Brier Island
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brier Island clear sky chart from 2005-11-06 to 2026-04-02.

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	20% 13% 5% 10% 5% 4 3 3 4 5% 27%	131239
	Night	21% 15% 5% 11% 5% 4 3 3 4 5% 24%	130659
Transparency	Day	58% 6% 10% 16% 9% 1	134799
	Night	63% 4 10% 15% 8% 1	135186
Seeing	Day	25% 10% 25% 26% 12% 2	18294
	Night	27% 13% 24% 24% 10% 1	26363

By Month

Percentage Cloud Cover NE (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% 19% 9% 18% 10% 4 3 3 3 4 10%	4244
	11th to 20th	Day	20% 19% 10% 18% 6% 3 2 2 2 3 14%	4827
	21st to end	Day	16% 14% 7% 23% 14% 7% 2 2 2 3 11%	5781
February	1st to 10th	Day	20% 17% 7% 17% 9% 4 4 3 3 5% 11%	1890
	11th to 20th	Day	18% 14% 5% 18% 10% 3 3 3 3 4 19%	1843
	21st to end	Day	13% 15% 6% 12% 8% 5% 4 3 5% 6% 25%	1708
March	1st to 10th	Day	25% 13% 4 11% 8% 3 3 3 4 5% 21%	4874
	11th to 20th	Day	18% 14% 3 8% 5% 4 4 3 5% 6% 30%	5090
	21st to end	Day	28% 11% 4 8% 5% 4 4 3 4 5% 24%	5587
April	1st to 10th	Day	22% 12% 3 6% 4 4 4 3 4 5% 33%	2306
	11th to 20th	Day	21% 9% 2 5% 3 3 3 3 4 6% 41%	2572
	21st to end	Day	22% 8% 2 6% 4 3 4 4 5% 6% 37%	2600
May	1st to 10th	Day	24% 15% 4 8% 5% 4 3 3 4 5% 24%	5886
	11th to 20th	Day	20% 10% 3 7% 4 3 4 3 5% 7% 33%	6030
	21st to end	Day	18% 13% 4 8% 5% 4 4 3 4 6% 32%	6948
June	1st to 10th	Day	23% 10% 3 6% 4 3 4 3 5% 6% 33%	2949
	11th to 20th	Day	21% 8% 4 7% 4 4 4 3 5% 6% 36%	3000
	21st to end	Day	25% 11% 3 7% 5% 4 3 3 5% 6% 28%	3000
July	1st to 10th	Day	26% 15% 6% 7% 4 3 3 3 4 6% 23%	6820
	11th to 20th	Day	21% 18% 5% 9% 4 3 3 2 4 6% 25%	6487
	21st to end	Day	14% 11% 3 10% 4 4 4 3 5% 8% 35%	6425
August	1st to 10th	Day	26% 14% 3 9% 3 3 3 3 4 5% 26%	4235
	11th to 20th	Day	15% 11% 4 9% 4 4 5% 4 5% 6% 32%	4175
	21st to end	Day	12% 8% 3 5% 3 4 3 3 4 6% 48%	4652
September	1st to 10th	Day	16% 8% 3 6% 4 3 3 3 4 7% 44%	2354
	11th to 20th	Day	15% 9% 3 5% 3 3 4 3 3 6% 47%	2200
	21st to end	Day	18% 8% 3 6% 3 3 3 3 4 7% 43%	2200
October	1st to 10th	Day	16% 8% 3 6% 4 3 4 3 5% 8% 42%	2040
	11th to 20th	Day	18% 9% 2 6% 4 4 4 3 5% 7% 37%	2000
	21st to end	Day	20% 11% 4 7% 5% 5% 4 3 5% 5% 33%	2120
November	1st to 10th	Day	22% 10% 3 7% 5% 4 5% 3 5% 5% 32%	1840
	11th to 20th	Day	21% 16% 5% 8% 4 3 3 3 5% 7% 26%	1680
