 12% 37% 30% 5%	747
	21st to end	Night	12% 4 15% 33% 28% 8%	880
September	1st to 10th	Night	18% 3 19% 30% 24% 6%	800
	11th to 20th	Night	16% 4 19% 31% 24% 8%	800
	21st to end	Night	20% 5% 23% 30% 19% 3	800
October	1st to 10th	Night	22% 9% 21% 27% 17% 5%	800
	11th to 20th	Night	22% 10% 27% 26% 13% 2	800
	21st to end	Night	23% 11% 33% 22% 10% 1	880
November	1st to 10th	Night	17% 11% 29% 27% 13% 2	820
	11th to 20th	Night	23% 17% 30% 21% 8%	840
	21st to end	Night	20% 18% 29% 23% 8% 1	840
December	1st to 10th	Night	25% 19% 30% 20% 5% 1	840
	11th to 20th	Night	24% 21% 31% 20% 5%	829
	21st to end	Night	24% 16% 35% 21% 4	880

Page generated at 2025-12-15 01:13:50. Version ac07

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