The Westport Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Westport Observatory clear sky chart from 2005-11-06 to 2024-01-12.

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	17% 6% 2 5% 5% 3 4 5% 5% 7% 41%	74766
	Night	17% 7% 3 5% 4 3 4 4 5% 7% 42%	74922
Transparency	Day	27% 38% 9% 15% 10% 1	74471
	Night	28% 38% 8% 15% 10% 1	74751
Seeing	Day	18% 5% 20% 32% 20% 5%	13887
	Night	20% 7% 28% 31% 13% 2	23079

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	15% 6% 2 4 5% 3 5% 5% 6% 6% 43%	1517
	11th to 20th	Day	17% 8% 3 5% 5% 3 5% 6% 6% 7% 37%	1448
	21st to end	Day	15% 6% 3 5% 4 3 5% 5% 5% 8% 42%	1665
February	1st to 10th	Day	18% 7% 3 5% 5% 3 5% 5% 5% 6% 38%	1663
	11th to 20th	Day	16% 6% 2 5% 4 3 4 5% 5% 7% 43%	1748
	21st to end	Day	21% 8% 2 6% 4 2 4 4 5% 6% 38%	1480
March	1st to 10th	Day	15% 6% 3 6% 4 3 4 5% 5% 7% 43%	1797
	11th to 20th	Day	16% 6% 3 4 6% 3 5% 5% 7% 7% 38%	1856
	21st to end	Day	21% 7% 3 6% 4 3 4 3 3 6% 40%	2376
April	1st to 10th	Day	19% 7% 3 5% 5% 2 4 4 5% 8% 39%	2160
	11th to 20th	Day	17% 6% 3 5% 4 3 4 4 5% 7% 42%	2134
	21st to end	Day	21% 6% 2 4 5% 3 5% 4 5% 6% 40%	2228
May	1st to 10th	Day	24% 7% 2 4 4 2 4 5% 6% 7% 35%	2340
	11th to 20th	Day	19% 6% 3 5% 5% 2 4 6% 6% 7% 39%	2419
	21st to end	Day	16% 5% 2 5% 5% 3 4 5% 6% 7% 43%	2725
June	1st to 10th	Day	19% 5% 2 5% 4 3 5% 6% 6% 8% 36%	2520
	11th to 20th	Day	16% 6% 3 4 5% 4 4 5% 6% 9% 37%	2520
	21st to end	Day	14% 6% 3 5% 5% 4 4 6% 6% 9% 40%	2520
July	1st to 10th	Day	12% 5% 2 4 5% 3 6% 6% 7% 9% 40%	2520
	11th to 20th	Day	11% 6% 2 5% 5% 3 4 6% 7% 9% 42%	2520
	21st to end	Day	13% 6% 3 5% 6% 3 5% 6% 6% 9% 39%	2744
August	1st to 10th	Day	11% 5% 2 5% 5% 3 5% 6% 7% 9% 41%	2405
	11th to 20th	Day	13% 5% 2 6% 5% 3 5% 5% 6% 9% 41%	2126
	21st to end	Day	12% 5% 2 5% 5% 3 5% 5% 6% 8% 43%	2376
September	1st to 10th	Day	14% 5% 3 4 5% 3 5% 5% 5% 8% 43%	2160
	11th to 20th	Day	13% 4 2 4 3 2 5% 5% 4 7% 51%	2052
	21st to end	Day	19% 4 2 5% 4 3 4 4 5% 7% 42%	1980
October	1st to 10th	Day	23% 5% 2 4 3 3 3 4 4 6% 42%	1926
	11th to 20th	Day	16% 5% 2 5% 3 3 4 4 5% 8% 45%	1800
	21st to end	Day	22% 5% 2 4 5% 3 4 5% 6% 6% 40%	1794
November	1st to 10th	Day	14% 6% 2 4 4 2 3 3 5% 5% 50%	1671
	11th to 20th	Day	19% 6% 2 4 3 2 3 4 4 6% 47%	1710
	21st to end	Day	17% 5% 2 4 4 2 4 4 5% 7% 44%	1668
December	1st to 10th	Day	19% 7% 2 5% 5% 3 5% 5% 5% 7% 37%	1520
	11th to 20th	Day	23% 6% 2 3 3 2 3 3 3 7% 44%	1906
	21st to end	Day	18% 11% 3 6% 6% 3 5% 4 5% 6% 33%	2772
January	1st to 10th	Night	16% 6% 2 4 4 2 3 4 4 7% 47%	2649
	11th to 20th	Night	20% 7% 3 4 4 2 3 4 4 7% 40%	2535
	21st to end	Night	14% 9% 3 5% 3 3 3 4 4 7% 45%	2686
February	1st to 10th	Night	18% 8% 3 6% 5% 3 4 4 5% 5% 40%	2295
	11th to 20th	Night	18% 7% 3 5% 4 3 3 3 5% 6% 41%	2098
	21st to end	Night	20% 7% 3 5% 5% 2 4 3 5% 4 42%	1776
