North Bay
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the North Bay clear sky chart from 2005-11-06 to 2025-09-04.

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	17% 11% 4 9% 5% 4 4 3 5% 7% 31%	126615
	Night	17% 14% 4 10% 5% 4 4 3 4 6% 28%	122151
Transparency	Day	53% 5% 12% 21% 9% 1	130159
	Night	57% 4 10% 18% 9% 1	126694
Seeing	Day	21% 9% 27% 28% 12% 3	16144
	Night	23% 8% 28% 29% 11% 1	25067

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% 22% 9% 14% 9% 6% 3 3 4 4 15%	3158
	11th to 20th	Day	11% 18% 8% 15% 8% 5% 4 3 4 6% 16%	3370
	21st to end	Day	12% 20% 8% 14% 6% 4 4 3 4 5% 19%	4466
February	1st to 10th	Day	10% 20% 7% 13% 7% 5% 5% 4 5% 6% 18%	1800
	11th to 20th	Day	13% 17% 6% 11% 8% 5% 4 3 5% 6% 24%	1753
	21st to end	Day	16% 16% 5% 11% 7% 6% 6% 3 4 5% 22%	1485
March	1st to 10th	Day	21% 14% 4 9% 4 4 4 3 4 6% 26%	4920
	11th to 20th	Day	15% 13% 4 12% 7% 4 5% 3 5% 7% 24%	4980
	21st to end	Day	16% 14% 6% 10% 5% 5% 3 3 4 5% 31%	5465
April	1st to 10th	Day	19% 13% 4 9% 4 4 3 3 5% 6% 28%	2382
	11th to 20th	Day	21% 11% 3 7% 4 4 3 2 5% 6% 34%	2509
	21st to end	Day	22% 11% 2 7% 4 3 3 3 4 5% 35%	2600
May	1st to 10th	Day	20% 9% 3 7% 5% 4 4 3 5% 6% 35%	6106
	11th to 20th	Day	14% 7% 3 7% 5% 5% 5% 3 5% 7% 40%	6318
	21st to end	Day	21% 9% 3 7% 4 4 3 3 4 6% 36%	6949
June	1st to 10th	Day	18% 9% 3 8% 5% 5% 5% 4 5% 7% 33%	2930
	11th to 20th	Day	15% 6% 3 6% 5% 4 5% 4 7% 8% 38%	3000
	21st to end	Day	17% 8% 3 7% 6% 5% 4 4 6% 8% 32%	3000
July	1st to 10th	Day	18% 6% 2 7% 4 5% 4 4 6% 9% 34%	6819
	11th to 20th	Day	14% 5% 2 5% 4 4 4 4 6% 9% 44%	6733
	21st to end	Day	11% 4 2 5% 4 3 4 4 7% 10% 47%	6819
August	1st to 10th	Day	12% 7% 3 7% 5% 5% 5% 4 7% 9% 36%	4234
	11th to 20th	Day	17% 6% 2 5% 4 4 5% 4 7% 9% 38%	4174
	21st to end	Day	10% 5% 2 6% 5% 5% 4 4 6% 9% 44%	4557
September	1st to 10th	Day	16% 7% 3 6% 4 4 4 4 5% 8% 39%	2316
	11th to 20th	Day	16% 6% 2 7% 4 4 3 3 6% 8% 42%	2185
	21st to end	Day	23% 7% 3 7% 4 4 4 3 5% 6% 33%	2043
October	1st to 10th	Day	19% 9% 2 7% 5% 5% 5% 4 5% 6% 34%	1900
	11th to 20th	Day	25% 14% 3 9% 5% 4 4 3 4 6% 23%	1900
	21st to end	Day	29% 14% 5% 8% 5% 3 3 3 5% 5% 20%	2062
November	1st to 10th	Day	22% 13% 4 8% 4 4 4 4 5% 7% 25%	1567
	11th to 20th	Day	20% 17% 5% 11% 6% 4 5% 4 4 6% 20%	1600
	21st to end	Day	26% 22% 5% 11% 5% 4 3 3 4 5% 14%	1600
December	1st to 10th	Day	16% 24% 7% 14% 7% 5% 4 3 4 4 10%	2600
	11th to 20th	Day	19% 18% 5% 14% 6% 4 3 2 3 4 23%	2839
	21st to end	Day	21% 22% 7% 17% 5% 3 3 2 3 4 15%	3476
January	1st to 10th	Night	12% 20% 8% 16% 8% 5% 4 3 4 5% 15%	6808
	11th to 20th	Night	12% 18% 8% 17% 7% 5% 4 3 4 5% 17%	6505
	21st to end	Night	11% 24% 9% 14% 5% 5% 4 3 4 5% 16%	6490
February	1st to 10th	Night	11% 16% 6% 14% 8% 6% 4 4 5% 5% 21%	2600
	11th to 20th	Night	13% 18% 7% 12% 7% 5% 4 3 4 5% 23%	2533
