Boyertown Planetarium
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Boyertown Planetarium clear sky chart from 2005-11-06 to 2025-07-21.

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	17% 6% 2 6% 4 4 4 4 5% 7% 40%	120175
	Night	17% 7% 2 6% 4 4 4 3 5% 7% 42%	118929
Transparency	Day	42% 6% 11% 23% 16% 3	123762
	Night	41% 5% 11% 23% 17% 3	123502
Seeing	Day	23% 15% 28% 20% 11% 3	15825
	Night	20% 13% 29% 27% 10% 1	25103

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% 2 5% 4 4 4 3 5% 6% 48%	3597
	11th to 20th	Day	14% 9% 4 10% 6% 4 4 4 4 5% 37%	3690
	21st to end	Day	17% 9% 3 8% 4 4 4 3 4 6% 39%	4466
February	1st to 10th	Day	16% 8% 3 7% 5% 4 4 3 6% 7% 37%	1800
	11th to 20th	Day	16% 9% 3 7% 5% 4 4 3 5% 8% 37%	1875
	21st to end	Day	19% 11% 2 6% 4 4 3 3 4 5% 38%	1650
March	1st to 10th	Day	21% 5% 2 4 4 4 4 3 4 6% 44%	4537
	11th to 20th	Day	13% 6% 2 6% 4 5% 4 4 6% 6% 44%	4980
	21st to end	Day	19% 6% 2 5% 4 4 4 3 5% 7% 43%	5758
April	1st to 10th	Day	18% 7% 2 7% 5% 4 4 4 5% 7% 38%	2400
	11th to 20th	Day	16% 7% 2 6% 5% 3 4 3 6% 7% 41%	2374
	21st to end	Day	16% 5% 2 6% 4 4 4 5% 6% 7% 40%	2400
May	1st to 10th	Day	27% 6% 2 5% 4 4 5% 4 6% 7% 29%	5874
	11th to 20th	Day	25% 8% 3 7% 4 3 4 3 4 7% 32%	6318
	21st to end	Day	20% 6% 2 5% 4 4 4 4 6% 9% 37%	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% 38%	2800
	21st to end	Day	12% 5% 2 6% 4 5% 5% 5% 6% 10% 41%	2800
July	1st to 10th	Day	8% 4 2 6% 5% 6% 6% 6% 8% 11% 38%	4580
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 5% 7% 10% 40%	4580
	21st to end	Day	12% 8% 3 8% 5% 5% 5% 5% 6% 8% 34%	4704
August	1st to 10th	Day	19% 5% 3 6% 5% 4 5% 4 6% 8% 34%	4104
	11th to 20th	Day	13% 5% 2 6% 4 4 5% 4 6% 9% 42%	3769
	21st to end	Day	12% 5% 2 5% 4 3 4 3 6% 9% 48%	4180
September	1st to 10th	Day	15% 7% 2 5% 3 4 4 3 5% 6% 45%	2280
	11th to 20th	Day	13% 5% 2 4 3 3 4 3 5% 7% 51%	2266
	21st to end	Day	20% 6% 3 5% 5% 4 4 3 5% 6% 38%	2065
October	1st to 10th	Day	18% 6% 2 5% 4 3 3 4 5% 7% 42%	1900
	11th to 20th	Day	16% 5% 2 5% 4 4 4 3 4 6% 47%	1900
	21st to end	Day	19% 6% 2 5% 3 3 3 3 4 5% 44%	2037
November	1st to 10th	Day	14% 6% 2 4 3 2 4 3 4 6% 52%	1761
	11th to 20th	Day	17% 5% 2 5% 3 4 4 3 6% 7% 47%	1800
	21st to end	Day	15% 7% 2 4 4 4 3 4 6% 8% 43%	1635
December	1st to 10th	Day	15% 9% 2 5% 4 4 5% 4 5% 7% 41%	2600
	11th to 20th	Day	22% 9% 2 4 3 3 3 3 5% 5% 41%	2986
	21st to end	Day	18% 6% 2 6% 5% 5% 4 4 6% 8% 37%	3960
January	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 5% 6% 48%	6349
	11th to 20th	Night	14% 7% 2 5% 4 3 4 3 5% 7% 45%	6177
	21st to end	Night	18% 8% 2 6% 4 3 3 3 5% 6% 42%	6490
February	1st to 10th	Night	17% 9% 2 6% 4 4 4 3 5% 6% 40%	2600
	11th to 20th	Night	18% 8% 3 6% 4 4 4 4 4 5% 40%	2409
	21st to end	Night	18% 7% 2 6% 4 3 4 4 5% 6% 41%	1980
