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 2026-06-01.

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	15% 11% 4 9% 5% 4 4 4 5% 7% 32%	132572
	Night	15% 12% 4 10% 5% 4 5% 4 5% 6% 31%	130554
Transparency	Day	32% 38% 8% 13% 8% 1	136187
	Night	35% 37% 7% 12% 7% 1	134997
Seeing	Day	17% 11% 26% 26% 15% 4	17133
	Night	21% 11% 26% 27% 14% 2	26671

By Month

Percentage Cloud Cover SE (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	16% 18% 7% 21% 6% 4 3 3 4 5% 12%	5258
	11th to 20th	Day	13% 19% 8% 16% 9% 6% 6% 2 3 4 14%	5280
	21st to end	Day	18% 17% 8% 14% 7% 5% 5% 3 4 4 15%	5780
February	1st to 10th	Day	16% 17% 5% 12% 6% 5% 4 4 4 5% 22%	1890
	11th to 20th	Day	15% 14% 7% 14% 7% 6% 3 3 5% 5% 22%	1952
	21st to end	Day	16% 14% 4 12% 5% 4 5% 3 5% 7% 24%	1730
March	1st to 10th	Day	18% 12% 6% 11% 5% 3 3 3 4 5% 29%	4831
	11th to 20th	Day	11% 12% 4 9% 5% 5% 4 3 4 6% 37%	5090
	21st to end	Day	17% 14% 5% 10% 4 5% 4 3 4 6% 28%	5586
April	1st to 10th	Day	15% 13% 4 8% 5% 4 4 3 6% 6% 32%	2343
	11th to 20th	Day	16% 11% 3 8% 5% 4 4 4 5% 6% 35%	2702
	21st to end	Day	17% 11% 3 7% 4 4 4 3 4 7% 35%	2730
May	1st to 10th	Day	20% 10% 4 8% 5% 5% 4 3 4 5% 32%	5992
	11th to 20th	Day	16% 9% 3 7% 5% 4 5% 4 6% 7% 35%	5991
	21st to end	Day	16% 7% 3 6% 4 5% 5% 4 6% 7% 37%	6576
June	1st to 10th	Day	13% 7% 2 6% 5% 4 5% 4 6% 8% 41%	2685
	11th to 20th	Day	11% 6% 3 6% 5% 5% 5% 5% 6% 8% 42%	2937
	21st to end	Day	10% 5% 2 5% 4 5% 5% 4 7% 9% 43%	2833
July	1st to 10th	Day	8% 5% 2 6% 5% 4 5% 4 6% 10% 44%	6360
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 10% 43%	5955
	21st to end	Day	10% 6% 2 7% 4 4 4 4 6% 8% 43%	6424
August	1st to 10th	Day	11% 6% 2 6% 4 4 5% 4 6% 8% 44%	4234
	11th to 20th	Day	10% 6% 2 6% 4 5% 6% 5% 6% 10% 41%	4174
	21st to end	Day	9% 5% 2 7% 4 4 5% 5% 6% 11% 42%	4660
September	1st to 10th	Day	14% 5% 2 5% 4 3 5% 3 5% 7% 46%	2230
	11th to 20th	Day	11% 5% 2 6% 5% 3 4 4 6% 8% 46%	2200
	21st to end	Day	18% 7% 2 6% 4 4 4 4 6% 6% 39%	2200
October	1st to 10th	Day	13% 7% 3 7% 4 4 3 3 6% 8% 41%	2105
	11th to 20th	Day	19% 7% 3 6% 4 5% 4 3 5% 7% 38%	2000
	21st to end	Day	20% 10% 3 8% 4 4 5% 4 6% 8% 28%	2135
November	1st to 10th	Day	13% 11% 4 7% 5% 4 4 4 5% 6% 37%	1852
	11th to 20th	Day	16% 17% 3 8% 4 4 3 3 4 6% 30%	1890
	21st to end	Day	17% 19% 6% 9% 6% 4 4 3 5% 5% 21%	1746
