Roxbury Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Roxbury Park clear sky chart from 2005-11-06 to 2025-05-11.

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	11% 6% 2 6% 4 4 5% 4 7% 8% 42%	114365
	Night	11% 6% 2 6% 4 4 4 4 6% 8% 45%	114604
Transparency	Day	36% 8% 14% 19% 16% 6%	118042
	Night	36% 7% 12% 18% 19% 8%	119066
Seeing	Day	13% 9% 26% 27% 19% 6%	16172
	Night	13% 8% 24% 31% 21% 4	26331

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	12% 5% 2 5% 3 4 4 3 5% 6% 53%	4056
	11th to 20th	Day	14% 6% 2 6% 3 3 3 3 4 7% 49%	4060
	21st to end	Day	14% 9% 3 6% 3 3 3 3 5% 6% 45%	4466
February	1st to 10th	Day	13% 8% 2 6% 5% 3 5% 4 5% 7% 42%	1800
	11th to 20th	Day	19% 7% 2 5% 3 3 3 4 5% 6% 44%	1944
	21st to end	Day	13% 6% 3 6% 4 3 4 3 4 6% 48%	1783
March	1st to 10th	Day	21% 8% 3 7% 4 3 3 3 5% 6% 37%	4935
	11th to 20th	Day	8% 4 1 4 3 4 4 3 6% 7% 56%	4980
	21st to end	Day	12% 5% 2 6% 5% 4 4 3 5% 5% 49%	5465
April	1st to 10th	Day	8% 4 2 5% 4 4 5% 4 6% 7% 52%	2200
	11th to 20th	Day	9% 4 2 5% 4 3 5% 3 6% 7% 52%	2373
	21st to end	Day	5% 4 1 4 3 5% 4 4 6% 10% 55%	2400
May	1st to 10th	Day	6% 4 2 5% 5% 5% 5% 5% 8% 11% 44%	4202
	11th to 20th	Day	14% 6% 3 7% 4 4 4 4 7% 8% 39%	4081
	21st to end	Day	6% 3 2 5% 4 4 5% 5% 7% 11% 49%	4476
June	1st to 10th	Day	8% 6% 3 7% 6% 6% 7% 5% 8% 10% 33%	2470
	11th to 20th	Day	7% 4 2 5% 5% 6% 6% 5% 9% 11% 38%	2470
	21st to end	Day	6% 5% 2 7% 6% 6% 7% 6% 10% 13% 32%	2470
July	1st to 10th	Day	12% 6% 3 7% 6% 6% 7% 6% 9% 11% 28%	4120
	11th to 20th	Day	5% 4 2 7% 6% 7% 7% 7% 10% 14% 32%	4120
	21st to end	Day	6% 5% 2 8% 6% 8% 8% 6% 10% 14% 26%	4466
August	1st to 10th	Day	20% 6% 3 8% 5% 6% 6% 6% 9% 10% 23%	4104
	11th to 20th	Day	6% 5% 2 7% 5% 6% 7% 6% 10% 12% 34%	3957
	21st to end	Day	9% 6% 3 8% 5% 5% 6% 5% 8% 10% 35%	4180
September	1st to 10th	Day	11% 6% 2 7% 6% 5% 7% 5% 8% 8% 34%	2128
	11th to 20th	Day	12% 7% 3 7% 6% 5% 6% 4 8% 9% 32%	2090
	21st to end	Day	12% 7% 2 7% 5% 4 5% 4 6% 8% 39%	2090
October	1st to 10th	Day	11% 5% 2 6% 4 5% 4 3 6% 6% 47%	1953
	11th to 20th	Day	8% 4 1 4 3 3 3 3 5% 6% 60%	1900
	21st to end	Day	9% 5% 2 5% 3 4 4 3 5% 5% 55%	2090
November	1st to 10th	Day	11% 5% 1 5% 4 4 4 3 5% 6% 52%	1957
	11th to 20th	Day	11% 7% 2 6% 4 3 4 3 5% 6% 50%	2000
	21st to end	Day	12% 6% 2 4 3 3 4 3 5% 6% 52%	1810
December	1st to 10th	Day	8% 4 2 5% 3 4 4 4 5% 7% 54%	2930
	11th to 20th	Day	14% 5% 2 6% 4 3 3 3 5% 6% 49%	3373
	21st to end	Day	19% 8% 3 6% 4 3 3 3 5% 6% 41%	4466
January	1st to 10th	Night	10% 5% 2 5% 3 4 4 3 4 5% 57%	5889
	11th to 20th	Night	13% 7% 2 6% 4 4 5% 4 5% 7% 43%	5900
	21st to end	Night	13% 8% 3 6% 4 4 3 3 5% 6% 45%	6490
February	1st to 10th	Night	14% 6% 2 6% 5% 4 4 4 5% 6% 44%	2600
	11th to 20th	Night	16% 8% 2 6% 5% 4 4 3 5% 6% 43%	2338
	21st to end	Night	11% 7% 4 7% 5% 4 4 3 5% 7% 44%	1845
