Fort Washington Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fort Washington Park clear sky chart from 2005-11-06 to 2025-09-19.

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	14% 6% 2 6% 4 4 5% 4 6% 8% 42%	126216
	Night	14% 7% 2 6% 4 4 4 4 5% 7% 43%	122852
Transparency	Day	39% 6% 11% 23% 17% 4	129809
	Night	39% 6% 11% 23% 18% 4	127329
Seeing	Day	18% 12% 27% 25% 13% 4	16157
	Night	17% 10% 30% 29% 12% 1	25637

By Month

Percentage Cloud Cover SE (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% 4 1 4 3 4 4 3 5% 5% 51%	3712
	11th to 20th	Day	15% 8% 3 6% 4 3 3 3 4 5% 47%	4060
	21st to end	Day	15% 8% 2 7% 4 4 4 3 5% 6% 42%	4466
February	1st to 10th	Day	15% 8% 2 7% 5% 5% 4 4 5% 6% 39%	1800
	11th to 20th	Day	20% 7% 2 5% 3 4 4 2 6% 7% 41%	1825
	21st to end	Day	18% 8% 3 7% 5% 3 3 3 5% 6% 41%	1650
March	1st to 10th	Day	18% 6% 2 5% 4 4 4 3 5% 7% 42%	4836
	11th to 20th	Day	11% 6% 2 5% 4 4 4 3 5% 6% 51%	4980
	21st to end	Day	17% 6% 2 6% 4 4 4 4 5% 6% 42%	5574
April	1st to 10th	Day	13% 7% 2 6% 4 4 4 3 6% 6% 43%	2400
	11th to 20th	Day	12% 7% 2 5% 5% 4 3 4 5% 6% 47%	2374
	21st to end	Day	16% 6% 2 5% 4 4 5% 4 6% 6% 42%	2400
May	1st to 10th	Day	19% 7% 3 7% 4 4 5% 4 7% 9% 31%	5714
	11th to 20th	Day	20% 7% 2 5% 4 5% 5% 4 6% 9% 31%	6166
	21st to end	Day	15% 5% 2 6% 5% 4 5% 4 6% 8% 39%	6934
June	1st to 10th	Day	11% 5% 2 6% 5% 4 4 4 8% 10% 42%	2800
	11th to 20th	Day	11% 5% 2 5% 4 4 5% 5% 7% 9% 43%	2800
	21st to end	Day	9% 4 2 5% 4 5% 5% 5% 8% 10% 43%	2800
July	1st to 10th	Day	7% 4 2 6% 4 5% 5% 5% 7% 11% 44%	6360
	11th to 20th	Day	10% 6% 2 8% 5% 5% 7% 5% 9% 12% 30%	6158
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 10% 40%	6424
August	1st to 10th	Day	16% 6% 2 6% 4 5% 6% 5% 7% 10% 33%	4234
	11th to 20th	Day	10% 5% 2 7% 5% 5% 7% 5% 7% 9% 38%	3960
	21st to end	Day	9% 5% 2 5% 4 4 5% 5% 6% 8% 48%	4312
September	1st to 10th	Day	13% 5% 2 5% 4 4 4 4 5% 7% 46%	2400
	11th to 20th	Day	11% 6% 2 6% 3 4 4 3 6% 9% 46%	2376
	21st to end	Day	18% 8% 3 6% 4 4 5% 4 5% 6% 37%	2004
October	1st to 10th	Day	15% 6% 2 6% 3 3 4 3 5% 6% 47%	1900
	11th to 20th	Day	16% 4 2 4 3 3 3 3 4 7% 51%	1900
	21st to end	Day	13% 6% 3 6% 4 3 4 2 5% 7% 49%	2090
November	1st to 10th	Day	12% 5% 1 3 4 4 4 3 5% 8% 52%	1852
	11th to 20th	Day	13% 6% 1 4 3 3 4 3 5% 8% 50%	1800
	21st to end	Day	14% 6% 2 6% 4 3 3 3 5% 5% 49%	1605
December	1st to 10th	Day	11% 4 2 6% 4 3 4 4 5% 7% 49%	2600
	11th to 20th	Day	18% 8% 2 5% 3 3 3 3 4 7% 44%	2990
	21st to end	Day	15% 8% 3 6% 4 4 5% 3 6% 8% 39%	3960
January	1st to 10th	Night	13% 8% 2 5% 3 3 4 3 4 5% 50%	6231
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 4 6% 53%	5900
	21st to end	Night	14% 9% 3 6% 4 3 3 3 3 6% 47%	6490
February	1st to 10th	Night	16% 7% 3 5% 4 4 5% 3 5% 8% 41%	2600
	11th to 20th	Night	20% 7% 3 5% 4 3 4 3 4 6% 43%	2458
