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 2024-12-09.

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	21% 12% 4 9% 5% 4 4 3 4 6% 29%	101764
	Night	22% 14% 5% 10% 5% 4 3 3 4 5% 25%	98048
Transparency	Day	55% 6% 11% 17% 10% 1	105338
	Night	61% 5% 10% 16% 8% 1	102564
Seeing	Day	25% 10% 25% 26% 12% 2	16597
	Night	28% 13% 24% 24% 10% 1	24561

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% 16% 7% 15% 7% 4 4 3 4 4 14%	2280
	11th to 20th	Day	19% 15% 7% 16% 8% 4 3 2 3 4 19%	2613
	21st to end	Day	20% 16% 7% 19% 8% 5% 3 2 3 3 14%	3124
February	1st to 10th	Day	21% 18% 7% 16% 8% 4 4 3 3 5% 11%	1710
	11th to 20th	Day	18% 14% 5% 17% 11% 3 3 3 3 4 17%	1663
	21st to end	Day	13% 16% 7% 12% 9% 5% 4 3 5% 6% 23%	1550
March	1st to 10th	Day	26% 14% 4 9% 5% 3 3 2 3 5% 24%	3324
	11th to 20th	Day	17% 14% 4 9% 5% 4 4 3 5% 7% 28%	3480
	21st to end	Day	29% 10% 4 8% 5% 4 4 2 4 5% 25%	3816
April	1st to 10th	Day	21% 12% 3 6% 4 4 4 3 4 5% 33%	2180
	11th to 20th	Day	21% 9% 2 5% 3 3 3 3 4 6% 42%	2442
	21st to end	Day	22% 9% 2 6% 4 3 4 4 5% 6% 36%	2470
May	1st to 10th	Day	19% 10% 4 7% 5% 4 4 4 6% 7% 31%	4106
	11th to 20th	Day	18% 7% 2 7% 4 4 5% 4 6% 8% 35%	4194
	21st to end	Day	19% 11% 4 8% 4 3 4 3 4 7% 33%	4836
June	1st to 10th	Day	24% 10% 3 6% 4 3 4 3 5% 6% 32%	2801
	11th to 20th	Day	21% 8% 4 7% 4 4 4 3 5% 6% 35%	2850
	21st to end	Day	25% 11% 3 7% 5% 4 3 3 5% 5% 28%	2850
July	1st to 10th	Day	25% 15% 6% 7% 3 3 3 3 4 6% 25%	4760
	11th to 20th	Day	24% 15% 4 8% 5% 3 3 3 5% 6% 24%	4525
	21st to end	Day	17% 10% 3 9% 5% 4 4 3 5% 8% 32%	4467
August	1st to 10th	Day	26% 15% 3 9% 3 3 3 3 4 5% 25%	4105
	11th to 20th	Day	16% 12% 4 9% 4 4 5% 4 5% 6% 31%	4045
	21st to end	Day	12% 8% 3 5% 3 4 3 3 4 6% 48%	4510
September	1st to 10th	Day	16% 8% 3 6% 4 3 3 3 4 6% 45%	2237
	11th to 20th	Day	15% 9% 3 6% 3 3 4 3 3 6% 46%	2090
	21st to end	Day	19% 8% 3 6% 3 3 3 2 4 7% 42%	2090
October	1st to 10th	Day	16% 8% 3 6% 4 3 4 3 5% 8% 40%	1938
	11th to 20th	Day	18% 8% 2 6% 4 4 4 3 5% 7% 38%	1900
	21st to end	Day	21% 11% 4 7% 5% 5% 3 3 5% 5% 33%	2014
November	1st to 10th	Day	22% 10% 3 7% 5% 4 5% 3 5% 5% 32%	1751
	11th to 20th	Day	21% 17% 5% 8% 4 3 3 3 5% 6% 26%	1600
	21st to end	Day	20% 18% 6% 11% 6% 4 3 3 4 6% 19%	1600
December	1st to 10th	Day	27% 21% 7% 10% 4 3 3 3 3 4 14%	1584
	11th to 20th	Day	24% 19% 7% 15% 6% 4 4 2 4 3 12%	1828
	21st to end	Day	25% 15% 9% 15% 5% 3 3 2 3 4 15%	2431
January	1st to 10th	Night	19% 20% 7% 19% 6% 5% 3 2 2 3 14%	4670
	11th to 20th	Night	18% 18% 8% 17% 7% 4 3 2 4 4 14%	4344
	21st to end	Night	21% 16% 9% 20% 7% 4 4 2 4 4 10%	4532
February	1st to 10th	Night	22% 19% 6% 16% 8% 4 3 2 3 4 13%	2470
	11th to 20th	Night	20% 16% 6% 16% 9% 4 3 2 4 3 16%	2403