March	1st to 10th	Night	19% 8% 2 5% 4 4 3 3 4 5% 43%	5300
	11th to 20th	Night	12% 7% 2 6% 4 4 3 3 5% 6% 50%	4979
	21st to end	Night	16% 8% 3 6% 3 4 3 3 5% 6% 43%	5174
April	1st to 10th	Night	18% 7% 2 6% 5% 4 4 4 6% 6% 38%	2000
	11th to 20th	Night	15% 7% 2 7% 4 4 4 4 5% 8% 40%	1975
	21st to end	Night	19% 5% 2 5% 3 3 3 3 5% 7% 45%	2000
May	1st to 10th	Night	25% 9% 2 6% 4 4 4 4 5% 7% 29%	4109
	11th to 20th	Night	23% 8% 4 7% 4 4 4 4 4 6% 32%	3575
	21st to end	Night	22% 6% 2 6% 4 4 4 3 5% 8% 37%	3926
June	1st to 10th	Night	15% 6% 2 5% 4 4 4 4 5% 8% 41%	1600
	11th to 20th	Night	13% 6% 2 6% 4 5% 4 5% 7% 9% 40%	1600
	21st to end	Night	12% 4 2 5% 5% 5% 4 4 7% 9% 43%	1600
July	1st to 10th	Night	8% 6% 3 6% 6% 5% 7% 5% 7% 9% 38%	2600
	11th to 20th	Night	10% 6% 3 8% 4 5% 5% 5% 7% 11% 36%	2600
	21st to end	Night	15% 7% 2 6% 5% 4 4 4 7% 8% 37%	2832
August	1st to 10th	Night	20% 6% 3 5% 4 4 5% 4 6% 9% 33%	2825
	11th to 20th	Night	15% 7% 3 6% 5% 4 4 3 5% 7% 41%	3025
	21st to end	Night	12% 4 2 5% 4 4 4 4 5% 8% 48%	3476
September	1st to 10th	Night	15% 6% 2 5% 5% 3 3 3 4 7% 47%	1900
	11th to 20th	Night	13% 5% 2 5% 2 4 3 2 5% 7% 52%	1914
	21st to end	Night	19% 6% 2 5% 4 4 4 4 5% 6% 41%	2112
October	1st to 10th	Night	19% 8% 2 5% 3 4 4 3 4 5% 44%	2280
	11th to 20th	Night	16% 5% 1 5% 3 4 3 3 4 7% 50%	2280
	21st to end	Night	18% 7% 1 4 3 3 3 3 4 6% 48%	2560
November	1st to 10th	Night	12% 5% 2 4 3 3 4 4 5% 6% 53%	2537
	11th to 20th	Night	16% 8% 2 5% 3 3 4 3 4 5% 47%	2600
	21st to end	Night	15% 5% 2 5% 5% 4 3 3 6% 8% 44%	2761
December	1st to 10th	Night	13% 8% 3 6% 4 3 4 3 5% 6% 45%	4580
	11th to 20th	Night	23% 6% 2 5% 3 3 3 3 4 6% 40%	5208
	21st to end	Night	20% 7% 3 6% 4 4 3 4 4 6% 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	38% 3 9% 26% 21% 2	3598
	11th to 20th	Day	48% 4 8% 20% 15% 5%	3720
	21st to end	Day	47% 5% 11% 22% 15% 1	4466
February	1st to 10th	Day	46% 6% 13% 22% 12% 2	1800
	11th to 20th	Day	47% 4 9% 24% 14% 2	1884
	21st to end	Day	48% 4 9% 23% 12% 3	1650
March	1st to 10th	Day	43% 3 8% 24% 15% 7%	4540
	11th to 20th	Day	40% 5% 10% 28% 13% 5%	4980
	21st to end	Day	43% 4 10% 17% 23% 4	5758
April	1st to 10th	Day	44% 6% 8% 22% 15% 5%	2400
	11th to 20th	Day	42% 5% 11% 21% 17% 4	2374
	21st to end	Day	41% 6% 10% 22% 17% 4	2400
May	1st to 10th	Day	52% 6% 12% 18% 10% 2	5883
	11th to 20th	Day	52% 6% 8% 22% 11% 1	6322
	21st to end	Day	44% 6% 10% 24% 14% 2	6949
June	1st to 10th	Day	39% 7% 12% 24% 15% 1	2800
	11th to 20th	Day	42% 8% 14% 24% 10% 3	2800
	21st to end	Day	37% 8% 14% 25% 15% 1	2800
July	1st to 10th	Day	38% 9% 15% 22% 14% 2	4510
	11th to 20th	Day	37% 8% 16% 24% 13% 1	4505
	21st to end	Day	47% 7% 9% 19% 17% 2	4554
August	1st to 10th	Day	47% 7% 13% 23% 10%	4091
	11th to 20th	Day	38% 6% 11% 30% 14% 1	3806
	21st to end	Day	34% 6% 11% 29% 20% 2	4180
