Prompton State Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Prompton State Park clear sky chart from 2005-11-06 to 2026-02-01.

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% 3 8% 5% 5% 5% 4 6% 7% 33%	127750
	Night	16% 8% 3 8% 5% 4 4 4 6% 7% 35%	124275
Transparency	Day	48% 7% 12% 20% 12% 2	131253
	Night	46% 7% 13% 20% 12% 1	128859
Seeing	Day	25% 17% 28% 18% 9% 3	16322
	Night	23% 16% 28% 24% 9% 1	25643

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	10% 11% 5% 16% 9% 5% 6% 3 4 6% 24%	3677
	11th to 20th	Day	12% 13% 7% 17% 7% 5% 5% 3 4 5% 22%	3650
	21st to end	Day	16% 12% 5% 9% 5% 4 4 3 5% 5% 31%	4518
February	1st to 10th	Day	15% 11% 4 10% 7% 6% 6% 4 5% 5% 27%	1800
	11th to 20th	Day	12% 10% 5% 9% 6% 5% 5% 4 6% 7% 30%	1860
	21st to end	Day	19% 11% 3 8% 7% 4 4 4 5% 5% 30%	1650
March	1st to 10th	Day	19% 10% 4 9% 5% 4 4 3 4 5% 33%	4476
	11th to 20th	Day	12% 8% 4 8% 5% 5% 5% 4 6% 6% 38%	4975
	21st to end	Day	17% 8% 2 6% 5% 5% 5% 4 6% 7% 34%	5786
April	1st to 10th	Day	17% 12% 3 8% 5% 4 5% 4 4 6% 33%	2400
	11th to 20th	Day	16% 10% 3 7% 5% 4 3 3 5% 7% 35%	2374
	21st to end	Day	17% 9% 2 8% 6% 4 4 4 5% 6% 36%	2400
May	1st to 10th	Day	29% 7% 2 7% 5% 4 4 4 6% 6% 26%	6101
	11th to 20th	Day	21% 8% 3 7% 5% 5% 5% 4 6% 6% 29%	6318
	21st to end	Day	20% 7% 2 6% 4 5% 6% 5% 7% 8% 30%	6949
June	1st to 10th	Day	15% 7% 2 7% 6% 6% 6% 5% 6% 9% 33%	2800
	11th to 20th	Day	17% 7% 3 7% 6% 5% 5% 4 6% 9% 32%	2800
	21st to end	Day	13% 6% 3 8% 5% 5% 5% 5% 7% 9% 35%	2800
July	1st to 10th	Day	8% 6% 3 8% 6% 6% 6% 5% 7% 10% 36%	6360
	11th to 20th	Day	9% 6% 3 7% 5% 6% 7% 5% 9% 12% 31%	6360
	21st to end	Day	12% 7% 2 6% 5% 4 5% 5% 6% 8% 39%	6926
August	1st to 10th	Day	21% 6% 3 7% 5% 5% 5% 4 6% 8% 30%	4272
	11th to 20th	Day	15% 6% 3 6% 5% 5% 6% 4 7% 9% 35%	3927
	21st to end	Day	17% 5% 2 5% 3 4 5% 4 7% 9% 41%	4312
September	1st to 10th	Day	15% 6% 2 7% 4 5% 5% 3 6% 7% 40%	2400
	11th to 20th	Day	13% 5% 3 5% 4 4 4 4 5% 8% 45%	2375
	21st to end	Day	19% 6% 2 7% 5% 4 4 4 7% 8% 34%	2184
October	1st to 10th	Day	17% 7% 2 7% 4 4 4 3 5% 8% 38%	2000
	11th to 20th	Day	15% 8% 3 6% 5% 5% 4 4 6% 7% 37%	2000
	21st to end	Day	19% 9% 3 7% 5% 5% 4 4 5% 7% 31%	2096
November	1st to 10th	Day	13% 8% 2 7% 5% 4 4 4 4 7% 43%	1851
	11th to 20th	Day	14% 11% 3 7% 6% 4 5% 4 5% 7% 34%	1879
	21st to end	Day	17% 10% 3 8% 7% 5% 5% 4 6% 7% 30%	1680
December	1st to 10th	Day	15% 11% 4 9% 5% 6% 5% 5% 7% 7% 27%	2680
	11th to 20th	Day	20% 12% 4 7% 5% 4 4 3 5% 5% 30%	3066
	21st to end	Day	17% 13% 5% 11% 5% 5% 4 4 6% 6% 24%	4048
January	1st to 10th	Night	12% 8% 5% 15% 8% 6% 5% 4 5% 7% 26%	6489
	11th to 20th	Night	11% 10% 4 15% 6% 6% 6% 4 6% 8% 25%	6455
	21st to end	Night	15% 9% 3 9% 6% 6% 6% 4 6% 6% 28%	6662
February	1st to 10th	Night	14% 10% 4 8% 6% 5% 4 4 5% 7% 32%	2601
	11th to 20th	Night	13% 9% 3 9% 7% 6% 5% 5% 7% 7% 29%	2425
