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 2024-11-04.

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	19% 17% 4 7% 4 3 3 3 4 5% 33%	105635
	Night	20% 19% 4 7% 4 3 3 3 3 4 29%	101970
Transparency	Day	53% 5% 9% 16% 12% 5%	105401
	Night	57% 5% 9% 14% 10% 4	101850
Seeing	Day	29% 14% 25% 22% 9% 2	14753
	Night	31% 9% 17% 26% 15% 2	23451

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% 34% 7% 9% 4 2 2 1 2 2 9%	2539
	11th to 20th	Day	27% 22% 4 8% 3 3 3 2 3 4 20%	2516
	21st to end	Day	31% 28% 5% 8% 3 3 3 2 3 3 13%	2954
February	1st to 10th	Day	27% 23% 5% 8% 5% 3 3 2 3 4 16%	1710
	11th to 20th	Day	23% 29% 5% 8% 3 3 3 2 3 3 18%	1665
	21st to end	Day	23% 32% 6% 8% 3 2 3 1 2 4 15%	1498
March	1st to 10th	Day	14% 28% 10% 11% 5% 3 4 3 3 4 16%	3226
	11th to 20th	Day	17% 24% 4 7% 4 2 2 2 2 3 34%	3490
	21st to end	Day	19% 30% 7% 11% 5% 4 3 2 3 3 12%	3945
April	1st to 10th	Day	25% 29% 5% 9% 4 2 3 2 4 4 12%	2280
	11th to 20th	Day	21% 27% 5% 9% 4 3 2 2 3 4 20%	2252
	21st to end	Day	25% 25% 4 7% 5% 3 3 2 3 4 17%	2464
May	1st to 10th	Day	20% 21% 4 8% 4 3 3 2 3 4 29%	4420
	11th to 20th	Day	14% 15% 4 9% 5% 4 5% 4 5% 6% 30%	4377
	21st to end	Day	28% 22% 4 8% 4 3 4 2 4 5% 16%	4813
June	1st to 10th	Day	31% 12% 3 6% 4 4 4 4 5% 5% 24%	2660
	11th to 20th	Day	29% 14% 3 8% 5% 5% 5% 4 5% 4 19%	2660
	21st to end	Day	16% 7% 2 6% 5% 4 4 4 5% 7% 40%	2841
July	1st to 10th	Day	6% 5% 2 3 2 2 3 2 4 5% 66%	4468
	11th to 20th	Day	7% 4 1 3 2 2 2 2 3 4 70%	4420
	21st to end	Day	9% 4 1 3 2 2 3 2 4 5% 64%	4790
August	1st to 10th	Day	5% 4 2 4 2 2 3 2 4 6% 66%	4248
	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	33% 19% 5% 8% 4 4 3 2 3 4 15%	1602
	11th to 20th	Day	35% 20% 4 6% 3 2 2 3 4 4 17%	1520
	21st to end	Day	28% 21% 5% 7% 3 3 2 2 3 3 21%	1520
December	1st to 10th	Day	23% 22% 5% 8% 4 3 4 2 2 3 24%	1520
	11th to 20th	Day	25% 23% 5% 8% 4 4 3 2 4 5% 18%	1925
	21st to end	Day	23% 25% 5% 10% 5% 3 3 3 4 4 15%	2794
January	1st to 10th	Night	30% 31% 6% 7% 4 3 3 2 2 2 10%	4406
	11th to 20th	Night	26% 19% 6% 8% 4 3 3 2 3 5% 19%	4382
	21st to end	Night	31% 25% 5% 8% 4 3 3 2 3 4 14%	4704
February	1st to 10th	Night	25% 23% 7% 8% 4 4 4 2 4 4 15%	2470
	11th to 20th	Night	22% 25% 5% 7% 4 3 3 2 3 3 22%	2402
	21st to end	Night	21% 30% 7% 8% 4 3 3 2 3 3 16%	1955
March	1st to 10th	Night	14% 23% 11% 13% 5% 4 5% 3 3 4 15%	3646
	11th to 20th	Night	15% 23% 5% 6% 3 2 2 2 3 3 35%	3489
	21st to end	Night	20% 35% 7% 10% 4 3 3 2 3 3 11%	3707
April	1st to 10th	Night	20% 28% 5% 10% 5% 4 3 2 3 3 16%	1900
	11th to 20th	Night	20% 24% 4 8% 4 4 3 2 4 5% 22%	1879
	21st to end	Night	25% 22% 4 8% 4 4 4 3 4 6% 17%	1714
May	1st to 10th	Night	20% 24% 4 7% 4 4 3 2 2 3 28%	2540
	11th to 20th	Night	12% 17% 3 8% 5% 5% 5% 4 5% 5% 31%	2516
	21st to end	Night	15% 28% 5% 8% 4 5% 4 3 4 5% 17%	2762
June	1st to 10th	Night	27% 13% 3 7% 4 4 4 2 3 6% 27%	1520
	11th to 20th	Night	24% 16% 4 7% 5% 3 4 4 4 6% 24%	1515
	21st to end	Night	14% 8% 3 7% 4 4 3 2 5% 6% 44%	1330
July	1st to 10th	Night	4 5% 2 4 2 3 4 2 4 4 67%	2466
	11th to 20th	Night	4 4 1 4 3 2 2 2 4 5% 69%	2540
	21st to end	Night	5% 4 2 5% 3 3 3 2 4 6% 64%	2762
August	1st to 10th	Night	3 4 2 3 3 2 3 3 4 5% 68%	2683
	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	35% 16% 4 8% 4 3 3 3 3 3 18%	2549
	11th to 20th	Night	33% 19% 4 7% 4 3 3 2 3 3 19%	2660
	21st to end	Night	26% 20% 5% 8% 4 3 3 2 3 3 23%	2660
December	1st to 10th	Night	24% 20% 4 8% 5% 4 4 3 3 5% 21%	2660
	11th to 20th	Night	25% 22% 5% 8% 5% 4 4 3 3 5% 17%	3280
	21st to end	Night	23% 26% 6% 11% 5% 4 4 2 3 3 14%	4862

