North Branch School
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the North Branch School clear sky chart from 2005-11-06 to 2026-05-02.

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	18% 12% 4 9% 5% 5% 4 4 5% 6% 29%	131587
	Night	18% 14% 5% 11% 6% 5% 4 3 4 5% 26%	130908
Transparency	Day	55% 6% 11% 18% 9% 1	135068
	Night	59% 6% 10% 16% 8% 1	135514
Seeing	Day	29% 20% 28% 15% 6% 2	16567
	Night	26% 18% 26% 21% 8% 1	25660

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	12% 16% 6% 18% 8% 5% 5% 4 4 4 17%	4661
	11th to 20th	Day	9% 16% 6% 20% 10% 6% 4 3 3 4 20%	4650
	21st to end	Day	10% 15% 7% 12% 8% 7% 6% 3 5% 4 23%	5124
February	1st to 10th	Day	14% 14% 5% 12% 8% 6% 4 4 5% 5% 23%	1859
	11th to 20th	Day	12% 15% 5% 13% 9% 5% 4 3 5% 4 24%	2029
	21st to end	Day	16% 16% 5% 12% 6% 4 4 3 4 5% 25%	1730
March	1st to 10th	Day	22% 15% 6% 11% 5% 4 4 3 4 4 23%	4576
	11th to 20th	Day	14% 17% 5% 9% 5% 4 4 4 5% 6% 28%	4783
	21st to end	Day	22% 13% 5% 10% 4 4 4 3 4 4 28%	6102
April	1st to 10th	Day	23% 16% 3 8% 4 3 3 4 4 5% 29%	2520
	11th to 20th	Day	19% 14% 3 7% 5% 4 4 3 5% 5% 32%	2494
	21st to end	Day	23% 12% 4 7% 4 3 3 3 5% 5% 30%	2688
May	1st to 10th	Day	33% 14% 3 8% 4 4 4 3 4 4 20%	6027
	11th to 20th	Day	19% 11% 3 8% 6% 5% 5% 4 6% 7% 27%	6318
	21st to end	Day	20% 10% 3 7% 5% 5% 4 4 6% 7% 29%	6949
June	1st to 10th	Day	23% 8% 3 7% 5% 5% 5% 4 5% 7% 28%	2800
	11th to 20th	Day	19% 7% 3 7% 5% 5% 5% 4 6% 8% 32%	2800
	21st to end	Day	21% 9% 3 7% 6% 5% 4 4 6% 7% 29%	2800
July	1st to 10th	Day	14% 7% 2 7% 4 5% 6% 5% 7% 10% 33%	6360
	11th to 20th	Day	15% 7% 2 6% 5% 5% 5% 4 7% 10% 33%	6360
	21st to end	Day	16% 6% 2 6% 4 4 5% 4 6% 8% 38%	6926
August	1st to 10th	Day	18% 8% 2 7% 4 5% 5% 4 6% 8% 32%	4543
	11th to 20th	Day	16% 9% 3 8% 5% 4 5% 4 6% 8% 32%	3995
	21st to end	Day	15% 6% 2 5% 4 4 4 4 6% 9% 40%	4312
September	1st to 10th	Day	15% 6% 2 4 4 4 4 4 6% 7% 45%	2400
	11th to 20th	Day	15% 6% 2 5% 3 3 4 3 4 7% 47%	2280
	21st to end	Day	18% 8% 2 6% 4 4 4 4 6% 7% 38%	2200
October	1st to 10th	Day	23% 10% 3 7% 4 4 3 3 4 5% 34%	2125
	11th to 20th	Day	18% 12% 4 7% 4 3 4 3 4 7% 34%	1949
	21st to end	Day	26% 13% 3 8% 4 3 4 3 4 5% 26%	1980
November	1st to 10th	Day	17% 14% 4 8% 5% 4 3 3 5% 6% 33%	1851
	11th to 20th	Day	17% 18% 4 8% 5% 5% 3 3 5% 5% 28%	1890
	21st to end	Day	19% 15% 5% 10% 5% 5% 5% 4 4 6% 23%	1712
December	1st to 10th	Day	16% 21% 6% 14% 7% 5% 4 4 5% 4 15%	2680
	11th to 20th	Day	20% 18% 5% 11% 4 4 4 2 4 5% 22%	3066
	21st to end	Day	18% 20% 5% 10% 5% 4 5% 3 3 4 21%	4048
January	1st to 10th	Night	12% 17% 8% 19% 7% 4 4 2 4 4 18%	8234
	11th to 20th	Night	10% 19% 9% 21% 10% 5% 4 3 3 3 12%	8235
	21st to end	Night	12% 13% 5% 15% 9% 9% 6% 4 5% 5% 17%	9057
February	1st to 10th	Night	15% 15% 5% 12% 8% 6% 5% 3 5% 5% 21%	2758
	11th to 20th	Night	12% 15% 5% 14% 9% 5% 5% 3 4 4 22%	2475
