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 2026-07-31.

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% 2 7% 5% 5% 5% 4 6% 8% 40%	137215
	Night	13% 6% 2 6% 4 4 4 4 6% 7% 44%	135785
Transparency	Day	41% 6% 13% 20% 16% 5%	140815
	Night	38% 5% 13% 20% 18% 5%	140255
Seeing	Day	17% 13% 28% 24% 14% 5%	17778
	Night	16% 11% 27% 30% 14% 2	27645

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	10% 5% 2 5% 3 3 3 2 3 6% 56%	5258
	11th to 20th	Day	14% 5% 2 5% 3 3 3 2 3 5% 56%	5280
	21st to end	Day	15% 11% 4 8% 4 4 4 3 4 6% 39%	5780
February	1st to 10th	Day	16% 7% 3 6% 4 3 5% 4 5% 6% 42%	1890
	11th to 20th	Day	19% 7% 3 6% 4 3 3 3 4 4 44%	2016
	21st to end	Day	14% 6% 2 6% 5% 4 5% 3 6% 6% 42%	1733
March	1st to 10th	Day	22% 7% 2 6% 4 4 4 3 4 6% 37%	5045
	11th to 20th	Day	10% 5% 2 5% 3 3 4 3 5% 6% 53%	5090
	21st to end	Day	14% 5% 2 5% 4 4 4 3 5% 6% 51%	5665
April	1st to 10th	Day	12% 5% 2 5% 4 4 3 3 5% 7% 51%	2520
	11th to 20th	Day	10% 4 2 5% 4 4 4 4 6% 8% 52%	2494
	21st to end	Day	10% 4 1 6% 4 4 5% 3 6% 8% 48%	2520
May	1st to 10th	Day	13% 6% 3 7% 5% 5% 5% 4 6% 8% 37%	7021
	11th to 20th	Day	14% 7% 3 7% 4 4 4 4 5% 8% 41%	7481
	21st to end	Day	13% 6% 3 7% 6% 6% 5% 4 7% 8% 34%	8666
June	1st to 10th	Day	10% 6% 3 7% 6% 5% 6% 5% 7% 9% 36%	2940
	11th to 20th	Day	9% 5% 2 7% 5% 6% 7% 5% 7% 11% 37%	2940
	21st to end	Day	8% 5% 2 6% 6% 6% 7% 6% 9% 11% 33%	2915
July	1st to 10th	Day	9% 5% 3 8% 6% 7% 7% 6% 9% 11% 29%	6030
	11th to 20th	Day	9% 7% 3 9% 7% 7% 8% 6% 8% 12% 23%	6030
	21st to end	Day	11% 6% 3 8% 7% 7% 7% 6% 9% 11% 25%	6554
August	1st to 10th	Day	22% 11% 4 9% 6% 5% 5% 5% 7% 8% 18%	4175
	11th to 20th	Day	11% 6% 3 9% 6% 6% 6% 5% 6% 8% 35%	3850
	21st to end	Day	8% 6% 3 7% 5% 5% 5% 5% 7% 10% 38%	4312
September	1st to 10th	Day	12% 7% 2 7% 4 5% 6% 5% 7% 9% 35%	2400
	11th to 20th	Day	17% 6% 3 7% 5% 6% 5% 4 6% 9% 34%	2400
	21st to end	Day	19% 8% 3 8% 5% 4 4 4 5% 7% 34%	2116
October	1st to 10th	Day	15% 7% 3 6% 3 4 5% 3 5% 5% 44%	2000
	11th to 20th	Day	13% 6% 2 5% 3 3 4 2 4 5% 52%	2000
	21st to end	Day	12% 5% 2 5% 4 3 4 3 4 5% 53%	2200
November	1st to 10th	Day	13% 5% 2 5% 3 3 4 3 5% 5% 51%	2057
	11th to 20th	Day	15% 5% 2 4 4 4 3 3 4 6% 50%	1948
	21st to end	Day	11% 6% 2 5% 4 3 3 3 4 7% 50%	1879
December	1st to 10th	Day	12% 4 2 5% 3 3 3 4 4 5% 53%	2680
	11th to 20th	Day	15% 7% 3 7% 4 3 3 3 4 5% 46%	3070
	21st to end	Day	12% 6% 3 6% 3 3 4 3 4 6% 49%	4260
January	1st to 10th	Night	10% 6% 2 5% 3 3 3 3 5% 6% 53%	7635
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 4 6% 53%	7680
	21st to end	Night	16% 8% 3 7% 4 4 4 3 4 5% 42%	8401
February	1st to 10th	Night	18% 6% 2 5% 3 3 4 3 5% 7% 43%	2730
	11th to 20th	Night	18% 9% 2 6% 3 3 3 3 5% 6% 41%	2489
	21st to end	Night	17% 6% 2 6% 4 4 4 4 5% 6% 43%	2070
March	1st to 10th	Night	20% 10% 3 6% 4 4 3 3 4 4 40%	5028