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	83% 4 5% 5% 3 1	2539
	11th to 20th	Day	67% 4 8% 12% 5% 3	2540
	21st to end	Day	78% 3 5% 7% 6% 1	2954
February	1st to 10th	Day	73% 4 7% 10% 5%	1710
	11th to 20th	Day	73% 4 6% 11% 4 1	1674
	21st to end	Day	76% 3 6% 8% 6% 1	1498
March	1st to 10th	Day	69% 4 10% 12% 5% 1	3226
	11th to 20th	Day	56% 4 6% 29% 4 1	3490
	21st to end	Day	76% 7% 8% 6% 3	3945
April	1st to 10th	Day	75% 5% 9% 9% 1	2280
	11th to 20th	Day	69% 3 10% 10% 8% 1	2252
	21st to end	Day	70% 5% 8% 10% 5% 1	2464
May	1st to 10th	Day	57% 5% 11% 15% 11%	4420
	11th to 20th	Day	45% 11% 20% 17% 7%	4380
	21st to end	Day	69% 7% 13% 10% 1	4814
June	1st to 10th	Day	59% 7% 14% 13% 7%	2660
	11th to 20th	Day	63% 7% 13% 10% 6% 1	2660
	21st to end	Day	40% 7% 15% 24% 11% 2	2841
July	1st to 10th	Day	20% 4 8% 16% 21% 30%	4399
	11th to 20th	Day	18% 3 6% 15% 28% 29%	4345
	21st to end	Day	21% 3 9% 26% 33% 8%	4639
August	1st to 10th	Day	20% 5% 7% 20% 30% 17%	4231
	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	75% 4 6% 10% 4 1	1602
	11th to 20th	Day	72% 3 8% 10% 6% 2	1520
	21st to end	Day	70% 3 3 10% 12% 2	1520
December	1st to 10th	Day	67% 4 5% 11% 10% 2	1520
	11th to 20th	Day	70% 6% 10% 10% 4	1928
	21st to end	Day	71% 6% 10% 9% 3 1	2794
January	1st to 10th	Night	81% 3 5% 6% 3 1	4406
	11th to 20th	Night	66% 5% 8% 14% 4 3	4420
	21st to end	Night	75% 4 7% 7% 5% 2	4704
February	1st to 10th	Night	71% 6% 8% 9% 5%	2470
	11th to 20th	Night	67% 5% 9% 11% 5% 2	2404
	21st to end	Night	73% 4 7% 9% 6% 1	1955
March	1st to 10th	Night	67% 8% 10% 9% 5%	3646
	11th to 20th	Night	53% 5% 10% 26% 5% 1	3490
	21st to end	Night	77% 6% 7% 8% 1	3707
April	1st to 10th	Night	73% 6% 9% 10% 3	1900
	11th to 20th	Night	64% 6% 10% 13% 7% 1	1879
	21st to end	Night	66% 9% 9% 11% 5% 1	1714
May	1st to 10th	Night	59% 5% 10% 16% 10%	2540
	11th to 20th	Night	43% 9% 21% 18% 8% 1	2516
	21st to end	Night	62% 13% 13% 10% 3	2762
June	1st to 10th	Night	58% 5% 14% 16% 7% 1	1520
	11th to 20th	Night	56% 8% 16% 14% 7%	1515
	21st to end	Night	38% 6% 16% 25% 11% 3	1330
July	1st to 10th	Night	19% 3 9% 16% 22% 31%	2426
	11th to 20th	Night	17% 3 8% 15% 25% 32%	2492
	21st to end	Night	19% 4 10% 21% 33% 14%	2674
August	1st to 10th	Night	19% 4 9% 19% 32% 17%	2675
	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% 11% 6% 1	2549
	11th to 20th	Night	72% 4 7% 8% 6% 2	2660
	21st to end	Night	67% 3 5% 8% 14% 3	2660
December	1st to 10th	Night	67% 4 8% 10% 9% 1	2660
	11th to 20th	Night	69% 6% 9% 10% 6%	3276
	21st to end	Night	74% 7% 8% 7% 3	4862

