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 2024-01-16.

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	20% 11% 4 8% 6% 3 4 4 4 6% 30%	74727
	Night	22% 13% 5% 9% 6% 3 4 4 4 6% 25%	75070
Transparency	Day	56% 6% 10% 17% 10% 1	74433
	Night	62% 5% 10% 15% 8% 1	74900
Seeing	Day	26% 10% 25% 26% 11% 2	15332
	Night	28% 13% 24% 24% 10% 1	23421

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	20% 18% 9% 13% 9% 4 4 4 3 4 12%	1376
	11th to 20th	Day	24% 18% 7% 14% 11% 3 3 3 3 4 11%	1531
	21st to end	Day	16% 14% 9% 16% 14% 5% 5% 3 3 4 12%	1782
February	1st to 10th	Day	22% 15% 7% 15% 10% 4 4 4 3 5% 11%	1620
	11th to 20th	Day	19% 13% 6% 15% 13% 3 4 3 3 4 17%	1573
	21st to end	Day	12% 14% 7% 11% 10% 3 5% 4 4 6% 22%	1461
March	1st to 10th	Day	17% 14% 5% 12% 8% 3 3 3 4 6% 26%	1907
	11th to 20th	Day	17% 12% 5% 9% 7% 3 3 3 4 7% 30%	1980
	21st to end	Day	18% 10% 5% 7% 6% 3 3 3 4 6% 36%	2178
April	1st to 10th	Day	21% 11% 4 6% 5% 2 4 4 4 6% 33%	2065
	11th to 20th	Day	21% 8% 3 5% 4 2 3 4 4 6% 41%	2312
	21st to end	Day	23% 8% 3 5% 5% 2 4 4 5% 6% 35%	2340
May	1st to 10th	Day	23% 10% 3 6% 6% 3 4 4 4 6% 32%	2340
	11th to 20th	Day	21% 7% 2 4 4 2 4 4 5% 8% 39%	2358
	21st to end	Day	21% 9% 3 6% 5% 3 4 4 5% 8% 33%	2724
June	1st to 10th	Day	25% 9% 4 6% 5% 3 3 4 5% 6% 31%	2653
	11th to 20th	Day	22% 7% 4 6% 5% 3 4 4 4 6% 35%	2700
	21st to end	Day	24% 10% 3 6% 7% 3 4 4 5% 6% 28%	2700
July	1st to 10th	Day	20% 12% 4 6% 4 3 5% 5% 5% 8% 30%	2700
	11th to 20th	Day	21% 11% 4 7% 6% 3 4 4 6% 6% 27%	2577
	21st to end	Day	21% 10% 3 8% 6% 3 4 5% 5% 7% 27%	2548
August	1st to 10th	Day	16% 9% 3 6% 4 2 5% 6% 6% 7% 35%	2327
	11th to 20th	Day	19% 11% 3 7% 5% 3 5% 5% 6% 6% 29%	2304
	21st to end	Day	16% 8% 2 5% 4 2 4 5% 5% 7% 42%	2566
September	1st to 10th	Day	17% 7% 3 6% 4 2 3 3 4 6% 45%	2120
	11th to 20th	Day	15% 9% 4 5% 4 2 4 4 3 6% 45%	1980
	21st to end	Day	19% 8% 3 6% 3 2 3 4 4 7% 42%	1980
October	1st to 10th	Day	17% 8% 3 5% 4 3 4 4 5% 8% 40%	1836
	11th to 20th	Day	19% 8% 3 5% 5% 3 4 4 5% 7% 37%	1800
	21st to end	Day	22% 10% 4 6% 5% 3 4 4 5% 5% 31%	1908
November	1st to 10th	Day	22% 9% 4 6% 6% 2 5% 4 5% 5% 32%	1662
	11th to 20th	Day	21% 15% 6% 7% 5% 2 3 4 4 7% 26%	1520
	21st to end	Day	20% 16% 6% 10% 7% 3 3 4 4 7% 20%	1520
December	1st to 10th	Day	28% 18% 9% 9% 5% 2 4 3 4 4 15%	1520
	11th to 20th	Day	24% 17% 9% 13% 8% 3 4 3 3 3 12%	1828
	21st to end	Day	25% 15% 10% 15% 6% 3 3 3 3 4 15%	2431
January	1st to 10th	Night	23% 20% 8% 12% 8% 3 3 3 3 4 12%	2786
	11th to 20th	Night	22% 18% 10% 14% 9% 3 3 2 4 4 11%	2548
	21st to end	Night	17% 16% 10% 18% 11% 3 3 3 3 4 11%	2574
February	1st to 10th	Night	22% 15% 7% 16% 10% 3 4 3 3 4 12%	2340
	11th to 20th	Night	22% 15% 7% 15% 11% 3 4 3 4 3 15%	2273
	21st to end	Night	15% 14% 8% 11% 9% 5% 4 5% 4 6% 20%	1794
