Vermilion
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Vermilion clear sky chart from 2005-11-06 to 2026-05-02.

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	10% 9% 4 9% 6% 5% 5% 4 6% 8% 35%	135084
	Night	8% 10% 4 10% 6% 6% 5% 4 6% 8% 33%	135984
Transparency	Day	45% 8% 17% 22% 8% 1	134780
	Night	47% 7% 16% 21% 9% 1	135869
Seeing	Day	13% 8% 27% 30% 17% 5%	17437
	Night	12% 6% 29% 36% 15% 2	26682

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	8% 11% 5% 14% 10% 8% 6% 5% 6% 6% 21%	4125
	11th to 20th	Day	4 8% 3 9% 7% 7% 8% 7% 7% 7% 31%	4119
	21st to end	Day	7% 13% 4 11% 9% 6% 5% 4 6% 8% 28%	4567
February	1st to 10th	Day	7% 15% 5% 14% 8% 5% 6% 3 7% 6% 24%	1737
	11th to 20th	Day	6% 13% 6% 14% 7% 6% 6% 5% 6% 7% 25%	1845
	21st to end	Day	5% 13% 4 13% 8% 7% 7% 6% 7% 8% 23%	1557
March	1st to 10th	Day	5% 11% 5% 10% 8% 6% 8% 5% 7% 8% 28%	4666
	11th to 20th	Day	6% 10% 6% 15% 7% 5% 4 4 5% 9% 29%	5109
	21st to end	Day	9% 15% 6% 16% 6% 4 4 4 5% 6% 24%	5607
April	1st to 10th	Day	8% 14% 4 8% 6% 5% 6% 4 6% 7% 33%	2626
	11th to 20th	Day	13% 11% 5% 10% 5% 5% 5% 4 5% 6% 31%	2700
	21st to end	Day	14% 12% 3 7% 4 4 5% 4 7% 8% 32%	2790
May	1st to 10th	Day	12% 9% 3 8% 5% 5% 6% 4 5% 6% 38%	6443
	11th to 20th	Day	17% 11% 3 9% 6% 5% 5% 5% 6% 7% 27%	6734
	21st to end	Day	11% 7% 3 7% 5% 4 5% 4 6% 7% 40%	7405
June	1st to 10th	Day	13% 8% 3 8% 6% 6% 5% 5% 7% 8% 32%	3000
	11th to 20th	Day	19% 10% 3 8% 6% 6% 6% 5% 7% 7% 22%	3061
	21st to end	Day	12% 6% 2 6% 5% 5% 6% 5% 7% 10% 34%	3000
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 5% 4 6% 9% 46%	6780
	11th to 20th	Day	11% 5% 2 7% 4 5% 6% 4 7% 9% 40%	6780
	21st to end	Day	9% 5% 2 6% 4 5% 5% 4 7% 9% 43%	7376
August	1st to 10th	Day	13% 5% 2 6% 3 4 5% 4 6% 7% 45%	4805
	11th to 20th	Day	8% 5% 2 6% 4 4 4 5% 7% 11% 44%	4158
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 6% 9% 46%	4664
September	1st to 10th	Day	10% 6% 3 6% 5% 5% 5% 4 6% 8% 44%	2500
	11th to 20th	Day	15% 8% 2 5% 4 3 4 4 6% 7% 42%	2275
	21st to end	Day	10% 7% 2 6% 3 4 4 4 6% 8% 46%	2194
October	1st to 10th	Day	8% 9% 3 7% 5% 5% 5% 3 5% 7% 42%	3270
	11th to 20th	Day	11% 8% 3 8% 4 4 5% 4 6% 10% 39%	3164
	21st to end	Day	7% 9% 4 9% 4 5% 5% 4 7% 8% 38%	2988
November	1st to 10th	Day	12% 15% 4 9% 8% 5% 4 4 5% 7% 26%	1648
	11th to 20th	Day	7% 12% 6% 15% 7% 5% 4 4 7% 7% 26%	1638
	21st to end	Day	5% 13% 6% 13% 9% 6% 5% 4 6% 8% 26%	1470
December	1st to 10th	Day	8% 14% 5% 11% 6% 6% 7% 5% 7% 8% 22%	2270
	11th to 20th	Day	8% 17% 5% 12% 7% 6% 6% 4 7% 7% 20%	2438
	21st to end	Day	3 9% 5% 13% 7% 6% 5% 4 5% 7% 36%	3575
January	1st to 10th	Night	7% 10% 4 14% 9% 8% 8% 6% 7% 6% 20%	8768
	11th to 20th	Night	3 8% 4 10% 7% 7% 7% 6% 7% 8% 34%	8779
	21st to end	Night	6% 12% 4 10% 7% 7% 6% 4 6% 8% 30%	9650
February	1st to 10th	Night	6% 12% 4 11% 8% 7% 7% 6% 6% 8% 25%	2887