March	1st to 10th	Night	17% 8% 3 8% 4 3 3 3 4 5% 41%	4918
	11th to 20th	Night	9% 5% 2 4 3 4 4 3 6% 8% 51%	4980
	21st to end	Night	12% 5% 2 5% 4 3 3 3 5% 7% 50%	5467
April	1st to 10th	Night	9% 5% 2 5% 4 4 4 3 5% 7% 51%	2200
	11th to 20th	Night	10% 4 2 5% 3 3 4 3 6% 7% 53%	1975
	21st to end	Night	6% 5% 2 4 4 3 4 4 6% 8% 53%	2000
May	1st to 10th	Night	6% 5% 2 5% 4 4 4 4 7% 10% 49%	2941
	11th to 20th	Night	11% 6% 2 6% 4 3 4 4 6% 9% 44%	2818
	21st to end	Night	6% 4 2 5% 4 4 5% 4 7% 8% 53%	3079
June	1st to 10th	Night	9% 5% 3 7% 5% 5% 5% 6% 9% 12% 34%	1710
	11th to 20th	Night	6% 5% 2 6% 6% 6% 5% 6% 10% 12% 38%	1710
	21st to end	Night	5% 5% 2 6% 6% 6% 7% 6% 9% 14% 34%	1710
July	1st to 10th	Night	10% 6% 3 8% 6% 5% 7% 6% 9% 12% 29%	2840
	11th to 20th	Night	6% 5% 3 8% 6% 6% 7% 7% 10% 13% 30%	2840
	21st to end	Night	8% 6% 3 8% 7% 7% 8% 6% 9% 13% 25%	3087
August	1st to 10th	Night	18% 7% 2 6% 4 5% 6% 6% 9% 13% 23%	2825
	11th to 20th	Night	7% 5% 3 6% 5% 5% 6% 6% 7% 13% 37%	2839
	21st to end	Night	7% 5% 2 7% 5% 5% 5% 4 6% 8% 45%	3476
September	1st to 10th	Night	9% 6% 3 7% 6% 5% 6% 5% 7% 9% 38%	2046
	11th to 20th	Night	10% 7% 3 6% 6% 5% 5% 5% 7% 8% 38%	2090
	21st to end	Night	13% 6% 3 7% 5% 5% 5% 3 6% 10% 38%	2090
October	1st to 10th	Night	12% 6% 3 6% 4 3 4 4 5% 6% 48%	2225
	11th to 20th	Night	9% 4 1 4 3 3 3 4 5% 7% 58%	2280
	21st to end	Night	8% 5% 2 6% 3 3 3 4 4 5% 57%	2508
November	1st to 10th	Night	9% 5% 2 5% 3 4 3 4 5% 7% 53%	2341
	11th to 20th	Night	12% 7% 2 6% 4 4 3 3 5% 5% 50%	2400
	21st to end	Night	11% 6% 3 6% 4 4 3 3 6% 7% 47%	2585
December	1st to 10th	Night	9% 5% 2 6% 3 3 3 2 4 6% 56%	4250
	11th to 20th	Night	14% 8% 3 8% 5% 4 3 3 4 5% 44%	4822
	21st to end	Night	16% 8% 2 6% 4 4 3 3 5% 6% 43%	6490

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	31% 6% 6% 17% 30% 10%	4057
	11th to 20th	Day	35% 5% 6% 17% 29% 9%	4060
	21st to end	Day	41% 4 6% 19% 20% 10%	4466
February	1st to 10th	Day	40% 6% 9% 20% 19% 6%	1800
	11th to 20th	Day	41% 5% 8% 16% 22% 8%	1954
	21st to end	Day	37% 5% 9% 18% 19% 11%	1783
March	1st to 10th	Day	47% 4 9% 13% 18% 9%	4935
	11th to 20th	Day	27% 6% 11% 16% 21% 19%	4980
	21st to end	Day	37% 6% 9% 20% 18% 10%	5465
April	1st to 10th	Day	29% 7% 13% 22% 19% 9%	2200
	11th to 20th	Day	30% 6% 10% 24% 19% 10%	2373
	21st to end	Day	24% 7% 15% 29% 20% 5%	2400
May	1st to 10th	Day	30% 10% 19% 27% 11% 3	4240
	11th to 20th	Day	39% 8% 15% 25% 12% 1	4085
	21st to end	Day	26% 10% 20% 25% 18% 1	4489
June	1st to 10th	Day	41% 13% 18% 16% 10%	2470
	11th to 20th	Day	34% 12% 21% 26% 7%	2470
	21st to end	Day	37% 15% 23% 17% 7% 1	2470
July	1st to 10th	Day	45% 12% 20% 19% 3 1	4055
	11th to 20th	Day	38% 18% 31% 10% 3	4051
	21st to end	Day	42% 17% 26% 14% 2	4324
August	1st to 10th	Day	52% 13% 22% 11% 2	4091
	11th to 20th	Day	37% 14% 20% 21% 8%	3996
	21st to end	Day	41% 10% 16% 30% 3	4180