March	1st to 10th	Night	17% 16% 5% 10% 8% 3 3 4 3 5% 25%	2039
	11th to 20th	Night	21% 13% 5% 9% 7% 3 5% 3 4 6% 24%	1979
	21st to end	Night	20% 10% 5% 7% 6% 2 3 4 4 5% 32%	2178
April	1st to 10th	Night	24% 11% 4 6% 6% 3 4 3 5% 6% 27%	1892
	11th to 20th	Night	23% 9% 2 6% 5% 2 5% 4 4 7% 32%	1598
	21st to end	Night	25% 9% 3 5% 5% 3 5% 4 5% 7% 30%	1620
May	1st to 10th	Night	22% 10% 4 6% 5% 3 4 4 4 7% 31%	1620
	11th to 20th	Night	25% 9% 3 5% 5% 2 5% 4 4 6% 32%	1532
	21st to end	Night	22% 11% 3 6% 6% 2 5% 4 5% 8% 29%	1550
June	1st to 10th	Night	28% 10% 4 6% 4 3 4 4 5% 7% 25%	1321
	11th to 20th	Night	22% 8% 5% 7% 4 2 4 4 5% 7% 32%	1260
	21st to end	Night	27% 11% 4 7% 5% 2 5% 4 4 6% 26%	1260
July	1st to 10th	Night	22% 13% 5% 6% 5% 3 5% 5% 5% 7% 25%	1260
	11th to 20th	Night	23% 12% 4 6% 6% 4 4 4 5% 7% 25%	1359
	21st to end	Night	22% 11% 5% 7% 6% 4 5% 4 5% 6% 26%	1770
August	1st to 10th	Night	17% 11% 4 6% 4 3 6% 4 6% 8% 33%	1612
	11th to 20th	Night	24% 14% 4 7% 6% 2 3 3 5% 5% 26%	1596
	21st to end	Night	16% 10% 4 5% 4 3 4 4 4 6% 40%	1790
September	1st to 10th	Night	17% 8% 2 4 4 3 4 4 5% 6% 43%	1838
	11th to 20th	Night	20% 6% 2 5% 3 2 4 4 4 6% 43%	1980
	21st to end	Night	21% 8% 3 5% 4 2 3 4 4 6% 40%	1980
October	1st to 10th	Night	21% 6% 3 5% 5% 3 4 4 4 6% 40%	2123
	11th to 20th	Night	17% 8% 3 6% 5% 3 4 4 5% 7% 37%	2160
	21st to end	Night	26% 12% 3 7% 5% 2 4 2 4 5% 31%	2443
November	1st to 10th	Night	22% 11% 4 6% 6% 3 4 4 4 6% 29%	2413
	11th to 20th	Night	21% 16% 5% 8% 5% 3 4 4 5% 7% 22%	2660
	21st to end	Night	21% 18% 6% 9% 6% 3 3 3 4 6% 22%	2660
December	1st to 10th	Night	29% 18% 8% 11% 5% 2 3 3 3 4 13%	2660
	11th to 20th	Night	24% 19% 7% 14% 7% 2 3 3 3 4 13%	3366
	21st to end	Night	25% 17% 6% 12% 6% 2 3 3 3 4 19%	5236

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	77% 5% 9% 7% 3	1377
	11th to 20th	Day	80% 4 9% 6% 2	1531
	21st to end	Day	79% 4 8% 6% 2	1782
February	1st to 10th	Day	77% 4 9% 6% 4	1620
	11th to 20th	Day	74% 5% 8% 10% 3	1581
	21st to end	Day	64% 5% 9% 14% 5% 1	1461
March	1st to 10th	Day	62% 4 11% 15% 7% 1	1911
	11th to 20th	Day	56% 4 11% 16% 11% 2	1980
	21st to end	Day	51% 4 11% 20% 11% 2	2178
April	1st to 10th	Day	53% 4 10% 17% 15% 1	2065
	11th to 20th	Day	45% 5% 11% 24% 15% 2	2312
	21st to end	Day	50% 4 11% 19% 14% 2	2340
May	1st to 10th	Day	53% 5% 9% 20% 12% 1	2340
	11th to 20th	Day	45% 6% 11% 22% 14% 2	2362
	21st to end	Day	51% 7% 11% 19% 11% 1	2724
June	1st to 10th	Day	53% 6% 9% 21% 9% 1	2653
	11th to 20th	Day	51% 6% 9% 19% 14% 2	2700
	21st to end	Day	56% 7% 10% 17% 9% 1	2700
July	1st to 10th	Day	51% 8% 11% 19% 10% 1	2625
	11th to 20th	Day	54% 8% 10% 18% 9% 1	2497
	21st to end	Day	52% 10% 11% 20% 7% 1	2406
August	1st to 10th	Day	46% 9% 12% 21% 11%	2314
	11th to 20th	Day	53% 8% 11% 18% 10%	2304