	21st to end	Night	15% 15% 6% 13% 7% 6% 3 4 5% 6% 20%	1903
March	1st to 10th	Night	24% 15% 4 9% 8% 4 3 3 3 5% 21%	3515
	11th to 20th	Night	18% 16% 4 9% 5% 5% 5% 4 5% 7% 23%	3479
	21st to end	Night	31% 11% 4 9% 5% 3 3 2 3 4 25%	3816
April	1st to 10th	Night	24% 12% 3 6% 5% 4 3 3 5% 6% 28%	1997
	11th to 20th	Night	24% 9% 2 7% 4 3 5% 3 5% 7% 32%	1688
	21st to end	Night	24% 9% 3 6% 5% 4 4 3 5% 7% 31%	1710
May	1st to 10th	Night	21% 12% 4 6% 4 3 4 3 5% 7% 32%	2834
	11th to 20th	Night	20% 9% 3 8% 5% 5% 5% 4 5% 7% 29%	2700
	21st to end	Night	18% 13% 3 8% 4 4 4 3 6% 8% 30%	2749
June	1st to 10th	Night	27% 11% 3 7% 3 4 4 3 5% 7% 25%	1394
	11th to 20th	Night	21% 9% 3 8% 4 3 3 3 5% 7% 33%	1330
	21st to end	Night	28% 11% 3 7% 4 4 5% 3 4 5% 25%	1330
July	1st to 10th	Night	30% 16% 4 5% 3 3 3 4 5% 6% 21%	2210
	11th to 20th	Night	29% 15% 3 6% 4 4 3 3 5% 6% 24%	2388
	21st to end	Night	16% 11% 4 9% 5% 5% 4 3 5% 7% 31%	3087
August	1st to 10th	Night	27% 15% 4 7% 3 4 3 2 4 6% 25%	2825
	11th to 20th	Night	20% 14% 5% 7% 5% 3 4 3 6% 6% 27%	2782
	21st to end	Night	12% 11% 4 6% 3 3 3 3 4 6% 45%	3146
September	1st to 10th	Night	17% 8% 2 5% 3 4 4 3 5% 5% 43%	1941
	11th to 20th	Night	19% 7% 2 6% 3 3 3 3 4 6% 44%	2090
	21st to end	Night	22% 8% 2 6% 3 3 4 3 4 6% 39%	2090
October	1st to 10th	Night	20% 7% 2 5% 4 4 3 3 4 7% 40%	2241
	11th to 20th	Night	16% 9% 3 7% 4 5% 3 3 5% 7% 38%	2280
	21st to end	Night	24% 13% 2 8% 4 3 3 2 4 5% 32%	2579
November	1st to 10th	Night	21% 12% 3 7% 5% 4 4 3 5% 6% 31%	2544
	11th to 20th	Night	20% 18% 5% 8% 5% 4 4 4 5% 6% 22%	2800
	21st to end	Night	21% 19% 5% 9% 5% 4 3 3 5% 5% 21%	2800
December	1st to 10th	Night	28% 20% 7% 11% 5% 3 3 2 4 4 13%	2779
	11th to 20th	Night	23% 21% 6% 16% 5% 3 3 2 3 4 13%	3366
	21st to end	Night	24% 18% 6% 13% 5% 3 3 3 3 4 18%	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	73% 6% 13% 6% 2	2281
	11th to 20th	Day	71% 4 9% 10% 5%	2613
	21st to end	Day	77% 4 12% 5% 2	3124
February	1st to 10th	Day	78% 4 9% 6% 4	1710
	11th to 20th	Day	73% 5% 8% 10% 3	1671
	21st to end	Day	63% 5% 9% 15% 5% 1	1550
March	1st to 10th	Day	64% 4 10% 16% 5% 2	3328
	11th to 20th	Day	56% 7% 13% 16% 7% 1	3480
	21st to end	Day	64% 3 8% 15% 9% 1	3816
April	1st to 10th	Day	53% 4 10% 17% 15% 1	2180
	11th to 20th	Day	45% 4 10% 24% 15% 1	2442
	21st to end	Day	49% 4 11% 19% 16% 2	2470
May	1st to 10th	Day	49% 10% 15% 18% 7% 1	4120
	11th to 20th	Day	45% 8% 17% 20% 9% 1	4198
	21st to end	Day	48% 10% 12% 15% 14% 1	4836
June	1st to 10th	Day	52% 7% 10% 21% 9% 1	2801
	11th to 20th	Day	50% 6% 10% 19% 14% 2	2850
	21st to end	Day	57% 7% 9% 16% 9% 2	2850
July	1st to 10th	Day	60% 6% 7% 15% 11% 1	4685
	11th to 20th	Day	59% 8% 9% 16% 8%	4445