	21st to end	Day	20% 18% 6% 11% 6% 4 3 3 4 6% 19%	1680
December	1st to 10th	Day	25% 17% 11% 13% 5% 3 4 3 3 4 12%	2690
	11th to 20th	Day	24% 15% 6% 15% 6% 4 4 3 4 4 16%	2953
	21st to end	Day	21% 16% 8% 20% 4 3 2 2 3 3 18%	3553
January	1st to 10th	Night	17% 23% 8% 22% 8% 3 2 2 2 3 11%	8669
	11th to 20th	Night	22% 20% 9% 17% 6% 4 3 2 3 3 12%	8042
	21st to end	Night	15% 15% 7% 23% 10% 6% 3 2 3 4 12%	8400
February	1st to 10th	Night	21% 18% 7% 18% 8% 4 3 2 3 4 12%	2730
	11th to 20th	Night	19% 16% 6% 16% 9% 4 3 2 4 3 18%	2663
	21st to end	Night	15% 15% 6% 12% 7% 5% 3 4 4 6% 22%	2097
March	1st to 10th	Night	25% 14% 4 11% 9% 4 3 2 3 5% 19%	5185
	11th to 20th	Night	18% 14% 5% 8% 4 4 4 4 5% 7% 26%	5089
	21st to end	Night	31% 11% 4 8% 4 3 3 2 3 4 26%	5587
April	1st to 10th	Night	25% 12% 4 6% 5% 4 3 3 5% 6% 28%	2114
	11th to 20th	Night	24% 9% 2 7% 4 3 5% 3 4 7% 32%	1778
	21st to end	Night	24% 9% 3 6% 5% 4 4 3 5% 6% 31%	1800
May	1st to 10th	Night	28% 13% 4 6% 4 3 3 3 4 5% 26%	4054
	11th to 20th	Night	22% 10% 4 7% 4 4 4 3 4 6% 31%	3868
	21st to end	Night	19% 15% 3 7% 4 4 4 3 5% 8% 28%	3948
June	1st to 10th	Night	26% 11% 3 7% 3 4 4 3 6% 7% 26%	1467
	11th to 20th	Night	21% 9% 3 8% 4 3 3 3 5% 7% 34%	1400
	21st to end	Night	27% 11% 3 7% 4 4 4 3 4 5% 27%	1400
July	1st to 10th	Night	29% 15% 5% 6% 4 4 4 4 5% 6% 19%	3160
	11th to 20th	Night	27% 18% 3 6% 4 4 3 2 4 5% 24%	3417
	21st to end	Night	16% 12% 4 8% 4 4 4 3 5% 6% 34%	4428
August	1st to 10th	Night	26% 15% 4 7% 3 3 3 2 4 6% 26%	2915
	11th to 20th	Night	19% 13% 5% 8% 5% 3 4 3 6% 6% 27%	2872
	21st to end	Night	12% 11% 4 6% 3 3 3 3 4 6% 45%	3246
September	1st to 10th	Night	17% 8% 2 5% 3 4 4 3 5% 6% 42%	2043
	11th to 20th	Night	18% 7% 2 6% 3 3 3 3 4 6% 44%	2200
	21st to end	Night	21% 8% 2 6% 3 3 4 4 4 6% 39%	2200
October	1st to 10th	Night	19% 7% 2 5% 4 4 3 3 4 7% 42%	2359
	11th to 20th	Night	16% 9% 3 7% 4 5% 3 3 5% 7% 37%	2400
	21st to end	Night	24% 12% 2 8% 4 3 3 2 4 5% 32%	2715
November	1st to 10th	Night	21% 12% 3 7% 5% 4 4 3 5% 6% 31%	2675
	11th to 20th	Night	20% 18% 5% 8% 5% 4 4 4 5% 6% 22%	2940
	21st to end	Night	21% 19% 5% 9% 5% 4 3 3 5% 5% 21%	2940
December	1st to 10th	Night	21% 19% 9% 14% 6% 4 4 3 4 4 13%	4720
	11th to 20th	Night	21% 18% 5% 17% 5% 3 3 3 4 5% 17%	5471
	21st to end	Night	22% 19% 6% 15% 4 3 2 2 3 3 20%	7667

