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 2024-11-05.

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% 7% 2 6% 4 4 4 4 6% 7% 40%	101254
	Night	18% 7% 2 6% 4 4 4 3 5% 6% 41%	97794
Transparency	Day	40% 4 13% 24% 16% 2	104800
	Night	43% 4 11% 23% 16% 2	102337
Seeing	Day	18% 5% 20% 32% 20% 6%	15021
	Night	20% 7% 27% 31% 13% 2	24086

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	16% 6% 2 6% 4 4 4 3 5% 5% 46%	2517
	11th to 20th	Day	16% 9% 2 7% 5% 5% 5% 4 5% 5% 36%	2520
	21st to end	Day	22% 9% 3 7% 5% 5% 4 4 4 5% 32%	2909
February	1st to 10th	Day	17% 7% 3 6% 4 5% 5% 4 5% 6% 39%	1755
	11th to 20th	Day	15% 6% 2 5% 4 4 4 3 5% 7% 44%	1848
	21st to end	Day	21% 8% 2 6% 4 3 4 4 5% 5% 37%	1570
March	1st to 10th	Day	27% 8% 3 5% 3 3 3 3 4 5% 35%	3116
	11th to 20th	Day	10% 4 2 4 5% 5% 4 5% 8% 8% 45%	3272
	21st to end	Day	21% 7% 2 6% 3 4 3 3 5% 6% 41%	4166
April	1st to 10th	Day	19% 8% 2 6% 4 3 4 4 5% 8% 37%	2280
	11th to 20th	Day	18% 7% 3 5% 4 4 3 4 5% 7% 41%	2254
	21st to end	Day	20% 6% 2 5% 4 4 5% 2 5% 6% 40%	2352
May	1st to 10th	Day	28% 9% 2 5% 3 3 3 3 5% 7% 31%	4106
	11th to 20th	Day	22% 8% 3 6% 4 3 5% 4 6% 7% 32%	4297
	21st to end	Day	15% 5% 2 5% 4 3 4 4 7% 8% 44%	4837
June	1st to 10th	Day	18% 6% 2 5% 4 4 5% 5% 7% 8% 37%	2660
	11th to 20th	Day	15% 6% 2 5% 4 5% 4 4 7% 9% 38%	2660
	21st to end	Day	13% 7% 2 6% 5% 5% 5% 4 6% 9% 39%	2660
July	1st to 10th	Day	12% 6% 3 6% 5% 5% 5% 5% 7% 9% 37%	4440
	11th to 20th	Day	13% 6% 2 6% 4 4 5% 5% 7% 9% 40%	4440
	21st to end	Day	15% 8% 2 7% 6% 5% 5% 4 6% 8% 33%	4814
August	1st to 10th	Day	19% 7% 2 7% 4 4 5% 4 6% 8% 33%	4225
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 6% 9% 43%	3730
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 7% 9% 47%	4180
September	1st to 10th	Day	13% 5% 2 5% 4 5% 5% 4 5% 7% 45%	2280
	11th to 20th	Day	13% 5% 2 5% 3 3 4 4 5% 6% 51%	2166
	21st to end	Day	20% 5% 2 6% 3 4 4 4 5% 6% 40%	2090
October	1st to 10th	Day	22% 6% 2 5% 3 3 3 3 5% 7% 42%	2033
	11th to 20th	Day	15% 5% 2 5% 3 4 4 3 5% 7% 48%	1900
	21st to end	Day	21% 5% 2 4 4 4 3 4 6% 6% 42%	1893
November	1st to 10th	Day	14% 6% 2 5% 3 3 3 3 5% 5% 50%	1708
	11th to 20th	Day	19% 6% 2 4 3 2 3 3 4 6% 47%	1710
	21st to end	Day	17% 5% 2 5% 4 4 3 3 5% 7% 44%	1668
December	1st to 10th	Day	19% 7% 2 6% 4 4 5% 4 6% 7% 37%	1520
	11th to 20th	Day	23% 6% 2 4 3 3 3 3 4 7% 44%	1906
	21st to end	Day	18% 12% 3 7% 5% 4 4 4 5% 6% 33%	2772
January	1st to 10th	Night	17% 6% 3 5% 4 3 3 3 4 6% 46%	4429
	11th to 20th	Night	17% 9% 3 6% 3 3 3 3 4 5% 43%	4440
	21st to end	Night	24% 12% 3 6% 4 3 3 3 3 5% 34%	4742
February	1st to 10th	Night	17% 9% 2 6% 4 3 4 3 5% 5% 41%	2423
	11th to 20th	Night	17% 8% 2 6% 4 4 3 3 5% 6% 42%	2218
