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 2024-02-05.

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	11% 9% 4 8% 7% 4 6% 5% 6% 8% 34%	77507
	Night	8% 9% 4 8% 8% 4 6% 5% 6% 8% 35%	81303
Transparency	Day	47% 8% 14% 22% 8% 1	77208
	Night	47% 7% 14% 22% 9% 1	81182
Seeing	Day	13% 7% 28% 30% 17% 5%	14765
	Night	13% 6% 30% 34% 15% 2	24334

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	6% 14% 6% 16% 12% 5% 7% 5% 6% 7% 19%	2031
	11th to 20th	Day	4 9% 5% 10% 14% 9% 10% 8% 9% 7% 16%	2199
	21st to end	Day	6% 13% 5% 11% 9% 4 6% 5% 5% 7% 29%	2759
February	1st to 10th	Day	7% 10% 5% 10% 9% 6% 9% 9% 8% 8% 19%	1486
	11th to 20th	Day	7% 12% 5% 9% 9% 4 9% 9% 7% 7% 23%	1575
	21st to end	Day	5% 10% 5% 13% 11% 5% 9% 7% 8% 8% 19%	1406
March	1st to 10th	Day	9% 13% 6% 11% 10% 6% 8% 7% 6% 6% 19%	1800
	11th to 20th	Day	8% 12% 6% 10% 7% 3 5% 5% 5% 9% 30%	1893
	21st to end	Day	10% 11% 5% 8% 7% 3 6% 5% 6% 7% 32%	2335
April	1st to 10th	Day	7% 9% 4 7% 7% 4 6% 5% 6% 8% 39%	2160
	11th to 20th	Day	14% 11% 4 9% 7% 3 5% 6% 6% 7% 29%	2198
	21st to end	Day	13% 10% 3 8% 6% 4 5% 5% 5% 7% 33%	2520
May	1st to 10th	Day	13% 7% 3 5% 5% 3 5% 5% 7% 9% 39%	2520
	11th to 20th	Day	9% 5% 3 6% 6% 4 6% 5% 4 9% 43%	2478
	21st to end	Day	19% 8% 3 6% 5% 4 5% 5% 6% 7% 33%	2968
June	1st to 10th	Day	15% 7% 3 6% 6% 3 5% 6% 7% 9% 35%	2880
	11th to 20th	Day	23% 8% 3 7% 6% 3 6% 5% 6% 8% 27%	2880
	21st to end	Day	13% 5% 2 5% 6% 3 6% 6% 7% 10% 37%	2880
July	1st to 10th	Day	9% 4 2 5% 5% 3 6% 6% 7% 9% 43%	2880
	11th to 20th	Day	12% 5% 2 5% 5% 3 5% 6% 6% 8% 43%	2744
	21st to end	Day	8% 4 2 5% 5% 3 5% 5% 5% 8% 48%	2804
August	1st to 10th	Day	9% 4 3 5% 5% 4 6% 5% 7% 9% 42%	2507
	11th to 20th	Day	10% 6% 2 5% 5% 3 5% 4 6% 9% 44%	2477
	21st to end	Day	13% 6% 3 4 4 3 4 5% 5% 6% 47%	2496
September	1st to 10th	Day	11% 5% 2 7% 5% 3 4 5% 5% 8% 45%	2160
	11th to 20th	Day	13% 7% 3 5% 6% 3 4 5% 5% 6% 41%	2160
	21st to end	Day	11% 6% 2 4 4 3 5% 4 5% 8% 47%	1933
October	1st to 10th	Day	15% 12% 3 5% 5% 3 3 4 5% 7% 38%	1800
	11th to 20th	Day	10% 10% 4 6% 5% 4 5% 4 5% 7% 39%	1715
	21st to end	Day	12% 12% 5% 9% 6% 4 6% 5% 5% 7% 29%	1782
November	1st to 10th	Day	18% 15% 4 10% 6% 3 4 4 4 6% 25%	1493
	11th to 20th	Day	9% 13% 7% 12% 12% 4 6% 4 4 5% 23%	1467
	21st to end	Day	9% 13% 8% 12% 9% 4 6% 5% 5% 5% 25%	1330
December	1st to 10th	Day	6% 11% 6% 13% 10% 6% 7% 7% 7% 6% 21%	1262
	11th to 20th	Day	8% 12% 7% 14% 11% 5% 7% 5% 4 6% 20%	1439
	21st to end	Day	4 6% 5% 8% 7% 4 6% 5% 7% 8% 42%	2090
January	1st to 10th	Night	4 13% 6% 14% 14% 5% 6% 6% 6% 7% 19%	4909
	11th to 20th	Night	4 7% 5% 11% 16% 9% 9% 6% 8% 8% 17%	4699
	21st to end	Night	6% 10% 4 10% 11% 6% 6% 6% 6% 7% 29%	4905
February	1st to 10th	Night	7% 10% 5% 10% 10% 5% 7% 7% 6% 9% 25%	2583
	11th to 20th	Night	4 9% 4 11% 7% 5% 8% 7% 7% 7% 31%	2276