	11th to 20th	Night	14% 5% 2 5% 3 3 4 3 5% 6% 50%	5090
	21st to end	Night	14% 5% 2 5% 4 4 3 2 5% 6% 51%	5507
April	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 5% 6% 48%	2100
	11th to 20th	Night	8% 5% 2 6% 4 4 4 4 6% 8% 51%	2075
	21st to end	Night	8% 4 1 5% 4 3 4 4 6% 9% 51%	2100
May	1st to 10th	Night	11% 6% 2 8% 5% 5% 5% 4 6% 9% 38%	5833
	11th to 20th	Night	12% 6% 2 6% 4 5% 4 3 5% 7% 44%	5420
	21st to end	Night	13% 6% 3 8% 5% 5% 5% 5% 7% 9% 34%	5469
June	1st to 10th	Night	9% 6% 3 6% 6% 4 5% 4 7% 11% 39%	1680
	11th to 20th	Night	7% 5% 2 6% 4 6% 7% 5% 8% 10% 41%	1680
	21st to end	Night	8% 6% 2 5% 4 5% 5% 5% 7% 11% 40%	1697
July	1st to 10th	Night	7% 5% 2 7% 5% 5% 6% 5% 8% 13% 35%	4148
	11th to 20th	Night	10% 5% 2 8% 6% 5% 7% 6% 10% 14% 26%	4147
	21st to end	Night	12% 6% 3 7% 6% 6% 6% 6% 9% 10% 29%	4517
August	1st to 10th	Night	17% 11% 3 8% 4 5% 6% 5% 7% 10% 23%	2942
	11th to 20th	Night	9% 6% 3 8% 5% 5% 6% 5% 7% 11% 35%	3197
	21st to end	Night	8% 5% 3 7% 4 5% 5% 4 7% 10% 41%	3586
September	1st to 10th	Night	11% 5% 2 6% 3 5% 5% 5% 7% 10% 41%	2000
	11th to 20th	Night	15% 6% 2 8% 5% 4 5% 3 6% 7% 39%	2000
	21st to end	Night	18% 6% 3 8% 5% 5% 5% 3 5% 6% 36%	2277
October	1st to 10th	Night	14% 7% 2 7% 4 3 3 3 5% 6% 47%	2400
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 4 5% 54%	2400
	21st to end	Night	12% 5% 1 5% 3 3 3 3 4 5% 56%	2640
November	1st to 10th	Night	13% 6% 2 4 3 4 3 3 5% 6% 50%	2461
	11th to 20th	Night	14% 7% 2 5% 4 3 3 3 3 6% 49%	2667
	21st to end	Night	11% 5% 2 5% 4 3 4 3 5% 8% 50%	2735
December	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 4 5% 56%	4720
	11th to 20th	Night	15% 7% 2 6% 4 4 4 3 6% 7% 42%	5345
	21st to end	Night	13% 6% 2 5% 3 3 3 3 5% 7% 50%	6919

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% 3 11% 23% 25% 7%	5259
	11th to 20th	Day	33% 3 5% 16% 31% 12%	5280
	21st to end	Day	49% 3 5% 17% 17% 9%	5780
February	1st to 10th	Day	43% 4 11% 21% 18% 5%	1890
	11th to 20th	Day	45% 3 8% 17% 22% 6%	2025
	21st to end	Day	42% 4 10% 20% 18% 6%	1733
March	1st to 10th	Day	49% 4 9% 14% 17% 8%	5045
	11th to 20th	Day	32% 4 8% 17% 25% 15%	5090
	21st to end	Day	35% 3 10% 24% 20% 8%	5665
April	1st to 10th	Day	34% 3 11% 22% 23% 7%	2520
	11th to 20th	Day	31% 4 12% 26% 20% 8%	2494
	21st to end	Day	33% 5% 13% 25% 20% 3	2520
May	1st to 10th	Day	43% 5% 19% 22% 9% 2	7053
	11th to 20th	Day	41% 5% 15% 24% 15% 1	7485
	21st to end	Day	45% 7% 13% 21% 12% 2	8681
June	1st to 10th	Day	42% 7% 14% 20% 15% 1	2940
	11th to 20th	Day	39% 8% 19% 22% 11% 1	2940
	21st to end	Day	38% 10% 19% 23% 9% 1	2915
July	1st to 10th	Day	43% 11% 24% 16% 6%	5965
	11th to 20th	Day	48% 14% 24% 11% 3	5961
	21st to end	Day	47% 11% 17% 18% 6%	6370
August	1st to 10th	Day	62% 7% 18% 10% 3	4162
	11th to 20th	Day	44% 8% 15% 22% 11%	3886
	21st to end	Day	38% 8% 18% 25% 10% 1	4312
