Colonial Dorchester State Historic Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Colonial Dorchester State Historic Site clear sky chart from 2005-11-06 to 2025-12-18.

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	12% 6% 2 7% 5% 5% 5% 4 6% 9% 39%	127132
	Night	11% 6% 2 6% 4 4 4 4 6% 8% 44%	123922
Transparency	Day	39% 8% 13% 18% 16% 6%	130765
	Night	36% 7% 13% 18% 18% 7%	128358
Seeing	Day	13% 9% 25% 25% 19% 8%	17043
	Night	13% 7% 23% 33% 21% 3	27128

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	13% 5% 1 4 3 3 3 3 5% 7% 52%	4056
	11th to 20th	Day	14% 6% 2 6% 3 3 3 3 4 7% 51%	4060
	21st to end	Day	15% 8% 3 6% 4 3 3 2 4 7% 45%	4466
February	1st to 10th	Day	14% 6% 2 5% 4 4 5% 4 6% 7% 42%	1800
	11th to 20th	Day	19% 7% 2 5% 4 3 3 3 4 7% 44%	1920
	21st to end	Day	14% 7% 2 6% 3 3 4 3 4 6% 46%	1791
March	1st to 10th	Day	22% 9% 3 6% 4 3 4 2 5% 5% 37%	4935
	11th to 20th	Day	8% 4 2 5% 4 4 4 3 5% 7% 54%	4980
	21st to end	Day	13% 6% 2 6% 3 4 4 3 4 5% 48%	5465
April	1st to 10th	Day	8% 4 2 5% 4 4 5% 3 6% 7% 51%	2214
	11th to 20th	Day	10% 4 2 7% 4 4 4 4 6% 7% 50%	2374
	21st to end	Day	7% 3 1 4 4 5% 5% 4 8% 8% 52%	2400
May	1st to 10th	Day	10% 6% 2 7% 5% 5% 6% 5% 7% 10% 37%	5831
	11th to 20th	Day	15% 7% 2 7% 5% 5% 5% 4 7% 10% 34%	5861
	21st to end	Day	8% 5% 2 7% 5% 5% 6% 5% 7% 10% 41%	6433
June	1st to 10th	Day	10% 6% 3 8% 7% 6% 6% 6% 8% 10% 29%	2600
	11th to 20th	Day	8% 5% 2 5% 6% 6% 6% 7% 8% 13% 34%	2600
	21st to end	Day	7% 5% 2 6% 7% 7% 7% 6% 10% 12% 31%	2600
July	1st to 10th	Day	11% 8% 3 10% 8% 7% 8% 6% 9% 11% 20%	5900
	11th to 20th	Day	5% 4 2 8% 7% 7% 9% 7% 11% 14% 25%	5900
	21st to end	Day	6% 6% 2 9% 7% 7% 8% 7% 10% 13% 25%	6424
August	1st to 10th	Day	22% 8% 3 8% 6% 7% 7% 6% 7% 9% 19%	4234
	11th to 20th	Day	8% 6% 3 8% 7% 7% 8% 6% 8% 11% 28%	4076
	21st to end	Day	10% 7% 3 8% 6% 6% 6% 5% 7% 10% 32%	4312
September	1st to 10th	Day	12% 6% 2 6% 6% 6% 6% 5% 7% 9% 35%	2270
	11th to 20th	Day	14% 5% 3 8% 6% 6% 6% 5% 8% 7% 32%	2200
	21st to end	Day	17% 7% 2 6% 5% 4 5% 4 5% 7% 37%	2200
October	1st to 10th	Day	14% 6% 2 6% 5% 4 5% 4 6% 7% 42%	2045
	11th to 20th	Day	9% 4 2 4 3 4 3 3 5% 6% 57%	2000
	21st to end	Day	10% 5% 2 5% 4 3 4 3 4 5% 55%	2200
November	1st to 10th	Day	11% 5% 2 5% 3 3 3 3 5% 6% 53%	2057
	11th to 20th	Day	12% 7% 2 6% 4 3 3 2 4 5% 52%	2100
	21st to end	Day	13% 5% 2 4 4 3 3 3 5% 7% 51%	1906
December	1st to 10th	Day	9% 5% 2 5% 5% 4 4 4 5% 6% 51%	3020
	11th to 20th	Day	14% 7% 2 5% 3 3 4 3 4 6% 48%	3436
	21st to end	Day	19% 8% 3 6% 4 3 3 2 4 6% 41%	4466
January	1st to 10th	Night	10% 5% 2 4 3 3 4 3 4 5% 56%	5889
	11th to 20th	Night	13% 7% 3 6% 4 4 4 4 5% 7% 43%	5900
	21st to end	Night	13% 7% 2 7% 4 4 4 3 5% 6% 45%	6490
February	1st to 10th	Night	14% 6% 3 5% 4 4 4 4 5% 6% 43%	2600
	11th to 20th	Night	17% 6% 3 6% 4 5% 4 3 4 5% 44%	2363
