Willow Grove Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Willow Grove Park 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	20% 10% 3 8% 5% 4 4 3 5% 6% 31%	106236
	Night	21% 10% 3 8% 5% 4 4 3 4 6% 31%	101873
Transparency	Day	28% 48% 6% 9% 7% 2	105997
	Night	29% 48% 7% 10% 5% 1	101758
Seeing	Day	22% 14% 27% 25% 10% 2	15155
	Night	25% 8% 20% 30% 15% 1	23569

By Month

Percentage Cloud Cover NW (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	38% 17% 4 8% 5% 5% 4 3 3 4 8%	2539
	11th to 20th	Day	34% 14% 4 8% 4 3 3 2 4 5% 19%	2516
	21st to end	Day	36% 16% 4 10% 6% 4 4 2 3 4 11%	3029
February	1st to 10th	Day	32% 14% 4 9% 6% 4 4 4 5% 6% 13%	1710
	11th to 20th	Day	30% 14% 3 10% 7% 4 5% 3 3 3 17%	1665
	21st to end	Day	31% 15% 4 9% 6% 4 3 3 4 4 17%	1463
March	1st to 10th	Day	20% 17% 4 12% 8% 5% 6% 4 5% 4 14%	3310
	11th to 20th	Day	24% 11% 3 8% 5% 4 4 2 3 5% 31%	3490
	21st to end	Day	25% 14% 5% 11% 7% 6% 6% 3 4 4 14%	3886
April	1st to 10th	Day	29% 12% 4 10% 6% 5% 5% 4 5% 6% 14%	2280
	11th to 20th	Day	27% 12% 4 9% 5% 5% 4 4 4 4 22%	2282
	21st to end	Day	24% 11% 3 10% 7% 6% 4 4 5% 5% 21%	2470
May	1st to 10th	Day	21% 8% 3 7% 5% 5% 4 3 5% 6% 33%	4358
	11th to 20th	Day	13% 9% 2 7% 4 5% 5% 5% 7% 9% 34%	4377
	21st to end	Day	23% 10% 3 10% 6% 6% 5% 5% 5% 6% 20%	4813
June	1st to 10th	Day	23% 9% 4 8% 6% 5% 5% 3 5% 6% 27%	2660
	11th to 20th	Day	22% 9% 3 9% 6% 6% 6% 4 6% 7% 22%	2805
	21st to end	Day	12% 7% 2 6% 5% 5% 5% 4 5% 7% 40%	2850
July	1st to 10th	Day	5% 4 2 5% 3 3 2 2 3 5% 64%	4722
	11th to 20th	Day	6% 3 2 4 3 3 3 2 4 6% 64%	4420
	21st to end	Day	9% 5% 2 5% 3 3 3 3 5% 7% 57%	4790
August	1st to 10th	Day	5% 4 1 5% 3 3 3 3 5% 8% 60%	4141
	11th to 20th	Day	8% 5% 2 6% 6% 6% 6% 5% 8% 8% 40%	4026
	21st to end	Day	13% 8% 3 6% 4 4 4 3 5% 6% 45%	4345
September	1st to 10th	Day	8% 6% 3 6% 3 4 4 4 5% 8% 49%	2280
	11th to 20th	Day	13% 9% 3 8% 5% 4 4 4 5% 7% 38%	2266
	21st to end	Day	18% 7% 4 7% 4 4 4 3 5% 7% 38%	2006
October	1st to 10th	Day	18% 7% 2 5% 4 4 4 4 6% 8% 39%	3180
	11th to 20th	Day	22% 9% 4 9% 5% 5% 5% 4 7% 7% 25%	3161
	21st to end	Day	26% 10% 4 10% 6% 6% 5% 4 4 6% 19%	3365
November	1st to 10th	Day	29% 15% 3 12% 5% 6% 5% 4 4 4 13%	1616
	11th to 20th	Day	35% 12% 4 9% 6% 5% 4 3 4 5% 15%	1600
	21st to end	Day	30% 12% 4 9% 6% 4 4 3 4 4 19%	1576
December	1st to 10th	Day	28% 14% 3 9% 7% 5% 5% 3 3 4 19%	1520
	11th to 20th	Day	30% 13% 5% 11% 6% 6% 5% 3 4 5% 12%	1925
	21st to end	Day	32% 16% 4 13% 6% 5% 5% 3 4 3 9%	2794
January	1st to 10th	Night	35% 16% 4 11% 6% 5% 4 3 3 3 10%	4406
	11th to 20th	Night	31% 14% 4 9% 4 4 4 3 4 4 19%	4382
	21st to end	Night	39% 15% 4 8% 5% 4 3 2 3 4 13%	4631
February	1st to 10th	Night	29% 14% 5% 9% 6% 5% 5% 3 4 4 17%	2470
	11th to 20th	Night	26% 13% 3 8% 6% 5% 4 3 4 5% 23%	2402