	11th to 20th	Night	6% 10% 4 11% 8% 7% 6% 6% 6% 6% 30%	2662
	21st to end	Night	3 9% 5% 13% 8% 6% 6% 5% 7% 9% 28%	2249
March	1st to 10th	Night	5% 11% 4 11% 7% 6% 6% 5% 8% 9% 28%	5426
	11th to 20th	Night	4 10% 5% 11% 8% 6% 5% 5% 7% 9% 28%	5110
	21st to end	Night	10% 16% 5% 12% 6% 5% 4 4 6% 7% 27%	5610
April	1st to 10th	Night	7% 9% 3 8% 5% 5% 5% 4 7% 8% 37%	1992
	11th to 20th	Night	9% 8% 4 11% 7% 6% 6% 5% 7% 8% 29%	1871
	21st to end	Night	12% 11% 4 8% 4 4 5% 4 6% 8% 35%	1829
May	1st to 10th	Night	14% 9% 4 7% 5% 4 5% 4 5% 6% 38%	3541
	11th to 20th	Night	14% 9% 3 9% 6% 5% 6% 5% 7% 8% 29%	3159
	21st to end	Night	12% 7% 3 8% 5% 5% 6% 4 7% 8% 35%	3470
June	1st to 10th	Night	12% 7% 3 9% 5% 6% 7% 5% 7% 7% 32%	1400
	11th to 20th	Night	18% 10% 3 7% 5% 5% 5% 5% 8% 9% 26%	1288
	21st to end	Night	10% 5% 2 8% 5% 5% 8% 6% 6% 7% 39%	1400
July	1st to 10th	Night	6% 4 2 5% 4 4 6% 4 8% 10% 48%	3180
	11th to 20th	Night	10% 4 1 6% 5% 5% 5% 4 6% 8% 46%	3180
	21st to end	Night	9% 4 2 5% 4 4 4 4 7% 8% 49%	3467
August	1st to 10th	Night	13% 4 2 6% 4 4 4 4 6% 9% 45%	2343
	11th to 20th	Night	8% 6% 2 6% 4 5% 5% 5% 7% 9% 44%	2901
	21st to end	Night	9% 6% 2 7% 4 4 3 3 6% 8% 48%	3245
September	1st to 10th	Night	10% 5% 2 6% 5% 3 4 3 5% 7% 49%	1900
	11th to 20th	Night	12% 5% 2 6% 4 4 4 3 6% 7% 47%	2123
	21st to end	Night	7% 6% 3 6% 4 4 4 4 6% 9% 48%	2206
October	1st to 10th	Night	8% 8% 3 7% 5% 5% 5% 4 6% 8% 43%	3920
	11th to 20th	Night	8% 9% 3 6% 4 3 4 4 6% 8% 45%	3982
	21st to end	Night	8% 12% 4 8% 4 4 4 4 6% 10% 36%	4780
November	1st to 10th	Night	13% 14% 5% 9% 5% 5% 5% 4 4 6% 28%	2870
	11th to 20th	Night	8% 12% 5% 15% 8% 7% 6% 4 6% 6% 25%	2980
	21st to end	Night	5% 12% 5% 13% 9% 6% 5% 4 6% 7% 29%	3150
December	1st to 10th	Night	9% 15% 5% 15% 8% 6% 6% 5% 6% 7% 19%	5107
	11th to 20th	Night	7% 16% 5% 15% 9% 7% 6% 5% 6% 7% 19%	5936
	21st to end	Night	3 8% 3 12% 8% 6% 5% 4 5% 6% 40%	7623

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	61% 6% 8% 16% 9%	4125
	11th to 20th	Day	47% 7% 16% 15% 14%	4140
	21st to end	Day	54% 7% 17% 15% 7%	4568
February	1st to 10th	Day	59% 5% 12% 16% 7%	1737
	11th to 20th	Day	56% 7% 12% 17% 8%	1854
	21st to end	Day	56% 6% 14% 16% 7%	1557
March	1st to 10th	Day	49% 8% 20% 18% 5%	4666
	11th to 20th	Day	51% 7% 18% 18% 6%	5110
	21st to end	Day	57% 8% 11% 18% 6%	5607
April	1st to 10th	Day	49% 7% 15% 19% 9%	2626
	11th to 20th	Day	50% 7% 14% 20% 9% 1	2700
	21st to end	Day	47% 8% 13% 21% 9% 1	2790
May	1st to 10th	Day	45% 7% 10% 23% 13% 3	6443
	11th to 20th	Day	52% 9% 11% 20% 8%	6738
	21st to end	Day	38% 11% 16% 22% 12% 2	7406
June	1st to 10th	Day	46% 9% 15% 20% 9% 1	3000
	11th to 20th	Day	56% 11% 13% 16% 4 1	3061
	21st to end	Day	40% 12% 21% 20% 7% 1	3000
July	1st to 10th	Day	33% 8% 23% 28% 7%	6706
	11th to 20th	Day	39% 8% 16% 28% 9%	6699
	21st to end	Day	35% 7% 23% 29% 6%	7169
August	1st to 10th	Day	37% 8% 19% 32% 4	4787
