Westport
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Westport clear sky chart from 2005-11-06 to 2026-06-16.

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	14% 9% 3 7% 5% 4 5% 4 6% 7% 36%	137032
	Night	15% 10% 3 8% 5% 4 4 3 5% 7% 37%	135147
Transparency	Day	46% 5% 11% 23% 14% 2	140661
	Night	46% 5% 10% 22% 15% 2	139593
Seeing	Day	18% 11% 27% 23% 15% 6%	17607
	Night	18% 8% 28% 29% 14% 2	27420

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% 13% 6% 9% 5% 4 4 3 5% 7% 25%	4691
	11th to 20th	Day	11% 14% 4 15% 6% 5% 5% 3 3 4 29%	4710
	21st to end	Day	19% 13% 6% 10% 7% 5% 6% 4 4 5% 21%	5699
February	1st to 10th	Day	19% 13% 5% 9% 5% 3 4 3 4 7% 29%	2084
	11th to 20th	Day	17% 11% 4 10% 6% 4 4 3 5% 5% 32%	2048
	21st to end	Day	18% 12% 5% 8% 5% 3 3 3 5% 6% 32%	1730
March	1st to 10th	Day	20% 13% 5% 7% 4 3 3 2 4 4 34%	4580
	11th to 20th	Day	13% 9% 2 6% 5% 4 4 4 6% 7% 39%	4991
	21st to end	Day	17% 11% 3 7% 5% 4 5% 4 5% 7% 33%	5713
April	1st to 10th	Day	15% 10% 3 7% 5% 4 4 4 6% 7% 35%	2520
	11th to 20th	Day	17% 8% 3 6% 5% 4 4 3 5% 7% 38%	2494
	21st to end	Day	18% 7% 2 7% 5% 5% 4 4 5% 7% 35%	2719
May	1st to 10th	Day	18% 8% 3 8% 5% 4 5% 3 6% 8% 30%	7616
	11th to 20th	Day	18% 8% 3 7% 4 4 4 3 5% 7% 38%	7626
	21st to end	Day	16% 7% 3 8% 6% 6% 6% 5% 7% 9% 27%	8391
June	1st to 10th	Day	12% 6% 2 6% 5% 4 5% 4 6% 7% 43%	2880
	11th to 20th	Day	9% 6% 2 6% 5% 5% 4 4 7% 8% 44%	2871
	21st to end	Day	10% 6% 2 6% 5% 4 5% 4 6% 9% 42%	2800
July	1st to 10th	Day	11% 6% 2 6% 4 4 5% 4 6% 10% 43%	6360
	11th to 20th	Day	9% 5% 2 7% 5% 5% 5% 4 8% 10% 39%	6360
	21st to end	Day	10% 8% 3 8% 6% 5% 6% 5% 8% 9% 32%	6498
August	1st to 10th	Day	8% 5% 2 5% 4 4 5% 4 7% 10% 47%	4234
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 5% 7% 11% 41%	4042
	21st to end	Day	5% 4 2 5% 3 4 4 4 6% 8% 56%	4312
September	1st to 10th	Day	12% 5% 2 6% 4 4 5% 4 5% 8% 47%	2333
	11th to 20th	Day	9% 5% 1 6% 4 5% 5% 3 6% 7% 48%	2200
	21st to end	Day	15% 5% 2 5% 4 3 4 3 5% 6% 48%	2200
October	1st to 10th	Day	10% 4 2 6% 4 4 4 3 4 6% 52%	2200
	11th to 20th	Day	12% 5% 2 6% 3 4 4 4 5% 6% 48%	2078
	21st to end	Day	19% 10% 3 7% 3 3 4 2 5% 6% 37%	1980
November	1st to 10th	Day	10% 6% 2 6% 4 4 3 3 4 7% 51%	1852
	11th to 20th	Day	15% 11% 3 7% 4 3 4 3 6% 6% 38%	1890
	21st to end	Day	19% 10% 4 9% 4 3 3 2 4 6% 36%	1890
December	1st to 10th	Day	17% 12% 3 7% 3 3 3 2 3 4 41%	3020
	11th to 20th	Day	17% 15% 5% 10% 5% 5% 5% 4 5% 5% 26%	3350
	21st to end	Day	25% 18% 5% 8% 4 4 4 3 5% 5% 19%	4070
January	1st to 10th	Night	16% 15% 6% 11% 6% 5% 4 3 5% 6% 24%	8232
	11th to 20th	Night	11% 16% 6% 14% 6% 5% 5% 3 4 5% 25%	8280
	21st to end	Night	18% 12% 4 8% 5% 4 4 3 4 6% 33%	8482
February	1st to 10th	Night	20% 11% 5% 9% 6% 4 4 4 5% 5% 28%	2535
	11th to 20th	Night	16% 12% 4 8% 6% 4 4 3 5% 7% 33%	2458
