Fred Bay Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fred Bay Observatory clear sky chart from 2005-11-06 to 2026-06-16.

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	14% 9% 3 8% 5% 5% 5% 4 5% 7% 35%	137901
	Night	15% 10% 4 8% 5% 4 4 3 5% 7% 36%	134112
Transparency	Day	47% 4 11% 22% 13% 1	141541
	Night	47% 4 10% 22% 14% 2	138544
Seeing	Day	18% 10% 26% 23% 16% 7%	17447
	Night	18% 8% 27% 30% 15% 2	27066

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	17% 16% 7% 12% 5% 4 4 3 4 5% 23%	4759
	11th to 20th	Day	12% 14% 6% 16% 7% 6% 4 3 3 4 25%	5280
	21st to end	Day	19% 13% 7% 11% 7% 6% 6% 4 5% 5% 19%	5780
February	1st to 10th	Day	18% 14% 6% 10% 6% 4 4 3 4 5% 27%	1967
	11th to 20th	Day	16% 12% 4 12% 7% 5% 4 3 5% 5% 28%	2048
	21st to end	Day	17% 13% 6% 9% 5% 4 4 3 4 5% 31%	1730
March	1st to 10th	Day	18% 14% 6% 9% 4 4 4 3 4 4 30%	4663
	11th to 20th	Day	13% 8% 3 8% 5% 4 4 4 5% 8% 40%	5090
	21st to end	Day	17% 10% 4 7% 5% 4 4 4 5% 7% 33%	5586
April	1st to 10th	Day	15% 11% 4 7% 4 5% 4 3 6% 8% 34%	2452
	11th to 20th	Day	15% 9% 3 8% 5% 4 4 3 5% 6% 38%	2576
	21st to end	Day	17% 8% 2 8% 5% 4 5% 3 5% 7% 35%	2730
May	1st to 10th	Day	18% 8% 3 8% 5% 5% 5% 4 6% 7% 31%	7616
	11th to 20th	Day	18% 7% 3 7% 4 5% 4 3 5% 8% 36%	7626
	21st to end	Day	17% 8% 3 8% 5% 5% 5% 4 7% 9% 29%	8391
June	1st to 10th	Day	11% 6% 2 7% 5% 4 5% 5% 7% 9% 39%	2730
	11th to 20th	Day	10% 5% 2 7% 5% 4 5% 4 7% 10% 41%	2839
	21st to end	Day	9% 6% 2 6% 6% 5% 4 5% 7% 10% 39%	2800
July	1st to 10th	Day	9% 6% 2 7% 4 5% 5% 4 7% 10% 41%	6360
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 4 7% 11% 37%	6066
	21st to end	Day	11% 7% 3 7% 5% 6% 7% 5% 7% 9% 33%	6424
August	1st to 10th	Day	9% 5% 2 6% 4 4 5% 4 8% 9% 43%	4234
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 4 8% 10% 41%	4174
	21st to end	Day	6% 4 2 5% 4 4 4 3 5% 9% 54%	4411
September	1st to 10th	Day	12% 5% 1 5% 3 5% 4 4 5% 7% 49%	2260
	11th to 20th	Day	8% 5% 2 5% 4 4 4 3 6% 10% 49%	2200
	21st to end	Day	15% 7% 2 6% 4 3 3 2 5% 6% 46%	2200
October	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 3 6% 6% 49%	2200
	11th to 20th	Day	14% 7% 2 6% 4 3 4 3 5% 6% 47%	2010
	21st to end	Day	21% 8% 3 6% 4 4 3 2 4 7% 38%	2067
November	1st to 10th	Day	10% 6% 2 6% 4 4 4 3 4 7% 50%	1852
	11th to 20th	Day	14% 11% 5% 8% 3 4 4 3 5% 6% 35%	1890
	21st to end	Day	19% 11% 4 9% 5% 4 3 3 4 5% 33%	1890
December	1st to 10th	Day	17% 10% 4 9% 5% 4 4 3 4 5% 35%	3020
	11th to 20th	Day	20% 15% 5% 10% 5% 5% 4 3 4 4 25%	3463
	21st to end	Day	21% 20% 6% 10% 5% 4 4 3 4 5% 18%	4517
January	1st to 10th	Night	13% 16% 6% 13% 7% 6% 5% 4 5% 5% 20%	8117
	11th to 20th	Night	11% 16% 7% 15% 8% 5% 4 3 4 5% 22%	7680
	21st to end	Night	18% 13% 4 8% 5% 6% 4 3 4 5% 30%	8401
February	1st to 10th	Night	20% 12% 4 9% 5% 4 5% 3 4 5% 29%	2651
	11th to 20th	Night	18% 12% 3 9% 5% 4 5% 3 4 6% 30%	2458
