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 2025-10-19.

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%	126496
	Night	18% 7% 2 6% 4 3 4 3 5% 6% 42%	123224
Transparency	Day	41% 4 13% 23% 16% 3	130024
	Night	43% 4 11% 23% 17% 3	127783
Seeing	Day	18% 5% 19% 32% 20% 6%	16257
	Night	20% 7% 27% 31% 13% 2	25286

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	13% 6% 2 5% 3 4 5% 4 5% 6% 47%	3597
	11th to 20th	Day	14% 9% 2 6% 4 4 5% 4 5% 5% 43%	3570
	21st to end	Day	18% 6% 3 8% 4 4 4 3 4 5% 40%	4167
February	1st to 10th	Day	17% 7% 3 5% 4 5% 5% 4 5% 6% 39%	1848
	11th to 20th	Day	15% 7% 2 6% 4 4 4 3 5% 7% 43%	1948
	21st to end	Day	20% 8% 2 6% 4 3 4 3 5% 5% 39%	1650
March	1st to 10th	Day	23% 7% 2 5% 3 3 4 3 4 6% 42%	4476
	11th to 20th	Day	13% 8% 2 5% 4 4 4 4 6% 7% 43%	4688
	21st to end	Day	20% 8% 2 6% 4 4 4 3 5% 6% 39%	5970
April	1st to 10th	Day	20% 8% 2 6% 4 3 4 4 5% 8% 37%	2400
	11th to 20th	Day	17% 7% 3 5% 4 4 4 3 5% 7% 41%	2374
	21st to end	Day	19% 6% 2 5% 4 4 5% 3 5% 6% 41%	2476
May	1st to 10th	Day	30% 8% 2 5% 3 3 4 4 6% 8% 28%	5886
	11th to 20th	Day	22% 9% 3 6% 4 3 4 3 5% 6% 35%	6175
	21st to end	Day	21% 6% 2 6% 4 4 4 4 6% 8% 35%	6949
June	1st to 10th	Day	18% 5% 2 6% 4 4 5% 5% 7% 8% 37%	2800
	11th to 20th	Day	16% 7% 3 5% 4 5% 4 4 7% 9% 37%	2800
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 4 6% 9% 39%	2800
July	1st to 10th	Day	13% 7% 3 6% 5% 5% 5% 5% 7% 9% 36%	6360
	11th to 20th	Day	12% 6% 2 7% 5% 4 5% 5% 8% 10% 35%	6360
	21st to end	Day	14% 6% 2 6% 5% 4 4 4 6% 8% 41%	6926
August	1st to 10th	Day	19% 6% 2 7% 4 4 5% 4 6% 8% 34%	4359
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 6% 9% 43%	3850
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 7% 9% 47%	4312
September	1st to 10th	Day	14% 5% 2 5% 4 4 4 4 5% 7% 45%	2400
	11th to 20th	Day	12% 5% 2 5% 3 3 4 4 5% 6% 51%	2280
	21st to end	Day	20% 5% 2 6% 4 5% 4 4 5% 7% 39%	2200
October	1st to 10th	Day	21% 5% 2 4 3 3 3 3 5% 7% 44%	2140
	11th to 20th	Day	16% 5% 2 5% 3 4 3 3 5% 8% 47%	1980
	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 2 5% 5% 51%	1761
	11th to 20th	Day	18% 6% 2 4 3 2 3 3 4 6% 49%	1800
	21st to end	Day	17% 5% 2 5% 3 3 3 4 5% 7% 44%	1755
December	1st to 10th	Day	16% 6% 2 5% 3 3 4 3 5% 7% 45%	2600
	11th to 20th	Day	22% 6% 2 4 3 3 3 2 4 6% 45%	2986
	21st to end	Day	18% 11% 2 6% 4 3 4 3 4 5% 39%	3960
January	1st to 10th	Night	14% 6% 2 4 3 3 3 3 5% 7% 50%	6349
	11th to 20th	Night	14% 8% 3 6% 3 3 3 3 4 5% 47%	6315
	21st to end	Night	17% 9% 3 7% 4 3 4 3 4 5% 40%	6784
February	1st to 10th	Night	17% 9% 2 6% 4 3 4 4 4 5% 41%	2550
	11th to 20th	Night	18% 8% 2 6% 4 4 3 3 5% 6% 42%	2338
	21st to end	Night	19% 8% 3 6% 4 3 3 3 5% 4 42%	1980
March	1st to 10th	Night	21% 9% 2 5% 3 3 3 2 4 6% 42%	5364
