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 2026-09-29.

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	16% 10% 3 8% 5% 5% 5% 4 5% 7% 32%	149964
	Night	16% 11% 4 9% 5% 5% 4 3 5% 6% 31%	141412
Transparency	Day	49% 6% 12% 21% 10% 1	153470
	Night	52% 6% 11% 20% 10% 1	145995
Seeing	Day	24% 16% 31% 18% 8% 2	16904
	Night	23% 14% 31% 23% 8% 1	25979

By Month

Percentage Cloud Cover NE (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	13% 16% 5% 12% 7% 6% 5% 4 5% 6% 22%	4661
	11th to 20th	Day	11% 20% 8% 15% 7% 5% 5% 3 3 4 18%	4650
	21st to end	Day	11% 16% 7% 14% 8% 7% 5% 3 4 5% 19%	5538
February	1st to 10th	Day	15% 14% 4 12% 6% 5% 4 5% 4 6% 24%	1890
	11th to 20th	Day	12% 15% 5% 11% 7% 5% 5% 4 5% 5% 27%	1843
	21st to end	Day	19% 15% 5% 10% 7% 5% 5% 3 5% 4 23%	1719
March	1st to 10th	Day	19% 13% 4 10% 6% 4 3 3 4 6% 28%	4670
	11th to 20th	Day	11% 13% 4 9% 5% 5% 5% 4 5% 7% 31%	5090
	21st to end	Day	18% 12% 4 8% 4 4 3 3 4 4 37%	5835
April	1st to 10th	Day	20% 14% 3 7% 5% 3 4 3 4 5% 32%	2520
	11th to 20th	Day	21% 11% 3 6% 4 3 3 3 4 5% 37%	2494
	21st to end	Day	21% 11% 3 8% 5% 4 4 3 5% 5% 32%	2628
May	1st to 10th	Day	22% 9% 3 8% 5% 5% 5% 4 5% 6% 28%	8237
	11th to 20th	Day	16% 8% 3 7% 5% 5% 5% 3 5% 8% 34%	8221
	21st to end	Day	19% 9% 3 6% 4 4 4 3 5% 8% 35%	9060
June	1st to 10th	Day	20% 8% 3 7% 5% 5% 5% 4 6% 7% 31%	2940
	11th to 20th	Day	14% 6% 3 6% 6% 4 5% 4 7% 9% 36%	2940
	21st to end	Day	16% 7% 3 7% 6% 4 5% 4 6% 8% 33%	2940
July	1st to 10th	Day	10% 6% 3 7% 5% 5% 5% 5% 7% 10% 36%	8280
	11th to 20th	Day	10% 5% 2 5% 3 4 4 4 6% 9% 47%	8280
	21st to end	Day	12% 5% 2 6% 4 4 5% 4 7% 10% 40%	8993
August	1st to 10th	Day	17% 9% 3 8% 4 4 4 4 5% 6% 35%	6211
	11th to 20th	Day	14% 6% 2 6% 4 5% 5% 5% 7% 9% 36%	5910
	21st to end	Day	14% 7% 3 7% 4 5% 5% 5% 7% 10% 34%	6080
September	1st to 10th	Day	13% 6% 2 6% 4 5% 4 4 6% 9% 41%	2520
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 5% 8% 48%	2520
	21st to end	Day	18% 6% 2 6% 4 5% 4 3 5% 6% 41%	2206
October	1st to 10th	Day	20% 8% 3 7% 4 3 3 3 5% 6% 38%	2000
	11th to 20th	Day	19% 9% 3 9% 5% 5% 4 4 6% 7% 30%	2000
	21st to end	Day	25% 9% 3 8% 5% 4 5% 4 5% 6% 27%	2084
November	1st to 10th	Day	15% 10% 3 7% 5% 4 5% 4 6% 8% 33%	1850
	11th to 20th	Day	16% 13% 4 10% 5% 4 4 3 4 6% 30%	1680
	21st to end	Day	20% 15% 4 10% 6% 5% 5% 3 5% 4 22%	1680
December	1st to 10th	Day	21% 17% 5% 12% 6% 5% 4 4 5% 5% 17%	2680
	11th to 20th	Day	18% 19% 4 11% 6% 4 4 3 4 4 22%	3066
	21st to end	Day	22% 17% 5% 13% 6% 3 3 2 3 4 22%	4048
January	1st to 10th	Night	14% 15% 7% 14% 7% 5% 5% 3 4 5% 21%	8234
	11th to 20th	Night	11% 18% 7% 16% 7% 7% 5% 4 4 5% 17%	8235
	21st to end	Night	13% 15% 6% 12% 6% 6% 5% 4 5% 5% 23%	8640
February	1st to 10th	Night	14% 15% 5% 9% 7% 5% 5% 4 6% 6% 23%	2730
	11th to 20th	Night	12% 15% 5% 10% 6% 5% 5% 3 4 6% 29%	2663
