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 2025-05-26.

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% 11% 3 8% 5% 4 4 4 5% 7% 32%	114155
	Night	17% 11% 4 9% 5% 5% 4 3 5% 6% 31%	115098
Transparency	Day	50% 6% 12% 21% 10% 1	117707
	Night	53% 6% 11% 19% 10% 1	119710
Seeing	Day	24% 17% 31% 19% 8% 2	15133
	Night	23% 15% 31% 23% 8% 1	24387

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% 15% 5% 11% 6% 6% 5% 3 4 6% 25%	3597
	11th to 20th	Day	11% 18% 7% 14% 6% 5% 5% 3 4 5% 21%	3570
	21st to end	Day	14% 18% 7% 14% 7% 5% 4 3 4 4 20%	4279
February	1st to 10th	Day	16% 15% 4 12% 6% 5% 4 5% 4 6% 24%	1800
	11th to 20th	Day	12% 15% 5% 11% 7% 5% 5% 3 5% 5% 27%	1753
	21st to end	Day	19% 14% 5% 10% 7% 5% 5% 3 5% 4 23%	1639
March	1st to 10th	Day	19% 14% 4 10% 6% 4 3 3 4 5% 28%	4568
	11th to 20th	Day	11% 13% 4 9% 5% 5% 5% 4 5% 7% 32%	4980
	21st to end	Day	18% 12% 4 8% 4 3 3 3 4 4 38%	5709
April	1st to 10th	Day	20% 14% 3 7% 5% 3 4 3 4 5% 33%	2400
	11th to 20th	Day	21% 10% 3 7% 4 3 3 3 5% 5% 37%	2374
	21st to end	Day	21% 11% 3 8% 5% 4 4 3 5% 5% 32%	2503
May	1st to 10th	Day	21% 8% 3 8% 5% 4 4 4 5% 6% 32%	4566
	11th to 20th	Day	16% 9% 4 8% 5% 5% 4 3 6% 8% 33%	4538
	21st to end	Day	14% 7% 2 6% 4 4 5% 4 6% 9% 40%	4907
June	1st to 10th	Day	20% 8% 3 7% 5% 5% 5% 4 6% 7% 30%	2660
	11th to 20th	Day	15% 6% 3 6% 5% 4 5% 4 7% 9% 37%	2660
	21st to end	Day	17% 8% 3 8% 6% 4 5% 4 6% 8% 33%	2660
July	1st to 10th	Day	11% 6% 3 7% 5% 5% 6% 5% 7% 10% 36%	4440
	11th to 20th	Day	13% 5% 2 6% 4 4 5% 5% 7% 9% 41%	4440
	21st to end	Day	10% 6% 2 6% 5% 5% 6% 4 7% 9% 40%	4814
August	1st to 10th	Day	17% 8% 2 8% 4 4 4 4 6% 7% 36%	4241
	11th to 20th	Day	17% 7% 2 6% 4 4 5% 4 7% 9% 36%	4017
	21st to end	Day	14% 6% 2 6% 4 4 4 4 7% 9% 39%	4180
September	1st to 10th	Day	13% 6% 2 6% 4 5% 4 4 5% 9% 41%	2280
	11th to 20th	Day	12% 6% 2 5% 4 4 4 4 5% 8% 48%	2280
	21st to end	Day	19% 6% 2 6% 4 5% 4 3 5% 7% 40%	2014
October	1st to 10th	Day	20% 8% 3 7% 4 4 3 3 6% 6% 36%	1900
	11th to 20th	Day	20% 9% 3 9% 5% 5% 4 3 5% 7% 30%	1900
	21st to end	Day	26% 9% 3 8% 5% 4 5% 4 5% 6% 26%	1980
November	1st to 10th	Day	14% 10% 3 7% 6% 5% 5% 4 6% 8% 33%	1760
	11th to 20th	Day	16% 13% 4 10% 5% 4 4 3 4 7% 30%	1600
	21st to end	Day	20% 15% 4 10% 6% 5% 6% 3 5% 4 22%	1600
December	1st to 10th	Day	21% 16% 4 12% 6% 5% 5% 4 5% 5% 17%	2600
	11th to 20th	Day	18% 19% 4 11% 6% 4 4 3 4 4 22%	2986
	21st to end	Day	22% 17% 5% 12% 6% 3 3 3 3 4 22%	3960
January	1st to 10th	Night	15% 15% 7% 13% 7% 5% 4 3 4 5% 22%	6349
	11th to 20th	Night	11% 16% 7% 15% 8% 7% 5% 4 4 5% 18%	6315
	21st to end	Night	16% 16% 7% 13% 6% 5% 4 3 4 4 20%	6674
February	1st to 10th	Night	14% 16% 5% 9% 7% 5% 5% 4 6% 6% 23%	2600
	11th to 20th	Night	13% 15% 5% 9% 6% 5% 5% 3 4 5% 29%	2533
	21st to end	Night	15% 14% 4 9% 6% 5% 4 4 5% 6% 29%	1990
