Weymouth Pines Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Weymouth Pines Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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	15% 11% 4 9% 5% 5% 5% 4 5% 7% 31%	127710
	Night	15% 11% 4 9% 5% 4 5% 4 5% 6% 31%	123921
Transparency	Day	50% 6% 13% 19% 10% 1	131293
	Night	50% 6% 13% 19% 11% 1	128396
Seeing	Day	18% 12% 29% 23% 13% 5%	16533
	Night	19% 10% 29% 29% 11% 1	25783

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% 18% 6% 22% 6% 6% 5% 3 4 4 14%	3689
	11th to 20th	Day	11% 22% 9% 22% 8% 6% 4 3 4 3 8%	4114
	21st to end	Day	20% 18% 7% 13% 6% 4 4 3 4 4 16%	4466
February	1st to 10th	Day	17% 16% 4 13% 9% 5% 4 4 5% 5% 18%	1800
	11th to 20th	Day	15% 13% 7% 15% 7% 6% 4 3 5% 5% 20%	1761
	21st to end	Day	16% 16% 5% 12% 6% 5% 4 4 5% 5% 21%	1650
March	1st to 10th	Day	17% 12% 7% 13% 7% 3 4 3 4 5% 26%	4798
	11th to 20th	Day	12% 12% 5% 9% 5% 4 4 4 5% 8% 31%	4980
	21st to end	Day	19% 13% 5% 10% 5% 4 4 3 4 5% 29%	5465
April	1st to 10th	Day	17% 13% 4 9% 5% 5% 4 3 5% 7% 29%	2265
	11th to 20th	Day	18% 10% 3 6% 5% 5% 3 4 5% 8% 32%	2562
	21st to end	Day	19% 9% 4 8% 5% 4 4 2 5% 6% 34%	2600
May	1st to 10th	Day	19% 10% 3 8% 5% 4 5% 3 5% 7% 32%	5862
	11th to 20th	Day	17% 9% 3 7% 4 4 4 3 6% 9% 34%	5861
	21st to end	Day	17% 8% 2 6% 4 4 4 4 5% 6% 38%	6889
June	1st to 10th	Day	12% 6% 2 7% 5% 5% 6% 4 6% 8% 38%	2800
	11th to 20th	Day	11% 6% 3 7% 5% 4 6% 5% 6% 9% 39%	2989
	21st to end	Day	11% 7% 2 5% 4 4 6% 4 7% 9% 40%	3000
July	1st to 10th	Day	9% 5% 2 7% 5% 5% 5% 5% 7% 10% 41%	6782
	11th to 20th	Day	9% 5% 2 7% 5% 5% 5% 4 7% 10% 43%	6039
	21st to end	Day	8% 5% 2 5% 4 5% 5% 4 6% 10% 45%	6424
August	1st to 10th	Day	14% 5% 2 6% 5% 5% 5% 4 7% 9% 38%	4234
	11th to 20th	Day	10% 7% 3 8% 5% 6% 5% 5% 7% 11% 34%	4174
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 5% 6% 9% 41%	4619
September	1st to 10th	Day	10% 6% 2 7% 6% 5% 5% 5% 7% 7% 41%	2332
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 6% 8% 48%	2200
	21st to end	Day	17% 8% 3 6% 4 5% 5% 4 7% 7% 36%	2200
October	1st to 10th	Day	16% 7% 2 7% 5% 3 5% 4 6% 9% 37%	2049
	11th to 20th	Day	20% 8% 3 9% 4 4 5% 4 6% 7% 30%	2000
	21st to end	Day	23% 11% 3 10% 6% 5% 4 4 5% 6% 24%	2155
November	1st to 10th	Day	15% 11% 3 8% 6% 4 4 4 6% 8% 31%	1852
	11th to 20th	Day	16% 14% 4 8% 4 4 5% 5% 6% 7% 26%	1738
	21st to end	Day	19% 17% 6% 10% 6% 5% 5% 4 5% 5% 17%	1680
December	1st to 10th	Day	18% 18% 6% 11% 6% 5% 4 4 6% 6% 15%	2680
	11th to 20th	Day	21% 20% 6% 10% 6% 5% 4 4 4 5% 16%	3070
	21st to end	Day	26% 21% 7% 13% 5% 4 4 3 4 4 9%	3931
January	1st to 10th	Night	14% 15% 6% 20% 7% 5% 5% 3 5% 4 16%	6447
	11th to 20th	Night	12% 17% 8% 18% 9% 6% 7% 4 4 4 11%	5979
	21st to end	Night	19% 16% 7% 13% 5% 4 4 3 5% 5% 19%	6490
February	1st to 10th	Night	17% 14% 5% 11% 7% 5% 4 4 5% 6% 20%	2600