	11th to 20th	Night	16% 9% 2 5% 4 4 4 3 4 6% 43%	5265
	21st to end	Night	20% 9% 3 6% 4 3 3 3 5% 5% 40%	4962
April	1st to 10th	Night	20% 7% 2 6% 5% 4 4 4 5% 5% 38%	2000
	11th to 20th	Night	20% 8% 2 5% 4 3 4 3 4 5% 41%	1975
	21st to end	Night	19% 6% 2 6% 4 3 4 3 5% 6% 42%	1922
May	1st to 10th	Night	29% 10% 3 7% 4 3 3 3 4 7% 27%	4054
	11th to 20th	Night	23% 10% 3 6% 3 3 3 3 5% 7% 34%	3759
	21st to end	Night	24% 8% 2 5% 4 4 4 3 5% 6% 36%	3926
June	1st to 10th	Night	18% 7% 2 5% 4 4 4 3 5% 7% 40%	1600
	11th to 20th	Night	16% 7% 3 6% 4 4 4 4 6% 8% 40%	1600
	21st to end	Night	12% 6% 2 5% 5% 4 4 3 6% 9% 43%	1600
July	1st to 10th	Night	14% 8% 3 8% 5% 4 5% 3 5% 7% 38%	3600
	11th to 20th	Night	18% 8% 2 7% 4 4 5% 4 6% 9% 32%	3600
	21st to end	Night	15% 6% 2 5% 3 4 4 4 6% 7% 43%	3926
August	1st to 10th	Night	23% 6% 3 7% 4 4 5% 5% 5% 6% 32%	2754
	11th to 20th	Night	15% 7% 3 6% 4 5% 5% 3 6% 7% 39%	3197
	21st to end	Night	9% 4 2 4 3 4 4 3 4 8% 55%	3586
September	1st to 10th	Night	14% 6% 2 6% 5% 4 4 3 4 6% 46%	2000
	11th to 20th	Night	14% 5% 1 5% 3 3 3 3 5% 6% 52%	2120
	21st to end	Night	19% 7% 3 5% 3 4 4 3 3 6% 44%	2200
October	1st to 10th	Night	18% 7% 2 6% 4 3 4 3 5% 6% 43%	2255
	11th to 20th	Night	16% 5% 2 5% 3 4 3 2 4 6% 49%	2377
	21st to end	Night	22% 7% 2 5% 3 4 3 3 4 6% 42%	2703
November	1st to 10th	Night	12% 5% 2 5% 4 4 4 3 6% 6% 50%	2537
	11th to 20th	Night	19% 7% 2 4 3 3 2 2 4 6% 48%	2600
	21st to end	Night	15% 5% 2 4 4 3 4 3 5% 7% 47%	2642
December	1st to 10th	Night	16% 7% 2 4 3 3 3 2 3 5% 49%	4580
	11th to 20th	Night	26% 6% 2 4 3 3 3 2 4 4 43%	5208
	21st to end	Night	19% 9% 3 6% 4 3 3 2 4 5% 42%	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	34% 4 8% 32% 20% 3	3598
	11th to 20th	Day	41% 3 11% 23% 15% 6%	3600
	21st to end	Day	46% 3 11% 24% 15% 1	4167
February	1st to 10th	Day	44% 3 14% 23% 15% 1	1848
	11th to 20th	Day	40% 4 9% 26% 17% 3	1957
	21st to end	Day	45% 4 10% 23% 15% 3	1650
March	1st to 10th	Day	43% 3 10% 20% 14% 10%	4479
	11th to 20th	Day	38% 3 15% 29% 13% 2	4688
	21st to end	Day	43% 4 9% 15% 23% 6%	5970
April	1st to 10th	Day	43% 5% 11% 21% 16% 4	2400
	11th to 20th	Day	41% 4 11% 22% 19% 3	2374
	21st to end	Day	42% 3 10% 23% 18% 4	2476
May	1st to 10th	Day	52% 4 18% 15% 10% 1	5897
	11th to 20th	Day	48% 4 13% 20% 13% 2	6179
	21st to end	Day	43% 6% 11% 24% 14% 1	6949
June	1st to 10th	Day	40% 5% 14% 24% 16% 1	2800
	11th to 20th	Day	39% 5% 15% 25% 14% 1	2800
	21st to end	Day	37% 6% 17% 24% 15% 2	2800
July	1st to 10th	Day	42% 8% 13% 20% 15% 1	6290
	11th to 20th	Day	40% 6% 19% 24% 10% 1	6285
	21st to end	Day	39% 5% 12% 23% 20% 1	6728
August	1st to 10th	Day	45% 5% 17% 24% 9% 1	4345
	11th to 20th	Day	35% 5% 16% 32% 12% 1	3886
	21st to end	Day	31% 5% 13% 29% 20% 2	4312
September	1st to 10th	Day	36% 5% 14% 24% 20% 2	2400
