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 2025-02-25.

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% 10% 3 8% 6% 5% 5% 4 6% 8% 35%	112687
	Night	9% 11% 4 10% 6% 5% 5% 4 6% 7% 32%	114640
Transparency	Day	46% 6% 16% 22% 9% 1	112429
	Night	49% 6% 14% 21% 9% 1	114544
Seeing	Day	14% 7% 25% 30% 19% 6%	16060
	Night	14% 6% 29% 35% 15% 2	24855

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	4 12% 5% 12% 8% 6% 5% 4 5% 7% 31%	3179
	11th to 20th	Day	5% 11% 4 11% 6% 5% 5% 6% 8% 8% 30%	3159
	21st to end	Day	6% 13% 5% 8% 5% 6% 5% 5% 5% 7% 34%	3893
February	1st to 10th	Day	9% 14% 5% 11% 6% 7% 6% 5% 6% 8% 24%	1600
	11th to 20th	Day	4 15% 6% 13% 6% 5% 5% 4 6% 6% 29%	1673
	21st to end	Day	3 13% 5% 14% 8% 7% 7% 6% 7% 6% 24%	1546
March	1st to 10th	Day	3 12% 6% 14% 10% 6% 6% 3 6% 6% 28%	3138
	11th to 20th	Day	5% 11% 3 6% 4 4 4 4 7% 9% 43%	3414
	21st to end	Day	6% 17% 7% 12% 6% 5% 5% 4 6% 7% 25%	3940
April	1st to 10th	Day	8% 14% 5% 8% 7% 6% 5% 6% 8% 7% 28%	2340
	11th to 20th	Day	12% 14% 4 8% 6% 5% 5% 5% 6% 8% 27%	2440
	21st to end	Day	11% 11% 3 9% 6% 6% 5% 5% 6% 8% 30%	2470
May	1st to 10th	Day	16% 11% 3 8% 5% 5% 5% 4 6% 7% 30%	4592
	11th to 20th	Day	10% 11% 5% 10% 6% 6% 5% 5% 7% 8% 28%	4694
	21st to end	Day	13% 8% 3 9% 7% 6% 6% 5% 8% 9% 26%	5161
June	1st to 10th	Day	17% 8% 2 7% 5% 5% 5% 4 8% 8% 30%	2850
	11th to 20th	Day	19% 10% 3 8% 7% 5% 6% 5% 6% 7% 23%	2970
	21st to end	Day	11% 5% 2 6% 5% 6% 6% 5% 8% 9% 38%	3040
July	1st to 10th	Day	11% 5% 2 5% 4 3 4 3 6% 8% 49%	4911
	11th to 20th	Day	9% 5% 2 5% 5% 4 4 4 6% 8% 49%	4740
	21st to end	Day	12% 4 2 6% 5% 4 5% 4 6% 9% 44%	5138
August	1st to 10th	Day	13% 6% 2 5% 4 4 5% 4 6% 8% 42%	4411
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 4 6% 10% 42%	4186
	21st to end	Day	12% 6% 2 7% 4 4 5% 4 6% 9% 41%	4521
September	1st to 10th	Day	13% 5% 2 7% 4 4 5% 4 6% 8% 42%	2378
	11th to 20th	Day	16% 8% 2 5% 3 4 4 3 5% 9% 42%	2106
	21st to end	Day	10% 7% 3 6% 4 6% 5% 4 6% 8% 42%	2090
October	1st to 10th	Day	10% 9% 4 8% 5% 5% 4 3 6% 7% 40%	3327
	11th to 20th	Day	11% 12% 3 8% 5% 5% 5% 4 6% 8% 34%	2842
	21st to end	Day	7% 12% 3 9% 6% 5% 5% 3 4 7% 38%	3101
November	1st to 10th	Day	10% 16% 4 11% 5% 6% 5% 3 6% 7% 27%	1592
	11th to 20th	Day	7% 15% 4 13% 6% 5% 4 3 5% 6% 34%	1415
	21st to end	Day	6% 11% 6% 11% 7% 6% 5% 6% 7% 8% 28%	1400
December	1st to 10th	Day	9% 16% 5% 12% 6% 7% 5% 3 5% 8% 23%	2290
	11th to 20th	Day	7% 13% 5% 13% 8% 6% 6% 3 6% 7% 25%	2642
	21st to end	Day	3 8% 3 9% 6% 5% 6% 4 6% 9% 40%	3498
January	1st to 10th	Night	4 11% 5% 16% 8% 6% 5% 4 5% 6% 29%	6766
	11th to 20th	Night	4 11% 4 12% 7% 7% 6% 4 6% 6% 33%	6739
	21st to end	Night	7% 14% 5% 9% 5% 5% 5% 4 6% 8% 32%	7108
February	1st to 10th	Night	6% 14% 6% 12% 7% 6% 5% 5% 6% 6% 26%	2800
	11th to 20th	Night	5% 14% 5% 11% 8% 6% 7% 5% 6% 7% 27%	2614
	21st to end	Night	5% 15% 4 10% 7% 6% 6% 5% 5% 7% 30%	1997
