Humboldt
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Humboldt clear sky chart from 2005-11-06 to 2025-02-25.

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	9% 9% 3 9% 6% 5% 5% 4 6% 8% 36%	111684
	Night	7% 9% 4 10% 7% 6% 5% 4 6% 8% 34%	115655
Transparency	Day	45% 7% 16% 24% 8% 1	111427
	Night	46% 7% 15% 23% 9% 1	115558
Seeing	Day	13% 8% 28% 30% 17% 5%	16193
	Night	13% 6% 31% 34% 14% 2	25754

By Month

Percentage Cloud Cover NW (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	5% 13% 5% 16% 10% 7% 7% 4 8% 8% 16%	2921
	11th to 20th	Day	3 7% 3 10% 10% 12% 8% 6% 11% 8% 21%	3159
	21st to end	Day	6% 13% 6% 13% 8% 6% 6% 3 5% 7% 26%	3955
February	1st to 10th	Day	7% 12% 5% 12% 8% 8% 8% 6% 8% 8% 17%	1615
	11th to 20th	Day	6% 12% 4 10% 7% 7% 8% 7% 7% 8% 25%	1755
	21st to end	Day	4 11% 5% 13% 10% 8% 8% 6% 8% 8% 20%	1528
March	1st to 10th	Day	6% 13% 6% 12% 9% 8% 7% 5% 6% 6% 22%	3130
	11th to 20th	Day	6% 12% 4 13% 8% 5% 5% 3 5% 8% 32%	3347
	21st to end	Day	7% 9% 3 6% 5% 5% 5% 4 7% 8% 40%	4098
April	1st to 10th	Day	6% 10% 3 8% 6% 5% 5% 4 6% 8% 39%	2280
	11th to 20th	Day	14% 13% 4 10% 6% 4 4 5% 5% 7% 29%	2323
	21st to end	Day	12% 10% 3 9% 5% 5% 5% 4 5% 8% 33%	2660
May	1st to 10th	Day	15% 12% 4 7% 4 3 4 3 5% 7% 36%	4430
	11th to 20th	Day	10% 7% 2 8% 6% 5% 6% 5% 6% 8% 37%	4388
	21st to end	Day	14% 11% 3 7% 5% 5% 5% 4 6% 7% 32%	5274
June	1st to 10th	Day	15% 8% 3 8% 5% 4 5% 5% 7% 8% 34%	3040
	11th to 20th	Day	22% 8% 2 8% 5% 5% 5% 4 6% 7% 27%	3040
	21st to end	Day	13% 5% 2 7% 5% 5% 5% 5% 8% 9% 36%	3040
July	1st to 10th	Day	8% 4 2 5% 4 5% 6% 4 7% 10% 44%	5060
	11th to 20th	Day	8% 4 1 5% 3 4 5% 4 6% 10% 51%	4812
	21st to end	Day	5% 3 2 5% 4 4 5% 4 6% 10% 53%	4903
August	1st to 10th	Day	10% 5% 3 7% 5% 5% 5% 4 6% 8% 43%	4412
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 3 6% 9% 47%	4343
	21st to end	Day	12% 6% 2 5% 2 4 4 3 5% 7% 50%	4379
September	1st to 10th	Day	10% 5% 2 7% 4 4 4 3 5% 8% 46%	2280
	11th to 20th	Day	14% 8% 3 6% 5% 4 4 4 5% 7% 40%	2280
	21st to end	Day	10% 7% 2 4 4 4 5% 3 5% 8% 48%	2040
October	1st to 10th	Day	11% 9% 2 6% 4 4 4 3 5% 8% 44%	3170
	11th to 20th	Day	7% 7% 2 6% 4 5% 6% 5% 8% 12% 38%	2997
	21st to end	Day	8% 10% 4 9% 6% 6% 5% 5% 6% 7% 33%	3091
November	1st to 10th	Day	17% 16% 4 11% 5% 4 4 4 4 6% 26%	1573
	11th to 20th	Day	9% 14% 7% 15% 10% 6% 4 3 5% 4 22%	1544
	21st to end	Day	8% 14% 7% 13% 8% 5% 5% 4 6% 5% 24%	1400
December	1st to 10th	Day	7% 11% 4 16% 10% 7% 6% 5% 7% 6% 22%	2166
	11th to 20th	Day	5% 12% 7% 18% 10% 9% 7% 4 4 5% 19%	2259
	21st to end	Day	3 8% 4 15% 8% 5% 5% 4 6% 7% 34%	2992
January	1st to 10th	Night	3 12% 5% 16% 12% 8% 6% 4 6% 9% 18%	7019
	11th to 20th	Night	3 9% 4 12% 12% 10% 7% 5% 8% 9% 21%	6739
	21st to end	Night	4 9% 4 13% 10% 8% 6% 4 6% 8% 28%	7019
February	1st to 10th	Night	6% 12% 5% 13% 9% 7% 6% 5% 6% 8% 24%	2800