	21st to end	Night	19% 9% 2 5% 4 3 3 3 5% 4 42%	1884
March	1st to 10th	Night	25% 11% 2 5% 3 3 3 3 4 5% 35%	3724
	11th to 20th	Night	12% 6% 2 6% 5% 5% 4 4 5% 7% 45%	3681
	21st to end	Night	20% 9% 3 5% 4 3 3 3 4 4 42%	3466
April	1st to 10th	Night	19% 7% 2 6% 5% 4 4 4 5% 5% 39%	1900
	11th to 20th	Night	19% 8% 2 6% 4 3 4 3 4 6% 42%	1875
	21st to end	Night	19% 7% 2 6% 4 3 4 3 4 5% 42%	1826
May	1st to 10th	Night	25% 10% 3 7% 5% 4 3 3 4 7% 29%	2834
	11th to 20th	Night	23% 9% 4 7% 3 4 4 3 5% 7% 30%	2623
	21st to end	Night	19% 7% 2 4 3 3 3 3 5% 6% 43%	2738
June	1st to 10th	Night	18% 6% 2 5% 4 4 4 3 5% 7% 40%	1520
	11th to 20th	Night	15% 7% 3 6% 4 3 4 4 6% 8% 41%	1520
	21st to end	Night	12% 5% 2 6% 4 4 4 3 7% 10% 42%	1520
July	1st to 10th	Night	13% 6% 3 8% 5% 5% 6% 4 6% 8% 37%	2520
	11th to 20th	Night	17% 6% 2 6% 4 3 5% 4 6% 10% 36%	2520
	21st to end	Night	20% 8% 3 6% 3 4 5% 4 6% 7% 34%	2739
August	1st to 10th	Night	23% 6% 3 7% 4 4 5% 5% 6% 7% 31%	2669
	11th to 20th	Night	15% 7% 3 6% 4 5% 5% 3 6% 7% 39%	3097
	21st to end	Night	9% 4 2 5% 3 4 4 3 4 8% 55%	3476
September	1st to 10th	Night	14% 6% 2 6% 5% 4 4 3 4 6% 46%	1900
	11th to 20th	Night	14% 5% 1 4 3 3 3 3 5% 6% 52%	2014
	21st to end	Night	19% 6% 3 5% 3 4 4 3 4 6% 44%	2090
October	1st to 10th	Night	19% 8% 2 6% 4 3 4 3 5% 6% 41%	2142
	11th to 20th	Night	15% 5% 2 5% 3 4 4 2 5% 7% 49%	2280
	21st to end	Night	22% 7% 2 5% 3 4 3 3 4 6% 42%	2703
November	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 5% 6% 49%	2470
	11th to 20th	Night	20% 7% 2 4 3 3 3 2 4 6% 47%	2470
	21st to end	Night	15% 5% 2 4 4 3 4 3 5% 7% 47%	2509
December	1st to 10th	Night	20% 7% 3 5% 4 4 4 3 5% 7% 39%	2660
	11th to 20th	Night	20% 6% 2 5% 3 4 3 3 5% 6% 42%	3288
	21st to end	Night	21% 10% 3 6% 4 3 3 3 5% 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	39% 4 10% 32% 13% 2	2518
	11th to 20th	Day	46% 4 12% 26% 11% 1	2520
	21st to end	Day	53% 3 11% 21% 11% 1	2909
February	1st to 10th	Day	44% 3 14% 23% 14% 1	1755
	11th to 20th	Day	39% 4 9% 26% 18% 3	1857
	21st to end	Day	46% 4 10% 22% 14% 3	1570
March	1st to 10th	Day	50% 3 11% 23% 11% 2	3119
	11th to 20th	Day	31% 4 15% 34% 15% 1	3272
	21st to end	Day	43% 3 7% 18% 22% 6%	4166
April	1st to 10th	Day	42% 5% 11% 22% 16% 4	2280
	11th to 20th	Day	41% 4 11% 22% 19% 3	2254
	21st to end	Day	42% 3 10% 23% 18% 4	2352
May	1st to 10th	Day	51% 3 17% 16% 12% 1	4120
	11th to 20th	Day	48% 3 17% 20% 9% 2	4301
	21st to end	Day	34% 6% 13% 27% 19% 2	4837
June	1st to 10th	Day	40% 5% 14% 24% 15% 1	2660
	11th to 20th	Day	38% 5% 16% 26% 14% 1	2660
	21st to end	Day	37% 6% 17% 25% 15% 1	2660
July	1st to 10th	Day	42% 8% 13% 20% 16% 1	4370
	11th to 20th	Day	37% 5% 19% 24% 13% 2	4365
