Merion Golf Club
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Merion Golf Club 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% 6% 2 6% 4 4 4 4 6% 8% 40%	127565
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 43%	124455
Transparency	Day	40% 6% 12% 23% 17% 3	131093
	Night	40% 5% 11% 23% 18% 4	129013
Seeing	Day	19% 13% 28% 23% 12% 4	16508
	Night	19% 10% 32% 29% 10% 1	25897

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% 5% 2 5% 4 4 4 3 6% 6% 49%	3677
	11th to 20th	Day	14% 8% 3 7% 5% 4 4 4 5% 5% 42%	3770
	21st to end	Day	18% 7% 2 6% 4 4 4 3 5% 6% 41%	4565
February	1st to 10th	Day	16% 7% 2 6% 5% 5% 4 3 5% 7% 39%	1800
	11th to 20th	Day	16% 9% 2 7% 3 4 4 4 6% 6% 40%	1898
	21st to end	Day	18% 9% 3 6% 4 4 3 3 5% 7% 38%	1650
March	1st to 10th	Day	20% 5% 1 5% 3 3 4 3 5% 6% 45%	4494
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 5% 7% 45%	4980
	21st to end	Day	18% 6% 2 6% 4 3 4 3 5% 6% 43%	5781
April	1st to 10th	Day	16% 8% 2 6% 4 4 5% 4 5% 8% 39%	2400
	11th to 20th	Day	15% 6% 2 5% 5% 4 5% 4 5% 8% 41%	2374
	21st to end	Day	16% 5% 2 6% 5% 4 5% 4 6% 8% 40%	2400
May	1st to 10th	Day	26% 8% 2 5% 4 4 5% 4 5% 7% 30%	5805
	11th to 20th	Day	25% 8% 3 6% 4 4 4 3 5% 7% 32%	6318
	21st to end	Day	20% 6% 2 6% 4 4 5% 4 6% 9% 36%	6949
June	1st to 10th	Day	13% 6% 2 6% 4 5% 5% 4 7% 9% 39%	2800
	11th to 20th	Day	14% 6% 2 6% 5% 5% 5% 5% 6% 9% 37%	2800
	21st to end	Day	10% 5% 2 6% 4 5% 5% 5% 7% 10% 42%	2800
July	1st to 10th	Day	9% 6% 2 6% 5% 6% 6% 5% 8% 10% 38%	6360
	11th to 20th	Day	12% 7% 3 8% 5% 6% 6% 5% 8% 10% 31%	6360
	21st to end	Day	11% 7% 2 7% 5% 4 5% 5% 6% 9% 39%	6794
August	1st to 10th	Day	19% 5% 2 7% 5% 4 5% 4 6% 8% 34%	4234
	11th to 20th	Day	13% 5% 2 6% 4 4 4 4 6% 9% 43%	3855
	21st to end	Day	11% 5% 2 5% 4 4 4 4 6% 8% 48%	4312
September	1st to 10th	Day	15% 6% 1 6% 4 4 4 4 5% 6% 45%	2400
	11th to 20th	Day	14% 5% 1 4 3 3 3 3 6% 8% 50%	2370
	21st to end	Day	19% 8% 3 6% 4 4 5% 3 5% 6% 37%	2196
October	1st to 10th	Day	18% 4 2 6% 3 3 3 4 5% 6% 45%	2000
	11th to 20th	Day	16% 5% 2 4 3 3 4 3 5% 7% 48%	2000
	21st to end	Day	18% 6% 3 4 3 3 3 3 6% 6% 46%	2135
November	1st to 10th	Day	12% 6% 2 4 3 3 3 3 4 7% 53%	1851
	11th to 20th	Day	16% 5% 2 5% 4 4 3 3 6% 7% 47%	1890
	21st to end	Day	16% 6% 2 5% 3 3 4 3 5% 7% 46%	1753
December	1st to 10th	Day	13% 7% 2 5% 4 3 5% 3 6% 7% 44%	2680
	11th to 20th	Day	20% 9% 2 4 2 3 3 2 4 6% 44%	3066
	21st to end	Day	16% 5% 2 6% 4 4 5% 4 6% 8% 39%	4048
January	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 4 5% 50%	6489
	11th to 20th	Night	15% 7% 2 5% 3 3 3 2 4 5% 50%	6332
	21st to end	Night	18% 7% 2 5% 4 3 4 4 4 6% 43%	6633
February	1st to 10th	Night	17% 9% 2 6% 3 4 4 4 4 6% 40%	2601
	11th to 20th	Night	19% 7% 2 6% 4 4 4 3 4 6% 41%	2385
	21st to end	Night	17% 7% 3 6% 4 3 4 3 5% 5% 42%	1980
