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 2024-11-10.

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	7% 6% 3 8% 7% 7% 7% 6% 8% 10% 32%	100659
	Night	6% 5% 2 7% 5% 5% 5% 5% 8% 10% 42%	98435
Transparency	Day	42% 9% 17% 19% 10% 2	104369
	Night	35% 9% 20% 22% 11% 2	102864
Seeing	Day	8% 3 15% 28% 25% 20%	16002
	Night	8% 3 13% 32% 32% 12%	26117

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	9% 8% 3 8% 4 5% 5% 4 7% 8% 39%	2836
	11th to 20th	Day	15% 7% 3 9% 6% 6% 7% 5% 7% 8% 27%	2840
	21st to end	Day	6% 6% 3 10% 7% 6% 6% 5% 7% 6% 39%	3188
February	1st to 10th	Day	9% 8% 2 7% 7% 5% 5% 4 7% 8% 40%	1900
	11th to 20th	Day	8% 4 1 5% 4 4 4 4 7% 10% 49%	1953
	21st to end	Day	5% 4 2 6% 5% 6% 6% 5% 7% 9% 45%	1727
March	1st to 10th	Day	6% 6% 3 8% 7% 7% 7% 6% 8% 10% 32%	3435
	11th to 20th	Day	4 3 1 5% 4 4 5% 5% 8% 9% 51%	3480
	21st to end	Day	10% 5% 2 6% 5% 4 4 4 6% 9% 46%	3815
April	1st to 10th	Day	4 3 2 5% 5% 6% 6% 6% 7% 12% 44%	2090
	11th to 20th	Day	6% 5% 3 8% 6% 6% 6% 5% 6% 9% 40%	2149
	21st to end	Day	7% 4 2 6% 6% 5% 6% 5% 6% 10% 43%	2280
May	1st to 10th	Day	3 2 1 4 5% 5% 5% 4 7% 9% 55%	3765
	11th to 20th	Day	6% 6% 2 7% 6% 6% 6% 6% 8% 10% 36%	3823
	21st to end	Day	6% 5% 2 7% 6% 6% 5% 6% 7% 9% 42%	4476
June	1st to 10th	Day	13% 10% 4 12% 9% 9% 7% 5% 7% 7% 16%	2470
	11th to 20th	Day	8% 8% 4 12% 9% 9% 8% 7% 8% 9% 19%	2470
	21st to end	Day	8% 8% 3 11% 10% 9% 9% 7% 9% 9% 19%	2470
July	1st to 10th	Day	6% 7% 3 10% 7% 8% 7% 7% 10% 12% 23%	4120
	11th to 20th	Day	5% 6% 3 9% 8% 8% 8% 7% 10% 12% 22%	4120
	21st to end	Day	5% 5% 3 8% 8% 7% 8% 6% 11% 14% 26%	4466
August	1st to 10th	Day	5% 5% 3 10% 8% 9% 10% 8% 10% 13% 19%	4042
	11th to 20th	Day	5% 4 2 8% 7% 9% 8% 8% 12% 14% 22%	3730
	21st to end	Day	5% 5% 2 9% 8% 9% 10% 8% 12% 14% 18%	3867
September	1st to 10th	Day	7% 6% 4 11% 10% 10% 9% 6% 8% 9% 19%	2090
	11th to 20th	Day	9% 7% 3 12% 8% 8% 8% 6% 9% 9% 20%	2090
	21st to end	Day	10% 9% 4 13% 8% 8% 8% 6% 7% 7% 19%	2090
October	1st to 10th	Day	9% 6% 3 11% 7% 8% 8% 7% 9% 9% 22%	2005
	11th to 20th	Day	9% 7% 3 9% 7% 7% 8% 7% 8% 8% 27%	1900
	21st to end	Day	12% 6% 2 8% 5% 6% 5% 4 7% 9% 35%	2090
November	1st to 10th	Day	7% 5% 3 6% 5% 6% 6% 6% 8% 9% 39%	1947
	11th to 20th	Day	9% 8% 2 8% 8% 7% 5% 5% 7% 8% 33%	1900
	21st to end	Day	8% 6% 2 6% 5% 6% 6% 6% 7% 9% 39%	1900
December	1st to 10th	Day	7% 5% 2 8% 5% 6% 6% 4 7% 7% 42%	1858
	11th to 20th	Day	9% 6% 3 6% 5% 6% 4 4 6% 8% 42%	2153
	21st to end	Day	12% 6% 2 8% 6% 5% 7% 6% 8% 9% 31%	3124
January	1st to 10th	Night	7% 6% 2 8% 6% 5% 5% 5% 7% 9% 39%	4109