March	1st to 10th	Night	18% 6% 2 3 4 3 4 4 5% 7% 45%	2150
	11th to 20th	Night	17% 6% 2 5% 5% 3 4 4 5% 7% 42%	2097
	21st to end	Night	17% 8% 3 4 5% 3 4 4 4 5% 43%	1980
April	1st to 10th	Night	19% 6% 3 5% 5% 3 4 5% 5% 5% 40%	1800
	11th to 20th	Night	19% 7% 2 5% 5% 2 4 4 4 6% 42%	1775
	21st to end	Night	20% 6% 3 5% 5% 2 4 4 4 5% 41%	1730
May	1st to 10th	Night	23% 7% 3 6% 6% 3 4 4 4 7% 34%	1620
	11th to 20th	Night	21% 7% 4 4 4 3 4 4 5% 7% 36%	1487
	21st to end	Night	18% 7% 2 5% 4 3 4 4 6% 7% 40%	1550
June	1st to 10th	Night	19% 6% 3 5% 5% 3 4 4 5% 7% 40%	1440
	11th to 20th	Night	15% 7% 3 5% 5% 2 4 4 6% 9% 40%	1440
	21st to end	Night	12% 5% 2 5% 5% 3 5% 4 7% 10% 44%	1440
July	1st to 10th	Night	13% 5% 2 5% 6% 3 6% 5% 6% 8% 41%	1440
	11th to 20th	Night	13% 5% 2 6% 5% 3 5% 7% 6% 10% 38%	1440
	21st to end	Night	11% 5% 2 4 4 3 6% 7% 7% 8% 41%	1574
August	1st to 10th	Night	8% 6% 3 5% 5% 3 5% 6% 6% 8% 44%	1511
	11th to 20th	Night	11% 6% 3 5% 7% 5% 6% 5% 7% 8% 36%	1774
	21st to end	Night	14% 5% 2 4 5% 3 5% 4 5% 8% 45%	1980
September	1st to 10th	Night	15% 6% 2 5% 6% 3 4 4 4 7% 45%	1800
	11th to 20th	Night	15% 5% 1 4 4 2 4 3 5% 6% 52%	1908
	21st to end	Night	19% 6% 3 4 4 3 4 4 3 6% 45%	1980
October	1st to 10th	Night	20% 7% 3 5% 4 2 4 3 4 6% 41%	2029
	11th to 20th	Night	16% 5% 2 5% 4 3 4 3 5% 7% 47%	2160
	21st to end	Night	23% 7% 2 4 4 3 3 4 4 6% 41%	2560
November	1st to 10th	Night	13% 5% 3 4 4 3 3 4 5% 6% 49%	2407
	11th to 20th	Night	20% 6% 2 4 3 2 3 3 4 6% 47%	2470
	21st to end	Night	15% 5% 2 4 5% 2 4 4 4 7% 47%	2509
December	1st to 10th	Night	20% 6% 3 4 5% 3 4 4 4 7% 40%	2660
	11th to 20th	Night	20% 6% 2 4 4 3 4 4 5% 6% 42%	3288
	21st to end	Night	21% 10% 3 5% 4 3 3 3 4 5% 38%	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	25% 40% 9% 19% 7%	1518
	11th to 20th	Day	30% 43% 8% 11% 6% 1	1448
	21st to end	Day	26% 38% 7% 18% 9% 1	1665
February	1st to 10th	Day	28% 37% 8% 15% 11% 1	1663
	11th to 20th	Day	27% 38% 9% 17% 9% 2	1757
	21st to end	Day	31% 36% 7% 14% 10% 1	1480
March	1st to 10th	Day	26% 37% 10% 17% 10% 2	1800
	11th to 20th	Day	28% 38% 9% 14% 9% 2	1856
	21st to end	Day	29% 37% 5% 14% 13% 3	2376
April	1st to 10th	Day	25% 38% 8% 14% 12% 3	2160
	11th to 20th	Day	28% 36% 9% 14% 11% 3	2134
	21st to end	Day	27% 37% 9% 13% 11% 3	2228
May	1st to 10th	Day	29% 39% 7% 13% 11% 1	2340
	11th to 20th	Day	27% 39% 8% 13% 11% 3	2423
	21st to end	Day	23% 41% 12% 13% 9% 2	2725
June	1st to 10th	Day	27% 39% 11% 13% 8% 1	2520
	11th to 20th	Day	26% 41% 9% 16% 6% 1	2520
	21st to end	Day	27% 40% 12% 15% 7% 1	2520
July	1st to 10th	Day	25% 41% 9% 14% 9% 1	2450
	11th to 20th	Day	22% 40% 10% 17% 9% 1	2445
	21st to end	Day	24% 42% 9% 12% 12% 1	2591
August	1st to 10th	Day	25% 41% 11% 15% 8%	2391
	11th to 20th	Day	25% 38% 10% 16% 10% 1	2126
	21st to end	Day	27% 40% 7% 12% 12% 2	2376