September	1st to 10th	Day	42% 13% 16% 15% 11% 3	2128
	11th to 20th	Day	44% 12% 14% 17% 11% 3	2090
	21st to end	Day	42% 8% 14% 19% 14% 4	2090
October	1st to 10th	Day	41% 5% 9% 13% 25% 7%	3239
	11th to 20th	Day	16% 4 7% 13% 31% 29%	3140
	21st to end	Day	23% 5% 7% 19% 37% 10%	3428
November	1st to 10th	Day	32% 6% 10% 16% 23% 12%	1957
	11th to 20th	Day	35% 5% 8% 17% 23% 12%	2000
	21st to end	Day	34% 4 9% 22% 22% 9%	1810
December	1st to 10th	Day	30% 6% 10% 18% 21% 15%	2930
	11th to 20th	Day	36% 4 10% 22% 23% 5%	3370
	21st to end	Day	44% 4 6% 22% 20% 3	4466
January	1st to 10th	Night	29% 4 6% 14% 36% 10%	5889
	11th to 20th	Night	40% 4 6% 17% 27% 7%	5900
	21st to end	Night	40% 4 8% 12% 24% 11%	6490
February	1st to 10th	Night	40% 5% 8% 20% 19% 8%	2600
	11th to 20th	Night	43% 5% 9% 14% 20% 10%	2339
	21st to end	Night	40% 5% 9% 18% 15% 12%	1845
March	1st to 10th	Night	46% 4 8% 13% 18% 10%	4918
	11th to 20th	Night	29% 5% 10% 14% 25% 16%	4980
	21st to end	Night	33% 5% 11% 19% 20% 11%	5467
April	1st to 10th	Night	32% 6% 13% 21% 19% 9%	2200
	11th to 20th	Night	30% 6% 9% 24% 20% 11%	1975
	21st to end	Night	27% 7% 14% 26% 22% 5%	2000
May	1st to 10th	Night	27% 9% 20% 29% 12% 3	2968
	11th to 20th	Night	35% 11% 13% 26% 14% 1	2819
	21st to end	Night	26% 9% 16% 30% 18% 1	3087
June	1st to 10th	Night	38% 13% 21% 19% 8%	1710
	11th to 20th	Night	35% 13% 21% 25% 7%	1710
	21st to end	Night	35% 15% 25% 20% 5% 1	1710
July	1st to 10th	Night	44% 13% 20% 20% 3 1	2795
	11th to 20th	Night	39% 17% 30% 11% 3	2786
	21st to end	Night	46% 14% 24% 14% 2	2988
August	1st to 10th	Night	48% 15% 24% 11% 2	2814
	11th to 20th	Night	35% 13% 24% 21% 6%	2866
	21st to end	Night	35% 10% 18% 33% 4	3476
September	1st to 10th	Night	39% 12% 18% 18% 10% 2	2046
	11th to 20th	Night	41% 10% 16% 19% 12% 2	2090
	21st to end	Night	41% 8% 14% 19% 14% 2	2090
October	1st to 10th	Night	36% 5% 10% 16% 26% 7%	3719
	11th to 20th	Night	16% 3 6% 14% 27% 34%	3779
	21st to end	Night	24% 4 6% 18% 32% 16%	4120
November	1st to 10th	Night	31% 5% 10% 17% 24% 13%	2341
	11th to 20th	Night	36% 4 7% 17% 24% 12%	2400
	21st to end	Night	36% 4 9% 18% 23% 10%	2585
December	1st to 10th	Night	31% 4 9% 16% 28% 12%	4250
	11th to 20th	Night	43% 5% 9% 15% 20% 7%	4824
	21st to end	Night	42% 5% 7% 20% 23% 3	6490

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	13% 19% 49% 15% 3	300
	11th to 20th	Day	13% 18% 42% 19% 7% 1	300
	21st to end	Day	21% 25% 40% 12% 2	330
February	1st to 10th	Day	16% 20% 40% 19% 6%	300
	11th to 20th	Day	28% 17% 38% 15% 2	300
	21st to end	Day	15% 20% 42% 18% 5%	382
March	1st to 10th	Day	16% 19% 35% 23% 8%	500
	11th to 20th	Day	14% 17% 42% 20% 6% 1	500
	21st to end	Day	14% 18% 38% 21% 7% 1	550
April	1st to 10th	Day	14% 17% 35% 24% 8% 1	500
	11th to 20th	Day	10% 14% 31% 27% 14% 4	492
