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 2025-02-21.

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% 4 4 4 4 5% 7% 37%	108599
	Night	15% 10% 4 7% 4 4 4 3 5% 6% 37%	110092
Transparency	Day	44% 6% 11% 23% 15% 2	112270
	Night	45% 5% 10% 22% 15% 2	114561
Seeing	Day	18% 11% 27% 24% 15% 6%	15789
	Night	18% 8% 28% 29% 14% 2	25707

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% 15% 7% 10% 5% 4 4 3 5% 6% 25%	3617
	11th to 20th	Day	14% 15% 5% 15% 5% 5% 5% 3 3 4 27%	3620
	21st to end	Day	21% 13% 7% 9% 5% 4 5% 3 3 4 25%	4398
February	1st to 10th	Day	19% 13% 5% 9% 5% 3 4 3 4 7% 29%	1985
	11th to 20th	Day	17% 11% 4 10% 6% 5% 4 3 5% 5% 32%	1948
	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	10% 5% 1 6% 4 5% 5% 3 6% 8% 47%	2090
	21st to end	Day	14% 5% 2 5% 4 3 4 3 5% 6% 48%	2090
October	1st to 10th	Day	10% 5% 2 6% 4 4 4 3 4 6% 51%	2090
	11th to 20th	Day	13% 6% 2 6% 3 4 4 3 5% 6% 47%	1975
	21st to end	Day	19% 10% 3 7% 3 3 4 2 5% 6% 37%	1881
November	1st to 10th	Day	9% 6% 2 6% 4 4 3 3 4 6% 52%	1762
	11th to 20th	Day	15% 11% 3 7% 4 3 4 3 6% 6% 38%	1800
	21st to end	Day	18% 10% 4 8% 4 3 4 2 4 6% 37%	1800
December	1st to 10th	Day	17% 12% 3 7% 3 3 3 2 4 4 42%	2930
	11th to 20th	Day	17% 15% 5% 9% 5% 5% 5% 4 5% 5% 26%	3263
	21st to end	Day	26% 18% 5% 7% 4 4 4 3 4 5% 19%	3982
January	1st to 10th	Night	17% 15% 7% 12% 7% 4 4 3 5% 5% 20%	6348
	11th to 20th	Night	13% 18% 7% 13% 6% 5% 5% 3 3 5% 23%	6360
	21st to end	Night	21% 13% 5% 7% 4 4 3 3 3 5% 32%	6557
February	1st to 10th	Night	21% 12% 5% 9% 5% 4 4 4 5% 5% 27%	2415
	11th to 20th	Night	16% 12% 4 9% 6% 4 4 3 5% 7% 32%	2338
	21st to end	Night	21% 12% 5% 9% 4 3 4 3 4 5% 31%	1885
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 3 4 6% 8% 54%	2090
	21st to end	Night	15% 5% 2 5% 4 3 3 2 4 6% 51%	2090
October	1st to 10th	Night	9% 4 2 5% 4 3 3 3 4 7% 57%	2090
	11th to 20th	Night	15% 6% 2 6% 3 3 4 3 5% 6% 46%	2200
	21st to end	Night	17% 8% 2 8% 4 4 4 3 4 6% 41%	2717
November	1st to 10th	Night	13% 7% 2 6% 4 3 3 4 5% 6% 48%	2536
	11th to 20th	Night	16% 9% 3 7% 5% 3 4 3 5% 6% 41%	2600
	21st to end	Night	19% 12% 3 8% 5% 4 3 3 4 5% 33%	2600
December	1st to 10th	Night	18% 11% 3 8% 4 3 3 3 4 5% 38%	4250
	11th to 20th	Night	19% 14% 6% 9% 5% 4 4 3 5% 5% 28%	4932
	21st to end	Night	26% 18% 6% 10% 5% 4 3 3 4 4 18%	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% 4 7% 21% 8% 1	3618
	11th to 20th	Day	60% 4 9% 15% 10% 2	3620
	21st to end	Day	64% 3 8% 17% 7% 2	4398
February	1st to 10th	Day	56% 4 11% 18% 10% 1	1985
	11th to 20th	Day	55% 5% 9% 20% 11% 1	1958
	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	33% 6% 12% 24% 21% 4	2090
	21st to end	Day	37% 5% 10% 24% 22% 3	2090
October	1st to 10th	Day	24% 3 7% 31% 34% 1	3480
	11th to 20th	Day	32% 3 8% 28% 28% 2	3255