	21st to end	Night	27% 15% 4 9% 6% 5% 5% 3 4 4 19%	1990
March	1st to 10th	Night	20% 15% 4 11% 7% 6% 6% 4 4 5% 18%	3561
	11th to 20th	Night	20% 10% 3 7% 4 5% 6% 4 4 6% 32%	3489
	21st to end	Night	27% 16% 5% 11% 6% 4 4 3 4 5% 15%	3754
April	1st to 10th	Night	25% 12% 4 9% 7% 6% 5% 3 4 5% 20%	1900
	11th to 20th	Night	22% 11% 3 8% 6% 5% 4 3 5% 7% 28%	1846
	21st to end	Night	22% 11% 3 7% 5% 4 5% 3 5% 7% 27%	1710
May	1st to 10th	Night	22% 9% 3 6% 4 4 4 3 3 6% 38%	2589
	11th to 20th	Night	10% 7% 2 6% 4 4 5% 4 6% 8% 44%	2516
	21st to end	Night	11% 9% 4 9% 8% 6% 6% 5% 6% 7% 30%	2762
June	1st to 10th	Night	18% 8% 3 7% 6% 5% 4 4 4 6% 37%	1520
	11th to 20th	Night	17% 8% 2 6% 4 4 5% 4 6% 8% 36%	1369
	21st to end	Night	9% 5% 3 4 3 3 3 4 5% 7% 53%	1330
July	1st to 10th	Night	3 2 1 3 2 2 3 3 4 6% 70%	2220
	11th to 20th	Night	3 3 1 4 3 3 3 3 4 6% 68%	2540
	21st to end	Night	3 3 1 4 2 2 3 3 4 7% 67%	2762
August	1st to 10th	Night	2 2 1 2 2 3 3 4 4 7% 69%	2782
	11th to 20th	Night	7% 4 3 6% 5% 4 4 3 5% 8% 51%	2812
	21st to end	Night	12% 6% 2 6% 3 4 3 3 4 6% 51%	3328
September	1st to 10th	Night	8% 4 1 5% 3 3 3 3 4 6% 60%	1900
	11th to 20th	Night	12% 6% 3 7% 6% 4 4 3 5% 6% 45%	1911
	21st to end	Night	13% 6% 2 6% 4 4 3 4 5% 7% 45%	2173
October	1st to 10th	Night	12% 6% 2 6% 4 4 4 4 5% 8% 45%	3800
	11th to 20th	Night	19% 9% 4 9% 5% 5% 5% 3 4 7% 31%	3777
	21st to end	Night	23% 9% 4 10% 6% 5% 5% 4 5% 6% 25%	4199
November	1st to 10th	Night	31% 10% 3 8% 6% 5% 5% 3 5% 5% 19%	2682
	11th to 20th	Night	34% 11% 3 8% 5% 4 4 4 4 4 18%	2800
	21st to end	Night	27% 11% 3 9% 6% 4 5% 4 4 4 23%	2758
December	1st to 10th	Night	28% 12% 3 7% 6% 5% 5% 3 5% 5% 21%	2660
	11th to 20th	Night	29% 14% 5% 11% 6% 4 5% 4 4 4 15%	3280
	21st to end	Night	30% 15% 5% 12% 7% 6% 5% 3 3 4 11%	4862

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% 40% 4 2 2 1	2539
	11th to 20th	Day	47% 39% 4 9% 2	2540
	21st to end	Day	52% 41% 3 2 1	3029
February	1st to 10th	Day	27% 65% 4 3 1	1710
	11th to 20th	Day	30% 65% 2 2 1	1674
	21st to end	Day	27% 65% 3 3 3	1463
March	1st to 10th	Day	40% 45% 8% 5% 2	3310
	11th to 20th	Day	27% 41% 6% 22% 4	3490
	21st to end	Day	41% 44% 7% 6% 2	3886
April	1st to 10th	Day	24% 67% 4 4 1	2280
	11th to 20th	Day	22% 67% 3 5% 3	2282
	21st to end	Day	24% 66% 3 5% 2	2470
May	1st to 10th	Day	28% 44% 8% 11% 8%	4358
	11th to 20th	Day	24% 44% 16% 13% 3	4380
	21st to end	Day	39% 46% 11% 5% 1	4814
June	1st to 10th	Day	16% 67% 7% 7% 3	2660
	11th to 20th	Day	19% 67% 5% 6% 3	2805
	21st to end	Day	15% 66% 6% 9% 4	2850
July	1st to 10th	Day	12% 36% 2 10% 17% 23%	4648
	11th to 20th	Day	9% 37% 4 14% 32% 5%	4345
	21st to end	Day	18% 36% 8% 21% 16% 1	4639
August	1st to 10th	Day	11% 38% 4 18% 21% 8%	4124
	11th to 20th	Day	26% 41% 15% 12% 4 2	4065
