Thomas J Boyce Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Thomas J Boyce Park clear sky chart from 2005-11-06 to 2025-03-27.

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% 8% 3 6% 5% 4 4 4 5% 7% 38%	108833
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 40%	110491
Transparency	Day	26% 44% 6% 13% 9% 1	112383
	Night	26% 42% 7% 13% 10% 2	115092
Seeing	Day	26% 18% 29% 18% 8% 2	15362
	Night	24% 16% 29% 22% 8% 1	24582

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	11% 8% 3 8% 5% 5% 4 4 5% 5% 42%	3597
	11th to 20th	Day	14% 10% 3 9% 6% 4 4 3 5% 5% 38%	3570
	21st to end	Day	17% 10% 4 7% 4 4 5% 3 5% 4 38%	4162
February	1st to 10th	Day	17% 8% 3 7% 6% 5% 4 5% 5% 6% 34%	1818
	11th to 20th	Day	14% 8% 3 8% 5% 5% 4 3 5% 7% 38%	1948
	21st to end	Day	18% 10% 3 7% 4 5% 3 3 4 6% 37%	1650
March	1st to 10th	Day	25% 10% 3 7% 5% 3 3 3 4 4 34%	3216
	11th to 20th	Day	11% 7% 2 7% 6% 5% 5% 4 6% 7% 40%	3397
	21st to end	Day	21% 8% 2 7% 5% 4 4 3 4 6% 37%	4238
April	1st to 10th	Day	18% 10% 3 6% 5% 4 5% 3 4 6% 35%	2280
	11th to 20th	Day	17% 8% 2 6% 5% 5% 4 3 5% 7% 39%	2254
	21st to end	Day	20% 7% 2 5% 4 4 4 3 5% 7% 39%	2351
May	1st to 10th	Day	27% 8% 3 6% 4 3 4 3 5% 7% 31%	4106
	11th to 20th	Day	21% 8% 3 7% 5% 4 5% 3 5% 7% 32%	4364
	21st to end	Day	13% 5% 2 5% 4 4 4 4 6% 9% 43%	4837
June	1st to 10th	Day	17% 6% 2 5% 5% 5% 6% 5% 7% 9% 35%	2660
	11th to 20th	Day	15% 7% 3 6% 4 5% 5% 4 7% 8% 36%	2660
	21st to end	Day	13% 7% 3 7% 6% 5% 4 4 6% 9% 37%	2660
July	1st to 10th	Day	11% 6% 3 8% 6% 5% 6% 5% 7% 8% 36%	4440
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 5% 7% 10% 38%	4440
	21st to end	Day	14% 8% 3 8% 5% 5% 5% 4 6% 7% 34%	4814
August	1st to 10th	Day	19% 7% 3 7% 5% 5% 5% 4 6% 8% 31%	4285
	11th to 20th	Day	12% 7% 3 6% 4 4 5% 4 6% 9% 41%	3730
	21st to end	Day	10% 5% 2 6% 4 4 4 4 6% 9% 45%	4180
September	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 4 6% 6% 43%	2280
	11th to 20th	Day	12% 6% 2 5% 3 4 4 3 6% 6% 49%	2179
	21st to end	Day	18% 6% 2 6% 4 4 4 3 6% 7% 40%	2090
October	1st to 10th	Day	23% 6% 2 5% 4 3 4 2 5% 5% 41%	2011
	11th to 20th	Day	14% 7% 2 5% 4 4 4 3 5% 7% 45%	1900
	21st to end	Day	22% 6% 2 5% 5% 4 4 3 5% 6% 37%	1895
November	1st to 10th	Day	13% 7% 3 5% 4 3 3 3 4 7% 49%	1761
	11th to 20th	Day	17% 7% 2 5% 4 2 4 3 6% 6% 45%	1800
	21st to end	Day	18% 6% 2 6% 3 4 4 3 5% 6% 41%	1714
December	1st to 10th	Day	15% 9% 3 6% 5% 4 3 3 5% 6% 42%	2600
	11th to 20th	Day	21% 7% 2 5% 4 3 3 3 5% 6% 41%	2986
	21st to end	Day	18% 11% 3 8% 5% 4 3 3 4 7% 35%	3960
January	1st to 10th	Night	13% 6% 2 6% 5% 6% 5% 4 6% 7% 40%	6349
	11th to 20th	Night	13% 8% 3 9% 5% 4 4 4 5% 6% 39%	6315
	21st to end	Night	17% 9% 3 7% 5% 4 4 3 4 6% 37%	6789
February	1st to 10th	Night	16% 10% 3 6% 4 5% 5% 3 5% 5% 38%	2580
	11th to 20th	Night	15% 8% 3 7% 5% 5% 5% 4 6% 6% 37%	2338