March	1st to 10th	Night	19% 8% 2 5% 3 3 3 3 4 6% 44%	5329
	11th to 20th	Night	11% 8% 4 5% 3 3 4 3 4 5% 51%	4979
	21st to end	Night	15% 8% 3 6% 4 4 4 3 4 6% 43%	5149
April	1st to 10th	Night	17% 7% 2 6% 4 4 4 4 6% 6% 39%	2000
	11th to 20th	Night	15% 7% 2 6% 4 4 4 4 5% 7% 42%	1975
	21st to end	Night	17% 5% 2 5% 3 3 4 3 5% 7% 44%	2000
May	1st to 10th	Night	23% 9% 3 7% 4 5% 4 4 5% 8% 28%	4176
	11th to 20th	Night	23% 9% 3 7% 4 4 3 3 5% 6% 32%	3575
	21st to end	Night	21% 6% 2 5% 4 3 3 3 6% 9% 37%	3926
June	1st to 10th	Night	16% 6% 3 5% 4 4 4 3 7% 8% 42%	1600
	11th to 20th	Night	12% 6% 2 6% 5% 5% 6% 5% 6% 9% 40%	1600
	21st to end	Night	10% 4 1 5% 4 4 5% 5% 8% 11% 41%	1600
July	1st to 10th	Night	9% 7% 3 7% 5% 5% 6% 5% 7% 10% 38%	3600
	11th to 20th	Night	15% 8% 3 8% 5% 5% 5% 5% 7% 9% 31%	3600
	21st to end	Night	12% 6% 2 5% 4 4 4 4 7% 8% 44%	4054
August	1st to 10th	Night	21% 6% 2 6% 4 4 5% 4 6% 8% 34%	2915
	11th to 20th	Night	14% 6% 2 6% 4 5% 4 4 6% 7% 42%	3158
	21st to end	Night	11% 4 2 5% 4 4 4 3 6% 8% 49%	3586
September	1st to 10th	Night	14% 7% 3 6% 4 3 4 3 5% 6% 47%	2000
	11th to 20th	Night	14% 4 2 4 4 3 4 2 5% 6% 53%	2029
	21st to end	Night	19% 6% 2 6% 4 3 5% 4 5% 7% 39%	2200
October	1st to 10th	Night	18% 6% 3 6% 4 3 3 3 4 6% 44%	2400
	11th to 20th	Night	16% 5% 1 5% 3 3 3 3 4 6% 51%	2400
	21st to end	Night	17% 6% 2 4 3 3 3 2 4 6% 49%	2704
November	1st to 10th	Night	11% 6% 2 4 3 3 3 3 4 6% 54%	2667
	11th to 20th	Night	17% 7% 2 5% 3 3 3 3 3 5% 48%	2730
	21st to end	Night	13% 6% 2 5% 4 5% 4 3 6% 7% 45%	2865
December	1st to 10th	Night	13% 8% 2 6% 3 3 4 3 4 6% 47%	4720
	11th to 20th	Night	22% 7% 2 5% 3 3 3 3 4 7% 42%	5348
	21st to end	Night	19% 7% 3 6% 4 4 3 3 5% 6% 40%	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	35% 4 10% 26% 22% 3	3678
	11th to 20th	Day	43% 4 9% 22% 17% 5%	3800
	21st to end	Day	43% 5% 12% 24% 15% 2	4565
February	1st to 10th	Day	43% 5% 14% 25% 11% 2	1800
	11th to 20th	Day	43% 5% 11% 24% 15% 2	1907
	21st to end	Day	45% 4 11% 22% 13% 4	1650
March	1st to 10th	Day	39% 3 10% 24% 17% 7%	4497
	11th to 20th	Day	36% 5% 10% 29% 15% 5%	4980
	21st to end	Day	42% 4 10% 16% 23% 5%	5781
April	1st to 10th	Day	42% 6% 11% 20% 16% 5%	2400
	11th to 20th	Day	40% 6% 12% 22% 16% 4	2374
	21st to end	Day	40% 6% 11% 19% 19% 4	2400
May	1st to 10th	Day	51% 6% 13% 18% 10% 2	5814
	11th to 20th	Day	51% 6% 9% 21% 12% 1	6322
	21st to end	Day	43% 6% 11% 23% 15% 1	6949
June	1st to 10th	Day	39% 8% 13% 23% 15% 1	2800
	11th to 20th	Day	40% 9% 12% 24% 12% 3	2800
	21st to end	Day	36% 8% 14% 23% 17% 1	2800
July	1st to 10th	Day	38% 11% 15% 18% 17% 2	6290
	11th to 20th	Day	44% 9% 16% 19% 10% 1	6285
	21st to end	Day	39% 7% 11% 20% 22% 1	6598
August	1st to 10th	Day	46% 7% 14% 22% 10%	4221
	11th to 20th	Day	37% 6% 12% 30% 14% 1	3891
	21st to end	Day	32% 6% 11% 27% 21% 2	4312