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	79% 4 10% 6% 1	4245
	11th to 20th	Day	78% 3 7% 8% 4	4857
	21st to end	Day	82% 2 8% 4 3	5781
February	1st to 10th	Day	78% 4 8% 6% 4	1890
	11th to 20th	Day	71% 5% 9% 11% 4	1851
	21st to end	Day	62% 5% 10% 15% 6% 1	1708
March	1st to 10th	Day	67% 3 9% 12% 6% 3	4878
	11th to 20th	Day	54% 7% 14% 18% 6% 1	5090
	21st to end	Day	64% 3 7% 15% 10% 1	5587
April	1st to 10th	Day	53% 4 9% 17% 15% 1	2306
	11th to 20th	Day	46% 5% 10% 24% 14% 2	2572
	21st to end	Day	48% 4 11% 20% 16% 2	2600
May	1st to 10th	Day	60% 8% 11% 15% 6% 1	5897
	11th to 20th	Day	51% 6% 13% 19% 9% 2	6034
	21st to end	Day	51% 9% 11% 16% 12% 1	6948
June	1st to 10th	Day	51% 7% 10% 21% 10% 1	2949
	11th to 20th	Day	50% 5% 9% 20% 14% 2	3000
	21st to end	Day	57% 7% 9% 17% 9% 2	3000
July	1st to 10th	Day	62% 6% 9% 14% 9%	6745
	11th to 20th	Day	59% 7% 9% 17% 7%	6407
	21st to end	Day	47% 8% 13% 21% 11% 1	6242
August	1st to 10th	Day	61% 6% 8% 15% 9%	4222
	11th to 20th	Day	51% 7% 16% 18% 7% 1	4214
	21st to end	Day	35% 7% 10% 25% 22% 1	4652
September	1st to 10th	Day	41% 7% 10% 23% 18% 1	2354
	11th to 20th	Day	39% 5% 11% 25% 18% 2	2200
	21st to end	Day	43% 5% 7% 25% 17% 3	2200
October	1st to 10th	Day	42% 6% 10% 31% 10% 1	3326
	11th to 20th	Day	39% 4 12% 29% 16%	3240
	21st to end	Day	42% 6% 12% 24% 14% 2	3409
November	1st to 10th	Day	54% 5% 13% 19% 8% 1	1840
	11th to 20th	Day	60% 4 12% 16% 7% 1	1680
	21st to end	Day	66% 6% 13% 12% 3	1680
December	1st to 10th	Day	77% 4 8% 9% 2	2690
	11th to 20th	Day	73% 4 11% 7% 4 2	2952
	21st to end	Day	73% 5% 10% 6% 5%	3553
January	1st to 10th	Night	82% 3 8% 6% 1	8669
	11th to 20th	Night	80% 3 6% 9% 3	8086
	21st to end	Night	78% 3 7% 8% 4	8400
February	1st to 10th	Night	78% 3 8% 7% 4	2730
	11th to 20th	Night	72% 3 7% 12% 5% 1	2666
	21st to end	Night	64% 5% 9% 14% 6% 1	2097
March	1st to 10th	Night	70% 3 7% 10% 8% 2	5194
	11th to 20th	Night	57% 6% 17% 14% 5%	5090
	21st to end	Night	64% 3 7% 15% 11% 1	5587
April	1st to 10th	Night	58% 3 8% 15% 13% 2	2114
	11th to 20th	Night	51% 5% 10% 20% 12% 1	1778
	21st to end	Night	54% 5% 9% 17% 14% 2	1800
May	1st to 10th	Night	61% 6% 10% 15% 7% 1	4060
	11th to 20th	Night	54% 6% 11% 19% 9% 2	3869
	21st to end	Night	54% 6% 12% 19% 8% 1	3949
June	1st to 10th	Night	57% 6% 9% 20% 7%	1467
	11th to 20th	Night	50% 6% 11% 20% 12% 2	1400
	21st to end	Night	60% 5% 8% 16% 9% 2	1400
July	1st to 10th	Night	66% 5% 8% 13% 8%	3125
	11th to 20th	Night	63% 4 9% 12% 10% 1	3375
	21st to end	Night	50% 6% 11% 19% 12% 2	4297
August	1st to 10th	Night	60% 5% 8% 19% 7%	2903
	11th to 20th	Night	56% 6% 14% 18% 7%	2899
	21st to end	Night	41% 5% 10% 26% 17% 1	3245
September	1st to 10th	Night	42% 6% 9% 25% 17% 2	2043
	11th to 20th	Night	42% 4 9% 27% 17% 1	2200
	21st to end	Night	46% 4 9% 22% 19% 1	2200
October	1st to 10th	Night	43% 4 13% 24% 14% 1	3853
	11th to 20th	Night	39% 3 13% 25% 19% 1	3899
	21st to end	Night	47% 3 10% 25% 13% 2	4376
November	1st to 10th	Night	54% 5% 12% 18% 10%	2675
	11th to 20th	Night	64% 6% 11% 14% 6%	2940
	21st to end	Night	66% 5% 13% 12% 4	2940
December	1st to 10th	Night	75% 4 9% 9% 3	4720
	11th to 20th	Night	71% 4 12% 10% 3 1	5473
	21st to end	Night	71% 4 8% 11% 4	7667

