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 2025-03-27.

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 7% 5% 5% 5% 4 6% 7% 41%	108667
	Night	13% 6% 2 6% 4 4 4 4 5% 7% 44%	110660
Transparency	Day	41% 5% 12% 20% 17% 5%	112311
	Night	38% 5% 12% 20% 19% 5%	115155
Seeing	Day	17% 13% 27% 24% 14% 5%	15931
	Night	16% 11% 27% 30% 14% 2	25918

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	12% 5% 2 5% 3 3 2 2 3 5% 57%	4056
	11th to 20th	Day	12% 5% 2 5% 3 3 3 2 4 5% 55%	4060
	21st to end	Day	12% 11% 4 8% 4 4 4 3 4 5% 41%	4466
February	1st to 10th	Day	16% 8% 2 6% 4 2 5% 4 5% 6% 41%	1800
	11th to 20th	Day	19% 7% 3 6% 4 3 3 3 4 4 44%	1917
	21st to end	Day	14% 6% 2 6% 5% 4 5% 3 6% 6% 43%	1653
March	1st to 10th	Day	24% 9% 3 6% 4 3 3 3 5% 6% 35%	3545
	11th to 20th	Day	11% 5% 2 6% 3 3 3 3 5% 6% 52%	3590
	21st to end	Day	17% 6% 2 5% 4 4 3 2 4 5% 48%	3931
April	1st to 10th	Day	13% 5% 2 5% 4 4 3 3 5% 6% 52%	2280
	11th to 20th	Day	11% 4 2 5% 4 4 4 4 5% 8% 50%	2254
	21st to end	Day	10% 4 1 6% 4 4 5% 3 6% 7% 49%	2280
May	1st to 10th	Day	9% 5% 3 7% 6% 6% 6% 5% 7% 10% 37%	3765
	11th to 20th	Day	17% 8% 3 9% 4 5% 5% 4 6% 7% 31%	4005
	21st to end	Day	7% 5% 2 7% 5% 6% 6% 4 8% 10% 41%	4625
June	1st to 10th	Day	10% 7% 3 8% 6% 5% 6% 5% 7% 9% 34%	2660
	11th to 20th	Day	9% 4 2 7% 5% 6% 7% 5% 7% 10% 38%	2660
	21st to end	Day	8% 5% 3 6% 6% 6% 8% 7% 9% 11% 31%	2637
July	1st to 10th	Day	9% 5% 2 7% 6% 6% 6% 6% 9% 12% 31%	4120
	11th to 20th	Day	11% 8% 3 10% 7% 7% 8% 6% 8% 10% 23%	4120
	21st to end	Day	11% 7% 3 9% 7% 7% 7% 6% 8% 9% 25%	4466
August	1st to 10th	Day	22% 11% 4 9% 6% 5% 5% 5% 7% 8% 18%	4047
	11th to 20th	Day	11% 5% 3 9% 6% 6% 6% 5% 7% 9% 35%	3730
	21st to end	Day	8% 6% 2 7% 5% 5% 5% 5% 7% 10% 38%	4180
September	1st to 10th	Day	12% 7% 2 7% 4 5% 6% 5% 7% 9% 34%	2280
	11th to 20th	Day	15% 6% 3 7% 5% 6% 5% 4 6% 9% 34%	2280
	21st to end	Day	18% 8% 3 8% 5% 4 4 4 5% 7% 34%	2011
October	1st to 10th	Day	16% 7% 3 6% 3 4 4 3 5% 5% 43%	1900
	11th to 20th	Day	12% 6% 2 5% 3 4 4 2 4 5% 53%	1900
	21st to end	Day	12% 5% 2 5% 4 3 4 3 4 5% 53%	2090
November	1st to 10th	Day	14% 5% 2 5% 3 3 4 3 5% 5% 50%	1957
	11th to 20th	Day	15% 6% 2 4 4 4 3 3 4 6% 49%	1855
	21st to end	Day	12% 6% 2 5% 4 3 4 3 4 7% 50%	1790
December	1st to 10th	Day	12% 4 2 5% 3 3 3 4 4 5% 54%	2600
	11th to 20th	Day	16% 7% 3 7% 4 4 3 3 4 5% 46%	2990
	21st to end	Day	12% 6% 2 6% 4 3 4 3 4 6% 49%	4167
January	1st to 10th	Night	11% 7% 3 5% 3 3 3 2 4 5% 54%	5889
	11th to 20th	Night	14% 5% 2 5% 4 3 3 3 4 6% 50%	5900
	21st to end	Night	13% 9% 3 7% 4 4 4 3 5% 6% 44%	6490
February	1st to 10th	Night	18% 6% 2 5% 3 3 5% 3 5% 7% 42%	2600
	11th to 20th	Night	19% 9% 2 6% 3 3 3 3 4 6% 41%	2368
	21st to end	Night	16% 6% 2 6% 4 4 4 4 5% 6% 44%	1974
March	1st to 10th	Night	23% 11% 3 6% 4 4 4 3 4 5% 33%	3528