	11th to 20th	Night	3 9% 4 12% 6% 8% 6% 5% 8% 7% 32%	2537
	21st to end	Night	4 9% 3 9% 7% 5% 6% 6% 8% 11% 32%	2019
March	1st to 10th	Night	6% 13% 4 12% 7% 6% 8% 6% 7% 8% 24%	3734
	11th to 20th	Night	7% 11% 4 8% 4 5% 5% 5% 7% 9% 34%	3626
	21st to end	Night	7% 7% 3 7% 6% 5% 5% 5% 7% 8% 40%	3546
April	1st to 10th	Night	6% 8% 3 7% 5% 5% 5% 4 7% 7% 42%	1900
	11th to 20th	Night	12% 12% 3 7% 7% 4 5% 4 7% 7% 33%	1807
	21st to end	Night	13% 9% 4 8% 6% 4 5% 3 6% 8% 33%	1520
May	1st to 10th	Night	18% 10% 3 7% 4 4 5% 4 5% 6% 34%	2540
	11th to 20th	Night	9% 8% 3 7% 5% 5% 5% 4 6% 8% 38%	2518
	21st to end	Night	13% 9% 3 8% 6% 4 5% 4 6% 7% 34%	2305
June	1st to 10th	Night	12% 5% 3 5% 4 4 4 3 7% 10% 43%	1140
	11th to 20th	Night	18% 8% 2 6% 5% 4 4 3 5% 8% 36%	1140
	21st to end	Night	11% 5% 2 7% 4 4 5% 4 6% 8% 44%	1140
July	1st to 10th	Night	8% 4 2 5% 4 5% 6% 6% 8% 13% 40%	1900
	11th to 20th	Night	6% 4 2 5% 3 3 3 3 6% 7% 58%	2146
	21st to end	Night	3 2 1 4 3 3 3 3 6% 9% 62%	2656
August	1st to 10th	Night	8% 3 2 5% 4 4 4 4 6% 9% 52%	2528
	11th to 20th	Night	6% 3 1 5% 3 4 5% 5% 7% 11% 50%	2496
	21st to end	Night	9% 4 2 4 4 4 4 4 5% 9% 52%	3284
September	1st to 10th	Night	9% 5% 1 5% 3 4 5% 4 6% 8% 50%	1900
	11th to 20th	Night	13% 6% 2 6% 4 4 4 4 5% 7% 45%	1900
	21st to end	Night	9% 5% 2 5% 4 3 4 4 5% 9% 50%	2137
October	1st to 10th	Night	11% 8% 3 5% 3 4 4 3 5% 7% 48%	3800
	11th to 20th	Night	6% 7% 2 7% 4 5% 5% 5% 6% 7% 46%	3900
	21st to end	Night	8% 9% 3 9% 5% 5% 5% 4 6% 8% 40%	4467
November	1st to 10th	Night	15% 16% 4 10% 5% 4 5% 3 5% 7% 26%	2722
	11th to 20th	Night	7% 14% 7% 14% 8% 6% 5% 4 5% 5% 24%	2855
	21st to end	Night	7% 13% 6% 12% 7% 6% 6% 5% 5% 8% 26%	3000
December	1st to 10th	Night	7% 14% 6% 16% 11% 8% 6% 5% 5% 5% 18%	5018
	11th to 20th	Night	5% 13% 6% 13% 9% 8% 7% 5% 7% 7% 21%	5933
	21st to end	Night	3 7% 3 14% 9% 6% 5% 4 5% 5% 38%	7964

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	63% 5% 9% 15% 8%	2921
	11th to 20th	Day	53% 6% 14% 19% 8%	3180
	21st to end	Day	57% 6% 17% 14% 5%	3955
February	1st to 10th	Day	60% 7% 11% 13% 8%	1615
	11th to 20th	Day	52% 6% 12% 19% 10%	1764
	21st to end	Day	56% 8% 11% 16% 9%	1528
March	1st to 10th	Day	61% 6% 12% 12% 8%	3130
	11th to 20th	Day	51% 7% 18% 18% 7%	3348
	21st to end	Day	40% 7% 12% 24% 17%	4098
April	1st to 10th	Day	42% 7% 15% 24% 12% 1	2280
	11th to 20th	Day	54% 7% 12% 18% 8% 2	2323
	21st to end	Day	48% 7% 14% 22% 8%	2660
May	1st to 10th	Day	47% 6% 14% 24% 9% 1	4430
	11th to 20th	Day	41% 9% 12% 24% 12% 1	4392
	21st to end	Day	47% 7% 14% 23% 8%	5274
June	1st to 10th	Day	44% 8% 14% 22% 10% 1	3040
	11th to 20th	Day	55% 9% 11% 18% 7% 1	3040
	21st to end	Day	41% 10% 17% 24% 7%	3040
July	1st to 10th	Day	32% 9% 24% 29% 5%	4981
	11th to 20th	Day	29% 7% 16% 42% 6%	4728
	21st to end	Day	27% 7% 20% 38% 8%	4748
