West Haven Pass Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the West Haven Pass 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	16% 8% 3 7% 6% 3 5% 5% 5% 7% 34%	74775
	Night	17% 10% 4 7% 6% 3 5% 4 5% 6% 33%	74946
Transparency	Day	33% 39% 9% 12% 6% 1	74480
	Night	35% 39% 8% 12% 6% 1	74775
Seeing	Day	23% 16% 30% 20% 8% 2	13650
	Night	22% 13% 29% 27% 9% 1	22762

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	14% 12% 6% 9% 7% 4 6% 5% 6% 6% 24%	1517
	11th to 20th	Day	16% 12% 4 8% 6% 5% 6% 7% 6% 6% 25%	1454
	21st to end	Day	11% 10% 7% 9% 8% 5% 6% 5% 6% 7% 27%	1584
February	1st to 10th	Day	13% 14% 5% 9% 8% 5% 5% 5% 5% 7% 23%	1619
	11th to 20th	Day	12% 12% 7% 9% 8% 4 5% 4 5% 6% 28%	1726
	21st to end	Day	18% 12% 5% 8% 7% 4 5% 4 4 5% 28%	1480
March	1st to 10th	Day	13% 11% 4 9% 7% 4 5% 5% 5% 5% 31%	1797
	11th to 20th	Day	17% 12% 6% 7% 5% 4 4 5% 5% 4 31%	1857
	21st to end	Day	16% 10% 4 7% 7% 3 4 4 4 5% 36%	2376
April	1st to 10th	Day	20% 10% 4 8% 4 3 4 4 4 6% 35%	2160
	11th to 20th	Day	17% 9% 3 6% 6% 3 5% 5% 5% 5% 37%	2134
	21st to end	Day	22% 9% 4 7% 5% 4 4 4 5% 6% 30%	2285
May	1st to 10th	Day	22% 8% 3 7% 6% 3 4 4 4 6% 33%	2389
	11th to 20th	Day	17% 7% 3 5% 5% 3 4 5% 5% 7% 38%	2478
	21st to end	Day	15% 6% 3 5% 6% 3 5% 5% 5% 8% 40%	2725
June	1st to 10th	Day	18% 8% 4 5% 6% 4 6% 6% 6% 9% 29%	2520
	11th to 20th	Day	17% 6% 3 5% 6% 4 5% 6% 5% 8% 36%	2520
	21st to end	Day	15% 7% 3 7% 6% 4 5% 6% 6% 7% 32%	2520
July	1st to 10th	Day	13% 6% 2 5% 5% 3 6% 5% 6% 10% 39%	2520
	11th to 20th	Day	11% 5% 2 7% 6% 4 5% 5% 7% 10% 36%	2520
	21st to end	Day	14% 5% 2 5% 5% 4 6% 6% 8% 8% 37%	2744
August	1st to 10th	Day	11% 5% 2 5% 6% 4 6% 7% 7% 9% 38%	2506
	11th to 20th	Day	12% 6% 3 5% 6% 3 5% 6% 7% 10% 36%	2183
	21st to end	Day	13% 5% 2 6% 5% 4 5% 6% 6% 9% 40%	2376
September	1st to 10th	Day	13% 5% 2 4 5% 3 5% 5% 6% 8% 44%	2160
	11th to 20th	Day	13% 5% 2 4 4 3 3 4 5% 7% 50%	2062
	21st to end	Day	14% 6% 2 6% 5% 3 5% 5% 6% 8% 42%	1980
October	1st to 10th	Day	21% 9% 2 7% 6% 3 4 5% 5% 5% 33%	1900
	11th to 20th	Day	16% 7% 3 7% 6% 3 4 5% 5% 8% 35%	1773
	21st to end	Day	25% 10% 3 6% 5% 4 4 5% 5% 5% 28%	1782
November	1st to 10th	Day	14% 10% 3 7% 6% 2 5% 5% 5% 7% 36%	1671
	11th to 20th	Day	17% 10% 3 7% 5% 3 5% 5% 6% 6% 33%	1710
	21st to end	Day	17% 9% 5% 8% 5% 3 6% 5% 5% 7% 29%	1549
December	1st to 10th	Day	21% 13% 5% 9% 8% 3 3 4 5% 7% 22%	1520
	11th to 20th	Day	21% 13% 5% 7% 5% 3 4 4 5% 6% 28%	1906
	21st to end	Day	27% 14% 5% 9% 6% 3 4 4 3 4 21%	2772
January	1st to 10th	Night	15% 14% 5% 10% 7% 4 5% 5% 5% 6% 25%	2649
	11th to 20th	Night	16% 14% 5% 8% 7% 5% 5% 5% 5% 6% 23%	2546
	21st to end	Night	11% 11% 5% 9% 7% 5% 6% 5% 5% 6% 31%	2772
February	1st to 10th	Night	14% 13% 5% 11% 9% 5% 6% 5% 5% 5% 23%	2340
	11th to 20th	Night	13% 11% 5% 9% 9% 5% 5% 5% 6% 6% 28%	2120