	21st to end	Night	20% 13% 5% 8% 4 3 3 3 3 5% 31%	2076
March	1st to 10th	Night	19% 12% 3 7% 4 3 4 3 4 5% 37%	5410
	11th to 20th	Night	12% 7% 3 6% 5% 4 5% 4 6% 7% 42%	5090
	21st to end	Night	18% 9% 4 8% 5% 4 4 4 5% 5% 34%	5588
April	1st to 10th	Night	16% 9% 2 6% 4 4 4 3 5% 6% 39%	2186
	11th to 20th	Night	13% 8% 3 6% 4 3 5% 4 7% 8% 40%	1992
	21st to end	Night	16% 8% 3 6% 4 4 4 4 6% 8% 39%	1890
May	1st to 10th	Night	21% 8% 3 8% 5% 4 5% 4 6% 7% 29%	5237
	11th to 20th	Night	15% 6% 2 7% 4 4 4 4 6% 8% 38%	5234
	21st to end	Night	16% 7% 3 7% 4 4 5% 4 7% 8% 35%	5761
June	1st to 10th	Night	9% 5% 2 5% 4 5% 4 3 5% 9% 49%	1890
	11th to 20th	Night	9% 5% 2 5% 4 5% 4 4 8% 10% 45%	1665
	21st to end	Night	6% 5% 1 4 5% 5% 7% 5% 7% 9% 46%	1600
July	1st to 10th	Night	9% 6% 2 5% 4 4 4 4 6% 8% 47%	3618
	11th to 20th	Night	9% 5% 2 6% 5% 4 5% 4 6% 10% 43%	3881
	21st to end	Night	10% 6% 2 6% 5% 5% 5% 5% 7% 9% 40%	4427
August	1st to 10th	Night	8% 7% 3 7% 5% 5% 5% 4 7% 8% 43%	2915
	11th to 20th	Night	7% 7% 3 8% 4 4 4 3 6% 8% 45%	2875
	21st to end	Night	4 3 1 4 4 4 4 4 6% 9% 57%	3483
September	1st to 10th	Night	12% 5% 2 4 4 4 3 3 5% 7% 52%	2118
	11th to 20th	Night	8% 4 2 4 3 3 4 3 6% 7% 55%	2200
	21st to end	Night	15% 6% 2 5% 4 3 4 3 5% 5% 50%	2200
October	1st to 10th	Night	9% 4 2 6% 3 4 3 3 5% 6% 54%	2200
	11th to 20th	Night	14% 8% 2 6% 4 3 4 3 4 5% 46%	2390
	21st to end	Night	19% 8% 2 7% 4 4 4 3 4 7% 39%	2770
November	1st to 10th	Night	13% 8% 2 5% 4 3 4 3 4 7% 47%	2666
	11th to 20th	Night	16% 9% 2 8% 4 4 4 3 6% 6% 39%	2730
	21st to end	Night	19% 13% 4 7% 5% 4 4 3 5% 6% 30%	2730
December	1st to 10th	Night	16% 11% 4 9% 4 3 4 3 4 6% 36%	4380
	11th to 20th	Night	19% 15% 6% 10% 4 5% 4 2 5% 5% 25%	4952
	21st to end	Night	22% 19% 6% 11% 6% 5% 4 2 4 5% 17%	6651

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	64% 3 12% 16% 5%	4760
	11th to 20th	Day	64% 3 8% 13% 11% 1	5280
	21st to end	Day	68% 3 9% 13% 6% 1	5780
February	1st to 10th	Day	60% 3 10% 17% 9% 1	1967
	11th to 20th	Day	58% 4 10% 19% 8% 1	2058
	21st to end	Day	56% 4 10% 20% 8% 1	1730
March	1st to 10th	Day	59% 3 6% 19% 11% 2	4663
	11th to 20th	Day	42% 5% 10% 28% 15% 1	5090
	21st to end	Day	50% 4 12% 21% 11% 3	5586
April	1st to 10th	Day	48% 6% 11% 23% 11% 1	2452
	11th to 20th	Day	47% 4 11% 22% 14% 3	2576
	21st to end	Day	48% 5% 11% 23% 11% 2	2730
May	1st to 10th	Day	52% 5% 11% 21% 10%	7651
	11th to 20th	Day	47% 4 12% 24% 12%	7630
	21st to end	Day	50% 6% 10% 20% 12% 3	8405
June	1st to 10th	Day	40% 7% 11% 23% 17% 2	2730
	11th to 20th	Day	38% 5% 16% 23% 16% 2	2839
	21st to end	Day	38% 8% 15% 21% 15% 3	2800
July	1st to 10th	Day	38% 5% 12% 26% 17% 1	6291
	11th to 20th	Day	40% 6% 13% 27% 13% 1	5993
	21st to end	Day	45% 6% 14% 20% 13% 2	6240
August	1st to 10th	Day	35% 6% 16% 31% 11% 1	4221