	11th to 20th	Day	32% 8% 25% 28% 7%	4197
	21st to end	Day	33% 7% 25% 29% 5% 1	4663
September	1st to 10th	Day	37% 8% 18% 26% 9% 2	2500
	11th to 20th	Day	40% 7% 17% 29% 6%	2275
	21st to end	Day	35% 8% 17% 28% 10% 2	2194
October	1st to 10th	Day	41% 5% 23% 26% 5%	3270
	11th to 20th	Day	41% 6% 15% 30% 8%	3164
	21st to end	Day	42% 8% 20% 26% 5%	2988
November	1st to 10th	Day	57% 7% 13% 18% 5%	1648
	11th to 20th	Day	55% 9% 16% 14% 5%	1638
	21st to end	Day	55% 8% 15% 18% 4	1470
December	1st to 10th	Day	58% 8% 17% 13% 4	2270
	11th to 20th	Day	61% 6% 15% 15% 3	2439
	21st to end	Day	47% 4 10% 27% 10% 1	3575
January	1st to 10th	Night	60% 7% 11% 12% 10%	8769
	11th to 20th	Night	45% 5% 18% 17% 13% 1	8820
	21st to end	Night	52% 7% 17% 17% 7%	9650
February	1st to 10th	Night	54% 7% 13% 16% 9%	2887
	11th to 20th	Night	51% 8% 12% 18% 12%	2663
	21st to end	Night	48% 9% 13% 21% 8% 1	2249
March	1st to 10th	Night	48% 7% 20% 17% 7%	5426
	11th to 20th	Night	48% 8% 17% 20% 6%	5110
	21st to end	Night	53% 8% 12% 17% 10%	5610
April	1st to 10th	Night	42% 8% 15% 20% 13% 1	1992
	11th to 20th	Night	49% 8% 13% 17% 10% 2	1871
	21st to end	Night	44% 8% 13% 22% 11% 2	1829
May	1st to 10th	Night	44% 6% 8% 23% 16% 3	3541
	11th to 20th	Night	47% 11% 13% 21% 8% 1	3159
	21st to end	Night	41% 11% 18% 21% 9% 1	3470
June	1st to 10th	Night	46% 9% 13% 21% 10% 1	1400
	11th to 20th	Night	50% 12% 14% 18% 6%	1288
	21st to end	Night	39% 11% 21% 22% 6% 1	1400
July	1st to 10th	Night	30% 7% 25% 25% 12% 1	3145
	11th to 20th	Night	35% 6% 15% 30% 14%	3138
	21st to end	Night	32% 7% 23% 29% 9%	3369
August	1st to 10th	Night	35% 8% 19% 34% 3	2336
	11th to 20th	Night	34% 8% 23% 27% 8%	2928
	21st to end	Night	34% 6% 21% 32% 8%	3245
September	1st to 10th	Night	34% 8% 17% 27% 11% 3	1900
	11th to 20th	Night	35% 7% 17% 30% 9% 1	2123
	21st to end	Night	32% 8% 15% 31% 12% 2	2206
October	1st to 10th	Night	38% 6% 21% 28% 7%	3920
	11th to 20th	Night	35% 5% 15% 35% 9%	3982
	21st to end	Night	43% 6% 22% 24% 5%	4780
November	1st to 10th	Night	56% 7% 13% 17% 7% 1	2870
	11th to 20th	Night	58% 7% 14% 15% 5%	2980
	21st to end	Night	53% 8% 16% 17% 5%	3150
December	1st to 10th	Night	63% 7% 14% 11% 4 1	5107
	11th to 20th	Night	63% 6% 16% 11% 4	5933
	21st to end	Night	44% 6% 13% 22% 13% 2	7623

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	10% 6% 27% 34% 20% 3	220
	11th to 20th	Day	12% 12% 25% 30% 20% 2	220
	21st to end	Day	14% 13% 29% 33% 7% 3	242
February	1st to 10th	Day	8% 10% 33% 30% 12% 6%	250
	11th to 20th	Day	9% 6% 35% 30% 16% 4	330
	21st to end	Day	9% 6% 32% 32% 16% 5%	273
March	1st to 10th	Day	12% 8% 32% 30% 15% 2	482
	11th to 20th	Day	11% 7% 33% 29% 16% 5%	540
	21st to end	Day	14% 8% 30% 29% 17% 2	594
April	1st to 10th	Day	13% 8% 33% 32% 11% 4	540
	11th to 20th	Day	16% 10% 29% 29% 13% 4	532
	21st to end	Day	18% 8% 28% 29% 14% 3	600
