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 2025-12-03.

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% 5% 2 8% 6% 6% 7% 6% 8% 10% 35%	126356
	Night	6% 5% 2 6% 5% 5% 5% 5% 7% 10% 43%	124335
Transparency	Day	40% 9% 17% 21% 11% 3	130022
	Night	35% 8% 20% 23% 12% 3	128738
Seeing	Day	8% 3 16% 28% 25% 20%	17485
	Night	8% 3 13% 32% 32% 12%	27585

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 6% 3 4 5% 3 7% 9% 49%	4056
	11th to 20th	Day	16% 7% 3 8% 6% 5% 6% 4 7% 9% 29%	4060
	21st to end	Day	11% 6% 3 8% 5% 5% 5% 4 6% 7% 40%	4558
February	1st to 10th	Day	9% 7% 2 6% 6% 5% 5% 4 7% 8% 41%	2000
	11th to 20th	Day	7% 4 1 5% 4 4 4 4 7% 10% 50%	2059
	21st to end	Day	5% 4 2 6% 5% 6% 6% 5% 7% 9% 44%	1815
March	1st to 10th	Day	5% 4 3 6% 6% 6% 7% 5% 7% 9% 42%	4935
	11th to 20th	Day	3 2 1 4 4 3 5% 4 7% 8% 59%	4980
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 9% 42%	5465
April	1st to 10th	Day	4 3 2 5% 5% 6% 6% 6% 7% 11% 45%	2200
	11th to 20th	Day	5% 5% 2 8% 6% 6% 6% 5% 6% 9% 42%	2264
	21st to end	Day	6% 4 2 6% 5% 5% 6% 5% 7% 11% 44%	2400
May	1st to 10th	Day	3 2 1 4 4 5% 5% 4 8% 11% 53%	5405
	11th to 20th	Day	6% 5% 2 6% 5% 5% 5% 5% 7% 9% 45%	5491
	21st to end	Day	4 4 2 7% 6% 6% 7% 7% 9% 11% 38%	6433
June	1st to 10th	Day	13% 10% 4 12% 9% 8% 8% 5% 7% 7% 16%	2600
	11th to 20th	Day	8% 8% 4 11% 8% 9% 8% 7% 8% 10% 20%	2600
	21st to end	Day	8% 8% 3 11% 10% 9% 9% 7% 9% 9% 19%	2600
July	1st to 10th	Day	5% 7% 4 12% 8% 8% 8% 7% 10% 12% 21%	5900
	11th to 20th	Day	5% 6% 3 10% 8% 8% 8% 7% 10% 12% 24%	5900
	21st to end	Day	4 4 2 7% 6% 6% 7% 6% 10% 15% 33%	6424
August	1st to 10th	Day	5% 5% 3 10% 8% 9% 10% 8% 10% 13% 19%	4170
	11th to 20th	Day	4 4 2 8% 7% 9% 8% 8% 12% 14% 22%	3850
	21st to end	Day	5% 5% 2 9% 8% 9% 10% 8% 12% 14% 19%	3989
September	1st to 10th	Day	7% 6% 4 11% 10% 10% 9% 6% 8% 10% 19%	2200
	11th to 20th	Day	9% 7% 3 12% 8% 8% 8% 6% 9% 9% 20%	2200
	21st to end	Day	10% 8% 4 13% 8% 8% 8% 6% 7% 8% 19%	2200
October	1st to 10th	Day	9% 6% 3 11% 8% 8% 8% 7% 9% 9% 23%	2110
	11th to 20th	Day	9% 6% 3 8% 7% 7% 8% 7% 8% 8% 28%	2000
	21st to end	Day	11% 6% 2 8% 5% 6% 5% 4 7% 9% 36%	2200
November	1st to 10th	Day	7% 5% 3 7% 5% 6% 6% 6% 8% 9% 38%	2057
	11th to 20th	Day	9% 7% 2 8% 7% 6% 5% 5% 7% 8% 35%	2100
	21st to end	Day	7% 5% 2 6% 5% 6% 6% 6% 7% 9% 41%	2100
December	1st to 10th	Day	5% 4 2 5% 4 4 5% 4 7% 6% 55%	3196
	11th to 20th	Day	9% 5% 2 6% 5% 6% 5% 5% 7% 10% 39%	3373
	21st to end	Day	11% 6% 3 8% 7% 6% 7% 6% 8% 10% 30%	4466
January	1st to 10th	Night	6% 5% 2 7% 5% 4 5% 4 7% 10% 45%	5889
	11th to 20th	Night	15% 10% 4 9% 6% 5% 4 4 5% 7% 31%	5900
	21st to end	Night	11% 7% 3 7% 5% 4 5% 4 5% 7% 43%	6380
February	1st to 10th	Night	8% 5% 2 6% 4 5% 4 4 7% 9% 47%	2400
	11th to 20th	Night	6% 4 2 4 4 3 5% 4 6% 7% 56%	2225
