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 2026-07-31.

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% 7% 3 6% 5% 4 4 4 5% 7% 38%	138162
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 39%	134869
Transparency	Day	30% 36% 7% 15% 10% 2	141666
	Night	30% 35% 7% 15% 11% 2	139441
Seeing	Day	26% 18% 29% 17% 7% 2	17153
	Night	24% 16% 29% 22% 8% 1	26177

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% 9% 3 8% 5% 5% 4 4 5% 5% 40%	4661
	11th to 20th	Day	14% 10% 3 9% 5% 4 3 3 4 5% 40%	4650
	21st to end	Day	15% 9% 3 6% 4 3 5% 3 5% 7% 40%	5395
February	1st to 10th	Day	16% 8% 3 7% 6% 5% 4 5% 5% 6% 35%	1909
	11th to 20th	Day	14% 8% 3 9% 5% 5% 4 3 5% 7% 37%	2048
	21st to end	Day	18% 10% 3 7% 4 5% 3 3 4 6% 37%	1730
March	1st to 10th	Day	22% 9% 3 6% 4 3 4 3 4 5% 38%	4576
	11th to 20th	Day	13% 7% 3 7% 5% 4 5% 4 5% 7% 41%	4812
	21st to end	Day	19% 8% 2 7% 5% 4 5% 4 4 6% 36%	6102
April	1st to 10th	Day	19% 9% 3 6% 5% 4 4 3 5% 6% 36%	2520
	11th to 20th	Day	16% 8% 2 6% 4 5% 4 3 5% 7% 39%	2494
	21st to end	Day	20% 7% 2 6% 4 4 4 3 5% 7% 39%	2599
May	1st to 10th	Day	27% 7% 2 5% 4 3 4 4 5% 7% 31%	7639
	11th to 20th	Day	18% 8% 3 6% 5% 4 4 3 5% 7% 38%	8160
	21st to end	Day	22% 7% 2 6% 4 4 5% 4 6% 8% 31%	9060
June	1st to 10th	Day	16% 6% 2 5% 5% 5% 6% 5% 7% 9% 36%	2940
	11th to 20th	Day	16% 7% 3 6% 4 5% 5% 4 7% 9% 35%	2940
	21st to end	Day	13% 7% 3 6% 6% 5% 4 5% 6% 9% 37%	2940
July	1st to 10th	Day	12% 7% 2 7% 6% 5% 5% 5% 7% 8% 35%	6500
	11th to 20th	Day	11% 6% 2 7% 5% 6% 6% 5% 7% 11% 34%	6500
	21st to end	Day	14% 6% 3 6% 4 4 4 4 5% 8% 41%	7066
August	1st to 10th	Day	19% 7% 3 7% 5% 5% 5% 4 6% 8% 32%	4420
	11th to 20th	Day	12% 7% 3 6% 4 4 5% 4 6% 8% 41%	3850
	21st to end	Day	10% 5% 2 6% 4 4 4 4 6% 9% 46%	4312
September	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 4 6% 6% 43%	2400
	11th to 20th	Day	12% 5% 2 5% 3 4 3 3 6% 6% 50%	2293
	21st to end	Day	17% 6% 2 6% 4 4 4 4 6% 7% 39%	2200
October	1st to 10th	Day	22% 6% 2 5% 4 3 3 2 5% 5% 42%	2117
	11th to 20th	Day	14% 7% 2 5% 4 4 5% 3 5% 7% 44%	2000
	21st to end	Day	22% 6% 2 5% 5% 4 4 3 5% 6% 38%	1995
November	1st to 10th	Day	12% 7% 3 5% 4 3 3 3 5% 7% 49%	1851
	11th to 20th	Day	17% 7% 2 5% 3 3 4 3 6% 6% 44%	1890
	21st to end	Day	18% 7% 3 6% 4 4 4 4 5% 6% 40%	1799
December	1st to 10th	Day	15% 8% 3 6% 5% 4 3 3 5% 6% 42%	2680
	11th to 20th	Day	20% 7% 2 5% 4 3 3 3 5% 6% 41%	3066
	21st to end	Day	18% 11% 3 8% 5% 4 3 3 4 7% 35%	4048
January	1st to 10th	Night	13% 8% 3 7% 5% 5% 5% 4 6% 7% 38%	8234
	11th to 20th	Night	14% 9% 3 9% 5% 4 4 4 4 6% 37%	8235
	21st to end	Night	15% 8% 3 7% 5% 5% 4 3 5% 6% 40%	8781
February	1st to 10th	Night	15% 10% 3 6% 4 5% 5% 3 5% 5% 39%	2709
	11th to 20th	Night	16% 8% 3 7% 5% 5% 5% 4 6% 6% 36%	2458
	21st to end	Night	17% 8% 3 7% 5% 3 4 3 6% 6% 38%	2076