	21st to end	Night	14% 16% 5% 12% 6% 5% 4 3 4 4 26%	2076
March	1st to 10th	Night	21% 14% 5% 10% 6% 5% 4 3 4 4 23%	5484
	11th to 20th	Night	15% 17% 5% 9% 5% 4 4 3 4 6% 29%	5391
	21st to end	Night	21% 17% 5% 10% 5% 4 3 2 3 4 26%	5072
April	1st to 10th	Night	22% 16% 3 9% 4 3 4 2 4 5% 28%	2100
	11th to 20th	Night	23% 14% 3 7% 5% 3 4 3 4 4 29%	2075
	21st to end	Night	22% 11% 4 7% 4 3 3 4 4 5% 33%	1931
May	1st to 10th	Night	32% 13% 3 6% 4 4 4 4 4 5% 20%	3980
	11th to 20th	Night	20% 13% 3 9% 5% 4 4 3 5% 6% 28%	3575
	21st to end	Night	22% 11% 3 6% 4 5% 4 4 6% 6% 29%	3926
June	1st to 10th	Night	23% 8% 3 7% 5% 4 4 3 6% 6% 32%	1600
	11th to 20th	Night	21% 7% 3 5% 4 5% 3 4 5% 8% 36%	1600
	21st to end	Night	21% 8% 3 7% 5% 5% 4 4 6% 7% 32%	1600
July	1st to 10th	Night	14% 7% 3 7% 5% 5% 6% 5% 7% 9% 33%	3600
	11th to 20th	Night	19% 10% 3 7% 4 5% 4 4 5% 6% 34%	3600
	21st to end	Night	12% 8% 3 7% 5% 4 4 4 5% 7% 41%	3926
August	1st to 10th	Night	22% 9% 2 6% 4 4 4 3 4 6% 36%	2586
	11th to 20th	Night	17% 8% 3 7% 5% 4 4 3 6% 6% 37%	3023
	21st to end	Night	14% 7% 2 5% 4 4 4 4 6% 8% 42%	3586
September	1st to 10th	Night	17% 6% 2 6% 3 4 3 3 5% 7% 45%	2000
	11th to 20th	Night	14% 6% 2 5% 3 4 4 2 5% 5% 50%	2120
	21st to end	Night	18% 7% 2 5% 3 5% 4 4 6% 5% 41%	2200
October	1st to 10th	Night	23% 10% 3 8% 4 3 4 3 5% 6% 34%	2270
	11th to 20th	Night	20% 13% 3 7% 4 4 5% 3 4 6% 31%	2450
	21st to end	Night	25% 14% 3 8% 5% 4 4 3 4 5% 25%	2860
November	1st to 10th	Night	21% 14% 4 7% 5% 4 5% 3 4 4 30%	2667
	11th to 20th	Night	19% 16% 4 8% 4 4 4 3 4 5% 29%	2730
	21st to end	Night	18% 17% 5% 11% 4 4 4 3 5% 5% 24%	2903
December	1st to 10th	Night	17% 21% 9% 18% 7% 4 4 3 3 3 12%	4720
	11th to 20th	Night	22% 20% 4 10% 5% 4 3 3 4 4 22%	5348
	21st to end	Night	20% 19% 6% 11% 5% 5% 4 3 3 4 20%	7150

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	69% 5% 10% 13% 4	4662
	11th to 20th	Day	69% 5% 7% 10% 8% 2	4680
	21st to end	Day	62% 6% 10% 13% 9% 1	5124
February	1st to 10th	Day	62% 6% 9% 15% 6% 2	1859
	11th to 20th	Day	63% 5% 8% 16% 7% 1	2038
	21st to end	Day	62% 5% 9% 12% 11% 1	1730
March	1st to 10th	Day	65% 5% 9% 10% 7% 4	4579
	11th to 20th	Day	55% 6% 15% 14% 8% 3	4783
	21st to end	Day	60% 3 7% 12% 16% 2	6102
April	1st to 10th	Day	58% 4 8% 17% 10% 2	2520
	11th to 20th	Day	54% 5% 8% 17% 14% 1	2494
	21st to end	Day	57% 5% 8% 16% 11% 2	2688
May	1st to 10th	Day	68% 5% 7% 14% 6% 1	6038
	11th to 20th	Day	55% 8% 13% 15% 9% 1	6322
	21st to end	Day	52% 8% 10% 21% 8% 1	6949
June	1st to 10th	Day	54% 7% 13% 16% 9% 1	2800
	11th to 20th	Day	49% 7% 11% 20% 12% 1	2800
	21st to end	Day	54% 7% 11% 18% 9% 1	2800
July	1st to 10th	Day	44% 8% 16% 23% 10%	6290
	11th to 20th	Day	45% 8% 13% 25% 9%	6285
	21st to end	Day	43% 7% 12% 31% 8%	6728
August	1st to 10th	Day	48% 9% 14% 21% 9%	4529