	11th to 20th	Night	13% 6% 2 6% 3 3 4 3 5% 6% 50%	3590
	21st to end	Night	17% 5% 2 4 3 3 3 2 5% 6% 50%	3838
April	1st to 10th	Night	12% 6% 2 6% 4 4 4 3 5% 6% 47%	1900
	11th to 20th	Night	9% 5% 2 6% 4 4 4 4 5% 7% 49%	1875
	21st to end	Night	9% 4 2 5% 3 3 4 4 6% 8% 52%	1900
May	1st to 10th	Night	8% 6% 2 8% 6% 6% 6% 4 6% 8% 39%	3130
	11th to 20th	Night	16% 8% 3 7% 5% 5% 4 3 5% 7% 37%	2916
	21st to end	Night	7% 5% 2 6% 4 4 5% 4 7% 10% 45%	2925
June	1st to 10th	Night	9% 6% 3 5% 6% 4 5% 5% 7% 10% 39%	1520
	11th to 20th	Night	7% 5% 2 6% 4 5% 6% 5% 8% 10% 42%	1520
	21st to end	Night	9% 6% 2 6% 5% 6% 5% 5% 7% 12% 37%	1535
July	1st to 10th	Night	8% 5% 3 7% 5% 6% 6% 5% 8% 13% 35%	2840
	11th to 20th	Night	10% 6% 2 8% 6% 5% 6% 5% 9% 13% 28%	2840
	21st to end	Night	12% 7% 3 8% 6% 6% 6% 6% 9% 9% 29%	3087
August	1st to 10th	Night	17% 11% 3 8% 4 5% 6% 5% 7% 10% 24%	2851
	11th to 20th	Night	9% 6% 3 8% 5% 6% 6% 5% 7% 11% 35%	3097
	21st to end	Night	8% 5% 3 7% 4 5% 5% 4 7% 10% 42%	3476
September	1st to 10th	Night	11% 5% 2 6% 3 5% 5% 5% 7% 10% 41%	1900
	11th to 20th	Night	14% 6% 2 8% 5% 4 5% 3 6% 7% 40%	1900
	21st to end	Night	18% 6% 3 7% 5% 5% 5% 3 5% 6% 36%	2162
October	1st to 10th	Night	14% 7% 2 6% 4 3 3 3 5% 6% 48%	2280
	11th to 20th	Night	11% 6% 2 5% 3 3 3 2 4 5% 54%	2280
	21st to end	Night	11% 5% 1 5% 3 3 3 3 4 5% 57%	2508
November	1st to 10th	Night	13% 6% 2 4 4 4 3 3 5% 6% 49%	2341
	11th to 20th	Night	15% 8% 2 6% 4 3 3 3 3 5% 48%	2540
	21st to end	Night	12% 5% 2 5% 4 3 4 3 5% 7% 50%	2605
December	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 4 5% 57%	4580
	11th to 20th	Night	15% 7% 2 7% 4 4 4 3 6% 7% 41%	5205
	21st to end	Night	13% 6% 2 5% 3 3 3 3 5% 7% 49%	6770

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	32% 2 6% 21% 30% 9%	4057
	11th to 20th	Day	33% 3 5% 17% 31% 10%	4060
	21st to end	Day	46% 4 6% 15% 20% 9%	4466
February	1st to 10th	Day	43% 4 11% 21% 17% 4	1800
	11th to 20th	Day	45% 3 8% 16% 22% 6%	1926
	21st to end	Day	41% 4 10% 20% 19% 7%	1653
March	1st to 10th	Day	51% 3 11% 14% 14% 7%	3545
	11th to 20th	Day	32% 4 8% 17% 24% 14%	3590
	21st to end	Day	40% 3 8% 26% 18% 6%	3931
April	1st to 10th	Day	35% 3 11% 21% 24% 7%	2280
	11th to 20th	Day	32% 4 12% 23% 21% 8%	2254
	21st to end	Day	33% 5% 13% 25% 20% 4	2280
May	1st to 10th	Day	41% 5% 21% 20% 10% 2	3800
	11th to 20th	Day	51% 4 11% 21% 13%	4009
	21st to end	Day	36% 6% 16% 27% 13% 2	4639
June	1st to 10th	Day	44% 8% 14% 20% 14% 1	2660
	11th to 20th	Day	38% 7% 19% 22% 12% 1	2660
	21st to end	Day	41% 10% 19% 21% 9% 1	2637
July	1st to 10th	Day	40% 11% 21% 19% 8% 1	4055
	11th to 20th	Day	52% 11% 20% 12% 4 1	4051
	21st to end	Day	50% 10% 15% 17% 7%	4324
August	1st to 10th	Day	61% 7% 18% 10% 3	4034
	11th to 20th	Day	43% 8% 15% 22% 11%	3766
	21st to end	Day	38% 8% 19% 25% 10% 1	4180
