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 2024-09-14.

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	15% 9% 3 7% 4 4 4 4 6% 7% 38%	100522
	Night	15% 9% 3 7% 4 4 4 3 5% 7% 38%	97389
Transparency	Day	44% 6% 11% 23% 14% 2	100323
	Night	45% 6% 11% 22% 14% 2	97259
Seeing	Day	18% 10% 26% 24% 15% 6%	15274
	Night	18% 8% 28% 29% 15% 2	25035

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	15% 16% 7% 8% 5% 4 5% 3 5% 5% 26%	2527
	11th to 20th	Day	17% 13% 4 14% 5% 7% 6% 3 4 4 23%	2530
	21st to end	Day	28% 16% 8% 8% 4 3 3 2 3 4 19%	3069
February	1st to 10th	Day	19% 13% 5% 9% 5% 3 4 3 4 6% 29%	1885
	11th to 20th	Day	16% 11% 3 9% 6% 5% 4 3 5% 6% 33%	1848
	21st to end	Day	19% 13% 5% 8% 4 3 4 3 5% 6% 31%	1570
March	1st to 10th	Day	24% 15% 4 7% 3 3 3 3 4 4 30%	3120
	11th to 20th	Day	15% 8% 3 5% 4 4 4 3 6% 6% 42%	3411
	21st to end	Day	15% 11% 3 7% 5% 4 5% 4 5% 7% 33%	3899
April	1st to 10th	Day	15% 10% 3 7% 5% 4 4 4 6% 7% 35%	2280
	11th to 20th	Day	18% 9% 3 6% 4 4 4 3 5% 7% 39%	2254
	21st to end	Day	19% 7% 2 7% 5% 5% 4 3 5% 7% 35%	2460
May	1st to 10th	Day	21% 8% 3 8% 5% 4 5% 3 5% 8% 30%	4082
	11th to 20th	Day	21% 8% 2 6% 3 4 4 3 5% 6% 38%	4081
	21st to end	Day	11% 6% 3 7% 5% 5% 6% 5% 7% 9% 36%	4476
June	1st to 10th	Day	12% 7% 2 6% 5% 4 5% 4 6% 7% 43%	2606
	11th to 20th	Day	9% 5% 2 5% 5% 5% 4 4 7% 8% 46%	2660
	21st to end	Day	10% 6% 2 6% 5% 4 5% 4 6% 9% 42%	2660
July	1st to 10th	Day	13% 6% 2 6% 4 5% 5% 4 7% 10% 39%	4440
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 7% 11% 48%	4440
	21st to end	Day	10% 8% 3 7% 6% 5% 6% 6% 8% 9% 32%	4512
August	1st to 10th	Day	8% 5% 2 5% 4 4 5% 4 7% 10% 46%	4104
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 5% 7% 11% 41%	3916
	21st to end	Day	6% 4 2 5% 3 4 4 4 6% 8% 55%	4180
September	1st to 10th	Day	12% 5% 2 6% 4 4 5% 4 5% 8% 46%	2217
	11th to 20th	Day	11% 5% 1 6% 5% 5% 5% 3 6% 7% 47%	2014
	21st to end	Day	13% 4 2 5% 4 3 4 3 5% 7% 51%	1980
October	1st to 10th	Day	11% 5% 2 6% 5% 4 4 3 4 6% 49%	1980
	11th to 20th	Day	13% 6% 2 6% 4 4 4 3 5% 6% 47%	1872
	21st to end	Day	20% 10% 3 7% 3 3 4 2 5% 6% 36%	1782
November	1st to 10th	Day	9% 6% 2 6% 3 4 3 3 4 6% 53%	1672
	11th to 20th	Day	15% 11% 4 7% 4 2 4 3 6% 6% 38%	1710
	21st to end	Day	18% 10% 4 8% 4 3 4 2 4 6% 38%	1710
December	1st to 10th	Day	21% 14% 4 9% 5% 4 4 3 4 5% 29%	1710
	11th to 20th	Day	16% 16% 6% 9% 4 4 4 3 5% 5% 28%	2082
	21st to end	Day	24% 20% 6% 7% 4 4 4 3 5% 5% 20%	2783
January	1st to 10th	Night	18% 17% 6% 9% 5% 4 3 3 5% 5% 24%	4428
	11th to 20th	Night	15% 17% 6% 14% 6% 5% 3 3 4 5% 23%	4440
	21st to end	Night	26% 18% 6% 7% 4 3 3 2 3 4 25%	4585
February	1st to 10th	Night	21% 12% 5% 9% 5% 4 3 4 5% 5% 28%	2295
	11th to 20th	Night	15% 12% 4 8% 6% 4 4 3 5% 7% 33%	2218
	21st to end	Night	21% 12% 5% 9% 4 3 4 3 4 5% 31%	1884