	21st to end	Day	26% 38% 8% 14% 13% 2	4344
September	1st to 10th	Day	16% 63% 5% 10% 5% 1	2280
	11th to 20th	Day	20% 62% 4 6% 6% 2	2266
	21st to end	Day	18% 61% 7% 8% 6%	2006
October	1st to 10th	Day	23% 38% 6% 24% 9%	3180
	11th to 20th	Day	37% 39% 8% 8% 7%	3161
	21st to end	Day	40% 38% 6% 10% 6%	3365
November	1st to 10th	Day	32% 58% 3 6%	1616
	11th to 20th	Day	31% 58% 4 3 3 1	1600
	21st to end	Day	27% 57% 2 4 10%	1576
December	1st to 10th	Day	31% 60% 2 4 3 1	1520
	11th to 20th	Day	37% 50% 6% 5% 2	1928
	21st to end	Day	48% 42% 6% 3 2	2794
January	1st to 10th	Night	52% 39% 3 2 2 1	4406
	11th to 20th	Night	45% 39% 5% 9% 1	4420
	21st to end	Night	52% 41% 3 3 1	4631
February	1st to 10th	Night	28% 66% 3 3 1	2470
	11th to 20th	Night	28% 64% 2 4 2	2404
	21st to end	Night	23% 66% 4 5% 3	1990
March	1st to 10th	Night	39% 44% 7% 9% 2	3561
	11th to 20th	Night	23% 44% 9% 19% 4	3490
	21st to end	Night	45% 43% 6% 6%	3754
April	1st to 10th	Night	23% 67% 5% 4 1	1900
	11th to 20th	Night	19% 67% 5% 5% 3	1846
	21st to end	Night	19% 66% 5% 7% 3	1710
May	1st to 10th	Night	27% 43% 9% 15% 6%	2589
	11th to 20th	Night	18% 44% 16% 13% 8%	2516
	21st to end	Night	29% 50% 14% 5% 1	2762
June	1st to 10th	Night	14% 67% 8% 9% 3	1520
	11th to 20th	Night	13% 69% 5% 11% 2	1369
	21st to end	Night	11% 66% 9% 12% 2	1330
July	1st to 10th	Night	7% 37% 5% 15% 13% 23%	2185
	11th to 20th	Night	10% 36% 3 15% 29% 7%	2492
	21st to end	Night	13% 36% 9% 24% 16% 1	2674
August	1st to 10th	Night	8% 38% 6% 20% 24% 5%	2774
	11th to 20th	Night	22% 40% 19% 15% 3 1	2839
	21st to end	Night	23% 37% 12% 14% 12% 1	3328
September	1st to 10th	Night	11% 62% 7% 12% 5% 2	1900
	11th to 20th	Night	18% 62% 4 8% 4 3	1911
	21st to end	Night	15% 62% 6% 11% 6%	2173
October	1st to 10th	Night	20% 38% 8% 27% 8%	3800
	11th to 20th	Night	35% 38% 10% 13% 4	3777
	21st to end	Night	37% 38% 8% 12% 5%	4199
November	1st to 10th	Night	30% 58% 5% 6% 2	2682
	11th to 20th	Night	31% 58% 4 4 3 2	2800
	21st to end	Night	26% 58% 3 4 9% 1	2758
December	1st to 10th	Night	28% 60% 3 5% 3 1	2660
	11th to 20th	Night	37% 52% 3 5% 4	3276
	21st to end	Night	45% 44% 6% 5% 1	4862

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	40% 19% 21% 10% 7% 2	180
	11th to 20th	Day	34% 20% 27% 11% 6% 3	180
	21st to end	Day	34% 13% 20% 23% 8% 2	342
February	1st to 10th	Day	44% 12% 16% 16% 10% 1	370
	11th to 20th	Day	39% 17% 23% 16% 4 1	365
	21st to end	Day	38% 18% 25% 14% 5%	332
March	1st to 10th	Day	33% 19% 27% 16% 6%	460
	11th to 20th	Day	30% 17% 24% 18% 8% 2	460
	21st to end	Day	31% 22% 23% 15% 7% 1	506
April	1st to 10th	Day	30% 20% 23% 18% 7% 2	460
	11th to 20th	Day	30% 21% 26% 14% 6% 2	452
	21st to end	Day	25% 19% 31% 17% 7% 1	460
May	1st to 10th	Day	17% 19% 28% 25% 8% 2	463
	11th to 20th	Day	27% 12% 26% 22% 10% 2	488