	21st to end	Night	15% 13% 5% 9% 7% 3 4 4 4 5% 30%	1776
March	1st to 10th	Night	15% 12% 4 9% 7% 4 5% 5% 5% 5% 30%	2150
	11th to 20th	Night	16% 11% 6% 7% 7% 3 4 4 5% 5% 32%	2101
	21st to end	Night	18% 10% 4 6% 6% 3 4 4 3 6% 37%	1980
April	1st to 10th	Night	19% 8% 5% 7% 5% 4 6% 5% 5% 5% 32%	1800
	11th to 20th	Night	20% 9% 4 6% 5% 3 5% 4 4 6% 35%	1775
	21st to end	Night	20% 9% 3 7% 6% 3 4 4 5% 5% 34%	1674
May	1st to 10th	Night	23% 10% 4 7% 6% 2 4 4 4 5% 31%	1586
	11th to 20th	Night	18% 8% 3 6% 6% 3 3 3 5% 6% 39%	1415
	21st to end	Night	15% 6% 3 6% 5% 2 5% 5% 6% 7% 40%	1550
June	1st to 10th	Night	19% 7% 3 6% 6% 3 5% 4 6% 6% 35%	1440
	11th to 20th	Night	18% 7% 3 5% 5% 2 4 5% 5% 8% 39%	1440
	21st to end	Night	16% 6% 3 6% 6% 4 5% 5% 5% 8% 36%	1440
July	1st to 10th	Night	12% 4 2 5% 6% 4 6% 5% 7% 10% 40%	1440
	11th to 20th	Night	13% 5% 2 5% 4 2 4 5% 7% 10% 42%	1440
	21st to end	Night	10% 5% 3 5% 5% 4 5% 6% 6% 8% 43%	1574
August	1st to 10th	Night	10% 5% 2 5% 4 4 5% 6% 5% 9% 45%	1432
	11th to 20th	Night	11% 7% 2 6% 7% 4 5% 6% 5% 8% 39%	1695
	21st to end	Night	13% 5% 3 5% 5% 3 5% 4 6% 8% 43%	1980
September	1st to 10th	Night	13% 5% 2 5% 5% 3 4 4 5% 7% 46%	1800
	11th to 20th	Night	14% 5% 2 4 4 2 4 3 4 5% 52%	1897
	21st to end	Night	15% 5% 2 5% 4 3 5% 5% 5% 6% 46%	1980
October	1st to 10th	Night	20% 8% 3 7% 7% 3 4 5% 4 6% 33%	2056
	11th to 20th	Night	17% 8% 3 7% 6% 3 5% 4 5% 6% 36%	2186
	21st to end	Night	25% 9% 3 7% 5% 3 5% 4 5% 6% 27%	2574
November	1st to 10th	Night	15% 11% 4 6% 6% 3 5% 4 5% 5% 36%	2407
	11th to 20th	Night	19% 8% 4 7% 5% 3 5% 4 6% 7% 34%	2470
	21st to end	Night	17% 11% 5% 7% 6% 4 5% 5% 5% 6% 31%	2629
December	1st to 10th	Night	19% 14% 6% 11% 7% 3 6% 5% 4 6% 20%	2660
	11th to 20th	Night	22% 11% 4 8% 5% 3 5% 4 5% 6% 28%	3288
	21st to end	Night	26% 16% 5% 8% 5% 3 4 4 4 4 19%	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	38% 41% 8% 10% 2	1518
	11th to 20th	Day	37% 44% 8% 6% 5%	1454
	21st to end	Day	37% 37% 10% 10% 5% 1	1584
February	1st to 10th	Day	39% 39% 8% 10% 4	1619
	11th to 20th	Day	39% 39% 8% 10% 4 1	1735
	21st to end	Day	41% 37% 7% 8% 7%	1480
March	1st to 10th	Day	34% 39% 9% 11% 5% 1	1800
	11th to 20th	Day	38% 37% 8% 9% 7% 1	1857
	21st to end	Day	33% 36% 6% 13% 10% 2	2376
April	1st to 10th	Day	33% 37% 7% 14% 7% 1	2160
	11th to 20th	Day	34% 37% 7% 11% 9% 1	2134
	21st to end	Day	35% 38% 7% 10% 8% 2	2285
May	1st to 10th	Day	34% 37% 7% 12% 9% 2	2389
	11th to 20th	Day	30% 39% 6% 12% 10% 2	2482
	21st to end	Day	27% 39% 11% 14% 8% 1	2725
June	1st to 10th	Day	34% 40% 11% 11% 4 1	2520
	11th to 20th	Day	30% 40% 10% 12% 8% 1	2520
	21st to end	Day	35% 41% 8% 9% 6%	2520
July	1st to 10th	Day	28% 41% 11% 15% 5%	2450
	11th to 20th	Day	29% 40% 11% 16% 4	2445
	21st to end	Day	27% 41% 10% 16% 6% 1	2591
