Weston
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Weston clear sky chart from 2005-11-06 to 2024-01-13.

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	8% 6% 3 7% 8% 5% 7% 7% 8% 9% 31%	74876
	Night	7% 5% 2 5% 6% 4 5% 6% 7% 10% 43%	74855
Transparency	Day	45% 8% 16% 17% 11% 3	74599
	Night	34% 9% 19% 22% 13% 3	74651
Seeing	Day	8% 3 15% 27% 26% 21%	14828
	Night	8% 3 13% 32% 32% 13%	24990

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	7% 6% 2 5% 5% 3 6% 6% 8% 9% 43%	1706
	11th to 20th	Day	7% 6% 3 6% 6% 5% 6% 6% 6% 9% 39%	1638
	21st to end	Day	8% 5% 3 7% 7% 4 6% 6% 7% 8% 39%	1818
February	1st to 10th	Day	9% 7% 2 5% 7% 4 5% 5% 7% 8% 39%	1800
	11th to 20th	Day	7% 4 2 4 5% 3 4 5% 6% 10% 50%	1848
	21st to end	Day	5% 4 2 6% 6% 5% 7% 6% 7% 9% 43%	1628
March	1st to 10th	Day	4 4 2 4 6% 4 6% 6% 6% 10% 46%	1980
	11th to 20th	Day	6% 4 2 5% 6% 3 5% 7% 8% 9% 44%	1980
	21st to end	Day	5% 5% 3 6% 6% 3 6% 6% 6% 9% 45%	2178
April	1st to 10th	Day	4 3 2 5% 6% 4 7% 7% 7% 12% 43%	1980
	11th to 20th	Day	6% 5% 3 7% 7% 5% 6% 7% 6% 9% 38%	2034
	21st to end	Day	7% 4 2 5% 6% 4 6% 6% 6% 11% 42%	2160
May	1st to 10th	Day	4 3 2 4 6% 4 8% 6% 7% 9% 47%	2160
	11th to 20th	Day	9% 7% 4 8% 8% 5% 8% 8% 8% 9% 28%	2155
	21st to end	Day	10% 7% 4 8% 9% 6% 7% 8% 7% 8% 26%	2531
June	1st to 10th	Day	14% 10% 5% 10% 10% 6% 9% 7% 7% 7% 15%	2340
	11th to 20th	Day	8% 7% 5% 10% 10% 7% 8% 9% 8% 10% 19%	2340
	21st to end	Day	8% 7% 4 8% 11% 7% 9% 8% 9% 9% 19%	2340
July	1st to 10th	Day	8% 7% 4 10% 9% 6% 9% 9% 10% 9% 19%	2340
	11th to 20th	Day	8% 6% 3 9% 10% 7% 9% 9% 10% 10% 20%	2340
	21st to end	Day	6% 6% 4 9% 10% 5% 9% 8% 10% 12% 21%	2547
August	1st to 10th	Day	8% 7% 3 9% 10% 6% 8% 9% 8% 12% 20%	2292
	11th to 20th	Day	5% 5% 3 8% 9% 7% 9% 10% 12% 12% 21%	2126
	21st to end	Day	9% 6% 4 10% 10% 7% 10% 9% 9% 11% 17%	2203
September	1st to 10th	Day	8% 6% 4 10% 11% 7% 10% 8% 8% 9% 19%	1980
	11th to 20th	Day	9% 7% 4 10% 10% 6% 8% 8% 9% 9% 21%	1980
	21st to end	Day	11% 9% 4 11% 10% 6% 7% 9% 7% 7% 19%	1980
October	1st to 10th	Day	9% 6% 3 9% 9% 5% 8% 9% 9% 9% 23%	1900
	11th to 20th	Day	9% 6% 3 7% 8% 6% 8% 8% 8% 8% 28%	1800
	21st to end	Day	12% 6% 2 7% 7% 4 6% 6% 7% 8% 34%	1980
November	1st to 10th	Day	7% 5% 3 5% 6% 5% 6% 7% 7% 9% 39%	1857
	11th to 20th	Day	9% 7% 3 7% 9% 5% 6% 6% 7% 8% 33%	1900
	21st to end	Day	8% 5% 3 6% 5% 4 6% 8% 7% 9% 39%	1900
December	1st to 10th	Day	7% 5% 3 7% 6% 4 7% 5% 7% 8% 42%	1858
	11th to 20th	Day	10% 5% 3 5% 7% 4 5% 5% 6% 8% 42%	2153
	21st to end	Day	12% 5% 2 7% 7% 4 7% 7% 8% 10% 31%	3124
January	1st to 10th	Night	7% 4 2 6% 4 3 4 5% 6% 9% 49%	2459
	11th to 20th	Night	7% 6% 3 5% 5% 4 6% 6% 7% 10% 42%	2367
	21st to end	Night	6% 5% 3 5% 5% 3 5% 5% 7% 9% 46%	2534
February	1st to 10th	Night	8% 5% 2 5% 4 4 4 5% 7% 9% 46%	2160
	11th to 20th	Night	6% 3 2 3 4 2 5% 5% 6% 8% 56%	1997