September	1st to 10th	Day	43% 8% 17% 16% 12% 4	2280
	11th to 20th	Day	47% 6% 12% 19% 14% 3	2280
	21st to end	Day	50% 5% 10% 18% 16% 3	2011
October	1st to 10th	Day	37% 6% 8% 21% 23% 6%	3160
	11th to 20th	Day	25% 2 5% 19% 25% 24%	3140
	21st to end	Day	27% 4 6% 27% 28% 9%	3428
November	1st to 10th	Day	36% 4 9% 20% 25% 6%	1957
	11th to 20th	Day	37% 3 8% 19% 25% 7%	1855
	21st to end	Day	36% 3 9% 24% 24% 5%	1790
December	1st to 10th	Day	34% 3 8% 20% 27% 8%	2600
	11th to 20th	Day	42% 3 11% 19% 18% 6%	2986
	21st to end	Day	38% 4 12% 25% 19% 1	4167
January	1st to 10th	Night	33% 3 5% 18% 34% 7%	5889
	11th to 20th	Night	36% 3 7% 16% 31% 7%	5900
	21st to end	Night	41% 4 7% 17% 24% 7%	6490
February	1st to 10th	Night	42% 4 9% 21% 19% 5%	2600
	11th to 20th	Night	46% 3 10% 16% 19% 6%	2370
	21st to end	Night	42% 3 9% 17% 21% 8%	1974
March	1st to 10th	Night	55% 2 9% 15% 14% 4	3528
	11th to 20th	Night	36% 4 9% 12% 24% 15%	3590
	21st to end	Night	37% 1 12% 25% 19% 5%	3838
April	1st to 10th	Night	37% 4 12% 17% 23% 7%	1900
	11th to 20th	Night	35% 4 14% 22% 20% 6%	1875
	21st to end	Night	30% 5% 15% 30% 17% 4	1900
May	1st to 10th	Night	42% 5% 21% 21% 10% 2	3160
	11th to 20th	Night	47% 4 13% 21% 13% 1	2917
	21st to end	Night	33% 6% 23% 25% 12% 1	2932
June	1st to 10th	Night	39% 7% 18% 22% 12% 1	1520
	11th to 20th	Night	32% 10% 20% 26% 10% 1	1520
	21st to end	Night	38% 9% 20% 24% 10%	1535
July	1st to 10th	Night	37% 11% 25% 20% 6% 1	2795
	11th to 20th	Night	45% 11% 28% 12% 4	2786
	21st to end	Night	48% 12% 16% 16% 8%	2988
August	1st to 10th	Night	54% 8% 22% 14% 2	2840
	11th to 20th	Night	41% 10% 20% 21% 8% 1	3127
	21st to end	Night	34% 8% 19% 29% 10% 1	3476
September	1st to 10th	Night	37% 10% 19% 18% 14% 3	1900
	11th to 20th	Night	43% 8% 13% 20% 13% 3	1900
	21st to end	Night	48% 6% 11% 21% 13% 1	2162
October	1st to 10th	Night	39% 5% 8% 20% 24% 4	3800
	11th to 20th	Night	23% 2 6% 19% 23% 28%	3779
	21st to end	Night	23% 4 7% 28% 30% 9%	4120
November	1st to 10th	Night	36% 4 9% 20% 25% 6%	2341
	11th to 20th	Night	41% 3 8% 17% 25% 7%	2540
	21st to end	Night	34% 3 8% 25% 24% 6%	2605
December	1st to 10th	Night	32% 3 8% 13% 33% 11%	4580
	11th to 20th	Night	42% 4 14% 14% 19% 6%	5208
	21st to end	Night	35% 4 12% 28% 19% 2	6770

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	23% 27% 36% 12% 2	300
	11th to 20th	Day	19% 29% 37% 14% 1	300
	21st to end	Day	22% 25% 38% 12% 4	330
February	1st to 10th	Day	23% 21% 39% 13% 3	300
	11th to 20th	Day	29% 23% 34% 11% 2	465
	21st to end	Day	18% 26% 34% 18% 4	414
March	1st to 10th	Day	20% 22% 35% 17% 6%	500
	11th to 20th	Day	23% 23% 34% 14% 6% 1	500
	21st to end	Day	22% 19% 35% 17% 6% 1	530
April	1st to 10th	Day	21% 25% 31% 16% 5% 2	460
	11th to 20th	Day	14% 19% 32% 22% 9% 4	452
	21st to end	Day	12% 15% 33% 24% 12% 3	460
May	1st to 10th	Day	16% 17% 32% 22% 10% 3	463