September	1st to 10th	Day	43% 8% 16% 17% 12% 4	2400
	11th to 20th	Day	48% 6% 11% 19% 14% 3	2400
	21st to end	Day	49% 5% 10% 18% 15% 3	2116
October	1st to 10th	Day	37% 6% 9% 21% 23% 5%	3260
	11th to 20th	Day	25% 2 5% 19% 25% 24%	3240
	21st to end	Day	27% 3 6% 26% 28% 9%	3538
November	1st to 10th	Day	35% 4 9% 20% 26% 6%	2057
	11th to 20th	Day	36% 3 9% 20% 25% 7%	1948
	21st to end	Day	35% 3 9% 24% 23% 5%	1879
December	1st to 10th	Day	34% 3 8% 20% 26% 8%	2680
	11th to 20th	Day	42% 4 11% 19% 18% 6%	3066
	21st to end	Day	37% 4 13% 25% 19% 2	4260
January	1st to 10th	Night	32% 4 11% 21% 27% 5%	7636
	11th to 20th	Night	33% 3 7% 16% 31% 10%	7680
	21st to end	Night	44% 4 6% 17% 21% 9%	8401
February	1st to 10th	Night	42% 4 9% 20% 20% 5%	2730
	11th to 20th	Night	45% 3 10% 17% 19% 6%	2491
	21st to end	Night	43% 3 9% 17% 20% 8%	2070
March	1st to 10th	Night	48% 3 9% 15% 19% 6%	5028
	11th to 20th	Night	36% 4 9% 13% 23% 16%	5090
	21st to end	Night	36% 2 12% 25% 20% 5%	5507
April	1st to 10th	Night	37% 4 12% 18% 22% 6%	2100
	11th to 20th	Night	32% 5% 14% 25% 19% 6%	2075
	21st to end	Night	30% 5% 15% 30% 17% 4	2100
May	1st to 10th	Night	42% 7% 20% 20% 9% 1	5863
	11th to 20th	Night	39% 5% 14% 25% 15% 1	5421
	21st to end	Night	45% 8% 17% 20% 9% 1	5476
June	1st to 10th	Night	38% 7% 18% 22% 13% 2	1680
	11th to 20th	Night	34% 11% 20% 26% 9% 1	1680
	21st to end	Night	36% 8% 20% 26% 10%	1697
July	1st to 10th	Night	37% 11% 29% 18% 4	4105
	11th to 20th	Night	45% 13% 29% 10% 3	4096
	21st to end	Night	45% 12% 18% 18% 7%	4387
August	1st to 10th	Night	54% 8% 22% 13% 2	2931
	11th to 20th	Night	41% 10% 20% 21% 8%	3227
	21st to end	Night	34% 8% 19% 29% 10% 1	3586
September	1st to 10th	Night	37% 9% 18% 19% 14% 3	2000
	11th to 20th	Night	44% 8% 13% 20% 13% 3	2000
	21st to end	Night	48% 6% 11% 21% 12% 1	2277
October	1st to 10th	Night	39% 5% 8% 20% 24% 4	3920
	11th to 20th	Night	24% 2 6% 19% 22% 28%	3899
	21st to end	Night	23% 4 7% 27% 30% 9%	4252
November	1st to 10th	Night	36% 4 9% 19% 26% 6%	2461
	11th to 20th	Night	39% 3 8% 18% 25% 7%	2667
	21st to end	Night	34% 4 8% 25% 24% 6%	2735
December	1st to 10th	Night	33% 3 8% 13% 33% 10%	4720
	11th to 20th	Night	42% 4 14% 14% 19% 7%	5348
	21st to end	Night	35% 4 13% 27% 19% 2	6919

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	22% 26% 37% 13% 2	330
	11th to 20th	Day	20% 28% 37% 14% 1	330
	21st to end	Day	23% 25% 38% 11% 3	363
February	1st to 10th	Day	24% 22% 39% 13% 3	330
	11th to 20th	Day	29% 24% 34% 11% 2	504
	21st to end	Day	20% 26% 33% 17% 4	446
March	1st to 10th	Day	19% 22% 35% 18% 6%	540
	11th to 20th	Day	23% 23% 34% 14% 5% 1	540
	21st to end	Day	20% 19% 37% 18% 6% 1	594
April	1st to 10th	Day	19% 24% 32% 17% 6% 1	540
	11th to 20th	Day	13% 20% 33% 22% 8% 3	532
	21st to end	Day	12% 15% 32% 27% 12% 3	540