	21st to end	Night	20% 11% 5% 8% 4 3 4 3 4 6% 32%	2076
March	1st to 10th	Night	20% 12% 3 7% 4 3 3 2 4 5% 37%	5493
	11th to 20th	Night	13% 6% 3 7% 4 4 3 3 5% 7% 45%	5189
	21st to end	Night	17% 9% 3 8% 5% 5% 4 4 5% 6% 34%	5500
April	1st to 10th	Night	15% 9% 3 7% 4 4 4 4 6% 7% 38%	2100
	11th to 20th	Night	15% 9% 2 6% 4 4 5% 4 5% 7% 39%	2075
	21st to end	Night	17% 7% 3 6% 5% 4 4 3 7% 9% 37%	1899
May	1st to 10th	Night	19% 9% 3 7% 4 4 5% 3 5% 8% 33%	5237
	11th to 20th	Night	14% 7% 3 7% 5% 5% 4 4 6% 8% 38%	5234
	21st to end	Night	15% 8% 3 9% 5% 5% 5% 4 6% 9% 32%	5761
June	1st to 10th	Night	10% 5% 2 7% 4 4 4 5% 5% 8% 47%	1735
	11th to 20th	Night	9% 5% 3 5% 3 4 4 5% 8% 10% 45%	1640
	21st to end	Night	6% 4 2 5% 3 5% 4 4 8% 10% 50%	1600
July	1st to 10th	Night	9% 5% 2 6% 4 4 5% 4 5% 8% 48%	3618
	11th to 20th	Night	9% 5% 2 6% 5% 5% 4 4 7% 9% 44%	3617
	21st to end	Night	9% 6% 3 7% 4 4 5% 5% 8% 10% 39%	4340
August	1st to 10th	Night	9% 5% 2 6% 4 4 4 3 6% 10% 48%	2915
	11th to 20th	Night	7% 5% 3 5% 4 4 5% 4 6% 8% 48%	3003
	21st to end	Night	3 3 1 4 4 3 5% 4 6% 9% 58%	3586
September	1st to 10th	Night	12% 5% 2 5% 3 3 4 4 4 6% 53%	2046
	11th to 20th	Night	9% 3 1 5% 3 3 3 4 6% 8% 55%	2200
	21st to end	Night	15% 6% 2 5% 4 3 3 2 4 6% 50%	2200
October	1st to 10th	Night	9% 4 2 5% 4 3 3 3 4 7% 57%	2200
	11th to 20th	Night	14% 6% 2 6% 3 3 4 3 5% 6% 47%	2316
	21st to end	Night	17% 8% 2 7% 4 4 4 3 4 6% 40%	2860
November	1st to 10th	Night	13% 7% 2 6% 4 3 3 4 5% 6% 48%	2666
	11th to 20th	Night	15% 9% 3 6% 5% 3 4 3 5% 6% 42%	2730
	21st to end	Night	19% 12% 3 8% 5% 4 3 3 4 5% 33%	2730
December	1st to 10th	Night	18% 11% 3 8% 4 3 3 3 4 5% 38%	4380
	11th to 20th	Night	19% 14% 6% 9% 5% 4 4 3 4 5% 28%	5064
	21st to end	Night	26% 18% 6% 10% 5% 4 3 3 4 4 18%	7150

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	59% 3 11% 20% 7%	4692
	11th to 20th	Day	58% 3 9% 17% 11% 2	4710
	21st to end	Day	66% 2 8% 15% 6% 3	5699
February	1st to 10th	Day	56% 4 11% 19% 10% 1	2084
	11th to 20th	Day	55% 5% 9% 20% 11% 1	2058
	21st to end	Day	53% 4 11% 22% 9% 2	1730
March	1st to 10th	Day	54% 4 5% 18% 16% 3	4580
	11th to 20th	Day	42% 5% 9% 29% 14% 2	4991
	21st to end	Day	50% 5% 12% 19% 11% 1	5715
April	1st to 10th	Day	46% 7% 12% 22% 12% 1	2520
	11th to 20th	Day	46% 5% 11% 22% 13% 3	2494
	21st to end	Day	46% 7% 9% 23% 12% 2	2719
May	1st to 10th	Day	51% 5% 12% 20% 11%	7651
	11th to 20th	Day	46% 4 11% 24% 14%	7630
	21st to end	Day	51% 6% 12% 19% 11% 2	8405
June	1st to 10th	Day	39% 7% 11% 24% 16% 3	2880
	11th to 20th	Day	36% 7% 13% 23% 19% 2	2871
	21st to end	Day	38% 7% 15% 20% 17% 3	2800
July	1st to 10th	Day	39% 5% 10% 25% 20% 2	6291
	11th to 20th	Day	40% 5% 14% 26% 15% 1	6285
	21st to end	Day	47% 5% 14% 20% 13% 2	6313
August	1st to 10th	Day	32% 6% 14% 35% 12% 1	4221
	11th to 20th	Day	36% 5% 11% 32% 15% 1	4081