	21st to end	Night	5% 8% 4 8% 8% 4 6% 7% 7% 11% 33%	1849
March	1st to 10th	Night	9% 10% 5% 8% 8% 4 7% 7% 6% 8% 29%	2160
	11th to 20th	Night	8% 10% 5% 7% 6% 4 6% 7% 7% 7% 34%	2064
	21st to end	Night	10% 9% 4 7% 8% 4 6% 4 6% 7% 34%	2021
April	1st to 10th	Night	7% 8% 3 7% 5% 4 5% 5% 6% 7% 43%	1800
	11th to 20th	Night	12% 10% 4 7% 7% 3 5% 4 6% 7% 34%	1712
	21st to end	Night	14% 9% 4 7% 7% 3 5% 4 6% 8% 32%	1440
May	1st to 10th	Night	11% 7% 3 6% 6% 3 5% 5% 6% 7% 42%	1440
	11th to 20th	Night	10% 8% 3 4 5% 3 4 3 4 9% 46%	1418
	21st to end	Night	15% 6% 3 6% 6% 3 5% 6% 6% 7% 37%	1305
June	1st to 10th	Night	12% 4 3 5% 4 3 4 4 7% 11% 44%	1080
	11th to 20th	Night	18% 7% 2 5% 6% 3 5% 4 4 8% 36%	1080
	21st to end	Night	10% 5% 2 6% 5% 3 5% 6% 6% 9% 43%	1080
July	1st to 10th	Night	7% 3 2 4 5% 3 4 7% 7% 11% 49%	1080
	11th to 20th	Night	8% 4 2 5% 4 2 4 5% 7% 7% 51%	1214
	21st to end	Night	5% 2 2 3 4 2 4 4 6% 9% 60%	1512
August	1st to 10th	Night	6% 2 1 3 4 3 4 5% 7% 11% 53%	1432
	11th to 20th	Night	7% 4 1 3 3 2 4 6% 6% 10% 53%	1424
	21st to end	Night	11% 5% 2 4 4 3 3 5% 5% 8% 49%	1856
September	1st to 10th	Night	10% 5% 1 4 4 3 5% 6% 6% 8% 49%	1800
	11th to 20th	Night	12% 6% 2 5% 5% 3 4 5% 5% 7% 46%	1800
	21st to end	Night	9% 5% 2 5% 4 2 4 5% 5% 9% 50%	2024
October	1st to 10th	Night	14% 10% 4 5% 5% 3 4 5% 5% 7% 39%	2160
	11th to 20th	Night	9% 9% 3 6% 5% 3 5% 5% 5% 7% 44%	2227
	21st to end	Night	9% 12% 5% 10% 6% 4 5% 4 5% 7% 32%	2574
November	1st to 10th	Night	16% 16% 5% 9% 6% 3 5% 4 5% 7% 25%	2582
	11th to 20th	Night	8% 12% 8% 12% 10% 4 6% 5% 5% 5% 24%	2712
	21st to end	Night	7% 12% 7% 11% 8% 5% 5% 6% 5% 8% 27%	2850
December	1st to 10th	Night	7% 10% 6% 10% 10% 5% 7% 7% 6% 7% 26%	2916
	11th to 20th	Night	7% 14% 5% 12% 11% 5% 6% 6% 6% 7% 20%	3753
	21st to end	Night	4 6% 3 8% 6% 4 5% 5% 5% 6% 47%	5566

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	64% 6% 11% 12% 7%	2031
	11th to 20th	Day	61% 6% 13% 14% 5%	2220
	21st to end	Day	54% 7% 13% 18% 7%	2759
February	1st to 10th	Day	59% 8% 12% 14% 7%	1486
	11th to 20th	Day	55% 6% 12% 17% 9%	1584
	21st to end	Day	58% 8% 10% 15% 9%	1406
March	1st to 10th	Day	63% 7% 9% 15% 5%	1800
	11th to 20th	Day	52% 6% 12% 21% 9%	1894
	21st to end	Day	51% 8% 12% 21% 7%	2335
April	1st to 10th	Day	43% 7% 14% 24% 12% 1	2160
	11th to 20th	Day	53% 7% 12% 18% 8% 2	2198
	21st to end	Day	49% 7% 15% 21% 8%	2520
May	1st to 10th	Day	40% 7% 13% 28% 11% 1	2520
	11th to 20th	Day	38% 7% 12% 27% 13% 2	2482
	21st to end	Day	48% 7% 13% 21% 11% 1	2968
June	1st to 10th	Day	44% 8% 15% 22% 10% 1	2880
	11th to 20th	Day	55% 9% 11% 18% 7% 1	2880
	21st to end	Day	40% 10% 18% 24% 7%	2880
July	1st to 10th	Day	33% 11% 20% 28% 8%	2801
	11th to 20th	Day	38% 9% 19% 27% 6%	2660
	21st to end	Day	34% 8% 20% 31% 7%	2648
August	1st to 10th	Day	37% 9% 20% 27% 8%	2491