	11th to 20th	Day	36% 6% 11% 32% 16% 1	4213
	21st to end	Day	28% 5% 14% 30% 20% 3	4410
September	1st to 10th	Day	35% 5% 10% 26% 20% 5%	2260
	11th to 20th	Day	32% 5% 11% 26% 23% 3	2200
	21st to end	Day	40% 4 10% 22% 22% 2	2200
October	1st to 10th	Day	24% 4 9% 30% 31% 2	3590
	11th to 20th	Day	30% 3 9% 31% 25% 2	3250
	21st to end	Day	38% 2 11% 24% 23% 2	3317
November	1st to 10th	Day	36% 4 10% 28% 20% 2	1852
	11th to 20th	Day	50% 3 10% 20% 15% 2	1890
	21st to end	Day	54% 3 10% 21% 11% 1	1890
December	1st to 10th	Day	53% 3 10% 25% 8%	3020
	11th to 20th	Day	63% 4 9% 18% 6%	3460
	21st to end	Day	70% 3 10% 11% 5%	4517
January	1st to 10th	Night	66% 3 10% 14% 7%	8118
	11th to 20th	Night	66% 3 7% 13% 10% 2	7680
	21st to end	Night	58% 2 10% 21% 8% 1	8401
February	1st to 10th	Night	58% 4 9% 19% 8% 1	2651
	11th to 20th	Night	55% 4 9% 20% 11% 1	2459
	21st to end	Night	56% 4 11% 17% 10% 2	2076
March	1st to 10th	Night	51% 4 6% 27% 11% 2	5410
	11th to 20th	Night	40% 5% 11% 29% 14% 1	5090
	21st to end	Night	51% 4 7% 25% 11% 2	5588
April	1st to 10th	Night	45% 6% 12% 24% 11% 2	2186
	11th to 20th	Night	41% 5% 13% 19% 18% 4	1992
	21st to end	Night	44% 5% 11% 24% 14% 3	1890
May	1st to 10th	Night	53% 5% 9% 21% 8% 3	5264
	11th to 20th	Night	43% 6% 12% 24% 14% 1	5235
	21st to end	Night	45% 5% 13% 21% 10% 5%	5771
June	1st to 10th	Night	33% 6% 11% 26% 21% 2	1890
	11th to 20th	Night	32% 7% 16% 22% 19% 4	1665
	21st to end	Night	31% 9% 17% 22% 17% 5%	1600
July	1st to 10th	Night	35% 6% 11% 25% 19% 4	3580
	11th to 20th	Night	36% 6% 15% 28% 14% 1	3834
	21st to end	Night	39% 5% 13% 24% 16% 3	4297
August	1st to 10th	Night	37% 7% 13% 30% 12% 1	2904
	11th to 20th	Night	36% 6% 10% 28% 19% 1	2902
	21st to end	Night	24% 5% 16% 31% 22% 3	3483
September	1st to 10th	Night	32% 5% 12% 25% 20% 6%	2118
	11th to 20th	Night	28% 5% 9% 27% 28% 3	2200
	21st to end	Night	37% 4 9% 21% 26% 2	2200
October	1st to 10th	Night	23% 3 7% 28% 36% 3	3590
	11th to 20th	Night	29% 3 9% 27% 30% 3	3889
	21st to end	Night	34% 3 8% 24% 26% 5%	4470
November	1st to 10th	Night	38% 4 11% 22% 21% 3	2666
	11th to 20th	Night	45% 4 9% 23% 17% 3	2730
	21st to end	Night	56% 3 11% 19% 10% 1	2730
December	1st to 10th	Night	52% 3 12% 25% 9%	4380
	11th to 20th	Night	62% 4 10% 17% 7%	4954
	21st to end	Night	72% 3 8% 14% 3	6651

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	25% 18% 35% 16% 5%	330
	11th to 20th	Day	25% 23% 36% 13% 3	330
	21st to end	Day	29% 21% 32% 16% 3	363
February	1st to 10th	Day	27% 16% 36% 17% 3 1	330
	11th to 20th	Day	28% 22% 33% 14% 3	330
	21st to end	Day	24% 20% 45% 10% 1	273
March	1st to 10th	Day	25% 21% 36% 15% 3	361
	11th to 20th	Day	26% 15% 36% 17% 6% 1	540
	21st to end	Day	27% 16% 34% 18% 5%	594
April	1st to 10th	Day	21% 17% 32% 21% 8% 1	540
	11th to 20th	Day	21% 17% 28% 22% 9% 3	532
	21st to end	Day	20% 13% 34% 21% 9% 3	540