	21st to end	Night	14% 20% 5% 11% 7% 6% 4 3 3 5% 22%	2145
March	1st to 10th	Night	17% 16% 4 9% 4 4 3 4 5% 6% 27%	4918
	11th to 20th	Night	17% 17% 4 8% 5% 4 4 4 6% 6% 25%	4979
	21st to end	Night	19% 14% 4 8% 5% 4 4 3 4 5% 30%	5467
April	1st to 10th	Night	20% 15% 4 8% 5% 4 4 3 4 5% 29%	2017
	11th to 20th	Night	20% 11% 4 6% 4 4 4 3 4 5% 35%	1857
	21st to end	Night	22% 10% 4 7% 5% 4 4 3 4 6% 31%	1800
May	1st to 10th	Night	17% 10% 3 8% 5% 4 4 3 5% 7% 35%	3833
	11th to 20th	Night	13% 6% 2 6% 4 4 4 3 5% 9% 43%	3575
	21st to end	Night	17% 8% 3 7% 5% 4 4 3 5% 8% 36%	3926
June	1st to 10th	Night	14% 9% 3 7% 6% 5% 5% 4 6% 7% 34%	1467
	11th to 20th	Night	15% 6% 1 6% 4 4 3 4 5% 9% 42%	1400
	21st to end	Night	16% 7% 2 7% 5% 4 5% 4 7% 7% 36%	1400
July	1st to 10th	Night	18% 8% 2 6% 4 4 5% 4 6% 8% 36%	3160
	11th to 20th	Night	17% 5% 2 5% 3 4 4 3 4 8% 45%	3239
	21st to end	Night	12% 6% 2 5% 4 3 4 3 5% 8% 49%	4030
August	1st to 10th	Night	14% 6% 2 6% 4 3 6% 4 6% 7% 42%	2915
	11th to 20th	Night	17% 7% 2 6% 4 4 4 4 5% 5% 42%	2875
	21st to end	Night	14% 5% 2 4 4 3 3 4 5% 8% 47%	3307
September	1st to 10th	Night	16% 5% 2 5% 4 3 3 3 4 7% 48%	1931
	11th to 20th	Night	15% 6% 2 5% 4 4 4 3 5% 7% 45%	1990
	21st to end	Night	19% 7% 2 6% 4 3 4 3 5% 7% 40%	2137
October	1st to 10th	Night	21% 8% 3 7% 4 5% 4 4 5% 5% 34%	2280
	11th to 20th	Night	27% 12% 3 7% 5% 4 4 3 5% 5% 27%	2280
	21st to end	Night	27% 14% 4 8% 5% 4 4 3 5% 5% 22%	2532
November	1st to 10th	Night	22% 13% 3 7% 4 4 3 4 4 5% 30%	2730
	11th to 20th	Night	19% 16% 4 10% 6% 5% 5% 4 4 5% 22%	2800
	21st to end	Night	25% 19% 5% 8% 5% 5% 5% 3 4 5% 16%	2800
December	1st to 10th	Night	18% 25% 6% 12% 6% 5% 5% 4 4 4 12%	4580
	11th to 20th	Night	20% 20% 6% 12% 5% 4 4 3 4 4 19%	5343
	21st to end	Night	21% 21% 6% 17% 5% 3 3 2 3 4 15%	7502

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% 4 7% 13% 3	3159
	11th to 20th	Day	69% 3 10% 10% 7%	3399
	21st to end	Day	69% 3 6% 13% 8%	4466
February	1st to 10th	Day	66% 6% 9% 14% 5% 1	1800
	11th to 20th	Day	62% 4 11% 14% 7% 1	1762
	21st to end	Day	65% 4 8% 15% 8% 1	1485
March	1st to 10th	Day	59% 5% 10% 12% 13% 2	4923
	11th to 20th	Day	59% 4 14% 14% 8% 1	4980
	21st to end	Day	56% 4 10% 21% 9% 1	5465
April	1st to 10th	Day	55% 5% 11% 19% 9% 2	2382
	11th to 20th	Day	52% 4 8% 23% 12% 1	2509
	21st to end	Day	51% 5% 9% 18% 13% 3	2600
May	1st to 10th	Day	50% 4 9% 20% 12% 5%	6115
	11th to 20th	Day	43% 6% 11% 24% 13% 2	6322
	21st to end	Day	50% 4 12% 19% 13% 2	6949
June	1st to 10th	Day	50% 6% 10% 22% 11% 1	2930
	11th to 20th	Day	41% 8% 12% 25% 13% 1	3000
	21st to end	Day	48% 8% 13% 20% 11% 1	3000
July	1st to 10th	Day	47% 4 16% 27% 6%	6746
	11th to 20th	Day	38% 3 16% 37% 6%	6652
	21st to end	Day	34% 4 17% 33% 12%	6623
August	1st to 10th	Day	43% 5% 13% 32% 7%	4221