	21st to end	Day	46% 6% 14% 18% 16% 2	4661
August	1st to 10th	Day	45% 5% 17% 23% 9%	4211
	11th to 20th	Day	35% 5% 15% 32% 12% 1	3766
	21st to end	Day	31% 5% 14% 29% 20% 2	4180
September	1st to 10th	Day	36% 6% 14% 23% 19% 2	2280
	11th to 20th	Day	32% 3 13% 25% 25% 2	2166
	21st to end	Day	43% 5% 10% 21% 18% 3	2090
October	1st to 10th	Day	35% 4 10% 22% 24% 6%	3384
	11th to 20th	Day	27% 3 11% 24% 31% 4	3140
	21st to end	Day	31% 3 15% 26% 23% 3	3093
November	1st to 10th	Day	37% 4 13% 24% 20% 2	1708
	11th to 20th	Day	39% 3 11% 23% 21% 3	1710
	21st to end	Day	38% 5% 14% 27% 13% 3	1668
December	1st to 10th	Day	46% 4 15% 20% 14% 1	1520
	11th to 20th	Day	42% 3 16% 21% 17% 2	1906
	21st to end	Day	52% 5% 10% 24% 9% 1	2772
January	1st to 10th	Night	40% 4 11% 30% 12% 4	4429
	11th to 20th	Night	45% 3 12% 25% 13% 3	4440
	21st to end	Night	55% 3 9% 22% 11% 2	4742
February	1st to 10th	Night	45% 4 13% 24% 13% 2	2423
	11th to 20th	Night	44% 4 9% 21% 17% 5%	2220
	21st to end	Night	45% 3 7% 25% 16% 4	1884
March	1st to 10th	Night	53% 2 9% 21% 12% 2	3734
	11th to 20th	Night	39% 4 13% 25% 16% 3	3682
	21st to end	Night	46% 4 6% 14% 23% 6%	3466
April	1st to 10th	Night	47% 4 9% 20% 17% 3	1900
	11th to 20th	Night	45% 3 10% 19% 22% 1	1875
	21st to end	Night	45% 4 9% 22% 16% 5%	1826
May	1st to 10th	Night	55% 4 13% 16% 10% 1	2840
	11th to 20th	Night	55% 4 14% 15% 10% 1	2624
	21st to end	Night	42% 5% 13% 24% 15% 2	2739
June	1st to 10th	Night	44% 5% 10% 21% 18% 2	1520
	11th to 20th	Night	40% 5% 15% 27% 12% 1	1520
	21st to end	Night	37% 7% 13% 28% 14% 1	1520
July	1st to 10th	Night	46% 7% 14% 19% 13% 1	2480
	11th to 20th	Night	43% 6% 17% 24% 11% 1	2472
	21st to end	Night	49% 6% 14% 16% 14% 2	2651
August	1st to 10th	Night	51% 6% 16% 17% 10% 1	2659
	11th to 20th	Night	43% 4 13% 29% 9% 1	3127
	21st to end	Night	29% 5% 13% 33% 20% 1	3476
September	1st to 10th	Night	38% 6% 9% 27% 18% 2	1900
	11th to 20th	Night	34% 5% 9% 25% 24% 3	2014
	21st to end	Night	43% 4 9% 21% 20% 3	2090
October	1st to 10th	Night	40% 4 10% 24% 18% 3	3571
	11th to 20th	Night	25% 3 10% 27% 31% 4	3779
	21st to end	Night	33% 4 12% 28% 22% 2	4453
November	1st to 10th	Night	37% 5% 11% 23% 20% 4	2470
	11th to 20th	Night	41% 4 9% 25% 19% 4	2470
	21st to end	Night	38% 4 13% 30% 13% 2	2509
December	1st to 10th	Night	46% 4 12% 22% 15% 1	2660
	11th to 20th	Night	44% 4 12% 19% 18% 3	3288
	21st to end	Night	49% 5% 10% 24% 12% 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	20% 9% 36% 25% 9% 1	180
	11th to 20th	Day	16% 11% 42% 24% 6% 1	180
	21st to end	Day	21% 5% 32% 31% 9% 2	238
February	1st to 10th	Day	23% 6% 33% 32% 5%	315
	11th to 20th	Day	19% 9% 38% 32% 3	456
	21st to end	Day	23% 11% 29% 27% 9% 1	382