	11th to 20th	Day	36% 9% 22% 27% 5%	2477
	21st to end	Day	37% 7% 14% 33% 9%	2496
September	1st to 10th	Day	37% 7% 16% 28% 11% 2	2160
	11th to 20th	Day	43% 8% 15% 26% 8% 1	2160
	21st to end	Day	34% 7% 15% 30% 12% 1	1933
October	1st to 10th	Day	46% 6% 17% 24% 7%	1800
	11th to 20th	Day	45% 7% 11% 26% 10% 1	1715
	21st to end	Day	55% 6% 15% 19% 5%	1782
November	1st to 10th	Day	62% 6% 11% 16% 5%	1493
	11th to 20th	Day	64% 7% 12% 12% 5%	1467
	21st to end	Day	61% 6% 14% 15% 4	1330
December	1st to 10th	Day	60% 8% 11% 13% 7% 1	1262
	11th to 20th	Day	64% 7% 16% 9% 5%	1440
	21st to end	Day	41% 6% 12% 19% 17% 6%	2090
January	1st to 10th	Night	62% 6% 13% 15% 4	4909
	11th to 20th	Night	62% 6% 13% 14% 4	4740
	21st to end	Night	52% 8% 14% 19% 6%	4905
February	1st to 10th	Night	55% 6% 12% 18% 8% 1	2583
	11th to 20th	Night	49% 7% 12% 20% 11% 1	2277
	21st to end	Night	43% 7% 14% 25% 11%	1849
March	1st to 10th	Night	51% 8% 14% 20% 8% 1	2160
	11th to 20th	Night	45% 8% 15% 22% 10% 1	2064
	21st to end	Night	47% 8% 12% 21% 11% 1	2021
April	1st to 10th	Night	39% 6% 14% 25% 14% 1	1800
	11th to 20th	Night	47% 6% 14% 18% 12% 3	1712
	21st to end	Night	49% 6% 14% 19% 11% 1	1440
May	1st to 10th	Night	40% 7% 8% 28% 16% 1	1440
	11th to 20th	Night	38% 6% 12% 24% 20% 1	1418
	21st to end	Night	44% 8% 12% 22% 14%	1305
June	1st to 10th	Night	34% 7% 19% 26% 11% 2	1080
	11th to 20th	Night	46% 9% 15% 19% 10% 1	1080
	21st to end	Night	35% 10% 21% 24% 9% 1	1080
July	1st to 10th	Night	26% 13% 20% 32% 9%	1050
	11th to 20th	Night	31% 9% 19% 34% 8%	1175
	21st to end	Night	20% 9% 22% 37% 11% 1	1428
August	1st to 10th	Night	23% 10% 23% 33% 10%	1424
	11th to 20th	Night	25% 10% 20% 35% 10%	1424
	21st to end	Night	32% 7% 15% 34% 12%	1856
September	1st to 10th	Night	32% 8% 17% 27% 14% 2	1800
	11th to 20th	Night	37% 7% 15% 31% 10% 1	1800
	21st to end	Night	32% 6% 15% 30% 16% 1	2024
October	1st to 10th	Night	44% 6% 14% 28% 8%	2160
	11th to 20th	Night	40% 6% 14% 28% 11% 1	2227
	21st to end	Night	51% 6% 13% 21% 8% 1	2574
November	1st to 10th	Night	60% 6% 12% 15% 6% 1	2582
	11th to 20th	Night	59% 8% 14% 15% 4	2712
	21st to end	Night	55% 8% 15% 17% 4	2850
December	1st to 10th	Night	55% 7% 13% 18% 6% 1	2916
	11th to 20th	Night	61% 6% 13% 13% 5%	3751
	21st to end	Night	37% 6% 11% 24% 17% 5%	5566

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% 7% 28% 29% 17% 5%	180
	11th to 20th	Day	9% 4 32% 40% 12% 3	180
	21st to end	Day	11% 8% 31% 33% 15% 2	287
February	1st to 10th	Day	9% 8% 40% 30% 10% 3	252
	11th to 20th	Day	9% 5% 29% 36% 18% 3	240
	21st to end	Day	7% 5% 31% 35% 19% 5%	198
March	1st to 10th	Day	12% 5% 29% 35% 16% 2	240
	11th to 20th	Day	11% 6% 31% 35% 14% 3	334
	21st to end	Day	15% 6% 30% 31% 15% 2	462
April	1st to 10th	Day	10% 6% 38% 27% 14% 5%	420
	11th to 20th	Day	17% 10% 27% 29% 14% 4	451
