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 2024-01-14.

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% 10% 4 7% 6% 4 5% 5% 5% 7% 31%	74987
	Night	16% 10% 4 7% 6% 3 5% 5% 5% 7% 32%	74722
Transparency	Day	50% 8% 13% 18% 10% 1	74695
	Night	50% 7% 13% 18% 10% 1	74534
Seeing	Day	18% 11% 28% 24% 14% 5%	13888
	Night	19% 10% 29% 29% 12% 1	23214

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	17% 15% 6% 10% 9% 5% 6% 4 4 5% 17%	1513
	11th to 20th	Day	16% 18% 8% 13% 7% 5% 5% 4 5% 4 16%	1647
	21st to end	Day	15% 18% 9% 12% 9% 3 5% 4 3 5% 16%	1782
February	1st to 10th	Day	19% 15% 5% 11% 11% 4 5% 5% 5% 5% 17%	1620
	11th to 20th	Day	16% 12% 7% 12% 10% 5% 5% 4 5% 5% 19%	1580
	21st to end	Day	17% 15% 6% 11% 6% 4 5% 5% 5% 5% 22%	1480
March	1st to 10th	Day	16% 12% 5% 9% 7% 3 5% 5% 5% 6% 26%	1922
	11th to 20th	Day	15% 12% 5% 10% 6% 3 4 4 4 6% 30%	1980
	21st to end	Day	20% 11% 5% 9% 5% 3 4 4 4 6% 29%	2178
April	1st to 10th	Day	17% 12% 5% 8% 6% 3 5% 4 5% 7% 29%	2038
	11th to 20th	Day	19% 10% 3 5% 5% 4 4 4 5% 8% 32%	2302
	21st to end	Day	20% 9% 4 7% 6% 3 4 3 5% 6% 33%	2340
May	1st to 10th	Day	17% 7% 3 6% 7% 3 5% 5% 6% 8% 33%	2340
	11th to 20th	Day	16% 9% 4 5% 5% 4 4 4 6% 7% 37%	2301
	21st to end	Day	12% 5% 2 5% 5% 3 5% 6% 6% 8% 41%	2708
June	1st to 10th	Day	13% 5% 3 6% 6% 4 5% 5% 6% 8% 39%	2520
	11th to 20th	Day	11% 6% 3 6% 5% 3 6% 6% 6% 8% 40%	2689
	21st to end	Day	11% 6% 3 5% 5% 3 5% 6% 7% 9% 40%	2700
July	1st to 10th	Day	9% 4 2 5% 5% 3 5% 6% 6% 10% 45%	2690
	11th to 20th	Day	11% 5% 2 6% 6% 3 6% 5% 7% 11% 38%	2395
	21st to end	Day	9% 6% 3 6% 6% 4 6% 6% 6% 10% 39%	2547
August	1st to 10th	Day	9% 4 2 5% 6% 4 5% 6% 7% 10% 40%	2327
	11th to 20th	Day	10% 6% 3 7% 6% 5% 5% 6% 6% 11% 35%	2303
	21st to end	Day	11% 6% 2 5% 5% 4 5% 5% 6% 9% 41%	2548
September	1st to 10th	Day	11% 6% 2 6% 6% 4 5% 6% 7% 7% 39%	2100
	11th to 20th	Day	11% 5% 3 5% 5% 4 4 5% 6% 9% 44%	1980
	21st to end	Day	17% 7% 3 5% 5% 3 5% 5% 7% 7% 37%	1980
October	1st to 10th	Day	17% 7% 3 6% 6% 3 4 5% 5% 8% 35%	1845
	11th to 20th	Day	21% 9% 3 8% 6% 4 5% 5% 5% 7% 27%	1800
	21st to end	Day	25% 11% 4 9% 7% 4 4 5% 4 6% 23%	1940
November	1st to 10th	Day	15% 10% 3 6% 7% 3 5% 4 5% 9% 32%	1672
	11th to 20th	Day	16% 13% 4 8% 4 3 6% 6% 6% 7% 27%	1573
	21st to end	Day	20% 15% 7% 9% 6% 3 6% 5% 5% 6% 18%	1520
December	1st to 10th	Day	17% 16% 7% 10% 7% 4 5% 4 6% 6% 18%	1520
	11th to 20th	Day	19% 17% 8% 11% 7% 5% 5% 5% 5% 5% 14%	1910
	21st to end	Day	28% 20% 7% 10% 7% 3 4 3 4 5% 9%	2697
January	1st to 10th	Night	18% 14% 7% 12% 8% 4 5% 4 5% 4 18%	2637
	11th to 20th	Night	21% 18% 6% 10% 7% 4 5% 4 4 5% 16%	2386
	21st to end	Night	15% 14% 7% 12% 8% 4 5% 5% 5% 6% 19%	2574
February	1st to 10th	Night	18% 14% 5% 10% 9% 4 4 5% 5% 6% 19%	2340
	11th to 20th	Night	17% 14% 5% 8% 7% 4 6% 5% 5% 6% 23%	2264
