Mount Saint Helens
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Saint Helens clear sky chart from 2005-11-06 to 2025-08-05.

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	18% 17% 4 7% 4 3 3 3 4 5% 34%	130300
	Night	18% 19% 5% 8% 4 3 3 2 4 4 30%	126462
Transparency	Day	52% 5% 9% 17% 13% 5%	130024
	Night	57% 5% 8% 15% 11% 4	126315
Seeing	Day	28% 14% 25% 22% 9% 2	15667
	Night	31% 9% 17% 26% 15% 2	24342

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	27% 33% 7% 8% 4 3 2 1 3 3 10%	3639
	11th to 20th	Day	19% 17% 6% 9% 3 3 3 3 3 5% 30%	3616
	21st to end	Day	23% 21% 4 7% 3 3 3 2 3 3 30%	4234
February	1st to 10th	Day	25% 24% 6% 9% 5% 3 3 2 3 4 15%	1800
	11th to 20th	Day	23% 29% 6% 8% 3 3 2 2 3 3 18%	1755
	21st to end	Day	23% 32% 6% 8% 3 2 3 1 2 4 16%	1575
March	1st to 10th	Day	13% 25% 8% 10% 5% 4 5% 4 5% 5% 18%	4645
	11th to 20th	Day	15% 30% 7% 10% 4 2 2 2 2 2 24%	5000
	21st to end	Day	21% 28% 8% 11% 6% 4 4 3 4 3 9%	5658
April	1st to 10th	Day	25% 29% 5% 10% 4 2 4 3 4 4 12%	2400
	11th to 20th	Day	20% 26% 5% 8% 4 3 2 2 3 3 23%	2372
	21st to end	Day	24% 24% 4 7% 5% 3 3 2 3 4 19%	2594
May	1st to 10th	Day	16% 17% 3 7% 4 3 3 3 4 5% 36%	6320
	11th to 20th	Day	20% 22% 5% 9% 4 4 4 3 4 4 21%	6277
	21st to end	Day	21% 18% 4 7% 4 3 4 3 5% 6% 25%	6903
June	1st to 10th	Day	29% 11% 3 6% 4 4 4 4 5% 5% 26%	2800
	11th to 20th	Day	28% 13% 3 8% 5% 5% 5% 4 6% 5% 20%	2800
	21st to end	Day	16% 8% 2 6% 5% 4 4 4 5% 7% 40%	2991
July	1st to 10th	Day	6% 4 2 3 2 2 3 2 4 5% 67%	6396
	11th to 20th	Day	5% 3 1 2 1 2 2 2 3 4 75%	6320
	21st to end	Day	7% 4 1 3 2 2 3 2 4 6% 67%	6874
August	1st to 10th	Day	5% 4 2 4 2 3 3 3 4 6% 65%	4305
	11th to 20th	Day	8% 8% 3 7% 4 5% 4 4 6% 7% 44%	4026
	21st to end	Day	15% 8% 2 5% 3 3 4 2 5% 6% 46%	4285
September	1st to 10th	Day	11% 4 2 4 3 3 4 3 5% 5% 55%	2280
	11th to 20th	Day	16% 8% 3 6% 5% 4 4 3 5% 5% 41%	2280
	21st to end	Day	20% 8% 3 6% 4 3 4 3 3 6% 41%	2010
October	1st to 10th	Day	20% 8% 2 4 3 3 3 3 4 6% 46%	3180
	11th to 20th	Day	24% 14% 3 7% 4 4 4 3 4 6% 27%	3161
	21st to end	Day	24% 20% 5% 9% 4 3 3 3 3 3 23%	3296
November	1st to 10th	Day	32% 20% 4 8% 4 4 4 2 3 4 15%	1659
	11th to 20th	Day	34% 21% 4 7% 4 2 2 3 4 4 16%	1600
	21st to end	Day	28% 21% 5% 8% 3 3 3 2 3 4 21%	1600
December	1st to 10th	Day	18% 18% 4 6% 3 3 4 3 3 3 35%	2620
	11th to 20th	Day	24% 25% 6% 11% 5% 4 3 2 3 4 12%	3025
	21st to end	Day	26% 29% 5% 9% 4 3 3 2 3 3 11%	4004
January	1st to 10th	Night	29% 28% 6% 7% 4 3 3 2 2 2 12%	6306
	11th to 20th	Night	18% 17% 7% 9% 3 3 3 2 4 5% 30%	6282
	21st to end	Night	23% 19% 4 7% 3 2 2 2 3 4 31%	6716
February	1st to 10th	Night	24% 23% 7% 9% 4 4 4 2 4 4 15%	2600
	11th to 20th	Night	22% 26% 5% 7% 4 3 3 2 3 3 22%	2532
	21st to end	Night	21% 29% 6% 8% 4 3 3 2 3 4 17%	2054