September	1st to 10th	Day	40% 6% 11% 20% 20% 3	2280
	11th to 20th	Day	32% 5% 10% 25% 24% 4	2266
	21st to end	Day	47% 5% 8% 18% 20% 2	2065
October	1st to 10th	Day	38% 4 8% 19% 26% 5%	3160
	11th to 20th	Day	26% 4 12% 27% 25% 6%	3140
	21st to end	Day	26% 3 13% 25% 29% 4	3339
November	1st to 10th	Day	34% 4 13% 25% 20% 4	1761
	11th to 20th	Day	38% 6% 9% 26% 20% 2	1800
	21st to end	Day	39% 6% 13% 29% 11% 2	1635
December	1st to 10th	Day	43% 5% 15% 24% 11% 2	2600
	11th to 20th	Day	45% 3 14% 28% 9% 1	2986
	21st to end	Day	45% 6% 12% 21% 15% 1	3960
January	1st to 10th	Night	37% 4 9% 26% 20% 4	6349
	11th to 20th	Night	39% 4 12% 22% 16% 6%	6222
	21st to end	Night	44% 3 10% 23% 17% 3	6490
February	1st to 10th	Night	45% 5% 12% 26% 11% 2	2600
	11th to 20th	Night	46% 4 9% 20% 18% 3	2411
	21st to end	Night	44% 5% 8% 22% 17% 4	1980
March	1st to 10th	Night	43% 4 8% 23% 12% 9%	5310
	11th to 20th	Night	35% 5% 13% 27% 14% 5%	4980
	21st to end	Night	43% 4 7% 17% 24% 5%	5174
April	1st to 10th	Night	45% 5% 10% 20% 16% 4	2000
	11th to 20th	Night	42% 5% 10% 20% 20% 2	1975
	21st to end	Night	39% 5% 10% 23% 18% 5%	2000
May	1st to 10th	Night	54% 6% 11% 16% 11% 2	4116
	11th to 20th	Night	52% 7% 10% 17% 12% 1	3576
	21st to end	Night	45% 7% 10% 22% 13% 2	3927
June	1st to 10th	Night	40% 6% 11% 22% 20% 2	1600
	11th to 20th	Night	38% 9% 14% 24% 14% 1	1600
	21st to end	Night	35% 8% 14% 28% 14% 2	1600
July	1st to 10th	Night	41% 9% 16% 17% 14% 3	2560
	11th to 20th	Night	40% 8% 16% 23% 12% 2	2552
	21st to end	Night	44% 8% 12% 19% 17% 1	2741
August	1st to 10th	Night	46% 8% 18% 18% 9% 1	2814
	11th to 20th	Night	43% 5% 12% 24% 16% 1	3054
	21st to end	Night	33% 6% 10% 29% 20% 1	3476
September	1st to 10th	Night	39% 6% 9% 21% 22% 3	1900
	11th to 20th	Night	31% 6% 10% 23% 27% 4	1914
	21st to end	Night	43% 6% 7% 21% 21% 2	2112
October	1st to 10th	Night	43% 4 9% 21% 21% 2	3800
	11th to 20th	Night	26% 4 9% 29% 28% 4	3779
	21st to end	Night	27% 3 11% 32% 24% 3	4208
November	1st to 10th	Night	32% 6% 12% 26% 20% 5%	2537
	11th to 20th	Night	40% 4 9% 24% 20% 4	2600
	21st to end	Night	38% 4 13% 27% 15% 2	2761
December	1st to 10th	Night	41% 4 12% 29% 12% 2	4580
	11th to 20th	Night	46% 4 10% 25% 12% 3	5208
	21st to end	Night	47% 5% 10% 20% 17% 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	36% 24% 24% 13% 4 1	200
	11th to 20th	Day	40% 25% 27% 7% 1	227
	21st to end	Day	28% 29% 27% 12% 3 1	330
February	1st to 10th	Day	30% 25% 32% 9% 3 1	300
	11th to 20th	Day	34% 25% 30% 9% 1	423
	21st to end	Day	35% 19% 29% 14% 3	414
March	1st to 10th	Day	33% 18% 29% 16% 4	500
	11th to 20th	Day	29% 24% 29% 13% 3 1	500
	21st to end	Day	30% 23% 31% 10% 5% 1	550
April	1st to 10th	Day	29% 23% 29% 13% 4 1	500
	11th to 20th	Day	26% 24% 27% 15% 5% 3	492
	21st to end	Day	28% 17% 33% 15% 5% 2	500