	21st to end	Night	16% 8% 3 9% 7% 4 5% 5% 6% 6% 31%	1980
March	1st to 10th	Night	17% 10% 5% 7% 5% 4 4 4 5% 6% 34%	5364
	11th to 20th	Night	12% 8% 4 7% 5% 4 5% 4 5% 7% 39%	4980
	21st to end	Night	14% 9% 3 8% 5% 5% 5% 4 6% 6% 34%	5146
April	1st to 10th	Night	16% 11% 4 8% 7% 5% 4 3 6% 6% 31%	2000
	11th to 20th	Night	17% 8% 3 6% 5% 4 5% 4 6% 6% 36%	1975
	21st to end	Night	19% 7% 2 7% 5% 3 4 3 5% 7% 39%	2000
May	1st to 10th	Night	32% 8% 2 5% 4 3 4 3 5% 7% 27%	3881
	11th to 20th	Night	22% 9% 3 6% 4 4 4 4 6% 5% 34%	3575
	21st to end	Night	21% 7% 3 6% 4 4 5% 4 6% 7% 32%	3926
June	1st to 10th	Night	16% 6% 2 7% 5% 5% 4 4 6% 7% 40%	1600
	11th to 20th	Night	15% 7% 2 6% 5% 4 4 4 6% 9% 40%	1600
	21st to end	Night	11% 3 1 4 4 5% 5% 4 8% 9% 45%	1600
July	1st to 10th	Night	10% 6% 3 7% 5% 5% 4 4 7% 8% 41%	3600
	11th to 20th	Night	13% 7% 2 7% 5% 5% 5% 4 7% 9% 36%	3600
	21st to end	Night	12% 6% 2 5% 4 4 4 4 5% 7% 48%	3926
August	1st to 10th	Night	20% 6% 2 7% 4 5% 5% 5% 7% 8% 33%	2873
	11th to 20th	Night	17% 6% 2 6% 4 4 4 3 6% 7% 42%	3089
	21st to end	Night	14% 4 1 5% 3 4 3 4 5% 8% 48%	3586
September	1st to 10th	Night	14% 6% 2 5% 4 4 4 3 5% 8% 45%	2000
	11th to 20th	Night	13% 5% 2 5% 2 3 4 3 4 7% 51%	2025
	21st to end	Night	17% 7% 3 6% 4 5% 4 2 5% 7% 40%	2213
October	1st to 10th	Night	19% 6% 2 5% 4 4 3 3 5% 7% 42%	2400
	11th to 20th	Night	18% 8% 2 5% 3 4 4 3 5% 6% 42%	2400
	21st to end	Night	20% 9% 2 5% 4 4 4 4 5% 8% 36%	2742
November	1st to 10th	Night	11% 8% 3 7% 5% 4 4 3 6% 6% 44%	2667
	11th to 20th	Night	18% 8% 3 6% 4 4 4 4 6% 7% 37%	2737
	21st to end	Night	14% 8% 3 7% 5% 5% 4 4 6% 7% 35%	2940
December	1st to 10th	Night	14% 9% 4 10% 4 4 4 4 6% 9% 31%	4720
	11th to 20th	Night	22% 10% 3 7% 5% 4 4 3 4 5% 34%	5348
	21st to end	Night	19% 13% 5% 9% 6% 4 4 3 6% 6% 24%	7150

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	59% 8% 9% 18% 5% 1	3678
	11th to 20th	Day	62% 6% 8% 14% 8% 2	3680
	21st to end	Day	54% 6% 8% 21% 11%	4518
February	1st to 10th	Day	55% 8% 11% 16% 10%	1800
	11th to 20th	Day	51% 8% 10% 20% 11% 1	1869
	21st to end	Day	54% 6% 11% 19% 9% 1	1650
March	1st to 10th	Day	52% 6% 10% 16% 11% 5%	4479
	11th to 20th	Day	42% 8% 15% 24% 7% 4	4975
	21st to end	Day	47% 7% 10% 12% 20% 4	5786
April	1st to 10th	Day	52% 6% 8% 20% 11% 3	2400
	11th to 20th	Day	47% 7% 11% 20% 13% 2	2374
	21st to end	Day	47% 7% 9% 20% 12% 4	2400
May	1st to 10th	Day	57% 7% 11% 15% 9% 1	6112
	11th to 20th	Day	52% 8% 10% 19% 10% 1	6322
	21st to end	Day	46% 9% 13% 19% 11% 1	6949
June	1st to 10th	Day	46% 8% 14% 20% 13%	2800
	11th to 20th	Day	47% 8% 14% 20% 9% 1	2800
	21st to end	Day	43% 9% 14% 24% 10%	2800
July	1st to 10th	Day	43% 9% 13% 22% 12% 1	6290
	11th to 20th	Day	41% 10% 15% 23% 10%	6285
	21st to end	Day	40% 7% 13% 22% 17% 1	6728
August	1st to 10th	Day	51% 7% 14% 18% 9%	4258