	21st to end	Night	15% 14% 4 9% 6% 5% 4 4 5% 6% 28%	2086
March	1st to 10th	Night	20% 13% 4 9% 5% 5% 4 2 4 4 30%	5376
	11th to 20th	Night	14% 13% 3 7% 4 4 5% 4 7% 8% 30%	5089
	21st to end	Night	19% 11% 3 8% 4 4 4 3 5% 5% 35%	5338
April	1st to 10th	Night	21% 10% 3 7% 5% 3 3 3 5% 4 34%	2100
	11th to 20th	Night	21% 10% 3 7% 4 4 4 3 4 5% 36%	2075
	21st to end	Night	21% 9% 3 7% 4 3 4 3 5% 6% 34%	2007
May	1st to 10th	Night	22% 10% 3 9% 5% 5% 5% 4 5% 7% 25%	4660
	11th to 20th	Night	14% 10% 3 7% 4 3 4 3 5% 7% 40%	4639
	21st to end	Night	18% 7% 2 6% 4 4 4 4 6% 8% 37%	5114
June	1st to 10th	Night	15% 7% 2 7% 5% 5% 5% 5% 7% 7% 34%	1680
	11th to 20th	Night	13% 7% 2 6% 4 6% 4 3 6% 8% 41%	1680
	21st to end	Night	17% 6% 2 7% 5% 4 5% 4 6% 8% 35%	1680
July	1st to 10th	Night	10% 6% 3 7% 5% 5% 5% 4 7% 10% 39%	4680
	11th to 20th	Night	9% 5% 2 5% 4 4 4 3 5% 8% 50%	4680
	21st to end	Night	11% 7% 2 7% 4 4 4 3 6% 7% 45%	5084
August	1st to 10th	Night	13% 8% 3 7% 4 5% 5% 4 6% 7% 38%	3922
	11th to 20th	Night	11% 7% 3 6% 4 4 4 4 6% 8% 44%	4095
	21st to end	Night	9% 6% 2 7% 6% 5% 5% 5% 8% 9% 38%	5040
September	1st to 10th	Night	11% 6% 2 7% 4 3 4 4 5% 8% 46%	2100
	11th to 20th	Night	12% 4 2 4 3 4 4 4 5% 7% 52%	2100
	21st to end	Night	16% 7% 2 5% 4 3 4 4 5% 6% 43%	2366
October	1st to 10th	Night	17% 8% 2 6% 5% 4 4 3 5% 6% 40%	2400
	11th to 20th	Night	20% 9% 2 8% 5% 5% 4 4 5% 6% 32%	2400
	21st to end	Night	21% 10% 3 7% 5% 5% 4 3 5% 6% 31%	2755
November	1st to 10th	Night	18% 11% 3 7% 4 4 4 4 6% 6% 35%	2667
	11th to 20th	Night	17% 11% 4 8% 5% 4 5% 3 5% 6% 32%	2939
	21st to end	Night	20% 13% 4 10% 5% 5% 4 3 5% 6% 24%	2940
December	1st to 10th	Night	19% 15% 5% 13% 6% 5% 5% 4 5% 6% 18%	4720
	11th to 20th	Night	22% 15% 4 9% 5% 4 4 3 5% 5% 26%	5348
	21st to end	Night	26% 16% 4 11% 5% 4 3 2 4 4 20%	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	62% 6% 12% 15% 5% 1	4662
	11th to 20th	Day	70% 4 9% 9% 8% 1	4680
	21st to end	Day	67% 5% 8% 10% 9% 1	5538
February	1st to 10th	Day	61% 5% 9% 17% 7% 1	1890
	11th to 20th	Day	59% 4 11% 17% 9% 1	1852
	21st to end	Day	65% 5% 8% 12% 9% 1	1719
March	1st to 10th	Day	58% 4 9% 14% 12% 3	4673
	11th to 20th	Day	51% 5% 18% 14% 11% 2	5090
	21st to end	Day	50% 3 6% 20% 18% 3	5835
April	1st to 10th	Day	56% 4 7% 18% 13% 3	2520
	11th to 20th	Day	51% 4 7% 20% 16% 2	2494
	21st to end	Day	54% 4 9% 16% 14% 3	2628
May	1st to 10th	Day	54% 7% 9% 18% 11% 2	8249
	11th to 20th	Day	46% 7% 12% 22% 12% 1	8225
	21st to end	Day	47% 6% 11% 24% 11% 1	9061
June	1st to 10th	Day	50% 7% 9% 19% 13% 1	2940
	11th to 20th	Day	43% 6% 11% 25% 13% 2	2940
	21st to end	Day	47% 8% 13% 20% 11% 1	2940
July	1st to 10th	Day	39% 7% 16% 27% 11%	8210
	11th to 20th	Day	31% 6% 15% 33% 15% 1	8205
	21st to end	Day	37% 7% 15% 32% 8%	8795
August	1st to 10th	Day	49% 5% 14% 19% 12% 2	6197