	21st to end	Day	26% 5% 13% 33% 18% 4	4311
September	1st to 10th	Day	35% 6% 11% 24% 18% 5%	2333
	11th to 20th	Day	33% 6% 11% 24% 22% 4	2200
	21st to end	Day	37% 5% 10% 24% 23% 2	2200
October	1st to 10th	Day	24% 3 7% 31% 34% 1	3590
	11th to 20th	Day	32% 3 8% 28% 27% 2	3358
	21st to end	Day	39% 2 8% 23% 25% 2	3180
November	1st to 10th	Day	34% 4 10% 29% 19% 3	1852
	11th to 20th	Day	47% 4 8% 22% 15% 4	1890
	21st to end	Day	51% 4 9% 22% 13% 1	1890
December	1st to 10th	Day	49% 2 9% 27% 13%	3020
	11th to 20th	Day	60% 4 8% 18% 8%	3347
	21st to end	Day	68% 4 10% 12% 6%	4070
January	1st to 10th	Night	62% 4 9% 17% 8%	8233
	11th to 20th	Night	63% 3 7% 12% 12% 3	8280
	21st to end	Night	55% 2 9% 22% 11% 1	8482
February	1st to 10th	Night	57% 5% 10% 19% 8% 1	2535
	11th to 20th	Night	52% 4 11% 20% 12% 1	2459
	21st to end	Night	54% 5% 10% 18% 11% 2	2076
March	1st to 10th	Night	51% 3 5% 25% 12% 3	5493
	11th to 20th	Night	38% 5% 10% 25% 20% 1	5189
	21st to end	Night	51% 5% 7% 26% 10% 1	5500
April	1st to 10th	Night	43% 7% 12% 24% 13% 2	2100
	11th to 20th	Night	43% 6% 10% 19% 17% 5%	2075
	21st to end	Night	43% 6% 10% 24% 14% 3	1899
May	1st to 10th	Night	50% 7% 10% 23% 9% 2	5264
	11th to 20th	Night	44% 6% 13% 23% 14%	5235
	21st to end	Night	48% 6% 14% 19% 10% 3	5771
June	1st to 10th	Night	34% 6% 12% 23% 21% 3	1735
	11th to 20th	Night	31% 9% 12% 23% 20% 5%	1640
	21st to end	Night	28% 9% 16% 23% 19% 5%	1600
July	1st to 10th	Night	35% 6% 10% 25% 20% 5%	3580
	11th to 20th	Night	36% 5% 15% 28% 15% 1	3572
	21st to end	Night	39% 4 16% 27% 12% 3	4212
August	1st to 10th	Night	32% 6% 13% 35% 13% 1	2904
	11th to 20th	Night	34% 4 12% 30% 18% 1	3030
	21st to end	Night	23% 5% 14% 32% 22% 3	3586
September	1st to 10th	Night	31% 7% 11% 25% 20% 7%	2046
	11th to 20th	Night	26% 5% 12% 25% 27% 4	2200
	21st to end	Night	36% 4 9% 22% 24% 3	2200
October	1st to 10th	Night	22% 4 6% 22% 43% 3	3590
	11th to 20th	Night	28% 3 8% 23% 35% 2	3775
	21st to end	Night	35% 3 7% 23% 26% 5%	4610
November	1st to 10th	Night	37% 5% 10% 23% 22% 3	2666
	11th to 20th	Night	43% 4 9% 21% 19% 4	2730
	21st to end	Night	54% 4 9% 20% 12% 1	2730
December	1st to 10th	Night	51% 3 10% 24% 12%	4380
	11th to 20th	Night	59% 4 10% 16% 10% 1	5066
	21st to end	Night	70% 5% 8% 12% 5%	7150

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	25% 17% 35% 16% 6%	330
	11th to 20th	Day	30% 21% 35% 12% 2	330
	21st to end	Day	29% 24% 32% 13% 2	363
February	1st to 10th	Day	26% 21% 36% 14% 3	330
	11th to 20th	Day	27% 24% 35% 12% 2	330
	21st to end	Day	29% 17% 44% 8% 3	273
March	1st to 10th	Day	25% 26% 35% 13% 2	330
	11th to 20th	Day	28% 18% 33% 16% 4 1	493
	21st to end	Day	29% 18% 32% 16% 4	594
April	1st to 10th	Day	23% 18% 32% 19% 7% 1	540
	11th to 20th	Day	20% 21% 27% 21% 9% 2	532
	21st to end	Day	23% 13% 37% 18% 6% 2	540