December	1st to 10th	Day	16% 15% 7% 14% 6% 5% 4 3 4 6% 20%	2680
	11th to 20th	Day	20% 19% 7% 11% 5% 4 4 3 4 5% 18%	3070
	21st to end	Day	23% 22% 6% 13% 5% 5% 4 3 3 3 12%	4471
January	1st to 10th	Night	16% 19% 7% 21% 7% 4 3 3 3 3 14%	7635
	11th to 20th	Night	9% 18% 8% 17% 8% 5% 6% 4 4 4 15%	7680
	21st to end	Night	18% 15% 6% 12% 5% 4 5% 3 5% 5% 21%	8401
February	1st to 10th	Night	18% 14% 5% 12% 6% 4 4 3 5% 5% 23%	2730
	11th to 20th	Night	16% 14% 6% 11% 6% 6% 5% 3 5% 5% 23%	2550
	21st to end	Night	20% 13% 5% 12% 5% 4 4 3 5% 5% 24%	2076
March	1st to 10th	Night	19% 13% 4 7% 4 4 4 3 4 6% 32%	5240
	11th to 20th	Night	11% 10% 2 6% 5% 4 5% 4 6% 7% 40%	5090
	21st to end	Night	16% 11% 4 9% 5% 4 4 3 5% 5% 33%	5588
April	1st to 10th	Night	16% 14% 3 8% 5% 4 5% 3 5% 6% 32%	2293
	11th to 20th	Night	15% 10% 3 7% 5% 5% 5% 4 6% 7% 34%	1867
	21st to end	Night	16% 9% 3 6% 5% 5% 4 4 6% 7% 35%	1890
May	1st to 10th	Night	20% 10% 4 9% 5% 4 5% 4 5% 6% 27%	4123
	11th to 20th	Night	15% 8% 3 7% 5% 4 4 4 5% 8% 37%	4122
	21st to end	Night	16% 6% 3 7% 4 4 5% 4 5% 8% 40%	4520
June	1st to 10th	Night	12% 6% 3 6% 5% 4 4 4 6% 7% 44%	1705
	11th to 20th	Night	9% 5% 2 6% 5% 5% 5% 5% 6% 10% 41%	1455
	21st to end	Night	9% 6% 2 6% 3 5% 5% 4 7% 9% 44%	1553
July	1st to 10th	Night	9% 4 2 7% 4 4 5% 4 7% 10% 44%	3618
	11th to 20th	Night	10% 4 2 6% 5% 5% 5% 4 7% 9% 43%	3999
	21st to end	Night	8% 5% 2 6% 4 4 4 4 6% 9% 48%	4427
August	1st to 10th	Night	10% 6% 3 8% 4 4 4 3 6% 9% 42%	2915
	11th to 20th	Night	11% 6% 2 7% 4 3 4 3 5% 7% 47%	2875
	21st to end	Night	7% 5% 3 7% 4 5% 5% 4 6% 8% 46%	3223
September	1st to 10th	Night	10% 7% 2 6% 4 4 4 4 6% 7% 46%	2145
	11th to 20th	Night	10% 5% 2 5% 4 4 4 3 5% 7% 52%	2200
	21st to end	Night	15% 6% 2 9% 4 4 4 4 6% 6% 40%	2200
October	1st to 10th	Night	15% 5% 2 6% 4 3 4 3 5% 6% 46%	2295
	11th to 20th	Night	20% 8% 2 8% 4 4 5% 3 4 6% 37%	2400
	21st to end	Night	25% 10% 3 6% 5% 3 4 3 5% 6% 30%	2700
November	1st to 10th	Night	15% 11% 3 6% 4 4 4 3 5% 6% 37%	2666
	11th to 20th	Night	17% 15% 4 9% 5% 4 4 3 5% 5% 29%	2730
	21st to end	Night	22% 17% 5% 9% 5% 4 4 3 5% 5% 22%	2870
December	1st to 10th	Night	15% 16% 7% 11% 6% 5% 5% 4 5% 7% 20%	4720
	11th to 20th	Night	19% 19% 7% 11% 4 4 4 3 5% 5% 20%	5345
	21st to end	Night	24% 19% 6% 12% 5% 5% 4 3 4 6% 13%	6708