	21st to end	Night	17% 8% 3 7% 5% 3 4 3 6% 6% 38%	1980
March	1st to 10th	Night	23% 11% 3 6% 3 4 4 3 4 5% 33%	3844
	11th to 20th	Night	11% 7% 2 7% 6% 5% 4 4 6% 7% 41%	3779
	21st to end	Night	19% 9% 3 6% 4 3 4 3 6% 5% 39%	3533
April	1st to 10th	Night	17% 9% 3 7% 5% 5% 4 4 6% 6% 33%	1900
	11th to 20th	Night	18% 7% 2 7% 5% 5% 4 3 4 6% 38%	1875
	21st to end	Night	20% 8% 3 6% 4 4 3 3 5% 5% 39%	1827
May	1st to 10th	Night	26% 8% 3 6% 5% 5% 4 3 4 7% 29%	2834
	11th to 20th	Night	21% 10% 3 8% 4 4 3 3 5% 6% 33%	2544
	21st to end	Night	15% 7% 2 6% 4 3 4 4 5% 7% 42%	2738
June	1st to 10th	Night	16% 6% 2 6% 4 5% 3 5% 5% 7% 41%	1520
	11th to 20th	Night	14% 7% 2 6% 4 4 4 4 6% 8% 41%	1520
	21st to end	Night	13% 5% 2 5% 4 4 4 4 6% 10% 42%	1520
July	1st to 10th	Night	11% 6% 2 6% 5% 5% 6% 5% 8% 7% 39%	2520
	11th to 20th	Night	14% 6% 2 6% 5% 5% 5% 4 6% 9% 37%	2520
	21st to end	Night	16% 9% 3 5% 5% 4 5% 4 5% 6% 38%	2739
August	1st to 10th	Night	22% 6% 3 7% 4 4 6% 4 5% 7% 31%	2611
	11th to 20th	Night	15% 7% 2 7% 5% 4 5% 3 5% 7% 41%	3097
	21st to end	Night	12% 5% 1 5% 3 3 3 3 5% 7% 53%	3476
September	1st to 10th	Night	16% 7% 2 5% 4 4 3 3 4 6% 47%	1900
	11th to 20th	Night	13% 6% 2 4 2 3 3 3 4 6% 54%	2000
	21st to end	Night	17% 7% 3 5% 3 4 3 3 5% 7% 43%	2090
October	1st to 10th	Night	20% 7% 2 7% 3 3 3 3 5% 6% 42%	2166
	11th to 20th	Night	14% 6% 2 6% 4 4 4 4 5% 6% 46%	2280
	21st to end	Night	21% 7% 2 5% 4 4 4 3 6% 6% 38%	2701
November	1st to 10th	Night	13% 7% 2 5% 4 3 4 4 5% 6% 47%	2537
	11th to 20th	Night	18% 7% 2 5% 4 3 4 3 5% 5% 43%	2600
	21st to end	Night	14% 7% 2 6% 4 3 5% 4 6% 7% 42%	2685
December	1st to 10th	Night	15% 9% 2 6% 4 3 4 3 5% 6% 43%	4580
	11th to 20th	Night	23% 9% 2 5% 4 4 3 3 4 3 38%	5208
	21st to end	Night	19% 11% 3 7% 4 4 3 3 4 5% 36%	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	34% 27% 4 22% 11% 3	3598
	11th to 20th	Day	35% 27% 6% 17% 9% 6%	3600
	21st to end	Day	39% 29% 4 15% 13% 1	4162
February	1st to 10th	Day	19% 63% 6% 8% 4	1818
	11th to 20th	Day	19% 61% 2 8% 8% 1	1957
	21st to end	Day	17% 61% 5% 10% 5% 1	1650
March	1st to 10th	Day	39% 39% 6% 8% 6% 2	3219
	11th to 20th	Day	21% 41% 11% 20% 5% 1	3397
	21st to end	Day	31% 39% 5% 8% 14% 3	4238
April	1st to 10th	Day	20% 65% 3 7% 4 1	2280
	11th to 20th	Day	15% 66% 3 8% 8%	2254
	21st to end	Day	18% 65% 2 8% 6% 1	2351
May	1st to 10th	Day	35% 42% 9% 8% 6%	4120
	11th to 20th	Day	34% 42% 9% 12% 3	4368
	21st to end	Day	22% 41% 7% 18% 12%	4837
June	1st to 10th	Day	16% 66% 4 9% 5% 1	2660
	11th to 20th	Day	14% 66% 6% 9% 5% 1	2660
	21st to end	Day	15% 65% 4 9% 7%	2660
July	1st to 10th	Day	29% 39% 6% 14% 11% 1	4370
	11th to 20th	Day	27% 38% 10% 16% 8% 1	4365
	21st to end	Day	35% 38% 7% 10% 9% 1	4661
August	1st to 10th	Day	35% 39% 9% 13% 4	4271
