Vulcan
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Vulcan clear sky chart from 2005-11-06 to 2025-11-18.

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	7% 8% 3 8% 5% 5% 5% 4 6% 8% 41%	131100
	Night	6% 9% 3 9% 5% 5% 5% 4 6% 8% 39%	127980
Transparency	Day	39% 7% 14% 25% 12% 2	130800
	Night	41% 7% 15% 23% 12% 2	127857
Seeing	Day	12% 9% 27% 29% 18% 5%	16756
	Night	11% 7% 30% 35% 15% 2	25831

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	2 13% 8% 14% 7% 5% 5% 4 6% 6% 29%	3179
	11th to 20th	Day	3 7% 5% 12% 6% 6% 6% 5% 6% 7% 37%	3225
	21st to end	Day	2 6% 3 9% 6% 5% 5% 4 6% 11% 43%	4004
February	1st to 10th	Day	6% 11% 4 12% 7% 8% 6% 5% 6% 6% 28%	1643
	11th to 20th	Day	5% 12% 5% 11% 8% 6% 6% 5% 7% 8% 28%	1755
	21st to end	Day	3 12% 6% 11% 7% 7% 5% 4 5% 8% 31%	1585
March	1st to 10th	Day	3 11% 4 11% 6% 5% 4 3 5% 11% 37%	4533
	11th to 20th	Day	5% 9% 4 10% 5% 6% 5% 4 5% 8% 40%	4999
	21st to end	Day	7% 18% 6% 13% 6% 5% 5% 5% 6% 8% 22%	5486
April	1st to 10th	Day	8% 12% 5% 9% 6% 5% 5% 5% 5% 8% 32%	2466
	11th to 20th	Day	9% 12% 3 9% 5% 5% 6% 5% 6% 8% 31%	2570
	21st to end	Day	10% 10% 3 9% 6% 5% 5% 4 7% 7% 34%	2600
May	1st to 10th	Day	14% 11% 3 6% 4 4 4 4 5% 6% 40%	6082
	11th to 20th	Day	11% 8% 3 10% 6% 6% 7% 5% 7% 9% 27%	6734
	21st to end	Day	10% 7% 3 8% 6% 6% 6% 5% 6% 7% 38%	7405
June	1st to 10th	Day	12% 8% 3 7% 5% 6% 5% 5% 8% 9% 34%	3000
	11th to 20th	Day	16% 8% 3 7% 5% 5% 6% 5% 7% 8% 30%	3000
	21st to end	Day	8% 5% 2 5% 4 5% 5% 4 8% 10% 44%	3000
July	1st to 10th	Day	7% 4 1 5% 3 4 4 4 6% 8% 54%	6780
	11th to 20th	Day	9% 4 1 5% 3 4 5% 4 6% 9% 50%	6780
	21st to end	Day	7% 4 2 4 3 3 4 3 5% 7% 57%	7207
August	1st to 10th	Day	8% 5% 2 6% 4 4 5% 4 7% 10% 44%	4495
	11th to 20th	Day	6% 5% 2 6% 4 3 5% 4 6% 9% 50%	4158
	21st to end	Day	8% 3 2 4 3 3 3 3 6% 8% 57%	4664
September	1st to 10th	Day	9% 5% 2 5% 3 3 4 4 5% 7% 53%	2475
	11th to 20th	Day	12% 7% 3 5% 4 3 4 3 5% 7% 47%	2200
	21st to end	Day	9% 6% 2 6% 3 4 4 4 6% 8% 48%	2200
October	1st to 10th	Day	8% 7% 2 6% 5% 5% 5% 5% 7% 8% 43%	3416
	11th to 20th	Day	6% 6% 2 5% 4 3 4 4 6% 9% 48%	3075
	21st to end	Day	6% 8% 3 6% 5% 4 5% 4 6% 7% 46%	3200
November	1st to 10th	Day	9% 13% 4 9% 4 4 5% 5% 6% 7% 33%	1708
	11th to 20th	Day	6% 10% 4 9% 5% 4 5% 4 6% 9% 39%	1646