	11th to 20th	Day	41% 7% 16% 27% 8% 1	5949
	21st to end	Day	42% 7% 18% 24% 8%	6080
September	1st to 10th	Day	39% 7% 14% 29% 11% 1	2520
	11th to 20th	Day	34% 6% 13% 31% 16% 1	2520
	21st to end	Day	43% 6% 12% 25% 13% 1	2206
October	1st to 10th	Day	42% 5% 13% 31% 9%	3260
	11th to 20th	Day	44% 5% 15% 26% 9% 1	3240
	21st to end	Day	48% 6% 15% 24% 7%	3348
November	1st to 10th	Day	49% 7% 13% 22% 9% 1	1850
	11th to 20th	Day	55% 6% 10% 17% 10% 1	1680
	21st to end	Day	63% 7% 11% 11% 8%	1680
December	1st to 10th	Day	68% 6% 14% 9% 3	2680
	11th to 20th	Day	65% 5% 10% 14% 6%	3066
	21st to end	Day	66% 4 8% 15% 6%	4048
January	1st to 10th	Night	65% 6% 12% 13% 5%	8234
	11th to 20th	Night	69% 6% 8% 10% 6%	8280
	21st to end	Night	61% 6% 8% 13% 11% 1	8640
February	1st to 10th	Night	61% 6% 10% 16% 7% 1	2730
	11th to 20th	Night	58% 4 11% 19% 8% 1	2665
	21st to end	Night	58% 5% 9% 17% 10% 1	2086
March	1st to 10th	Night	59% 4 6% 17% 10% 4	5386
	11th to 20th	Night	49% 6% 19% 15% 8% 3	5090
	21st to end	Night	51% 4 8% 21% 14% 2	5338
April	1st to 10th	Night	52% 5% 8% 20% 14% 2	2100
	11th to 20th	Night	50% 5% 7% 20% 17% 1	2075
	21st to end	Night	52% 4 9% 16% 16% 3	2007
May	1st to 10th	Night	57% 6% 9% 17% 9% 2	4666
	11th to 20th	Night	44% 7% 13% 22% 14% 1	4640
	21st to end	Night	44% 6% 12% 21% 16% 1	5115
June	1st to 10th	Night	45% 8% 11% 21% 14% 1	1680
	11th to 20th	Night	40% 6% 13% 22% 18% 2	1680
	21st to end	Night	45% 9% 12% 20% 12% 1	1680
July	1st to 10th	Night	39% 7% 18% 22% 14% 1	4640
	11th to 20th	Night	31% 7% 14% 33% 15% 1	4632
	21st to end	Night	38% 7% 12% 28% 14% 1	4967
August	1st to 10th	Night	45% 6% 15% 19% 12% 3	3912
	11th to 20th	Night	38% 7% 15% 32% 8% 1	4122
	21st to end	Night	37% 9% 18% 26% 9% 1	5040
September	1st to 10th	Night	37% 7% 12% 30% 13% 1	2100
	11th to 20th	Night	31% 7% 13% 34% 14% 1	2100
	21st to end	Night	41% 6% 10% 28% 13% 1	2366
October	1st to 10th	Night	39% 6% 12% 28% 15%	3920
	11th to 20th	Night	43% 6% 15% 26% 10%	3899
	21st to end	Night	48% 6% 14% 25% 8%	4441
November	1st to 10th	Night	49% 6% 13% 21% 10% 1	2667
	11th to 20th	Night	52% 5% 12% 20% 11% 2	2939
	21st to end	Night	61% 7% 11% 16% 5%	2940
December	1st to 10th	Night	67% 6% 12% 12% 3	4720
	11th to 20th	Night	61% 6% 9% 15% 9% 1	5348
	21st to end	Night	67% 5% 7% 11% 9%	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	24% 21% 39% 12% 3	220
	11th to 20th	Day	36% 17% 30% 15% 2	220
	21st to end	Day	25% 25% 31% 16% 4	318
February	1st to 10th	Day	32% 25% 30% 11% 2	330
	11th to 20th	Day	29% 26% 36% 8%	330
	21st to end	Day	34% 20% 34% 11% 1	435
March	1st to 10th	Day	29% 21% 32% 14% 4	540
	11th to 20th	Day	32% 20% 30% 13% 4	540
	21st to end	Day	34% 20% 28% 12% 5% 1	594
April	1st to 10th	Day	33% 22% 29% 11% 5%	540
	11th to 20th	Day	35% 19% 23% 14% 7% 2	532
	21st to end	Day	28% 24% 30% 11% 5% 2	540