	11th to 20th	Night	16% 15% 5% 9% 6% 6% 5% 4 5% 7% 23%	2523
	21st to end	Night	16% 13% 5% 11% 7% 6% 5% 4 5% 5% 24%	1980
March	1st to 10th	Night	15% 11% 4 9% 6% 5% 5% 3 6% 6% 32%	5054
	11th to 20th	Night	10% 9% 4 9% 6% 4 4 3 6% 9% 36%	4980
	21st to end	Night	18% 11% 4 9% 5% 4 4 3 4 4 35%	5467
April	1st to 10th	Night	18% 11% 4 6% 5% 4 4 4 6% 6% 32%	2133
	11th to 20th	Night	18% 9% 3 6% 5% 4 4 3 6% 7% 35%	1802
	21st to end	Night	15% 9% 2 7% 5% 5% 5% 4 5% 7% 35%	1800
May	1st to 10th	Night	20% 9% 3 8% 5% 3 4 3 5% 6% 34%	4033
	11th to 20th	Night	14% 8% 3 8% 5% 5% 5% 4 5% 7% 36%	4032
	21st to end	Night	18% 7% 2 4 3 3 4 3 5% 7% 43%	3948
June	1st to 10th	Night	13% 5% 2 6% 3 4 5% 4 6% 9% 43%	1600
	11th to 20th	Night	9% 5% 2 6% 4 4 5% 5% 8% 9% 43%	1406
	21st to end	Night	10% 5% 2 5% 5% 4 5% 4 6% 7% 47%	1400
July	1st to 10th	Night	7% 5% 2 6% 4 5% 4 4 6% 10% 46%	3192
	11th to 20th	Night	9% 6% 2 5% 4 4 4 3 5% 9% 49%	3920
	21st to end	Night	7% 5% 2 6% 3 4 4 4 6% 8% 51%	4427
August	1st to 10th	Night	11% 6% 2 6% 4 4 4 3 6% 8% 45%	2915
	11th to 20th	Night	10% 8% 2 6% 3 3 4 3 5% 7% 48%	2875
	21st to end	Night	8% 5% 2 6% 4 3 5% 4 6% 9% 48%	3244
September	1st to 10th	Night	8% 7% 3 7% 4 4 4 4 6% 7% 48%	2065
	11th to 20th	Night	9% 4 2 5% 4 4 4 3 5% 7% 54%	2200
	21st to end	Night	16% 7% 3 7% 4 5% 4 3 4 6% 41%	2200
October	1st to 10th	Night	13% 7% 3 6% 4 4 4 3 5% 7% 44%	2350
	11th to 20th	Night	19% 9% 3 7% 4 3 4 3 4 7% 36%	2400
	21st to end	Night	21% 10% 4 8% 5% 4 5% 4 5% 6% 29%	2680
November	1st to 10th	Night	15% 9% 3 7% 5% 4 4 4 6% 6% 38%	2666
	11th to 20th	Night	16% 13% 4 8% 6% 5% 5% 4 6% 8% 26%	2880
	21st to end	Night	21% 15% 5% 9% 5% 5% 5% 4 5% 6% 21%	2940
December	1st to 10th	Night	14% 17% 7% 13% 6% 6% 6% 5% 5% 6% 14%	4720
	11th to 20th	Night	15% 22% 7% 11% 5% 5% 4 3 3 5% 20%	5345
	21st to end	Night	23% 20% 6% 12% 6% 4 5% 3 4 4 13%	7228

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	72% 7% 9% 9% 3	3690
	11th to 20th	Day	78% 8% 6% 4 3	4114
	21st to end	Day	71% 5% 11% 7% 6% 1	4466
February	1st to 10th	Day	67% 6% 11% 11% 4 1	1800
	11th to 20th	Day	65% 6% 10% 13% 6%	1770
	21st to end	Day	63% 8% 11% 12% 5% 1	1650
March	1st to 10th	Day	61% 5% 14% 16% 5%	4798
	11th to 20th	Day	50% 6% 17% 19% 7% 1	4980
	21st to end	Day	57% 5% 11% 17% 8% 1	5465
April	1st to 10th	Day	54% 6% 11% 19% 10% 1	2265
	11th to 20th	Day	49% 6% 13% 18% 12% 1	2562
	21st to end	Day	49% 7% 13% 17% 12% 2	2600
May	1st to 10th	Day	49% 6% 13% 18% 10% 5%	5897
	11th to 20th	Day	47% 8% 14% 20% 11% 1	5865
	21st to end	Day	45% 6% 11% 18% 17% 4	6904
June	1st to 10th	Day	40% 8% 14% 24% 12% 2	2800
	11th to 20th	Day	38% 9% 14% 21% 16% 2	2989
	21st to end	Day	38% 9% 13% 25% 15% 1	3000
July	1st to 10th	Day	38% 4 13% 26% 16% 2	6708
	11th to 20th	Day	36% 4 14% 30% 15% 1	5966
	21st to end	Day	34% 5% 15% 25% 19% 1	6240
August	1st to 10th	Day	41% 6% 16% 23% 14% 1	4221