March	1st to 10th	Night	3 9% 5% 14% 8% 8% 6% 4 6% 8% 28%	3736
	11th to 20th	Night	5% 10% 3 7% 5% 6% 6% 5% 8% 10% 36%	3565
	21st to end	Night	5% 16% 7% 17% 8% 6% 6% 4 5% 6% 21%	3710
April	1st to 10th	Night	8% 13% 4 10% 9% 6% 5% 5% 8% 7% 26%	1840
	11th to 20th	Night	10% 13% 4 9% 6% 6% 5% 6% 7% 7% 27%	1691
	21st to end	Night	12% 11% 4 8% 5% 4 4 5% 6% 9% 32%	1710
May	1st to 10th	Night	21% 13% 4 9% 4 3 4 4 4 6% 28%	2364
	11th to 20th	Night	11% 10% 4 9% 6% 6% 5% 6% 7% 8% 28%	2199
	21st to end	Night	15% 12% 3 9% 7% 6% 6% 5% 6% 7% 24%	2414
June	1st to 10th	Night	17% 9% 3 9% 6% 6% 6% 5% 6% 7% 27%	1330
	11th to 20th	Night	18% 10% 4 8% 6% 7% 6% 5% 7% 6% 24%	1205
	21st to end	Night	14% 7% 2 7% 6% 5% 6% 6% 8% 8% 33%	1140
July	1st to 10th	Night	12% 5% 2 7% 5% 5% 4 4 6% 7% 42%	2046
	11th to 20th	Night	8% 6% 2 6% 6% 5% 6% 4 7% 10% 39%	2220
	21st to end	Night	14% 6% 2 6% 4 5% 5% 5% 7% 8% 39%	2414
August	1st to 10th	Night	15% 6% 2 6% 4 4 5% 4 8% 8% 38%	2518
	11th to 20th	Night	12% 6% 3 7% 4 5% 5% 5% 7% 9% 37%	2646
	21st to end	Night	13% 5% 3 7% 4 4 5% 4 7% 9% 40%	3146
September	1st to 10th	Night	13% 5% 2 6% 4 4 5% 4 6% 9% 42%	1800
	11th to 20th	Night	14% 7% 3 6% 4 4 5% 3 6% 6% 43%	2073
	21st to end	Night	10% 7% 3 6% 4 4 4 4 7% 8% 42%	2090
October	1st to 10th	Night	10% 8% 3 7% 5% 5% 6% 5% 5% 7% 39%	3647
	11th to 20th	Night	11% 8% 3 8% 5% 4 5% 4 7% 8% 37%	4086
	21st to end	Night	7% 10% 4 9% 7% 6% 5% 4 6% 7% 35%	4457
November	1st to 10th	Night	12% 15% 6% 10% 6% 5% 5% 4 5% 7% 26%	2686
	11th to 20th	Night	7% 14% 5% 12% 7% 5% 5% 4 6% 7% 28%	2985
	21st to end	Night	7% 13% 4 10% 8% 6% 5% 4 6% 8% 27%	3000
December	1st to 10th	Night	11% 17% 5% 10% 7% 5% 5% 4 6% 6% 26%	4890
	11th to 20th	Night	8% 15% 5% 13% 7% 6% 5% 4 6% 7% 24%	5550
	21st to end	Night	3 8% 4 9% 6% 6% 5% 4 6% 8% 41%	7458

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	51% 4 15% 19% 9% 2	3179
	11th to 20th	Day	49% 5% 9% 15% 19% 4	3180
	21st to end	Day	48% 6% 15% 25% 6%	3893
February	1st to 10th	Day	56% 6% 12% 15% 10% 1	1600
	11th to 20th	Day	53% 6% 13% 17% 10% 1	1681
	21st to end	Day	57% 5% 10% 17% 10% 1	1546
March	1st to 10th	Day	55% 5% 13% 16% 10% 1	3138
	11th to 20th	Day	36% 7% 18% 20% 18% 1	3415
	21st to end	Day	56% 6% 12% 18% 6%	3940
April	1st to 10th	Day	51% 9% 14% 18% 8% 1	2340
	11th to 20th	Day	53% 6% 12% 21% 8%	2440
	21st to end	Day	49% 7% 14% 22% 7% 2	2470
May	1st to 10th	Day	53% 6% 10% 23% 8%	4592
	11th to 20th	Day	50% 8% 12% 22% 8%	4698
	21st to end	Day	50% 9% 16% 17% 8% 1	5162
June	1st to 10th	Day	47% 7% 17% 22% 6% 1	2850
	11th to 20th	Day	57% 7% 14% 16% 6%	2970
	21st to end	Day	39% 9% 22% 23% 6% 1	3040
July	1st to 10th	Day	33% 5% 16% 31% 13% 2	4836
	11th to 20th	Day	32% 6% 18% 33% 10%	4659
	21st to end	Day	37% 6% 21% 22% 13% 1	4976
August	1st to 10th	Day	38% 7% 20% 32% 2	4393
	11th to 20th	Day	36% 6% 23% 27% 8%	4228