	21st to end	Day	40% 8% 11% 23% 17% 1	2566
September	1st to 10th	Day	41% 7% 10% 23% 19% 1	2120
	11th to 20th	Day	41% 5% 10% 25% 16% 2	1980
	21st to end	Day	43% 5% 7% 25% 17% 2	1980
October	1st to 10th	Day	44% 5% 11% 27% 12% 1	1836
	11th to 20th	Day	46% 7% 14% 24% 9%	1800
	21st to end	Day	56% 5% 10% 18% 11% 1	1908
November	1st to 10th	Day	55% 5% 12% 19% 8% 1	1662
	11th to 20th	Day	60% 4 12% 16% 7% 1	1520
	21st to end	Day	66% 7% 13% 12% 3	1520
December	1st to 10th	Day	76% 4 8% 9% 3	1520
	11th to 20th	Day	78% 5% 7% 7% 3	1827
	21st to end	Day	75% 6% 12% 5% 2	2431
January	1st to 10th	Night	79% 3 8% 8% 2	2786
	11th to 20th	Night	80% 5% 7% 6% 2	2548
	21st to end	Night	80% 4 7% 8% 2	2574
February	1st to 10th	Night	78% 3 8% 7% 4	2340
	11th to 20th	Night	76% 3 7% 9% 4 1	2276
	21st to end	Night	66% 6% 10% 11% 6% 1	1794
March	1st to 10th	Night	64% 4 9% 13% 8% 1	2048
	11th to 20th	Night	63% 4 12% 11% 10% 1	1980
	21st to end	Night	55% 5% 10% 18% 10% 2	2178
April	1st to 10th	Night	59% 4 9% 14% 12% 3	1892
	11th to 20th	Night	51% 5% 11% 20% 12% 1	1598
	21st to end	Night	55% 5% 10% 17% 12% 2	1620
May	1st to 10th	Night	54% 5% 10% 20% 12% 1	1620
	11th to 20th	Night	54% 6% 6% 21% 12% 1	1533
	21st to end	Night	55% 6% 11% 17% 11% 1	1551
June	1st to 10th	Night	59% 6% 9% 19% 7% 1	1321
	11th to 20th	Night	51% 6% 11% 18% 11% 2	1260
	21st to end	Night	60% 6% 8% 16% 8% 3	1260
July	1st to 10th	Night	56% 8% 10% 18% 9%	1225
	11th to 20th	Night	59% 6% 10% 15% 10% 1	1317
	21st to end	Night	56% 8% 10% 17% 8% 1	1672
August	1st to 10th	Night	48% 7% 12% 24% 9%	1600
	11th to 20th	Night	59% 7% 9% 17% 8%	1596
	21st to end	Night	46% 5% 11% 23% 14% 1	1790
September	1st to 10th	Night	41% 6% 10% 26% 15% 1	1838
	11th to 20th	Night	42% 4 9% 27% 16% 2	1980
	21st to end	Night	45% 4 9% 22% 19% 1	1980
October	1st to 10th	Night	45% 5% 11% 23% 13% 2	2123
	11th to 20th	Night	47% 5% 14% 24% 10%	2160
	21st to end	Night	59% 4 10% 17% 9% 1	2443
November	1st to 10th	Night	56% 6% 12% 17% 9%	2413
	11th to 20th	Night	63% 6% 11% 14% 6%	2660
	21st to end	Night	66% 6% 13% 12% 4	2660
December	1st to 10th	Night	77% 5% 8% 7% 3	2660
	11th to 20th	Night	76% 5% 8% 8% 2	3368
	21st to end	Night	72% 5% 10% 11% 2	5236

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	31% 20% 32% 12% 5%	180
	11th to 20th	Day	32% 19% 31% 15% 2	221
	21st to end	Day	22% 20% 37% 19% 2	374
February	1st to 10th	Day	30% 20% 33% 14% 3	340
	11th to 20th	Day	28% 22% 31% 17% 2	336
	21st to end	Day	17% 22% 36% 22% 3	338
March	1st to 10th	Day	22% 20% 36% 20% 3	420
	11th to 20th	Day	23% 20% 30% 20% 7%	420
	21st to end	Day	24% 15% 31% 20% 8% 1	462
April	1st to 10th	Day	30% 14% 28% 22% 5% 1	420
	11th to 20th	Day	25% 11% 32% 25% 7%	412
	21st to end	Day	26% 11% 26% 24% 11% 2	420