	11th to 20th	Day	43% 6% 16% 27% 8% 1	4213
	21st to end	Day	38% 5% 15% 31% 11% 1	4619
September	1st to 10th	Day	38% 8% 16% 23% 13% 1	2332
	11th to 20th	Day	33% 6% 15% 31% 15%	2200
	21st to end	Day	45% 7% 14% 23% 11% 1	2200
October	1st to 10th	Day	36% 6% 14% 26% 18% 1	3335
	11th to 20th	Day	42% 5% 13% 23% 17%	3240
	21st to end	Day	50% 5% 16% 22% 7%	3457
November	1st to 10th	Day	49% 7% 14% 21% 8% 2	1852
	11th to 20th	Day	53% 8% 16% 13% 9% 1	1738
	21st to end	Day	66% 7% 13% 12% 2	1680
December	1st to 10th	Day	67% 6% 14% 10% 2	2680
	11th to 20th	Day	69% 6% 11% 8% 6%	3066
	21st to end	Day	79% 6% 8% 6% 1	3931
January	1st to 10th	Night	65% 8% 11% 12% 4	6447
	11th to 20th	Night	74% 7% 9% 7% 3	5979
	21st to end	Night	64% 6% 10% 11% 9%	6490
February	1st to 10th	Night	63% 9% 11% 13% 5%	2600
	11th to 20th	Night	59% 6% 14% 16% 6%	2525
	21st to end	Night	60% 7% 12% 15% 6%	1980
March	1st to 10th	Night	52% 5% 15% 18% 9% 1	5054
	11th to 20th	Night	42% 6% 19% 22% 10% 1	4980
	21st to end	Night	50% 5% 11% 18% 14% 1	5467
April	1st to 10th	Night	50% 7% 11% 18% 12% 2	2133
	11th to 20th	Night	47% 6% 11% 19% 16% 2	1802
	21st to end	Night	46% 6% 12% 18% 15% 3	1800
May	1st to 10th	Night	51% 5% 10% 14% 12% 7%	4060
	11th to 20th	Night	46% 7% 12% 21% 11% 3	4033
	21st to end	Night	40% 6% 12% 21% 17% 3	3955
June	1st to 10th	Night	36% 7% 12% 27% 15% 3	1600
	11th to 20th	Night	34% 8% 15% 24% 18% 2	1406
	21st to end	Night	34% 7% 14% 24% 20% 1	1400
July	1st to 10th	Night	35% 3 14% 27% 17% 5%	3159
	11th to 20th	Night	34% 3 12% 28% 20% 2	3873
	21st to end	Night	31% 5% 14% 22% 24% 4	4297
August	1st to 10th	Night	36% 5% 14% 25% 18% 1	2904
	11th to 20th	Night	36% 4 14% 32% 13% 1	2902
	21st to end	Night	32% 5% 12% 33% 17% 1	3244
September	1st to 10th	Night	34% 7% 15% 27% 16% 2	2065
	11th to 20th	Night	29% 4 14% 32% 19% 1	2200
	21st to end	Night	44% 6% 14% 22% 13% 1	2200
October	1st to 10th	Night	29% 5% 9% 29% 26% 1	3844
	11th to 20th	Night	44% 3 15% 21% 16%	3899
	21st to end	Night	44% 4 15% 25% 12% 1	4316
November	1st to 10th	Night	43% 6% 14% 24% 12% 1	2666
	11th to 20th	Night	52% 8% 14% 16% 8% 1	2880
	21st to end	Night	60% 7% 16% 13% 3 1	2940
December	1st to 10th	Night	69% 5% 16% 8% 2	4720
	11th to 20th	Night	66% 6% 10% 11% 7% 1	5348
	21st to end	Night	74% 5% 10% 9% 2	7228

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	22% 23% 34% 16% 5% 1	300
	11th to 20th	Day	19% 23% 42% 12% 3 1	318
	21st to end	Day	28% 16% 35% 18% 4	330
February	1st to 10th	Day	25% 21% 37% 14% 3	300
	11th to 20th	Day	27% 22% 36% 14% 1	300
	21st to end	Day	25% 25% 37% 12% 1	249
March	1st to 10th	Day	23% 16% 39% 17% 5% 1	436
	11th to 20th	Day	21% 18% 32% 24% 4 1	500
	21st to end	Day	29% 15% 31% 20% 5% 1	550
April	1st to 10th	Day	21% 18% 32% 21% 8% 1	500
	11th to 20th	Day	25% 16% 28% 18% 9% 3	492
	21st to end	Day	23% 13% 31% 23% 8% 2	500
