Breton
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Breton clear sky chart from 2005-11-06 to 2026-07-01.

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	10% 9% 3 8% 5% 5% 5% 4 6% 8% 35%	142986
	Night	9% 11% 4 10% 6% 5% 5% 4 6% 7% 33%	138073
Transparency	Day	45% 6% 16% 23% 9% 1	142684
	Night	49% 6% 14% 21% 9% 1	137958
Seeing	Day	14% 7% 25% 30% 18% 6%	17970
	Night	14% 6% 29% 34% 14% 2	26496

By Month

Percentage Cloud Cover NW (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	5% 13% 4 13% 8% 6% 5% 4 5% 7% 29%	4125
	11th to 20th	Day	4 8% 4 10% 6% 5% 5% 5% 7% 8% 37%	4119
	21st to end	Day	5% 13% 6% 10% 5% 6% 5% 5% 5% 7% 34%	5047
February	1st to 10th	Day	8% 13% 5% 11% 6% 7% 6% 4 6% 8% 25%	1680
	11th to 20th	Day	4 15% 5% 12% 6% 5% 5% 4 6% 6% 29%	1759
	21st to end	Day	3 13% 6% 14% 8% 7% 7% 6% 7% 6% 23%	1662
March	1st to 10th	Day	3 9% 4 11% 8% 6% 6% 4 6% 9% 34%	4618
	11th to 20th	Day	6% 12% 4 9% 5% 4 4 3 6% 8% 39%	5005
	21st to end	Day	9% 18% 6% 11% 5% 5% 5% 4 5% 7% 24%	5778
April	1st to 10th	Day	8% 14% 5% 8% 7% 6% 5% 6% 8% 7% 28%	2586
	11th to 20th	Day	11% 14% 4 8% 6% 6% 5% 5% 7% 8% 27%	2700
	21st to end	Day	10% 11% 4 9% 5% 6% 5% 5% 6% 8% 31%	2730
May	1st to 10th	Day	10% 8% 3 7% 5% 5% 5% 4 6% 7% 40%	8505
	11th to 20th	Day	12% 10% 4 10% 6% 6% 5% 5% 7% 9% 26%	8755
	21st to end	Day	12% 6% 2 8% 6% 6% 6% 5% 7% 9% 33%	9649
June	1st to 10th	Day	16% 8% 2 7% 5% 5% 5% 4 8% 8% 31%	3150
	11th to 20th	Day	19% 9% 3 8% 7% 6% 6% 5% 6% 7% 23%	3283
	21st to end	Day	12% 5% 2 6% 5% 6% 6% 5% 8% 9% 37%	3360
July	1st to 10th	Day	10% 4 2 5% 4 4 5% 4 7% 9% 47%	7022
	11th to 20th	Day	11% 5% 2 7% 5% 5% 5% 4 6% 8% 42%	6780
	21st to end	Day	13% 5% 2 6% 5% 5% 5% 4 6% 8% 41%	7376
August	1st to 10th	Day	13% 6% 2 5% 4 4 5% 4 6% 8% 41%	4551
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 4 6% 9% 42%	4321
	21st to end	Day	12% 5% 2 6% 4 4 5% 4 6% 8% 43%	4664
September	1st to 10th	Day	13% 5% 2 7% 4 4 5% 4 6% 8% 43%	2503
	11th to 20th	Day	15% 7% 2 5% 3 4 4 3 5% 9% 44%	2217
	21st to end	Day	10% 7% 3 6% 4 6% 6% 4 5% 8% 42%	2200
October	1st to 10th	Day	9% 8% 3 8% 5% 5% 4 3 5% 7% 40%	3433
	11th to 20th	Day	10% 11% 3 8% 5% 5% 5% 4 6% 8% 35%	2932
	21st to end	Day	7% 12% 3 9% 6% 5% 5% 3 4 7% 39%	3200
November	1st to 10th	Day	9% 16% 4 11% 5% 6% 5% 3 6% 7% 27%	1673