	21st to end	Night	18% 13% 5% 8% 8% 4 5% 4 4 5% 24%	1776
March	1st to 10th	Night	14% 10% 4 8% 7% 4 5% 5% 6% 7% 30%	2037
	11th to 20th	Night	15% 9% 5% 7% 6% 3 4 4 5% 7% 34%	1980
	21st to end	Night	19% 10% 4 8% 5% 3 4 4 4 5% 34%	2178
April	1st to 10th	Night	18% 9% 4 5% 5% 3 5% 5% 6% 6% 34%	1921
	11th to 20th	Night	18% 8% 3 5% 5% 3 4 4 6% 8% 36%	1620
	21st to end	Night	15% 9% 3 6% 6% 4 5% 5% 5% 7% 35%	1620
May	1st to 10th	Night	18% 8% 3 6% 5% 3 4 5% 6% 7% 36%	1620
	11th to 20th	Night	13% 6% 3 6% 5% 3 6% 5% 5% 8% 40%	1592
	21st to end	Night	9% 4 2 4 4 2 5% 5% 6% 7% 51%	1564
June	1st to 10th	Night	13% 4 2 5% 4 3 5% 5% 6% 9% 44%	1440
	11th to 20th	Night	9% 5% 2 5% 5% 3 4 6% 8% 9% 43%	1266
	21st to end	Night	10% 5% 2 4 5% 3 5% 5% 6% 7% 47%	1260
July	1st to 10th	Night	6% 4 2 4 5% 3 4 4 5% 9% 55%	1266
	11th to 20th	Night	9% 5% 2 4 4 3 3 4 6% 10% 49%	1561
	21st to end	Night	7% 4 3 5% 5% 4 4 5% 6% 9% 47%	1771
August	1st to 10th	Night	7% 5% 3 5% 6% 3 5% 4 7% 8% 47%	1611
	11th to 20th	Night	7% 5% 2 5% 4 3 5% 5% 6% 9% 49%	1598
	21st to end	Night	8% 5% 2 4 5% 3 4 6% 5% 9% 49%	1788
September	1st to 10th	Night	9% 6% 3 6% 5% 3 5% 4 6% 7% 46%	1858
	11th to 20th	Night	11% 4 2 4 4 3 4 4 5% 7% 52%	1980
	21st to end	Night	16% 6% 3 6% 5% 4 5% 4 5% 7% 42%	1980
October	1st to 10th	Night	14% 7% 3 6% 5% 4 4 4 5% 7% 41%	2114
	11th to 20th	Night	20% 9% 3 7% 6% 2 4 4 5% 7% 34%	2160
	21st to end	Night	23% 10% 4 7% 6% 3 4 5% 5% 6% 26%	2412
November	1st to 10th	Night	15% 7% 3 6% 5% 3 4 5% 6% 7% 39%	2406
	11th to 20th	Night	16% 11% 5% 7% 7% 3 6% 5% 6% 8% 27%	2605
	21st to end	Night	21% 14% 5% 7% 6% 3 6% 4 5% 7% 22%	2660
December	1st to 10th	Night	20% 14% 6% 10% 6% 3 5% 5% 5% 6% 19%	2660
	11th to 20th	Night	15% 19% 7% 10% 6% 4 5% 5% 4 6% 21%	3285
	21st to end	Night	27% 20% 7% 10% 7% 3 5% 3 3 4 12%	4932

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	68% 8% 10% 10% 3	1514
	11th to 20th	Day	69% 8% 10% 7% 4 1	1647
	21st to end	Day	70% 7% 9% 10% 4 1	1782
February	1st to 10th	Day	68% 6% 11% 10% 4	1620
	11th to 20th	Day	66% 6% 10% 12% 6%	1589
	21st to end	Day	63% 8% 11% 12% 6% 1	1480
March	1st to 10th	Day	57% 8% 13% 15% 7% 1	1922
	11th to 20th	Day	54% 6% 11% 17% 10% 2	1980
	21st to end	Day	58% 5% 10% 17% 10% 1	2178
April	1st to 10th	Day	54% 6% 11% 19% 10% 1	2038
	11th to 20th	Day	49% 7% 13% 17% 13% 1	2302
	21st to end	Day	50% 7% 13% 16% 11% 2	2340
May	1st to 10th	Day	47% 7% 13% 19% 11% 2	2340
	11th to 20th	Day	46% 7% 13% 19% 13% 2	2305
	21st to end	Day	38% 9% 15% 22% 14% 2	2708
June	1st to 10th	Day	40% 8% 14% 24% 12% 2	2520
	11th to 20th	Day	37% 10% 14% 21% 16% 2	2689
	21st to end	Day	37% 9% 13% 25% 15% 1	2700
July	1st to 10th	Day	33% 8% 13% 25% 18% 3	2616
	11th to 20th	Day	36% 7% 17% 25% 14% 1	2322
	21st to end	Day	36% 10% 16% 22% 14% 2	2405
August	1st to 10th	Day	36% 10% 17% 26% 10% 1	2314
