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 2025-02-06.

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%	109110
	Night	16% 12% 4 9% 5% 4 4 3 5% 6% 31%	109113
Transparency	Day	29% 45% 7% 11% 6% 1	112769
	Night	32% 43% 7% 11% 6% 1	113581
Seeing	Day	18% 12% 26% 26% 15% 4	15370
	Night	21% 11% 25% 27% 14% 2	24966

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	15% 19% 8% 22% 5% 5% 3 3 4 4 12%	4056
	11th to 20th	Day	14% 20% 8% 16% 8% 6% 6% 3 3 4 13%	4060
	21st to end	Day	21% 19% 9% 13% 7% 4 3 2 3 4 14%	4466
February	1st to 10th	Day	17% 17% 4 12% 6% 5% 4 4 4 5% 21%	1755
	11th to 20th	Day	15% 14% 6% 13% 7% 6% 3 3 5% 5% 22%	1761
	21st to end	Day	17% 14% 4 11% 6% 4 5% 3 5% 7% 24%	1570
March	1st to 10th	Day	22% 14% 5% 9% 5% 3 3 3 4 6% 27%	3296
	11th to 20th	Day	12% 13% 5% 8% 5% 5% 4 3 4 5% 34%	3480
	21st to end	Day	19% 12% 5% 10% 4 4 4 3 4 5% 29%	3815
April	1st to 10th	Day	13% 13% 4 8% 5% 4 4 3 6% 7% 32%	2119
	11th to 20th	Day	17% 11% 3 7% 5% 4 4 4 5% 6% 36%	2442
	21st to end	Day	18% 11% 3 7% 3 4 4 3 4 7% 35%	2470
May	1st to 10th	Day	18% 9% 4 8% 5% 4 5% 4 4 6% 33%	4082
	11th to 20th	Day	17% 8% 3 6% 4 4 5% 4 6% 7% 36%	4081
	21st to end	Day	10% 5% 2 7% 5% 5% 5% 4 7% 8% 42%	4476
June	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 4 6% 8% 40%	2551
	11th to 20th	Day	10% 5% 3 6% 5% 5% 5% 5% 6% 8% 43%	2790
	21st to end	Day	10% 5% 2 6% 4 5% 5% 4 7% 9% 43%	2692
July	1st to 10th	Day	9% 6% 3 6% 5% 4 5% 4 7% 10% 40%	4440
	11th to 20th	Day	8% 4 2 5% 4 4 5% 4 6% 9% 48%	4161
	21st to end	Day	8% 6% 2 7% 5% 4 5% 4 7% 8% 43%	4466
August	1st to 10th	Day	11% 6% 2 6% 4 4 5% 4 6% 8% 43%	4104
	11th to 20th	Day	9% 6% 2 6% 4 5% 6% 5% 7% 10% 41%	4044
	21st to end	Day	9% 5% 2 7% 4 4 4 5% 6% 11% 42%	4517
September	1st to 10th	Day	14% 5% 2 5% 4 4 5% 3 6% 7% 45%	2119
	11th to 20th	Day	12% 5% 2 6% 5% 3 4 4 6% 8% 45%	2090
	21st to end	Day	18% 7% 2 6% 4 4 4 4 6% 6% 39%	2090
October	1st to 10th	Day	14% 8% 3 7% 4 4 3 3 6% 8% 40%	2000
	11th to 20th	Day	19% 8% 3 6% 4 5% 4 3 5% 7% 36%	1900
	21st to end	Day	21% 11% 3 8% 4 4 5% 4 6% 8% 28%	2028
November	1st to 10th	Day	13% 11% 4 7% 5% 4 4 4 5% 7% 38%	1762
	11th to 20th	Day	16% 17% 3 8% 4 5% 3 3 4 6% 30%	1800
	21st to end	Day	17% 19% 6% 9% 6% 4 4 3 5% 5% 21%	1663
December	1st to 10th	Day	16% 15% 7% 13% 6% 5% 5% 3 4 6% 20%	2600
	11th to 20th	Day	20% 19% 7% 11% 5% 4 4 3 4 5% 18%	2990
	21st to end	Day	23% 22% 6% 13% 5% 5% 4 3 4 3 12%	4374
January	1st to 10th	Night	15% 18% 8% 22% 6% 4 3 3 3 4 14%	5889
	11th to 20th	Night	11% 20% 9% 17% 9% 5% 6% 4 4 4 11%	5900
	21st to end	Night	21% 16% 7% 13% 5% 3 3 3 4 4 20%	6490
February	1st to 10th	Night	19% 14% 5% 12% 6% 4 4 3 5% 5% 22%	2536
	11th to 20th	Night	16% 15% 5% 12% 6% 6% 5% 3 5% 4 24%	2302
	21st to end	Night	21% 12% 4 12% 6% 4 4 3 5% 5% 24%	1884
March	1st to 10th	Night	21% 14% 4 6% 4 3 4 3 3 5% 33%	3555
	11th to 20th	Night	13% 11% 3 6% 5% 4 5% 4 5% 6% 38%	3480
	21st to end	Night	17% 11% 4 9% 4 4 4 3 5% 5% 33%	3817
April	1st to 10th	Night	15% 14% 3 7% 5% 4 5% 3 5% 7% 33%	2075
	11th to 20th	Night	16% 10% 3 7% 5% 4 5% 4 6% 7% 34%	1687
	21st to end	Night	16% 9% 3 6% 5% 5% 4 4 6% 7% 35%	1710
May	1st to 10th	Night	20% 9% 4 8% 5% 4 5% 3 5% 7% 29%	2813
	11th to 20th	Night	17% 8% 3 6% 5% 4 5% 4 5% 8% 36%	2812
	21st to end	Night	9% 4 2 6% 5% 4 5% 4 5% 9% 48%	3079
June	1st to 10th	Night	12% 6% 3 5% 5% 4 4 4 6% 7% 43%	1620
	11th to 20th	Night	9% 5% 3 6% 5% 5% 5% 5% 6% 10% 42%	1382
	21st to end	Night	10% 6% 2 6% 3 5% 5% 4 7% 9% 43%	1475
July	1st to 10th	Night	10% 4 2 6% 4 4 5% 4 6% 9% 47%	2530
	11th to 20th	Night	9% 3 1 5% 4 4 5% 5% 7% 9% 49%	2795
	21st to end	Night	7% 4 2 6% 5% 5% 5% 5% 7% 10% 45%	3087
August	1st to 10th	Night	10% 7% 3 8% 4 4 4 3 6% 9% 42%	2825
	11th to 20th	Night	12% 6% 2 7% 4 3 4 3 6% 7% 46%	2785
	21st to end	Night	7% 5% 3 7% 4 5% 5% 4 6% 7% 46%	3124
September	1st to 10th	Night	10% 7% 2 6% 4 5% 4 4 6% 7% 45%	2038
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 5% 6% 51%	2090
	21st to end	Night	15% 6% 2 9% 4 4 4 4 6% 6% 40%	2090
October	1st to 10th	Night	15% 5% 2 6% 4 3 4 3 5% 7% 44%	2180
	11th to 20th	Night	21% 8% 2 8% 4 4 5% 3 4 6% 36%	2280
	21st to end	Night	25% 10% 3 7% 5% 4 4 3 5% 6% 29%	2565
November	1st to 10th	Night	15% 11% 3 6% 4 4 4 3 5% 6% 37%	2536
	11th to 20th	Night	17% 15% 4 9% 5% 4 4 3 5% 5% 29%	2600
	21st to end	Night	22% 17% 4 9% 5% 4 4 3 5% 5% 23%	2733
December	1st to 10th	Night	15% 16% 7% 11% 6% 6% 4 4 5% 7% 19%	4580
	11th to 20th	Night	19% 19% 7% 11% 4 4 4 3 5% 5% 20%	5205
	21st to end	Night	24% 19% 6% 11% 5% 5% 4 3 4 6% 12%	6564