March	1st to 10th	Night	23% 12% 2 7% 3 4 3 3 4 5% 34%	3733
	11th to 20th	Night	13% 7% 3 6% 4 4 3 3 5% 7% 46%	3549
	21st to end	Night	17% 8% 2 7% 5% 4 3 4 5% 6% 40%	3759
April	1st to 10th	Night	14% 9% 3 7% 4 4 5% 4 6% 7% 38%	1900
	11th to 20th	Night	16% 9% 2 6% 4 3 4 4 5% 7% 40%	1875
	21st to end	Night	17% 7% 3 6% 5% 4 4 3 6% 8% 37%	1719
May	1st to 10th	Night	20% 11% 3 8% 5% 5% 5% 4 6% 7% 28%	2813
	11th to 20th	Night	17% 7% 2 6% 4 4 4 4 5% 7% 40%	2812
	21st to end	Night	10% 5% 2 7% 3 4 5% 4 6% 10% 43%	3079
June	1st to 10th	Night	9% 5% 2 7% 4 4 4 5% 5% 8% 47%	1570
	11th to 20th	Night	8% 4 3 5% 3 3 4 5% 8% 10% 46%	1520
	21st to end	Night	7% 4 2 5% 3 5% 4 4 8% 10% 49%	1520
July	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 5% 8% 48%	2530
	11th to 20th	Night	6% 3 2 4 4 4 4 4 6% 8% 55%	2530
	21st to end	Night	10% 7% 3 7% 4 4 6% 5% 8% 10% 37%	3026
August	1st to 10th	Night	9% 5% 2 5% 4 3 4 3 6% 10% 48%	2825
	11th to 20th	Night	7% 5% 3 6% 5% 4 5% 4 6% 8% 48%	2909
	21st to end	Night	3 3 1 4 4 4 5% 4 6% 10% 57%	3476
September	1st to 10th	Night	12% 5% 2 5% 3 3 5% 4 4 6% 52%	1943
	11th to 20th	Night	9% 3 1 5% 3 4 4 4 6% 8% 54%	2024
	21st to end	Night	13% 5% 2 5% 4 3 3 2 4 6% 53%	1980
October	1st to 10th	Night	9% 4 2 5% 4 3 3 3 4 7% 55%	1980
	11th to 20th	Night	15% 6% 3 6% 4 4 4 3 5% 6% 44%	2083
	21st to end	Night	18% 8% 2 8% 4 4 4 3 4 6% 40%	2574
November	1st to 10th	Night	13% 7% 2 6% 4 3 3 3 5% 6% 48%	2406
	11th to 20th	Night	15% 9% 3 7% 5% 3 4 3 5% 6% 41%	2470
	21st to end	Night	19% 11% 3 7% 5% 4 3 3 4 6% 34%	2470
December	1st to 10th	Night	19% 13% 4 9% 4 4 3 4 4 6% 30%	2470
	11th to 20th	Night	19% 13% 5% 8% 4 4 4 3 4 5% 31%	3110
	21st to end	Night	22% 19% 6% 10% 5% 4 4 3 4 5% 18%	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	58% 5% 9% 18% 9%	2528
	11th to 20th	Day	61% 6% 12% 16% 5%	2530
	21st to end	Day	69% 4 8% 14% 3 1	3069
February	1st to 10th	Day	56% 4 10% 18% 10% 1	1885
	11th to 20th	Day	53% 5% 9% 21% 11% 1	1858
	21st to end	Day	54% 4 10% 21% 8% 3	1570
March	1st to 10th	Day	57% 5% 6% 18% 11% 2	3120
	11th to 20th	Day	40% 7% 10% 27% 15% 2	3411
	21st to end	Day	47% 8% 13% 22% 9% 1	3899
April	1st to 10th	Day	45% 7% 12% 23% 12% 1	2280
	11th to 20th	Day	45% 6% 11% 22% 13% 3	2254
	21st to end	Day	47% 8% 10% 22% 12% 2	2460
May	1st to 10th	Day	50% 9% 14% 20% 7%	4120
	11th to 20th	Day	46% 7% 12% 23% 12%	4085
	21st to end	Day	39% 11% 14% 18% 16% 2	4489
June	1st to 10th	Day	39% 7% 11% 24% 16% 3	2606
	11th to 20th	Day	35% 8% 14% 23% 19% 2	2660
	21st to end	Day	38% 7% 15% 20% 17% 3	2660
July	1st to 10th	Day	41% 6% 12% 20% 19% 2	4371
	11th to 20th	Day	30% 5% 11% 31% 20% 1	4365
	21st to end	Day	46% 6% 15% 16% 14% 3	4369
August	1st to 10th	Day	32% 6% 14% 34% 13% 2	4091
	11th to 20th	Day	36% 5% 11% 32% 15% 1	3955
	21st to end	Day	27% 6% 13% 33% 18% 4	4179