	11th to 20th	Day	41% 9% 17% 22% 10% 1	2303
	21st to end	Day	38% 8% 15% 25% 13% 1	2548
September	1st to 10th	Day	40% 9% 17% 21% 12% 1	2100
	11th to 20th	Day	35% 7% 15% 30% 14%	1980
	21st to end	Day	44% 7% 14% 23% 10% 1	1980
October	1st to 10th	Day	44% 8% 14% 20% 12% 1	1845
	11th to 20th	Day	53% 8% 15% 16% 8% 1	1800
	21st to end	Day	61% 7% 12% 13% 6%	1940
November	1st to 10th	Day	49% 8% 14% 20% 7% 2	1672
	11th to 20th	Day	53% 9% 17% 12% 8% 1	1573
	21st to end	Day	64% 8% 14% 12% 2	1520
December	1st to 10th	Day	65% 8% 13% 11% 3	1520
	11th to 20th	Day	68% 9% 12% 9% 2	1906
	21st to end	Day	78% 7% 10% 4 1	2697
January	1st to 10th	Night	66% 8% 10% 11% 5%	2637
	11th to 20th	Night	69% 7% 12% 8% 4	2386
	21st to end	Night	64% 7% 11% 13% 4 1	2574
February	1st to 10th	Night	64% 8% 11% 12% 5%	2340
	11th to 20th	Night	60% 5% 13% 15% 6%	2266
	21st to end	Night	60% 6% 12% 15% 6%	1776
March	1st to 10th	Night	51% 7% 13% 19% 8% 1	2037
	11th to 20th	Night	49% 6% 11% 21% 12% 2	1980
	21st to end	Night	53% 6% 8% 20% 11% 2	2178
April	1st to 10th	Night	48% 7% 11% 19% 13% 2	1921
	11th to 20th	Night	45% 6% 11% 19% 16% 2	1620
	21st to end	Night	47% 6% 12% 17% 15% 3	1620
May	1st to 10th	Night	46% 6% 11% 20% 13% 4	1620
	11th to 20th	Night	42% 7% 12% 21% 14% 4	1593
	21st to end	Night	31% 9% 14% 26% 17% 4	1565
June	1st to 10th	Night	35% 7% 13% 27% 15% 3	1440
	11th to 20th	Night	33% 9% 16% 22% 19% 2	1266
	21st to end	Night	33% 8% 14% 25% 19% 1	1260
July	1st to 10th	Night	28% 6% 10% 30% 22% 5%	1231
	11th to 20th	Night	31% 7% 13% 28% 17% 3	1511
	21st to end	Night	30% 10% 17% 23% 17% 3	1672
August	1st to 10th	Night	31% 9% 15% 30% 14% 1	1600
	11th to 20th	Night	31% 7% 16% 31% 14% 2	1598
	21st to end	Night	30% 8% 14% 29% 17% 2	1788
September	1st to 10th	Night	35% 7% 16% 24% 15% 2	1858
	11th to 20th	Night	31% 5% 15% 31% 17% 1	1980
	21st to end	Night	42% 6% 15% 23% 13% 1	1980
October	1st to 10th	Night	41% 7% 13% 22% 16% 1	2114
	11th to 20th	Night	50% 5% 14% 20% 10% 1	2160
	21st to end	Night	57% 7% 11% 16% 9% 1	2412
November	1st to 10th	Night	41% 7% 15% 24% 12% 1	2406
	11th to 20th	Night	51% 8% 15% 17% 8% 1	2605
	21st to end	Night	59% 8% 17% 13% 3 1	2660
December	1st to 10th	Night	64% 7% 16% 10% 3	2660
	11th to 20th	Night	63% 8% 14% 11% 4	3288
	21st to end	Night	75% 7% 11% 6% 1	4932

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	21% 22% 31% 19% 6% 1	245
	11th to 20th	Day	20% 18% 44% 12% 4 1	249
	21st to end	Day	26% 16% 37% 17% 5%	264
February	1st to 10th	Day	28% 20% 38% 13% 1	240
	11th to 20th	Day	26% 22% 38% 13% 1	240
	21st to end	Day	24% 30% 34% 10% 2	198
March	1st to 10th	Day	23% 15% 39% 17% 6% 1	362
	11th to 20th	Day	20% 16% 34% 25% 5% 1	420
	21st to end	Day	29% 13% 31% 20% 6% 1	462
April	1st to 10th	Day	19% 18% 32% 21% 9% 1	420
	11th to 20th	Day	25% 13% 28% 20% 9% 4	412