	21st to end	Day	4 11% 5% 10% 5% 4 4 6% 8% 10% 34%	1400
December	1st to 10th	Day	3 13% 6% 12% 5% 5% 5% 4 4 7% 36%	2290
	11th to 20th	Day	4 9% 4 11% 8% 7% 6% 5% 7% 8% 31%	2642
	21st to end	Day	3 9% 4 9% 5% 5% 5% 5% 6% 9% 40%	3498
January	1st to 10th	Night	3 14% 6% 13% 7% 7% 6% 4 6% 8% 25%	6766
	11th to 20th	Night	2 8% 5% 11% 7% 7% 6% 5% 6% 8% 35%	6716
	21st to end	Night	3 8% 4 10% 6% 5% 5% 4 6% 9% 42%	6952
February	1st to 10th	Night	5% 12% 4 11% 7% 7% 5% 5% 6% 8% 30%	2755
	11th to 20th	Night	5% 12% 5% 12% 7% 7% 5% 5% 7% 8% 28%	2532
	21st to end	Night	4 11% 5% 11% 8% 6% 6% 4 6% 7% 33%	2040
March	1st to 10th	Night	2 8% 4 12% 7% 5% 6% 4 6% 9% 36%	5360
	11th to 20th	Night	4 9% 4 10% 6% 5% 5% 5% 7% 9% 38%	5000
	21st to end	Night	9% 18% 6% 11% 6% 5% 5% 4 7% 8% 21%	5489
April	1st to 10th	Night	8% 12% 4 10% 6% 5% 6% 4 6% 9% 31%	1930
	11th to 20th	Night	9% 10% 5% 10% 7% 5% 4 5% 6% 8% 33%	1781
	21st to end	Night	10% 11% 3 9% 6% 6% 5% 4 6% 8% 33%	1800
May	1st to 10th	Night	14% 12% 3 7% 4 5% 5% 3 5% 7% 35%	3890
	11th to 20th	Night	10% 9% 4 8% 5% 6% 6% 5% 7% 9% 30%	3159
	21st to end	Night	9% 6% 3 8% 5% 6% 5% 5% 6% 8% 39%	3470
June	1st to 10th	Night	12% 9% 2 7% 5% 5% 5% 5% 7% 7% 36%	1400
	11th to 20th	Night	16% 9% 2 8% 5% 5% 5% 4 7% 8% 32%	1400
	21st to end	Night	6% 4 2 6% 3 4 5% 5% 8% 10% 46%	1400
July	1st to 10th	Night	6% 4 1 6% 4 4 5% 4 7% 8% 50%	3180
	11th to 20th	Night	7% 4 2 6% 4 5% 5% 4 8% 10% 46%	3180
	21st to end	Night	7% 4 2 5% 4 3 3 4 7% 9% 53%	3620
August	1st to 10th	Night	6% 6% 2 7% 5% 5% 5% 4 5% 8% 47%	2656
	11th to 20th	Night	5% 5% 2 7% 4 5% 5% 5% 7% 12% 44%	2901
	21st to end	Night	6% 3 1 4 3 3 3 3 5% 10% 59%	3245
September	1st to 10th	Night	9% 4 2 6% 4 4 4 4 5% 7% 51%	1922
	11th to 20th	Night	12% 6% 2 5% 3 4 3 3 5% 7% 51%	2200
	21st to end	Night	7% 6% 2 6% 4 4 5% 4 6% 8% 48%	2200
October	1st to 10th	Night	7% 5% 2 5% 4 3 4 4 6% 7% 52%	3777
	11th to 20th	Night	7% 7% 2 6% 4 4 4 3 6% 8% 49%	4069
	21st to end	Night	5% 6% 3 7% 5% 5% 5% 4 6% 8% 47%	4600
November	1st to 10th	Night	8% 11% 4 9% 5% 5% 5% 4 6% 8% 35%	2788
	11th to 20th	Night	4 9% 4 9% 5% 5% 5% 5% 7% 9% 39%	2904
	21st to end	Night	5% 11% 3 7% 5% 5% 5% 4 7% 10% 38%	3000
December	1st to 10th	Night	3 10% 4 10% 7% 6% 7% 5% 8% 8% 33%	4890
	11th to 20th	Night	3 10% 4 11% 7% 7% 6% 5% 7% 9% 29%	5550
