Lassen National Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lassen National Park 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	3 6% 2 5% 3 3 3 3 4 6% 60%	135027
	Night	2 6% 3 6% 4 4 4 3 5% 6% 57%	131085
Transparency	Day	26% 5% 7% 19% 25% 18%	129929
	Night	28% 6% 8% 19% 23% 16%	127173
Seeing	Day	14% 17% 29% 24% 13% 3	16076
	Night	10% 12% 20% 32% 23% 4	25236

By Month

Percentage Cloud Cover SW (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	3 10% 5% 9% 6% 5% 5% 5% 7% 7% 37%	3639
	11th to 20th	Day	2 8% 3 9% 5% 5% 4 3 4 5% 54%	3847
	21st to end	Day	3 10% 4 9% 5% 4 4 3 4 6% 49%	4510
February	1st to 10th	Day	4 9% 5% 8% 6% 6% 5% 4 6% 6% 41%	1800
	11th to 20th	Day	2 13% 5% 8% 5% 5% 5% 4 6% 8% 39%	1825
	21st to end	Day	4 13% 5% 9% 6% 5% 5% 3 4 7% 40%	1650
March	1st to 10th	Day	2 16% 7% 14% 6% 4 3 3 3 5% 36%	4682
	11th to 20th	Day	2 8% 5% 8% 5% 4 5% 4 6% 9% 45%	5000
	21st to end	Day	3 15% 5% 11% 6% 5% 5% 4 5% 7% 35%	5678
April	1st to 10th	Day	5% 13% 4 8% 5% 5% 5% 4 6% 8% 37%	2400
	11th to 20th	Day	5% 13% 3 7% 4 4 4 4 5% 7% 44%	2369
	21st to end	Day	5% 11% 3 5% 4 3 4 3 6% 7% 49%	2400
May	1st to 10th	Day	3 7% 2 4 2 2 2 3 4 7% 65%	5873
	11th to 20th	Day	3 5% 2 4 3 3 3 2 4 6% 65%	6277
	21st to end	Day	4 5% 1 3 2 2 3 2 4 7% 66%	6902
June	1st to 10th	Day	6% 6% 2 5% 4 4 4 4 5% 6% 53%	2800
	11th to 20th	Day	3 4 1 3 2 2 3 3 5% 6% 69%	2800
	21st to end	Day	2 2 1 2 3 2 3 2 3 4 77%	2800
July	1st to 10th	Day	1 1 1 1 1 1 2 2 4 86%	6319
	11th to 20th	Day	1 1 1 2 1 2 2 1 2 3 83%	6320
	21st to end	Day	1 1 1 3 3 4 4 3 4 5% 72%	6583
August	1st to 10th	Day	1 2 1 2 2 2 2 2 4 5% 77%	5777
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 87%	5584
	21st to end	Day	1 1 1 2 2 3 3 2 3 5% 77%	5981
September	1st to 10th	Day	1 1 1 2 2 3 2 3 5% 81%	2317
	11th to 20th	Day	3 3 1 4 3 3 3 3 5% 7% 65%	2280
	21st to end	Day	3 3 1 3 2 1 2 2 4 6% 72%	2037
October	1st to 10th	Day	3 3 1 3 2 2 2 3 3 5% 72%	3180
	11th to 20th	Day	4 4 2 4 3 3 3 3 5% 7% 62%	3161
	21st to end	Day	3 5% 2 4 4 4 4 3 6% 8% 56%	3448
November	1st to 10th	Day	7% 8% 2 6% 5% 5% 5% 4 5% 8% 45%	1776
	11th to 20th	Day	4 7% 3 6% 5% 6% 6% 6% 8% 8% 40%	1760
	21st to end	Day	2 8% 4 8% 4 4 5% 4 6% 7% 48%	1600
December	1st to 10th	Day	2 7% 2 6% 4 5% 5% 4 5% 7% 52%	2620
	11th to 20th	Day	3 11% 4 10% 6% 7% 8% 7% 8% 9% 27%	3028
	21st to end	Day	3 11% 4 10% 6% 5% 7% 5% 8% 9% 32%	4004
January	1st to 10th	Night	1 6% 3 9% 6% 7% 8% 6% 8% 9% 36%	6306
	11th to 20th	Night	2 8% 3 8% 5% 4 4 3 5% 6% 53%	6117
	21st to end	Night	3 9% 5% 10% 6% 5% 5% 4 4 6% 45%	6468
February	1st to 10th	Night	4 8% 3 9% 6% 6% 5% 4 6% 8% 41%	2600
	11th to 20th	Night	2 9% 5% 10% 7% 5% 5% 4 5% 7% 41%	2461
	21st to end	Night	3 13% 4 10% 5% 4 5% 4 5% 6% 41%	1980
March	1st to 10th	Night	2 12% 7% 16% 8% 5% 4 3 4 6% 33%	5210
