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 2024-10-20.

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% 8% 39%	100511
	Night	13% 6% 2 6% 4 4 4 4 6% 8% 42%	98106
Transparency	Day	42% 6% 13% 20% 16% 4	100317
	Night	40% 5% 13% 20% 17% 4	97970
Seeing	Day	16% 13% 27% 25% 14% 5%	15417
	Night	16% 11% 26% 30% 14% 2	25286

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 5% 3 3 3 3 3 7% 51%	2836
	11th to 20th	Day	11% 6% 2 6% 4 4 4 3 5% 5% 51%	2840
	21st to end	Day	13% 9% 3 8% 4 4 4 3 4 6% 41%	3124
February	1st to 10th	Day	15% 8% 2 6% 5% 2 5% 4 5% 6% 42%	1710
	11th to 20th	Day	18% 7% 3 6% 4 3 3 3 4 4 45%	1818
	21st to end	Day	14% 6% 3 6% 5% 4 5% 3 6% 6% 42%	1573
March	1st to 10th	Day	25% 9% 3 6% 4 3 3 3 5% 5% 35%	3435
	11th to 20th	Day	10% 5% 2 6% 3 3 3 3 5% 6% 53%	3480
	21st to end	Day	17% 6% 2 5% 4 4 3 2 4 6% 47%	3865
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%	1890
	21st to end	Day	12% 6% 2 5% 4 3 4 3 4 5% 52%	1980
November	1st to 10th	Day	14% 5% 2 5% 3 3 4 3 5% 5% 51%	1857
	11th to 20th	Day	15% 6% 2 4 4 3 3 3 4 6% 50%	1763
	21st to end	Day	12% 6% 2 5% 4 3 4 3 5% 7% 49%	1701
December	1st to 10th	Day	18% 6% 2 7% 5% 4 3 4 5% 5% 40%	1520
	11th to 20th	Day	16% 6% 2 4 3 2 3 2 4 4 53%	1910
	21st to end	Day	13% 8% 3 6% 4 3 4 3 4 6% 46%	2909
January	1st to 10th	Night	14% 7% 2 6% 4 3 4 3 5% 6% 46%	4109
	11th to 20th	Night	15% 6% 3 6% 4 4 4 4 5% 7% 42%	4120
	21st to end	Night	15% 8% 3 6% 4 4 4 3 5% 6% 43%	4532
February	1st to 10th	Night	18% 6% 2 5% 3 4 4 4 5% 7% 43%	2470
	11th to 20th	Night	18% 9% 2 6% 3 3 3 3 4 6% 41%	2247
	21st to end	Night	17% 6% 2 6% 4 5% 4 4 5% 6% 42%	1879
March	1st to 10th	Night	24% 11% 3 7% 4 4 3 3 4 5% 32%	3418
	11th to 20th	Night	12% 6% 2 6% 3 3 4 3 4 6% 50%	3480
	21st to end	Night	17% 5% 2 4 3 3 3 2 5% 6% 50%	3765
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%	2275
	21st to end	Night	11% 5% 1 5% 4 3 3 3 4 5% 55%	2376
November	1st to 10th	Night	13% 6% 2 4 4 4 3 3 5% 6% 49%	2221
	11th to 20th	Night	15% 7% 2 6% 4 3 3 3 3 5% 48%	2413
	21st to end	Night	12% 5% 2 5% 4 3 4 3 5% 7% 50%	2474
December	1st to 10th	Night	16% 7% 3 6% 4 4 4 4 6% 6% 40%	2660
	11th to 20th	Night	16% 6% 2 5% 4 3 3 3 5% 6% 47%	3285
	21st to end	Night	14% 7% 2 6% 4 3 3 3 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	35% 3 8% 26% 23% 5%	2837
	11th to 20th	Day	36% 3 7% 19% 26% 9%	2840
	21st to end	Day	46% 4 7% 16% 17% 9%	3124
February	1st to 10th	Day	42% 4 11% 22% 18% 4	1710
	11th to 20th	Day	44% 3 8% 17% 23% 6%	1827
	21st to end	Day	42% 4 10% 19% 18% 7%	1573
March	1st to 10th	Day	52% 3 11% 13% 14% 7%	3435
	11th to 20th	Day	32% 4 8% 17% 24% 14%	3480
	21st to end	Day	40% 3 8% 26% 18% 5%	3865
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	42% 4 9% 19% 24% 2	1900
	11th to 20th	Day	35% 3 6% 23% 24% 9%	1890
	21st to end	Day	35% 3 6% 20% 29% 6%	1980
November	1st to 10th	Day	36% 4 9% 19% 26% 6%	1857
	11th to 20th	Day	37% 3 8% 19% 25% 7%	1763
	21st to end	Day	36% 3 9% 24% 23% 5%	1701
December	1st to 10th	Day	46% 3 11% 22% 14% 4	1520
	11th to 20th	Day	37% 2 7% 23% 23% 8%	1906
	21st to end	Day	41% 3 10% 21% 22% 2	2909
January	1st to 10th	Night	38% 4 7% 22% 23% 5%	4109
	11th to 20th	Night	42% 4 8% 18% 21% 6%	4120
	21st to end	Night	42% 4 8% 17% 21% 8%	4532
February	1st to 10th	Night	42% 4 9% 21% 19% 5%	2470
	11th to 20th	Night	45% 3 9% 16% 19% 7%	2249
	21st to end	Night	43% 3 10% 17% 19% 7%	1879
March	1st to 10th	Night	56% 2 9% 15% 14% 4	3418
	11th to 20th	Night	36% 4 9% 12% 24% 14%	3480
	21st to end	Night	37% 1 12% 25% 18% 6%	3765
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	37% 5% 9% 21% 23% 5%	2280
	11th to 20th	Night	33% 3 9% 23% 24% 9%	2275
	21st to end	Night	32% 3 7% 23% 27% 8%	2376
November	1st to 10th	Night	37% 4 9% 18% 26% 7%	2221
	11th to 20th	Night	40% 3 8% 17% 24% 8%	2413
	21st to end	Night	34% 3 8% 24% 24% 6%	2474
December	1st to 10th	Night	44% 5% 13% 17% 17% 5%	2660
	11th to 20th	Night	39% 2 8% 19% 25% 6%	3288
	21st to end	Night	38% 3 8% 24% 23% 3	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	27% 23% 35% 12% 2	426
	21st to end	Day	19% 26% 35% 15% 4 1	382
March	1st to 10th	Day	19% 22% 36% 17% 6%	460
	11th to 20th	Day	22% 24% 34% 13% 6% 1	460
	21st to end	Day	21% 19% 35% 17% 6% 1	506
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% 12% 29% 29% 13% 5%	486
	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	743
	21st to end	Night	20% 20% 31% 25% 4	628
March	1st to 10th	Night	18% 16% 37% 24% 5%	760
	11th to 20th	Night	20% 17% 36% 21% 5% 1	760
	21st to end	Night	18% 16% 37% 23% 6%	836
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% 27% 30% 16% 3	759
	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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