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 2024-11-28.

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%	101519
	Night	17% 7% 2 6% 4 4 4 3 5% 7% 42%	98028
Transparency	Day	40% 6% 12% 23% 16% 3	105065
	Night	40% 6% 12% 23% 17% 3	102570
Seeing	Day	19% 12% 28% 24% 13% 4	15066
	Night	18% 9% 32% 29% 10% 1	24356

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	15% 6% 2 5% 4 4 4 3 5% 6% 46%	2517
	11th to 20th	Day	14% 10% 3 8% 5% 4 4 4 5% 6% 37%	2595
	21st to end	Day	21% 8% 2 7% 4 4 4 3 5% 6% 35%	3124
February	1st to 10th	Day	16% 7% 2 6% 5% 5% 4 3 5% 7% 39%	1710
	11th to 20th	Day	16% 8% 2 7% 4 4 3 4 6% 6% 41%	1800
	21st to end	Day	18% 9% 3 6% 4 4 3 3 5% 7% 37%	1570
March	1st to 10th	Day	25% 6% 2 6% 4 4 4 3 5% 6% 37%	3134
	11th to 20th	Day	10% 5% 2 5% 5% 5% 5% 4 6% 7% 46%	3480
	21st to end	Day	22% 6% 3 6% 4 3 3 2 4 6% 43%	4033
April	1st to 10th	Day	15% 8% 2 6% 4 4 5% 4 5% 8% 40%	2280
	11th to 20th	Day	15% 7% 2 5% 5% 4 4 4 5% 8% 42%	2254
	21st to end	Day	17% 5% 2 5% 5% 4 5% 4 6% 7% 39%	2280
May	1st to 10th	Day	27% 9% 2 6% 4 4 4 3 5% 7% 28%	4053
	11th to 20th	Day	25% 9% 3 7% 5% 5% 4 4 6% 6% 27%	4398
	21st to end	Day	13% 5% 1 6% 4 3 5% 4 6% 10% 44%	4837
June	1st to 10th	Day	13% 5% 2 5% 4 5% 5% 4 7% 9% 39%	2660
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 5% 7% 10% 37%	2660
	21st to end	Day	10% 5% 2 5% 4 5% 5% 5% 7% 10% 42%	2660
July	1st to 10th	Day	7% 6% 2 6% 5% 6% 7% 6% 9% 11% 36%	4440
	11th to 20th	Day	10% 6% 2 6% 4 5% 6% 5% 7% 10% 39%	4440
	21st to end	Day	12% 8% 3 8% 5% 5% 5% 6% 6% 8% 33%	4724
August	1st to 10th	Day	19% 5% 2 7% 5% 5% 5% 4 6% 8% 33%	4104
	11th to 20th	Day	12% 5% 2 5% 4 4 4 4 6% 9% 43%	3735
	21st to end	Day	11% 5% 2 5% 4 4 4 4 6% 8% 48%	4180
September	1st to 10th	Day	15% 6% 1 6% 4 5% 4 4 5% 6% 45%	2280
	11th to 20th	Day	13% 5% 1 4 3 3 3 3 6% 8% 50%	2252
	21st to end	Day	19% 8% 3 6% 3 4 5% 4 5% 6% 37%	2086
October	1st to 10th	Day	19% 5% 2 6% 3 3 3 4 5% 6% 44%	1900
	11th to 20th	Day	16% 5% 2 4 3 3 4 3 5% 7% 49%	1900
	21st to end	Day	18% 6% 3 4 3 3 3 3 6% 6% 46%	2028
November	1st to 10th	Day	13% 7% 2 4 3 3 2 3 4 6% 53%	1761
	11th to 20th	Day	16% 5% 2 5% 3 3 3 3 6% 7% 48%	1800
	21st to end	Day	16% 6% 2 5% 3 3 3 3 5% 7% 46%	1646
December	1st to 10th	Day	18% 7% 2 6% 4 3 4 3 6% 7% 39%	1520
	11th to 20th	Day	20% 7% 2 4 2 3 3 3 5% 7% 43%	1906
	21st to end	Day	17% 6% 2 6% 5% 5% 6% 5% 6% 7% 36%	2772
January	1st to 10th	Night	17% 6% 2 6% 4 4 4 3 4 5% 45%	4429
	11th to 20th	Night	18% 9% 2 5% 4 3 4 2 4 5% 44%	4362
	21st to end	Night	24% 8% 2 5% 4 3 4 3 4 6% 36%	4532
February	1st to 10th	Night	17% 9% 3 6% 3 4 4 4 4 6% 41%	2470
	11th to 20th	Night	18% 8% 3 6% 4 4 4 3 4 6% 41%	2263
	21st to end	Night	18% 7% 3 6% 4 3 3 3 5% 5% 42%	1884
March	1st to 10th	Night	22% 9% 2 6% 3 4 3 3 4 6% 37%	3703