	11th to 20th	Day	42% 5% 17% 26% 9%	4213
	21st to end	Day	37% 4 14% 35% 9%	4557
September	1st to 10th	Day	43% 7% 15% 25% 11%	2316
	11th to 20th	Day	41% 6% 15% 28% 9%	2185
	21st to end	Day	52% 6% 12% 23% 6%	2043
October	1st to 10th	Day	50% 5% 11% 28% 7%	3160
	11th to 20th	Day	53% 6% 15% 21% 5%	3140
	21st to end	Day	67% 4 14% 10% 4	3376
November	1st to 10th	Day	59% 6% 14% 17% 5%	1567
	11th to 20th	Day	65% 7% 11% 11% 5% 1	1600
	21st to end	Day	76% 5% 9% 8% 3	1600
December	1st to 10th	Day	78% 4 10% 7% 1	2600
	11th to 20th	Day	67% 4 7% 17% 4	2838
	21st to end	Day	75% 3 5% 13% 4	3476
January	1st to 10th	Night	73% 4 7% 12% 4	6808
	11th to 20th	Night	70% 5% 9% 9% 7%	6551
	21st to end	Night	71% 3 5% 9% 10% 1	6490
February	1st to 10th	Night	64% 5% 10% 14% 6% 1	2600
	11th to 20th	Night	65% 4 10% 15% 5% 1	2535
	21st to end	Night	66% 4 8% 15% 7% 1	2145
March	1st to 10th	Night	57% 4 9% 13% 13% 4	4928
	11th to 20th	Night	59% 5% 13% 15% 7% 1	4980
	21st to end	Night	57% 4 7% 18% 11% 2	5467
April	1st to 10th	Night	58% 4 7% 18% 11% 2	2017
	11th to 20th	Night	53% 3 7% 21% 14% 2	1857
	21st to end	Night	55% 5% 8% 15% 14% 3	1800
May	1st to 10th	Night	47% 5% 7% 21% 15% 3	3840
	11th to 20th	Night	38% 6% 13% 22% 17% 3	3576
	21st to end	Night	47% 5% 10% 22% 13% 3	3927
June	1st to 10th	Night	48% 8% 10% 21% 11% 3	1467
	11th to 20th	Night	39% 6% 11% 26% 17% 1	1400
	21st to end	Night	44% 8% 14% 21% 11% 1	1400
July	1st to 10th	Night	47% 4 12% 28% 9% 1	3125
	11th to 20th	Night	40% 3 15% 32% 9% 1	3197
	21st to end	Night	35% 4 12% 35% 14%	3911
August	1st to 10th	Night	41% 5% 14% 28% 11%	2904
	11th to 20th	Night	43% 6% 15% 26% 9% 1	2902
	21st to end	Night	37% 4 14% 32% 12% 1	3307
September	1st to 10th	Night	37% 6% 16% 29% 11% 1	1931
	11th to 20th	Night	40% 5% 16% 28% 10%	1990
	21st to end	Night	44% 5% 15% 26% 9%	2137
October	1st to 10th	Night	46% 4 9% 31% 10%	3800
	11th to 20th	Night	52% 3 14% 27% 5%	3779
	21st to end	Night	60% 5% 18% 12% 5%	4168
November	1st to 10th	Night	56% 6% 14% 18% 5%	2730
	11th to 20th	Night	65% 6% 12% 12% 5% 1	2800
	21st to end	Night	71% 6% 11% 9% 2	2800
December	1st to 10th	Night	77% 3 9% 9% 1	4580
	11th to 20th	Night	69% 4 7% 11% 9%	5343
	21st to end	Night	76% 3 5% 12% 5%	7502

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	25% 15% 30% 22% 9% 1	200
	11th to 20th	Day	20% 12% 30% 27% 9% 2	210
	21st to end	Day	25% 9% 29% 26% 9% 2	330
February	1st to 10th	Day	27% 15% 37% 16% 5% 1	300
	11th to 20th	Day	23% 14% 37% 24% 2	300
	21st to end	Day	32% 13% 33% 22%	249
March	1st to 10th	Day	22% 12% 35% 26% 5%	489
	11th to 20th	Day	18% 12% 35% 28% 6% 1	500
	21st to end	Day	23% 12% 29% 29% 6% 1	550
April	1st to 10th	Day	23% 9% 35% 22% 9% 2	500
	11th to 20th	Day	29% 13% 24% 20% 10% 5%	492
	21st to end	Day	28% 12% 26% 25% 8% 1	500
May	1st to 10th	Day	25% 6% 26% 29% 13% 1	503
