Fairfield Harbor
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fairfield Harbor clear sky chart from 2005-11-06 to 2024-02-15.

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	13% 6% 3 6% 5% 3 5% 5% 6% 8% 41%	78420
	Night	13% 6% 2 5% 5% 3 4 5% 5% 8% 44%	80632
Transparency	Day	27% 37% 8% 13% 12% 3	78141
	Night	26% 36% 8% 14% 13% 3	80430
Seeing	Day	17% 12% 27% 25% 15% 5%	14425
	Night	16% 11% 26% 30% 14% 2	24416

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	14% 6% 2 4 4 2 3 3 3 7% 52%	2836
	11th to 20th	Day	11% 6% 2 5% 4 3 4 4 5% 5% 51%	2840
	21st to end	Day	13% 9% 3 7% 5% 3 4 4 4 6% 41%	3124
February	1st to 10th	Day	15% 7% 3 5% 5% 2 4 5% 5% 7% 42%	1710
	11th to 20th	Day	19% 6% 3 5% 4 2 3 3 4 4 46%	1758
	21st to end	Day	15% 6% 3 5% 6% 3 5% 4 5% 6% 42%	1482
March	1st to 10th	Day	13% 6% 3 4 4 3 3 4 4 6% 51%	1980
	11th to 20th	Day	15% 7% 3 5% 5% 3 4 4 5% 8% 41%	1980
	21st to end	Day	16% 6% 3 5% 5% 4 4 4 5% 6% 42%	2201
April	1st to 10th	Day	13% 4 2 5% 4 3 3 3 5% 6% 52%	2160
	11th to 20th	Day	11% 4 2 4 4 3 4 4 6% 8% 50%	2134
	21st to end	Day	10% 4 2 5% 5% 3 5% 4 6% 7% 49%	2160
May	1st to 10th	Day	10% 4 2 5% 6% 4 6% 5% 7% 9% 42%	2160
	11th to 20th	Day	14% 6% 3 7% 6% 4 6% 6% 6% 6% 37%	2253
	21st to end	Day	10% 7% 3 7% 6% 4 6% 6% 8% 10% 35%	2611
June	1st to 10th	Day	11% 6% 3 7% 6% 4 6% 7% 7% 9% 34%	2520
	11th to 20th	Day	9% 4 2 6% 5% 4 7% 7% 7% 10% 36%	2520
	21st to end	Day	9% 5% 3 5% 7% 5% 8% 8% 9% 11% 31%	2499
July	1st to 10th	Day	10% 5% 3 6% 7% 4 6% 7% 10% 11% 31%	2340
	11th to 20th	Day	8% 7% 3 7% 6% 4 8% 8% 8% 11% 30%	2340
	21st to end	Day	8% 5% 3 7% 7% 5% 6% 9% 8% 11% 31%	2547
August	1st to 10th	Day	12% 7% 3 7% 7% 4 7% 7% 8% 10% 27%	2297
	11th to 20th	Day	12% 6% 3 7% 7% 5% 7% 7% 7% 9% 30%	2126
	21st to end	Day	13% 7% 3 6% 7% 4 6% 8% 7% 10% 29%	2376
September	1st to 10th	Day	12% 6% 2 6% 5% 5% 6% 7% 7% 9% 34%	2160
	11th to 20th	Day	14% 6% 3 6% 6% 5% 6% 5% 6% 9% 35%	2160
	21st to end	Day	18% 8% 3 7% 6% 3 4 5% 5% 8% 34%	1906
October	1st to 10th	Day	16% 7% 3 5% 4 3 4 4 5% 5% 43%	1800
	11th to 20th	Day	12% 5% 2 5% 4 3 5% 3 4 5% 52%	1800
	21st to end	Day	12% 5% 2 4 4 3 4 4 4 5% 52%	1980
November	1st to 10th	Day	14% 5% 2 5% 4 2 4 4 5% 5% 51%	1857
	11th to 20th	Day	16% 5% 2 4 4 2 3 4 4 6% 50%	1763
	21st to end	Day	12% 5% 3 5% 4 2 4 4 4 7% 49%	1701
December	1st to 10th	Day	18% 5% 2 6% 5% 3 3 5% 5% 6% 41%	1520
	11th to 20th	Day	16% 6% 2 4 4 2 3 2 4 5% 53%	1910
	21st to end	Day	13% 7% 3 5% 4 2 4 4 4 5% 46%	2909
January	1st to 10th	Night	14% 6% 3 5% 5% 2 4 4 5% 6% 46%	4109
	11th to 20th	Night	15% 5% 3 5% 5% 3 4 5% 5% 7% 42%	4120
	21st to end	Night	15% 7% 3 5% 5% 3 5% 4 4 6% 43%	4532
February	1st to 10th	Night	18% 6% 2 4 4 2 5% 4 4 7% 43%	2470
	11th to 20th	Night	19% 8% 2 5% 4 2 4 4 4 6% 42%	2182
	21st to end	Night	17% 5% 2 5% 5% 3 4 5% 5% 5% 43%	1772