	11th to 20th	Day	31% 3 13% 25% 25% 2	2280
	21st to end	Day	44% 5% 10% 21% 18% 3	2200
October	1st to 10th	Day	34% 4 10% 22% 24% 6%	3491
	11th to 20th	Day	27% 3 11% 24% 30% 4	3220
	21st to end	Day	31% 3 15% 26% 23% 3	3093
November	1st to 10th	Day	36% 4 13% 24% 21% 2	1761
	11th to 20th	Day	38% 3 10% 23% 23% 4	1800
	21st to end	Day	38% 4 14% 28% 12% 3	1755
December	1st to 10th	Day	40% 3 15% 28% 12% 1	2600
	11th to 20th	Day	42% 2 14% 21% 18% 3	2986
	21st to end	Day	48% 4 7% 22% 15% 3	3960
January	1st to 10th	Night	36% 3 8% 26% 23% 3	6349
	11th to 20th	Night	41% 3 13% 21% 16% 6%	6360
	21st to end	Night	48% 2 10% 20% 15% 5%	6784
February	1st to 10th	Night	45% 4 12% 24% 13% 2	2550
	11th to 20th	Night	44% 4 9% 21% 17% 4	2340
	21st to end	Night	45% 3 7% 24% 17% 4	1980
March	1st to 10th	Night	46% 2 10% 19% 14% 8%	5374
	11th to 20th	Night	44% 3 14% 23% 12% 4	5266
	21st to end	Night	48% 3 6% 13% 23% 6%	4962
April	1st to 10th	Night	48% 4 9% 19% 17% 3	2000
	11th to 20th	Night	45% 3 10% 18% 22% 1	1975
	21st to end	Night	44% 3 8% 22% 18% 5%	1922
May	1st to 10th	Night	58% 4 14% 13% 10% 1	4060
	11th to 20th	Night	52% 4 14% 14% 13% 3	3760
	21st to end	Night	50% 4 12% 21% 11% 1	3927
June	1st to 10th	Night	44% 5% 10% 21% 19% 2	1600
	11th to 20th	Night	41% 5% 15% 26% 12% 1	1600
	21st to end	Night	37% 7% 13% 27% 14% 2	1600
July	1st to 10th	Night	47% 6% 13% 22% 11% 1	3560
	11th to 20th	Night	47% 5% 17% 20% 10% 1	3552
	21st to end	Night	41% 5% 13% 24% 16% 1	3809
August	1st to 10th	Night	50% 6% 15% 18% 10% 1	2744
	11th to 20th	Night	43% 4 13% 29% 9% 1	3227
	21st to end	Night	29% 5% 12% 33% 20% 1	3586
September	1st to 10th	Night	38% 6% 9% 27% 18% 2	2000
	11th to 20th	Night	34% 5% 9% 25% 24% 3	2120
	21st to end	Night	43% 4 9% 21% 20% 3	2200
October	1st to 10th	Night	40% 4 10% 24% 19% 3	3684
	11th to 20th	Night	26% 3 10% 27% 30% 4	3876
	21st to end	Night	33% 4 12% 28% 22% 2	4453
November	1st to 10th	Night	36% 5% 11% 23% 20% 4	2537
	11th to 20th	Night	40% 3 8% 24% 20% 4	2600
	21st to end	Night	38% 4 13% 30% 13% 2	2642
December	1st to 10th	Night	40% 3 12% 31% 14% 2	4580
	11th to 20th	Night	47% 2 10% 21% 17% 3	5208
	21st to end	Night	46% 4 9% 21% 17% 4	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	20% 9% 39% 24% 9% 1	200
	11th to 20th	Day	16% 10% 43% 26% 6% 1	200
	21st to end	Day	20% 5% 35% 31% 8% 2	265
February	1st to 10th	Day	22% 6% 32% 35% 5%	348
	11th to 20th	Day	19% 10% 38% 31% 3	496
	21st to end	Day	22% 10% 29% 30% 8%	414
March	1st to 10th	Day	16% 7% 34% 33% 9%	500
	11th to 20th	Day	15% 7% 35% 33% 10% 1	500
	21st to end	Day	21% 6% 30% 33% 9% 2	550
April	1st to 10th	Day	23% 8% 26% 31% 11% 1	500
	11th to 20th	Day	19% 5% 24% 37% 15% 1	492
	21st to end	Day	23% 4 19% 38% 14% 1	500
May	1st to 10th	Day	27% 3 19% 35% 14% 2	503