	11th to 20th	Day	42% 9% 15% 26% 8%	3965
	21st to end	Day	39% 7% 12% 32% 10% 1	4312
September	1st to 10th	Day	42% 7% 12% 22% 15% 1	2400
	11th to 20th	Day	36% 6% 12% 23% 21% 1	2375
	21st to end	Day	46% 6% 13% 18% 15% 1	2184
October	1st to 10th	Day	44% 6% 8% 22% 18% 2	3260
	11th to 20th	Day	38% 5% 10% 25% 17% 4	3240
	21st to end	Day	34% 6% 17% 27% 14% 3	3360
November	1st to 10th	Day	41% 6% 13% 23% 15% 2	1851
	11th to 20th	Day	47% 7% 11% 20% 12% 2	1879
	21st to end	Day	52% 7% 15% 19% 7% 1	1680
December	1st to 10th	Day	54% 8% 13% 19% 6% 1	2680
	11th to 20th	Day	55% 5% 13% 16% 10% 1	3066
	21st to end	Day	59% 6% 10% 14% 10% 1	4048
January	1st to 10th	Night	55% 7% 12% 16% 9% 1	6489
	11th to 20th	Night	53% 9% 14% 15% 7% 2	6500
	21st to end	Night	53% 7% 10% 14% 14% 2	6662
February	1st to 10th	Night	50% 8% 14% 19% 9% 1	2601
	11th to 20th	Night	50% 9% 12% 18% 10% 2	2427
	21st to end	Night	50% 8% 10% 20% 11% 2	1980
March	1st to 10th	Night	50% 5% 13% 16% 13% 2	5374
	11th to 20th	Night	42% 9% 19% 20% 7% 3	4981
	21st to end	Night	48% 7% 8% 18% 18% 2	5146
April	1st to 10th	Night	53% 7% 10% 18% 11% 2	2000
	11th to 20th	Night	47% 7% 11% 20% 15%	1975
	21st to end	Night	45% 7% 10% 19% 15% 3	2000
May	1st to 10th	Night	57% 6% 11% 16% 9% 2	3887
	11th to 20th	Night	50% 8% 12% 17% 11% 1	3576
	21st to end	Night	47% 10% 12% 20% 10% 1	3927
June	1st to 10th	Night	42% 7% 12% 23% 16%	1600
	11th to 20th	Night	40% 7% 19% 24% 9% 1	1600
	21st to end	Night	32% 11% 19% 25% 13% 1	1600
July	1st to 10th	Night	40% 8% 16% 20% 15% 1	3560
	11th to 20th	Night	43% 9% 14% 23% 11% 1	3552
	21st to end	Night	34% 6% 13% 28% 18% 1	3809
August	1st to 10th	Night	47% 9% 16% 20% 7% 1	2863
	11th to 20th	Night	41% 8% 15% 26% 10%	3117
	21st to end	Night	33% 7% 12% 34% 14%	3586
September	1st to 10th	Night	37% 8% 13% 25% 17% 1	2000
	11th to 20th	Night	32% 7% 12% 26% 22% 1	2025
	21st to end	Night	43% 6% 11% 21% 17% 1	2213
October	1st to 10th	Night	43% 7% 10% 23% 17% 1	3920
	11th to 20th	Night	33% 6% 13% 24% 20% 4	3899
	21st to end	Night	34% 6% 17% 27% 15% 1	4428
November	1st to 10th	Night	40% 6% 12% 22% 18% 2	2667
	11th to 20th	Night	44% 6% 14% 20% 13% 2	2737
	21st to end	Night	46% 8% 16% 22% 9%	2940
December	1st to 10th	Night	48% 8% 14% 25% 5%	4720
	11th to 20th	Night	53% 5% 13% 16% 12% 1	5348
	21st to end	Night	58% 7% 10% 14% 10% 1	7150

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	32% 29% 26% 11% 1	220
	11th to 20th	Day	42% 27% 22% 8% 1	220
	21st to end	Day	32% 32% 26% 6% 4	352
February	1st to 10th	Day	33% 30% 30% 6% 1	300
	11th to 20th	Day	39% 27% 29% 4 1	407
	21st to end	Day	37% 24% 28% 9% 2	414
March	1st to 10th	Day	34% 21% 29% 13% 2	500
	11th to 20th	Day	33% 23% 26% 14% 3 1	500
	21st to end	Day	32% 26% 26% 10% 4 1	550
April	1st to 10th	Day	35% 25% 25% 11% 4 1	500
	11th to 20th	Day	30% 25% 26% 9% 7% 2	492
