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 2024-11-02.

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% 4 4 4 5% 7% 33%	101569
	Night	17% 11% 3 8% 5% 4 4 4 5% 6% 32%	97401
Transparency	Day	50% 6% 12% 21% 10% 1	101327
	Night	52% 6% 11% 19% 10% 1	97263
Seeing	Day	23% 17% 30% 19% 8% 2	14501
	Night	23% 14% 31% 23% 8% 1	23623

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	14% 17% 6% 9% 6% 5% 5% 3 4 5% 26%	2517
	11th to 20th	Day	13% 16% 6% 14% 6% 5% 5% 3 4 5% 22%	2520
	21st to end	Day	18% 20% 7% 13% 7% 5% 4 3 3 4 17%	2993
February	1st to 10th	Day	16% 15% 4 12% 7% 5% 4 5% 5% 6% 23%	1710
	11th to 20th	Day	12% 15% 5% 11% 6% 5% 5% 3 5% 5% 28%	1663
	21st to end	Day	20% 14% 4 9% 7% 5% 5% 3 5% 4 24%	1559
March	1st to 10th	Day	21% 13% 4 9% 6% 4 3 2 4 5% 28%	3180
	11th to 20th	Day	11% 16% 5% 9% 6% 5% 5% 3 4 6% 30%	3480
	21st to end	Day	12% 11% 3 7% 4 4 3 3 4 6% 42%	3989
April	1st to 10th	Day	20% 14% 3 7% 5% 3 4 3 4 5% 33%	2280
	11th to 20th	Day	21% 10% 3 6% 4 3 4 3 4 5% 37%	2254
	21st to end	Day	22% 11% 3 8% 5% 4 4 3 5% 5% 32%	2378
May	1st to 10th	Day	21% 7% 3 8% 5% 4 4 4 5% 6% 32%	4426
	11th to 20th	Day	16% 9% 4 8% 5% 5% 4 3 5% 8% 33%	4398
	21st to end	Day	13% 6% 2 6% 4 4 5% 4 6% 9% 40%	4837
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	25% 9% 3 8% 5% 4 5% 4 4 6% 26%	1980
November	1st to 10th	Day	15% 10% 3 7% 5% 4 5% 4 6% 8% 33%	1681
	11th to 20th	Day	17% 13% 4 10% 5% 4 4 3 4 6% 29%	1520
	21st to end	Day	20% 15% 4 10% 6% 5% 5% 3 5% 4 22%	1520
December	1st to 10th	Day	22% 17% 4 11% 5% 4 5% 4 5% 5% 18%	1520
	11th to 20th	Day	21% 15% 4 10% 6% 4 5% 3 5% 4 24%	1906
	21st to end	Day	26% 17% 5% 13% 5% 3 3 2 2 3 20%	2772
January	1st to 10th	Night	18% 14% 6% 12% 6% 5% 5% 4 5% 5% 20%	4429
	11th to 20th	Night	13% 15% 5% 14% 8% 7% 6% 4 4 5% 20%	4440
	21st to end	Night	20% 17% 6% 12% 6% 6% 4 3 3 4 20%	4660
February	1st to 10th	Night	15% 16% 5% 9% 7% 5% 5% 4 6% 6% 22%	2470
	11th to 20th	Night	12% 15% 5% 9% 6% 5% 5% 3 4 5% 29%	2403
	21st to end	Night	15% 14% 4 9% 6% 5% 4 4 5% 6% 29%	1894
March	1st to 10th	Night	20% 12% 3 9% 5% 4 4 3 4 5% 32%	3646
	11th to 20th	Night	17% 17% 4 8% 4 4 5% 3 5% 6% 26%	3479
	21st to end	Night	15% 10% 3 7% 4 3 4 3 5% 5% 41%	3642
April	1st to 10th	Night	21% 10% 3 7% 5% 3 3 3 5% 4 35%	1900
	11th to 20th	Night	20% 9% 3 7% 4 4 4 3 4 5% 37%	1875
	21st to end	Night	21% 10% 3 8% 4 3 4 3 5% 6% 33%	1815
May	1st to 10th	Night	22% 10% 3 7% 4 5% 4 4 5% 6% 31%	2514
	11th to 20th	Night	18% 10% 3 7% 4 3 4 4 5% 6% 37%	2495
	21st to end	Night	14% 6% 2 6% 4 4 4 4 6% 8% 42%	2738
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% 11% 3 7% 5% 5% 4 3 5% 6% 29%	2617
November	1st to 10th	Night	18% 10% 3 6% 4 4 4 4 6% 6% 35%	2431
	11th to 20th	Night	18% 11% 3 8% 5% 4 5% 3 5% 6% 32%	2659
	21st to end	Night	21% 13% 3 9% 5% 5% 4 3 5% 6% 25%	2660
December	1st to 10th	Night	20% 16% 5% 12% 6% 4 4 3 4 5% 20%	2660
	11th to 20th	Night	21% 15% 4 9% 6% 4 3 3 4 4 27%	3288
	21st to end	Night	30% 16% 4 10% 5% 4 4 3 4 4 16%	4884