March	1st to 10th	Night	12% 4 2 4 3 2 4 4 5% 5% 53%	1980
	11th to 20th	Night	15% 6% 3 6% 5% 3 5% 4 6% 8% 39%	1980
	21st to end	Night	14% 5% 2 5% 5% 3 5% 3 5% 7% 46%	2153
April	1st to 10th	Night	12% 6% 2 5% 5% 3 4 4 5% 6% 48%	1800
	11th to 20th	Night	9% 5% 3 5% 5% 3 4 5% 5% 7% 49%	1775
	21st to end	Night	9% 4 2 4 4 2 4 4 5% 9% 53%	1800
May	1st to 10th	Night	10% 5% 2 5% 6% 4 5% 5% 5% 8% 46%	1800
	11th to 20th	Night	12% 6% 3 5% 5% 4 4 5% 6% 9% 41%	1654
	21st to end	Night	9% 5% 2 5% 4 3 5% 5% 6% 10% 45%	1659
June	1st to 10th	Night	9% 5% 4 4 7% 3 5% 6% 7% 10% 40%	1440
	11th to 20th	Night	7% 5% 2 5% 4 4 6% 6% 8% 11% 40%	1440
	21st to end	Night	9% 5% 3 5% 5% 4 6% 7% 7% 12% 37%	1454
July	1st to 10th	Night	7% 4 3 6% 6% 5% 7% 6% 8% 12% 36%	1620
	11th to 20th	Night	6% 3 2 6% 6% 4 6% 6% 8% 13% 39%	1620
	21st to end	Night	7% 5% 3 7% 6% 4 6% 7% 8% 11% 37%	1771
August	1st to 10th	Night	8% 6% 2 6% 5% 3 6% 7% 8% 12% 35%	1623
	11th to 20th	Night	9% 6% 3 7% 6% 4 6% 6% 7% 11% 35%	1774
	21st to end	Night	12% 6% 3 6% 5% 3 5% 6% 7% 12% 35%	1980
September	1st to 10th	Night	11% 5% 2 5% 4 4 5% 6% 7% 10% 42%	1800
	11th to 20th	Night	13% 5% 2 7% 6% 3 5% 4 6% 7% 42%	1800
	21st to end	Night	18% 6% 3 6% 6% 4 5% 4 5% 7% 36%	2047
October	1st to 10th	Night	14% 6% 2 5% 4 2 3 4 5% 6% 48%	2160
	11th to 20th	Night	12% 6% 2 5% 4 3 3 3 4 6% 53%	2160
	21st to end	Night	11% 5% 1 4 4 2 3 4 4 5% 55%	2376
November	1st to 10th	Night	13% 5% 2 4 4 3 4 4 5% 6% 49%	2221
	11th to 20th	Night	15% 7% 2 5% 4 2 4 3 3 6% 48%	2413
	21st to end	Night	12% 5% 2 5% 4 2 4 4 5% 7% 50%	2474
December	1st to 10th	Night	16% 7% 3 5% 5% 3 4 5% 6% 6% 40%	2660
	11th to 20th	Night	16% 6% 2 4 4 2 3 3 5% 6% 48%	3285
	21st to end	Night	14% 7% 2 5% 4 3 3 4 5% 7% 47%	4728

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	26% 25% 6% 22% 19% 3	2837
	11th to 20th	Day	25% 25% 5% 16% 21% 8%	2840
	21st to end	Day	38% 25% 6% 11% 13% 7%	3124
February	1st to 10th	Day	27% 35% 7% 14% 13% 3	1710
	11th to 20th	Day	28% 36% 5% 10% 16% 5%	1767
	21st to end	Day	28% 36% 7% 13% 12% 4	1482
March	1st to 10th	Day	21% 36% 7% 15% 17% 5%	1980
	11th to 20th	Day	28% 38% 8% 14% 9% 3	1980
	21st to end	Day	29% 38% 7% 12% 11% 2	2201
April	1st to 10th	Day	21% 37% 6% 14% 17% 5%	2160
	11th to 20th	Day	20% 36% 8% 15% 15% 6%	2134
	21st to end	Day	23% 38% 8% 14% 14% 3	2160
May	1st to 10th	Day	23% 39% 12% 16% 9% 1	2160
	11th to 20th	Day	32% 40% 7% 12% 9% 1	2257
	21st to end	Day	28% 40% 10% 14% 6% 1	2611
June	1st to 10th	Day	29% 39% 9% 15% 7%	2520
	11th to 20th	Day	28% 40% 10% 15% 7% 1	2520
	21st to end	Day	27% 41% 11% 13% 7% 1	2499
July	1st to 10th	Day	27% 41% 11% 12% 7% 1	2275
	11th to 20th	Day	27% 44% 13% 10% 6% 1	2271
	21st to end	Day	26% 42% 16% 14% 2	2405
August	1st to 10th	Day	28% 40% 16% 12% 4	2284
	11th to 20th	Day	31% 39% 11% 13% 7% 1	2126