	21st to end	Day	28% 22% 31% 13% 5% 2	500
May	1st to 10th	Day	36% 16% 27% 12% 8% 2	503
	11th to 20th	Day	26% 16% 29% 19% 8% 2	527
	21st to end	Day	22% 13% 29% 21% 12% 3	573
June	1st to 10th	Day	21% 14% 32% 21% 9% 3	530
	11th to 20th	Day	22% 15% 25% 24% 12% 2	530
	21st to end	Day	16% 13% 35% 23% 10% 2	530
July	1st to 10th	Day	12% 7% 31% 28% 17% 4	525
	11th to 20th	Day	11% 9% 27% 32% 17% 4	524
	21st to end	Day	15% 10% 35% 26% 11% 4	576
August	1st to 10th	Day	13% 9% 26% 26% 19% 7%	526
	11th to 20th	Day	16% 8% 24% 30% 18% 6%	519
	21st to end	Day	16% 8% 29% 24% 14% 8%	583
September	1st to 10th	Day	20% 12% 21% 22% 18% 7%	530
	11th to 20th	Day	14% 7% 27% 24% 18% 11%	530
	21st to end	Day	24% 12% 27% 21% 13% 3	530
October	1st to 10th	Day	24% 12% 23% 20% 14% 7%	530
	11th to 20th	Day	21% 20% 28% 22% 7% 2	530
	21st to end	Day	28% 20% 26% 18% 7% 1	525
November	1st to 10th	Day	24% 20% 24% 20% 10% 2	425
	11th to 20th	Day	30% 22% 28% 16% 4	419
	21st to end	Day	38% 21% 30% 9% 3	220
December	1st to 10th	Day	33% 28% 25% 9% 5%	220
	11th to 20th	Day	41% 23% 24% 10% 2	220
	21st to end	Day	32% 24% 28% 12% 3	242
January	1st to 10th	Night	26% 26% 32% 14% 2	840
	11th to 20th	Night	29% 21% 30% 18% 2	840
	21st to end	Night	22% 25% 35% 16% 2	924
February	1st to 10th	Night	28% 19% 33% 17% 2	801
	11th to 20th	Night	29% 25% 32% 13% 1	783
	21st to end	Night	29% 23% 31% 14% 3	660
March	1st to 10th	Night	27% 23% 32% 16% 3	800
	11th to 20th	Night	28% 23% 28% 18% 4	800
	21st to end	Night	23% 24% 32% 15% 5%	880
April	1st to 10th	Night	26% 22% 30% 18% 3	800
	11th to 20th	Night	25% 18% 32% 19% 5% 1	791
	21st to end	Night	25% 17% 31% 22% 5%	800
May	1st to 10th	Night	30% 14% 26% 22% 8%	531
	11th to 20th	Night	22% 10% 32% 27% 8% 1	394
	21st to end	Night	18% 7% 26% 36% 12% 1	433
June	1st to 10th	Night	18% 9% 24% 36% 14% 1	400
	11th to 20th	Night	17% 8% 27% 34% 14%	400
	21st to end	Night	12% 6% 22% 46% 14%	400
July	1st to 10th	Night	13% 7% 20% 39% 21% 1	390
	11th to 20th	Night	13% 4 19% 45% 17% 1	388
	21st to end	Night	15% 4 20% 42% 19%	438
August	1st to 10th	Night	14% 2 21% 39% 21% 2	568
	11th to 20th	Night	16% 4 15% 41% 22% 3	718
	21st to end	Night	16% 6% 19% 34% 24% 1	880
September	1st to 10th	Night	18% 7% 24% 29% 20% 2	800
	11th to 20th	Night	15% 5% 22% 29% 24% 5%	800
	21st to end	Night	21% 9% 22% 30% 16% 2	800
October	1st to 10th	Night	24% 12% 22% 23% 19% 1	800
	11th to 20th	Night	24% 13% 27% 28% 8%	800
	21st to end	Night	27% 15% 30% 21% 6%	880
November	1st to 10th	Night	19% 15% 31% 25% 9% 1	820
	11th to 20th	Night	27% 19% 27% 20% 6% 1	840
	21st to end	Night	22% 21% 33% 19% 5%	840
December	1st to 10th	Night	28% 22% 30% 17% 4	840
	11th to 20th	Night	27% 22% 29% 19% 3	840
	21st to end	Night	28% 23% 29% 17% 3	924

Page generated at 2026-01-29 01:21:46. Version ac07

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