	11th to 20th	Day	7% 14% 4 12% 6% 5% 5% 3 5% 7% 35%	1486
	21st to end	Day	6% 11% 6% 11% 7% 6% 5% 5% 7% 8% 27%	1470
December	1st to 10th	Day	9% 17% 5% 12% 6% 7% 5% 3 5% 8% 23%	2360
	11th to 20th	Day	7% 13% 5% 13% 8% 6% 6% 3 6% 7% 25%	2712
	21st to end	Day	3 8% 3 9% 6% 5% 6% 4 6% 9% 39%	3575
January	1st to 10th	Night	7% 13% 6% 15% 8% 6% 5% 4 5% 6% 25%	8768
	11th to 20th	Night	3 9% 4 11% 6% 6% 5% 4 6% 7% 38%	8779
	21st to end	Night	6% 12% 5% 11% 5% 5% 5% 4 6% 8% 32%	9192
February	1st to 10th	Night	6% 14% 6% 12% 7% 6% 5% 4 6% 7% 27%	2940
	11th to 20th	Night	5% 14% 5% 11% 8% 6% 7% 5% 6% 7% 27%	2748
	21st to end	Night	5% 15% 4 11% 7% 6% 6% 5% 5% 7% 30%	2139
March	1st to 10th	Night	4 9% 4 11% 6% 6% 6% 4 7% 9% 34%	5496
	11th to 20th	Night	4 12% 4 9% 6% 7% 7% 5% 8% 9% 29%	5214
	21st to end	Night	9% 16% 6% 15% 7% 5% 5% 4 4 5% 24%	5436
April	1st to 10th	Night	8% 13% 4 9% 8% 6% 5% 5% 8% 7% 28%	2034
	11th to 20th	Night	10% 13% 4 9% 6% 6% 6% 6% 7% 7% 26%	1871
	21st to end	Night	12% 11% 4 8% 5% 4 3 5% 6% 9% 33%	1890
May	1st to 10th	Night	14% 10% 3 8% 4 4 4 4 5% 8% 37%	4410
	11th to 20th	Night	13% 9% 4 10% 7% 7% 6% 5% 7% 7% 25%	4105
	21st to end	Night	11% 8% 2 7% 5% 6% 5% 4 6% 9% 35%	4522
June	1st to 10th	Night	18% 9% 3 8% 5% 5% 6% 5% 6% 7% 28%	1470
	11th to 20th	Night	18% 10% 4 9% 6% 7% 6% 5% 7% 6% 23%	1331
	21st to end	Night	14% 7% 2 7% 6% 5% 6% 5% 8% 8% 33%	1260
July	1st to 10th	Night	11% 5% 2 7% 5% 5% 4 4 6% 9% 42%	2922
	11th to 20th	Night	13% 7% 3 6% 6% 6% 6% 3 6% 9% 36%	3180
	21st to end	Night	14% 6% 2 7% 4 4 5% 4 6% 8% 41%	3467
August	1st to 10th	Night	15% 6% 2 6% 4 4 5% 4 8% 9% 37%	2598
	11th to 20th	Night	12% 6% 3 7% 5% 5% 5% 5% 7% 9% 37%	2731
	21st to end	Night	13% 5% 3 7% 4 4 4 4 7% 9% 41%	3245
September	1st to 10th	Night	13% 5% 2 6% 4 4 5% 4 6% 8% 44%	1895
	11th to 20th	Night	13% 6% 3 6% 4 4 5% 3 5% 6% 44%	2182
	21st to end	Night	10% 7% 3 6% 4 4 4 4 7% 8% 42%	2200
October	1st to 10th	Night	10% 8% 3 7% 5% 5% 6% 4 5% 8% 39%	3761
	11th to 20th	Night	11% 8% 3 7% 5% 4 5% 4 7% 8% 37%	4216
	21st to end	Night	7% 10% 4 8% 7% 6% 5% 4 6% 7% 36%	4600
November	1st to 10th	Night	12% 15% 6% 10% 6% 5% 5% 4 5% 7% 27%	2824
	11th to 20th	Night	7% 13% 5% 11% 7% 6% 5% 4 6% 8% 28%	3134
	21st to end	Night	7% 13% 4 10% 8% 5% 5% 4 6% 8% 28%	3150