March	1st to 10th	Night	20% 9% 3 5% 3 4 4 3 5% 6% 39%	5484
	11th to 20th	Night	15% 8% 3 7% 5% 4 4 3 5% 6% 41%	5364
	21st to end	Night	19% 11% 3 6% 4 4 4 3 5% 5% 37%	5072
April	1st to 10th	Night	18% 9% 3 7% 5% 5% 4 4 6% 6% 33%	2100
	11th to 20th	Night	18% 8% 2 7% 5% 5% 4 3 4 6% 38%	2075
	21st to end	Night	20% 8% 3 6% 4 4 3 3 5% 5% 40%	2019
May	1st to 10th	Night	27% 8% 3 6% 4 4 4 3 5% 6% 30%	5259
	11th to 20th	Night	18% 9% 3 8% 4 4 4 4 5% 7% 35%	4742
	21st to end	Night	22% 8% 3 7% 4 4 4 4 5% 6% 33%	5114
June	1st to 10th	Night	16% 6% 2 6% 4 5% 4 4 4 7% 42%	1680
	11th to 20th	Night	15% 6% 3 6% 4 4 4 4 6% 9% 39%	1680
	21st to end	Night	14% 5% 2 5% 4 4 4 4 6% 10% 42%	1680
July	1st to 10th	Night	13% 7% 3 6% 5% 4 5% 4 7% 7% 39%	3680
	11th to 20th	Night	14% 8% 3 7% 5% 5% 5% 4 6% 9% 33%	3680
	21st to end	Night	13% 7% 2 5% 4 4 4 3 5% 6% 47%	4006
August	1st to 10th	Night	21% 6% 3 7% 4 4 6% 4 5% 7% 31%	2695
	11th to 20th	Night	15% 7% 2 7% 5% 4 5% 4 5% 7% 41%	3197
	21st to end	Night	12% 5% 1 5% 3 3 3 3 5% 7% 53%	3586
September	1st to 10th	Night	16% 6% 2 5% 4 4 3 3 4 6% 47%	2000
	11th to 20th	Night	12% 6% 2 4 2 3 3 3 5% 6% 54%	2106
	21st to end	Night	17% 8% 3 5% 3 4 3 3 4 7% 43%	2200
October	1st to 10th	Night	19% 7% 2 7% 3 3 3 2 4 6% 43%	2280
	11th to 20th	Night	15% 6% 2 6% 4 3 4 4 5% 6% 46%	2400
	21st to end	Night	21% 7% 2 5% 4 4 4 3 5% 6% 39%	2843
November	1st to 10th	Night	13% 7% 2 5% 4 4 4 4 5% 6% 46%	2667
	11th to 20th	Night	17% 7% 2 5% 4 3 4 3 5% 5% 43%	2730
	21st to end	Night	14% 7% 2 6% 4 3 5% 4 6% 7% 42%	2819
December	1st to 10th	Night	15% 9% 2 6% 4 3 4 3 5% 6% 43%	4720
	11th to 20th	Night	23% 9% 2 5% 4 3 3 3 4 3 39%	5348
	21st to end	Night	19% 11% 3 7% 4 4 3 3 4 5% 36%	7150

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% 22% 6% 20% 12% 2	4662
	11th to 20th	Day	36% 21% 6% 18% 12% 6%	4680
	21st to end	Day	36% 23% 6% 19% 15% 1	5395
February	1st to 10th	Day	20% 60% 5% 9% 5% 1	1909
	11th to 20th	Day	21% 58% 3 9% 9% 1	2057
	21st to end	Day	19% 58% 5% 11% 5% 1	1730
March	1st to 10th	Day	38% 28% 7% 10% 9% 8%	4579
	11th to 20th	Day	28% 30% 12% 22% 6% 2	4812
	21st to end	Day	36% 29% 6% 9% 16% 4	6102
April	1st to 10th	Day	23% 59% 3 8% 6% 2	2520
	11th to 20th	Day	17% 60% 5% 9% 9%	2494
	21st to end	Day	20% 59% 3 9% 8% 1	2599
May	1st to 10th	Day	43% 25% 10% 13% 9%	7651
	11th to 20th	Day	38% 24% 10% 18% 10%	8164
	21st to end	Day	42% 24% 8% 16% 9%	9061
June	1st to 10th	Day	18% 60% 4 11% 7% 1	2940
	11th to 20th	Day	17% 60% 6% 10% 6% 1	2940
	21st to end	Day	17% 59% 5% 10% 8% 1	2940
July	1st to 10th	Day	34% 29% 8% 16% 12% 1	6430
	11th to 20th	Day	33% 29% 12% 18% 8%	6425
	21st to end	Day	32% 27% 7% 18% 15% 1	6868
August	1st to 10th	Day	35% 38% 9% 14% 4	4406
	11th to 20th	Day	24% 39% 10% 22% 5%	3886