May	1st to 10th	Day	21% 15% 32% 21% 7% 4	543
	11th to 20th	Day	21% 7% 28% 23% 15% 6%	567
	21st to end	Day	15% 5% 25% 26% 18% 10%	617
June	1st to 10th	Day	13% 5% 22% 28% 25% 8%	570
	11th to 20th	Day	12% 5% 20% 31% 25% 7%	724
	21st to end	Day	9% 5% 23% 30% 23% 9%	730
July	1st to 10th	Day	12% 4 16% 24% 26% 19%	721
	11th to 20th	Day	10% 3 20% 26% 27% 14%	595
	21st to end	Day	11% 2 15% 30% 27% 16%	576
August	1st to 10th	Day	11% 3 13% 27% 28% 20%	526
	11th to 20th	Day	11% 5% 18% 27% 26% 14%	519
	21st to end	Day	10% 3 11% 29% 31% 16%	583
September	1st to 10th	Day	12% 3 15% 25% 32% 13%	530
	11th to 20th	Day	9% 3 14% 23% 36% 16%	530
	21st to end	Day	18% 8% 19% 23% 23% 10%	530
October	1st to 10th	Day	10% 5% 21% 33% 22% 9%	530
	11th to 20th	Day	16% 10% 30% 26% 14% 5%	530
	21st to end	Day	23% 12% 36% 21% 7% 1	450
November	1st to 10th	Day	15% 11% 35% 26% 9% 4	330
	11th to 20th	Day	22% 17% 31% 18% 9% 2	330
	21st to end	Day	28% 12% 37% 18% 4	330
December	1st to 10th	Day	22% 20% 35% 17% 6% 1	330
	11th to 20th	Day	23% 15% 41% 17% 4	330
	21st to end	Day	31% 17% 33% 17% 3	363
January	1st to 10th	Night	21% 16% 35% 22% 6%	1013
	11th to 20th	Night	26% 16% 34% 21% 3	840
	21st to end	Night	22% 13% 39% 24% 3	924
February	1st to 10th	Night	27% 10% 34% 27% 3	840
	11th to 20th	Night	24% 13% 36% 24% 2	823
	21st to end	Night	26% 12% 36% 23% 2	692
March	1st to 10th	Night	22% 12% 35% 25% 5%	840
	11th to 20th	Night	21% 13% 33% 26% 6%	840
	21st to end	Night	23% 10% 34% 25% 7%	924
April	1st to 10th	Night	20% 12% 33% 29% 6% 1	716
	11th to 20th	Night	16% 11% 34% 28% 10% 1	623
	21st to end	Night	17% 8% 35% 30% 9% 1	630
May	1st to 10th	Night	22% 8% 31% 28% 11% 1	630
	11th to 20th	Night	17% 5% 26% 33% 16% 3	621
	21st to end	Night	12% 3 21% 36% 24% 4	683
June	1st to 10th	Night	10% 2 19% 45% 23% 1	630
	11th to 20th	Night	10% 4 14% 44% 25% 2	615
	21st to end	Night	7% 3 18% 43% 27% 3	600
July	1st to 10th	Night	10% 2 11% 36% 32% 9%	585
	11th to 20th	Night	7% 3 11% 38% 35% 6%	582
	21st to end	Night	11% 2 9% 42% 33% 4	657
August	1st to 10th	Night	8% 3 11% 38% 36% 5%	597
	11th to 20th	Night	10% 1 11% 43% 31% 5%	594
	21st to end	Night	8% 1 11% 37% 38% 6%	660
September	1st to 10th	Night	14% 2 13% 32% 34% 5%	718
	11th to 20th	Night	10% 2 12% 36% 35% 6%	800
	21st to end	Night	17% 4 20% 34% 24% 2	800
October	1st to 10th	Night	12% 4 21% 36% 23% 4	800
	11th to 20th	Night	17% 8% 26% 34% 13% 1	800
	21st to end	Night	21% 7% 33% 28% 10%	880
November	1st to 10th	Night	17% 7% 30% 30% 14% 1	820
	11th to 20th	Night	19% 13% 32% 26% 9% 1	840
	21st to end	Night	26% 10% 28% 29% 7%	840
December	1st to 10th	Night	26% 10% 33% 25% 5%	840
	11th to 20th	Night	26% 9% 35% 24% 5% 1	840
	21st to end	Night	30% 13% 31% 24% 3	929

Page generated at 2026-06-13 02:31:40. Version ac07

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