	11th to 20th	Day	18% 10% 27% 28% 14% 3	487
	21st to end	Day	14% 6% 25% 29% 19% 7%	529
June	1st to 10th	Day	11% 8% 28% 31% 17% 4	490
	11th to 20th	Day	11% 5% 23% 34% 20% 7%	490
	21st to end	Day	10% 5% 20% 37% 21% 8%	490
July	1st to 10th	Day	13% 4 12% 35% 28% 9%	485
	11th to 20th	Day	9% 4 12% 36% 29% 10%	484
	21st to end	Day	10% 3 19% 37% 25% 7%	532
August	1st to 10th	Day	17% 5% 17% 29% 23% 9%	486
	11th to 20th	Day	13% 4 14% 29% 27% 12%	479
	21st to end	Day	12% 4 17% 31% 24% 12%	539
September	1st to 10th	Day	15% 5% 19% 28% 20% 13%	490
	11th to 20th	Day	16% 6% 20% 33% 16% 8%	490
	21st to end	Day	24% 4 23% 24% 18% 7%	490
October	1st to 10th	Day	20% 7% 26% 22% 18% 8%	490
	11th to 20th	Day	13% 11% 29% 29% 13% 4	490
	21st to end	Day	12% 12% 28% 27% 17% 4	539
November	1st to 10th	Day	17% 12% 33% 26% 9% 3	495
	11th to 20th	Day	19% 15% 37% 19% 9% 2	427
	21st to end	Day	16% 17% 30% 21% 15% 1	390
December	1st to 10th	Day	22% 21% 37% 16% 4 1	200
	11th to 20th	Day	17% 23% 35% 18% 6% 3	200
	21st to end	Day	20% 19% 36% 14% 9% 2	265
January	1st to 10th	Night	18% 22% 32% 22% 5%	800
	11th to 20th	Night	19% 18% 28% 29% 6%	800
	21st to end	Night	20% 18% 36% 23% 3	880
February	1st to 10th	Night	23% 16% 33% 23% 5%	800
	11th to 20th	Night	27% 20% 32% 19% 2	783
	21st to end	Night	20% 20% 31% 25% 4	660
March	1st to 10th	Night	18% 18% 36% 23% 5%	800
	11th to 20th	Night	21% 17% 36% 21% 5% 1	800
	21st to end	Night	18% 16% 37% 23% 6%	863
April	1st to 10th	Night	15% 21% 35% 23% 6%	760
	11th to 20th	Night	12% 15% 37% 25% 11% 1	751
	21st to end	Night	11% 12% 39% 31% 7%	760
May	1st to 10th	Night	13% 12% 36% 29% 9% 1	760
	11th to 20th	Night	17% 8% 29% 35% 11% 1	626
	21st to end	Night	10% 3 18% 41% 23% 4	527
June	1st to 10th	Night	10% 3 23% 48% 14% 1	380
	11th to 20th	Night	8% 3 21% 46% 19% 3	380
	21st to end	Night	10% 2 19% 44% 23% 1	395
July	1st to 10th	Night	8% 2 13% 42% 30% 5%	555
	11th to 20th	Night	7% 1 11% 44% 31% 6%	552
	21st to end	Night	8% 4 14% 44% 26% 3	624
August	1st to 10th	Night	12% 2 11% 40% 30% 6%	580
	11th to 20th	Night	12% 3 13% 39% 27% 7%	750
	21st to end	Night	13% 4 12% 35% 30% 7%	836
September	1st to 10th	Night	14% 3 14% 30% 31% 8%	760
	11th to 20th	Night	17% 4 18% 33% 22% 5%	760
	21st to end	Night	23% 4 16% 31% 21% 4	760
October	1st to 10th	Night	17% 4 21% 31% 22% 5%	760
	11th to 20th	Night	14% 10% 26% 30% 16% 3	760
	21st to end	Night	14% 9% 25% 29% 20% 3	836
November	1st to 10th	Night	15% 10% 27% 32% 14% 2	780
	11th to 20th	Night	20% 12% 30% 25% 12% 1	800
	21st to end	Night	14% 13% 31% 26% 15% 1	800
December	1st to 10th	Night	21% 15% 29% 28% 7%	800
	11th to 20th	Night	18% 14% 29% 30% 10% 1	800
	21st to end	Night	17% 15% 29% 31% 7% 1	880

Page generated at 2025-03-24 09:04:27. Version ac07

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