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	30% 20% 33% 13% 4	220
	11th to 20th	Day	30% 23% 29% 15% 3	280
	21st to end	Day	22% 20% 36% 19% 3	473
February	1st to 10th	Day	28% 19% 31% 16% 6%	430
	11th to 20th	Day	26% 22% 32% 18% 2	426
	21st to end	Day	18% 20% 35% 23% 4	435
March	1st to 10th	Day	24% 19% 34% 20% 4	540
	11th to 20th	Day	24% 18% 29% 21% 7%	540
	21st to end	Day	27% 16% 31% 19% 7% 1	594
April	1st to 10th	Day	32% 13% 28% 23% 4 1	504
	11th to 20th	Day	25% 11% 32% 25% 6%	492
	21st to end	Day	23% 11% 26% 29% 10% 1	500
May	1st to 10th	Day	25% 8% 33% 25% 8% 1	503
	11th to 20th	Day	22% 4 28% 30% 15% 1	592
	21st to end	Day	29% 3 21% 32% 13% 2	790
June	1st to 10th	Day	26% 4 23% 30% 15% 1	730
	11th to 20th	Day	23% 3 21% 35% 17% 2	730
	21st to end	Day	32% 3 17% 34% 12% 2	730
July	1st to 10th	Day	28% 3 15% 35% 17% 2	720
	11th to 20th	Day	28% 4 13% 33% 18% 4	618
	21st to end	Day	23% 2 14% 38% 22% 2	576
August	1st to 10th	Day	20% 2 13% 36% 26% 3	526
	11th to 20th	Day	23% 1 14% 33% 22% 6%	519
	21st to end	Day	18% 3 17% 33% 22% 6%	583
September	1st to 10th	Day	18% 2 20% 36% 20% 3	530
	11th to 20th	Day	21% 6% 21% 27% 19% 6%	530
	21st to end	Day	20% 5% 24% 30% 17% 5%	530
October	1st to 10th	Day	19% 9% 21% 28% 20% 2	442
	11th to 20th	Day	22% 9% 27% 27% 13% 2	420
	21st to end	Day	25% 11% 29% 23% 10% 1	462
November	1st to 10th	Day	25% 16% 27% 24% 8% 1	425
	11th to 20th	Day	29% 22% 27% 17% 5%	430
	21st to end	Day	31% 18% 30% 15% 4	430
December	1st to 10th	Day	34% 17% 30% 15% 3	430
	11th to 20th	Day	33% 24% 27% 14% 2	372
	21st to end	Day	26% 25% 33% 10% 4 2	242
January	1st to 10th	Night	32% 24% 26% 14% 3	920
	11th to 20th	Night	33% 22% 25% 16% 3	840
	21st to end	Night	23% 25% 33% 16% 3	924
February	1st to 10th	Night	30% 24% 28% 15% 3	840
	11th to 20th	Night	28% 25% 28% 15% 3 1	823
	21st to end	Night	22% 20% 32% 21% 5%	692
March	1st to 10th	Night	26% 17% 35% 18% 3	840
	11th to 20th	Night	25% 17% 28% 23% 7%	840
	21st to end	Night	28% 16% 26% 22% 6% 1	924
April	1st to 10th	Night	30% 15% 28% 21% 5%	725
	11th to 20th	Night	26% 8% 31% 28% 7% 1	594
	21st to end	Night	28% 6% 26% 29% 10% 2	600
May	1st to 10th	Night	28% 6% 25% 31% 10% 2	600
	11th to 20th	Night	29% 4 23% 31% 11% 1	592
	21st to end	Night	26% 6% 21% 32% 14% 2	649
June	1st to 10th	Night	30% 5% 20% 28% 17% 1	467
	11th to 20th	Night	26% 4 17% 33% 19% 2	400
	21st to end	Night	31% 2 18% 35% 13% 1	400
July	1st to 10th	Night	28% 5% 15% 34% 17% 2	390
	11th to 20th	Night	29% 3 14% 30% 21% 3	403
	21st to end	Night	24% 3 12% 40% 19% 1	656
August	1st to 10th	Night	21% 4 14% 34% 24% 2	598
	11th to 20th	Night	29% 1 16% 28% 24% 2	592
	21st to end	Night	18% 3 12% 37% 26% 3	660
September	1st to 10th	Night	20% 3 17% 37% 21% 2	643
	11th to 20th	Night	22% 4 20% 26% 23% 5%	800
	21st to end	Night	25% 4 20% 29% 19% 4	800
October	1st to 10th	Night	24% 7% 19% 29% 19% 3	800
	11th to 20th	Night	20% 10% 29% 25% 13% 3	800
	21st to end	Night	31% 13% 25% 23% 7%	880
November	1st to 10th	Night	28% 15% 27% 21% 8% 1	826
	11th to 20th	Night	29% 19% 26% 19% 8%	950
	21st to end	Night	31% 20% 26% 17% 6% 1	950
December	1st to 10th	Night	37% 16% 26% 16% 4	950
	11th to 20th	Night	32% 26% 27% 14% 2	950
	21st to end	Night	30% 23% 26% 18% 3 1	1045

Page generated at 2026-03-30 03:18:45. Version ac07

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