May	1st to 10th	Day	33% 18% 26% 15% 5% 2	503
	11th to 20th	Day	25% 14% 31% 18% 9% 2	527
	21st to end	Day	21% 11% 27% 23% 15% 3	573
June	1st to 10th	Day	17% 13% 31% 22% 13% 4	530
	11th to 20th	Day	18% 11% 29% 24% 14% 4	530
	21st to end	Day	14% 10% 30% 27% 15% 4	530
July	1st to 10th	Day	12% 5% 23% 32% 22% 7%	525
	11th to 20th	Day	11% 3 24% 33% 24% 5%	524
	21st to end	Day	13% 6% 30% 29% 19% 4	532
August	1st to 10th	Day	12% 7% 24% 29% 21% 7%	486
	11th to 20th	Day	13% 6% 20% 34% 19% 8%	479
	21st to end	Day	13% 7% 23% 28% 17% 11%	539
September	1st to 10th	Day	19% 9% 21% 26% 16% 9%	490
	11th to 20th	Day	15% 5% 26% 26% 17% 11%	490
	21st to end	Day	24% 13% 23% 23% 12% 4	490
October	1st to 10th	Day	26% 8% 25% 21% 14% 6%	490
	11th to 20th	Day	20% 18% 25% 24% 12% 2	490
	21st to end	Day	22% 16% 33% 16% 11% 3	511
November	1st to 10th	Day	21% 13% 30% 22% 11% 3	395
	11th to 20th	Day	24% 20% 31% 20% 5% 1	400
	21st to end	Day	29% 17% 31% 17% 4 1	235
December	1st to 10th	Day	35% 21% 29% 10% 5% 1	200
	11th to 20th	Day	38% 23% 24% 12% 3	200
	21st to end	Day	29% 19% 31% 13% 8% 1	220
January	1st to 10th	Night	21% 23% 36% 17% 3	800
	11th to 20th	Night	27% 17% 30% 23% 3	800
	21st to end	Night	20% 20% 36% 21% 2	880
February	1st to 10th	Night	23% 18% 35% 23% 1	800
	11th to 20th	Night	28% 24% 33% 14% 1	783
	21st to end	Night	27% 18% 36% 16% 3	660
March	1st to 10th	Night	25% 21% 29% 22% 4	800
	11th to 20th	Night	23% 22% 31% 18% 5%	800
	21st to end	Night	22% 17% 37% 19% 5%	880
April	1st to 10th	Night	24% 21% 34% 18% 4	800
	11th to 20th	Night	20% 15% 36% 22% 6%	791
	21st to end	Night	23% 14% 30% 27% 6%	800
May	1st to 10th	Night	27% 11% 30% 26% 7%	630
	11th to 20th	Night	21% 7% 31% 31% 9%	394
	21st to end	Night	18% 8% 22% 38% 13% 1	433
June	1st to 10th	Night	16% 5% 26% 37% 15%	400
	11th to 20th	Night	14% 5% 28% 41% 13% 1	400
	21st to end	Night	12% 3 24% 45% 15%	400
July	1st to 10th	Night	12% 3 18% 44% 21% 2	390
	11th to 20th	Night	9% 3 21% 40% 26% 2	388
	21st to end	Night	12% 3 18% 41% 23% 2	473
August	1st to 10th	Night	12% 3 15% 42% 27% 2	567
	11th to 20th	Night	14% 4 17% 35% 27% 3	702
	21st to end	Night	15% 3 17% 39% 24% 3	836
September	1st to 10th	Night	19% 4 23% 32% 19% 3	760
	11th to 20th	Night	14% 4 18% 36% 23% 4	760
	21st to end	Night	22% 6% 23% 30% 19% 1	760
October	1st to 10th	Night	23% 7% 23% 26% 17% 3	760
	11th to 20th	Night	18% 10% 29% 29% 13%	760
	21st to end	Night	23% 12% 28% 27% 10%	836
November	1st to 10th	Night	16% 11% 32% 28% 13% 1	780
	11th to 20th	Night	22% 16% 27% 27% 7% 1	800
	21st to end	Night	19% 18% 32% 24% 7%	800
December	1st to 10th	Night	26% 16% 31% 21% 5% 1	800
	11th to 20th	Night	23% 17% 34% 22% 6%	800
	21st to end	Night	24% 17% 31% 26% 3	880

Page generated at 2025-07-18 01:10:04. Version ac07

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