September	1st to 10th	Day	38% 6% 11% 20% 22% 3	2400
	11th to 20th	Day	32% 4 12% 25% 23% 4	2370
	21st to end	Day	48% 5% 8% 18% 19% 2	2196
October	1st to 10th	Day	35% 4 9% 19% 28% 5%	3260
	11th to 20th	Day	25% 3 12% 23% 30% 7%	3240
	21st to end	Day	25% 3 11% 28% 29% 5%	3425
November	1st to 10th	Day	32% 3 14% 25% 21% 4	1851
	11th to 20th	Day	37% 5% 10% 24% 21% 3	1890
	21st to end	Day	38% 3 13% 31% 12% 2	1753
December	1st to 10th	Day	39% 4 16% 25% 13% 2	2680
	11th to 20th	Day	44% 3 11% 29% 11% 2	3066
	21st to end	Day	40% 7% 12% 22% 17% 2	4048
January	1st to 10th	Night	36% 4 8% 25% 21% 6%	6489
	11th to 20th	Night	38% 3 10% 24% 18% 7%	6377
	21st to end	Night	42% 4 9% 23% 19% 3	6633
February	1st to 10th	Night	44% 5% 13% 24% 13% 2	2601
	11th to 20th	Night	44% 4 9% 20% 18% 4	2387
	21st to end	Night	42% 5% 9% 22% 17% 6%	1980
March	1st to 10th	Night	41% 4 9% 23% 13% 10%	5339
	11th to 20th	Night	35% 6% 13% 26% 15% 5%	4980
	21st to end	Night	43% 4 7% 16% 25% 5%	5149
April	1st to 10th	Night	43% 5% 11% 20% 18% 4	2000
	11th to 20th	Night	41% 6% 10% 19% 20% 4	1975
	21st to end	Night	39% 5% 10% 20% 20% 5%	2000
May	1st to 10th	Night	53% 7% 11% 16% 11% 2	4183
	11th to 20th	Night	51% 7% 10% 18% 13% 1	3576
	21st to end	Night	44% 7% 12% 20% 14% 2	3927
June	1st to 10th	Night	39% 6% 12% 21% 21% 2	1600
	11th to 20th	Night	40% 9% 12% 24% 14% 1	1600
	21st to end	Night	34% 9% 16% 24% 16% 2	1600
July	1st to 10th	Night	41% 9% 16% 19% 13% 2	3560
	11th to 20th	Night	47% 9% 17% 18% 8% 1	3552
	21st to end	Night	36% 7% 14% 23% 20% 1	3935
August	1st to 10th	Night	47% 8% 17% 18% 9% 1	2904
	11th to 20th	Night	40% 6% 12% 26% 15% 1	3188
	21st to end	Night	32% 5% 11% 28% 21% 2	3586
September	1st to 10th	Night	38% 7% 10% 20% 22% 3	2000
	11th to 20th	Night	33% 5% 9% 23% 26% 4	2029
	21st to end	Night	44% 6% 9% 19% 20% 2	2200
October	1st to 10th	Night	40% 4 10% 22% 21% 3	3920
	11th to 20th	Night	25% 3 10% 28% 29% 5%	3899
	21st to end	Night	26% 3 12% 31% 24% 4	4364
November	1st to 10th	Night	32% 4 12% 25% 20% 6%	2667
	11th to 20th	Night	39% 4 9% 21% 21% 5%	2730
	21st to end	Night	38% 5% 12% 27% 17% 2	2865
December	1st to 10th	Night	38% 4 12% 30% 14% 2	4720
	11th to 20th	Night	44% 4 11% 23% 15% 4	5348
	21st to end	Night	43% 5% 12% 20% 18% 2	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	26% 23% 34% 14% 2 1	220
	11th to 20th	Day	26% 34% 26% 12% 2	250
	21st to end	Day	23% 25% 31% 16% 5%	363
February	1st to 10th	Day	24% 24% 35% 13% 4	300
	11th to 20th	Day	30% 20% 36% 12% 2	446
	21st to end	Day	24% 20% 36% 16% 3	414
March	1st to 10th	Day	23% 21% 32% 19% 4 1	500
	11th to 20th	Day	21% 22% 32% 19% 4 1	500
	21st to end	Day	25% 20% 31% 18% 4 1	550
April	1st to 10th	Day	25% 18% 37% 13% 6% 1	500
	11th to 20th	Day	21% 19% 32% 19% 7% 2	492
	21st to end	Day	23% 15% 32% 23% 6% 1	500