	21st to end	Day	48% 8% 13% 20% 10% 1	4325
August	1st to 10th	Day	63% 6% 8% 14% 8%	4092
	11th to 20th	Day	51% 7% 16% 18% 7%	4084
	21st to end	Day	35% 7% 10% 24% 23% 1	4510
September	1st to 10th	Day	41% 7% 10% 23% 19% 1	2237
	11th to 20th	Day	40% 5% 11% 25% 17% 2	2090
	21st to end	Day	43% 5% 8% 25% 16% 2	2090
October	1st to 10th	Day	43% 6% 10% 31% 10% 1	3224
	11th to 20th	Day	38% 4 12% 29% 17%	3140
	21st to end	Day	42% 6% 13% 23% 14% 2	3303
November	1st to 10th	Day	55% 5% 12% 19% 8% 1	1751
	11th to 20th	Day	61% 4 12% 16% 7% 1	1600
	21st to end	Day	67% 7% 13% 11% 3	1600
December	1st to 10th	Day	76% 4 8% 9% 3	1584
	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	77% 4 10% 8% 2	4670
	11th to 20th	Night	77% 4 7% 9% 3	4344
	21st to end	Night	79% 5% 7% 7% 1	4532
February	1st to 10th	Night	78% 3 8% 7% 4	2470
	11th to 20th	Night	75% 3 7% 10% 4	2406
	21st to end	Night	65% 6% 10% 13% 6% 1	1903
March	1st to 10th	Night	68% 3 9% 12% 7% 1	3524
	11th to 20th	Night	59% 7% 14% 12% 7% 1	3480
	21st to end	Night	66% 3 8% 14% 8% 2	3816
April	1st to 10th	Night	58% 4 8% 15% 12% 2	1997
	11th to 20th	Night	51% 5% 10% 20% 11% 1	1688
	21st to end	Night	53% 5% 10% 17% 14% 2	1710
May	1st to 10th	Night	52% 8% 13% 19% 7% 1	2840
	11th to 20th	Night	52% 8% 14% 18% 7% 1	2701
	21st to end	Night	53% 7% 13% 20% 7% 1	2750
June	1st to 10th	Night	58% 6% 10% 19% 7% 1	1394
	11th to 20th	Night	50% 6% 12% 19% 11% 2	1330
	21st to end	Night	61% 5% 8% 15% 8% 3	1330
July	1st to 10th	Night	63% 6% 7% 13% 11%	2175
	11th to 20th	Night	62% 5% 8% 14% 10%	2346
	21st to end	Night	51% 8% 11% 18% 11% 2	2989
August	1st to 10th	Night	61% 5% 8% 19% 6%	2813
	11th to 20th	Night	56% 6% 14% 17% 6%	2809
	21st to end	Night	41% 5% 10% 26% 17% 1	3145
September	1st to 10th	Night	41% 6% 9% 26% 16% 1	1941
	11th to 20th	Night	42% 4 10% 27% 16% 1	2090
	21st to end	Night	46% 4 9% 21% 18% 1	2090
October	1st to 10th	Night	44% 5% 14% 23% 13% 1	3735
	11th to 20th	Night	39% 3 13% 25% 20% 1	3779
	21st to end	Night	47% 3 10% 25% 13% 2	4240
November	1st to 10th	Night	54% 5% 13% 19% 9%	2544
	11th to 20th	Night	64% 5% 11% 13% 6%	2800
	21st to end	Night	66% 5% 13% 12% 4	2800
December	1st to 10th	Night	77% 5% 8% 8% 3	2779
	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	31% 21% 30% 15% 3	234
	21st to end	Day	23% 19% 35% 20% 3	407
February	1st to 10th	Day	30% 19% 32% 15% 5%	370
	11th to 20th	Day	27% 22% 32% 17% 2	366
	21st to end	Day	18% 22% 34% 23% 4	373
March	1st to 10th	Day	23% 20% 34% 19% 4	460
	11th to 20th	Day	24% 20% 29% 20% 7%	460
	21st to end	Day	25% 16% 31% 19% 8% 1	506
April	1st to 10th	Day	31% 13% 28% 23% 4 1	460
	11th to 20th	Day	25% 11% 33% 25% 7%	452
	21st to end	Day	24% 11% 26% 27% 10% 2	460
