Coatesville
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Coatesville clear sky chart from 2005-11-06 to 2024-01-12.

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	15% 6% 3 5% 5% 3 5% 5% 6% 8% 40%	74718
	Night	15% 6% 2 5% 5% 3 4 4 5% 7% 43%	75000
Transparency	Day	41% 7% 12% 22% 15% 3	74429
	Night	40% 7% 12% 22% 16% 3	74823
Seeing	Day	20% 17% 29% 21% 10% 3	13828
	Night	19% 16% 30% 25% 9% 1	23269

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% 7% 3 5% 6% 3 5% 5% 5% 7% 41%	1517
	11th to 20th	Day	15% 7% 3 5% 7% 4 5% 5% 6% 5% 38%	1535
	21st to end	Day	14% 8% 2 6% 5% 3 5% 5% 5% 6% 39%	1782
February	1st to 10th	Day	15% 7% 3 6% 6% 3 4 4 5% 9% 37%	1620
	11th to 20th	Day	16% 7% 3 6% 5% 4 4 5% 4 7% 38%	1678
	21st to end	Day	18% 10% 4 5% 5% 3 4 4 5% 5% 37%	1480
March	1st to 10th	Day	13% 5% 2 5% 6% 3 4 4 4 6% 47%	1837
	11th to 20th	Day	17% 6% 3 6% 6% 3 5% 5% 5% 7% 38%	1980
	21st to end	Day	20% 7% 3 5% 6% 3 4 3 5% 6% 38%	2276
April	1st to 10th	Day	14% 8% 3 6% 5% 3 4 4 5% 7% 41%	2160
	11th to 20th	Day	15% 5% 2 5% 6% 3 5% 5% 5% 7% 43%	2134
	21st to end	Day	16% 5% 3 6% 5% 4 4 4 6% 7% 39%	2160
May	1st to 10th	Day	18% 7% 2 6% 5% 3 6% 5% 5% 7% 36%	2305
	11th to 20th	Day	19% 6% 3 5% 5% 3 4 5% 6% 8% 36%	2478
	21st to end	Day	13% 7% 3 5% 5% 3 4 5% 6% 8% 40%	2725
June	1st to 10th	Day	13% 5% 3 5% 5% 4 5% 6% 7% 11% 38%	2520
	11th to 20th	Day	14% 6% 3 6% 6% 3 4 5% 7% 9% 37%	2520
	21st to end	Day	11% 4 2 5% 5% 3 6% 6% 7% 10% 42%	2520
July	1st to 10th	Day	11% 5% 3 4 6% 4 6% 6% 7% 10% 38%	2520
	11th to 20th	Day	7% 4 2 5% 6% 3 6% 6% 8% 10% 43%	2520
	21st to end	Day	10% 5% 3 6% 7% 4 6% 7% 7% 8% 36%	2650
August	1st to 10th	Day	10% 5% 2 5% 6% 4 7% 5% 8% 8% 39%	2327
	11th to 20th	Day	11% 6% 2 5% 7% 4 4 5% 6% 10% 39%	2158
	21st to end	Day	13% 6% 2 5% 4 4 5% 6% 6% 8% 41%	2376
September	1st to 10th	Day	16% 5% 2 5% 4 3 5% 5% 5% 6% 43%	2160
	11th to 20th	Day	13% 5% 2 4 4 2 3 5% 5% 7% 50%	2156
	21st to end	Day	18% 6% 3 5% 5% 4 5% 4 5% 6% 39%	1940
October	1st to 10th	Day	17% 6% 2 6% 4 3 4 4 5% 7% 43%	1800
	11th to 20th	Day	16% 6% 2 4 5% 3 4 4 4 6% 47%	1800
	21st to end	Day	18% 6% 3 4 4 3 4 4 5% 6% 44%	1953
November	1st to 10th	Day	13% 7% 2 4 3 2 3 3 4 7% 52%	1671
	11th to 20th	Day	16% 5% 2 4 4 3 4 5% 5% 7% 46%	1710
	21st to end	Day	16% 5% 3 4 4 2 5% 5% 5% 7% 44%	1552
December	1st to 10th	Day	18% 8% 3 5% 5% 3 3 4 5% 7% 38%	1520
	11th to 20th	Day	19% 7% 3 5% 4 3 3 4 4 7% 41%	1906
	21st to end	Day	16% 6% 2 6% 7% 4 6% 6% 5% 7% 35%	2772
January	1st to 10th	Night	14% 7% 2 5% 5% 3 4 4 5% 7% 44%	2649
	11th to 20th	Night	19% 7% 2 5% 5% 3 5% 4 5% 6% 38%	2462
	21st to end	Night	13% 7% 2 5% 5% 3 4 5% 6% 7% 44%	2574
February	1st to 10th	Night	17% 8% 3 6% 5% 3 4 5% 5% 6% 40%	2340
	11th to 20th	Night	17% 7% 4 5% 4 4 4 4 5% 5% 40%	2168
	21st to end	Night	17% 8% 3 5% 4 2 5% 5% 5% 6% 40%	1776