	21st to end	Night	17% 8% 2 5% 3 3 3 3 5% 6% 43%	1980
March	1st to 10th	Night	16% 6% 2 6% 4 3 3 3 5% 5% 47%	5012
	11th to 20th	Night	11% 5% 2 5% 3 3 4 3 5% 8% 50%	4980
	21st to end	Night	13% 7% 3 6% 4 4 4 3 5% 6% 44%	5358
April	1st to 10th	Night	16% 7% 2 5% 6% 4 4 3 5% 7% 41%	2000
	11th to 20th	Night	13% 5% 2 6% 3 4 4 3 6% 7% 47%	1975
	21st to end	Night	16% 6% 2 5% 3 3 4 4 5% 8% 42%	2000
May	1st to 10th	Night	18% 9% 3 8% 5% 4 6% 4 6% 7% 30%	4224
	11th to 20th	Night	21% 8% 3 6% 4 4 4 4 6% 8% 32%	3723
	21st to end	Night	17% 6% 2 7% 4 4 5% 4 6% 9% 35%	3920
June	1st to 10th	Night	10% 5% 2 5% 4 4 5% 5% 8% 10% 42%	1600
	11th to 20th	Night	11% 7% 2 6% 4 5% 4 4 7% 10% 40%	1600
	21st to end	Night	9% 5% 2 6% 5% 5% 4 4 8% 10% 43%	1600
July	1st to 10th	Night	8% 5% 2 8% 5% 5% 6% 5% 7% 10% 39%	3598
	11th to 20th	Night	10% 8% 3 8% 5% 5% 6% 5% 8% 10% 30%	3781
	21st to end	Night	9% 6% 2 6% 5% 5% 5% 5% 8% 9% 39%	4427
August	1st to 10th	Night	16% 6% 2 7% 5% 4 5% 5% 6% 9% 34%	2915
	11th to 20th	Night	11% 7% 3 8% 5% 5% 6% 5% 7% 7% 35%	3054
	21st to end	Night	9% 5% 2 5% 3 3 4 4 5% 8% 51%	3586
September	1st to 10th	Night	13% 5% 2 6% 4 4 5% 3 5% 8% 46%	2000
	11th to 20th	Night	10% 7% 2 6% 3 3 5% 4 6% 8% 47%	1981
	21st to end	Night	18% 8% 3 7% 3 5% 3 3 5% 7% 37%	2171
October	1st to 10th	Night	16% 7% 2 6% 3 3 4 3 4 6% 46%	2280
	11th to 20th	Night	14% 5% 2 5% 4 3 3 3 4 6% 52%	2280
	21st to end	Night	14% 5% 2 5% 3 3 4 3 5% 7% 50%	2508
November	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 4 6% 53%	2446
	11th to 20th	Night	15% 7% 2 4 4 3 3 3 4 6% 49%	2600
	21st to end	Night	13% 4 2 5% 4 3 4 4 5% 7% 49%	2793
December	1st to 10th	Night	10% 6% 2 5% 4 3 3 3 5% 6% 53%	4580
	11th to 20th	Night	20% 7% 2 6% 4 4 4 3 4 7% 40%	5205
	21st to end	Night	16% 7% 3 6% 4 4 4 4 6% 7% 39%	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	35% 4 6% 27% 22% 7%	3713
	11th to 20th	Day	40% 4 6% 23% 19% 8%	4060
	21st to end	Day	43% 5% 8% 28% 14% 3	4466
February	1st to 10th	Day	44% 5% 12% 25% 13% 2	1800
	11th to 20th	Day	44% 4 8% 24% 18% 3	1834
	21st to end	Day	45% 5% 11% 20% 16% 4	1650
March	1st to 10th	Day	41% 5% 8% 22% 16% 8%	4836
	11th to 20th	Day	33% 5% 8% 26% 21% 6%	4980
	21st to end	Day	43% 5% 10% 19% 17% 7%	5574
April	1st to 10th	Day	40% 5% 10% 20% 19% 6%	2400
	11th to 20th	Day	37% 6% 9% 22% 20% 7%	2374
	21st to end	Day	40% 6% 10% 22% 17% 4	2400
May	1st to 10th	Day	48% 7% 14% 19% 11% 1	5747
	11th to 20th	Day	47% 8% 11% 24% 9% 1	6170
	21st to end	Day	41% 8% 12% 22% 16% 2	6949
June	1st to 10th	Day	35% 7% 14% 26% 16% 2	2800
	11th to 20th	Day	35% 10% 13% 24% 16% 2	2800
	21st to end	Day	32% 9% 15% 24% 18% 2	2800
July	1st to 10th	Day	33% 9% 20% 20% 14% 4	6290
	11th to 20th	Day	43% 10% 17% 21% 7% 1	6083
	21st to end	Day	37% 7% 11% 22% 22% 1	6240
August	1st to 10th	Day	44% 7% 16% 25% 8%	4221
	11th to 20th	Day	39% 8% 13% 26% 14% 1	3999