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% 24% 7% 6% 2	5259
	11th to 20th	Day	60% 22% 8% 4 5%	5280
	21st to end	Day	57% 24% 7% 9% 3	5780
February	1st to 10th	Day	26% 60% 4 7% 2	1890
	11th to 20th	Day	25% 59% 4 9% 3	1961
	21st to end	Day	22% 60% 6% 7% 3 1	1730
March	1st to 10th	Day	40% 31% 7% 15% 7%	4831
	11th to 20th	Day	31% 32% 13% 18% 5% 1	5090
	21st to end	Day	38% 32% 9% 11% 9%	5586
April	1st to 10th	Day	24% 60% 4 7% 4	2343
	11th to 20th	Day	20% 60% 5% 8% 6% 1	2702
	21st to end	Day	20% 60% 5% 8% 5% 1	2730
May	1st to 10th	Day	39% 31% 7% 12% 10% 1	6027
	11th to 20th	Day	33% 32% 12% 16% 6% 1	5995
	21st to end	Day	31% 32% 8% 15% 10% 4	6590
June	1st to 10th	Day	16% 63% 4 11% 6%	2685
	11th to 20th	Day	17% 63% 5% 8% 5% 1	2937
	21st to end	Day	13% 62% 7% 11% 7% 1	2833
July	1st to 10th	Day	26% 28% 9% 22% 13% 2	6291
	11th to 20th	Day	25% 29% 12% 21% 13% 1	5884
	21st to end	Day	27% 28% 11% 17% 17% 1	6240
August	1st to 10th	Day	23% 39% 11% 17% 10%	4221
	11th to 20th	Day	23% 39% 8% 24% 6%	4213
	21st to end	Day	22% 39% 11% 21% 7%	4660
September	1st to 10th	Day	13% 59% 7% 14% 7% 1	2230
	11th to 20th	Day	13% 60% 5% 14% 9%	2200
	21st to end	Day	18% 58% 5% 11% 8%	2200
October	1st to 10th	Day	14% 37% 12% 22% 14%	3430
	11th to 20th	Day	26% 38% 6% 15% 16%	3240
	21st to end	Day	24% 37% 8% 17% 14%	3424
November	1st to 10th	Day	26% 55% 4 10% 4	1852
	11th to 20th	Day	27% 54% 4 9% 5% 1	1890
	21st to end	Day	32% 56% 5% 5% 2	1746
December	1st to 10th	Day	47% 37% 9% 5% 2	2680
	11th to 20th	Day	51% 33% 8% 7% 1	3066
	21st to end	Day	58% 28% 5% 5% 3	4471
January	1st to 10th	Night	61% 24% 5% 7% 3	7636
	11th to 20th	Night	58% 22% 8% 7% 4 1	7680
	21st to end	Night	50% 24% 10% 10% 5%	8401
February	1st to 10th	Night	28% 59% 4 6% 2	2730
	11th to 20th	Night	25% 59% 4 8% 3	2552
	21st to end	Night	26% 60% 4 7% 3	2076
March	1st to 10th	Night	39% 29% 11% 14% 6%	5240
	11th to 20th	Night	28% 32% 12% 20% 7% 1	5090
	21st to end	Night	35% 31% 9% 13% 11% 1	5588
April	1st to 10th	Night	25% 60% 5% 7% 3	2293
	11th to 20th	Night	22% 60% 5% 7% 6%	1867
	21st to end	Night	19% 60% 4 10% 6% 1	1890
May	1st to 10th	Night	42% 32% 6% 10% 8% 3	4150
	11th to 20th	Night	32% 32% 9% 18% 8%	4123
	21st to end	Night	31% 31% 9% 15% 12% 1	4528
June	1st to 10th	Night	16% 63% 3 10% 7%	1705
	11th to 20th	Night	17% 62% 4 10% 6% 1	1455
	21st to end	Night	14% 61% 6% 11% 7% 1	1553
July	1st to 10th	Night	26% 29% 11% 19% 12% 3	3580
	11th to 20th	Night	28% 29% 10% 20% 11% 3	3950
	21st to end	Night	24% 28% 9% 18% 20% 2	4297
August	1st to 10th	Night	28% 38% 8% 16% 9% 1	2904
	11th to 20th	Night	25% 39% 6% 22% 9%	2902
	21st to end	Night	24% 39% 8% 20% 8% 2	3223
September	1st to 10th	Night	15% 58% 7% 13% 6% 1	2145
	11th to 20th	Night	12% 58% 5% 13% 11% 1	2200
	21st to end	Night	19% 58% 5% 9% 9% 1	2200
October	1st to 10th	Night	15% 36% 6% 22% 21%	3750
	11th to 20th	Night	28% 37% 6% 15% 14%	3899
	21st to end	Night	23% 37% 7% 19% 13% 1	4348
November	1st to 10th	Night	28% 53% 5% 9% 4 1	2666
	11th to 20th	Night	26% 54% 4 9% 6% 1	2730
	21st to end	Night	32% 55% 5% 6% 2	2870
December	1st to 10th	Night	46% 36% 9% 7% 3	4720
	11th to 20th	Night	49% 33% 9% 9% 1	5348
	21st to end	Night	55% 29% 8% 7% 1	6708