March	1st to 10th	Night	14% 5% 2 4 5% 3 4 4 5% 8% 46%	2108
	11th to 20th	Night	17% 6% 3 6% 5% 3 4 4 4 4 44%	1979
	21st to end	Night	16% 7% 3 6% 4 3 4 3 4 6% 44%	2079
April	1st to 10th	Night	16% 7% 3 5% 5% 2 5% 5% 5% 7% 41%	1800
	11th to 20th	Night	14% 5% 3 5% 5% 2 4 4 5% 8% 43%	1775
	21st to end	Night	18% 6% 2 5% 4 3 4 4 4 7% 43%	1800
May	1st to 10th	Night	17% 7% 2 7% 6% 3 4 5% 5% 7% 36%	1670
	11th to 20th	Night	18% 6% 4 5% 4 3 6% 5% 5% 7% 37%	1415
	21st to end	Night	15% 6% 3 5% 4 3 4 5% 6% 9% 41%	1550
June	1st to 10th	Night	15% 5% 3 5% 5% 3 4 5% 5% 8% 43%	1440
	11th to 20th	Night	12% 6% 2 5% 6% 3 5% 6% 5% 10% 39%	1440
	21st to end	Night	11% 4 1 5% 4 3 4 5% 8% 11% 43%	1440
July	1st to 10th	Night	10% 5% 2 6% 7% 4 6% 6% 7% 9% 39%	1440
	11th to 20th	Night	7% 5% 1 5% 5% 4 6% 6% 7% 9% 46%	1440
	21st to end	Night	10% 4 3 6% 5% 3 5% 5% 9% 10% 41%	1664
August	1st to 10th	Night	9% 5% 3 4 5% 4 5% 6% 7% 9% 42%	1611
	11th to 20th	Night	13% 6% 2 6% 6% 3 5% 5% 6% 7% 40%	1723
	21st to end	Night	13% 5% 2 4 5% 3 5% 5% 6% 7% 45%	1980
September	1st to 10th	Night	15% 6% 3 5% 4 2 4 3 4 7% 46%	1800
	11th to 20th	Night	12% 6% 2 4 3 2 3 4 4 6% 53%	1804
	21st to end	Night	17% 6% 2 4 5% 3 4 4 6% 6% 42%	2016
October	1st to 10th	Night	19% 6% 2 6% 5% 2 3 4 4 6% 43%	2160
	11th to 20th	Night	17% 5% 2 4 4 2 3 4 5% 6% 49%	2160
	21st to end	Night	18% 6% 2 4 3 2 3 3 4 6% 48%	2402
November	1st to 10th	Night	11% 5% 2 4 4 2 4 4 5% 6% 52%	2407
	11th to 20th	Night	17% 6% 2 4 4 2 4 5% 5% 6% 46%	2470
	21st to end	Night	13% 4 2 4 5% 3 5% 5% 6% 7% 46%	2626
December	1st to 10th	Night	18% 6% 3 5% 5% 3 5% 4 5% 7% 39%	2660
	11th to 20th	Night	18% 5% 2 5% 5% 3 4 4 6% 6% 42%	3288
	21st to end	Night	16% 7% 3 6% 5% 3 5% 5% 6% 8% 36%	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	41% 5% 15% 21% 14% 4	1518
	11th to 20th	Day	45% 5% 14% 21% 12% 2	1535
	21st to end	Day	45% 5% 10% 23% 14% 3	1782
February	1st to 10th	Day	45% 6% 14% 23% 10% 2	1620
	11th to 20th	Day	46% 5% 10% 25% 13% 2	1687
	21st to end	Day	48% 4 10% 21% 12% 5%	1480
March	1st to 10th	Day	38% 6% 11% 26% 15% 2	1840
	11th to 20th	Day	44% 6% 11% 20% 15% 5%	1980
	21st to end	Day	48% 5% 8% 19% 17% 3	2276
April	1st to 10th	Day	41% 6% 12% 20% 15% 5%	2160
	11th to 20th	Day	39% 7% 11% 22% 17% 4	2134
	21st to end	Day	42% 7% 12% 18% 17% 4	2160
May	1st to 10th	Day	46% 7% 11% 19% 15% 4	2305
	11th to 20th	Day	46% 8% 11% 20% 13% 2	2482
	21st to end	Day	40% 8% 16% 22% 13% 2	2725
June	1st to 10th	Day	39% 9% 14% 22% 14% 1	2520
	11th to 20th	Day	42% 9% 13% 22% 11% 3	2520
	21st to end	Day	36% 9% 15% 24% 16% 1	2520
July	1st to 10th	Day	38% 11% 13% 21% 14% 3	2450
	11th to 20th	Day	31% 10% 16% 25% 15% 2	2445
	21st to end	Day	41% 10% 12% 20% 15% 2	2502
August	1st to 10th	Day	39% 10% 14% 22% 14% 1	2314
	11th to 20th	Day	38% 9% 15% 21% 14% 2	2158