	21st to end	Night	4 9% 4 9% 5% 5% 5% 4 7% 8% 41%	7458

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	53% 6% 11% 21% 7% 1	3179
	11th to 20th	Day	44% 7% 13% 17% 17% 3	3246
	21st to end	Day	35% 6% 13% 33% 12% 1	4004
February	1st to 10th	Day	54% 8% 12% 15% 9% 2	1643
	11th to 20th	Day	50% 7% 15% 19% 8% 2	1764
	21st to end	Day	50% 6% 11% 19% 11% 1	1585
March	1st to 10th	Day	41% 5% 13% 23% 17% 1	4533
	11th to 20th	Day	42% 5% 16% 21% 15% 1	5000
	21st to end	Day	57% 7% 15% 15% 6%	5486
April	1st to 10th	Day	49% 7% 13% 21% 9% 1	2466
	11th to 20th	Day	47% 8% 13% 21% 10% 2	2570
	21st to end	Day	45% 9% 14% 21% 10% 1	2600
May	1st to 10th	Day	43% 5% 9% 22% 15% 5%	6082
	11th to 20th	Day	45% 10% 15% 22% 8%	6738
	21st to end	Day	40% 8% 15% 25% 11% 1	7406
June	1st to 10th	Day	42% 9% 16% 20% 11% 1	3000
	11th to 20th	Day	47% 8% 14% 20% 10% 2	3000
	21st to end	Day	31% 8% 18% 29% 12% 2	3000
July	1st to 10th	Day	26% 6% 16% 27% 20% 5%	6706
	11th to 20th	Day	29% 7% 20% 29% 14% 2	6699
	21st to end	Day	26% 5% 14% 27% 18% 10%	7003
August	1st to 10th	Day	31% 8% 14% 35% 11% 2	4478
	11th to 20th	Day	28% 7% 20% 34% 9% 2	4197
	21st to end	Day	25% 4 15% 39% 15% 2	4663
September	1st to 10th	Day	30% 6% 13% 28% 17% 7%	2475
	11th to 20th	Day	36% 6% 12% 30% 13% 3	2200
	21st to end	Day	33% 6% 14% 31% 13% 3	2200
October	1st to 10th	Day	37% 6% 11% 35% 11% 1	3416
	11th to 20th	Day	32% 5% 19% 27% 15% 2	3075
	21st to end	Day	36% 6% 11% 31% 13% 3	3200
November	1st to 10th	Day	48% 8% 13% 20% 9% 2	1708
	11th to 20th	Day	41% 7% 18% 24% 10% 1	1646
	21st to end	Day	41% 8% 17% 22% 10% 2	1400
December	1st to 10th	Day	50% 6% 11% 25% 7% 2	2290
	11th to 20th	Day	48% 6% 18% 21% 5% 1	2644
	21st to end	Day	39% 6% 11% 23% 19% 2	3498
January	1st to 10th	Night	54% 7% 11% 20% 5% 1	6766
	11th to 20th	Night	46% 6% 13% 17% 15% 3	6757
	21st to end	Night	37% 6% 14% 27% 15% 1	6952
February	1st to 10th	Night	51% 7% 12% 16% 12% 2	2755
	11th to 20th	Night	51% 8% 12% 19% 9% 1	2533
	21st to end	Night	48% 7% 13% 20% 10% 2	2040
March	1st to 10th	Night	41% 8% 17% 20% 11% 2	5360
	11th to 20th	Night	41% 7% 15% 24% 12% 1	5000
	21st to end	Night	59% 7% 14% 13% 7% 1	5489
April	1st to 10th	Night	48% 8% 12% 20% 12%	1930
	11th to 20th	Night	47% 7% 13% 22% 10% 1	1781