	11th to 20th	Day	24% 40% 10% 22% 5%	3766
	21st to end	Day	20% 38% 8% 25% 9% 1	4180
September	1st to 10th	Day	15% 62% 4 11% 8%	2280
	11th to 20th	Day	16% 60% 2 9% 11% 1	2179
	21st to end	Day	18% 61% 5% 9% 6% 1	2090
October	1st to 10th	Day	25% 38% 4 11% 19% 3	3336
	11th to 20th	Day	16% 37% 3 16% 24% 4	3140
	21st to end	Day	16% 37% 11% 18% 16% 2	3095
November	1st to 10th	Day	18% 56% 6% 11% 7% 1	1761
	11th to 20th	Day	15% 56% 4 10% 13% 2	1800
	21st to end	Day	17% 57% 7% 13% 4 1	1714
December	1st to 10th	Day	27% 36% 12% 20% 6%	2600
	11th to 20th	Day	32% 31% 9% 14% 12% 2	2986
	21st to end	Day	38% 27% 3 15% 12% 4	3960
January	1st to 10th	Night	31% 27% 7% 21% 13% 2	6349
	11th to 20th	Night	32% 27% 11% 14% 11% 4	6360
	21st to end	Night	38% 29% 7% 11% 12% 3	6789
February	1st to 10th	Night	18% 62% 6% 10% 4 1	2580
	11th to 20th	Night	18% 60% 4 6% 10% 2	2340
	21st to end	Night	17% 62% 4 8% 7% 2	1980
March	1st to 10th	Night	38% 40% 5% 9% 7% 2	3854
	11th to 20th	Night	21% 42% 12% 14% 10% 2	3780
	21st to end	Night	31% 39% 4 7% 16% 3	3533
April	1st to 10th	Night	18% 66% 3 8% 5% 1	1900
	11th to 20th	Night	14% 67% 4 7% 7%	1875
	21st to end	Night	17% 65% 3 7% 7% 1	1827
May	1st to 10th	Night	35% 43% 8% 10% 4	2840
	11th to 20th	Night	35% 44% 9% 10% 3	2545
	21st to end	Night	25% 41% 9% 16% 7% 1	2739
June	1st to 10th	Night	15% 65% 2 9% 7% 1	1520
	11th to 20th	Night	13% 66% 7% 9% 5%	1520
	21st to end	Night	14% 64% 4 11% 7%	1520
July	1st to 10th	Night	26% 39% 10% 12% 12% 1	2480
	11th to 20th	Night	28% 38% 9% 16% 8%	2472
	21st to end	Night	33% 37% 8% 9% 12% 1	2651
August	1st to 10th	Night	39% 40% 7% 10% 4	2601
	11th to 20th	Night	27% 39% 8% 22% 4	3127
	21st to end	Night	16% 37% 9% 28% 9%	3476
September	1st to 10th	Night	15% 61% 3 13% 7% 1	1900
	11th to 20th	Night	14% 60% 3 10% 12% 1	2000
	21st to end	Night	20% 60% 3 9% 7% 1	2090
October	1st to 10th	Night	25% 39% 7% 14% 14% 1	3608
	11th to 20th	Night	14% 37% 5% 18% 23% 4	3779
	21st to end	Night	18% 37% 10% 22% 12% 1	4451
November	1st to 10th	Night	18% 56% 5% 10% 10% 1	2537
	11th to 20th	Night	16% 56% 4 9% 11% 3	2600
	21st to end	Night	17% 57% 6% 14% 6% 1	2685
December	1st to 10th	Night	27% 36% 8% 24% 5%	4580
	11th to 20th	Night	35% 31% 9% 10% 12% 2	5208
	21st to end	Night	38% 27% 6% 11% 13% 4	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% 27% 24% 12% 2	200
	11th to 20th	Day	39% 26% 25% 11% 1	200
	21st to end	Day	30% 30% 29% 8% 2 1	263
February	1st to 10th	Day	38% 30% 25% 6% 1	318
	11th to 20th	Day	34% 28% 28% 9% 1	496
	21st to end	Day	36% 23% 29% 9% 2	414
March	1st to 10th	Day	31% 24% 32% 11% 2	500
	11th to 20th	Day	31% 27% 30% 11% 2 1	500
	21st to end	Day	34% 23% 29% 8% 5%	530
April	1st to 10th	Day	35% 24% 24% 9% 6% 1	460
	11th to 20th	Day	32% 25% 25% 11% 5% 1	452
	21st to end	Day	29% 19% 36% 12% 4 1	460