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	24% 20% 34% 17% 5% 1	330
	11th to 20th	Day	25% 22% 39% 10% 4	330
	21st to end	Day	23% 23% 32% 18% 3 1	363
February	1st to 10th	Day	22% 19% 36% 19% 4	330
	11th to 20th	Day	25% 21% 35% 16% 3	330
	21st to end	Day	20% 27% 38% 13% 2	273
March	1st to 10th	Day	20% 18% 34% 24% 4	438
	11th to 20th	Day	19% 20% 33% 22% 6% 1	540
	21st to end	Day	24% 16% 31% 24% 4 1	594
April	1st to 10th	Day	19% 15% 36% 22% 8% 1	540
	11th to 20th	Day	18% 18% 31% 21% 8% 3	532
	21st to end	Day	21% 14% 30% 25% 9% 2	540
May	1st to 10th	Day	23% 10% 28% 27% 9% 2	543
	11th to 20th	Day	18% 7% 29% 28% 15% 4	567
	21st to end	Day	15% 6% 25% 30% 20% 5%	617
June	1st to 10th	Day	15% 5% 23% 38% 16% 3	530
	11th to 20th	Day	12% 5% 22% 36% 22% 3	667
	21st to end	Day	10% 6% 22% 34% 24% 4	730
July	1st to 10th	Day	8% 3 17% 35% 26% 10%	721
	11th to 20th	Day	10% 4 16% 34% 26% 9%	551
	21st to end	Day	9% 3 20% 32% 29% 7%	576
August	1st to 10th	Day	10% 3 17% 29% 31% 10%	526
	11th to 20th	Day	11% 4 16% 31% 30% 9%	519
	21st to end	Day	10% 4 17% 31% 29% 8%	583
September	1st to 10th	Day	16% 4 16% 26% 26% 13%	530
	11th to 20th	Day	10% 3 13% 28% 32% 14%	530
	21st to end	Day	19% 9% 20% 26% 18% 7%	530
October	1st to 10th	Day	15% 6% 20% 28% 23% 8%	530
	11th to 20th	Day	20% 13% 29% 26% 9% 3	530
	21st to end	Day	26% 17% 28% 22% 6% 1	518
November	1st to 10th	Day	16% 18% 29% 23% 11% 2	330
	11th to 20th	Day	28% 22% 28% 16% 5% 1	330
	21st to end	Day	30% 21% 31% 16% 2	254
December	1st to 10th	Day	22% 23% 32% 17% 5% 1	220
	11th to 20th	Day	25% 25% 33% 16% 1	220
	21st to end	Day	28% 19% 34% 17% 3	341
January	1st to 10th	Night	27% 17% 33% 19% 5%	840
	11th to 20th	Night	29% 16% 34% 17% 4	840
	21st to end	Night	24% 15% 34% 22% 4	924
February	1st to 10th	Night	26% 13% 32% 23% 5%	840
	11th to 20th	Night	23% 15% 34% 24% 4	823
	21st to end	Night	25% 16% 35% 21% 3	692
March	1st to 10th	Night	23% 13% 32% 25% 7%	840
	11th to 20th	Night	20% 15% 32% 25% 7% 1	840
	21st to end	Night	22% 11% 30% 28% 9% 1	924
April	1st to 10th	Night	22% 13% 31% 28% 6%	824
	11th to 20th	Night	20% 9% 33% 26% 10% 2	623
	21st to end	Night	19% 8% 29% 33% 10% 1	630
May	1st to 10th	Night	20% 9% 23% 34% 12% 1	630
	11th to 20th	Night	21% 6% 23% 31% 18% 2	621
	21st to end	Night	13% 4 19% 35% 25% 5%	683
June	1st to 10th	Night	13% 4 22% 39% 20% 1	505
	11th to 20th	Night	11% 2 21% 42% 23% 1	400
	21st to end	Night	11% 4 21% 36% 26% 2	553
July	1st to 10th	Night	9% 3 12% 40% 31% 5%	585
	11th to 20th	Night	11% 3 17% 34% 29% 6%	582
	21st to end	Night	9% 3 15% 39% 31% 3	657
August	1st to 10th	Night	11% 3 12% 38% 30% 6%	597
	11th to 20th	Night	12% 3 12% 36% 30% 6%	594
	21st to end	Night	10% 4 15% 33% 32% 5%	660
September	1st to 10th	Night	12% 6% 14% 33% 28% 6%	780
	11th to 20th	Night	12% 4 12% 32% 34% 7%	800
	21st to end	Night	19% 8% 18% 28% 23% 4	800
October	1st to 10th	Night	18% 8% 20% 29% 22% 4	800
	11th to 20th	Night	26% 12% 26% 24% 11% 2	800
	21st to end	Night	31% 14% 26% 23% 7%	880
November	1st to 10th	Night	22% 15% 27% 24% 10% 1	820
	11th to 20th	Night	28% 15% 30% 19% 6% 2	840
	21st to end	Night	32% 16% 29% 17% 5%	840
December	1st to 10th	Night	29% 20% 27% 20% 5%	840
	11th to 20th	Night	26% 18% 34% 17% 3	840
	21st to end	Night	32% 14% 31% 21% 3	924

Page generated at 2026-05-29 01:53:31. Version ac07

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