	21st to end	Night	46% 9% 13% 22% 9% 2	1800
May	1st to 10th	Night	48% 6% 9% 22% 12% 3	3887
	11th to 20th	Night	43% 10% 14% 27% 5% 1	3159
	21st to end	Night	39% 10% 16% 24% 9% 2	3470
June	1st to 10th	Night	43% 9% 14% 22% 12% 1	1400
	11th to 20th	Night	47% 8% 15% 20% 9% 3	1400
	21st to end	Night	30% 9% 20% 26% 14% 2	1400
July	1st to 10th	Night	28% 7% 17% 22% 23% 3	3145
	11th to 20th	Night	30% 8% 19% 27% 12% 3	3138
	21st to end	Night	27% 7% 15% 26% 16% 9%	3519
August	1st to 10th	Night	34% 7% 15% 28% 15% 2	2648
	11th to 20th	Night	32% 7% 19% 29% 12% 2	2928
	21st to end	Night	22% 5% 14% 38% 17% 4	3245
September	1st to 10th	Night	30% 7% 13% 26% 17% 6%	1922
	11th to 20th	Night	32% 6% 14% 28% 17% 5%	2200
	21st to end	Night	31% 6% 16% 28% 15% 4	2200
October	1st to 10th	Night	29% 5% 13% 32% 18% 3	3777
	11th to 20th	Night	34% 5% 16% 25% 18% 3	4069
	21st to end	Night	34% 5% 12% 32% 16% 1	4600
November	1st to 10th	Night	46% 7% 12% 23% 11% 1	2788
	11th to 20th	Night	39% 8% 17% 21% 14% 1	2904
	21st to end	Night	40% 9% 19% 19% 11% 3	3000
December	1st to 10th	Night	47% 7% 17% 19% 8% 2	4890
	11th to 20th	Night	49% 8% 21% 17% 5%	5547
	21st to end	Night	40% 5% 11% 25% 16% 3	7458

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	5% 9% 33% 30% 14% 11%	200
	11th to 20th	Day	7% 9% 37% 28% 14% 7%	200
	21st to end	Day	6% 9% 33% 34% 15% 3	220
February	1st to 10th	Day	9% 11% 35% 31% 11% 3	223
	11th to 20th	Day	7% 8% 34% 33% 14% 4	300
	21st to end	Day	11% 13% 28% 29% 16% 3	349
March	1st to 10th	Day	9% 13% 35% 26% 13% 4	500
	11th to 20th	Day	10% 10% 33% 32% 12% 3	500
	21st to end	Day	12% 15% 30% 26% 12% 4	550
April	1st to 10th	Day	13% 15% 31% 26% 13% 2	500
	11th to 20th	Day	13% 14% 28% 27% 15% 3	492
	21st to end	Day	16% 12% 28% 27% 14% 4	500
May	1st to 10th	Day	16% 11% 24% 27% 16% 6%	553
	11th to 20th	Day	16% 6% 21% 30% 21% 5%	725
	21st to end	Day	17% 8% 19% 30% 18% 7%	789
June	1st to 10th	Day	13% 9% 27% 32% 16% 3	730
	11th to 20th	Day	18% 10% 27% 28% 15% 2	730
	21st to end	Day	10% 4 24% 33% 23% 6%	730
July	1st to 10th	Day	7% 5% 21% 35% 27% 5%	721
	11th to 20th	Day	11% 5% 23% 35% 21% 6%	718
	21st to end	Day	6% 4 24% 34% 24% 7%	726
August	1st to 10th	Day	9% 5% 22% 31% 25% 7%	527
	11th to 20th	Day	9% 5% 21% 32% 24% 9%	517
	21st to end	Day	8% 8% 23% 32% 20% 10%	583