	11th to 20th	Night	16% 10% 4 8% 5% 6% 5% 4 6% 8% 28%	4120
	21st to end	Night	6% 7% 3 9% 6% 5% 6% 5% 7% 7% 39%	4464
February	1st to 10th	Night	8% 6% 2 6% 4 5% 4 4 7% 9% 46%	2280
	11th to 20th	Night	6% 4 2 4 3 3 5% 4 6% 7% 56%	2111
	21st to end	Night	5% 2 1 4 4 4 4 4 5% 8% 59%	1727
March	1st to 10th	Night	6% 6% 2 8% 5% 5% 6% 5% 7% 9% 41%	3418
	11th to 20th	Night	4 3 1 5% 3 4 4 4 6% 11% 54%	3480
	21st to end	Night	6% 5% 1 4 4 3 4 4 6% 8% 54%	3817
April	1st to 10th	Night	4 2 1 4 3 4 4 3 7% 10% 58%	2090
	11th to 20th	Night	5% 3 1 5% 3 4 5% 5% 7% 9% 52%	1979
	21st to end	Night	6% 4 2 4 4 4 5% 4 6% 8% 53%	1900
May	1st to 10th	Night	2 2 1 3 3 3 3 3 5% 9% 65%	3130
	11th to 20th	Night	5% 4 2 5% 4 4 5% 5% 7% 11% 48%	3082
	21st to end	Night	5% 5% 2 5% 4 4 5% 5% 8% 10% 46%	3079
June	1st to 10th	Night	11% 7% 3 9% 7% 7% 7% 6% 7% 11% 25%	1710
	11th to 20th	Night	6% 7% 2 6% 6% 5% 6% 6% 9% 13% 33%	1710
	21st to end	Night	5% 5% 3 7% 7% 6% 8% 6% 10% 13% 31%	1710
July	1st to 10th	Night	4 5% 2 8% 5% 6% 6% 5% 9% 12% 38%	2840
	11th to 20th	Night	4 5% 2 7% 4 6% 7% 6% 10% 13% 36%	2840
	21st to end	Night	3 4 2 7% 5% 4 5% 6% 10% 14% 41%	3087
August	1st to 10th	Night	3 4 2 7% 6% 7% 7% 6% 9% 14% 35%	2852
	11th to 20th	Night	3 4 2 7% 5% 6% 7% 6% 10% 15% 35%	3097
	21st to end	Night	4 4 2 7% 5% 7% 8% 8% 12% 16% 27%	3782
September	1st to 10th	Night	5% 5% 3 7% 7% 7% 8% 6% 11% 12% 29%	2090
	11th to 20th	Night	7% 7% 3 9% 6% 6% 6% 5% 9% 11% 29%	2090
	21st to end	Night	8% 7% 3 9% 7% 7% 8% 6% 9% 10% 27%	2090
October	1st to 10th	Night	8% 6% 3 9% 7% 6% 7% 6% 9% 11% 29%	2175
	11th to 20th	Night	8% 6% 2 7% 4 5% 5% 5% 8% 10% 40%	2280
	21st to end	Night	10% 6% 2 6% 5% 5% 5% 4 6% 8% 43%	2508
November	1st to 10th	Night	8% 4 2 5% 5% 4 4 4 6% 8% 49%	2336
	11th to 20th	Night	7% 6% 3 6% 6% 6% 6% 5% 6% 8% 41%	2280
	21st to end	Night	6% 6% 2 6% 4 4 5% 4 6% 9% 45%	2280
December	1st to 10th	Night	6% 4 2 6% 5% 4 5% 4 6% 9% 50%	2318
	11th to 20th	Night	11% 5% 2 6% 4 3 4 3 5% 8% 48%	3042
	21st to end	Night	9% 8% 3 8% 6% 6% 6% 5% 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	41% 5% 10% 24% 16% 5%	2837
	11th to 20th	Day	53% 6% 14% 15% 9% 3	2840
	21st to end	Day	43% 5% 9% 21% 16% 6%	3188
February	1st to 10th	Day	40% 5% 10% 24% 16% 5%	1900
	11th to 20th	Day	29% 5% 9% 28% 22% 7%	1963
	21st to end	Day	33% 6% 10% 19% 24% 9%	1727
March	1st to 10th	Day	41% 9% 14% 21% 12% 3	3435
	11th to 20th	Day	26% 6% 17% 36% 12% 3	3480
	21st to end	Day	34% 5% 8% 28% 19% 6%	3815
April	1st to 10th	Day	30% 6% 16% 29% 16% 3	2090
	11th to 20th	Day	38% 6% 16% 21% 16% 3	2149
	21st to end	Day	35% 7% 10% 24% 20% 4	2280