	21st to end	Day	15% 10% 27% 31% 15% 3	600
May	1st to 10th	Day	12% 5% 27% 32% 21% 4	603
	11th to 20th	Day	13% 7% 27% 27% 20% 5%	625
	21st to end	Day	17% 8% 20% 31% 18% 6%	679
June	1st to 10th	Day	14% 8% 28% 28% 15% 6%	630
	11th to 20th	Day	23% 9% 26% 23% 15% 4	630
	21st to end	Day	13% 7% 25% 26% 21% 7%	630
July	1st to 10th	Day	10% 4 22% 34% 23% 6%	621
	11th to 20th	Day	11% 6% 23% 33% 20% 6%	618
	21st to end	Day	8% 6% 24% 29% 22% 10%	684
August	1st to 10th	Day	10% 6% 21% 31% 24% 8%	626
	11th to 20th	Day	11% 8% 19% 30% 23% 9%	615
	21st to end	Day	14% 6% 23% 32% 19% 5%	524
September	1st to 10th	Day	14% 12% 30% 26% 15% 2	450
	11th to 20th	Day	18% 6% 26% 33% 15% 2	450
	21st to end	Day	14% 9% 28% 32% 15% 3	274
October	1st to 10th	Day	19% 4 28% 25% 20% 3	270
	11th to 20th	Day	13% 10% 32% 29% 14% 3	270
	21st to end	Day	18% 11% 33% 25% 11% 2	297
November	1st to 10th	Day	24% 7% 31% 24% 10% 4	270
	11th to 20th	Day	16% 6% 45% 27% 6% 1	270
	21st to end	Day	12% 8% 40% 29% 9% 3	270
December	1st to 10th	Day	7% 6% 30% 35% 18% 4	237
	11th to 20th	Day	12% 10% 36% 33% 8%	180
	21st to end	Day	8% 3 35% 28% 21% 6%	198
January	1st to 10th	Night	12% 7% 36% 31% 12% 1	950
	11th to 20th	Night	12% 7% 40% 30% 10% 1	950
	21st to end	Night	13% 8% 37% 32% 9% 1	1045
February	1st to 10th	Night	13% 5% 32% 35% 13% 2	923
	11th to 20th	Night	9% 6% 30% 37% 14% 3	708
	21st to end	Night	7% 4 27% 37% 23% 3	592
March	1st to 10th	Night	13% 4 30% 34% 16% 2	720
	11th to 20th	Night	11% 6% 32% 36% 13% 2	720
	21st to end	Night	14% 6% 26% 35% 16% 3	635
April	1st to 10th	Night	9% 6% 29% 35% 17% 3	540
	11th to 20th	Night	16% 7% 27% 36% 15% 1	533
	21st to end	Night	18% 8% 29% 33% 11% 1	540
May	1st to 10th	Night	14% 5% 28% 34% 17% 2	540
	11th to 20th	Night	14% 7% 29% 32% 18% 1	531
	21st to end	Night	18% 4 24% 35% 15% 4	501
June	1st to 10th	Night	13% 6% 27% 35% 18% 1	360
	11th to 20th	Night	22% 6% 24% 29% 17% 3	360
	21st to end	Night	14% 4 23% 38% 21% 1	360
July	1st to 10th	Night	7% 4 23% 41% 25% 1	350
	11th to 20th	Night	10% 3 24% 43% 19% 2	348
	21st to end	Night	6% 3 23% 39% 24% 5%	527
August	1st to 10th	Night	6% 4 24% 35% 27% 4	537
	11th to 20th	Night	9% 3 19% 37% 30% 2	534
	21st to end	Night	14% 4 25% 37% 20% 1	594
September	1st to 10th	Night	12% 3 26% 38% 17% 3	540
	11th to 20th	Night	15% 4 23% 40% 16% 2	540
	21st to end	Night	12% 6% 32% 35% 14% 1	589
October	1st to 10th	Night	21% 4 28% 34% 13% 1	720
	11th to 20th	Night	12% 6% 31% 36% 13% 2	787
	21st to end	Night	17% 9% 32% 32% 9% 1	990
November	1st to 10th	Night	25% 6% 30% 31% 6% 1	925
	11th to 20th	Night	14% 8% 37% 29% 9% 2	950
	21st to end	Night	12% 8% 41% 30% 7% 1	950
December	1st to 10th	Night	11% 5% 35% 32% 14% 4	950
	11th to 20th	Night	13% 9% 35% 31% 11% 1	950
	21st to end	Night	12% 4 29% 35% 16% 4	1045

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