March	1st to 10th	Night	20% 13% 4 9% 5% 5% 4 2 4 4 29%	5258
	11th to 20th	Night	13% 13% 3 7% 4 4 5% 4 7% 8% 30%	4979
	21st to end	Night	19% 10% 3 8% 4 4 4 3 5% 5% 35%	5222
April	1st to 10th	Night	21% 10% 3 8% 5% 3 3 3 5% 4 34%	2000
	11th to 20th	Night	20% 9% 3 7% 4 4 4 3 4 5% 37%	1975
	21st to end	Night	21% 10% 3 8% 5% 3 4 3 5% 6% 33%	1911
May	1st to 10th	Night	22% 10% 3 7% 4 5% 5% 4 5% 6% 30%	2594
	11th to 20th	Night	18% 9% 3 7% 3 3 4 4 5% 6% 38%	2575
	21st to end	Night	14% 6% 2 6% 4 4 4 4 6% 8% 41%	2784
June	1st to 10th	Night	15% 7% 2 7% 5% 5% 5% 5% 7% 8% 33%	1520
	11th to 20th	Night	13% 7% 3 5% 4 6% 3 3 6% 8% 41%	1520
	21st to end	Night	18% 6% 2 7% 4 4 5% 4 6% 8% 35%	1520
July	1st to 10th	Night	10% 5% 3 8% 5% 5% 7% 4 7% 8% 37%	2520
	11th to 20th	Night	13% 6% 2 5% 4 4 4 3 6% 7% 45%	2520
	21st to end	Night	10% 6% 2 7% 4 4 4 4 6% 7% 46%	2739
August	1st to 10th	Night	11% 7% 2 7% 4 4 5% 5% 5% 7% 42%	2686
	11th to 20th	Night	12% 8% 3 6% 4 4 4 4 6% 7% 43%	2785
	21st to end	Night	10% 5% 2 6% 5% 4 5% 5% 8% 10% 40%	3471
September	1st to 10th	Night	12% 6% 2 6% 4 3 4 4 4 8% 47%	1900
	11th to 20th	Night	12% 4 2 4 3 4 4 4 5% 7% 51%	1900
	21st to end	Night	17% 7% 2 5% 4 3 4 4 5% 6% 42%	2161
October	1st to 10th	Night	18% 8% 2 6% 5% 4 5% 3 5% 6% 38%	2280
	11th to 20th	Night	21% 9% 2 8% 5% 4 4 3 5% 6% 32%	2280
	21st to end	Night	22% 10% 3 7% 5% 5% 4 3 5% 6% 29%	2617
November	1st to 10th	Night	18% 10% 3 7% 4 4 4 4 6% 6% 35%	2537
	11th to 20th	Night	17% 11% 3 8% 5% 4 5% 3 5% 6% 32%	2799
	21st to end	Night	20% 13% 3 9% 5% 5% 4 4 5% 6% 25%	2800
December	1st to 10th	Night	19% 15% 5% 13% 6% 5% 5% 4 5% 6% 17%	4580
	11th to 20th	Night	22% 15% 4 9% 5% 4 3 3 5% 5% 26%	5208
	21st to end	Night	26% 16% 4 11% 5% 4 3 2 4 4 20%	6996

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	60% 6% 11% 17% 5% 1	3598
	11th to 20th	Day	66% 4 11% 11% 8%	3600
	21st to end	Day	66% 6% 8% 10% 9% 1	4279
February	1st to 10th	Day	62% 5% 8% 17% 7% 1	1800
	11th to 20th	Day	59% 4 11% 16% 9% 1	1762
	21st to end	Day	64% 5% 8% 12% 9% 1	1639
March	1st to 10th	Day	58% 4 9% 13% 12% 3	4571
	11th to 20th	Day	50% 5% 18% 14% 11% 2	4980
	21st to end	Day	50% 3 6% 20% 18% 3	5709
April	1st to 10th	Day	55% 4 7% 18% 13% 3	2400
	11th to 20th	Day	50% 4 7% 20% 17% 2	2374
	21st to end	Day	54% 5% 8% 16% 14% 3	2503
May	1st to 10th	Day	51% 7% 10% 20% 11% 2	4580
	11th to 20th	Day	48% 7% 12% 22% 9% 1	4542
	21st to end	Day	40% 8% 12% 25% 14% 2	4907
June	1st to 10th	Day	51% 8% 10% 19% 12% 1	2660
	11th to 20th	Day	42% 6% 11% 25% 13% 2	2660
	21st to end	Day	48% 8% 13% 19% 11% 1	2660
July	1st to 10th	Day	40% 7% 14% 27% 12% 1	4370
	11th to 20th	Day	37% 7% 16% 32% 9%	4365
	21st to end	Day	37% 8% 14% 29% 10%	4661
August	1st to 10th	Day	46% 6% 15% 23% 11% 1	4227
	11th to 20th	Day	42% 7% 14% 27% 10% 1	4056