May	1st to 10th	Day	21% 6% 14% 33% 22% 4	3800
	11th to 20th	Day	37% 8% 21% 28% 6%	3827
	21st to end	Day	35% 7% 15% 30% 12%	4489
June	1st to 10th	Day	62% 9% 15% 11% 3	2470
	11th to 20th	Day	56% 12% 18% 12% 1	2470
	21st to end	Day	54% 13% 23% 9% 1	2470
July	1st to 10th	Day	44% 13% 29% 14%	4055
	11th to 20th	Day	46% 12% 31% 10%	4051
	21st to end	Day	39% 14% 32% 15%	4324
August	1st to 10th	Day	49% 16% 24% 10% 1	4029
	11th to 20th	Day	42% 15% 29% 14%	3766
	21st to end	Day	46% 16% 32% 6% 1	3867
September	1st to 10th	Day	54% 15% 19% 11% 1	2090
	11th to 20th	Day	54% 12% 21% 9% 3	2090
	21st to end	Day	60% 10% 14% 11% 4 1	2090
October	1st to 10th	Day	52% 11% 17% 15% 5% 1	3330
	11th to 20th	Day	49% 8% 14% 17% 11% 1	3140
	21st to end	Day	39% 5% 10% 26% 17% 3	3428
November	1st to 10th	Day	38% 5% 12% 20% 18% 7%	1947
	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	40% 5% 14% 23% 15% 3	4109
	11th to 20th	Night	54% 5% 13% 16% 9% 2	4120
	21st to end	Night	41% 6% 10% 22% 15% 6%	4464
February	1st to 10th	Night	32% 5% 13% 29% 17% 4	2280
	11th to 20th	Night	25% 6% 11% 28% 23% 7%	2112
	21st to end	Night	23% 6% 11% 26% 27% 7%	1727
March	1st to 10th	Night	37% 8% 15% 22% 14% 4	3418
	11th to 20th	Night	23% 6% 26% 31% 12% 3	3480
	21st to end	Night	26% 6% 13% 26% 25% 4	3817
April	1st to 10th	Night	20% 6% 18% 37% 16% 3	2090
	11th to 20th	Night	26% 6% 19% 26% 20% 3	1979
	21st to end	Night	26% 6% 15% 29% 20% 4	1900
May	1st to 10th	Night	16% 5% 20% 41% 15% 3	3160
	11th to 20th	Night	27% 8% 23% 32% 9%	3083
	21st to end	Night	28% 9% 21% 34% 9%	3087
June	1st to 10th	Night	50% 11% 22% 14% 2	1710
	11th to 20th	Night	39% 13% 28% 18% 3	1710
	21st to end	Night	38% 15% 35% 11% 1	1710
July	1st to 10th	Night	36% 13% 34% 17%	2795
	11th to 20th	Night	35% 13% 41% 11%	2786
	21st to end	Night	29% 14% 39% 16% 1	2988
August	1st to 10th	Night	37% 14% 30% 16% 2	2841
	11th to 20th	Night	31% 16% 34% 18%	3127
	21st to end	Night	37% 17% 39% 6% 1	3782
September	1st to 10th	Night	41% 17% 28% 13% 1	2090
	11th to 20th	Night	45% 15% 26% 12% 2	2090
	21st to end	Night	48% 14% 18% 15% 5%	2090
October	1st to 10th	Night	49% 11% 18% 17% 4 1	3630
	11th to 20th	Night	45% 7% 18% 18% 12% 1	3779
	21st to end	Night	37% 5% 12% 29% 15% 3	4120
November	1st to 10th	Night	32% 5% 14% 23% 19% 7%	2336
	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% 32% 36% 12% 2	270
	11th to 20th	Day	12% 7% 33% 31% 13% 3	270
	21st to end	Day	11% 6% 46% 27% 8% 1	297
February	1st to 10th	Day	10% 7% 28% 39% 13% 3	270
	11th to 20th	Day	8% 3 26% 38% 19% 6%	376
	21st to end	Day	5% 5% 22% 37% 27% 3	382