March	1st to 10th	Night	13% 22% 9% 12% 6% 5% 4 3 4 4 18%	5247
	11th to 20th	Night	15% 29% 8% 9% 4 3 2 2 3 3 25%	4999
	21st to end	Night	22% 31% 7% 10% 4 3 3 2 4 4 10%	5316
April	1st to 10th	Night	20% 28% 6% 10% 5% 4 3 2 3 3 16%	2000
	11th to 20th	Night	19% 23% 4 8% 4 4 3 3 4 5% 23%	1979
	21st to end	Night	24% 22% 4 8% 4 4 4 3 4 6% 18%	1803
May	1st to 10th	Night	17% 21% 5% 7% 3 3 3 3 3 4 32%	3640
	11th to 20th	Night	14% 27% 4 8% 4 4 5% 4 4 4 22%	3616
	21st to end	Night	13% 24% 5% 7% 4 4 4 3 5% 7% 23%	3972
June	1st to 10th	Night	26% 13% 3 7% 4 4 4 2 4 6% 29%	1600
	11th to 20th	Night	23% 15% 4 8% 5% 3 4 4 4 6% 25%	1595
	21st to end	Night	14% 8% 3 7% 4 5% 3 2 5% 6% 44%	1400
July	1st to 10th	Night	3 5% 2 4 2 3 3 3 4 5% 66%	3538
	11th to 20th	Night	3 3 1 3 2 2 2 2 3 4 75%	3640
	21st to end	Night	4 3 1 4 2 3 3 2 4 7% 66%	3969
August	1st to 10th	Night	3 4 2 3 3 2 3 3 4 5% 68%	2715
	11th to 20th	Night	7% 11% 4 8% 5% 3 4 3 4 5% 47%	2812
	21st to end	Night	14% 11% 2 6% 3 3 3 2 3 5% 49%	3389
September	1st to 10th	Night	10% 5% 2 6% 4 3 3 3 4 5% 56%	1900
	11th to 20th	Night	15% 10% 3 7% 4 4 3 2 4 5% 41%	1900
	21st to end	Night	16% 10% 3 7% 3 3 3 4 4 5% 41%	2166
October	1st to 10th	Night	16% 8% 2 6% 3 3 3 3 4 7% 45%	3800
	11th to 20th	Night	23% 12% 3 7% 3 4 4 3 4 7% 30%	3777
	21st to end	Night	25% 20% 5% 7% 3 3 3 3 3 4 23%	4267
November	1st to 10th	Night	34% 17% 4 8% 4 3 3 3 3 3 18%	2640
	11th to 20th	Night	33% 20% 4 7% 4 3 3 2 3 3 18%	2800
	21st to end	Night	26% 21% 5% 8% 4 3 3 2 3 3 23%	2800
December	1st to 10th	Night	17% 17% 4 7% 4 3 4 3 4 5% 33%	4560
	11th to 20th	Night	24% 26% 6% 10% 5% 4 3 2 3 4 11%	5180
	21st to end	Night	25% 29% 7% 11% 4 4 3 2 2 2 11%	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	81% 3 4 6% 5%	3639
	11th to 20th	Day	57% 3 7% 14% 16% 3	3640
	21st to end	Day	61% 2 4 12% 16% 5%	4234
February	1st to 10th	Day	74% 4 7% 9% 5%	1800
	11th to 20th	Day	73% 4 6% 10% 5% 2	1764
	21st to end	Day	75% 3 6% 8% 6% 1	1575
March	1st to 10th	Day	65% 4 11% 11% 8% 2	4645
	11th to 20th	Day	68% 3 5% 21% 3	5000
	21st to end	Day	78% 6% 8% 6% 2	5658
April	1st to 10th	Day	75% 5% 9% 10% 1	2400
	11th to 20th	Day	66% 3 10% 12% 8% 1	2372
	21st to end	Day	68% 5% 8% 12% 5% 1	2594
May	1st to 10th	Day	49% 4 9% 21% 15% 1	6320
	11th to 20th	Day	60% 8% 14% 12% 5%	6280
	21st to end	Day	57% 6% 16% 16% 3 1	6904
June	1st to 10th	Day	57% 6% 14% 14% 7% 1	2800
	11th to 20th	Day	62% 7% 13% 11% 7% 1	2800
	21st to end	Day	40% 7% 15% 24% 11% 3	2991
July	1st to 10th	Day	20% 3 8% 25% 22% 22%	6327
	11th to 20th	Day	15% 2 7% 21% 32% 22%	6245
	21st to end	Day	19% 3 10% 37% 25% 5%	6681
August	1st to 10th	Day	21% 5% 7% 20% 30% 17%	4288
	11th to 20th	Day	33% 6% 13% 31% 13% 4	4065
	21st to end	Day	37% 5% 10% 22% 20% 7%	4284
September	1st to 10th	Day	28% 5% 12% 25% 22% 9%	2280