	11th to 20th	Day	50% 8% 15% 22% 5%	4034
	21st to end	Day	40% 8% 17% 22% 12% 1	4312
September	1st to 10th	Day	39% 7% 11% 30% 13% 1	2400
	11th to 20th	Day	37% 5% 9% 31% 17% 1	2280
	21st to end	Day	44% 6% 14% 22% 11% 2	2200
October	1st to 10th	Day	49% 5% 8% 24% 13% 1	3463
	11th to 20th	Day	47% 4 13% 22% 10% 3	3152
	21st to end	Day	48% 5% 11% 26% 9% 1	3180
November	1st to 10th	Day	52% 6% 13% 19% 10%	1851
	11th to 20th	Day	59% 5% 11% 13% 11% 2	1890
	21st to end	Day	62% 6% 13% 14% 4 1	1712
December	1st to 10th	Day	70% 9% 8% 9% 3	2680
	11th to 20th	Day	66% 4 9% 13% 7%	3066
	21st to end	Day	66% 4 8% 11% 8% 3	4048
January	1st to 10th	Night	69% 6% 11% 9% 4	8234
	11th to 20th	Night	76% 4 8% 6% 4 1	8280
	21st to end	Night	67% 4 10% 10% 8% 1	9057
February	1st to 10th	Night	64% 6% 9% 12% 8% 1	2758
	11th to 20th	Night	63% 6% 9% 14% 8% 1	2477
	21st to end	Night	61% 5% 10% 15% 7% 2	2076
March	1st to 10th	Night	63% 4 10% 12% 10% 1	5494
	11th to 20th	Night	56% 6% 15% 13% 8% 2	5392
	21st to end	Night	62% 3 7% 12% 14% 2	5072
April	1st to 10th	Night	59% 5% 7% 16% 12% 1	2100
	11th to 20th	Night	57% 5% 9% 17% 12% 1	2075
	21st to end	Night	53% 5% 8% 18% 12% 3	1931
May	1st to 10th	Night	65% 7% 8% 13% 8% 1	3986
	11th to 20th	Night	56% 8% 10% 18% 8% 1	3576
	21st to end	Night	53% 8% 10% 16% 11% 1	3927
June	1st to 10th	Night	50% 9% 11% 18% 11% 1	1600
	11th to 20th	Night	46% 7% 14% 20% 11% 2	1600
	21st to end	Night	50% 7% 12% 22% 9% 1	1600
July	1st to 10th	Night	44% 8% 18% 20% 9%	3560
	11th to 20th	Night	49% 9% 9% 23% 10%	3552
	21st to end	Night	41% 7% 14% 30% 8%	3809
August	1st to 10th	Night	50% 6% 12% 22% 9% 1	2576
	11th to 20th	Night	46% 7% 15% 22% 9%	3050
	21st to end	Night	37% 8% 17% 25% 11% 1	3586
September	1st to 10th	Night	39% 8% 13% 28% 12% 1	2000
	11th to 20th	Night	36% 5% 13% 30% 15% 1	2120
	21st to end	Night	42% 6% 13% 23% 14% 2	2200
October	1st to 10th	Night	54% 6% 9% 21% 9% 1	3712
	11th to 20th	Night	49% 6% 11% 19% 12% 4	3986
	21st to end	Night	52% 6% 12% 22% 7%	4610
November	1st to 10th	Night	56% 5% 11% 17% 9% 2	2667
	11th to 20th	Night	56% 6% 11% 16% 9% 3	2730
	21st to end	Night	60% 6% 13% 15% 5%	2903
December	1st to 10th	Night	76% 7% 7% 9% 2	4720
	11th to 20th	Night	66% 5% 9% 9% 11%	5348
	21st to end	Night	68% 5% 7% 11% 6% 4	7150

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	29% 28% 29% 12% 2	220
	11th to 20th	Day	34% 26% 30% 8% 1 1	220
	21st to end	Day	30% 32% 29% 8% 2	242
February	1st to 10th	Day	37% 36% 21% 5% 1	314
	11th to 20th	Day	32% 31% 27% 8% 2	517
	21st to end	Day	34% 27% 27% 9% 2	446
March	1st to 10th	Day	34% 24% 29% 12% 2	540
	11th to 20th	Day	34% 29% 25% 11% 2 1	540
	21st to end	Day	38% 25% 22% 9% 4 2	594
April	1st to 10th	Day	41% 22% 23% 10% 3 1	540
	11th to 20th	Day	38% 25% 21% 10% 4 1	532
	21st to end	Day	36% 22% 26% 11% 4 1	540