	11th to 20th	Day	20% 7% 26% 27% 17% 3	527
	21st to end	Day	20% 6% 27% 28% 16% 3	573
June	1st to 10th	Day	20% 6% 30% 32% 11% 1	660
	11th to 20th	Day	15% 7% 28% 32% 13% 5%	730
	21st to end	Day	19% 8% 25% 32% 12% 3	730
July	1st to 10th	Day	13% 4 23% 40% 17% 2	721
	11th to 20th	Day	16% 4 23% 32% 20% 4	683
	21st to end	Day	12% 5% 22% 36% 20% 5%	576
August	1st to 10th	Day	15% 4 19% 35% 21% 5%	526
	11th to 20th	Day	17% 4 15% 32% 25% 8%	519
	21st to end	Day	16% 8% 23% 31% 16% 7%	583
September	1st to 10th	Day	18% 7% 24% 28% 18% 6%	502
	11th to 20th	Day	16% 5% 21% 29% 21% 8%	490
	21st to end	Day	25% 8% 21% 25% 18% 4	490
October	1st to 10th	Day	23% 10% 27% 24% 13% 2	490
	11th to 20th	Day	26% 15% 32% 21% 5% 1	490
	21st to end	Day	34% 12% 29% 22% 3 1	511
November	1st to 10th	Day	31% 18% 30% 18% 4 1	200
	11th to 20th	Day	25% 19% 35% 17% 5% 1	200
	21st to end	Day	35% 15% 33% 16% 2 1	200
December	1st to 10th	Day	27% 16% 31% 25% 2 1	200
	11th to 20th	Day	29% 12% 36% 20% 4 1	200
	21st to end	Day	31% 14% 31% 16% 7% 1	220
January	1st to 10th	Night	27% 7% 30% 28% 7%	1000
	11th to 20th	Night	29% 8% 26% 30% 8%	912
	21st to end	Night	19% 6% 29% 33% 11% 2	880
February	1st to 10th	Night	17% 10% 37% 30% 6%	800
	11th to 20th	Night	21% 10% 32% 29% 7% 1	783
	21st to end	Night	23% 11% 28% 34% 4	660
March	1st to 10th	Night	22% 8% 35% 29% 6%	800
	11th to 20th	Night	22% 10% 35% 27% 7% 1	800
	21st to end	Night	22% 6% 32% 31% 8% 1	880
April	1st to 10th	Night	28% 8% 26% 31% 6%	800
	11th to 20th	Night	24% 9% 28% 28% 10% 1	673
	21st to end	Night	25% 8% 28% 30% 10% 1	600
May	1st to 10th	Night	22% 5% 27% 34% 11% 1	510
	11th to 20th	Night	18% 5% 28% 37% 11% 1	394
	21st to end	Night	19% 4 26% 37% 13%	433
June	1st to 10th	Night	15% 4 30% 37% 15%	400
	11th to 20th	Night	18% 5% 24% 38% 16% 1	400
	21st to end	Night	18% 6% 20% 36% 20% 1	400
July	1st to 10th	Night	12% 3 24% 47% 15% 1	390
	11th to 20th	Night	13% 3 23% 38% 21% 1	388
	21st to end	Night	11% 4 20% 44% 20% 1	480
August	1st to 10th	Night	17% 3 18% 37% 23% 2	597
	11th to 20th	Night	15% 3 17% 37% 26% 3	594
	21st to end	Night	19% 6% 21% 31% 18% 4	705
September	1st to 10th	Night	18% 5% 21% 32% 23% 2	773
	11th to 20th	Night	17% 6% 22% 28% 23% 3	760
	21st to end	Night	22% 6% 22% 29% 18% 2	760
October	1st to 10th	Night	24% 7% 27% 29% 11% 1	760
	11th to 20th	Night	31% 12% 26% 23% 6% 1	760
	21st to end	Night	34% 12% 28% 22% 4	836
November	1st to 10th	Night	29% 10% 30% 23% 6% 1	780
	11th to 20th	Night	27% 14% 33% 19% 7%	800
	21st to end	Night	35% 11% 27% 22% 5%	800
December	1st to 10th	Night	31% 13% 30% 20% 6%	800
	11th to 20th	Night	28% 9% 34% 23% 5% 2	859
	21st to end	Night	31% 9% 28% 25% 6% 1	1100

Page generated at 2025-09-01 01:51:04. Version ac07

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