	21st to end	Night	5% 3 1 4 4 4 4 3 5% 8% 59%	1815
March	1st to 10th	Night	4 4 2 6% 4 4 5% 4 6% 9% 51%	4918
	11th to 20th	Night	4 3 1 5% 3 3 4 4 5% 9% 59%	4980
	21st to end	Night	6% 4 2 6% 5% 4 5% 4 6% 8% 49%	5467
April	1st to 10th	Night	4 2 1 4 3 4 4 3 7% 10% 58%	2200
	11th to 20th	Night	5% 3 1 5% 3 4 5% 4 7% 9% 54%	2084
	21st to end	Night	5% 4 2 4 4 4 5% 4 7% 8% 53%	2000
May	1st to 10th	Night	3 2 1 3 3 3 3 4 6% 11% 60%	4490
	11th to 20th	Night	6% 4 2 5% 4 4 5% 4 6% 10% 51%	4428
	21st to end	Night	4 4 2 6% 4 5% 6% 5% 9% 11% 43%	4421
June	1st to 10th	Night	11% 7% 3 9% 7% 7% 7% 6% 7% 11% 26%	1800
	11th to 20th	Night	6% 7% 2 6% 6% 5% 6% 6% 8% 14% 34%	1800
	21st to end	Night	5% 5% 3 7% 7% 6% 8% 6% 10% 13% 32%	1800
July	1st to 10th	Night	5% 5% 3 8% 7% 6% 6% 5% 9% 12% 34%	4058
	11th to 20th	Night	4 5% 2 7% 4 6% 6% 6% 9% 13% 38%	4057
	21st to end	Night	2 4 2 5% 4 4 4 5% 8% 13% 49%	4427
August	1st to 10th	Night	3 5% 2 7% 6% 7% 6% 6% 9% 14% 35%	2943
	11th to 20th	Night	3 4 2 6% 5% 6% 7% 6% 10% 14% 36%	3197
	21st to end	Night	4 4 2 7% 5% 7% 8% 8% 12% 16% 27%	3902
September	1st to 10th	Night	6% 5% 3 7% 7% 7% 8% 6% 11% 12% 28%	2200
	11th to 20th	Night	7% 7% 3 9% 7% 7% 7% 5% 9% 11% 29%	2200
	21st to end	Night	8% 7% 3 9% 7% 7% 7% 6% 9% 10% 27%	2200
October	1st to 10th	Night	8% 6% 3 9% 7% 6% 7% 6% 9% 11% 29%	2290
	11th to 20th	Night	8% 6% 2 7% 4 5% 5% 5% 7% 10% 40%	2400
	21st to end	Night	10% 6% 2 6% 5% 5% 5% 4 6% 8% 44%	2640
November	1st to 10th	Night	7% 5% 2 6% 5% 4 4 4 6% 8% 48%	2461
	11th to 20th	Night	7% 6% 3 6% 6% 5% 5% 5% 6% 8% 43%	2520
	21st to end	Night	6% 6% 2 6% 4 4 5% 4 6% 9% 48%	2520
December	1st to 10th	Night	5% 4 2 5% 3 3 4 3 5% 8% 57%	4011
	11th to 20th	Night	12% 7% 2 7% 5% 4 5% 4 6% 8% 40%	4822
	21st to end	Night	8% 8% 3 10% 6% 6% 6% 5% 7% 9% 31%	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 9% 30% 20% 7%	4057
	11th to 20th	Day	51% 5% 14% 19% 8% 2	4060
	21st to end	Day	42% 4 6% 28% 15% 4	4558
February	1st to 10th	Day	39% 5% 10% 24% 16% 5%	2000
	11th to 20th	Day	29% 5% 10% 28% 22% 6%	2069
	21st to end	Day	33% 6% 10% 19% 24% 8%	1815
March	1st to 10th	Day	35% 8% 12% 19% 22% 4	4935
	11th to 20th	Day	21% 4 13% 29% 20% 12%	4980
	21st to end	Day	35% 6% 9% 28% 17% 6%	5465
April	1st to 10th	Day	29% 5% 16% 28% 18% 4	2200
	11th to 20th	Day	36% 6% 16% 21% 17% 5%	2264
	21st to end	Day	33% 6% 10% 25% 22% 4	2400
May	1st to 10th	Day	21% 6% 17% 34% 19% 3	5437
	11th to 20th	Day	33% 6% 18% 35% 7% 2	5495
	21st to end	Day	34% 9% 18% 29% 9%	6447
June	1st to 10th	Day	62% 9% 16% 11% 3	2600
	11th to 20th	Day	54% 12% 19% 13% 2	2600
	21st to end	Day	54% 13% 23% 10% 1	2600
July	1st to 10th	Day	47% 13% 29% 11%	5835
	11th to 20th	Day	46% 12% 28% 13% 1	5831
	21st to end	Day	32% 12% 32% 21% 2	6240
August	1st to 10th	Day	48% 16% 25% 11% 1	4157
	11th to 20th	Day	41% 15% 29% 14% 1	3886