	11th to 20th	Night	2 7% 3 9% 6% 4 4 4 7% 8% 45%	5000
	21st to end	Night	1 11% 5% 10% 6% 5% 5% 4 6% 7% 38%	5297
April	1st to 10th	Night	2 12% 4 8% 6% 5% 5% 5% 6% 7% 42%	2000
	11th to 20th	Night	4 11% 3 6% 4 4 3 4 6% 8% 47%	1979
	21st to end	Night	4 11% 3 4 4 4 3 4 5% 8% 50%	2000
May	1st to 10th	Night	2 7% 3 5% 3 2 3 2 4 5% 64%	4086
	11th to 20th	Night	2 6% 1 4 3 3 3 3 4 4 68%	3616
	21st to end	Night	4 3 1 3 2 2 3 2 3 5% 72%	3972
June	1st to 10th	Night	4 7% 2 5% 4 3 3 3 5% 8% 55%	1600
	11th to 20th	Night	3 3 1 3 3 2 3 2 4 7% 70%	1600
	21st to end	Night	2 2 1 2 2 3 4 2 2 3 77%	1600
July	1st to 10th	Night	1 1 1 1 1 2 3 90%	3640
	11th to 20th	Night	1 1 1 1 1 2 1 2 2 88%	3640
	21st to end	Night	1 2 1 2 1 2 2 2 4 4 79%	4279
August	1st to 10th	Night	2 1 1 2 1 2 2 2 5% 5% 78%	4035
	11th to 20th	Night	1 1 1 1 1 1 1 2 2 89%	4272
	21st to end	Night	1 2 1 2 2 2 2 2 2 3 82%	5015
September	1st to 10th	Night	2 1 1 1 2 2 1 2 4 84%	1930
	11th to 20th	Night	2 2 1 3 2 2 2 3 3 6% 74%	1900
	21st to end	Night	2 2 1 2 2 2 3 2 4 6% 73%	2140
October	1st to 10th	Night	3 3 1 2 2 3 2 2 2 3 77%	3800
	11th to 20th	Night	2 3 1 3 2 2 3 3 4 6% 69%	3777
	21st to end	Night	2 5% 2 5% 3 4 4 4 6% 9% 56%	4121
November	1st to 10th	Night	6% 7% 3 7% 6% 5% 5% 4 6% 7% 44%	2521
	11th to 20th	Night	4 7% 3 7% 6% 5% 6% 5% 8% 8% 42%	2635
	21st to end	Night	2 7% 4 7% 5% 4 4 4 7% 9% 47%	2800
December	1st to 10th	Night	2 5% 2 4 4 4 4 4 5% 8% 58%	4560
	11th to 20th	Night	3 9% 3 10% 7% 7% 8% 7% 9% 11% 27%	5176
	21st to end	Night	3 10% 4 10% 6% 7% 7% 5% 7% 9% 34%	6952

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	42% 9% 12% 18% 14% 5%	3639
	11th to 20th	Day	35% 4 9% 15% 24% 13%	3847
	21st to end	Day	40% 3 8% 12% 20% 15%	4510
February	1st to 10th	Day	42% 6% 11% 20% 13% 8%	1800
	11th to 20th	Day	44% 6% 9% 19% 18% 5%	1834
	21st to end	Day	44% 5% 10% 20% 13% 7%	1650
March	1st to 10th	Day	51% 6% 5% 13% 19% 5%	4682
	11th to 20th	Day	35% 8% 11% 29% 16% 2	5000
	21st to end	Day	47% 7% 13% 22% 10% 1	5678
April	1st to 10th	Day	44% 6% 13% 19% 16% 3	2400
	11th to 20th	Day	38% 5% 10% 24% 17% 5%	2372
	21st to end	Day	33% 5% 14% 28% 17% 3	2400
May	1st to 10th	Day	20% 3 9% 23% 40% 6%	5873
	11th to 20th	Day	19% 5% 9% 23% 37% 6%	6280
	21st to end	Day	18% 4 8% 33% 34% 2	6904
June	1st to 10th	Day	30% 5% 10% 26% 25% 4	2800
	11th to 20th	Day	16% 4 8% 24% 35% 13%	2800
	21st to end	Day	11% 4 4 20% 36% 26%	2800
July	1st to 10th	Day	5% 2 3 12% 22% 56%	6251
	11th to 20th	Day	8% 2 2 7% 20% 61%	6245
	21st to end	Day	13% 6% 4 16% 28% 33%	6393
August	1st to 10th	Day	13% 3 3 13% 20% 49%	4206
	11th to 20th	Day	5% 2 2 11% 46% 34%	4043
	21st to end	Day	8% 3 2 20% 36% 30%	4311
September	1st to 10th	Day	9% 2 2 10% 32% 44%	2317
	11th to 20th	Day	18% 4 6% 20% 32% 20%	2280