August	1st to 10th	Day	28% 41% 11% 13% 6%	2492
	11th to 20th	Day	28% 38% 11% 16% 6% 1	2183
	21st to end	Day	28% 40% 9% 16% 7% 1	2376
September	1st to 10th	Day	27% 38% 9% 18% 7%	2160
	11th to 20th	Day	25% 38% 8% 19% 10%	2062
	21st to end	Day	27% 40% 9% 16% 7% 1	1980
October	1st to 10th	Day	34% 38% 8% 12% 7% 1	1900
	11th to 20th	Day	31% 39% 11% 13% 4 1	1773
	21st to end	Day	36% 39% 8% 9% 7%	1782
November	1st to 10th	Day	27% 43% 10% 14% 7%	1671
	11th to 20th	Day	32% 42% 9% 10% 5% 1	1710
	21st to end	Day	34% 43% 10% 11% 3	1549
December	1st to 10th	Day	39% 42% 9% 7% 3	1520
	11th to 20th	Day	39% 35% 11% 12% 3	1906
	21st to end	Day	52% 27% 6% 10% 4	2772
January	1st to 10th	Night	39% 42% 9% 7% 4	2649
	11th to 20th	Night	41% 42% 7% 6% 3	2546
	21st to end	Night	37% 38% 7% 11% 6% 1	2772
February	1st to 10th	Night	41% 38% 8% 8% 4 1	2340
	11th to 20th	Night	38% 39% 8% 11% 3 1	2122
	21st to end	Night	39% 37% 8% 11% 4 1	1776
March	1st to 10th	Night	37% 38% 8% 12% 5%	2160
	11th to 20th	Night	37% 38% 8% 11% 5%	2102
	21st to end	Night	32% 37% 7% 12% 10% 3	1980
April	1st to 10th	Night	34% 37% 7% 13% 7% 2	1800
	11th to 20th	Night	34% 38% 6% 13% 7% 1	1775
	21st to end	Night	33% 38% 6% 13% 8% 3	1674
May	1st to 10th	Night	35% 37% 5% 10% 11% 2	1586
	11th to 20th	Night	32% 38% 7% 12% 10% 2	1416
	21st to end	Night	27% 40% 9% 14% 9% 2	1551
June	1st to 10th	Night	33% 41% 11% 10% 5% 1	1440
	11th to 20th	Night	30% 39% 10% 14% 6% 1	1440
	21st to end	Night	33% 39% 9% 12% 6%	1440
July	1st to 10th	Night	28% 42% 11% 13% 6%	1400
	11th to 20th	Night	26% 41% 14% 14% 6%	1392
	21st to end	Night	24% 42% 11% 15% 7% 1	1486
August	1st to 10th	Night	26% 40% 12% 16% 7%	1422
	11th to 20th	Night	27% 40% 10% 16% 7%	1695
	21st to end	Night	28% 38% 10% 15% 8% 1	1980
September	1st to 10th	Night	27% 39% 8% 18% 7%	1800
	11th to 20th	Night	25% 38% 9% 20% 9%	1897
	21st to end	Night	29% 38% 8% 14% 9% 1	1980
October	1st to 10th	Night	34% 39% 7% 13% 7%	2056
	11th to 20th	Night	34% 40% 9% 12% 4	2186
	21st to end	Night	37% 39% 8% 10% 6% 1	2574
November	1st to 10th	Night	31% 42% 8% 11% 7% 1	2407
	11th to 20th	Night	30% 42% 9% 13% 5% 2	2470
	21st to end	Night	34% 42% 8% 11% 4	2629
December	1st to 10th	Night	40% 42% 8% 8% 1	2660
	11th to 20th	Night	39% 36% 10% 10% 5%	3288
	21st to end	Night	51% 29% 7% 11% 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	24% 27% 24% 22% 3 1	180
	11th to 20th	Day	27% 26% 29% 13% 3 1	164
	21st to end	Day	20% 30% 32% 15% 3	176
February	1st to 10th	Day	32% 29% 30% 8% 1	240
	11th to 20th	Day	26% 25% 34% 13% 2	394
	21st to end	Day	30% 20% 32% 16% 2	346
March	1st to 10th	Day	27% 16% 35% 19% 3	420
	11th to 20th	Day	27% 24% 32% 14% 3	420
	21st to end	Day	29% 23% 26% 14% 7% 2	462
April	1st to 10th	Day	34% 18% 25% 16% 6% 1	420
	11th to 20th	Day	33% 20% 26% 13% 6% 2	412