	11th to 20th	Night	11% 6% 3 5% 4 4 4 3 4 5% 51%	3479
	21st to end	Night	17% 9% 3 6% 4 3 3 2 3 4 45%	3597
April	1st to 10th	Night	16% 7% 2 6% 4 4 4 4 6% 7% 40%	1900
	11th to 20th	Night	15% 7% 3 6% 4 4 4 4 5% 6% 42%	1875
	21st to end	Night	18% 6% 2 5% 3 3 3 3 5% 7% 43%	1900
May	1st to 10th	Night	23% 10% 3 7% 4 4 4 4 5% 8% 29%	2914
	11th to 20th	Night	24% 9% 3 7% 5% 4 3 3 6% 6% 29%	2495
	21st to end	Night	14% 6% 2 5% 5% 4 3 4 6% 10% 43%	2738
June	1st to 10th	Night	16% 6% 3 5% 4 4 3 3 7% 8% 42%	1520
	11th to 20th	Night	11% 5% 2 6% 5% 5% 6% 5% 6% 9% 40%	1520
	21st to end	Night	10% 4 1 5% 4 4 5% 5% 8% 11% 41%	1520
July	1st to 10th	Night	8% 6% 3 6% 6% 5% 6% 5% 8% 10% 38%	2520
	11th to 20th	Night	12% 6% 2 6% 4 5% 5% 5% 7% 10% 37%	2520
	21st to end	Night	15% 7% 3 6% 5% 4 4 4 8% 8% 36%	2825
August	1st to 10th	Night	21% 7% 2 6% 4 4 5% 4 6% 8% 33%	2825
	11th to 20th	Night	14% 6% 2 6% 4 5% 4 3 6% 7% 42%	3059
	21st to end	Night	11% 4 2 5% 4 4 4 3 6% 8% 49%	3476
September	1st to 10th	Night	14% 7% 3 5% 4 3 4 3 5% 6% 47%	1900
	11th to 20th	Night	14% 5% 2 3 3 3 4 2 5% 6% 53%	1927
	21st to end	Night	19% 6% 2 6% 4 3 4 4 5% 7% 40%	2090
October	1st to 10th	Night	19% 6% 3 6% 4 3 3 3 4 6% 43%	2280
	11th to 20th	Night	15% 5% 1 5% 3 3 3 3 4 6% 51%	2280
	21st to end	Night	17% 7% 2 4 3 3 3 2 4 5% 49%	2569
November	1st to 10th	Night	11% 6% 2 4 3 3 3 3 4 6% 54%	2537
	11th to 20th	Night	17% 6% 2 5% 3 3 4 3 3 5% 48%	2600
	21st to end	Night	13% 6% 2 5% 4 5% 4 3 6% 7% 45%	2687
December	1st to 10th	Night	19% 8% 2 5% 4 4 4 3 5% 6% 39%	2660
	11th to 20th	Night	17% 6% 2 6% 4 4 4 3 5% 7% 42%	3288
	21st to end	Night	17% 7% 3 6% 5% 4 4 4 6% 7% 37%	4884

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	38% 5% 12% 27% 13% 4	2518
	11th to 20th	Day	45% 6% 11% 22% 13% 3	2595
	21st to end	Day	48% 6% 12% 22% 10% 1	3124
February	1st to 10th	Day	44% 5% 14% 24% 11% 2	1710
	11th to 20th	Day	42% 5% 11% 25% 15% 3	1809
	21st to end	Day	46% 5% 11% 21% 13% 4	1570
March	1st to 10th	Day	46% 4 10% 27% 10% 2	3137
	11th to 20th	Day	33% 6% 11% 31% 17% 3	3480
	21st to end	Day	44% 4 9% 18% 21% 3	4033
April	1st to 10th	Day	41% 6% 12% 21% 16% 5%	2280
	11th to 20th	Day	39% 6% 12% 22% 16% 4	2254
	21st to end	Day	41% 6% 11% 19% 18% 4	2280
May	1st to 10th	Day	54% 7% 10% 17% 10% 2	4065
	11th to 20th	Day	54% 7% 13% 17% 8% 1	4402
	21st to end	Day	34% 8% 13% 27% 17% 2	4837
June	1st to 10th	Day	39% 8% 14% 23% 15% 1	2660
	11th to 20th	Day	39% 10% 12% 25% 12% 3	2660
	21st to end	Day	36% 8% 14% 23% 17% 1	2660
July	1st to 10th	Day	37% 12% 16% 18% 14% 3	4370
	11th to 20th	Day	38% 8% 16% 23% 14% 2	4365
	21st to end	Day	45% 8% 12% 19% 16% 1	4573
August	1st to 10th	Day	46% 8% 14% 22% 9% 1	4091
	11th to 20th	Day	36% 6% 12% 30% 14% 1	3771
	21st to end	Day	33% 6% 11% 27% 21% 2	4180
September	1st to 10th	Day	39% 7% 11% 19% 22% 3	2280