May	1st to 10th	Day	24% 8% 34% 25% 7% 1	423
	11th to 20th	Day	22% 4 29% 31% 12% 1	504
	21st to end	Day	31% 3 22% 30% 14% 2	680
June	1st to 10th	Day	28% 4 22% 30% 14% 2	630
	11th to 20th	Day	24% 3 21% 35% 16% 2	630
	21st to end	Day	31% 3 18% 32% 13% 3	630
July	1st to 10th	Day	27% 3 15% 36% 16% 2	620
	11th to 20th	Day	27% 4 14% 34% 18% 3	529
	21st to end	Day	24% 2 15% 36% 20% 2	488
August	1st to 10th	Day	19% 2 14% 38% 25% 3	446
	11th to 20th	Day	26% 2 16% 33% 19% 5%	439
	21st to end	Day	20% 3 17% 32% 22% 5%	495
September	1st to 10th	Day	20% 2 20% 37% 18% 3	450
	11th to 20th	Day	23% 7% 24% 27% 14% 4	450
	21st to end	Day	21% 6% 26% 29% 14% 5%	450
October	1st to 10th	Day	21% 8% 22% 29% 17% 2	378
	11th to 20th	Day	22% 9% 28% 26% 12% 2	360
	21st to end	Day	28% 12% 29% 22% 8% 1	396
November	1st to 10th	Day	25% 15% 26% 24% 9% 1	365
	11th to 20th	Day	28% 22% 26% 18% 5%	370
	21st to end	Day	31% 19% 30% 16% 4 1	370
December	1st to 10th	Day	35% 16% 32% 14% 3	370
	11th to 20th	Day	34% 24% 28% 12% 1	318
	21st to end	Day	28% 27% 31% 9% 3 2	198
January	1st to 10th	Night	33% 24% 26% 14% 3	826
	11th to 20th	Night	33% 22% 26% 16% 3	744
	21st to end	Night	24% 23% 33% 16% 3	792
February	1st to 10th	Night	31% 25% 28% 15% 2	720
	11th to 20th	Night	30% 26% 27% 14% 3 1	703
	21st to end	Night	22% 22% 31% 21% 5%	592
March	1st to 10th	Night	26% 18% 36% 18% 3	720
	11th to 20th	Night	26% 17% 28% 22% 7%	720
	21st to end	Night	26% 17% 26% 24% 7% 2	792
April	1st to 10th	Night	29% 15% 29% 21% 6%	648
	11th to 20th	Night	26% 8% 31% 27% 7% 1	534
	21st to end	Night	30% 5% 27% 26% 10% 2	540
May	1st to 10th	Night	27% 6% 24% 31% 10% 2	540
	11th to 20th	Night	30% 4 24% 32% 10% 1	532
	21st to end	Night	27% 5% 21% 32% 13% 2	583
June	1st to 10th	Night	30% 4 21% 28% 17%	421
	11th to 20th	Night	26% 4 18% 32% 18% 2	360
	21st to end	Night	30% 2 19% 34% 13% 1	360
July	1st to 10th	Night	27% 5% 15% 34% 18% 2	350
	11th to 20th	Night	28% 4 15% 30% 20% 3	361
	21st to end	Night	25% 3 12% 39% 20% 1	590
August	1st to 10th	Night	21% 4 14% 35% 24% 2	538
	11th to 20th	Night	31% 1 16% 28% 21% 2	532
	21st to end	Night	19% 3 13% 37% 25% 3	594
September	1st to 10th	Night	20% 3 17% 37% 20% 2	578
	11th to 20th	Night	23% 4 20% 27% 21% 4	720
	21st to end	Night	25% 4 21% 28% 17% 4	720
October	1st to 10th	Night	26% 6% 20% 29% 17% 2	720
	11th to 20th	Night	20% 10% 30% 25% 12% 3	720
	21st to end	Night	33% 13% 25% 22% 6%	792
November	1st to 10th	Night	28% 15% 26% 22% 9% 1	744
	11th to 20th	Night	28% 19% 25% 19% 8%	850
	21st to end	Night	30% 20% 25% 18% 6% 1	850
December	1st to 10th	Night	38% 17% 26% 15% 4	850
	11th to 20th	Night	32% 26% 26% 13% 2	850
	21st to end	Night	31% 23% 26% 16% 3 1	935

Page generated at 2024-01-13 10:37:56. Version ac07

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