May	1st to 10th	Day	17% 17% 34% 20% 9% 3	543
	11th to 20th	Day	18% 11% 29% 26% 14% 3	567
	21st to end	Day	16% 8% 25% 28% 17% 6%	617
June	1st to 10th	Day	12% 8% 29% 31% 16% 4	570
	11th to 20th	Day	11% 5% 25% 36% 18% 6%	570
	21st to end	Day	8% 5% 21% 37% 21% 8%	570
July	1st to 10th	Day	11% 4 14% 35% 28% 8%	565
	11th to 20th	Day	9% 4 12% 36% 29% 10%	564
	21st to end	Day	10% 3 19% 35% 24% 8%	616
August	1st to 10th	Day	17% 5% 17% 30% 23% 8%	526
	11th to 20th	Day	13% 5% 14% 29% 28% 12%	519
	21st to end	Day	12% 5% 19% 31% 22% 11%	583
September	1st to 10th	Day	15% 6% 20% 28% 19% 12%	530
	11th to 20th	Day	18% 6% 21% 32% 16% 8%	530
	21st to end	Day	24% 4 24% 24% 17% 7%	530
October	1st to 10th	Day	19% 8% 26% 23% 18% 7%	530
	11th to 20th	Day	14% 13% 28% 28% 13% 4	530
	21st to end	Day	13% 14% 28% 25% 16% 3	583
November	1st to 10th	Day	16% 13% 33% 26% 9% 3	535
	11th to 20th	Day	19% 15% 37% 19% 9% 2	460
	21st to end	Day	16% 18% 32% 20% 14% 1	419
December	1st to 10th	Day	23% 20% 37% 15% 4	220
	11th to 20th	Day	18% 22% 35% 18% 5% 2	220
	21st to end	Day	20% 18% 39% 13% 8% 2	292
January	1st to 10th	Night	18% 22% 33% 23% 5%	840
	11th to 20th	Night	19% 19% 29% 27% 6%	840
	21st to end	Night	20% 18% 36% 22% 3	924
February	1st to 10th	Night	23% 16% 34% 23% 4	840
	11th to 20th	Night	26% 20% 32% 19% 2	823
	21st to end	Night	21% 21% 31% 24% 4	692
March	1st to 10th	Night	18% 18% 36% 24% 5%	840
	11th to 20th	Night	21% 17% 36% 21% 5%	840
	21st to end	Night	17% 16% 38% 23% 6%	924
April	1st to 10th	Night	16% 21% 34% 24% 6%	840
	11th to 20th	Night	11% 15% 37% 26% 10%	831
	21st to end	Night	11% 12% 38% 33% 7%	840
May	1st to 10th	Night	13% 13% 37% 29% 8% 1	840
	11th to 20th	Night	16% 9% 28% 34% 11% 1	692
	21st to end	Night	12% 4 20% 39% 22% 4	583
June	1st to 10th	Night	10% 4 24% 47% 14% 1	420
	11th to 20th	Night	8% 3 21% 45% 19% 3	420
	21st to end	Night	10% 2 19% 45% 23% 1	437
July	1st to 10th	Night	8% 2 14% 42% 30% 4	615
	11th to 20th	Night	7% 1 10% 43% 31% 6%	612
	21st to end	Night	9% 5% 15% 42% 26% 5%	687
August	1st to 10th	Night	12% 3 11% 39% 29% 6%	611
	11th to 20th	Night	12% 4 12% 38% 28% 7%	790
	21st to end	Night	13% 4 13% 35% 29% 6%	880
September	1st to 10th	Night	14% 3 16% 30% 30% 8%	800
	11th to 20th	Night	18% 4 19% 32% 22% 5%	800
	21st to end	Night	23% 5% 16% 32% 21% 4	800
October	1st to 10th	Night	16% 5% 20% 32% 22% 5%	800
	11th to 20th	Night	16% 10% 26% 30% 15% 3	800
	21st to end	Night	15% 9% 25% 29% 19% 3	880
November	1st to 10th	Night	15% 10% 28% 32% 14% 2	820
	11th to 20th	Night	20% 13% 30% 25% 11% 1	840
	21st to end	Night	14% 14% 31% 27% 14% 1	840
December	1st to 10th	Night	21% 15% 28% 28% 7%	840
	11th to 20th	Night	18% 14% 29% 30% 10%	840
	21st to end	Night	17% 15% 30% 31% 7% 1	924

Page generated at 2026-07-28 03:25:23. Version ac07

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