	21st to end	Night	5% 2 1 3 5% 2 4 5% 5% 8% 58%	1628
March	1st to 10th	Night	3 3 1 4 3 2 4 5% 6% 9% 60%	1980
	11th to 20th	Night	6% 3 1 4 4 3 4 6% 6% 9% 54%	1980
	21st to end	Night	5% 4 2 4 5% 2 4 4 7% 9% 55%	2178
April	1st to 10th	Night	4 2 1 4 3 3 3 4 7% 10% 59%	1980
	11th to 20th	Night	5% 3 1 5% 4 3 5% 6% 7% 9% 51%	1874
	21st to end	Night	6% 3 2 4 4 3 4 6% 6% 9% 52%	1800
May	1st to 10th	Night	3 2 1 3 5% 2 4 5% 5% 11% 59%	1800
	11th to 20th	Night	7% 5% 2 5% 6% 3 6% 7% 7% 12% 40%	1750
	21st to end	Night	8% 5% 3 5% 6% 3 6% 5% 8% 13% 38%	1744
June	1st to 10th	Night	12% 7% 4 7% 8% 5% 8% 8% 7% 11% 24%	1620
	11th to 20th	Night	6% 6% 3 5% 7% 4 6% 7% 8% 13% 35%	1620
	21st to end	Night	5% 5% 3 6% 7% 4 8% 8% 10% 13% 32%	1620
July	1st to 10th	Night	5% 4 3 7% 7% 5% 6% 7% 9% 13% 34%	1620
	11th to 20th	Night	5% 5% 3 6% 6% 5% 6% 7% 9% 12% 37%	1620
	21st to end	Night	3 4 2 6% 7% 4 6% 7% 10% 13% 39%	1771
August	1st to 10th	Night	5% 5% 2 5% 6% 4 6% 6% 8% 15% 38%	1624
	11th to 20th	Night	4 3 2 5% 6% 5% 7% 8% 10% 13% 36%	1774
	21st to end	Night	6% 6% 3 7% 7% 4 6% 9% 9% 13% 30%	2146
September	1st to 10th	Night	5% 4 3 6% 8% 5% 7% 9% 11% 12% 30%	1980
	11th to 20th	Night	7% 6% 4 7% 8% 4 7% 6% 9% 11% 30%	1980
	21st to end	Night	9% 7% 3 8% 8% 6% 7% 8% 8% 11% 25%	1980
October	1st to 10th	Night	8% 6% 3 6% 8% 4 7% 8% 8% 11% 31%	2060
	11th to 20th	Night	8% 5% 2 6% 4 3 5% 6% 7% 11% 41%	2160
	21st to end	Night	10% 6% 2 6% 6% 3 6% 5% 6% 8% 43%	2376
November	1st to 10th	Night	8% 4 2 5% 5% 3 4 5% 6% 9% 50%	2221
	11th to 20th	Night	8% 5% 3 6% 7% 4 7% 6% 6% 8% 41%	2280
	21st to end	Night	6% 5% 2 6% 5% 3 5% 5% 6% 10% 46%	2280
December	1st to 10th	Night	6% 4 2 5% 5% 3 5% 5% 6% 9% 50%	2318
	11th to 20th	Night	12% 5% 2 5% 4 2 4 5% 5% 9% 48%	3042
	21st to end	Night	9% 7% 3 7% 7% 4 6% 7% 7% 9% 34%	4532

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	35% 6% 11% 22% 17% 8%	1707
	11th to 20th	Day	41% 5% 13% 22% 15% 5%	1638
	21st to end	Day	40% 5% 11% 20% 18% 6%	1818
February	1st to 10th	Day	41% 5% 10% 24% 15% 5%	1800
	11th to 20th	Day	28% 4 10% 28% 23% 6%	1858
	21st to end	Day	34% 6% 10% 19% 24% 8%	1628
March	1st to 10th	Day	31% 6% 11% 26% 21% 5%	1980
	11th to 20th	Day	33% 5% 11% 27% 18% 5%	1980
	21st to end	Day	34% 5% 10% 24% 20% 7%	2178
April	1st to 10th	Day	31% 6% 17% 28% 16% 3	1980
	11th to 20th	Day	40% 6% 16% 21% 15% 2	2034
	21st to end	Day	36% 6% 10% 24% 20% 4	2160
May	1st to 10th	Day	29% 6% 14% 25% 18% 8%	2160
	11th to 20th	Day	49% 8% 14% 22% 7%	2159
	21st to end	Day	52% 8% 13% 15% 11% 1	2531
June	1st to 10th	Day	64% 9% 15% 10% 3	2340
	11th to 20th	Day	56% 13% 18% 11% 1	2340
	21st to end	Day	54% 13% 23% 9% 1	2340
July	1st to 10th	Day	52% 13% 26% 8%	2275
	11th to 20th	Day	52% 11% 26% 11%	2271
	21st to end	Day	49% 13% 28% 10%	2405