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% 7% 14% 12% 6% 2	2518
	11th to 20th	Day	66% 5% 11% 10% 8%	2520
	21st to end	Day	71% 6% 8% 9% 5% 1	2993
February	1st to 10th	Day	62% 5% 8% 17% 6% 1	1710
	11th to 20th	Day	58% 4 11% 16% 9% 1	1672
	21st to end	Day	64% 5% 9% 12% 9% 1	1559
March	1st to 10th	Day	60% 4 9% 18% 9% 1	3183
	11th to 20th	Day	54% 5% 14% 14% 11% 2	3480
	21st to end	Day	43% 4 8% 25% 17% 4	3989
April	1st to 10th	Day	55% 4 7% 18% 13% 3	2280
	11th to 20th	Day	50% 4 7% 20% 17% 2	2254
	21st to end	Day	54% 5% 9% 15% 14% 4	2378
May	1st to 10th	Day	51% 7% 10% 20% 11% 2	4440
	11th to 20th	Day	48% 7% 12% 22% 9% 1	4402
	21st to end	Day	39% 8% 12% 25% 14% 2	4837
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	48% 5% 12% 23% 12% 1	1900
	11th to 20th	Day	53% 6% 13% 20% 7% 1	1900
	21st to end	Day	58% 5% 10% 18% 7%	1980
November	1st to 10th	Day	48% 7% 14% 21% 9% 1	1681
	11th to 20th	Day	55% 7% 10% 17% 10% 1	1520
	21st to end	Day	63% 7% 11% 11% 8%	1520
December	1st to 10th	Day	67% 7% 11% 11% 4 1	1520
	11th to 20th	Day	64% 6% 14% 12% 4	1906
	21st to end	Day	71% 4 7% 14% 4	2772
January	1st to 10th	Night	63% 7% 14% 12% 4	4429
	11th to 20th	Night	66% 7% 11% 11% 5%	4440
	21st to end	Night	68% 6% 9% 10% 6% 1	4660
February	1st to 10th	Night	62% 6% 10% 15% 6% 1	2470
	11th to 20th	Night	57% 4 11% 19% 9%	2405
	21st to end	Night	57% 6% 9% 17% 10% 1	1894
March	1st to 10th	Night	55% 5% 8% 22% 9% 1	3656
	11th to 20th	Night	57% 6% 14% 14% 8% 1	3480
	21st to end	Night	46% 3 9% 23% 17% 3	3642
April	1st to 10th	Night	52% 4 7% 20% 14% 3	1900
	11th to 20th	Night	49% 6% 7% 19% 17% 1	1875
	21st to end	Night	52% 4 9% 16% 15% 3	1815
May	1st to 10th	Night	53% 7% 10% 18% 10% 3	2520
	11th to 20th	Night	47% 8% 11% 23% 10% 2	2496
	21st to end	Night	39% 7% 13% 21% 18% 2	2739
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	47% 6% 10% 22% 14% 1	2280
	11th to 20th	Night	52% 6% 13% 19% 10% 1	2280
	21st to end	Night	56% 6% 13% 16% 9%	2617
November	1st to 10th	Night	49% 6% 14% 21% 10% 1	2431
	11th to 20th	Night	51% 5% 12% 19% 11% 2	2659
	21st to end	Night	60% 7% 12% 16% 6%	2660
December	1st to 10th	Night	67% 6% 10% 14% 4	2660
	11th to 20th	Night	61% 5% 11% 15% 7% 1	3288
	21st to end	Night	71% 6% 8% 12% 3	4884