	11th to 20th	Day	32% 4 12% 24% 24% 4	2252
	21st to end	Day	47% 5% 8% 18% 20% 2	2086
October	1st to 10th	Day	36% 4 9% 18% 28% 5%	3160
	11th to 20th	Day	24% 3 12% 23% 30% 7%	3140
	21st to end	Day	25% 3 11% 28% 29% 5%	3318
November	1st to 10th	Day	33% 3 14% 25% 21% 4	1761
	11th to 20th	Day	36% 5% 10% 25% 21% 3	1800
	21st to end	Day	38% 3 14% 31% 12% 2	1646
December	1st to 10th	Day	44% 5% 16% 22% 13% 1	1520
	11th to 20th	Day	42% 4 11% 28% 12% 2	1906
	21st to end	Day	43% 7% 14% 23% 12% 1	2772
January	1st to 10th	Night	41% 5% 9% 29% 13% 3	4429
	11th to 20th	Night	44% 4 11% 25% 14% 3	4362
	21st to end	Night	49% 5% 8% 25% 12% 2	4532
February	1st to 10th	Night	44% 5% 13% 24% 13% 2	2470
	11th to 20th	Night	44% 4 9% 20% 18% 5%	2265
	21st to end	Night	42% 5% 9% 22% 16% 5%	1884
March	1st to 10th	Night	48% 5% 10% 23% 12% 2	3713
	11th to 20th	Night	34% 6% 12% 28% 17% 3	3480
	21st to end	Night	43% 4 7% 17% 23% 5%	3597
April	1st to 10th	Night	42% 5% 11% 20% 18% 4	1900
	11th to 20th	Night	41% 6% 10% 20% 19% 4	1875
	21st to end	Night	40% 5% 10% 20% 19% 5%	1900
May	1st to 10th	Night	52% 7% 10% 19% 11% 1	2921
	11th to 20th	Night	53% 8% 13% 14% 10% 2	2496
	21st to end	Night	36% 9% 15% 20% 16% 3	2739
June	1st to 10th	Night	38% 7% 12% 22% 20% 2	1520
	11th to 20th	Night	39% 9% 12% 25% 14% 1	1520
	21st to end	Night	33% 9% 16% 24% 15% 2	1520
July	1st to 10th	Night	39% 10% 20% 16% 12% 2	2480
	11th to 20th	Night	39% 9% 17% 23% 11% 1	2472
	21st to end	Night	44% 7% 15% 19% 14% 1	2735
August	1st to 10th	Night	47% 8% 18% 18% 9% 1	2814
	11th to 20th	Night	40% 6% 12% 26% 15% 1	3089
	21st to end	Night	33% 5% 11% 28% 20% 2	3476
September	1st to 10th	Night	38% 7% 11% 19% 22% 3	1900
	11th to 20th	Night	32% 5% 9% 23% 27% 4	1927
	21st to end	Night	43% 6% 8% 20% 21% 2	2090
October	1st to 10th	Night	41% 4 10% 21% 21% 3	3800
	11th to 20th	Night	24% 3 10% 28% 29% 5%	3779
	21st to end	Night	26% 3 12% 31% 24% 4	4229
November	1st to 10th	Night	32% 4 12% 25% 20% 6%	2537
	11th to 20th	Night	39% 5% 8% 22% 21% 5%	2600
	21st to end	Night	38% 5% 12% 27% 16% 2	2687
December	1st to 10th	Night	45% 5% 13% 23% 14% 1	2660
	11th to 20th	Night	41% 5% 12% 20% 17% 4	3288
	21st to end	Night	43% 7% 14% 24% 11% 1	4884

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% 23% 34% 16% 3 2	180
	11th to 20th	Day	22% 35% 25% 15% 3	204
	21st to end	Day	23% 24% 29% 18% 5%	297
February	1st to 10th	Day	24% 23% 34% 14% 4	270
	11th to 20th	Day	27% 19% 38% 13% 2	408
	21st to end	Day	25% 19% 35% 17% 3 1	382
March	1st to 10th	Day	22% 21% 32% 20% 5% 1	460
	11th to 20th	Day	22% 22% 30% 20% 4 2	460
	21st to end	Day	25% 18% 32% 19% 5% 1	506
April	1st to 10th	Day	23% 18% 37% 14% 7% 1	460
	11th to 20th	Day	19% 19% 33% 19% 8% 2	452
	21st to end	Day	24% 15% 32% 22% 6% 1	460