	21st to end	Day	37% 9% 14% 21% 15% 4	2376
September	1st to 10th	Day	40% 8% 11% 20% 18% 3	2160
	11th to 20th	Day	33% 6% 10% 25% 22% 4	2156
	21st to end	Day	46% 6% 8% 18% 20% 3	1940
October	1st to 10th	Day	40% 7% 12% 22% 17% 1	1800
	11th to 20th	Day	38% 5% 12% 22% 21% 2	1800
	21st to end	Day	41% 5% 10% 25% 17% 2	1953
November	1st to 10th	Day	34% 4 13% 25% 19% 4	1671
	11th to 20th	Day	36% 6% 12% 23% 21% 2	1710
	21st to end	Day	39% 5% 12% 30% 11% 2	1552
December	1st to 10th	Day	45% 5% 16% 20% 12% 1	1520
	11th to 20th	Day	42% 6% 12% 29% 10% 2	1906
	21st to end	Day	45% 9% 14% 21% 10% 1	2772
January	1st to 10th	Night	40% 5% 11% 24% 17% 4	2649
	11th to 20th	Night	45% 6% 13% 22% 12% 2	2462
	21st to end	Night	39% 5% 11% 26% 16% 2	2574
February	1st to 10th	Night	45% 5% 13% 24% 11% 2	2340
	11th to 20th	Night	45% 5% 10% 20% 17% 4	2170
	21st to end	Night	44% 5% 10% 21% 15% 5%	1776
March	1st to 10th	Night	36% 5% 12% 25% 18% 3	2118
	11th to 20th	Night	42% 6% 11% 22% 15% 4	1980
	21st to end	Night	43% 5% 11% 18% 18% 5%	2079
April	1st to 10th	Night	43% 5% 12% 19% 17% 5%	1800
	11th to 20th	Night	38% 7% 12% 20% 19% 4	1775
	21st to end	Night	42% 6% 10% 19% 18% 5%	1800
May	1st to 10th	Night	46% 8% 11% 16% 15% 3	1670
	11th to 20th	Night	46% 8% 12% 14% 18% 3	1416
	21st to end	Night	40% 9% 15% 18% 15% 2	1551
June	1st to 10th	Night	38% 9% 10% 23% 18% 1	1440
	11th to 20th	Night	39% 9% 15% 21% 15% 1	1440
	21st to end	Night	32% 11% 13% 26% 15% 2	1440
July	1st to 10th	Night	40% 11% 16% 17% 13% 3	1400
	11th to 20th	Night	31% 12% 16% 23% 15% 2	1392
	21st to end	Night	35% 10% 17% 21% 16% 1	1571
August	1st to 10th	Night	35% 12% 16% 22% 15% 1	1600
	11th to 20th	Night	41% 9% 13% 18% 16% 2	1723
	21st to end	Night	36% 8% 16% 20% 17% 3	1980
September	1st to 10th	Night	39% 6% 11% 21% 19% 3	1800
	11th to 20th	Night	32% 4 11% 23% 26% 4	1804
	21st to end	Night	41% 7% 9% 20% 22% 2	2016
October	1st to 10th	Night	44% 5% 12% 22% 16% 2	2160
	11th to 20th	Night	35% 6% 12% 28% 17% 2	2160
	21st to end	Night	38% 5% 9% 27% 18% 2	2402
November	1st to 10th	Night	32% 6% 11% 26% 19% 5%	2407
	11th to 20th	Night	39% 5% 9% 22% 20% 5%	2470
	21st to end	Night	36% 6% 12% 28% 16% 2	2626
December	1st to 10th	Night	45% 5% 12% 22% 14% 1	2660
	11th to 20th	Night	40% 6% 13% 22% 16% 3	3288
	21st to end	Night	42% 9% 15% 22% 10% 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	26% 30% 28% 14% 1 1	180
	11th to 20th	Day	29% 30% 30% 11% 1	200
	21st to end	Day	23% 35% 27% 11% 3	264
February	1st to 10th	Day	30% 30% 30% 8% 1	240
	11th to 20th	Day	31% 27% 35% 6% 1	346
	21st to end	Day	33% 27% 27% 12% 2	346
March	1st to 10th	Day	25% 27% 32% 14% 3	420
	11th to 20th	Day	27% 27% 30% 11% 3 1	420
	21st to end	Day	29% 25% 30% 11% 4 1	462
April	1st to 10th	Day	22% 29% 34% 10% 4 1	420
	11th to 20th	Day	23% 24% 29% 17% 5% 2	412