	21st to end	Day	15% 3 6% 22% 33% 22%	2037
October	1st to 10th	Day	16% 3 4 15% 31% 31%	3180
	11th to 20th	Day	21% 5% 6% 25% 33% 11%	3161
	21st to end	Day	25% 6% 9% 24% 26% 10%	3448
November	1st to 10th	Day	37% 5% 10% 19% 18% 12%	1776
	11th to 20th	Day	37% 6% 13% 20% 17% 8%	1760
	21st to end	Day	35% 5% 10% 19% 21% 11%	1600
December	1st to 10th	Day	31% 5% 6% 18% 29% 12%	2620
	11th to 20th	Day	50% 11% 11% 14% 12% 2	3028
	21st to end	Day	46% 9% 13% 15% 13% 3	4004
January	1st to 10th	Night	39% 10% 14% 19% 13% 4	6306
	11th to 20th	Night	34% 5% 9% 13% 26% 13%	6117
	21st to end	Night	42% 4 6% 11% 20% 17%	6468
February	1st to 10th	Night	40% 5% 9% 20% 17% 9%	2600
	11th to 20th	Night	41% 6% 10% 19% 19% 6%	2463
	21st to end	Night	42% 6% 11% 19% 15% 7%	1980
March	1st to 10th	Night	50% 8% 7% 13% 16% 6%	5210
	11th to 20th	Night	33% 9% 10% 24% 20% 4	5000
	21st to end	Night	42% 9% 19% 18% 10% 1	5297
April	1st to 10th	Night	40% 7% 12% 23% 14% 4	2000
	11th to 20th	Night	34% 5% 14% 24% 17% 6%	1979
	21st to end	Night	30% 5% 16% 28% 17% 2	2000
May	1st to 10th	Night	23% 5% 7% 26% 33% 7%	4083
	11th to 20th	Night	19% 5% 9% 30% 29% 8%	3616
	21st to end	Night	16% 4 9% 40% 30% 2	3972
June	1st to 10th	Night	26% 5% 12% 32% 22% 3	1600
	11th to 20th	Night	15% 5% 9% 26% 36% 10%	1600
	21st to end	Night	14% 4 5% 20% 34% 25%	1600
July	1st to 10th	Night	3 1 3 13% 25% 55%	3600
	11th to 20th	Night	7% 1 2 6% 19% 66%	3592
	21st to end	Night	10% 3 3 18% 34% 33%	4157
August	1st to 10th	Night	13% 3 3 11% 23% 47%	2929
	11th to 20th	Night	5% 2 1 11% 43% 38%	3084
	21st to end	Night	7% 2 3 17% 42% 28%	3608
September	1st to 10th	Night	8% 2 2 9% 32% 47%	1930
	11th to 20th	Night	13% 4 7% 21% 32% 23%	1900
	21st to end	Night	13% 2 6% 23% 33% 22%	2140
October	1st to 10th	Night	15% 2 4 13% 33% 32%	3800
	11th to 20th	Night	15% 4 7% 32% 31% 11%	3777
	21st to end	Night	21% 8% 9% 22% 28% 12%	4121
November	1st to 10th	Night	37% 6% 12% 18% 18% 10%	2521
	11th to 20th	Night	36% 7% 12% 20% 17% 7%	2635
	21st to end	Night	33% 5% 10% 21% 20% 12%	2800
December	1st to 10th	Night	26% 4 6% 17% 28% 19%	4560
	11th to 20th	Night	47% 11% 9% 14% 16% 3	5176
	21st to end	Night	45% 11% 12% 15% 12% 5%	6952

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	32% 23% 24% 15% 6% 1	200
	11th to 20th	Day	20% 26% 26% 20% 7% 2	290
	21st to end	Day	19% 20% 29% 18% 12% 2	440
February	1st to 10th	Day	27% 20% 26% 18% 8% 1	400
	11th to 20th	Day	28% 20% 28% 17% 6%	430
	21st to end	Day	21% 23% 30% 21% 5%	414
March	1st to 10th	Day	28% 31% 23% 12% 4 1	500
	11th to 20th	Day	28% 19% 26% 17% 9% 1	500
	21st to end	Day	24% 30% 26% 15% 6%	550
April	1st to 10th	Day	20% 25% 29% 18% 7% 1	500
	11th to 20th	Day	23% 23% 25% 18% 9% 2	492
	21st to end	Day	17% 21% 29% 21% 10% 2	500
May	1st to 10th	Day	15% 24% 32% 18% 9% 2	503