August	1st to 10th	Day	50% 12% 25% 12% 1	2279
	11th to 20th	Day	46% 13% 29% 11%	2126
	21st to end	Day	54% 15% 22% 8% 1	2203
September	1st to 10th	Day	55% 14% 18% 12% 1	1980
	11th to 20th	Day	54% 12% 22% 10% 3	1980
	21st to end	Day	61% 10% 14% 10% 4 1	1980
October	1st to 10th	Day	51% 10% 16% 16% 7% 1	1900
	11th to 20th	Day	49% 8% 12% 17% 12% 2	1800
	21st to end	Day	45% 6% 9% 18% 17% 5%	1980
November	1st to 10th	Day	38% 5% 12% 20% 18% 7%	1857
	11th to 20th	Day	47% 6% 10% 18% 14% 5%	1900
	21st to end	Day	40% 6% 11% 19% 21% 3	1900
December	1st to 10th	Day	38% 6% 12% 24% 18% 2	1858
	11th to 20th	Day	39% 5% 9% 20% 19% 7%	2150
	21st to end	Day	46% 6% 13% 19% 14% 2	3124
January	1st to 10th	Night	31% 4 14% 29% 17% 4	2459
	11th to 20th	Night	37% 6% 14% 25% 15% 4	2367
	21st to end	Night	33% 7% 13% 25% 17% 6%	2534
February	1st to 10th	Night	33% 5% 13% 29% 17% 4	2160
	11th to 20th	Night	25% 6% 11% 28% 23% 7%	1998
	21st to end	Night	25% 6% 11% 26% 27% 6%	1628
March	1st to 10th	Night	21% 5% 14% 31% 23% 7%	1980
	11th to 20th	Night	26% 5% 16% 30% 18% 4	1980
	21st to end	Night	25% 5% 13% 30% 22% 5%	2178
April	1st to 10th	Night	20% 5% 19% 36% 17% 3	1980
	11th to 20th	Night	27% 7% 19% 25% 19% 3	1874
	21st to end	Night	27% 6% 15% 29% 19% 4	1800
May	1st to 10th	Night	20% 6% 16% 35% 18% 5%	1800
	11th to 20th	Night	36% 8% 19% 25% 11%	1751
	21st to end	Night	37% 8% 21% 24% 11%	1745
June	1st to 10th	Night	51% 11% 22% 14% 2	1620
	11th to 20th	Night	38% 13% 29% 17% 3	1620
	21st to end	Night	38% 15% 35% 11% 1	1620
July	1st to 10th	Night	39% 15% 33% 12% 1	1575
	11th to 20th	Night	36% 12% 37% 15% 1	1566
	21st to end	Night	32% 14% 38% 14% 2	1672
August	1st to 10th	Night	33% 13% 35% 17% 2	1613
	11th to 20th	Night	32% 18% 35% 15%	1774
	21st to end	Night	41% 15% 32% 11% 2	2146
September	1st to 10th	Night	40% 17% 28% 14% 1	1980
	11th to 20th	Night	44% 15% 27% 13% 2	1980
	21st to end	Night	50% 14% 18% 13% 6%	1980
October	1st to 10th	Night	43% 10% 19% 20% 7% 1	2060
	11th to 20th	Night	37% 7% 20% 22% 13% 1	2160
	21st to end	Night	38% 6% 12% 23% 15% 6%	2376
November	1st to 10th	Night	32% 5% 14% 23% 19% 7%	2221
	11th to 20th	Night	40% 5% 12% 24% 14% 5%	2280
	21st to end	Night	34% 6% 14% 23% 19% 4	2280
December	1st to 10th	Night	31% 6% 14% 29% 17% 3	2318
	11th to 20th	Night	34% 5% 9% 24% 19% 9%	3044
	21st to end	Night	43% 8% 15% 18% 14% 2	4532

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	11% 8% 33% 35% 12% 2	270
	11th to 20th	Day	11% 6% 33% 33% 15% 3	246
	21st to end	Day	12% 6% 44% 28% 9% 1	264
February	1st to 10th	Day	10% 6% 28% 38% 14% 3	240
	11th to 20th	Day	7% 3 24% 39% 21% 6%	341
	21st to end	Day	6% 5% 20% 38% 29% 3	346
March	1st to 10th	Day	4 7% 30% 35% 22% 3	420
	11th to 20th	Day	7% 5% 25% 37% 22% 4	420
	21st to end	Day	5% 3 26% 37% 24% 6%	462
April	1st to 10th	Day	4 5% 29% 31% 24% 7%	420