	21st to end	Day	23% 13% 30% 23% 8% 3	420
May	1st to 10th	Day	23% 11% 28% 25% 10% 3	423
	11th to 20th	Day	17% 9% 27% 24% 17% 5%	447
	21st to end	Day	12% 7% 25% 28% 19% 8%	485
June	1st to 10th	Day	14% 5% 32% 30% 14% 4	450
	11th to 20th	Day	13% 8% 26% 32% 16% 5%	619
	21st to end	Day	12% 8% 27% 33% 16% 4	630
July	1st to 10th	Day	9% 6% 20% 30% 26% 10%	621
	11th to 20th	Day	15% 5% 22% 30% 20% 8%	495
	21st to end	Day	10% 4 21% 32% 24% 9%	488
August	1st to 10th	Day	9% 4 18% 28% 26% 15%	446
	11th to 20th	Day	13% 2 17% 27% 26% 15%	439
	21st to end	Day	12% 5% 20% 26% 25% 11%	495
September	1st to 10th	Day	12% 4 23% 29% 20% 12%	450
	11th to 20th	Day	10% 3 22% 24% 28% 14%	450
	21st to end	Day	16% 11% 22% 26% 15% 9%	450
October	1st to 10th	Day	20% 8% 26% 22% 18% 6%	450
	11th to 20th	Day	19% 15% 36% 22% 7% 1	450
	21st to end	Day	26% 14% 35% 19% 5% 1	455
November	1st to 10th	Day	21% 16% 34% 17% 10% 3	270
	11th to 20th	Day	25% 20% 30% 16% 6% 2	205
	21st to end	Day	27% 17% 36% 17% 3	180
December	1st to 10th	Day	24% 19% 33% 17% 6% 1	180
	11th to 20th	Day	26% 23% 30% 16% 5%	180
	21st to end	Day	30% 19% 32% 16% 2 1	198
January	1st to 10th	Night	23% 17% 37% 16% 5% 1	886
	11th to 20th	Night	30% 15% 30% 20% 4	735
	21st to end	Night	20% 12% 36% 25% 7%	792
February	1st to 10th	Night	25% 14% 36% 22% 2	720
	11th to 20th	Night	23% 15% 31% 25% 5%	703
	21st to end	Night	25% 14% 38% 20% 3	592
March	1st to 10th	Night	19% 12% 37% 27% 6%	720
	11th to 20th	Night	18% 13% 35% 25% 7% 1	720
	21st to end	Night	23% 11% 28% 28% 8% 1	792
April	1st to 10th	Night	21% 14% 32% 27% 5%	720
	11th to 20th	Night	21% 11% 32% 25% 10% 1	556
	21st to end	Night	19% 8% 34% 30% 8% 1	540
May	1st to 10th	Night	20% 7% 28% 32% 13% 1	540
	11th to 20th	Night	14% 5% 27% 41% 11% 2	531
	21st to end	Night	11% 3 29% 39% 17% 1	404
June	1st to 10th	Night	14% 4 29% 41% 13% 1	360
	11th to 20th	Night	13% 3 26% 43% 16%	360
	21st to end	Night	9% 4 24% 47% 15% 1	360
July	1st to 10th	Night	6% 2 19% 48% 23% 2	356
	11th to 20th	Night	11% 2 17% 41% 26% 2	522
	21st to end	Night	9% 3 17% 47% 23% 2	591
August	1st to 10th	Night	9% 1 12% 45% 30% 4	537
	11th to 20th	Night	8% 2 14% 38% 33% 5%	534
	21st to end	Night	11% 3 18% 37% 28% 3	600
September	1st to 10th	Night	10% 5% 20% 36% 23% 6%	720
	11th to 20th	Night	11% 5% 16% 36% 28% 4	720
	21st to end	Night	17% 7% 22% 32% 18% 5%	720
October	1st to 10th	Night	17% 6% 26% 31% 18% 2	720
	11th to 20th	Night	24% 11% 27% 27% 10% 1	720
	21st to end	Night	27% 10% 32% 26% 4	792
November	1st to 10th	Night	18% 12% 27% 30% 11% 1	740
	11th to 20th	Night	22% 11% 38% 21% 6% 2	760
	21st to end	Night	27% 13% 30% 24% 6%	760
December	1st to 10th	Night	29% 14% 33% 18% 5%	760
	11th to 20th	Night	23% 17% 38% 18% 3	760
	21st to end	Night	28% 13% 36% 21% 3	871

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