August	1st to 10th	Day	38% 7% 20% 27% 7%	4396
	11th to 20th	Day	33% 7% 20% 34% 6%	4385
	21st to end	Day	34% 5% 20% 33% 7%	4378
September	1st to 10th	Day	35% 7% 15% 29% 12% 2	2280
	11th to 20th	Day	43% 7% 15% 26% 7% 1	2280
	21st to end	Day	34% 7% 15% 30% 12% 1	2040
October	1st to 10th	Day	39% 5% 21% 27% 8%	3170
	11th to 20th	Day	38% 6% 18% 28% 9% 1	2997
	21st to end	Day	48% 6% 19% 22% 4	3091
November	1st to 10th	Day	60% 6% 12% 17% 5%	1573
	11th to 20th	Day	65% 7% 11% 13% 4	1544
	21st to end	Day	60% 6% 14% 16% 4	1400
December	1st to 10th	Day	61% 7% 13% 13% 6% 1	2166
	11th to 20th	Day	68% 5% 13% 10% 3	2260
	21st to end	Day	49% 4 9% 20% 13% 4	2992
January	1st to 10th	Night	61% 5% 11% 19% 4	7019
	11th to 20th	Night	57% 6% 13% 16% 8%	6780
	21st to end	Night	52% 8% 18% 16% 4	7019
February	1st to 10th	Night	57% 6% 11% 17% 9% 1	2800
	11th to 20th	Night	46% 7% 12% 22% 13% 1	2538
	21st to end	Night	43% 7% 15% 24% 11%	2019
March	1st to 10th	Night	53% 8% 15% 18% 5%	3734
	11th to 20th	Night	43% 9% 19% 20% 9% 1	3626
	21st to end	Night	37% 7% 11% 23% 20% 1	3546
April	1st to 10th	Night	38% 6% 16% 25% 14% 1	1900
	11th to 20th	Night	47% 6% 13% 19% 12% 2	1807
	21st to end	Night	47% 6% 14% 20% 12% 1	1520
May	1st to 10th	Night	48% 9% 9% 24% 10% 1	2540
	11th to 20th	Night	40% 8% 13% 21% 17% 1	2518
	21st to end	Night	46% 8% 12% 25% 8%	2305
June	1st to 10th	Night	35% 7% 19% 26% 11% 2	1140
	11th to 20th	Night	45% 9% 15% 19% 10% 1	1140
	21st to end	Night	35% 9% 20% 25% 10% 1	1140
July	1st to 10th	Night	31% 12% 24% 27% 5%	1870
	11th to 20th	Night	25% 7% 13% 47% 8%	2107
	21st to end	Night	18% 7% 20% 43% 11% 1	2572
August	1st to 10th	Night	28% 8% 22% 31% 10%	2520
	11th to 20th	Night	26% 8% 20% 36% 9%	2520
	21st to end	Night	29% 6% 20% 33% 12%	3284
September	1st to 10th	Night	30% 8% 16% 28% 14% 3	1900
	11th to 20th	Night	38% 7% 15% 30% 10% 1	1900
	21st to end	Night	32% 6% 14% 30% 16% 2	2137
October	1st to 10th	Night	36% 5% 17% 34% 8%	3800
	11th to 20th	Night	35% 5% 15% 33% 12% 1	3900
	21st to end	Night	42% 6% 18% 28% 6%	4467
November	1st to 10th	Night	59% 6% 13% 15% 6% 1	2722
	11th to 20th	Night	60% 8% 13% 15% 4	2855
	21st to end	Night	55% 8% 15% 18% 4	3000
December	1st to 10th	Night	66% 6% 11% 13% 4	5018
	11th to 20th	Night	60% 5% 16% 15% 4	5931
	21st to end	Night	47% 5% 9% 22% 14% 3	7964

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	13% 8% 29% 29% 18% 5%	200
	11th to 20th	Day	8% 6% 34% 40% 11% 3	200
	21st to end	Day	11% 9% 34% 30% 13% 2	319
February	1st to 10th	Day	10% 10% 37% 30% 10% 3	300
	11th to 20th	Day	7% 4 29% 35% 19% 6%	300
	21st to end	Day	6% 6% 32% 33% 18% 4	237
March	1st to 10th	Day	11% 8% 30% 34% 14% 3	270
	11th to 20th	Day	10% 8% 32% 33% 14% 3	370
	21st to end	Day	14% 6% 29% 33% 17% 2	506
April	1st to 10th	Day	10% 8% 37% 26% 13% 6%	460
	11th to 20th	Day	17% 11% 27% 29% 12% 3	494