	21st to end	Day	30% 20% 28% 14% 6% 2	420
May	1st to 10th	Day	30% 20% 26% 15% 8% 1	423
	11th to 20th	Day	19% 17% 33% 24% 6% 2	447
	21st to end	Day	19% 15% 32% 21% 11% 1	485
June	1st to 10th	Day	22% 15% 32% 22% 8% 1	450
	11th to 20th	Day	20% 14% 33% 23% 8% 2	450
	21st to end	Day	18% 14% 35% 22% 9% 1	450
July	1st to 10th	Day	15% 9% 30% 29% 14% 2	445
	11th to 20th	Day	15% 8% 34% 29% 13% 2	444
	21st to end	Day	14% 8% 40% 28% 8% 2	488
August	1st to 10th	Day	12% 7% 32% 29% 16% 4	446
	11th to 20th	Day	16% 7% 27% 31% 14% 5%	439
	21st to end	Day	14% 13% 26% 28% 15% 5%	495
September	1st to 10th	Day	14% 12% 28% 25% 14% 6%	450
	11th to 20th	Day	15% 12% 28% 20% 15% 10%	450
	21st to end	Day	17% 13% 31% 21% 12% 6%	450
October	1st to 10th	Day	26% 15% 25% 19% 11% 4	450
	11th to 20th	Day	19% 18% 29% 23% 9% 2	436
	21st to end	Day	33% 14% 29% 20% 4	396
November	1st to 10th	Day	24% 19% 26% 21% 9%	365
	11th to 20th	Day	29% 17% 28% 19% 6% 1	370
	21st to end	Day	27% 18% 33% 19% 3	209
December	1st to 10th	Day	29% 23% 27% 18% 1 1	180
	11th to 20th	Day	28% 25% 36% 11%	180
	21st to end	Day	32% 16% 38% 11% 3 1	198
January	1st to 10th	Night	24% 18% 34% 22% 2	760
	11th to 20th	Night	26% 20% 29% 22% 3	728
	21st to end	Night	18% 15% 37% 26% 3 1	792
February	1st to 10th	Night	23% 16% 39% 20% 2	720
	11th to 20th	Night	20% 19% 38% 20% 3	703
	21st to end	Night	24% 18% 35% 21% 3	592
March	1st to 10th	Night	23% 15% 33% 25% 3	720
	11th to 20th	Night	23% 19% 32% 21% 5%	720
	21st to end	Night	24% 18% 30% 21% 7%	792
April	1st to 10th	Night	26% 15% 29% 26% 3	720
	11th to 20th	Night	26% 19% 28% 20% 7%	711
	21st to end	Night	28% 13% 26% 27% 6%	594
May	1st to 10th	Night	31% 11% 24% 25% 8%	506
	11th to 20th	Night	19% 12% 31% 31% 7% 1	354
	21st to end	Night	19% 8% 27% 36% 11%	389
June	1st to 10th	Night	21% 6% 29% 32% 13%	360
	11th to 20th	Night	19% 6% 26% 36% 12%	360
	21st to end	Night	17% 8% 25% 39% 10% 1	360
July	1st to 10th	Night	14% 3 20% 46% 16% 1	350
	11th to 20th	Night	13% 2 24% 41% 18% 1	348
	21st to end	Night	11% 3 22% 47% 16% 1	394
August	1st to 10th	Night	11% 3 20% 44% 20% 1	358
	11th to 20th	Night	14% 4 20% 40% 19% 2	631
	21st to end	Night	15% 5% 21% 38% 20% 1	792
September	1st to 10th	Night	15% 6% 26% 36% 16% 2	720
	11th to 20th	Night	18% 8% 22% 29% 21% 3	720
	21st to end	Night	18% 7% 29% 28% 18% 2	720
October	1st to 10th	Night	25% 11% 20% 29% 14% 1	720
	11th to 20th	Night	21% 14% 25% 28% 10% 1	720
	21st to end	Night	29% 12% 28% 24% 6%	792
November	1st to 10th	Night	21% 14% 33% 25% 7%	740
	11th to 20th	Night	23% 15% 30% 24% 8% 1	760
	21st to end	Night	22% 16% 33% 22% 6%	760
December	1st to 10th	Night	28% 18% 30% 20% 4	760
	11th to 20th	Night	25% 15% 34% 22% 4	760
	21st to end	Night	32% 12% 34% 19% 2	836

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