May	1st to 10th	Day	27% 17% 27% 16% 10% 2	463
	11th to 20th	Day	21% 10% 30% 25% 11% 2	487
	21st to end	Day	19% 6% 21% 30% 18% 5%	529
June	1st to 10th	Day	13% 9% 30% 28% 16% 4	490
	11th to 20th	Day	13% 9% 27% 29% 18% 3	490
	21st to end	Day	13% 7% 30% 30% 14% 6%	490
July	1st to 10th	Day	11% 4 20% 32% 25% 9%	485
	11th to 20th	Day	8% 3 20% 35% 27% 8%	484
	21st to end	Day	12% 6% 23% 34% 19% 6%	532
August	1st to 10th	Day	13% 4 22% 30% 22% 9%	486
	11th to 20th	Day	14% 3 17% 35% 22% 9%	479
	21st to end	Day	12% 6% 25% 28% 20% 10%	539
September	1st to 10th	Day	17% 8% 20% 28% 19% 7%	490
	11th to 20th	Day	15% 3 24% 29% 18% 11%	490
	21st to end	Day	23% 13% 20% 27% 12% 5%	490
October	1st to 10th	Day	25% 6% 24% 22% 16% 7%	490
	11th to 20th	Day	16% 14% 29% 25% 13% 3	490
	21st to end	Day	20% 10% 35% 20% 12% 3	506
November	1st to 10th	Day	16% 10% 35% 24% 12% 3	395
	11th to 20th	Day	19% 15% 34% 26% 6%	400
	21st to end	Day	19% 17% 34% 23% 6% 1	264
December	1st to 10th	Day	24% 22% 35% 11% 8%	180
	11th to 20th	Day	28% 22% 32% 13% 4	180
	21st to end	Day	27% 18% 30% 15% 9% 1	198
January	1st to 10th	Night	19% 14% 41% 23% 4	760
	11th to 20th	Night	24% 13% 34% 25% 3	760
	21st to end	Night	19% 13% 41% 25% 2	836
February	1st to 10th	Night	21% 13% 38% 26% 2	760
	11th to 20th	Night	24% 17% 41% 18% 1	743
	21st to end	Night	22% 16% 40% 19% 3	628
March	1st to 10th	Night	20% 14% 37% 24% 4	760
	11th to 20th	Night	20% 15% 40% 20% 5% 1	760
	21st to end	Night	21% 11% 39% 23% 6%	836
April	1st to 10th	Night	20% 17% 38% 19% 5%	760
	11th to 20th	Night	18% 12% 37% 28% 6%	751
	21st to end	Night	20% 10% 34% 30% 6%	760
May	1st to 10th	Night	23% 9% 33% 29% 6%	625
	11th to 20th	Night	19% 8% 35% 31% 6%	374
	21st to end	Night	17% 3 22% 41% 15% 1	411
June	1st to 10th	Night	18% 3 28% 38% 12% 1	380
	11th to 20th	Night	12% 5% 27% 44% 11% 1	380
	21st to end	Night	8% 3 26% 48% 14%	380
July	1st to 10th	Night	11% 4 24% 39% 20% 1	370
	11th to 20th	Night	7% 4 20% 44% 24% 2	368
	21st to end	Night	10% 4 24% 40% 20% 2	466
August	1st to 10th	Night	11% 2 16% 44% 23% 2	567
	11th to 20th	Night	14% 3 17% 39% 25% 2	724
	21st to end	Night	14% 3 18% 38% 23% 3	836
September	1st to 10th	Night	16% 4 24% 35% 20% 2	760
	11th to 20th	Night	16% 2 20% 36% 23% 4	760
	21st to end	Night	22% 5% 22% 30% 19% 1	760
October	1st to 10th	Night	22% 7% 24% 26% 17% 4	760
	11th to 20th	Night	18% 8% 28% 33% 14% 1	760
	21st to end	Night	21% 7% 33% 28% 10%	836
November	1st to 10th	Night	15% 7% 36% 31% 11% 1	780
	11th to 20th	Night	21% 11% 34% 26% 8% 1	800
	21st to end	Night	16% 13% 35% 28% 7% 1	789
December	1st to 10th	Night	24% 12% 34% 23% 6% 1	760
	11th to 20th	Night	19% 11% 37% 27% 6%	760
	21st to end	Night	21% 12% 36% 28% 3	836

Page generated at 2024-11-25 00:22:21. Version ac07

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