	11th to 20th	Day	19% 4 15% 38% 21% 3	527
	21st to end	Day	18% 2 8% 35% 28% 8%	573
June	1st to 10th	Day	17% 3 10% 33% 30% 7%	530
	11th to 20th	Day	18% 2 10% 36% 26% 8%	530
	21st to end	Day	13% 2 10% 36% 32% 8%	530
July	1st to 10th	Day	12% 3 7% 29% 35% 14%	525
	11th to 20th	Day	10% 2 5% 29% 38% 15%	524
	21st to end	Day	13% 3 7% 32% 39% 6%	576
August	1st to 10th	Day	11% 2 5% 36% 34% 12%	526
	11th to 20th	Day	14% 2 4 29% 37% 13%	519
	21st to end	Day	10% 3 9% 35% 29% 14%	583
September	1st to 10th	Day	18% 3 11% 28% 26% 14%	530
	11th to 20th	Day	14% 2 12% 26% 31% 14%	530
	21st to end	Day	22% 2 15% 33% 22% 5%	530
October	1st to 10th	Day	23% 2 15% 27% 21% 12%	530
	11th to 20th	Day	17% 5% 24% 31% 18% 5%	522
	21st to end	Day	24% 6% 25% 26% 15% 3	434
November	1st to 10th	Day	14% 7% 23% 35% 18% 4	395
	11th to 20th	Day	18% 10% 34% 28% 10%	400
	21st to end	Day	20% 9% 30% 30% 10%	355
December	1st to 10th	Day	20% 8% 34% 26% 12% 1	200
	11th to 20th	Day	26% 14% 26% 28% 7%	200
	21st to end	Day	21% 8% 32% 28% 10% 1	220
January	1st to 10th	Night	19% 14% 41% 21% 5%	800
	11th to 20th	Night	23% 12% 34% 28% 4	800
	21st to end	Night	19% 12% 40% 26% 3	880
February	1st to 10th	Night	21% 10% 38% 29% 2	800
	11th to 20th	Night	23% 10% 42% 23% 2	783
	21st to end	Night	23% 11% 36% 26% 5%	660
March	1st to 10th	Night	21% 7% 37% 29% 7%	800
	11th to 20th	Night	20% 7% 35% 31% 7% 1	800
	21st to end	Night	22% 7% 35% 29% 6% 1	880
April	1st to 10th	Night	23% 6% 31% 33% 7% 1	800
	11th to 20th	Night	23% 4 28% 34% 10%	791
	21st to end	Night	21% 3 27% 38% 9% 1	722
May	1st to 10th	Night	26% 5% 22% 34% 12% 2	600
	11th to 20th	Night	23% 3 24% 38% 12% 1	474
	21st to end	Night	22% 3 10% 38% 25% 2	433
June	1st to 10th	Night	19% 5% 15% 38% 23% 2	400
	11th to 20th	Night	18% 3 11% 43% 25% 1	400
	21st to end	Night	12% 2 12% 49% 24% 2	400
July	1st to 10th	Night	14% 2 11% 39% 31% 4	390
	11th to 20th	Night	13% 3 8% 41% 31% 4	388
	21st to end	Night	15% 3 10% 42% 29% 1	438
August	1st to 10th	Night	10% 3 11% 42% 31% 3	488
	11th to 20th	Night	15% 4 9% 34% 34% 6%	790
	21st to end	Night	14% 2 13% 35% 31% 5%	880
September	1st to 10th	Night	17% 2 17% 37% 22% 6%	800
	11th to 20th	Night	16% 2 14% 35% 25% 7%	800
	21st to end	Night	22% 3 18% 34% 19% 4	800
October	1st to 10th	Night	22% 6% 25% 25% 17% 5%	800
	11th to 20th	Night	19% 6% 28% 33% 13% 2	793
	21st to end	Night	24% 9% 31% 26% 9% 1	836
November	1st to 10th	Night	15% 7% 33% 31% 12% 2	780
	11th to 20th	Night	21% 10% 33% 26% 9% 1	800
	21st to end	Night	17% 13% 33% 29% 8% 1	800
December	1st to 10th	Night	24% 11% 34% 23% 6% 1	800
	11th to 20th	Night	24% 12% 34% 26% 4	800
	21st to end	Night	23% 11% 36% 25% 5%	880

Page generated at 2025-10-16 01:50:28. Version ac07

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