	21st to end	Day	46% 16% 32% 6% 1	3989
September	1st to 10th	Day	54% 15% 19% 11% 1	2200
	11th to 20th	Day	54% 12% 21% 10% 2	2200
	21st to end	Day	59% 11% 15% 11% 5% 1	2200
October	1st to 10th	Day	51% 11% 17% 15% 5% 1	3435
	11th to 20th	Day	48% 8% 15% 17% 12% 1	3240
	21st to end	Day	38% 5% 10% 27% 17% 4	3538
November	1st to 10th	Day	38% 6% 12% 20% 18% 7%	2057
	11th to 20th	Day	44% 6% 9% 18% 17% 6%	2100
	21st to end	Day	37% 6% 11% 20% 22% 4	2100
December	1st to 10th	Day	27% 5% 12% 29% 22% 5%	3196
	11th to 20th	Day	37% 6% 12% 21% 20% 4	3370
	21st to end	Day	46% 6% 15% 17% 14% 2	4466
January	1st to 10th	Night	34% 4 13% 28% 18% 3	5889
	11th to 20th	Night	52% 5% 17% 16% 9% 2	5900
	21st to end	Night	40% 5% 8% 29% 14% 4	6380
February	1st to 10th	Night	32% 5% 13% 28% 18% 4	2400
	11th to 20th	Night	25% 6% 11% 28% 23% 7%	2226
	21st to end	Night	24% 5% 11% 26% 27% 7%	1815
March	1st to 10th	Night	29% 6% 14% 22% 25% 5%	4918
	11th to 20th	Night	22% 5% 19% 27% 18% 10%	4980
	21st to end	Night	30% 6% 12% 27% 21% 3	5467
April	1st to 10th	Night	20% 5% 18% 36% 18% 4	2200
	11th to 20th	Night	25% 6% 18% 24% 21% 5%	2084
	21st to end	Night	25% 6% 15% 30% 21% 4	2000
May	1st to 10th	Night	16% 6% 25% 38% 13% 2	4520
	11th to 20th	Night	27% 7% 19% 35% 12% 1	4429
	21st to end	Night	29% 10% 26% 29% 6%	4429
June	1st to 10th	Night	50% 11% 22% 15% 2	1800
	11th to 20th	Night	38% 12% 29% 18% 3	1800
	21st to end	Night	38% 15% 35% 11% 1	1800
July	1st to 10th	Night	40% 13% 35% 12%	4015
	11th to 20th	Night	35% 13% 39% 13%	4006
	21st to end	Night	24% 11% 40% 22% 3	4297
August	1st to 10th	Night	37% 14% 30% 17% 2	2932
	11th to 20th	Night	31% 16% 34% 19%	3227
	21st to end	Night	37% 17% 38% 6% 1	3902
September	1st to 10th	Night	42% 18% 27% 12% 1	2200
	11th to 20th	Night	45% 15% 25% 12% 2	2200
	21st to end	Night	48% 14% 18% 14% 5%	2200
October	1st to 10th	Night	49% 11% 18% 17% 4 1	3745
	11th to 20th	Night	45% 7% 18% 17% 12% 1	3899
	21st to end	Night	36% 5% 12% 29% 15% 4	4252
November	1st to 10th	Night	33% 5% 14% 23% 18% 7%	2461
	11th to 20th	Night	38% 5% 12% 24% 16% 5%	2520
	21st to end	Night	33% 6% 14% 23% 20% 5%	2520
December	1st to 10th	Night	26% 5% 14% 33% 18% 4	4011
	11th to 20th	Night	40% 6% 11% 21% 16% 5%	4824
	21st to end	Night	46% 8% 15% 17% 13% 2	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	10% 7% 35% 34% 12% 2	300
	11th to 20th	Day	14% 7% 35% 29% 12% 2	300
	21st to end	Day	13% 6% 43% 29% 8% 1	330
February	1st to 10th	Day	9% 6% 26% 39% 15% 5%	300
	11th to 20th	Day	7% 3 27% 39% 18% 5%	412
	21st to end	Day	6% 5% 24% 37% 26% 2	414
March	1st to 10th	Day	5% 7% 31% 35% 20% 2	500
	11th to 20th	Day	6% 6% 28% 37% 20% 3	500
	21st to end	Day	6% 3 29% 35% 22% 5%	550
April	1st to 10th	Day	3 5% 31% 32% 22% 6%	500
	11th to 20th	Day	8% 4 28% 32% 21% 8%	492