May	1st to 10th	Day	27% 18% 27% 16% 10% 2	503
	11th to 20th	Day	23% 10% 30% 24% 10% 2	527
	21st to end	Day	22% 6% 21% 28% 17% 5%	573
June	1st to 10th	Day	14% 10% 29% 28% 16% 3	530
	11th to 20th	Day	16% 9% 27% 27% 17% 3	530
	21st to end	Day	13% 6% 30% 30% 16% 5%	530
July	1st to 10th	Day	11% 4 20% 32% 24% 9%	525
	11th to 20th	Day	8% 3 20% 33% 28% 8%	524
	21st to end	Day	11% 5% 23% 35% 20% 6%	576
August	1st to 10th	Day	13% 4 24% 30% 21% 8%	526
	11th to 20th	Day	14% 3 17% 36% 22% 8%	519
	21st to end	Day	11% 6% 26% 27% 20% 9%	583
September	1st to 10th	Day	17% 9% 21% 28% 19% 7%	530
	11th to 20th	Day	15% 3 24% 31% 17% 10%	530
	21st to end	Day	23% 12% 21% 27% 12% 5%	530
October	1st to 10th	Day	23% 6% 24% 23% 16% 8%	530
	11th to 20th	Day	17% 15% 29% 23% 13% 3	530
	21st to end	Day	20% 12% 34% 20% 12% 2	547
November	1st to 10th	Day	16% 12% 34% 24% 12% 2	425
	11th to 20th	Day	21% 15% 33% 25% 6%	430
	21st to end	Day	20% 19% 33% 21% 5% 1	293
December	1st to 10th	Day	25% 23% 36% 10% 6%	220
	11th to 20th	Day	29% 22% 34% 11% 4	220
	21st to end	Day	26% 20% 31% 14% 8%	242
January	1st to 10th	Night	19% 15% 41% 22% 4	840
	11th to 20th	Night	24% 15% 33% 24% 3	840
	21st to end	Night	20% 14% 41% 24% 2	924
February	1st to 10th	Night	22% 13% 37% 25% 2	801
	11th to 20th	Night	25% 17% 40% 17% 1	783
	21st to end	Night	22% 16% 40% 19% 3	660
March	1st to 10th	Night	20% 16% 37% 24% 4	800
	11th to 20th	Night	20% 15% 40% 20% 5% 1	800
	21st to end	Night	21% 12% 40% 23% 5%	880
April	1st to 10th	Night	21% 18% 38% 19% 5%	800
	11th to 20th	Night	19% 13% 36% 27% 5%	791
	21st to end	Night	20% 10% 34% 30% 6%	800
May	1st to 10th	Night	22% 9% 34% 29% 6%	658
	11th to 20th	Night	19% 9% 35% 31% 6%	394
	21st to end	Night	19% 3 23% 39% 14% 1	433
June	1st to 10th	Night	18% 3 30% 38% 11% 1	400
	11th to 20th	Night	14% 5% 26% 44% 11% 1	400
	21st to end	Night	9% 3 27% 47% 15%	400
July	1st to 10th	Night	11% 5% 24% 39% 21% 1	390
	11th to 20th	Night	7% 4 19% 44% 24% 2	388
	21st to end	Night	10% 3 24% 40% 21% 2	491
August	1st to 10th	Night	12% 2 16% 44% 23% 2	597
	11th to 20th	Night	14% 3 18% 38% 25% 2	763
	21st to end	Night	14% 4 19% 38% 23% 3	880
September	1st to 10th	Night	16% 4 24% 35% 20% 2	800
	11th to 20th	Night	15% 2 20% 36% 23% 4	800
	21st to end	Night	22% 5% 22% 31% 19% 1	800
October	1st to 10th	Night	22% 7% 24% 26% 18% 4	800
	11th to 20th	Night	19% 9% 27% 32% 13% 1	800
	21st to end	Night	21% 9% 33% 28% 10%	880
November	1st to 10th	Night	15% 7% 35% 31% 10% 1	820
	11th to 20th	Night	20% 12% 34% 25% 8% 1	840
	21st to end	Night	16% 14% 34% 28% 7% 1	840
December	1st to 10th	Night	24% 12% 35% 23% 6% 1	840
	11th to 20th	Night	21% 12% 36% 26% 6%	840
	21st to end	Night	22% 12% 35% 29% 3	924

Page generated at 2026-01-29 01:21:41. Version ac07

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