	21st to end	Day	39% 7% 27% 23% 5%	4520
September	1st to 10th	Day	38% 6% 20% 26% 7% 2	2378
	11th to 20th	Day	41% 4 18% 28% 9%	2106
	21st to end	Day	40% 7% 16% 24% 10% 2	2090
October	1st to 10th	Day	42% 5% 21% 26% 5% 1	3327
	11th to 20th	Day	46% 5% 13% 24% 11%	2842
	21st to end	Day	47% 4 14% 29% 6%	3101
November	1st to 10th	Day	56% 5% 14% 18% 6% 1	1592
	11th to 20th	Day	52% 6% 14% 18% 9%	1415
	21st to end	Day	51% 8% 15% 17% 6% 2	1400
December	1st to 10th	Day	61% 4 17% 12% 5% 1	2290
	11th to 20th	Day	57% 6% 16% 15% 5% 1	2644
	21st to end	Day	39% 6% 11% 25% 17% 2	3498
January	1st to 10th	Night	56% 4 12% 17% 9% 1	6766
	11th to 20th	Night	50% 4 11% 15% 16% 4	6780
	21st to end	Night	49% 6% 17% 20% 7%	7108
February	1st to 10th	Night	57% 4 10% 19% 9% 1	2800
	11th to 20th	Night	54% 6% 11% 16% 12% 1	2616
	21st to end	Night	53% 5% 10% 20% 10% 2	1997
March	1st to 10th	Night	51% 6% 11% 21% 10%	3736
	11th to 20th	Night	39% 7% 17% 21% 14% 1	3565
	21st to end	Night	61% 6% 11% 16% 5% 1	3710
April	1st to 10th	Night	53% 8% 11% 17% 9% 1	1840
	11th to 20th	Night	53% 6% 12% 19% 9% 2	1691
	21st to end	Night	47% 7% 15% 22% 8% 1	1710
May	1st to 10th	Night	56% 5% 9% 19% 10%	2364
	11th to 20th	Night	49% 8% 12% 22% 8%	2199
	21st to end	Night	55% 9% 14% 13% 8% 2	2414
June	1st to 10th	Night	52% 7% 15% 19% 6% 1	1330
	11th to 20th	Night	56% 8% 16% 16% 4	1205
	21st to end	Night	44% 9% 19% 21% 6% 1	1140
July	1st to 10th	Night	38% 7% 15% 21% 17% 1	2012
	11th to 20th	Night	38% 8% 18% 27% 9%	2178
	21st to end	Night	40% 7% 18% 23% 10% 1	2337
August	1st to 10th	Night	41% 8% 21% 28% 3	2511
	11th to 20th	Night	41% 7% 20% 22% 9% 1	2670
	21st to end	Night	40% 6% 21% 28% 6%	3146
September	1st to 10th	Night	38% 7% 18% 25% 11% 2	1800
	11th to 20th	Night	41% 6% 14% 28% 8% 3	2073
	21st to end	Night	38% 6% 15% 26% 13% 2	2090
October	1st to 10th	Night	43% 5% 19% 25% 7%	3647
	11th to 20th	Night	43% 6% 13% 30% 8%	4086
	21st to end	Night	46% 6% 15% 26% 7% 1	4457
November	1st to 10th	Night	57% 6% 11% 19% 7% 1	2686
	11th to 20th	Night	54% 6% 12% 17% 9% 1	2985
	21st to end	Night	53% 6% 15% 18% 6% 2	3000
December	1st to 10th	Night	60% 5% 13% 16% 6% 1	4890
	11th to 20th	Night	59% 5% 16% 15% 5%	5547
	21st to end	Night	40% 4 10% 27% 16% 2	7458

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	6% 7% 31% 34% 18% 6%	200
	11th to 20th	Day	7% 9% 39% 24% 15% 7%	200
	21st to end	Day	15% 8% 32% 29% 12% 4	220
February	1st to 10th	Day	15% 10% 30% 28% 15% 4	200
	11th to 20th	Day	13% 5% 34% 33% 11% 4	257
	21st to end	Day	7% 7% 36% 28% 16% 5%	334
March	1st to 10th	Day	11% 10% 33% 28% 16% 3	460
	11th to 20th	Day	9% 7% 34% 30% 15% 6%	460
	21st to end	Day	15% 8% 28% 29% 16% 4	506
April	1st to 10th	Day	10% 14% 32% 29% 12% 2	460
	11th to 20th	Day	17% 6% 24% 31% 16% 5%	452
	21st to end	Day	15% 9% 25% 34% 13% 4	460