	21st to end	Day	39% 6% 15% 28% 11%	4180
September	1st to 10th	Day	39% 7% 14% 27% 12% 1	2280
	11th to 20th	Day	35% 6% 13% 30% 16% 1	2280
	21st to end	Day	44% 7% 13% 26% 11%	2014
October	1st to 10th	Day	43% 5% 13% 30% 8%	3160
	11th to 20th	Day	44% 6% 15% 26% 9%	3140
	21st to end	Day	48% 6% 15% 24% 6%	3244
November	1st to 10th	Day	48% 7% 13% 22% 9% 1	1760
	11th to 20th	Day	54% 6% 10% 18% 11% 1	1600
	21st to end	Day	63% 7% 11% 11% 8%	1600
December	1st to 10th	Day	68% 6% 14% 9% 3	2600
	11th to 20th	Day	65% 5% 10% 14% 5%	2986
	21st to end	Day	66% 4 8% 15% 6%	3960
January	1st to 10th	Night	64% 6% 12% 13% 5%	6349
	11th to 20th	Night	68% 6% 10% 10% 6%	6360
	21st to end	Night	66% 6% 8% 9% 10% 1	6674
February	1st to 10th	Night	61% 6% 10% 16% 6% 1	2600
	11th to 20th	Night	57% 4 11% 18% 9% 1	2535
	21st to end	Night	57% 5% 9% 17% 10% 1	1990
March	1st to 10th	Night	59% 4 6% 17% 10% 4	5268
	11th to 20th	Night	48% 6% 19% 16% 8% 3	4980
	21st to end	Night	51% 4 8% 21% 14% 2	5222
April	1st to 10th	Night	53% 4 7% 20% 14% 3	2000
	11th to 20th	Night	49% 6% 7% 20% 17% 1	1975
	21st to end	Night	52% 4 9% 16% 15% 4	1911
May	1st to 10th	Night	54% 7% 9% 18% 10% 2	2600
	11th to 20th	Night	46% 8% 12% 23% 10% 2	2576
	21st to end	Night	39% 7% 13% 21% 18% 2	2785
June	1st to 10th	Night	46% 9% 12% 21% 12% 1	1520
	11th to 20th	Night	40% 6% 13% 22% 18% 2	1520
	21st to end	Night	45% 9% 12% 20% 13% 1	1520
July	1st to 10th	Night	42% 7% 13% 21% 15% 2	2480
	11th to 20th	Night	37% 7% 13% 34% 10%	2472
	21st to end	Night	37% 9% 14% 25% 13% 2	2651
August	1st to 10th	Night	40% 7% 14% 26% 13% 1	2676
	11th to 20th	Night	40% 7% 13% 31% 9% 1	2812
	21st to end	Night	35% 9% 16% 27% 11% 1	3471
September	1st to 10th	Night	37% 7% 13% 29% 14% 1	1900
	11th to 20th	Night	32% 7% 13% 33% 13% 1	1900
	21st to end	Night	41% 6% 11% 30% 12% 1	2161
October	1st to 10th	Night	40% 6% 12% 28% 14%	3800
	11th to 20th	Night	43% 6% 15% 26% 10%	3779
	21st to end	Night	48% 6% 14% 24% 8%	4303
November	1st to 10th	Night	48% 6% 13% 22% 10% 1	2537
	11th to 20th	Night	51% 5% 12% 19% 11% 2	2799
	21st to end	Night	60% 7% 11% 16% 5%	2800
December	1st to 10th	Night	68% 6% 12% 12% 3	4580
	11th to 20th	Night	61% 6% 9% 15% 9% 1	5208
	21st to end	Night	67% 5% 7% 11% 9%	6996

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% 22% 38% 11% 4 1	200
	11th to 20th	Day	34% 17% 31% 16% 2 1	200
	21st to end	Day	25% 26% 31% 15% 3	289
February	1st to 10th	Day	34% 26% 29% 9% 1	300
	11th to 20th	Day	31% 27% 35% 7%	300
	21st to end	Day	34% 20% 34% 11% 1	403
March	1st to 10th	Day	29% 21% 32% 14% 4	500
	11th to 20th	Day	29% 21% 32% 14% 4	500
	21st to end	Day	34% 20% 28% 13% 5% 1	550
April	1st to 10th	Day	32% 22% 29% 12% 5%	500
	11th to 20th	Day	34% 20% 23% 14% 8% 2	492
	21st to end	Day	28% 24% 30% 10% 5% 2	500