September	1st to 10th	Day	25% 39% 8% 13% 13% 2	2160
	11th to 20th	Day	20% 38% 10% 17% 15% 1	2052
	21st to end	Day	27% 38% 6% 13% 13% 2	1980
October	1st to 10th	Day	27% 37% 8% 16% 13%	1926
	11th to 20th	Day	25% 38% 10% 16% 10% 1	1800
	21st to end	Day	30% 37% 7% 15% 10% 1	1794
November	1st to 10th	Day	21% 41% 7% 16% 14% 1	1671
	11th to 20th	Day	25% 40% 8% 15% 11% 2	1710
	21st to end	Day	27% 40% 9% 16% 8% 1	1668
December	1st to 10th	Day	27% 41% 10% 13% 7% 1	1520
	11th to 20th	Day	30% 32% 12% 14% 12% 1	1906
	21st to end	Day	39% 29% 10% 16% 6%	2772
January	1st to 10th	Night	25% 40% 8% 17% 9% 1	2649
	11th to 20th	Night	30% 42% 8% 14% 5% 1	2535
	21st to end	Night	25% 37% 8% 18% 11% 1	2686
February	1st to 10th	Night	30% 37% 8% 14% 10% 1	2295
	11th to 20th	Night	30% 38% 9% 13% 9% 3	2100
	21st to end	Night	31% 35% 5% 16% 11% 2	1776
March	1st to 10th	Night	24% 37% 9% 18% 10% 2	2160
	11th to 20th	Night	30% 38% 7% 15% 8% 2	2098
	21st to end	Night	28% 37% 6% 14% 12% 3	1980
April	1st to 10th	Night	29% 38% 8% 11% 12% 2	1800
	11th to 20th	Night	33% 36% 6% 11% 14% 1	1775
	21st to end	Night	28% 38% 7% 13% 11% 3	1730
May	1st to 10th	Night	33% 37% 6% 11% 11% 2	1620
	11th to 20th	Night	29% 38% 7% 11% 13% 2	1488
	21st to end	Night	27% 40% 9% 14% 10% 1	1551
June	1st to 10th	Night	29% 38% 9% 12% 11% 1	1440
	11th to 20th	Night	27% 38% 11% 16% 6% 1	1440
	21st to end	Night	25% 40% 11% 16% 7% 1	1440
July	1st to 10th	Night	28% 41% 10% 13% 7% 1	1400
	11th to 20th	Night	25% 41% 10% 15% 8% 1	1392
	21st to end	Night	23% 42% 10% 13% 10% 2	1486
August	1st to 10th	Night	24% 41% 11% 15% 9%	1501
	11th to 20th	Night	29% 39% 8% 16% 9% 1	1774
	21st to end	Night	27% 40% 9% 13% 10% 1	1980
September	1st to 10th	Night	27% 38% 7% 15% 12% 1	1800
	11th to 20th	Night	23% 37% 7% 16% 15% 2	1908
	21st to end	Night	26% 38% 6% 13% 15% 2	1980
October	1st to 10th	Night	29% 37% 7% 15% 10% 1	2029
	11th to 20th	Night	27% 37% 10% 17% 8% 1	2160
	21st to end	Night	30% 38% 8% 13% 10% 1	2560
November	1st to 10th	Night	20% 42% 8% 14% 13% 3	2407
	11th to 20th	Night	26% 40% 6% 17% 11% 1	2470
	21st to end	Night	25% 40% 8% 18% 9%	2509
December	1st to 10th	Night	26% 41% 8% 14% 10% 1	2660
	11th to 20th	Night	29% 33% 9% 12% 14% 2	3288
	21st to end	Night	37% 26% 11% 18% 8%	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	19% 9% 37% 25% 9% 1	180
	11th to 20th	Day	15% 9% 43% 26% 7% 1	162
	21st to end	Day	20% 5% 35% 31% 8% 2	212
February	1st to 10th	Day	26% 7% 34% 30% 4	283
	11th to 20th	Day	20% 8% 39% 30% 3	416
	21st to end	Day	24% 11% 31% 26% 8%	346
March	1st to 10th	Day	15% 6% 36% 35% 8%	420
	11th to 20th	Day	16% 6% 36% 32% 8% 1	420
	21st to end	Day	22% 6% 30% 31% 8% 2	462
April	1st to 10th	Day	22% 8% 27% 30% 12% 2	420
	11th to 20th	Day	18% 5% 23% 37% 17% 1	412
	21st to end	Day	26% 4 20% 37% 12% 1	420