	21st to end	Day	33% 41% 9% 11% 5% 1	2376
September	1st to 10th	Day	28% 40% 11% 10% 9% 3	2160
	11th to 20th	Day	27% 38% 9% 13% 11% 2	2160
	21st to end	Day	32% 37% 6% 11% 11% 3	1906
October	1st to 10th	Day	27% 37% 5% 10% 20% 1	1800
	11th to 20th	Day	26% 36% 3 13% 16% 5%	1800
	21st to end	Day	24% 38% 4 14% 16% 4	1980
November	1st to 10th	Day	19% 41% 6% 11% 19% 5%	1857
	11th to 20th	Day	23% 39% 6% 13% 15% 4	1763
	21st to end	Day	24% 39% 6% 13% 14% 4	1701
December	1st to 10th	Day	25% 40% 8% 14% 9% 3	1520
	11th to 20th	Day	26% 31% 5% 15% 17% 6%	1906
	21st to end	Day	29% 26% 8% 16% 20% 1	2909
January	1st to 10th	Night	28% 26% 6% 17% 19% 3	4109
	11th to 20th	Night	32% 26% 6% 13% 17% 5%	4120
	21st to end	Night	34% 25% 7% 13% 16% 6%	4532
February	1st to 10th	Night	29% 35% 6% 12% 15% 4	2470
	11th to 20th	Night	28% 36% 7% 10% 14% 5%	2184
	21st to end	Night	28% 37% 7% 12% 11% 6%	1772
March	1st to 10th	Night	21% 36% 6% 16% 16% 5%	1980
	11th to 20th	Night	29% 39% 8% 10% 10% 3	1980
	21st to end	Night	26% 36% 10% 14% 12% 2	2153
April	1st to 10th	Night	24% 37% 7% 11% 16% 5%	1800
	11th to 20th	Night	20% 36% 9% 14% 16% 5%	1775
	21st to end	Night	20% 37% 11% 17% 13% 3	1800
May	1st to 10th	Night	23% 38% 13% 16% 9% 1	1800
	11th to 20th	Night	28% 39% 12% 11% 10% 1	1655
	21st to end	Night	24% 41% 13% 15% 7% 1	1660
June	1st to 10th	Night	25% 40% 11% 16% 7%	1440
	11th to 20th	Night	25% 41% 12% 16% 5%	1440
	21st to end	Night	26% 41% 12% 14% 7%	1454
July	1st to 10th	Night	25% 43% 11% 12% 7% 1	1575
	11th to 20th	Night	21% 42% 18% 13% 5%	1566
	21st to end	Night	24% 43% 16% 15% 1	1672
August	1st to 10th	Night	23% 40% 19% 15% 3	1612
	11th to 20th	Night	28% 42% 12% 13% 5% 1	1774
	21st to end	Night	28% 41% 14% 13% 4 1	1980
September	1st to 10th	Night	24% 40% 11% 12% 12% 2	1800
	11th to 20th	Night	26% 39% 9% 14% 11% 2	1800
	21st to end	Night	30% 38% 5% 14% 11% 1	2047
October	1st to 10th	Night	23% 37% 5% 13% 18% 4	2160
	11th to 20th	Night	25% 37% 6% 14% 14% 5%	2160
	21st to end	Night	22% 37% 5% 14% 17% 5%	2376
November	1st to 10th	Night	18% 42% 5% 12% 18% 5%	2221
	11th to 20th	Night	25% 39% 5% 11% 16% 3	2413
	21st to end	Night	24% 39% 5% 12% 16% 4	2474
December	1st to 10th	Night	25% 40% 8% 11% 13% 3	2660
	11th to 20th	Night	28% 32% 6% 11% 18% 4	3288
	21st to end	Night	27% 25% 6% 20% 20% 2	4728

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% 28% 37% 13% 3	270
	11th to 20th	Day	18% 27% 38% 15% 1	270
	21st to end	Day	21% 25% 37% 12% 4	297
February	1st to 10th	Day	22% 22% 39% 14% 3	270
	11th to 20th	Day	29% 22% 34% 12% 2	402
	21st to end	Day	18% 27% 35% 15% 4 1	346
March	1st to 10th	Day	16% 23% 37% 17% 6%	420
	11th to 20th	Day	24% 23% 33% 13% 6% 1	420
	21st to end	Day	21% 19% 33% 18% 7% 1	462
April	1st to 10th	Day	21% 23% 31% 17% 6% 2	420
	11th to 20th	Day	13% 20% 32% 21% 9% 4	412