May	1st to 10th	Day	25% 18% 32% 18% 6% 2	503
	11th to 20th	Day	19% 15% 37% 20% 8% 2	527
	21st to end	Day	21% 13% 32% 22% 9% 2	549
June	1st to 10th	Day	23% 12% 36% 20% 7% 2	490
	11th to 20th	Day	17% 16% 34% 20% 11% 2	490
	21st to end	Day	21% 15% 30% 23% 8% 3	490
July	1st to 10th	Day	14% 10% 30% 29% 14% 3	485
	11th to 20th	Day	15% 12% 32% 24% 14% 4	484
	21st to end	Day	13% 10% 33% 29% 11% 4	532
August	1st to 10th	Day	15% 9% 31% 28% 13% 3	486
	11th to 20th	Day	16% 7% 26% 30% 15% 6%	479
	21st to end	Day	16% 11% 32% 24% 11% 6%	539
September	1st to 10th	Day	14% 14% 32% 22% 12% 6%	490
	11th to 20th	Day	12% 11% 23% 24% 19% 12%	490
	21st to end	Day	19% 11% 28% 24% 13% 3	490
October	1st to 10th	Day	26% 18% 26% 17% 11% 4	490
	11th to 20th	Day	29% 19% 29% 16% 6% 2	490
	21st to end	Day	30% 20% 28% 18% 4	481
November	1st to 10th	Day	26% 19% 29% 19% 7%	394
	11th to 20th	Day	29% 20% 33% 12% 5% 2	200
	21st to end	Day	32% 22% 31% 13% 3	200
December	1st to 10th	Day	35% 19% 32% 11% 5%	200
	11th to 20th	Day	36% 23% 31% 7% 4 1	200
	21st to end	Day	33% 20% 30% 12% 4	220
January	1st to 10th	Night	29% 20% 30% 18% 3	800
	11th to 20th	Night	29% 18% 30% 19% 4	800
	21st to end	Night	20% 17% 37% 22% 5%	880
February	1st to 10th	Night	24% 21% 35% 18% 2	800
	11th to 20th	Night	23% 22% 37% 16% 3	783
	21st to end	Night	25% 19% 35% 18% 3	660
March	1st to 10th	Night	25% 17% 35% 20% 3	800
	11th to 20th	Night	26% 21% 28% 20% 4 1	800
	21st to end	Night	24% 17% 33% 19% 7%	880
April	1st to 10th	Night	27% 16% 33% 21% 3	800
	11th to 20th	Night	25% 20% 29% 18% 6% 1	791
	21st to end	Night	26% 14% 29% 22% 7% 1	711
May	1st to 10th	Night	26% 11% 27% 26% 9% 1	400
	11th to 20th	Night	15% 12% 35% 31% 6%	394
	21st to end	Night	13% 9% 39% 28% 10%	423
June	1st to 10th	Night	15% 8% 32% 36% 9% 1	380
	11th to 20th	Night	14% 11% 31% 30% 12% 1	380
	21st to end	Night	19% 7% 27% 35% 12% 1	380
July	1st to 10th	Night	11% 6% 32% 36% 15%	370
	11th to 20th	Night	12% 4 31% 38% 14% 1	368
	21st to end	Night	10% 5% 31% 42% 12% 1	416
August	1st to 10th	Night	13% 5% 28% 34% 20% 1	480
	11th to 20th	Night	13% 5% 24% 32% 24% 2	564
	21st to end	Night	12% 6% 31% 34% 15% 2	831
September	1st to 10th	Night	14% 6% 33% 29% 16% 1	760
	11th to 20th	Night	14% 8% 23% 27% 24% 5%	760
	21st to end	Night	20% 9% 25% 26% 18% 2	760
October	1st to 10th	Night	22% 13% 27% 21% 15% 2	760
	11th to 20th	Night	26% 14% 30% 22% 7% 1	760
	21st to end	Night	29% 15% 28% 23% 5%	836
November	1st to 10th	Night	27% 15% 31% 20% 7%	780
	11th to 20th	Night	26% 17% 32% 16% 8%	800
	21st to end	Night	29% 20% 28% 17% 6%	800
December	1st to 10th	Night	30% 20% 31% 16% 3	800
	11th to 20th	Night	27% 20% 33% 17% 3 1	800
	21st to end	Night	34% 16% 30% 18% 2	880

Page generated at 2025-05-23 09:21:04. Version ac07

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