May	1st to 10th	Day	35% 21% 24% 13% 5% 2	463
	11th to 20th	Day	24% 21% 30% 20% 4 1	487
	21st to end	Day	23% 12% 30% 24% 10% 2	529
June	1st to 10th	Day	21% 14% 32% 20% 11% 1	490
	11th to 20th	Day	18% 15% 32% 22% 10% 2	490
	21st to end	Day	17% 18% 30% 24% 10% 2	490
July	1st to 10th	Day	18% 10% 32% 25% 13% 2	485
	11th to 20th	Day	14% 7% 33% 30% 13% 3	484
	21st to end	Day	15% 11% 36% 25% 11% 2	532
August	1st to 10th	Day	16% 9% 31% 27% 12% 5%	486
	11th to 20th	Day	17% 8% 30% 27% 12% 6%	479
	21st to end	Day	14% 12% 30% 25% 13% 6%	539
September	1st to 10th	Day	21% 12% 25% 21% 15% 6%	490
	11th to 20th	Day	15% 10% 28% 25% 12% 9%	490
	21st to end	Day	22% 13% 30% 21% 9% 5%	490
October	1st to 10th	Day	31% 9% 27% 17% 12% 4	490
	11th to 20th	Day	20% 21% 29% 20% 8% 1	490
	21st to end	Day	31% 15% 25% 21% 7% 1	436
November	1st to 10th	Day	25% 18% 26% 21% 9% 1	395
	11th to 20th	Day	31% 24% 26% 15% 4 1	400
	21st to end	Day	29% 26% 26% 14% 4	314
December	1st to 10th	Day	33% 26% 26% 11% 5%	200
	11th to 20th	Day	42% 21% 26% 10% 1	200
	21st to end	Day	31% 25% 27% 13% 3	220
January	1st to 10th	Night	25% 23% 36% 13% 3	800
	11th to 20th	Night	30% 22% 29% 17% 3	800
	21st to end	Night	24% 24% 34% 16% 3	880
February	1st to 10th	Night	27% 22% 32% 19% 2	800
	11th to 20th	Night	28% 25% 35% 11% 1	783
	21st to end	Night	31% 21% 31% 15% 3	660
March	1st to 10th	Night	27% 22% 34% 15% 3	800
	11th to 20th	Night	23% 27% 30% 17% 3	800
	21st to end	Night	27% 23% 31% 16% 4	863
April	1st to 10th	Night	28% 21% 30% 19% 3	760
	11th to 20th	Night	25% 21% 31% 18% 5%	751
	21st to end	Night	24% 21% 31% 18% 5%	687
May	1st to 10th	Night	29% 17% 29% 20% 5% 1	570
	11th to 20th	Night	22% 16% 36% 21% 4	410
	21st to end	Night	20% 9% 28% 33% 9% 1	411
June	1st to 10th	Night	20% 8% 31% 30% 10% 1	380
	11th to 20th	Night	20% 8% 29% 29% 15%	380
	21st to end	Night	17% 6% 27% 37% 13%	380
July	1st to 10th	Night	16% 6% 20% 40% 17% 1	370
	11th to 20th	Night	16% 4 22% 40% 17% 1	368
	21st to end	Night	14% 6% 27% 37% 17%	416
August	1st to 10th	Night	15% 4 23% 41% 15% 2	431
	11th to 20th	Night	15% 5% 22% 37% 19% 2	750
	21st to end	Night	18% 5% 21% 34% 20% 2	836
September	1st to 10th	Night	18% 8% 27% 31% 14% 3	760
	11th to 20th	Night	16% 8% 24% 30% 19% 3	760
	21st to end	Night	23% 8% 26% 27% 14% 3	760
October	1st to 10th	Night	25% 12% 24% 26% 11% 2	760
	11th to 20th	Night	19% 15% 32% 24% 9% 1	760
	21st to end	Night	28% 15% 28% 22% 7%	836
November	1st to 10th	Night	20% 16% 32% 22% 8% 1	780
	11th to 20th	Night	26% 19% 30% 18% 7% 1	800
	21st to end	Night	23% 21% 29% 21% 6% 1	800
December	1st to 10th	Night	29% 20% 28% 18% 5% 1	800
	11th to 20th	Night	28% 22% 28% 19% 3	800
	21st to end	Night	30% 19% 33% 16% 2	880

Page generated at 2025-03-24 09:04:28. Version ac07

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