September	1st to 10th	Day	36% 6% 11% 23% 18% 6%	2217
	11th to 20th	Day	34% 7% 12% 23% 20% 4	2014
	21st to end	Day	34% 5% 10% 25% 23% 3	1980
October	1st to 10th	Day	36% 5% 10% 23% 23% 2	1980
	11th to 20th	Day	37% 6% 9% 25% 20% 3	1872
	21st to end	Day	50% 4 8% 22% 14% 2	1782
November	1st to 10th	Day	33% 4 11% 29% 19% 4	1672
	11th to 20th	Day	46% 5% 9% 21% 15% 4	1710
	21st to end	Day	48% 4 9% 23% 14% 1	1710
December	1st to 10th	Day	59% 4 10% 18% 9%	1710
	11th to 20th	Day	57% 6% 8% 18% 11%	2079
	21st to end	Day	66% 6% 9% 12% 7% 1	2783
January	1st to 10th	Night	59% 8% 10% 15% 8%	4428
	11th to 20th	Night	65% 5% 10% 13% 7%	4440
	21st to end	Night	64% 4 7% 16% 8% 1	4585
February	1st to 10th	Night	58% 5% 10% 18% 8% 1	2295
	11th to 20th	Night	51% 5% 12% 19% 12% 1	2219
	21st to end	Night	55% 5% 10% 17% 11% 2	1884
March	1st to 10th	Night	53% 5% 7% 22% 11% 1	3733
	11th to 20th	Night	36% 8% 12% 23% 19% 2	3549
	21st to end	Night	44% 7% 9% 27% 11% 1	3759
April	1st to 10th	Night	42% 7% 11% 24% 13% 2	1900
	11th to 20th	Night	42% 7% 9% 19% 18% 5%	1875
	21st to end	Night	43% 7% 11% 22% 14% 3	1719
May	1st to 10th	Night	52% 11% 10% 18% 7% 2	2840
	11th to 20th	Night	42% 9% 15% 25% 9% 1	2813
	21st to end	Night	32% 11% 15% 24% 14% 4	3087
June	1st to 10th	Night	33% 7% 11% 25% 20% 3	1570
	11th to 20th	Night	29% 9% 13% 23% 20% 5%	1520
	21st to end	Night	28% 9% 16% 23% 19% 6%	1520
July	1st to 10th	Night	35% 8% 11% 24% 18% 5%	2490
	11th to 20th	Night	26% 5% 13% 33% 21% 2	2482
	21st to end	Night	41% 4 15% 25% 11% 4	2929
August	1st to 10th	Night	32% 6% 13% 35% 13% 1	2814
	11th to 20th	Night	34% 5% 12% 30% 19% 1	2936
	21st to end	Night	23% 6% 14% 32% 22% 3	3476
September	1st to 10th	Night	31% 7% 11% 24% 19% 8%	1943
	11th to 20th	Night	27% 6% 13% 25% 26% 4	2024
	21st to end	Night	34% 5% 10% 23% 25% 4	1980
October	1st to 10th	Night	29% 6% 10% 26% 24% 5%	1980
	11th to 20th	Night	39% 6% 8% 23% 21% 3	2083
	21st to end	Night	45% 5% 9% 21% 17% 3	2574
November	1st to 10th	Night	36% 5% 11% 22% 22% 4	2406
	11th to 20th	Night	43% 4 10% 20% 19% 4	2470
	21st to end	Night	52% 4 9% 21% 13% 1	2470
December	1st to 10th	Night	54% 6% 10% 20% 10% 1	2470
	11th to 20th	Night	54% 6% 9% 17% 13% 1	3112
	21st to end	Night	68% 7% 8% 11% 6% 1	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	25% 17% 34% 19% 6%	270
	11th to 20th	Day	29% 19% 36% 14% 3	270
	21st to end	Day	30% 21% 33% 13% 3	297
February	1st to 10th	Day	27% 21% 34% 15% 2	270
	11th to 20th	Day	25% 23% 36% 13% 3	270
	21st to end	Day	30% 16% 44% 8% 3	225
March	1st to 10th	Day	24% 27% 33% 14% 2	270
	11th to 20th	Day	28% 17% 33% 17% 5% 1	417
	21st to end	Day	30% 17% 32% 17% 5%	506
April	1st to 10th	Day	21% 18% 31% 21% 8% 1	460
	11th to 20th	Day	20% 19% 25% 22% 10% 3	452
	21st to end	Day	23% 14% 39% 15% 7% 2	460
May	1st to 10th	Day	22% 15% 32% 19% 9% 4	463