	21st to end	Day	20% 37% 8% 25% 9% 1	4312
September	1st to 10th	Day	16% 59% 4 12% 9%	2400
	11th to 20th	Day	17% 57% 3 10% 12% 1	2293
	21st to end	Day	20% 58% 5% 10% 6% 1	2200
October	1st to 10th	Day	24% 36% 5% 12% 20% 3	3442
	11th to 20th	Day	17% 36% 4 17% 23% 4	3240
	21st to end	Day	16% 36% 11% 18% 16% 2	3195
November	1st to 10th	Day	20% 54% 6% 12% 8% 1	1851
	11th to 20th	Day	16% 54% 4 11% 13% 2	1890
	21st to end	Day	20% 54% 7% 14% 5% 1	1799
December	1st to 10th	Day	27% 35% 11% 20% 6%	2680
	11th to 20th	Day	32% 31% 9% 14% 12% 2	3066
	21st to end	Day	38% 27% 4 16% 12% 4	4048
January	1st to 10th	Night	36% 21% 9% 20% 12% 2	8234
	11th to 20th	Night	38% 21% 9% 14% 12% 5%	8280
	21st to end	Night	37% 23% 6% 15% 16% 3	8781
February	1st to 10th	Night	18% 59% 6% 11% 5% 1	2709
	11th to 20th	Night	20% 57% 4 8% 10% 2	2460
	21st to end	Night	18% 59% 4 10% 7% 2	2076
March	1st to 10th	Night	36% 29% 8% 11% 12% 5%	5494
	11th to 20th	Night	30% 30% 13% 15% 8% 3	5365
	21st to end	Night	38% 28% 4 9% 18% 3	5072
April	1st to 10th	Night	22% 60% 4 8% 6% 1	2100
	11th to 20th	Night	18% 61% 5% 8% 8%	2075
	21st to end	Night	20% 59% 3 8% 9% 1	2019
May	1st to 10th	Night	44% 25% 10% 11% 9%	5265
	11th to 20th	Night	40% 26% 10% 16% 7% 1	4743
	21st to end	Night	43% 24% 9% 17% 6% 1	5115
June	1st to 10th	Night	18% 59% 2 12% 9% 1	1680
	11th to 20th	Night	17% 60% 7% 10% 6%	1680
	21st to end	Night	16% 58% 5% 12% 8% 1	1680
July	1st to 10th	Night	32% 28% 11% 16% 12% 1	3640
	11th to 20th	Night	37% 28% 11% 15% 8% 1	3632
	21st to end	Night	28% 26% 8% 21% 16% 1	3889
August	1st to 10th	Night	39% 38% 7% 11% 5%	2685
	11th to 20th	Night	27% 38% 8% 22% 4	3227
	21st to end	Night	17% 36% 9% 29% 10%	3586
September	1st to 10th	Night	16% 58% 4 14% 8% 1	2000
	11th to 20th	Night	15% 57% 4 11% 13% 1	2106
	21st to end	Night	21% 58% 4 9% 8% 1	2200
October	1st to 10th	Night	25% 38% 7% 15% 15% 1	3722
	11th to 20th	Night	15% 36% 5% 18% 23% 3	3899
	21st to end	Night	18% 36% 10% 22% 13% 1	4593
November	1st to 10th	Night	19% 54% 5% 11% 10% 1	2667
	11th to 20th	Night	17% 54% 4 10% 12% 3	2730
	21st to end	Night	18% 54% 6% 14% 6% 1	2819
December	1st to 10th	Night	27% 35% 8% 24% 5%	4720
	11th to 20th	Night	35% 31% 9% 10% 13% 2	5348
	21st to end	Night	38% 26% 6% 12% 13% 4	7150

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% 11% 2	220
	11th to 20th	Day	40% 25% 25% 10% 1	220
	21st to end	Day	31% 32% 28% 8% 1 1	290
February	1st to 10th	Day	36% 30% 27% 6% 1	349
	11th to 20th	Day	33% 27% 31% 8% 1	536
	21st to end	Day	35% 24% 29% 9% 2	446
March	1st to 10th	Day	32% 23% 31% 12% 1	540
	11th to 20th	Day	32% 26% 29% 11% 2 1	540
	21st to end	Day	35% 25% 28% 8% 4	594
April	1st to 10th	Day	38% 23% 24% 9% 6% 1	540
	11th to 20th	Day	33% 24% 27% 10% 5% 1	532
	21st to end	Day	30% 20% 34% 11% 4 1	540