	21st to end	Day	32% 6% 14% 28% 18% 2	4312
September	1st to 10th	Day	36% 7% 13% 19% 21% 4	2400
	11th to 20th	Day	34% 6% 11% 25% 20% 4	2376
	21st to end	Day	46% 6% 9% 19% 17% 3	2004
October	1st to 10th	Day	38% 4 7% 17% 29% 5%	3160
	11th to 20th	Day	25% 4 6% 29% 27% 10%	3140
	21st to end	Day	23% 2 8% 30% 27% 10%	3428
November	1st to 10th	Day	32% 4 11% 27% 21% 5%	1852
	11th to 20th	Day	34% 4 11% 25% 22% 5%	1800
	21st to end	Day	37% 4 9% 27% 19% 3	1605
December	1st to 10th	Day	36% 4 12% 27% 17% 4	2600
	11th to 20th	Day	41% 4 13% 29% 11% 2	2986
	21st to end	Day	43% 6% 12% 22% 16% 1	3960
January	1st to 10th	Night	36% 3 6% 25% 25% 5%	6231
	11th to 20th	Night	35% 3 7% 25% 23% 7%	5900
	21st to end	Night	40% 4 7% 28% 19% 3	6490
February	1st to 10th	Night	42% 5% 12% 24% 15% 2	2600
	11th to 20th	Night	44% 3 8% 21% 18% 5%	2460
	21st to end	Night	41% 4 10% 22% 17% 6%	1980
March	1st to 10th	Night	39% 4 7% 24% 17% 9%	5012
	11th to 20th	Night	32% 5% 10% 25% 21% 7%	4980
	21st to end	Night	40% 5% 8% 22% 20% 6%	5358
April	1st to 10th	Night	42% 5% 9% 19% 21% 4	2000
	11th to 20th	Night	37% 6% 11% 21% 20% 6%	1975
	21st to end	Night	39% 6% 10% 24% 16% 5%	2000
May	1st to 10th	Night	52% 7% 12% 17% 9% 3	4254
	11th to 20th	Night	48% 8% 11% 21% 11% 1	3724
	21st to end	Night	43% 8% 13% 22% 11% 2	3927
June	1st to 10th	Night	35% 9% 11% 26% 17% 2	1600
	11th to 20th	Night	37% 9% 13% 25% 13% 2	1600
	21st to end	Night	34% 7% 15% 21% 19% 3	1600
July	1st to 10th	Night	39% 10% 17% 19% 14% 2	3560
	11th to 20th	Night	46% 11% 19% 17% 7% 1	3736
	21st to end	Night	37% 9% 14% 22% 18%	4297
August	1st to 10th	Night	45% 9% 16% 21% 8% 1	2904
	11th to 20th	Night	44% 7% 12% 24% 12% 1	3081
	21st to end	Night	32% 5% 11% 30% 20% 2	3586
September	1st to 10th	Night	38% 7% 10% 22% 18% 6%	2000
	11th to 20th	Night	33% 6% 9% 26% 22% 3	1981
	21st to end	Night	46% 6% 9% 19% 16% 4	2171
October	1st to 10th	Night	41% 5% 9% 21% 19% 7%	3800
	11th to 20th	Night	24% 4 10% 21% 32% 9%	3779
	21st to end	Night	22% 4 9% 31% 26% 9%	4120
November	1st to 10th	Night	33% 5% 9% 26% 21% 6%	2446
	11th to 20th	Night	37% 4 9% 22% 21% 7%	2600
	21st to end	Night	34% 5% 11% 25% 22% 3	2793
December	1st to 10th	Night	33% 4 11% 27% 22% 3	4580
	11th to 20th	Night	45% 4 12% 20% 15% 5%	5208
	21st to end	Night	43% 6% 13% 22% 15% 1	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	31% 18% 32% 14% 4	225
	11th to 20th	Day	29% 22% 32% 14% 3	300
	21st to end	Day	22% 23% 35% 15% 5%	330
February	1st to 10th	Day	24% 25% 36% 12% 3	300
	11th to 20th	Day	36% 21% 32% 10% 1	372
	21st to end	Day	28% 21% 31% 18% 2	414
March	1st to 10th	Day	23% 23% 31% 20% 3	500
	11th to 20th	Day	27% 19% 32% 16% 5% 1	500
	21st to end	Day	23% 17% 34% 21% 4 1	550
April	1st to 10th	Day	22% 23% 31% 18% 5% 1	500
	11th to 20th	Day	17% 19% 32% 20% 9% 3	492
	21st to end	Day	24% 12% 32% 23% 7% 1	500