September	1st to 10th	Day	12% 6% 27% 25% 22% 8%	530
	11th to 20th	Day	18% 5% 22% 28% 19% 7%	530
	21st to end	Day	15% 13% 31% 24% 13% 4	530
October	1st to 10th	Day	15% 8% 23% 27% 21% 6%	424
	11th to 20th	Day	10% 15% 31% 22% 17% 6%	330
	21st to end	Day	13% 16% 29% 26% 12% 2	363
November	1st to 10th	Day	18% 11% 29% 28% 11% 3	330
	11th to 20th	Day	11% 15% 34% 24% 13% 2	316
	21st to end	Day	6% 16% 31% 26% 15% 9%	200
December	1st to 10th	Day	9% 12% 29% 31% 16% 5%	200
	11th to 20th	Day	11% 8% 40% 25% 13% 4	200
	21st to end	Day	9% 8% 27% 29% 21% 5%	220
January	1st to 10th	Night	9% 11% 34% 31% 13% 2	1000
	11th to 20th	Night	8% 9% 36% 35% 12% 1	990
	21st to end	Night	10% 8% 35% 33% 11% 1	880
February	1st to 10th	Night	10% 10% 30% 34% 15% 3	800
	11th to 20th	Night	9% 7% 32% 38% 13% 2	784
	21st to end	Night	10% 7% 25% 35% 20% 3	660
March	1st to 10th	Night	10% 7% 32% 34% 14% 2	800
	11th to 20th	Night	11% 7% 36% 32% 12% 2	800
	21st to end	Night	13% 8% 31% 32% 12% 4	880
April	1st to 10th	Night	13% 8% 34% 30% 13% 2	725
	11th to 20th	Night	14% 7% 34% 32% 11% 1	593
	21st to end	Night	15% 9% 30% 34% 13% 1	600
May	1st to 10th	Night	17% 8% 23% 35% 16% 1	570
	11th to 20th	Night	13% 7% 22% 35% 22% 1	394
	21st to end	Night	18% 6% 22% 30% 22% 2	433
June	1st to 10th	Night	15% 7% 28% 34% 15% 1	400
	11th to 20th	Night	19% 9% 27% 31% 14%	400
	21st to end	Night	9% 3 22% 42% 21% 2	400
July	1st to 10th	Night	7% 3 19% 45% 26% 1	390
	11th to 20th	Night	8% 3 22% 46% 21% 1	388
	21st to end	Night	7% 3 23% 42% 24% 2	438
August	1st to 10th	Night	8% 5% 18% 41% 27% 2	419
	11th to 20th	Night	7% 3 21% 45% 22% 2	594
	21st to end	Night	10% 4 22% 43% 19% 1	660
September	1st to 10th	Night	12% 2 22% 39% 23% 2	674
	11th to 20th	Night	15% 5% 20% 39% 21% 2	800
	21st to end	Night	11% 7% 29% 36% 14% 3	800
October	1st to 10th	Night	14% 6% 27% 34% 18% 2	800
	11th to 20th	Night	12% 8% 31% 33% 14% 3	800
	21st to end	Night	10% 9% 36% 32% 12% 1	880
November	1st to 10th	Night	14% 9% 39% 30% 7% 1	943
	11th to 20th	Night	10% 12% 39% 27% 10% 1	1036
	21st to end	Night	10% 9% 35% 34% 10% 2	1000
December	1st to 10th	Night	9% 8% 32% 35% 14% 2	1000
	11th to 20th	Night	8% 9% 34% 36% 11% 1	1000
	21st to end	Night	11% 8% 33% 34% 12% 2	1100

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