May	1st to 10th	Day	24% 9% 33% 26% 7% 1	463
	11th to 20th	Day	21% 4 28% 31% 15% 1	548
	21st to end	Day	29% 3 22% 32% 13% 1	735
June	1st to 10th	Day	27% 4 23% 30% 14% 1	680
	11th to 20th	Day	23% 3 20% 35% 17% 2	680
	21st to end	Day	32% 3 18% 32% 12% 2	680
July	1st to 10th	Day	28% 3 15% 35% 17% 2	670
	11th to 20th	Day	28% 4 13% 34% 18% 3	573
	21st to end	Day	23% 2 15% 36% 21% 2	532
August	1st to 10th	Day	22% 2 13% 36% 25% 3	486
	11th to 20th	Day	24% 1 15% 33% 21% 5%	479
	21st to end	Day	19% 3 17% 32% 22% 7%	539
September	1st to 10th	Day	19% 3 21% 36% 18% 3	490
	11th to 20th	Day	23% 6% 22% 27% 17% 6%	490
	21st to end	Day	21% 5% 25% 29% 16% 5%	490
October	1st to 10th	Day	20% 8% 22% 29% 19% 2	410
	11th to 20th	Day	21% 10% 28% 27% 12% 2	390
	21st to end	Day	26% 11% 29% 23% 9% 1	429
November	1st to 10th	Day	24% 15% 27% 24% 9% 1	395
	11th to 20th	Day	29% 22% 28% 17% 5%	400
	21st to end	Day	32% 19% 30% 15% 4 1	400
December	1st to 10th	Day	34% 16% 31% 15% 4	394
	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% 23% 25% 16% 3	760
	21st to end	Night	24% 22% 33% 17% 3	836
February	1st to 10th	Night	31% 24% 27% 15% 3	760
	11th to 20th	Night	29% 26% 28% 14% 3 1	743
	21st to end	Night	22% 21% 31% 21% 5%	628
March	1st to 10th	Night	26% 18% 35% 18% 3	760
	11th to 20th	Night	25% 17% 28% 23% 7%	760
	21st to end	Night	27% 17% 25% 23% 7% 1	836
April	1st to 10th	Night	29% 15% 29% 22% 6%	684
	11th to 20th	Night	27% 8% 31% 27% 7% 1	564
	21st to end	Night	29% 5% 27% 28% 10% 2	570
May	1st to 10th	Night	27% 6% 25% 31% 9% 2	570
	11th to 20th	Night	28% 4 23% 32% 11% 1	562
	21st to end	Night	26% 6% 21% 32% 13% 2	616
June	1st to 10th	Night	30% 4 21% 28% 17%	444
	11th to 20th	Night	25% 4 18% 33% 18% 2	380
	21st to end	Night	32% 2 18% 34% 13% 1	380
July	1st to 10th	Night	28% 5% 14% 34% 17% 2	370
	11th to 20th	Night	28% 4 15% 31% 20% 3	382
	21st to end	Night	25% 3 12% 39% 20% 1	623
August	1st to 10th	Night	22% 4 14% 34% 24% 2	568
	11th to 20th	Night	30% 1 16% 28% 22% 2	562
	21st to end	Night	19% 3 13% 36% 26% 3	627
September	1st to 10th	Night	20% 3 18% 37% 20% 2	611
	11th to 20th	Night	23% 4 19% 26% 23% 4	760
	21st to end	Night	26% 4 21% 28% 18% 4	760
October	1st to 10th	Night	25% 6% 19% 30% 18% 3	760
	11th to 20th	Night	20% 10% 30% 24% 13% 3	760
	21st to end	Night	32% 13% 25% 23% 7%	836
November	1st to 10th	Night	27% 15% 27% 22% 9% 1	785
	11th to 20th	Night	29% 19% 26% 19% 8%	900
	21st to end	Night	31% 20% 25% 17% 6% 1	900
December	1st to 10th	Night	37% 16% 26% 16% 4	893
	11th to 20th	Night	32% 26% 26% 13% 2	850
	21st to end	Night	31% 23% 26% 16% 3 1	935

Page generated at 2024-12-06 04:54:48. Version ac07

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