	21st to end	Night	12% 7% 3 6% 4 4 4 4 5% 6% 44%	1836
March	1st to 10th	Night	19% 10% 3 7% 4 3 3 3 4 5% 39%	4918
	11th to 20th	Night	10% 4 2 5% 3 3 3 3 5% 9% 52%	4980
	21st to end	Night	13% 5% 2 5% 3 3 4 3 5% 7% 50%	5467
April	1st to 10th	Night	11% 5% 1 5% 4 4 4 4 5% 8% 50%	2185
	11th to 20th	Night	12% 4 2 4 4 3 3 4 5% 8% 52%	1975
	21st to end	Night	7% 5% 2 5% 4 4 4 4 6% 8% 53%	2000
May	1st to 10th	Night	6% 5% 2 6% 4 5% 5% 4 7% 9% 46%	4092
	11th to 20th	Night	15% 6% 2 5% 4 4 4 4 7% 10% 39%	4032
	21st to end	Night	10% 5% 2 6% 5% 4 5% 4 7% 9% 44%	4421
June	1st to 10th	Night	8% 6% 2 8% 5% 5% 6% 5% 9% 11% 35%	1800
	11th to 20th	Night	7% 5% 2 6% 4 6% 7% 5% 9% 11% 38%	1800
	21st to end	Night	5% 5% 2 7% 5% 5% 7% 6% 9% 13% 37%	1800
July	1st to 10th	Night	10% 7% 3 8% 7% 6% 7% 5% 8% 11% 29%	4058
	11th to 20th	Night	5% 5% 3 7% 6% 5% 7% 6% 9% 14% 34%	4057
	21st to end	Night	7% 6% 3 7% 6% 7% 7% 6% 9% 12% 30%	4427
August	1st to 10th	Night	18% 8% 3 7% 5% 4 5% 5% 9% 12% 24%	2915
	11th to 20th	Night	8% 6% 2 8% 5% 5% 6% 5% 10% 11% 35%	2937
	21st to end	Night	6% 5% 2 7% 5% 5% 5% 4 6% 9% 45%	3586
September	1st to 10th	Night	9% 5% 2 7% 6% 6% 5% 5% 7% 10% 40%	2125
	11th to 20th	Night	10% 6% 3 8% 6% 5% 5% 4 6% 8% 38%	2200
	21st to end	Night	14% 7% 3 6% 5% 5% 4 4 5% 8% 39%	2200
October	1st to 10th	Night	13% 6% 2 6% 4 3 4 3 5% 6% 48%	2355
	11th to 20th	Night	11% 4 1 3 3 2 3 4 4 7% 58%	2400
	21st to end	Night	9% 5% 2 5% 3 3 4 3 5% 5% 56%	2640
November	1st to 10th	Night	11% 6% 1 5% 4 3 3 3 5% 5% 55%	2461
	11th to 20th	Night	13% 7% 2 4 3 3 3 3 4 6% 52%	2520
	21st to end	Night	10% 6% 2 6% 4 4 5% 4 6% 7% 47%	2709
December	1st to 10th	Night	10% 6% 3 5% 3 3 3 3 3 5% 56%	4380
	11th to 20th	Night	17% 8% 3 6% 4 3 3 3 4 4 44%	4914
	21st to end	Night	17% 7% 2 6% 3 4 4 4 4 6% 44%	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% 4 8% 16% 33% 8%	4057
	11th to 20th	Day	34% 3 6% 16% 30% 10%	4060
	21st to end	Day	40% 4 7% 18% 20% 11%	4466
February	1st to 10th	Day	40% 7% 9% 20% 19% 6%	1800
	11th to 20th	Day	42% 4 8% 15% 22% 9%	1930
	21st to end	Day	39% 4 9% 19% 19% 11%	1791
March	1st to 10th	Day	49% 4 6% 12% 18% 10%	4935
	11th to 20th	Day	28% 5% 10% 16% 23% 19%	4980
	21st to end	Day	38% 5% 9% 17% 21% 11%	5465
April	1st to 10th	Day	31% 6% 12% 21% 20% 10%	2214
	11th to 20th	Day	32% 5% 9% 23% 18% 12%	2374
	21st to end	Day	26% 7% 13% 28% 21% 5%	2400
May	1st to 10th	Day	37% 10% 16% 24% 10% 3	5866
	11th to 20th	Day	43% 8% 19% 19% 10% 1	5865
	21st to end	Day	35% 10% 15% 19% 20% 1	6447
June	1st to 10th	Day	45% 10% 17% 17% 11%	2600
	11th to 20th	Day	37% 12% 20% 23% 8% 1	2600
	21st to end	Day	38% 14% 22% 17% 8% 1	2600
July	1st to 10th	Day	51% 15% 18% 13% 3 1	5835
	11th to 20th	Day	41% 18% 30% 9% 2	5831
	21st to end	Day	43% 16% 19% 20% 3	6240
August	1st to 10th	Day	58% 11% 19% 10% 2	4221
	11th to 20th	Day	43% 14% 16% 20% 7%	4115