May	1st to 10th	Day	22% 13% 28% 24% 10% 3	503
	11th to 20th	Day	17% 9% 28% 24% 17% 6%	527
	21st to end	Day	15% 8% 25% 27% 18% 7%	573
June	1st to 10th	Day	13% 7% 33% 29% 14% 4	530
	11th to 20th	Day	12% 8% 29% 32% 15% 4	719
	21st to end	Day	12% 9% 28% 32% 15% 4	730
July	1st to 10th	Day	10% 7% 23% 29% 24% 8%	721
	11th to 20th	Day	15% 5% 24% 29% 18% 8%	581
	21st to end	Day	10% 4 21% 32% 24% 9%	576
August	1st to 10th	Day	10% 5% 18% 29% 25% 14%	526
	11th to 20th	Day	14% 3 19% 26% 24% 14%	519
	21st to end	Day	11% 5% 22% 27% 24% 10%	583
September	1st to 10th	Day	12% 5% 24% 27% 21% 12%	530
	11th to 20th	Day	9% 3 19% 23% 32% 15%	530
	21st to end	Day	17% 11% 23% 27% 15% 8%	530
October	1st to 10th	Day	19% 8% 27% 24% 18% 5%	530
	11th to 20th	Day	18% 14% 36% 23% 7% 2	530
	21st to end	Day	25% 16% 35% 19% 6% 1	538
November	1st to 10th	Day	23% 17% 34% 15% 8% 2	330
	11th to 20th	Day	28% 20% 29% 16% 5% 2	250
	21st to end	Day	28% 19% 33% 16% 3	220
December	1st to 10th	Day	25% 18% 33% 19% 5%	220
	11th to 20th	Day	27% 25% 27% 15% 5%	220
	21st to end	Day	31% 19% 32% 14% 2	242
January	1st to 10th	Night	23% 18% 37% 16% 5% 1	978
	11th to 20th	Night	28% 17% 31% 20% 4	825
	21st to end	Night	21% 13% 36% 24% 6%	880
February	1st to 10th	Night	24% 15% 36% 23% 2	800
	11th to 20th	Night	23% 15% 32% 25% 5%	783
	21st to end	Night	23% 15% 39% 20% 3	660
March	1st to 10th	Night	19% 13% 37% 26% 5%	800
	11th to 20th	Night	18% 14% 37% 24% 7% 1	800
	21st to end	Night	23% 13% 29% 27% 8% 1	880
April	1st to 10th	Night	22% 15% 32% 26% 5%	800
	11th to 20th	Night	22% 12% 32% 25% 9% 1	618
	21st to end	Night	18% 10% 35% 29% 8% 1	600
May	1st to 10th	Night	20% 7% 29% 32% 12% 1	600
	11th to 20th	Night	14% 6% 27% 40% 12% 2	591
	21st to end	Night	12% 4 29% 38% 16% 1	449
June	1st to 10th	Night	14% 4 29% 40% 12% 1	400
	11th to 20th	Night	13% 3 26% 44% 15%	400
	21st to end	Night	10% 5% 24% 46% 15% 1	400
July	1st to 10th	Night	6% 2 19% 49% 22% 2	398
	11th to 20th	Night	11% 3 17% 41% 26% 2	582
	21st to end	Night	9% 2 17% 47% 24% 2	657
August	1st to 10th	Night	9% 2 13% 43% 30% 3	597
	11th to 20th	Night	9% 2 14% 39% 31% 5%	594
	21st to end	Night	10% 3 18% 39% 28% 2	668
September	1st to 10th	Night	9% 6% 21% 34% 25% 5%	800
	11th to 20th	Night	10% 5% 15% 37% 31% 4	800
	21st to end	Night	17% 7% 21% 33% 17% 5%	800
October	1st to 10th	Night	16% 6% 26% 31% 18% 2	800
	11th to 20th	Night	23% 11% 27% 27% 11% 1	800
	21st to end	Night	25% 11% 32% 27% 5%	880
November	1st to 10th	Night	19% 14% 27% 29% 10% 1	820
	11th to 20th	Night	22% 13% 37% 21% 6% 2	840
	21st to end	Night	28% 13% 30% 23% 5%	840
December	1st to 10th	Night	28% 14% 33% 19% 5%	840
	11th to 20th	Night	23% 18% 38% 18% 3	840
	21st to end	Night	28% 13% 36% 20% 3	963

Page generated at 2026-01-14 01:14:29. Version ac07

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