	11th to 20th	Day	43% 6% 11% 17% 17% 7%	2280
	21st to end	Day	44% 5% 12% 19% 15% 5%	2010
October	1st to 10th	Day	39% 3 6% 24% 25% 3	3180
	11th to 20th	Day	58% 5% 10% 15% 12% 1	3161
	21st to end	Day	67% 3 4 13% 10% 2	3296
November	1st to 10th	Day	74% 4 6% 11% 4 1	1659
	11th to 20th	Day	73% 3 7% 10% 6% 2	1600
	21st to end	Day	69% 3 3 10% 12% 2	1600
December	1st to 10th	Day	55% 3 4 10% 22% 6%	2620
	11th to 20th	Day	77% 5% 7% 8% 3	3028
	21st to end	Day	77% 5% 8% 7% 2	4004
January	1st to 10th	Night	79% 3 4 7% 6% 1	6306
	11th to 20th	Night	57% 4 6% 16% 15% 3	6320
	21st to end	Night	59% 3 5% 10% 16% 7%	6716
February	1st to 10th	Night	71% 6% 8% 9% 5%	2600
	11th to 20th	Night	67% 5% 9% 11% 5% 3	2534
	21st to end	Night	72% 4 7% 9% 6% 1	2054
March	1st to 10th	Night	66% 6% 9% 11% 6% 3	5247
	11th to 20th	Night	66% 4 7% 18% 4 1	5000
	21st to end	Night	78% 5% 7% 9% 1	5316
April	1st to 10th	Night	72% 6% 9% 10% 3	2000
	11th to 20th	Night	62% 6% 10% 14% 7% 1	1979
	21st to end	Night	64% 8% 9% 12% 5% 1	1803
May	1st to 10th	Night	54% 4 8% 20% 14%	3637
	11th to 20th	Night	57% 8% 15% 14% 6%	3616
	21st to end	Night	56% 10% 13% 15% 4 1	3972
June	1st to 10th	Night	56% 5% 14% 17% 7% 1	1600
	11th to 20th	Night	55% 7% 15% 14% 8%	1595
	21st to end	Night	39% 6% 16% 25% 11% 3	1400
July	1st to 10th	Night	19% 3 8% 25% 20% 24%	3498
	11th to 20th	Night	14% 3 7% 21% 30% 25%	3592
	21st to end	Night	17% 3 13% 30% 27% 10%	3857
August	1st to 10th	Night	19% 4 9% 19% 32% 17%	2707
	11th to 20th	Night	37% 5% 12% 27% 15% 4	2839
	21st to end	Night	38% 4 10% 22% 21% 5%	3389
September	1st to 10th	Night	29% 5% 11% 24% 22% 9%	1900
	11th to 20th	Night	44% 6% 8% 18% 15% 9%	1900
	21st to end	Night	42% 5% 15% 16% 15% 6%	2166
October	1st to 10th	Night	38% 4 8% 26% 22% 3	3800
	11th to 20th	Night	54% 5% 10% 18% 12% 2	3777
	21st to end	Night	66% 4 6% 12% 12% 1	4267
November	1st to 10th	Night	71% 5% 6% 10% 7% 1	2640
	11th to 20th	Night	73% 4 7% 8% 6% 2	2800
	21st to end	Night	67% 3 5% 9% 13% 3	2800
December	1st to 10th	Night	53% 4 7% 11% 19% 6%	4560
	11th to 20th	Night	77% 5% 7% 7% 4	5176
	21st to end	Night	80% 5% 6% 6% 2	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	49% 14% 24% 8% 6% 1	200
	11th to 20th	Day	36% 23% 21% 13% 7% 1	200
	21st to end	Day	37% 19% 18% 16% 10% 1	320
February	1st to 10th	Day	42% 17% 20% 16% 6% 1	400
	11th to 20th	Day	45% 17% 22% 14% 2	395
	21st to end	Day	41% 21% 20% 16% 2	377
March	1st to 10th	Day	38% 17% 28% 14% 3	500
	11th to 20th	Day	37% 18% 23% 14% 6% 1	500
	21st to end	Day	39% 21% 23% 14% 3 1	550
April	1st to 10th	Day	40% 19% 22% 14% 5% 1	500
	11th to 20th	Day	38% 21% 22% 13% 5% 1	492
	21st to end	Day	35% 18% 24% 15% 6% 1	500
May	1st to 10th	Day	28% 19% 23% 22% 7% 1	503
	11th to 20th	Day	36% 13% 25% 17% 7% 2	528