May	1st to 10th	Day	13% 11% 26% 29% 16% 5%	709
	11th to 20th	Day	13% 7% 26% 31% 16% 7%	725
	21st to end	Day	16% 8% 20% 31% 18% 7%	789
June	1st to 10th	Day	16% 8% 27% 33% 12% 4	730
	11th to 20th	Day	19% 8% 25% 25% 15% 7%	730
	21st to end	Day	12% 5% 22% 31% 21% 9%	730
July	1st to 10th	Day	11% 6% 17% 32% 26% 7%	721
	11th to 20th	Day	11% 6% 23% 32% 20% 8%	718
	21st to end	Day	11% 4 22% 31% 24% 8%	794
August	1st to 10th	Day	12% 5% 17% 31% 25% 10%	721
	11th to 20th	Day	10% 7% 23% 33% 21% 6%	517
	21st to end	Day	11% 7% 22% 30% 21% 9%	583
September	1st to 10th	Day	11% 7% 32% 27% 17% 6%	530
	11th to 20th	Day	20% 6% 25% 29% 17% 3	530
	21st to end	Day	12% 13% 30% 27% 15% 3	524
October	1st to 10th	Day	13% 7% 33% 28% 17% 3	330
	11th to 20th	Day	18% 7% 31% 28% 12% 3	330
	21st to end	Day	15% 12% 37% 23% 10% 3	363
November	1st to 10th	Day	18% 8% 31% 30% 11% 2	330
	11th to 20th	Day	11% 9% 41% 29% 10% 1	308
	21st to end	Day	6% 6% 40% 30% 11% 6%	220
December	1st to 10th	Day	10% 5% 34% 34% 13% 4	220
	11th to 20th	Day	12% 12% 33% 28% 13% 2	220
	21st to end	Day	10% 3 31% 34% 17% 5%	242
January	1st to 10th	Night	11% 7% 31% 37% 13% 1	1050
	11th to 20th	Night	11% 9% 34% 34% 12% 1	1050
	21st to end	Night	16% 9% 34% 30% 9% 1	1155
February	1st to 10th	Night	9% 8% 29% 36% 15% 3	845
	11th to 20th	Night	9% 4 31% 36% 17% 2	824
	21st to end	Night	6% 5% 27% 38% 20% 4	692
March	1st to 10th	Night	12% 5% 29% 40% 13% 2	840
	11th to 20th	Night	10% 5% 32% 37% 14% 2	840
	21st to end	Night	13% 7% 27% 35% 15% 2	924
April	1st to 10th	Night	10% 8% 27% 39% 14% 3	643
	11th to 20th	Night	13% 6% 31% 35% 13% 1	623
	21st to end	Night	16% 7% 23% 38% 16%	630
May	1st to 10th	Night	13% 6% 28% 35% 16% 2	561
	11th to 20th	Night	12% 6% 22% 41% 16% 2	394
	21st to end	Night	18% 4 22% 33% 22% 2	433
June	1st to 10th	Night	14% 6% 27% 37% 16% 2	400
	11th to 20th	Night	23% 6% 19% 31% 20% 1	400
	21st to end	Night	11% 4 20% 36% 26% 3	400
July	1st to 10th	Night	10% 4 17% 42% 26% 1	390
	11th to 20th	Night	11% 3 19% 44% 20% 2	388
	21st to end	Night	8% 4 17% 44% 23% 4	438
August	1st to 10th	Night	10% 2 17% 37% 29% 5%	420
	11th to 20th	Night	11% 5% 20% 40% 22% 2	594
	21st to end	Night	10% 4 22% 40% 23% 1	660
September	1st to 10th	Night	14% 4 22% 38% 21% 1	600
	11th to 20th	Night	14% 4 21% 43% 18% 1	723
	21st to end	Night	9% 7% 32% 35% 16% 1	800
October	1st to 10th	Night	13% 6% 30% 37% 13% 2	800
	11th to 20th	Night	12% 5% 34% 35% 12% 2	800
	21st to end	Night	15% 8% 35% 32% 10% 1	985
November	1st to 10th	Night	19% 7% 31% 32% 10% 2	1025
	11th to 20th	Night	12% 7% 34% 32% 12% 3	1050
	21st to end	Night	9% 6% 37% 36% 11% 2	1050
December	1st to 10th	Night	12% 7% 36% 27% 13% 4	1050
	11th to 20th	Night	13% 7% 35% 33% 11% 1	1050
	21st to end	Night	11% 5% 30% 37% 15% 3	1155

Page generated at 2026-04-29 00:21:46. Version ac07

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