March	1st to 10th	Day	17% 6% 34% 34% 9%	460
	11th to 20th	Day	14% 7% 37% 32% 8% 1	460
	21st to end	Day	22% 7% 29% 32% 8% 2	506
April	1st to 10th	Day	22% 8% 26% 30% 12% 2	460
	11th to 20th	Day	19% 5% 23% 36% 16% 1	452
	21st to end	Day	24% 4 19% 37% 14% 2	460
May	1st to 10th	Day	26% 3 21% 35% 12% 3	463
	11th to 20th	Day	20% 3 15% 38% 22% 3	487
	21st to end	Day	17% 2 9% 35% 29% 9%	529
June	1st to 10th	Day	17% 3 11% 34% 29% 7%	490
	11th to 20th	Day	16% 2 11% 36% 27% 8%	490
	21st to end	Day	12% 2 10% 37% 32% 7%	490
July	1st to 10th	Day	12% 3 7% 29% 35% 14%	485
	11th to 20th	Day	10% 2 5% 30% 38% 14%	484
	21st to end	Day	14% 3 7% 33% 37% 6%	532
August	1st to 10th	Day	12% 2 5% 36% 34% 12%	486
	11th to 20th	Day	15% 2 5% 31% 36% 12%	479
	21st to end	Day	11% 2 9% 34% 29% 14%	539
September	1st to 10th	Day	18% 3 11% 28% 26% 15%	490
	11th to 20th	Day	15% 2 12% 27% 28% 15%	490
	21st to end	Day	22% 2 16% 34% 20% 6%	490
October	1st to 10th	Day	24% 3 16% 27% 20% 10%	490
	11th to 20th	Day	16% 5% 24% 32% 19% 5%	490
	21st to end	Day	24% 6% 25% 26% 15% 3	434
November	1st to 10th	Day	15% 7% 23% 33% 19% 4	378
	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	19% 13% 40% 22% 6%	760
	11th to 20th	Night	23% 12% 34% 27% 4	760
	21st to end	Night	20% 12% 39% 25% 3	836
February	1st to 10th	Night	21% 10% 37% 30% 3	760
	11th to 20th	Night	22% 9% 43% 24% 2	743
	21st to end	Night	24% 11% 35% 25% 5%	628
March	1st to 10th	Night	22% 7% 36% 28% 7%	760
	11th to 20th	Night	20% 7% 37% 30% 6% 1	760
	21st to end	Night	22% 7% 35% 29% 6% 1	836
April	1st to 10th	Night	22% 6% 31% 34% 7% 1	760
	11th to 20th	Night	22% 4 28% 35% 11%	751
	21st to end	Night	22% 3 27% 38% 8% 1	686
May	1st to 10th	Night	26% 5% 23% 33% 12% 2	570
	11th to 20th	Night	23% 2 24% 38% 13% 1	450
	21st to end	Night	21% 3 10% 37% 26% 2	411
June	1st to 10th	Night	18% 5% 15% 37% 23% 2	380
	11th to 20th	Night	17% 3 11% 43% 25% 1	380
	21st to end	Night	12% 1 13% 50% 23% 2	380
July	1st to 10th	Night	14% 2 11% 39% 30% 4	370
	11th to 20th	Night	13% 3 8% 42% 31% 3	368
	21st to end	Night	16% 3 10% 43% 27% 1	416
August	1st to 10th	Night	10% 3 12% 43% 30% 3	463
	11th to 20th	Night	15% 4 9% 34% 33% 5%	750
	21st to end	Night	14% 2 12% 34% 32% 6%	836
September	1st to 10th	Night	17% 2 17% 37% 21% 6%	760
	11th to 20th	Night	17% 2 15% 35% 24% 7%	760
	21st to end	Night	22% 3 19% 34% 19% 4	760
October	1st to 10th	Night	23% 6% 25% 25% 16% 4	760
	11th to 20th	Night	18% 6% 28% 34% 13% 1	760
	21st to end	Night	24% 9% 31% 26% 9% 1	836
November	1st to 10th	Night	15% 7% 33% 30% 13% 2	760
	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 2024-11-03 02:11:07. Version ac07

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