	11th to 20th	Day	18% 17% 32% 19% 10% 4	528
	21st to end	Day	16% 17% 25% 27% 12% 4	573
June	1st to 10th	Day	10% 18% 27% 25% 16% 4	530
	11th to 20th	Day	10% 22% 31% 23% 12% 2	530
	21st to end	Day	4 13% 32% 31% 15% 5%	530
July	1st to 10th	Day	2 9% 40% 28% 16% 5%	526
	11th to 20th	Day	1 7% 36% 32% 16% 8%	524
	21st to end	Day	1 7% 32% 34% 18% 8%	575
August	1st to 10th	Day	4 9% 29% 33% 17% 9%	527
	11th to 20th	Day	2 12% 30% 30% 16% 10%	517
	21st to end	Day	3 8% 30% 34% 18% 6%	583
September	1st to 10th	Day	3 10% 35% 32% 17% 2	503
	11th to 20th	Day	6% 14% 30% 29% 18% 3	490
	21st to end	Day	7% 13% 32% 34% 12% 2	490
October	1st to 10th	Day	7% 16% 33% 25% 18% 1	490
	11th to 20th	Day	14% 15% 24% 28% 18% 2	490
	21st to end	Day	16% 19% 24% 25% 13% 3	539
November	1st to 10th	Day	17% 16% 22% 25% 17% 3	312
	11th to 20th	Day	14% 18% 26% 21% 18% 3	280
	21st to end	Day	17% 18% 27% 22% 14% 4	200
December	1st to 10th	Day	22% 16% 23% 18% 15% 7%	200
	11th to 20th	Day	21% 21% 25% 17% 13% 5%	200
	21st to end	Day	25% 18% 25% 20% 12%	220
January	1st to 10th	Night	14% 18% 26% 27% 12% 4	800
	11th to 20th	Night	13% 18% 25% 29% 13% 2	800
	21st to end	Night	13% 15% 27% 27% 16% 3	880
February	1st to 10th	Night	16% 16% 24% 29% 14% 2	800
	11th to 20th	Night	14% 17% 24% 28% 14% 3	784
	21st to end	Night	17% 14% 25% 32% 10% 1	660
March	1st to 10th	Night	18% 21% 23% 25% 11% 3	800
	11th to 20th	Night	17% 16% 19% 25% 20% 3	800
	21st to end	Night	15% 22% 24% 24% 13% 2	880
April	1st to 10th	Night	12% 17% 24% 33% 13% 1	800
	11th to 20th	Night	13% 15% 26% 29% 14% 3	791
	21st to end	Night	11% 11% 23% 33% 20% 3	800
May	1st to 10th	Night	10% 13% 29% 29% 18% 3	599
	11th to 20th	Night	10% 8% 18% 35% 25% 3	394
	21st to end	Night	12% 8% 17% 30% 30% 3	433
June	1st to 10th	Night	8% 7% 17% 38% 30% 2	400
	11th to 20th	Night	4 11% 21% 35% 28% 2	400
	21st to end	Night	3 5% 13% 41% 36% 3	400
July	1st to 10th	Night	1 2 8% 41% 47% 2	390
	11th to 20th	Night	2 1 7% 44% 43% 3	388
	21st to end	Night	1 1 5% 39% 48% 5%	568
August	1st to 10th	Night	3 1 7% 35% 49% 6%	597
	11th to 20th	Night	2 2 11% 40% 40% 6%	684
	21st to end	Night	3 2 9% 41% 42% 4	880
September	1st to 10th	Night	2 3 15% 41% 34% 4	772
	11th to 20th	Night	2 6% 12% 35% 36% 8%	760
	21st to end	Night	5% 6% 15% 37% 29% 9%	760
October	1st to 10th	Night	7% 9% 18% 34% 28% 5%	760
	11th to 20th	Night	9% 8% 16% 33% 29% 5%	760
	21st to end	Night	9% 13% 18% 30% 23% 7%	836
November	1st to 10th	Night	12% 11% 20% 31% 21% 5%	780
	11th to 20th	Night	12% 15% 23% 31% 16% 3	800
	21st to end	Night	10% 15% 24% 30% 17% 4	800
December	1st to 10th	Night	13% 15% 23% 30% 17% 3	800
	11th to 20th	Night	17% 15% 24% 28% 14% 3	800
	21st to end	Night	21% 17% 25% 27% 9% 1	880

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