May	1st to 10th	Day	25% 15% 28% 20% 10% 2	503
	11th to 20th	Day	22% 12% 30% 23% 11% 2	527
	21st to end	Day	18% 9% 24% 25% 19% 5%	573
June	1st to 10th	Day	12% 9% 29% 25% 19% 6%	530
	11th to 20th	Day	14% 8% 25% 33% 16% 4	530
	21st to end	Day	10% 5% 27% 35% 19% 5%	530
July	1st to 10th	Day	11% 3 17% 36% 22% 11%	525
	11th to 20th	Day	5% 2 16% 41% 26% 10%	524
	21st to end	Day	9% 3 19% 39% 23% 7%	576
August	1st to 10th	Day	11% 5% 22% 31% 23% 8%	526
	11th to 20th	Day	12% 4 18% 33% 24% 9%	519
	21st to end	Day	9% 4 21% 31% 23% 12%	583
September	1st to 10th	Day	15% 6% 23% 25% 18% 12%	530
	11th to 20th	Day	14% 4 20% 35% 17% 10%	522
	21st to end	Day	24% 12% 23% 24% 12% 4	490
October	1st to 10th	Day	21% 6% 23% 24% 18% 7%	490
	11th to 20th	Day	16% 14% 26% 27% 14% 3	490
	21st to end	Day	17% 11% 34% 27% 9% 4	539
November	1st to 10th	Day	16% 12% 31% 29% 10% 3	442
	11th to 20th	Day	18% 15% 35% 23% 9% 2	400
	21st to end	Day	22% 21% 27% 20% 9% 1	205
December	1st to 10th	Day	19% 19% 41% 15% 5% 2	200
	11th to 20th	Day	24% 20% 29% 21% 7% 1	200
	21st to end	Day	25% 17% 31% 21% 6%	220
January	1st to 10th	Night	21% 19% 38% 18% 4	800
	11th to 20th	Night	22% 14% 35% 24% 5%	800
	21st to end	Night	19% 17% 37% 24% 2	880
February	1st to 10th	Night	20% 15% 35% 26% 3	800
	11th to 20th	Night	26% 19% 37% 16% 1	783
	21st to end	Night	25% 17% 35% 21% 2	660
March	1st to 10th	Night	20% 15% 39% 23% 3	800
	11th to 20th	Night	22% 15% 38% 20% 6% 1	800
	21st to end	Night	19% 14% 41% 22% 3	880
April	1st to 10th	Night	20% 18% 39% 21% 4	800
	11th to 20th	Night	16% 14% 34% 28% 6%	791
	21st to end	Night	21% 9% 34% 28% 8%	800
May	1st to 10th	Night	19% 10% 31% 29% 10%	690
	11th to 20th	Night	18% 7% 31% 34% 10%	464
	21st to end	Night	15% 4 26% 36% 18% 1	433
June	1st to 10th	Night	10% 4 27% 38% 20% 2	400
	11th to 20th	Night	14% 5% 21% 42% 18% 1	400
	21st to end	Night	10% 3 24% 44% 18% 1	400
July	1st to 10th	Night	9% 2 22% 38% 28% 2	390
	11th to 20th	Night	7% 2 13% 44% 31% 4	468
	21st to end	Night	8% 2 19% 39% 29% 3	657
August	1st to 10th	Night	10% 2 13% 44% 28% 3	597
	11th to 20th	Night	13% 3 17% 39% 25% 2	695
	21st to end	Night	12% 2 17% 36% 28% 4	880
September	1st to 10th	Night	15% 3 19% 34% 26% 4	800
	11th to 20th	Night	13% 3 16% 38% 28% 4	793
	21st to end	Night	21% 7% 22% 32% 17% 1	760
October	1st to 10th	Night	19% 6% 20% 30% 19% 5%	760
	11th to 20th	Night	16% 10% 27% 30% 17% 1	760
	21st to end	Night	17% 10% 31% 28% 13% 1	836
November	1st to 10th	Night	13% 8% 34% 30% 12% 2	780
	11th to 20th	Night	19% 13% 30% 29% 9% 1	800
	21st to end	Night	17% 14% 33% 26% 10% 1	800
December	1st to 10th	Night	20% 13% 35% 23% 8% 1	800
	11th to 20th	Night	19% 11% 40% 26% 5%	800
	21st to end	Night	21% 12% 34% 30% 4	880

Page generated at 2025-09-16 01:49:11. Version ac07

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