May	1st to 10th	Day	22% 15% 32% 19% 8% 3	543
	11th to 20th	Day	22% 10% 27% 24% 12% 5%	567
	21st to end	Day	16% 7% 26% 25% 16% 10%	617
June	1st to 10th	Day	13% 6% 27% 29% 20% 6%	720
	11th to 20th	Day	13% 6% 20% 33% 22% 7%	756
	21st to end	Day	12% 5% 22% 30% 23% 8%	730
July	1st to 10th	Day	12% 4 14% 27% 26% 17%	721
	11th to 20th	Day	8% 3 22% 32% 23% 12%	718
	21st to end	Day	11% 2 18% 29% 27% 12%	611
August	1st to 10th	Day	9% 3 17% 28% 25% 20%	526
	11th to 20th	Day	13% 3 18% 28% 27% 11%	519
	21st to end	Day	9% 2 15% 29% 29% 15%	583
September	1st to 10th	Day	10% 4 17% 33% 27% 9%	530
	11th to 20th	Day	9% 2 15% 28% 31% 15%	530
	21st to end	Day	18% 7% 20% 25% 23% 8%	530
October	1st to 10th	Day	11% 4 24% 33% 20% 7%	530
	11th to 20th	Day	13% 13% 30% 27% 14% 3	515
	21st to end	Day	27% 14% 34% 22% 2	363
November	1st to 10th	Day	14% 16% 35% 24% 8% 3	330
	11th to 20th	Day	23% 21% 29% 18% 7% 2	330
	21st to end	Day	29% 11% 36% 20% 4	330
December	1st to 10th	Day	22% 19% 39% 14% 5% 1	330
	11th to 20th	Day	26% 17% 40% 12% 4 1	330
	21st to end	Day	33% 18% 34% 12% 2 1	363
January	1st to 10th	Night	23% 14% 34% 24% 5%	1050
	11th to 20th	Night	25% 16% 37% 20% 3	1050
	21st to end	Night	23% 14% 38% 23% 3	953
February	1st to 10th	Night	25% 13% 37% 23% 3	840
	11th to 20th	Night	23% 14% 39% 21% 2	823
	21st to end	Night	26% 13% 38% 22% 2	692
March	1st to 10th	Night	24% 13% 33% 25% 5%	840
	11th to 20th	Night	23% 12% 37% 23% 5% 1	840
	21st to end	Night	24% 11% 32% 28% 5%	887
April	1st to 10th	Night	18% 11% 37% 28% 6%	630
	11th to 20th	Night	18% 11% 33% 27% 9% 1	623
	21st to end	Night	21% 6% 38% 27% 8% 1	630
May	1st to 10th	Night	19% 9% 35% 26% 10% 1	630
	11th to 20th	Night	15% 6% 27% 32% 16% 3	621
	21st to end	Night	10% 2 25% 35% 25% 2	683
June	1st to 10th	Night	12% 2 22% 42% 20% 2	630
	11th to 20th	Night	9% 3 19% 42% 24% 2	615
	21st to end	Night	7% 1 21% 40% 29% 2	600
July	1st to 10th	Night	10% 3 10% 33% 35% 9%	585
	11th to 20th	Night	8% 3 13% 37% 33% 5%	582
	21st to end	Night	11% 1 12% 39% 33% 4	657
August	1st to 10th	Night	8% 2 12% 35% 37% 5%	597
	11th to 20th	Night	8% 2 11% 41% 33% 5%	594
	21st to end	Night	5% 1 14% 36% 38% 6%	660
September	1st to 10th	Night	12% 2 13% 38% 29% 5%	646
	11th to 20th	Night	9% 2 14% 36% 34% 5%	800
	21st to end	Night	16% 4 22% 31% 25% 3	800
October	1st to 10th	Night	10% 5% 23% 34% 24% 4	800
	11th to 20th	Night	17% 8% 27% 34% 13% 1	800
	21st to end	Night	21% 9% 35% 25% 9% 1	880
November	1st to 10th	Night	16% 7% 33% 30% 13% 1	820
	11th to 20th	Night	19% 14% 32% 26% 8%	840
	21st to end	Night	25% 12% 27% 30% 6%	840
December	1st to 10th	Night	25% 10% 38% 22% 5%	840
	11th to 20th	Night	26% 10% 34% 22% 6% 1	887
	21st to end	Night	31% 13% 34% 19% 3	1155

Page generated at 2026-06-13 02:31:42. Version ac07

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