	11th to 20th	Day	9% 4 25% 32% 21% 8%	412
	21st to end	Day	7% 3 21% 33% 26% 9%	420
May	1st to 10th	Day	4 1 17% 33% 29% 15%	423
	11th to 20th	Day	9% 2 16% 30% 28% 15%	447
	21st to end	Day	12% 4 6% 26% 27% 24%	485
June	1st to 10th	Day	16% 3 8% 22% 29% 22%	450
	11th to 20th	Day	8% 3 3 19% 30% 37%	450
	21st to end	Day	6% 1 1 17% 24% 51%	450
July	1st to 10th	Day	5% 2 1 11% 29% 52%	445
	11th to 20th	Day	7% 2 13% 31% 47%	444
	21st to end	Day	5% 3 2 15% 28% 48%	488
August	1st to 10th	Day	8% 2 1 13% 29% 47%	446
	11th to 20th	Day	7% 3 2 14% 28% 46%	439
	21st to end	Day	9% 2 2 21% 29% 35%	495
September	1st to 10th	Day	9% 3 2 13% 29% 44%	450
	11th to 20th	Day	6% 2 3 14% 28% 46%	450
	21st to end	Day	13% 3 4 19% 28% 33%	450
October	1st to 10th	Day	11% 2 8% 35% 31% 13%	450
	11th to 20th	Day	11% 15% 30% 29% 15%	450
	21st to end	Day	11% 3 15% 34% 28% 10%	495
November	1st to 10th	Day	9% 4 20% 34% 24% 8%	455
	11th to 20th	Day	12% 3 22% 31% 25% 8%	460
	21st to end	Day	6% 2 23% 35% 29% 5%	460
December	1st to 10th	Day	11% 3 18% 39% 24% 5%	418
	11th to 20th	Day	9% 5% 24% 37% 19% 6%	270
	21st to end	Day	11% 6% 27% 35% 15% 6%	297
January	1st to 10th	Night	9% 5% 19% 41% 24% 2	760
	11th to 20th	Night	9% 5% 24% 41% 20% 1	729
	21st to end	Night	8% 5% 27% 43% 16% 1	792
February	1st to 10th	Night	10% 4 21% 43% 20% 2	720
	11th to 20th	Night	7% 4 22% 43% 23% 2	703
	21st to end	Night	7% 3 20% 48% 22% 1	592
March	1st to 10th	Night	6% 3 27% 43% 19% 2	720
	11th to 20th	Night	8% 3 19% 46% 23% 2	720
	21st to end	Night	7% 4 22% 44% 23% 2	792
April	1st to 10th	Night	4 2 17% 50% 25% 2	720
	11th to 20th	Night	7% 3 20% 46% 23% 2	711
	21st to end	Night	8% 2 17% 46% 25% 3	720
May	1st to 10th	Night	4 2 14% 44% 33% 3	720
	11th to 20th	Night	9% 2 10% 42% 33% 4	689
	21st to end	Night	8% 2 4 30% 44% 11%	584
June	1st to 10th	Night	12% 2 3 21% 46% 16%	540
	11th to 20th	Night	6% 2 2 12% 47% 31%	540
	21st to end	Night	5% 2 1 9% 44% 39%	540
July	1st to 10th	Night	5% 1 1 8% 40% 45%	525
	11th to 20th	Night	5% 2 8% 44% 41%	522
	21st to end	Night	2 1 6% 45% 46%	591
August	1st to 10th	Night	7% 2 7% 46% 38%	550
	11th to 20th	Night	4 2 2 10% 43% 39%	710
	21st to end	Night	9% 2 2 17% 40% 29%	792
September	1st to 10th	Night	6% 2 3 12% 41% 36%	720
	11th to 20th	Night	9% 3 2 14% 43% 30%	720
	21st to end	Night	12% 2 3 18% 41% 24%	720
October	1st to 10th	Night	10% 2 7% 35% 37% 11%	720
	11th to 20th	Night	11% 2 6% 32% 40% 10%	720
	21st to end	Night	14% 2 12% 34% 31% 8%	792
November	1st to 10th	Night	10% 3 18% 35% 30% 4	740
	11th to 20th	Night	9% 2 17% 37% 30% 5%	760
	21st to end	Night	8% 3 16% 43% 27% 3	760
December	1st to 10th	Night	7% 2 14% 40% 33% 4	760
	11th to 20th	Night	10% 4 17% 38% 28% 3	760
	21st to end	Night	8% 2 20% 39% 28% 3	836

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