	21st to end	Day	27% 20% 37% 13% 4	420
May	1st to 10th	Day	27% 21% 27% 17% 6% 1	423
	11th to 20th	Day	22% 16% 35% 20% 6% 1	447
	21st to end	Day	19% 10% 26% 27% 16% 2	485
June	1st to 10th	Day	14% 12% 35% 24% 12% 3	450
	11th to 20th	Day	18% 11% 31% 26% 11% 2	450
	21st to end	Day	13% 13% 26% 32% 12% 4	450
July	1st to 10th	Day	13% 7% 23% 30% 21% 7%	445
	11th to 20th	Day	7% 4 25% 38% 22% 4	444
	21st to end	Day	11% 7% 28% 33% 17% 3	488
August	1st to 10th	Day	11% 6% 26% 30% 19% 7%	446
	11th to 20th	Day	11% 5% 23% 34% 19% 8%	439
	21st to end	Day	11% 8% 25% 29% 16% 10%	495
September	1st to 10th	Day	20% 10% 21% 26% 16% 8%	450
	11th to 20th	Day	16% 5% 30% 24% 16% 9%	450
	21st to end	Day	22% 14% 24% 26% 11% 3	450
October	1st to 10th	Day	23% 9% 28% 23% 12% 6%	450
	11th to 20th	Day	18% 16% 30% 24% 11% 1	450
	21st to end	Day	20% 20% 34% 16% 8% 2	481
November	1st to 10th	Day	19% 16% 32% 20% 10% 3	365
	11th to 20th	Day	19% 24% 31% 20% 5%	370
	21st to end	Day	21% 24% 32% 20% 3	212
December	1st to 10th	Day	28% 26% 30% 13% 4	180
	11th to 20th	Day	34% 23% 29% 9% 3	180
	21st to end	Day	26% 26% 29% 14% 5% 1	198
January	1st to 10th	Night	20% 27% 36% 15% 2	760
	11th to 20th	Night	24% 22% 31% 20% 2	728
	21st to end	Night	17% 26% 38% 17% 1	792
February	1st to 10th	Night	22% 24% 35% 18% 1	720
	11th to 20th	Night	25% 30% 32% 12% 1	703
	21st to end	Night	25% 25% 35% 13% 2	592
March	1st to 10th	Night	21% 25% 34% 18% 3	720
	11th to 20th	Night	23% 23% 33% 16% 5% 1	720
	21st to end	Night	21% 18% 39% 17% 5%	792
April	1st to 10th	Night	21% 25% 34% 16% 4	720
	11th to 20th	Night	18% 16% 36% 24% 5%	711
	21st to end	Night	21% 19% 32% 22% 6%	720
May	1st to 10th	Night	22% 14% 34% 25% 6%	590
	11th to 20th	Night	21% 12% 33% 28% 6%	354
	21st to end	Night	17% 7% 22% 39% 13% 1	389
June	1st to 10th	Night	16% 6% 28% 39% 11%	360
	11th to 20th	Night	13% 5% 30% 41% 11%	360
	21st to end	Night	10% 2 30% 45% 13%	360
July	1st to 10th	Night	12% 4 25% 38% 21% 1	350
	11th to 20th	Night	7% 3 19% 45% 26% 1	348
	21st to end	Night	10% 4 23% 42% 19% 2	484
August	1st to 10th	Night	11% 2 21% 40% 23% 3	537
	11th to 20th	Night	16% 3 18% 35% 25% 2	659
	21st to end	Night	13% 5% 19% 38% 22% 2	792
September	1st to 10th	Night	17% 6% 24% 32% 18% 3	720
	11th to 20th	Night	14% 5% 21% 33% 23% 4	720
	21st to end	Night	19% 6% 26% 28% 19% 1	720
October	1st to 10th	Night	21% 10% 26% 25% 15% 4	720
	11th to 20th	Night	19% 12% 29% 29% 11%	720
	21st to end	Night	23% 15% 31% 22% 8%	792
November	1st to 10th	Night	14% 15% 34% 26% 10% 1	740
	11th to 20th	Night	20% 20% 29% 23% 7% 1	760
	21st to end	Night	18% 20% 32% 23% 7% 1	760
December	1st to 10th	Night	24% 21% 33% 16% 6% 1	760
	11th to 20th	Night	19% 20% 36% 20% 4	760
	21st to end	Night	21% 21% 34% 23% 1	836

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