December	1st to 10th	Night	11% 17% 5% 10% 7% 5% 5% 4 5% 6% 25%	5040
	11th to 20th	Night	8% 15% 5% 13% 7% 6% 5% 4 6% 7% 24%	5700
	21st to end	Night	4 8% 4 9% 6% 6% 5% 4 6% 8% 41%	7623

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	54% 4 12% 17% 12% 1	4125
	11th to 20th	Day	43% 4 13% 18% 18% 3	4140
	21st to end	Day	50% 5% 15% 25% 4	5048
February	1st to 10th	Day	55% 6% 12% 15% 11% 1	1680
	11th to 20th	Day	53% 6% 13% 17% 11% 1	1767
	21st to end	Day	57% 5% 10% 17% 9% 1	1662
March	1st to 10th	Day	47% 5% 15% 18% 14% 1	4618
	11th to 20th	Day	43% 6% 13% 22% 16% 1	5006
	21st to end	Day	58% 5% 9% 18% 9% 1	5778
April	1st to 10th	Day	51% 8% 14% 18% 8% 1	2586
	11th to 20th	Day	52% 6% 12% 21% 8%	2700
	21st to end	Day	49% 6% 13% 22% 7% 1	2730
May	1st to 10th	Day	42% 5% 11% 28% 14% 1	8505
	11th to 20th	Day	51% 7% 11% 23% 7%	8759
	21st to end	Day	44% 7% 19% 22% 7% 1	9650
June	1st to 10th	Day	47% 7% 16% 23% 7% 1	3150
	11th to 20th	Day	57% 7% 14% 16% 5%	3283
	21st to end	Day	39% 9% 21% 24% 6% 1	3360
July	1st to 10th	Day	32% 6% 19% 30% 11% 1	6947
	11th to 20th	Day	39% 6% 18% 30% 7%	6699
	21st to end	Day	39% 6% 20% 24% 9% 1	7169
August	1st to 10th	Day	38% 7% 20% 32% 2	4533
	11th to 20th	Day	37% 7% 22% 27% 8%	4363
	21st to end	Day	38% 6% 26% 24% 6%	4663
September	1st to 10th	Day	37% 6% 20% 27% 7% 2	2503
	11th to 20th	Day	39% 4 18% 29% 9%	2217
	21st to end	Day	40% 6% 16% 25% 10% 2	2200
October	1st to 10th	Day	42% 5% 20% 27% 5% 1	3433
	11th to 20th	Day	46% 5% 13% 24% 11%	2932
	21st to end	Day	46% 4 14% 29% 6%	3200
November	1st to 10th	Day	56% 5% 14% 18% 6% 1	1673
	11th to 20th	Day	51% 5% 15% 19% 9%	1486
	21st to end	Day	51% 7% 15% 18% 7% 2	1470
December	1st to 10th	Day	61% 4 17% 12% 5% 1	2360
	11th to 20th	Day	57% 6% 16% 15% 5% 1	2714
	21st to end	Day	39% 6% 11% 25% 16% 2	3575
January	1st to 10th	Night	60% 4 10% 15% 10% 1	8769
	11th to 20th	Night	45% 4 14% 18% 15% 3	8820
	21st to end	Night	49% 5% 19% 20% 7%	9192
February	1st to 10th	Night	56% 4 10% 19% 9% 1	2940
	11th to 20th	Night	54% 6% 11% 16% 12% 1	2750
	21st to end	Night	53% 5% 10% 21% 10% 2	2139
March	1st to 10th	Night	45% 5% 15% 22% 12%	5496
	11th to 20th	Night	48% 7% 14% 20% 11% 1	5214
	21st to end	Night	61% 5% 9% 18% 7%	5436
April	1st to 10th	Night	52% 8% 12% 18% 9% 1	2034
	11th to 20th	Night	53% 6% 12% 19% 9% 2	1871