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	47% 15% 22% 8% 6% 1	180
	11th to 20th	Day	40% 22% 19% 12% 7%	180
	21st to end	Day	40% 19% 16% 16% 9% 1	293
February	1st to 10th	Day	43% 16% 19% 15% 6% 1	370
	11th to 20th	Day	45% 17% 22% 14% 2	365
	21st to end	Day	41% 21% 20% 16% 2	348
March	1st to 10th	Day	40% 17% 27% 14% 3	460
	11th to 20th	Day	37% 17% 23% 15% 6% 1	460
	21st to end	Day	38% 22% 22% 15% 3 1	506
April	1st to 10th	Day	40% 18% 23% 13% 5% 1	460
	11th to 20th	Day	40% 21% 20% 12% 5% 2	452
	21st to end	Day	38% 20% 23% 13% 6% 1	460
May	1st to 10th	Day	29% 18% 23% 22% 7% 1	463
	11th to 20th	Day	33% 13% 26% 18% 8% 3	488
	21st to end	Day	34% 14% 22% 20% 9% 1	529
June	1st to 10th	Day	31% 13% 22% 23% 9% 2	490
	11th to 20th	Day	31% 18% 27% 16% 6% 1	490
	21st to end	Day	16% 13% 30% 28% 11% 2	671
July	1st to 10th	Day	13% 9% 34% 32% 9% 2	521
	11th to 20th	Day	11% 9% 32% 31% 13% 3	484
	21st to end	Day	8% 7% 30% 36% 15% 5%	531
August	1st to 10th	Day	8% 9% 26% 37% 14% 6%	487
	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	39% 20% 19% 17% 5%	232
	11th to 20th	Day	43% 16% 17% 13% 9% 2	180
	21st to end	Day	36% 16% 19% 16% 13%	180
December	1st to 10th	Day	44% 12% 14% 18% 10% 1	180
	11th to 20th	Day	43% 12% 24% 16% 4	180
	21st to end	Day	43% 11% 27% 14% 5%	198
January	1st to 10th	Night	47% 13% 16% 16% 8%	760
	11th to 20th	Night	39% 12% 19% 19% 10% 1	760
	21st to end	Night	36% 11% 21% 23% 8% 1	836
February	1st to 10th	Night	38% 10% 23% 22% 6% 1	760
	11th to 20th	Night	38% 9% 19% 26% 8%	744
	21st to end	Night	39% 15% 19% 22% 4	628
March	1st to 10th	Night	34% 14% 18% 22% 10% 1	760
	11th to 20th	Night	38% 10% 19% 23% 8% 1	760
	21st to end	Night	39% 14% 20% 19% 8% 1	836
April	1st to 10th	Night	37% 12% 19% 20% 11% 1	760
	11th to 20th	Night	32% 13% 20% 22% 12% 2	751
	21st to end	Night	34% 10% 15% 29% 10% 1	574
May	1st to 10th	Night	29% 11% 17% 25% 18% 1	380
	11th to 20th	Night	26% 6% 13% 30% 22% 2	374
	21st to end	Night	28% 8% 15% 24% 23% 2	411
June	1st to 10th	Night	32% 7% 13% 28% 18% 2	380
	11th to 20th	Night	30% 6% 18% 29% 17%	380
	21st to end	Night	17% 4 15% 35% 28% 2	380
July	1st to 10th	Night	11% 2 15% 37% 32% 2	370
	11th to 20th	Night	8% 2 14% 46% 26% 3	368
	21st to end	Night	4 3 12% 42% 35% 4	416
August	1st to 10th	Night	8% 4 9% 33% 41% 6%	467
	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% 5%	754
	11th to 20th	Night	44% 11% 18% 20% 6% 1	760
	21st to end	Night	38% 11% 20% 21% 8% 1	760
December	1st to 10th	Night	35% 11% 22% 23% 8% 1	760
	11th to 20th	Night	44% 10% 20% 21% 5% 1	760
	21st to end	Night	47% 15% 19% 14% 5%	836

Page generated at 2024-11-01 19:47:30. Version ac07

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