May	1st to 10th	Day	39% 19% 22% 13% 6% 1	507
	11th to 20th	Day	25% 20% 35% 15% 5% 1	527
	21st to end	Day	24% 18% 32% 20% 6% 1	573
June	1st to 10th	Day	28% 18% 32% 16% 6%	530
	11th to 20th	Day	24% 19% 33% 17% 7% 1	530
	21st to end	Day	28% 17% 32% 17% 6% 1	530
July	1st to 10th	Day	18% 14% 35% 22% 8% 2	525
	11th to 20th	Day	20% 11% 37% 22% 9% 1	524
	21st to end	Day	19% 15% 39% 20% 6% 1	576
August	1st to 10th	Day	17% 12% 31% 27% 11% 2	526
	11th to 20th	Day	21% 13% 26% 24% 13% 4	519
	21st to end	Day	16% 17% 31% 24% 9% 4	583
September	1st to 10th	Day	18% 15% 28% 22% 10% 6%	530
	11th to 20th	Day	18% 13% 24% 22% 15% 8%	530
	21st to end	Day	21% 18% 29% 18% 11% 4	530
October	1st to 10th	Day	30% 15% 25% 15% 11% 4	530
	11th to 20th	Day	28% 19% 28% 17% 7% 1	501
	21st to end	Day	35% 21% 26% 15% 3	462
November	1st to 10th	Day	33% 23% 23% 16% 6%	425
	11th to 20th	Day	30% 26% 24% 14% 4 1	430
	21st to end	Day	42% 19% 27% 10% 2	252
December	1st to 10th	Day	35% 26% 25% 11% 2	220
	11th to 20th	Day	42% 26% 25% 6% 1	220
	21st to end	Day	36% 23% 31% 9% 2	242
January	1st to 10th	Night	26% 27% 31% 14% 1	840
	11th to 20th	Night	30% 25% 26% 15% 3	840
	21st to end	Night	22% 25% 30% 20% 3	924
February	1st to 10th	Night	24% 26% 33% 17% 1	840
	11th to 20th	Night	26% 26% 31% 15% 2	823
	21st to end	Night	27% 24% 31% 16% 2	692
March	1st to 10th	Night	29% 22% 30% 17% 2	840
	11th to 20th	Night	29% 25% 25% 17% 4	840
	21st to end	Night	32% 21% 24% 17% 6%	924
April	1st to 10th	Night	32% 21% 25% 18% 4	840
	11th to 20th	Night	32% 24% 23% 16% 6%	831
	21st to end	Night	29% 20% 25% 20% 5% 1	671
May	1st to 10th	Night	33% 15% 27% 18% 7%	572
	11th to 20th	Night	21% 15% 32% 25% 7% 1	394
	21st to end	Night	22% 9% 30% 29% 9% 1	433
June	1st to 10th	Night	26% 8% 29% 26% 10% 1	400
	11th to 20th	Night	24% 10% 24% 31% 11% 1	400
	21st to end	Night	23% 9% 23% 34% 10% 1	400
July	1st to 10th	Night	18% 5% 24% 36% 16% 1	390
	11th to 20th	Night	19% 4 23% 36% 16% 2	388
	21st to end	Night	13% 7% 23% 42% 14%	438
August	1st to 10th	Night	18% 5% 19% 40% 18% 1	398
	11th to 20th	Night	19% 7% 21% 32% 18% 3	678
	21st to end	Night	18% 9% 22% 31% 18% 2	880
September	1st to 10th	Night	21% 9% 25% 30% 14% 2	800
	11th to 20th	Night	18% 9% 21% 29% 20% 4	800
	21st to end	Night	21% 12% 25% 23% 17% 2	800
October	1st to 10th	Night	30% 11% 21% 22% 14% 2	800
	11th to 20th	Night	28% 17% 22% 22% 10% 1	800
	21st to end	Night	35% 18% 25% 17% 5%	880
November	1st to 10th	Night	29% 21% 26% 17% 7%	820
	11th to 20th	Night	28% 23% 26% 15% 6% 1	840
	21st to end	Night	28% 27% 25% 15% 5% 1	840
December	1st to 10th	Night	31% 26% 25% 14% 3	840
	11th to 20th	Night	32% 23% 24% 18% 3	840
	21st to end	Night	33% 23% 28% 13% 2	924

Page generated at 2026-04-29 02:21:40. Version ac07

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