	21st to end	Day	44% 9% 14% 28% 4	4312
September	1st to 10th	Day	43% 10% 16% 15% 14% 3	2270
	11th to 20th	Day	46% 10% 12% 17% 11% 4	2200
	21st to end	Day	45% 7% 13% 18% 14% 4	2200
October	1st to 10th	Day	41% 5% 9% 13% 26% 6%	3331
	11th to 20th	Day	18% 3 6% 14% 32% 28%	3240
	21st to end	Day	25% 4 7% 17% 37% 11%	3538
November	1st to 10th	Day	31% 4 10% 17% 25% 13%	2057
	11th to 20th	Day	36% 3 8% 17% 24% 12%	2100
	21st to end	Day	34% 5% 10% 21% 23% 9%	1906
December	1st to 10th	Day	33% 5% 9% 17% 22% 14%	3020
	11th to 20th	Day	37% 5% 9% 21% 23% 5%	3433
	21st to end	Day	44% 3 7% 22% 21% 3	4466
January	1st to 10th	Night	29% 4 7% 14% 36% 9%	5889
	11th to 20th	Night	39% 5% 6% 16% 27% 7%	5900
	21st to end	Night	40% 4 7% 12% 25% 11%	6490
February	1st to 10th	Night	40% 5% 9% 19% 19% 8%	2600
	11th to 20th	Night	43% 5% 9% 14% 20% 10%	2364
	21st to end	Night	39% 6% 10% 17% 16% 12%	1836
March	1st to 10th	Night	47% 5% 8% 12% 19% 9%	4918
	11th to 20th	Night	30% 4 10% 13% 26% 17%	4980
	21st to end	Night	33% 5% 11% 19% 20% 11%	5467
April	1st to 10th	Night	32% 6% 12% 20% 20% 9%	2185
	11th to 20th	Night	30% 5% 10% 23% 20% 12%	1975
	21st to end	Night	28% 6% 14% 26% 21% 5%	2000
May	1st to 10th	Night	32% 9% 20% 27% 9% 2	4119
	11th to 20th	Night	38% 9% 21% 20% 11% 1	4033
	21st to end	Night	34% 9% 16% 24% 16% 1	4429
June	1st to 10th	Night	38% 12% 21% 20% 10%	1800
	11th to 20th	Night	34% 13% 23% 23% 7%	1800
	21st to end	Night	33% 13% 26% 22% 5% 1	1800
July	1st to 10th	Night	46% 12% 24% 16% 2 1	4015
	11th to 20th	Night	37% 16% 36% 9% 3	4006
	21st to end	Night	41% 15% 21% 20% 2	4297
August	1st to 10th	Night	49% 14% 23% 11% 2	2904
	11th to 20th	Night	38% 13% 22% 21% 6%	2964
	21st to end	Night	35% 9% 16% 35% 5%	3586
September	1st to 10th	Night	38% 10% 18% 19% 11% 3	2125
	11th to 20th	Night	41% 10% 15% 19% 13% 2	2200
	21st to end	Night	43% 7% 15% 20% 13% 3	2200
October	1st to 10th	Night	36% 5% 10% 18% 25% 6%	3849
	11th to 20th	Night	16% 3 7% 14% 26% 34%	3899
	21st to end	Night	25% 4 7% 19% 29% 16%	4252
November	1st to 10th	Night	32% 5% 8% 17% 25% 13%	2461
	11th to 20th	Night	35% 4 7% 17% 25% 12%	2520
	21st to end	Night	35% 5% 9% 17% 23% 10%	2709
December	1st to 10th	Night	32% 4 9% 14% 29% 12%	4380
	11th to 20th	Night	43% 5% 9% 14% 20% 7%	4916
	21st to end	Night	41% 5% 8% 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	18% 20% 43% 15% 3	300
	11th to 20th	Day	17% 17% 40% 18% 7% 1	300
	21st to end	Day	23% 25% 37% 13% 2	330
February	1st to 10th	Day	17% 19% 41% 18% 4	300
	11th to 20th	Day	29% 17% 40% 13% 2	300
	21st to end	Day	18% 16% 40% 22% 5%	390
March	1st to 10th	Day	20% 16% 31% 25% 7%	500
	11th to 20th	Day	15% 17% 41% 18% 8% 1	500
	21st to end	Day	14% 17% 40% 19% 9% 1	550
April	1st to 10th	Day	15% 18% 35% 20% 10% 2	500
	11th to 20th	Day	10% 15% 30% 22% 16% 6%	492