May	1st to 10th	Day	26% 3 21% 35% 11% 3	423
	11th to 20th	Day	20% 3 16% 39% 19% 2	447
	21st to end	Day	17% 2 8% 33% 29% 9%	485
June	1st to 10th	Day	18% 3 10% 34% 29% 7%	450
	11th to 20th	Day	18% 2 11% 37% 25% 7%	450
	21st to end	Day	13% 2 10% 37% 31% 7%	450
July	1st to 10th	Day	13% 3 7% 30% 34% 13%	445
	11th to 20th	Day	11% 2 6% 30% 36% 15%	444
	21st to end	Day	13% 3 7% 33% 38% 6%	488
August	1st to 10th	Day	11% 1 5% 38% 34% 12%	446
	11th to 20th	Day	15% 2 5% 31% 36% 11%	439
	21st to end	Day	12% 3 10% 34% 28% 14%	495
September	1st to 10th	Day	18% 4 11% 27% 25% 16%	450
	11th to 20th	Day	15% 2 13% 29% 27% 13%	450
	21st to end	Day	20% 2 17% 33% 21% 6%	450
October	1st to 10th	Day	26% 3 16% 26% 19% 10%	450
	11th to 20th	Day	17% 5% 24% 31% 19% 4	450
	21st to end	Day	25% 7% 26% 25% 14% 2	401
November	1st to 10th	Day	15% 7% 22% 33% 18% 4	365
	11th to 20th	Day	19% 10% 34% 27% 9%	370
	21st to end	Day	19% 9% 29% 31% 11%	328
December	1st to 10th	Day	20% 7% 35% 26% 12% 1	180
	11th to 20th	Day	27% 13% 26% 28% 6%	180
	21st to end	Day	22% 9% 34% 27% 8% 1	198
January	1st to 10th	Night	20% 13% 40% 22% 6%	760
	11th to 20th	Night	24% 11% 34% 28% 4	725
	21st to end	Night	19% 13% 40% 26% 3	792
February	1st to 10th	Night	22% 10% 37% 28% 2	720
	11th to 20th	Night	23% 9% 43% 24% 2	703
	21st to end	Night	24% 11% 36% 25% 5%	592
March	1st to 10th	Night	21% 7% 37% 29% 6%	720
	11th to 20th	Night	20% 7% 36% 30% 6% 1	720
	21st to end	Night	21% 6% 35% 29% 7% 1	792
April	1st to 10th	Night	22% 6% 31% 33% 7% 1	720
	11th to 20th	Night	22% 4 27% 35% 11%	711
	21st to end	Night	23% 3 27% 37% 8% 1	650
May	1st to 10th	Night	26% 4 24% 32% 12% 2	540
	11th to 20th	Night	22% 3 25% 38% 12% 1	426
	21st to end	Night	21% 3 11% 37% 26% 2	389
June	1st to 10th	Night	19% 5% 16% 36% 23% 2	360
	11th to 20th	Night	18% 3 11% 44% 24% 1	360
	21st to end	Night	12% 1 11% 51% 23% 2	360
July	1st to 10th	Night	15% 2 12% 38% 29% 4	350
	11th to 20th	Night	13% 2 8% 43% 31% 3	348
	21st to end	Night	15% 2 10% 44% 27% 2	394
August	1st to 10th	Night	8% 2 12% 44% 31% 3	437
	11th to 20th	Night	15% 4 9% 34% 33% 5%	710
	21st to end	Night	14% 2 12% 35% 31% 6%	792
September	1st to 10th	Night	18% 2 17% 37% 21% 6%	720
	11th to 20th	Night	18% 2 15% 36% 23% 6%	720
	21st to end	Night	21% 3 19% 33% 19% 4	720
October	1st to 10th	Night	24% 6% 25% 25% 16% 4	720
	11th to 20th	Night	19% 6% 27% 33% 13% 1	720
	21st to end	Night	25% 9% 32% 25% 8% 1	792
November	1st to 10th	Night	15% 7% 33% 30% 13% 2	740
	11th to 20th	Night	22% 10% 32% 26% 9% 1	760
	21st to end	Night	17% 13% 33% 29% 8% 1	760
December	1st to 10th	Night	24% 12% 34% 24% 6% 1	760
	11th to 20th	Night	23% 12% 35% 26% 5%	760
	21st to end	Night	24% 11% 36% 24% 4	836

Page generated at 2024-01-08 22:31:23. Version ac07

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