	11th to 20th	Day	21% 9% 26% 25% 13% 6%	487
	21st to end	Day	13% 6% 25% 26% 17% 12%	529
June	1st to 10th	Day	12% 6% 26% 29% 19% 7%	626
	11th to 20th	Day	11% 6% 19% 33% 23% 8%	680
	21st to end	Day	13% 5% 23% 29% 22% 7%	680
July	1st to 10th	Day	13% 4 15% 26% 25% 17%	671
	11th to 20th	Day	7% 3 22% 32% 23% 12%	668
	21st to end	Day	12% 2 19% 29% 27% 12%	565
August	1st to 10th	Day	9% 3 16% 28% 25% 19%	486
	11th to 20th	Day	13% 3 19% 28% 25% 11%	479
	21st to end	Day	10% 3 14% 28% 30% 16%	539
September	1st to 10th	Day	10% 4 17% 33% 28% 9%	490
	11th to 20th	Day	11% 2 16% 27% 32% 13%	463
	21st to end	Day	14% 8% 20% 26% 23% 8%	450
October	1st to 10th	Day	12% 5% 26% 34% 18% 6%	450
	11th to 20th	Day	14% 13% 33% 26% 11% 2	437
	21st to end	Day	29% 14% 33% 22% 2	297
November	1st to 10th	Day	13% 16% 32% 26% 10% 3	270
	11th to 20th	Day	21% 20% 29% 20% 7% 3	270
	21st to end	Day	26% 11% 36% 21% 5%	270
December	1st to 10th	Day	21% 19% 37% 16% 6% 1	270
	11th to 20th	Day	24% 16% 42% 13% 4 1	270
	21st to end	Day	32% 20% 32% 12% 3 1	297
January	1st to 10th	Night	23% 14% 34% 23% 5%	950
	11th to 20th	Night	25% 14% 37% 20% 3	950
	21st to end	Night	24% 12% 37% 24% 3	863
February	1st to 10th	Night	25% 13% 36% 23% 3	760
	11th to 20th	Night	22% 14% 40% 22% 2	743
	21st to end	Night	27% 13% 37% 21% 2	628
March	1st to 10th	Night	23% 13% 34% 25% 5%	760
	11th to 20th	Night	23% 11% 37% 23% 5% 1	760
	21st to end	Night	24% 10% 32% 28% 6%	803
April	1st to 10th	Night	17% 11% 37% 29% 6%	570
	11th to 20th	Night	18% 10% 33% 27% 10% 1	563
	21st to end	Night	21% 6% 38% 26% 8% 1	570
May	1st to 10th	Night	18% 9% 34% 26% 11% 1	570
	11th to 20th	Night	16% 5% 25% 34% 18% 3	561
	21st to end	Night	9% 2 24% 36% 27% 3	617
June	1st to 10th	Night	10% 3 22% 42% 20% 2	570
	11th to 20th	Night	8% 4 19% 42% 25% 3	570
	21st to end	Night	7% 1 22% 40% 29% 1	570
July	1st to 10th	Night	11% 3 11% 32% 34% 9%	555
	11th to 20th	Night	7% 3 13% 38% 34% 5%	552
	21st to end	Night	11% 1 12% 40% 32% 4	624
August	1st to 10th	Night	8% 2 12% 35% 37% 5%	567
	11th to 20th	Night	7% 2 12% 43% 31% 5%	564
	21st to end	Night	6% 1 14% 34% 39% 6%	627
September	1st to 10th	Night	13% 2 13% 38% 29% 5%	613
	11th to 20th	Night	10% 2 15% 35% 34% 5%	736
	21st to end	Night	14% 4 23% 31% 26% 3	720
October	1st to 10th	Night	10% 6% 24% 33% 23% 5%	720
	11th to 20th	Night	17% 8% 28% 33% 13% 1	720
	21st to end	Night	21% 10% 35% 26% 9% 1	792
November	1st to 10th	Night	15% 7% 33% 30% 14% 1	740
	11th to 20th	Night	19% 14% 31% 27% 9% 1	760
	21st to end	Night	24% 11% 28% 30% 6% 1	760
December	1st to 10th	Night	24% 10% 38% 22% 5%	760
	11th to 20th	Night	26% 10% 33% 23% 7% 1	802
	21st to end	Night	30% 13% 33% 20% 3	1045

Page generated at 2024-09-11 10:00:01. Version ac07

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