	21st to end	Day	33% 13% 24% 20% 9% 2	573
June	1st to 10th	Day	29% 12% 24% 23% 9% 2	530
	11th to 20th	Day	29% 18% 29% 17% 6% 1	530
	21st to end	Day	16% 13% 30% 27% 11% 2	721
July	1st to 10th	Day	13% 9% 34% 33% 9% 2	563
	11th to 20th	Day	10% 9% 33% 32% 13% 3	524
	21st to end	Day	7% 6% 29% 37% 15% 5%	575
August	1st to 10th	Day	8% 10% 27% 36% 14% 6%	504
	11th to 20th	Day	8% 8% 26% 35% 17% 6%	477
	21st to end	Day	13% 15% 27% 29% 11% 5%	539
September	1st to 10th	Day	13% 7% 32% 33% 11% 3	490
	11th to 20th	Day	18% 12% 29% 28% 12% 1	490
	21st to end	Day	23% 13% 24% 29% 10% 1	490
October	1st to 10th	Day	28% 11% 26% 21% 14% 1	490
	11th to 20th	Day	34% 12% 19% 22% 11% 2	490
	21st to end	Day	35% 17% 20% 17% 8% 3	449
November	1st to 10th	Day	42% 17% 17% 17% 6%	247
	11th to 20th	Day	46% 16% 17% 12% 9% 2	200
	21st to end	Day	35% 16% 19% 16% 15%	200
December	1st to 10th	Day	42% 11% 15% 21% 11% 2	200
	11th to 20th	Day	42% 14% 26% 16% 4	200
	21st to end	Day	44% 12% 26% 13% 5%	220
January	1st to 10th	Night	47% 13% 17% 16% 7%	800
	11th to 20th	Night	37% 13% 20% 20% 10% 1	800
	21st to end	Night	35% 10% 21% 24% 9% 1	880
February	1st to 10th	Night	36% 11% 24% 22% 6% 1	800
	11th to 20th	Night	37% 10% 20% 25% 8%	784
	21st to end	Night	39% 15% 19% 23% 4	660
March	1st to 10th	Night	34% 14% 19% 23% 10% 1	800
	11th to 20th	Night	38% 11% 19% 23% 8% 1	800
	21st to end	Night	38% 15% 20% 20% 7% 1	880
April	1st to 10th	Night	36% 12% 19% 21% 11% 1	800
	11th to 20th	Night	30% 13% 21% 22% 12% 2	791
	21st to end	Night	33% 10% 15% 29% 12% 1	603
May	1st to 10th	Night	28% 10% 18% 26% 18% 1	400
	11th to 20th	Night	28% 6% 14% 29% 21% 2	394
	21st to end	Night	27% 8% 14% 25% 23% 2	433
June	1st to 10th	Night	31% 7% 14% 27% 19% 3	400
	11th to 20th	Night	29% 6% 18% 31% 17%	400
	21st to end	Night	17% 4 14% 35% 28% 2	400
July	1st to 10th	Night	11% 2 15% 38% 32% 2	390
	11th to 20th	Night	8% 2 15% 47% 26% 3	388
	21st to end	Night	3 3 11% 41% 36% 5%	438
August	1st to 10th	Night	8% 4 9% 32% 42% 6%	475
	11th to 20th	Night	8% 2 8% 31% 42% 9%	564
	21st to end	Night	14% 5% 12% 38% 27% 4	766
September	1st to 10th	Night	14% 3 14% 40% 25% 4	760
	11th to 20th	Night	19% 4 13% 30% 29% 5%	760
	21st to end	Night	21% 6% 15% 34% 21% 3	760
October	1st to 10th	Night	25% 5% 16% 32% 19% 3	760
	11th to 20th	Night	30% 7% 15% 28% 18% 3	760
	21st to end	Night	35% 9% 18% 24% 11% 3	836
November	1st to 10th	Night	44% 8% 19% 23% 6%	780
	11th to 20th	Night	46% 10% 18% 20% 6% 1	800
	21st to end	Night	38% 12% 19% 22% 9% 1	800
December	1st to 10th	Night	34% 11% 21% 24% 10% 1	800
	11th to 20th	Night	44% 10% 20% 20% 5% 1	800
	21st to end	Night	48% 15% 19% 13% 4	880

Page generated at 2025-08-01 22:32:22. Version ac07

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