	21st to end	Night	47% 6% 14% 23% 9% 1	1890
May	1st to 10th	Night	44% 5% 11% 24% 14% 2	4407
	11th to 20th	Night	54% 7% 13% 19% 6%	4105
	21st to end	Night	44% 7% 19% 22% 7% 1	4526
June	1st to 10th	Night	53% 7% 14% 19% 7% 1	1470
	11th to 20th	Night	57% 8% 16% 15% 4	1331
	21st to end	Night	44% 9% 19% 23% 5% 1	1260
July	1st to 10th	Night	37% 7% 20% 20% 15% 1	2888
	11th to 20th	Night	44% 7% 15% 27% 7%	3138
	21st to end	Night	40% 7% 21% 23% 8% 1	3369
August	1st to 10th	Night	41% 8% 21% 28% 3	2591
	11th to 20th	Night	42% 7% 20% 22% 9% 1	2755
	21st to end	Night	38% 6% 20% 28% 7%	3245
September	1st to 10th	Night	37% 6% 17% 27% 11% 2	1895
	11th to 20th	Night	40% 6% 14% 28% 11% 2	2182
	21st to end	Night	38% 6% 15% 26% 13% 2	2200
October	1st to 10th	Night	42% 5% 20% 25% 7%	3761
	11th to 20th	Night	43% 5% 13% 30% 8% 1	4216
	21st to end	Night	46% 6% 15% 26% 7% 1	4600
November	1st to 10th	Night	56% 6% 11% 19% 8% 1	2824
	11th to 20th	Night	53% 6% 13% 18% 10% 1	3134
	21st to end	Night	53% 6% 14% 19% 7% 2	3150
December	1st to 10th	Night	60% 5% 13% 16% 5% 1	5040
	11th to 20th	Night	59% 5% 16% 15% 6%	5697
	21st to end	Night	40% 4 10% 27% 15% 2	7623

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	7% 6% 31% 34% 16% 5%	220
	11th to 20th	Day	7% 10% 40% 24% 14% 6%	220
	21st to end	Day	13% 8% 32% 29% 14% 4	242
February	1st to 10th	Day	14% 9% 33% 28% 14% 3	220
	11th to 20th	Day	14% 6% 32% 34% 11% 4	283
	21st to end	Day	7% 8% 38% 26% 15% 4	378
March	1st to 10th	Day	12% 9% 34% 27% 15% 3	540
	11th to 20th	Day	9% 7% 35% 30% 14% 5%	540
	21st to end	Day	16% 8% 29% 27% 16% 4	594
April	1st to 10th	Day	12% 14% 30% 28% 13% 3	540
	11th to 20th	Day	17% 7% 26% 30% 16% 5%	532
	21st to end	Day	15% 9% 24% 33% 15% 4	540
May	1st to 10th	Day	16% 10% 21% 31% 17% 5%	650
	11th to 20th	Day	12% 8% 21% 32% 21% 5%	775
	21st to end	Day	16% 5% 21% 30% 22% 7%	844
June	1st to 10th	Day	18% 9% 22% 33% 15% 4	780
	11th to 20th	Day	20% 7% 19% 28% 19% 6%	913
	21st to end	Day	12% 5% 22% 28% 24% 9%	990
July	1st to 10th	Day	13% 3 20% 34% 22% 8%	832
	11th to 20th	Day	14% 4 19% 31% 25% 7%	718
	21st to end	Day	12% 4 16% 31% 27% 11%	794
August	1st to 10th	Day	9% 5% 20% 29% 24% 13%	537
	11th to 20th	Day	9% 5% 23% 32% 25% 7%	517
	21st to end	Day	16% 5% 15% 28% 26% 10%	583
September	1st to 10th	Day	15% 5% 25% 24% 22% 9%	530