	21st to end	Day	14% 9% 27% 31% 16% 3	650
May	1st to 10th	Day	13% 6% 26% 31% 19% 4	653
	11th to 20th	Day	13% 7% 26% 27% 20% 6%	675
	21st to end	Day	16% 8% 21% 30% 19% 6%	734
June	1st to 10th	Day	15% 9% 28% 27% 15% 5%	680
	11th to 20th	Day	23% 10% 26% 22% 15% 4	680
	21st to end	Day	14% 8% 25% 26% 21% 7%	680
July	1st to 10th	Day	9% 4 21% 33% 25% 7%	671
	11th to 20th	Day	10% 6% 22% 33% 21% 8%	668
	21st to end	Day	8% 6% 23% 30% 23% 10%	739
August	1st to 10th	Day	10% 7% 22% 31% 23% 8%	676
	11th to 20th	Day	11% 7% 19% 30% 23% 9%	665
	21st to end	Day	13% 7% 25% 31% 19% 5%	569
September	1st to 10th	Day	13% 12% 29% 27% 15% 3	490
	11th to 20th	Day	18% 7% 27% 32% 14% 2	490
	21st to end	Day	14% 10% 29% 31% 13% 3	304
October	1st to 10th	Day	19% 4 30% 25% 19% 3	300
	11th to 20th	Day	12% 10% 33% 29% 14% 2	300
	21st to end	Day	17% 11% 37% 23% 10% 2	330
November	1st to 10th	Day	23% 7% 34% 23% 9% 4	300
	11th to 20th	Day	17% 8% 42% 26% 5% 1	300
	21st to end	Day	12% 8% 38% 30% 9% 2	300
December	1st to 10th	Day	7% 7% 33% 33% 16% 3	263
	11th to 20th	Day	11% 9% 39% 34% 8%	200
	21st to end	Day	8% 4 36% 29% 19% 5%	220
January	1st to 10th	Night	11% 7% 36% 32% 12% 2	1000
	11th to 20th	Night	11% 8% 41% 30% 9% 1	1000
	21st to end	Night	13% 9% 39% 31% 9% 1	1100
February	1st to 10th	Night	13% 6% 32% 35% 13% 2	1000
	11th to 20th	Night	8% 6% 29% 39% 14% 3	789
	21st to end	Night	7% 5% 28% 36% 21% 3	646
March	1st to 10th	Night	13% 5% 30% 34% 15% 2	760
	11th to 20th	Night	11% 7% 33% 35% 12% 2	760
	21st to end	Night	14% 5% 26% 36% 16% 3	670
April	1st to 10th	Night	9% 6% 30% 35% 17% 3	570
	11th to 20th	Night	16% 7% 27% 35% 14% 1	563
	21st to end	Night	18% 8% 29% 34% 11% 1	570
May	1st to 10th	Night	15% 5% 28% 33% 17% 2	570
	11th to 20th	Night	14% 6% 30% 33% 17% 1	561
	21st to end	Night	17% 4 23% 36% 16% 4	529
June	1st to 10th	Night	13% 7% 27% 35% 17% 1	380
	11th to 20th	Night	22% 6% 24% 29% 17% 3	380
	21st to end	Night	14% 4 22% 38% 21% 1	380
July	1st to 10th	Night	7% 4 21% 41% 26% 1	370
	11th to 20th	Night	10% 4 23% 43% 20% 2	368
	21st to end	Night	6% 4 23% 39% 24% 5%	557
August	1st to 10th	Night	6% 4 24% 36% 26% 4	567
	11th to 20th	Night	9% 3 19% 37% 30% 2	564
	21st to end	Night	14% 4 26% 36% 19% 1	627
September	1st to 10th	Night	11% 3 27% 39% 17% 3	570
	11th to 20th	Night	15% 4 23% 39% 15% 2	570
	21st to end	Night	12% 6% 32% 36% 13% 1	622
October	1st to 10th	Night	20% 4 28% 34% 13% 1	760
	11th to 20th	Night	12% 7% 32% 36% 12% 2	831
	21st to end	Night	16% 9% 33% 31% 9% 1	1045
November	1st to 10th	Night	24% 7% 30% 32% 6% 1	975
	11th to 20th	Night	14% 8% 38% 29% 9% 2	1000
	21st to end	Night	12% 8% 40% 30% 7% 2	1000
December	1st to 10th	Night	11% 5% 35% 31% 14% 4	1000
	11th to 20th	Night	13% 8% 35% 31% 11% 1	1000
	21st to end	Night	12% 4 29% 36% 16% 3	1100

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