	21st to end	Day	39% 2 8% 23% 25% 2	3081
November	1st to 10th	Day	34% 4 11% 29% 19% 4	1762
	11th to 20th	Day	46% 4 9% 22% 15% 4	1800
	21st to end	Day	49% 4 9% 22% 14% 1	1800
December	1st to 10th	Day	48% 2 8% 28% 13%	2930
	11th to 20th	Day	61% 4 8% 18% 8%	3260
	21st to end	Day	68% 4 10% 12% 6%	3982
January	1st to 10th	Night	64% 6% 8% 15% 8%	6348
	11th to 20th	Night	66% 4 8% 11% 9% 3	6360
	21st to end	Night	56% 3 7% 22% 11% 1	6557
February	1st to 10th	Night	59% 5% 10% 18% 8% 1	2415
	11th to 20th	Night	53% 4 11% 18% 12% 1	2339
	21st to end	Night	55% 5% 9% 17% 11% 2	1885
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	26% 5% 13% 25% 26% 5%	2090
	21st to end	Night	36% 4 10% 23% 24% 3	2090
October	1st to 10th	Night	22% 4 6% 22% 43% 3	3480
	11th to 20th	Night	27% 3 8% 23% 36% 2	3659
	21st to end	Night	35% 3 7% 23% 26% 6%	4467
November	1st to 10th	Night	37% 5% 10% 22% 22% 4	2536
	11th to 20th	Night	43% 4 9% 20% 19% 4	2600
	21st to end	Night	54% 4 9% 20% 12% 1	2600
December	1st to 10th	Night	50% 3 10% 25% 12%	4250
	11th to 20th	Night	59% 4 10% 16% 11% 1	4934
	21st to end	Night	70% 5% 8% 12% 5%	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	26% 18% 33% 18% 6%	300
	11th to 20th	Day	29% 20% 36% 13% 2	300
	21st to end	Day	29% 23% 33% 13% 2	330
February	1st to 10th	Day	26% 22% 35% 14% 2	300
	11th to 20th	Day	26% 24% 36% 11% 3	300
	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	10% 2 16% 29% 31% 13%	490
	21st to end	Day	17% 7% 20% 25% 22% 8%	490
October	1st to 10th	Day	11% 4 25% 34% 19% 6%	490
	11th to 20th	Day	13% 14% 32% 25% 13% 3	476
	21st to end	Day	27% 13% 34% 23% 3	330
November	1st to 10th	Day	13% 15% 34% 25% 9% 3	300
	11th to 20th	Day	22% 20% 30% 19% 7% 2	300
	21st to end	Day	27% 12% 36% 21% 4	300
December	1st to 10th	Day	21% 19% 39% 15% 5% 1	300
	11th to 20th	Day	25% 16% 41% 12% 4 1	300
	21st to end	Day	33% 19% 32% 13% 3 1	330
January	1st to 10th	Night	23% 15% 34% 24% 5%	1000
	11th to 20th	Night	25% 15% 37% 20% 3	1000
	21st to end	Night	23% 13% 38% 24% 3	908
February	1st to 10th	Night	25% 13% 37% 22% 3	800
	11th to 20th	Night	24% 14% 39% 21% 2	783
	21st to end	Night	27% 13% 37% 21% 2	629
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	9% 2 14% 36% 34% 5%	760
	21st to end	Night	16% 4 23% 30% 25% 3	760
October	1st to 10th	Night	9% 5% 24% 34% 23% 5%	760
	11th to 20th	Night	17% 8% 27% 33% 13% 1	760
	21st to end	Night	20% 9% 35% 25% 9% 1	836
November	1st to 10th	Night	16% 7% 33% 29% 13% 1	780
	11th to 20th	Night	19% 14% 32% 26% 9% 1	800
	21st to end	Night	25% 12% 28% 30% 6% 1	800
December	1st to 10th	Night	25% 10% 38% 22% 5%	800
	11th to 20th	Night	26% 10% 34% 22% 7% 1	845
	21st to end	Night	30% 13% 33% 20% 3	1100

Page generated at 2025-02-18 03:26:31. Version ac07

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