	21st to end	Day	13% 15% 34% 24% 12% 3	420
May	1st to 10th	Day	17% 17% 33% 21% 9% 3	423
	11th to 20th	Day	18% 9% 26% 28% 15% 3	447
	21st to end	Day	15% 6% 23% 30% 19% 7%	485
June	1st to 10th	Day	12% 8% 28% 31% 17% 4	450
	11th to 20th	Day	12% 5% 21% 33% 21% 8%	450
	21st to end	Day	10% 5% 20% 36% 21% 8%	450
July	1st to 10th	Day	13% 4 12% 35% 27% 8%	445
	11th to 20th	Day	10% 3 12% 35% 30% 11%	444
	21st to end	Day	9% 1 19% 37% 26% 7%	488
August	1st to 10th	Day	14% 5% 16% 30% 25% 10%	446
	11th to 20th	Day	14% 3 11% 30% 28% 13%	439
	21st to end	Day	13% 4 15% 31% 24% 13%	495
September	1st to 10th	Day	15% 4 17% 29% 21% 14%	450
	11th to 20th	Day	14% 5% 21% 34% 16% 9%	450
	21st to end	Day	24% 4 23% 24% 18% 8%	450
October	1st to 10th	Day	21% 6% 25% 22% 18% 8%	450
	11th to 20th	Day	14% 11% 28% 30% 14% 5%	450
	21st to end	Day	13% 13% 29% 27% 15% 3	495
November	1st to 10th	Day	18% 12% 32% 25% 10% 4	455
	11th to 20th	Day	19% 16% 36% 19% 9% 2	395
	21st to end	Day	17% 16% 29% 21% 16% 1	361
December	1st to 10th	Day	23% 19% 36% 17% 4 1	180
	11th to 20th	Day	17% 23% 34% 17% 6% 2	180
	21st to end	Day	21% 18% 33% 15% 10% 2	238
January	1st to 10th	Night	18% 21% 32% 23% 5%	760
	11th to 20th	Night	19% 18% 27% 29% 7%	760
	21st to end	Night	20% 18% 35% 23% 4	836
February	1st to 10th	Night	23% 16% 33% 24% 5%	760
	11th to 20th	Night	27% 20% 32% 19% 2	722
	21st to end	Night	20% 21% 31% 24% 4	592
March	1st to 10th	Night	17% 17% 37% 24% 6%	720
	11th to 20th	Night	20% 17% 36% 21% 6% 1	720
	21st to end	Night	18% 16% 36% 24% 7%	792
April	1st to 10th	Night	15% 20% 35% 23% 6%	720
	11th to 20th	Night	12% 15% 37% 25% 11% 1	711
	21st to end	Night	11% 12% 39% 31% 7%	720
May	1st to 10th	Night	14% 12% 37% 28% 9% 1	720
	11th to 20th	Night	16% 8% 28% 35% 12% 1	593
	21st to end	Night	11% 3 17% 41% 24% 4	499
June	1st to 10th	Night	10% 3 24% 47% 15% 1	360
	11th to 20th	Night	9% 3 21% 45% 19% 3	360
	21st to end	Night	10% 2 20% 44% 22% 1	374
July	1st to 10th	Night	8% 2 13% 42% 29% 5%	525
	11th to 20th	Night	7% 1 9% 43% 32% 7%	522
	21st to end	Night	8% 4 14% 44% 27% 3	591
August	1st to 10th	Night	11% 2 10% 40% 31% 6%	549
	11th to 20th	Night	11% 3 12% 39% 28% 7%	710
	21st to end	Night	13% 4 12% 34% 31% 7%	792
September	1st to 10th	Night	14% 2 13% 31% 32% 8%	720
	11th to 20th	Night	16% 3 19% 34% 22% 6%	720
	21st to end	Night	24% 5% 16% 30% 22% 4	720
October	1st to 10th	Night	17% 4 20% 31% 22% 5%	720
	11th to 20th	Night	15% 10% 25% 30% 16% 3	720
	21st to end	Night	15% 9% 26% 29% 18% 3	792
November	1st to 10th	Night	15% 10% 26% 32% 14% 2	740
	11th to 20th	Night	20% 12% 30% 25% 12% 1	760
	21st to end	Night	14% 13% 30% 26% 15% 1	760
December	1st to 10th	Night	21% 15% 28% 28% 8%	760
	11th to 20th	Night	17% 14% 29% 29% 10% 1	760
	21st to end	Night	17% 16% 28% 31% 7% 1	836

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