	21st to end	Day	6% 11% 31% 26% 17% 9%	500
May	1st to 10th	Day	9% 13% 32% 25% 15% 6%	503
	11th to 20th	Day	13% 5% 26% 28% 20% 8%	527
	21st to end	Day	11% 5% 20% 31% 20% 13%	573
June	1st to 10th	Day	10% 4 20% 31% 23% 11%	530
	11th to 20th	Day	8% 5% 17% 27% 27% 16%	530
	21st to end	Day	6% 3 14% 32% 29% 16%	530
July	1st to 10th	Day	7% 4 8% 26% 35% 20%	525
	11th to 20th	Day	7% 2 6% 22% 41% 21%	524
	21st to end	Day	5% 3 12% 30% 34% 16%	576
August	1st to 10th	Day	15% 4 11% 25% 28% 17%	526
	11th to 20th	Day	8% 2 8% 26% 34% 22%	519
	21st to end	Day	16% 3 11% 25% 26% 18%	583
September	1st to 10th	Day	11% 4 14% 29% 27% 15%	530
	11th to 20th	Day	15% 3 16% 31% 25% 10%	530
	21st to end	Day	17% 3 15% 32% 23% 10%	530
October	1st to 10th	Day	17% 6% 23% 29% 16% 8%	530
	11th to 20th	Day	11% 5% 22% 35% 20% 7%	530
	21st to end	Day	12% 8% 24% 33% 19% 4	583
November	1st to 10th	Day	15% 7% 28% 28% 17% 4	535
	11th to 20th	Day	17% 10% 36% 25% 11% 2	540
	21st to end	Day	14% 13% 37% 27% 10% 1	346
December	1st to 10th	Day	20% 17% 36% 20% 7%	330
	11th to 20th	Day	17% 17% 36% 18% 9% 1	321
	21st to end	Day	25% 15% 34% 19% 7% 1	330
January	1st to 10th	Night	14% 11% 36% 32% 6% 1	800
	11th to 20th	Night	14% 13% 31% 33% 9%	800
	21st to end	Night	18% 11% 39% 29% 4	880
February	1st to 10th	Night	17% 11% 34% 31% 8%	800
	11th to 20th	Night	21% 12% 34% 28% 5%	783
	21st to end	Night	16% 12% 37% 29% 6%	660
March	1st to 10th	Night	15% 12% 33% 33% 8%	800
	11th to 20th	Night	14% 11% 35% 33% 8%	800
	21st to end	Night	14% 9% 36% 32% 10%	880
April	1st to 10th	Night	13% 11% 35% 31% 9% 1	800
	11th to 20th	Night	13% 10% 30% 31% 16% 2	791
	21st to end	Night	11% 7% 27% 38% 17% 1	800
May	1st to 10th	Night	6% 5% 29% 40% 19% 1	633
	11th to 20th	Night	9% 6% 22% 35% 25% 3	591
	21st to end	Night	9% 2 14% 36% 34% 6%	650
June	1st to 10th	Night	10% 3 12% 43% 28% 4	600
	11th to 20th	Night	8% 2 10% 35% 36% 10%	600
	21st to end	Night	5% 2 10% 37% 39% 7%	600
July	1st to 10th	Night	8% 3 6% 32% 45% 7%	585
	11th to 20th	Night	9% 1 3 30% 46% 12%	582
	21st to end	Night	5% 2 6% 35% 44% 8%	657
August	1st to 10th	Night	10% 3 6% 29% 42% 10%	597
	11th to 20th	Night	7% 3 7% 28% 44% 11%	630
	21st to end	Night	11% 3 9% 30% 35% 13%	880
September	1st to 10th	Night	10% 2 10% 30% 42% 6%	800
	11th to 20th	Night	12% 3 10% 34% 35% 6%	800
	21st to end	Night	18% 2 12% 31% 31% 5%	800
October	1st to 10th	Night	14% 4 20% 32% 25% 5%	800
	11th to 20th	Night	12% 4 21% 35% 25% 3	800
	21st to end	Night	12% 5% 22% 34% 24% 3	880
November	1st to 10th	Night	13% 6% 24% 36% 19% 2	820
	11th to 20th	Night	16% 8% 26% 34% 15% 2	840
	21st to end	Night	12% 8% 28% 34% 17% 1	840
December	1st to 10th	Night	17% 8% 29% 35% 10% 1	840
	11th to 20th	Night	19% 8% 29% 28% 14% 1	829
	21st to end	Night	21% 10% 30% 29% 10%	880

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