May	1st to 10th	Day	26% 18% 31% 18% 6% 2	543
	11th to 20th	Day	19% 16% 36% 19% 8% 2	567
	21st to end	Day	22% 15% 33% 21% 8% 2	617
June	1st to 10th	Day	22% 13% 37% 19% 7% 2	570
	11th to 20th	Day	17% 16% 37% 18% 9% 2	570
	21st to end	Day	20% 14% 31% 23% 8% 3	570
July	1st to 10th	Day	13% 10% 33% 28% 13% 3	565
	11th to 20th	Day	14% 13% 33% 23% 13% 4	564
	21st to end	Day	13% 10% 33% 28% 11% 4	620
August	1st to 10th	Day	15% 9% 32% 28% 13% 3	566
	11th to 20th	Day	16% 7% 28% 30% 14% 6%	559
	21st to end	Day	16% 11% 33% 24% 11% 5%	627
September	1st to 10th	Day	15% 14% 32% 22% 11% 6%	570
	11th to 20th	Day	11% 11% 25% 26% 17% 10%	570
	21st to end	Day	18% 12% 30% 24% 13% 3	562
October	1st to 10th	Day	25% 17% 25% 18% 11% 3	530
	11th to 20th	Day	28% 19% 31% 15% 5% 2	530
	21st to end	Day	29% 19% 29% 18% 4	519
November	1st to 10th	Day	27% 19% 29% 18% 7%	424
	11th to 20th	Day	30% 18% 34% 12% 5% 1	220
	21st to end	Day	34% 20% 32% 11% 3	220
December	1st to 10th	Day	38% 17% 31% 10% 4	220
	11th to 20th	Day	36% 22% 32% 6% 3	220
	21st to end	Day	34% 20% 30% 12% 4	242
January	1st to 10th	Night	28% 19% 32% 18% 3	840
	11th to 20th	Night	31% 18% 29% 19% 4	840
	21st to end	Night	20% 17% 36% 21% 5%	924
February	1st to 10th	Night	23% 20% 34% 19% 3	840
	11th to 20th	Night	23% 21% 37% 16% 3	823
	21st to end	Night	26% 19% 35% 17% 3	692
March	1st to 10th	Night	26% 17% 34% 20% 3	840
	11th to 20th	Night	28% 21% 28% 20% 3 1	840
	21st to end	Night	25% 17% 33% 18% 7%	924
April	1st to 10th	Night	28% 16% 32% 21% 3	840
	11th to 20th	Night	26% 20% 29% 19% 6% 1	831
	21st to end	Night	26% 14% 29% 23% 7% 1	747
May	1st to 10th	Night	25% 12% 27% 25% 10% 1	420
	11th to 20th	Night	15% 13% 35% 31% 6%	414
	21st to end	Night	14% 9% 40% 27% 9%	455
June	1st to 10th	Night	15% 7% 33% 35% 9%	420
	11th to 20th	Night	15% 12% 32% 29% 11% 1	420
	21st to end	Night	19% 7% 27% 35% 11%	420
July	1st to 10th	Night	10% 6% 33% 37% 13%	410
	11th to 20th	Night	11% 6% 31% 38% 13% 1	408
	21st to end	Night	11% 5% 30% 41% 12% 1	460
August	1st to 10th	Night	14% 4 27% 34% 20% 1	531
	11th to 20th	Night	13% 5% 25% 33% 23% 2	624
	21st to end	Night	12% 7% 31% 34% 15% 2	919
September	1st to 10th	Night	13% 7% 33% 29% 16% 1	840
	11th to 20th	Night	14% 8% 23% 29% 23% 4	840
	21st to end	Night	20% 9% 25% 27% 17% 2	833
October	1st to 10th	Night	21% 13% 28% 21% 16% 2	800
	11th to 20th	Night	26% 14% 31% 22% 7% 1	800
	21st to end	Night	28% 15% 28% 24% 5%	880
November	1st to 10th	Night	28% 15% 31% 20% 7%	820
	11th to 20th	Night	25% 17% 33% 16% 8%	840
	21st to end	Night	29% 20% 28% 17% 5%	840
December	1st to 10th	Night	29% 20% 31% 16% 3	840
	11th to 20th	Night	28% 20% 33% 16% 3 1	840
	21st to end	Night	34% 17% 29% 17% 2	924

Page generated at 2026-09-26 04:18:38. Version ac07

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