	21st to end	Day	21% 14% 29% 25% 10% 2	529
June	1st to 10th	Day	24% 12% 26% 27% 10% 1	490
	11th to 20th	Day	18% 22% 33% 21% 5% 1	635
	21st to end	Day	9% 14% 33% 33% 10% 1	680
July	1st to 10th	Day	9% 8% 37% 34% 10% 2	666
	11th to 20th	Day	8% 9% 31% 36% 14% 2	484
	21st to end	Day	6% 5% 30% 38% 17% 3	531
August	1st to 10th	Day	7% 7% 27% 42% 14% 4	487
	11th to 20th	Day	5% 5% 25% 40% 19% 5%	477
	21st to end	Day	10% 10% 31% 32% 13% 4	539
September	1st to 10th	Day	9% 8% 30% 37% 13% 3	490
	11th to 20th	Day	16% 9% 26% 33% 13% 3	490
	21st to end	Day	18% 15% 23% 29% 13% 2	490
October	1st to 10th	Day	24% 12% 23% 25% 15% 2	490
	11th to 20th	Day	24% 13% 23% 22% 16% 2	490
	21st to end	Day	32% 16% 24% 17% 10% 2	492
November	1st to 10th	Day	29% 21% 25% 16% 7% 1	225
	11th to 20th	Day	41% 14% 18% 18% 9% 1	200
	21st to end	Day	30% 23% 19% 16% 11% 1	194
December	1st to 10th	Day	33% 19% 20% 19% 7% 3	180
	11th to 20th	Day	35% 18% 27% 14% 6%	180
	21st to end	Day	39% 16% 24% 14% 7% 1	198
January	1st to 10th	Night	42% 10% 19% 17% 10% 2	760
	11th to 20th	Night	38% 11% 15% 22% 14% 1	760
	21st to end	Night	36% 7% 15% 26% 14% 2	836
February	1st to 10th	Night	33% 10% 18% 27% 11% 1	760
	11th to 20th	Night	30% 10% 20% 27% 13% 1	744
	21st to end	Night	32% 11% 21% 25% 11%	628
March	1st to 10th	Night	30% 11% 22% 24% 13% 1	760
	11th to 20th	Night	31% 10% 20% 26% 12%	760
	21st to end	Night	34% 13% 18% 25% 10% 1	836
April	1st to 10th	Night	29% 13% 22% 23% 12% 1	760
	11th to 20th	Night	23% 12% 25% 27% 11% 2	718
	21st to end	Night	26% 9% 19% 31% 14% 1	570
May	1st to 10th	Night	18% 10% 25% 31% 15%	416
	11th to 20th	Night	18% 5% 24% 34% 17% 2	374
	21st to end	Night	12% 7% 26% 35% 18% 1	411
June	1st to 10th	Night	18% 4 27% 41% 11%	380
	11th to 20th	Night	16% 9% 27% 37% 11% 1	380
	21st to end	Night	10% 4 28% 42% 16%	380
July	1st to 10th	Night	6% 3 25% 53% 14%	370
	11th to 20th	Night	4 2 34% 46% 13% 1	368
	21st to end	Night	4 2 26% 50% 18%	416
August	1st to 10th	Night	5% 2 18% 47% 26% 2	527
	11th to 20th	Night	6% 2 15% 43% 32% 3	564
	21st to end	Night	11% 4 22% 41% 20% 2	731
September	1st to 10th	Night	11% 4 21% 43% 21% 1	760
	11th to 20th	Night	13% 5% 17% 34% 28% 4	760
	21st to end	Night	15% 6% 21% 38% 19% 2	760
October	1st to 10th	Night	19% 4 18% 36% 20% 2	760
	11th to 20th	Night	23% 7% 16% 29% 22% 3	760
	21st to end	Night	29% 8% 17% 27% 17% 1	836
November	1st to 10th	Night	31% 10% 19% 28% 10% 1	780
	11th to 20th	Night	37% 11% 17% 24% 11% 1	800
	21st to end	Night	31% 11% 17% 26% 13% 2	788
December	1st to 10th	Night	31% 9% 19% 25% 13% 2	760
	11th to 20th	Night	37% 11% 20% 20% 10% 1	760
	21st to end	Night	41% 14% 22% 16% 7% 1	836

Page generated at 2024-11-24 21:22:20. Version ac07

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