May	1st to 10th	Day	17% 9% 19% 32% 18% 5%	560
	11th to 20th	Day	11% 8% 21% 33% 21% 6%	675
	21st to end	Day	16% 4 20% 30% 22% 7%	734
June	1st to 10th	Day	18% 8% 22% 34% 16% 3	680
	11th to 20th	Day	21% 7% 19% 28% 20% 6%	800
	21st to end	Day	12% 4 22% 28% 24% 9%	870
July	1st to 10th	Day	13% 3 19% 34% 22% 8%	776
	11th to 20th	Day	14% 4 17% 32% 26% 7%	668
	21st to end	Day	12% 4 16% 30% 27% 12%	739
August	1st to 10th	Day	9% 5% 20% 29% 25% 12%	497
	11th to 20th	Day	8% 4 21% 33% 26% 7%	477
	21st to end	Day	17% 6% 16% 29% 24% 8%	539
September	1st to 10th	Day	16% 6% 26% 23% 21% 8%	490
	11th to 20th	Day	20% 4 23% 30% 17% 7%	490
	21st to end	Day	14% 9% 31% 26% 17% 3	490
October	1st to 10th	Day	18% 5% 28% 28% 16% 6%	409
	11th to 20th	Day	12% 10% 32% 29% 13% 5%	300
	21st to end	Day	15% 12% 33% 28% 10% 2	330
November	1st to 10th	Day	19% 6% 28% 33% 12% 3	300
	11th to 20th	Day	12% 6% 34% 35% 11% 2	207
	21st to end	Day	10% 7% 33% 36% 11% 5%	200
December	1st to 10th	Day	12% 8% 35% 29% 11% 6%	200
	11th to 20th	Day	10% 10% 33% 30% 16% 2	200
	21st to end	Day	9% 5% 27% 32% 21% 6%	220
January	1st to 10th	Night	11% 8% 34% 31% 15% 2	1000
	11th to 20th	Night	12% 8% 34% 32% 13% 1	1000
	21st to end	Night	14% 11% 34% 28% 11% 1	945
February	1st to 10th	Night	12% 10% 26% 32% 17% 2	800
	11th to 20th	Night	12% 7% 32% 35% 13% 1	784
	21st to end	Night	10% 6% 27% 34% 17% 5%	646
March	1st to 10th	Night	11% 5% 33% 34% 15% 2	760
	11th to 20th	Night	11% 5% 32% 37% 13% 2	760
	21st to end	Night	12% 6% 29% 37% 14% 3	836
April	1st to 10th	Night	14% 7% 32% 33% 12% 1	700
	11th to 20th	Night	17% 6% 31% 33% 13% 1	563
	21st to end	Night	16% 7% 26% 37% 12% 1	570
May	1st to 10th	Night	21% 6% 23% 35% 13% 2	380
	11th to 20th	Night	15% 4 26% 37% 17% 1	374
	21st to end	Night	20% 4 22% 36% 17% 1	411
June	1st to 10th	Night	21% 6% 24% 36% 12% 2	380
	11th to 20th	Night	22% 3 18% 38% 17% 2	380
	21st to end	Night	16% 4 19% 36% 23% 2	380
July	1st to 10th	Night	15% 2 13% 45% 23% 1	370
	11th to 20th	Night	13% 3 17% 45% 20% 2	368
	21st to end	Night	15% 4 12% 41% 25% 2	416
August	1st to 10th	Night	16% 2 16% 37% 25% 4	378
	11th to 20th	Night	12% 3 18% 38% 27% 2	460
	21st to end	Night	14% 3 19% 42% 22% 1	627
September	1st to 10th	Night	15% 3 18% 45% 18% 2	660
	11th to 20th	Night	14% 4 20% 40% 20% 1	760
	21st to end	Night	13% 5% 31% 36% 13% 3	760
October	1st to 10th	Night	17% 4 28% 37% 13% 1	760
	11th to 20th	Night	16% 5% 33% 31% 14% 1	760
	21st to end	Night	14% 6% 32% 35% 12% 1	836
November	1st to 10th	Night	19% 8% 31% 30% 11% 2	931
	11th to 20th	Night	13% 7% 40% 28% 10% 2	1000
	21st to end	Night	13% 5% 39% 31% 11% 2	1000
December	1st to 10th	Night	15% 9% 33% 29% 12% 3	1000
	11th to 20th	Night	12% 7% 33% 32% 14% 1	1000
	21st to end	Night	11% 4 33% 34% 15% 2	1100

Page generated at 2025-02-22 04:51:33. Version ac07

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