May	1st to 10th	Day	36% 22% 25% 11% 4 1	543
	11th to 20th	Day	24% 23% 29% 19% 4 1	567
	21st to end	Day	27% 13% 29% 21% 9% 2	617
June	1st to 10th	Day	21% 15% 31% 21% 11% 2	570
	11th to 20th	Day	21% 15% 32% 20% 9% 2	570
	21st to end	Day	18% 16% 31% 23% 10% 2	570
July	1st to 10th	Day	18% 10% 32% 26% 12% 2	565
	11th to 20th	Day	14% 9% 33% 30% 13% 2	564
	21st to end	Day	17% 10% 36% 24% 11% 2	616
August	1st to 10th	Day	15% 9% 31% 27% 12% 6%	526
	11th to 20th	Day	17% 7% 29% 28% 13% 6%	519
	21st to end	Day	13% 13% 31% 25% 12% 6%	583
September	1st to 10th	Day	21% 12% 26% 21% 15% 5%	530
	11th to 20th	Day	14% 10% 28% 27% 13% 8%	530
	21st to end	Day	22% 12% 31% 22% 8% 5%	530
October	1st to 10th	Day	30% 9% 26% 17% 13% 5%	530
	11th to 20th	Day	22% 22% 28% 19% 8% 2	530
	21st to end	Day	31% 16% 25% 20% 7% 1	470
November	1st to 10th	Day	25% 20% 26% 20% 8% 1	425
	11th to 20th	Day	30% 25% 25% 15% 4	430
	21st to end	Day	31% 26% 25% 14% 4	339
December	1st to 10th	Day	34% 25% 26% 10% 5%	220
	11th to 20th	Day	42% 21% 26% 10% 1	220
	21st to end	Day	34% 25% 26% 13% 3	242
January	1st to 10th	Night	25% 23% 36% 14% 3	840
	11th to 20th	Night	31% 22% 29% 16% 3	840
	21st to end	Night	24% 25% 33% 15% 3	924
February	1st to 10th	Night	26% 22% 31% 19% 2	840
	11th to 20th	Night	28% 24% 35% 12% 1	823
	21st to end	Night	30% 21% 32% 15% 3	692
March	1st to 10th	Night	27% 22% 34% 15% 3	840
	11th to 20th	Night	24% 27% 29% 17% 3	840
	21st to end	Night	27% 23% 31% 14% 3	924
April	1st to 10th	Night	29% 21% 30% 17% 3	840
	11th to 20th	Night	26% 20% 32% 17% 5%	831
	21st to end	Night	24% 22% 31% 18% 5%	759
May	1st to 10th	Night	29% 17% 30% 19% 5%	630
	11th to 20th	Night	22% 15% 35% 22% 4	454
	21st to end	Night	22% 9% 29% 31% 9% 1	455
June	1st to 10th	Night	19% 8% 31% 31% 11%	420
	11th to 20th	Night	21% 8% 29% 28% 14%	420
	21st to end	Night	17% 6% 26% 37% 13%	420
July	1st to 10th	Night	17% 5% 20% 41% 16% 1	410
	11th to 20th	Night	16% 4 23% 39% 16% 1	408
	21st to end	Night	14% 6% 26% 37% 17%	458
August	1st to 10th	Night	15% 4 22% 41% 16% 2	455
	11th to 20th	Night	15% 5% 22% 37% 19% 2	790
	21st to end	Night	18% 5% 22% 34% 19% 2	880
September	1st to 10th	Night	18% 8% 27% 31% 14% 3	800
	11th to 20th	Night	15% 8% 23% 31% 19% 4	800
	21st to end	Night	23% 8% 25% 27% 14% 3	800
October	1st to 10th	Night	24% 12% 24% 26% 12% 2	800
	11th to 20th	Night	21% 15% 31% 24% 9% 1	800
	21st to end	Night	27% 15% 29% 22% 6%	880
November	1st to 10th	Night	21% 16% 31% 22% 8% 1	820
	11th to 20th	Night	26% 19% 30% 18% 7% 1	840
	21st to end	Night	23% 21% 29% 21% 5%	840
December	1st to 10th	Night	29% 20% 28% 18% 4	840
	11th to 20th	Night	28% 22% 28% 19% 3	840
	21st to end	Night	31% 19% 32% 16% 2	924

Page generated at 2026-07-28 03:25:20. Version ac07

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