March	1st to 10th	Day	4 7% 30% 35% 21% 3	460
	11th to 20th	Day	7% 5% 27% 37% 21% 3	460
	21st to end	Day	6% 3 27% 36% 22% 6%	506
April	1st to 10th	Day	3 5% 31% 31% 23% 6%	460
	11th to 20th	Day	8% 4 28% 32% 21% 8%	452
	21st to end	Day	7% 3 22% 34% 26% 9%	460
May	1st to 10th	Day	4 1 18% 33% 29% 15%	463
	11th to 20th	Day	9% 2 17% 32% 27% 14%	487
	21st to end	Day	11% 3 6% 27% 27% 25%	529
June	1st to 10th	Day	16% 3 7% 24% 28% 22%	490
	11th to 20th	Day	10% 3 4 20% 28% 34%	490
	21st to end	Day	6% 2 1 17% 25% 49%	490
July	1st to 10th	Day	5% 2 1 13% 29% 50%	485
	11th to 20th	Day	6% 2 1 14% 31% 46%	484
	21st to end	Day	4 3 2 15% 28% 47%	532
August	1st to 10th	Day	8% 2 3 15% 29% 44%	486
	11th to 20th	Day	6% 2 3 15% 28% 46%	479
	21st to end	Day	9% 3 3 22% 29% 35%	539
September	1st to 10th	Day	8% 3 3 14% 29% 43%	490
	11th to 20th	Day	7% 2 5% 15% 27% 45%	490
	21st to end	Day	12% 3 6% 21% 27% 32%	490
October	1st to 10th	Day	12% 3 8% 34% 30% 13%	490
	11th to 20th	Day	12% 1 15% 30% 28% 14%	490
	21st to end	Day	11% 3 17% 35% 26% 9%	539
November	1st to 10th	Day	9% 4 20% 35% 24% 8%	491
	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	10% 5% 24% 41% 19% 1	760
	21st to end	Night	8% 5% 28% 43% 15% 1	836
February	1st to 10th	Night	10% 5% 22% 42% 19% 2	760
	11th to 20th	Night	7% 4 24% 43% 22% 1	743
	21st to end	Night	7% 3 20% 48% 21% 1	628
March	1st to 10th	Night	5% 3 27% 44% 19% 1	760
	11th to 20th	Night	7% 3 20% 46% 22% 1	760
	21st to end	Night	7% 3 23% 43% 22% 2	836
April	1st to 10th	Night	4 2 18% 50% 24% 2	760
	11th to 20th	Night	6% 3 21% 46% 22% 2	751
	21st to end	Night	8% 2 17% 46% 25% 3	760
May	1st to 10th	Night	3 2 14% 45% 33% 3	760
	11th to 20th	Night	9% 2 11% 42% 32% 3	727
	21st to end	Night	8% 2 4 30% 45% 10%	617
June	1st to 10th	Night	11% 2 3 22% 46% 15%	570
	11th to 20th	Night	7% 2 2 14% 45% 30%	570
	21st to end	Night	5% 2 2 10% 44% 38%	570
July	1st to 10th	Night	5% 1 1 8% 42% 43%	555
	11th to 20th	Night	5% 2 8% 44% 41%	552
	21st to end	Night	2 1 6% 44% 46%	624
August	1st to 10th	Night	6% 2 1 9% 46% 37%	581
	11th to 20th	Night	3 2 2 10% 44% 39%	750
	21st to end	Night	9% 2 2 18% 41% 28%	836
September	1st to 10th	Night	6% 2 4 12% 41% 35%	760
	11th to 20th	Night	10% 3 2 14% 43% 29%	760
	21st to end	Night	12% 2 4 18% 41% 24%	760
October	1st to 10th	Night	11% 2 6% 33% 37% 11%	760
	11th to 20th	Night	11% 3 6% 32% 39% 9%	760
	21st to end	Night	13% 2 13% 35% 29% 7%	836
November	1st to 10th	Night	10% 3 18% 36% 30% 4	779
	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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