	21st to end	Day	7% 3 21% 34% 26% 9%	500
May	1st to 10th	Day	4 1 19% 34% 29% 14%	503
	11th to 20th	Day	8% 2 17% 31% 27% 14%	527
	21st to end	Day	10% 3 7% 28% 28% 24%	573
June	1st to 10th	Day	15% 3 8% 24% 28% 22%	530
	11th to 20th	Day	10% 3 4 22% 29% 33%	530
	21st to end	Day	6% 2 2 18% 25% 47%	530
July	1st to 10th	Day	5% 2 2 13% 29% 48%	525
	11th to 20th	Day	6% 2 2 15% 31% 45%	524
	21st to end	Day	4 3 3 17% 28% 46%	576
August	1st to 10th	Day	7% 2 3 15% 29% 45%	526
	11th to 20th	Day	6% 2 3 16% 29% 45%	519
	21st to end	Day	9% 2 3 21% 30% 35%	583
September	1st to 10th	Day	9% 2 3 14% 28% 43%	530
	11th to 20th	Day	7% 2 5% 16% 28% 43%	530
	21st to end	Day	11% 3 6% 22% 27% 31%	530
October	1st to 10th	Day	11% 3 8% 34% 30% 13%	530
	11th to 20th	Day	12% 1 15% 31% 29% 13%	530
	21st to end	Day	10% 3 18% 35% 25% 9%	583
November	1st to 10th	Day	9% 4 21% 35% 24% 7%	535
	11th to 20th	Day	12% 3 21% 32% 26% 7%	540
	21st to end	Day	6% 3 24% 35% 29% 4	540
December	1st to 10th	Day	10% 4 19% 39% 23% 5%	463
	11th to 20th	Day	8% 5% 25% 39% 18% 5%	300
	21st to end	Day	11% 6% 29% 34% 15% 6%	330
January	1st to 10th	Night	9% 5% 21% 41% 23% 2	800
	11th to 20th	Night	10% 5% 25% 40% 19% 1	800
	21st to end	Night	9% 5% 27% 43% 15% 1	880
February	1st to 10th	Night	10% 5% 21% 41% 22% 2	800
	11th to 20th	Night	7% 4 23% 42% 22% 2	783
	21st to end	Night	7% 3 21% 48% 20% 1	660
March	1st to 10th	Night	5% 4 28% 44% 18% 1	800
	11th to 20th	Night	7% 4 21% 45% 21% 1	800
	21st to end	Night	7% 3 24% 43% 21% 2	880
April	1st to 10th	Night	4 2 18% 51% 24% 2	800
	11th to 20th	Night	6% 3 21% 46% 22% 2	791
	21st to end	Night	7% 2 17% 46% 27% 2	800
May	1st to 10th	Night	3 2 14% 46% 32% 3	800
	11th to 20th	Night	9% 2 11% 42% 32% 3	766
	21st to end	Night	8% 2 4 30% 46% 10%	650
June	1st to 10th	Night	11% 2 3 22% 46% 15%	600
	11th to 20th	Night	7% 2 2 13% 46% 30%	600
	21st to end	Night	5% 2 2 10% 45% 37%	600
July	1st to 10th	Night	6% 1 1 8% 43% 42%	585
	11th to 20th	Night	5% 2 8% 45% 40%	582
	21st to end	Night	2 1 6% 45% 45%	657
August	1st to 10th	Night	6% 2 1 8% 46% 37%	612
	11th to 20th	Night	4 2 2 10% 44% 38%	790
	21st to end	Night	9% 2 2 18% 41% 29%	880
September	1st to 10th	Night	7% 2 4 11% 41% 34%	800
	11th to 20th	Night	10% 2 3 15% 43% 28%	800
	21st to end	Night	12% 2 4 19% 41% 23%	800
October	1st to 10th	Night	11% 2 7% 33% 38% 10%	800
	11th to 20th	Night	11% 3 6% 32% 40% 9%	800
	21st to end	Night	13% 2 13% 36% 29% 7%	880
November	1st to 10th	Night	10% 3 19% 36% 29% 4	820
	11th to 20th	Night	9% 2 16% 37% 31% 5%	840
	21st to end	Night	7% 3 16% 43% 28% 3	840
December	1st to 10th	Night	7% 3 14% 40% 32% 3	809
	11th to 20th	Night	10% 4 17% 38% 28% 3	800
	21st to end	Night	8% 2 21% 39% 27% 3	880

Page generated at 2025-11-30 01:00:55. Version ac07

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