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	26% 23% 35% 12% 4 1	180
	11th to 20th	Day	36% 17% 29% 16% 2 1	180
	21st to end	Day	27% 22% 31% 17% 4	260
February	1st to 10th	Day	34% 26% 29% 10% 1	270
	11th to 20th	Day	29% 28% 37% 6%	270
	21st to end	Day	35% 20% 33% 11% 2	371
March	1st to 10th	Day	28% 22% 32% 15% 5%	460
	11th to 20th	Day	29% 20% 33% 14% 4	460
	21st to end	Day	33% 21% 28% 13% 5% 1	506
April	1st to 10th	Day	31% 21% 29% 13% 5%	460
	11th to 20th	Day	35% 19% 22% 14% 7% 2	452
	21st to end	Day	28% 24% 30% 11% 5% 2	460
May	1st to 10th	Day	25% 18% 30% 19% 6% 2	463
	11th to 20th	Day	19% 16% 35% 20% 8% 2	487
	21st to end	Day	19% 14% 33% 23% 9% 2	529
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	29% 20% 28% 18% 4	481
November	1st to 10th	Day	25% 19% 29% 20% 7%	369
	11th to 20th	Day	32% 19% 31% 13% 4 2	180
	21st to end	Day	29% 23% 31% 13% 3	180
December	1st to 10th	Day	35% 19% 32% 10% 4	180
	11th to 20th	Day	35% 22% 32% 8% 3 1	180
	21st to end	Day	34% 21% 32% 10% 3 1	198
January	1st to 10th	Night	29% 20% 29% 19% 3	760
	11th to 20th	Night	29% 18% 29% 19% 4	760
	21st to end	Night	20% 17% 36% 22% 5%	836
February	1st to 10th	Night	24% 21% 35% 18% 2	760
	11th to 20th	Night	22% 22% 37% 16% 3	743
	21st to end	Night	25% 20% 35% 18% 3	628
March	1st to 10th	Night	24% 17% 35% 21% 3	760
	11th to 20th	Night	27% 21% 28% 20% 4 1	760
	21st to end	Night	23% 17% 33% 19% 7%	836
April	1st to 10th	Night	27% 16% 33% 21% 3	760
	11th to 20th	Night	25% 21% 30% 18% 6% 1	751
	21st to end	Night	26% 14% 29% 22% 8% 1	675
May	1st to 10th	Night	25% 11% 26% 27% 10% 1	380
	11th to 20th	Night	16% 11% 35% 32% 6%	374
	21st to end	Night	12% 9% 39% 28% 10%	411
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% 14% 31% 21% 7%	748
	11th to 20th	Night	26% 18% 32% 15% 9%	760
	21st to end	Night	29% 20% 28% 18% 6%	760
December	1st to 10th	Night	30% 20% 31% 16% 3	760
	11th to 20th	Night	27% 20% 33% 17% 3 1	760
	21st to end	Night	35% 17% 30% 17% 1	836

Page generated at 2024-10-30 13:19:21. Version ac07

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