	21st to end	Day	6% 9% 32% 32% 15% 6%	500
May	1st to 10th	Day	6% 15% 34% 28% 12% 5%	503
	11th to 20th	Day	10% 6% 27% 32% 19% 5%	487
	21st to end	Day	10% 5% 19% 34% 24% 8%	529
June	1st to 10th	Day	11% 4 21% 34% 23% 7%	490
	11th to 20th	Day	8% 3 20% 30% 27% 13%	490
	21st to end	Day	7% 2 17% 32% 31% 11%	490
July	1st to 10th	Day	7% 2 9% 31% 35% 16%	485
	11th to 20th	Day	8% 8% 31% 36% 17%	484
	21st to end	Day	5% 2 11% 38% 33% 10%	532
August	1st to 10th	Day	13% 2 11% 28% 33% 13%	486
	11th to 20th	Day	6% 4 8% 28% 38% 16%	479
	21st to end	Day	15% 3 11% 26% 31% 15%	539
September	1st to 10th	Day	14% 3 11% 30% 32% 10%	490
	11th to 20th	Day	16% 3 16% 33% 24% 8%	490
	21st to end	Day	15% 2 16% 32% 25% 10%	490
October	1st to 10th	Day	16% 6% 23% 30% 16% 9%	490
	11th to 20th	Day	9% 6% 26% 32% 20% 7%	490
	21st to end	Day	12% 6% 24% 34% 19% 5%	539
November	1st to 10th	Day	15% 6% 31% 28% 18% 3	495
	11th to 20th	Day	16% 10% 38% 24% 10% 2	500
	21st to end	Day	11% 16% 31% 31% 9% 2	310
December	1st to 10th	Day	17% 14% 37% 24% 7% 1	300
	11th to 20th	Day	20% 14% 34% 21% 10% 1	300
	21st to end	Day	25% 13% 32% 21% 8% 1	330
January	1st to 10th	Night	14% 16% 34% 29% 7%	800
	11th to 20th	Night	14% 17% 30% 29% 10%	800
	21st to end	Night	18% 15% 38% 25% 4	880
February	1st to 10th	Night	17% 12% 33% 31% 7%	800
	11th to 20th	Night	21% 15% 33% 27% 4	783
	21st to end	Night	15% 13% 35% 30% 6%	660
March	1st to 10th	Night	14% 11% 35% 33% 7%	800
	11th to 20th	Night	15% 12% 33% 32% 9%	800
	21st to end	Night	11% 15% 35% 29% 10% 1	880
April	1st to 10th	Night	11% 13% 34% 31% 11% 1	800
	11th to 20th	Night	11% 11% 30% 30% 16% 2	791
	21st to end	Night	9% 7% 28% 37% 18% 1	800
May	1st to 10th	Night	6% 7% 26% 37% 21% 2	625
	11th to 20th	Night	10% 5% 25% 33% 23% 5%	563
	21st to end	Night	8% 3 16% 32% 34% 7%	617
June	1st to 10th	Night	11% 3 11% 43% 26% 6%	570
	11th to 20th	Night	7% 2 11% 32% 35% 12%	570
	21st to end	Night	7% 1 12% 37% 34% 9%	570
July	1st to 10th	Night	7% 2 10% 35% 36% 11%	555
	11th to 20th	Night	9% 1 7% 26% 45% 12%	552
	21st to end	Night	7% 1 7% 39% 38% 8%	624
August	1st to 10th	Night	10% 2 7% 31% 40% 10%	567
	11th to 20th	Night	6% 3 7% 31% 41% 12%	592
	21st to end	Night	12% 3 9% 26% 36% 14%	836
September	1st to 10th	Night	10% 3 11% 28% 39% 8%	760
	11th to 20th	Night	13% 3 12% 31% 34% 6%	760
	21st to end	Night	16% 3 13% 29% 32% 7%	760
October	1st to 10th	Night	14% 3 20% 32% 24% 6%	760
	11th to 20th	Night	11% 5% 23% 32% 25% 4	760
	21st to end	Night	10% 6% 24% 32% 23% 3	836
November	1st to 10th	Night	13% 6% 24% 34% 20% 3	780
	11th to 20th	Night	15% 10% 28% 30% 15% 1	800
	21st to end	Night	14% 10% 29% 29% 17% 2	800
December	1st to 10th	Night	15% 12% 29% 31% 11% 1	800
	11th to 20th	Night	19% 10% 30% 26% 15% 1	800
	21st to end	Night	21% 10% 31% 27% 10%	880

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