	11th to 20th	Day	18% 4 21% 30% 19% 8%	530
	21st to end	Day	13% 9% 33% 25% 16% 3	530
October	1st to 10th	Day	16% 5% 29% 28% 16% 6%	445
	11th to 20th	Day	11% 9% 32% 30% 13% 4	330
	21st to end	Day	14% 12% 36% 27% 9% 2	363
November	1st to 10th	Day	18% 7% 29% 32% 11% 2	330
	11th to 20th	Day	11% 6% 37% 32% 11% 2	228
	21st to end	Day	9% 7% 32% 35% 11% 5%	220
December	1st to 10th	Day	13% 8% 35% 29% 10% 5%	220
	11th to 20th	Day	10% 11% 33% 30% 15% 2	220
	21st to end	Day	10% 5% 28% 31% 20% 6%	242
January	1st to 10th	Night	12% 8% 34% 30% 14% 2	1050
	11th to 20th	Night	12% 9% 36% 31% 12% 1	1050
	21st to end	Night	14% 11% 34% 29% 10% 1	992
February	1st to 10th	Night	12% 10% 28% 32% 17% 2	840
	11th to 20th	Night	12% 7% 32% 35% 13% 1	824
	21st to end	Night	10% 7% 29% 33% 17% 4	692
March	1st to 10th	Night	10% 6% 33% 34% 14% 2	840
	11th to 20th	Night	11% 5% 33% 37% 13% 2	840
	21st to end	Night	13% 6% 29% 37% 13% 2	924
April	1st to 10th	Night	14% 6% 32% 33% 12% 1	774
	11th to 20th	Night	17% 6% 32% 33% 12%	623
	21st to end	Night	16% 8% 25% 38% 12% 1	630
May	1st to 10th	Night	20% 5% 25% 36% 12% 2	420
	11th to 20th	Night	15% 4 25% 37% 18% 1	414
	21st to end	Night	19% 4 23% 36% 16% 1	455
June	1st to 10th	Night	22% 6% 24% 36% 10% 2	420
	11th to 20th	Night	22% 4 19% 38% 16% 2	420
	21st to end	Night	17% 4 19% 36% 22% 2	420
July	1st to 10th	Night	15% 3 14% 45% 22% 1	390
	11th to 20th	Night	14% 4 17% 45% 19% 2	388
	21st to end	Night	15% 4 12% 41% 26% 2	438
August	1st to 10th	Night	17% 2 16% 36% 25% 4	398
	11th to 20th	Night	12% 3 18% 39% 26% 2	485
	21st to end	Night	14% 3 18% 43% 22% 1	660
September	1st to 10th	Night	14% 3 17% 45% 19% 2	695
	11th to 20th	Night	14% 4 20% 41% 20% 1	800
	21st to end	Night	13% 5% 32% 36% 12% 3	800
October	1st to 10th	Night	16% 4 28% 38% 13% 1	800
	11th to 20th	Night	16% 5% 33% 30% 14% 1	800
	21st to end	Night	14% 7% 33% 34% 11% 1	880
November	1st to 10th	Night	19% 9% 32% 29% 10% 2	979
	11th to 20th	Night	13% 7% 40% 28% 10% 2	1050
	21st to end	Night	13% 5% 38% 31% 11% 2	1050
December	1st to 10th	Night	15% 10% 34% 28% 11% 2	1050
	11th to 20th	Night	12% 7% 34% 32% 13% 1	1050
	21st to end	Night	12% 5% 33% 34% 15% 2	1155

Page generated at 2026-06-28 00:10:47. Version ac07

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