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	57% 30% 5% 5% 3	4057
	11th to 20th	Day	58% 28% 8% 2 4	4060
	21st to end	Day	56% 29% 6% 7% 3	4466
February	1st to 10th	Day	25% 64% 4 6% 2	1755
	11th to 20th	Day	20% 65% 3 9% 2	1770
	21st to end	Day	18% 66% 5% 6% 3 1	1570
March	1st to 10th	Day	36% 44% 5% 13% 3	3296
	11th to 20th	Day	27% 44% 11% 14% 3	3480
	21st to end	Day	30% 44% 10% 8% 7%	3815
April	1st to 10th	Day	20% 66% 4 6% 3	2119
	11th to 20th	Day	17% 66% 4 7% 5% 1	2442
	21st to end	Day	17% 66% 5% 7% 4 1	2470
May	1st to 10th	Day	31% 43% 6% 14% 5% 1	4120
	11th to 20th	Day	27% 44% 11% 16% 3	4085
	21st to end	Day	21% 45% 7% 11% 11% 6%	4489
June	1st to 10th	Day	15% 66% 3 11% 5%	2551
	11th to 20th	Day	15% 66% 5% 8% 6% 1	2790
	21st to end	Day	12% 65% 6% 10% 6% 1	2692
July	1st to 10th	Day	26% 38% 9% 15% 10% 2	4371
	11th to 20th	Day	18% 39% 8% 21% 13% 1	4090
	21st to end	Day	24% 38% 11% 11% 14% 2	4324
August	1st to 10th	Day	23% 40% 10% 17% 9%	4091
	11th to 20th	Day	23% 40% 8% 23% 6%	4083
	21st to end	Day	22% 40% 11% 20% 7%	4517
September	1st to 10th	Day	13% 61% 7% 12% 6% 1	2119
	11th to 20th	Day	13% 62% 4 11% 9%	2090
	21st to end	Day	17% 60% 5% 10% 7%	2090
October	1st to 10th	Day	14% 38% 12% 22% 14%	3325
	11th to 20th	Day	26% 39% 6% 14% 16%	3140
	21st to end	Day	24% 38% 8% 17% 13%	3317
November	1st to 10th	Day	25% 57% 4 9% 4	1762
	11th to 20th	Day	26% 57% 4 8% 5%	1800
	21st to end	Day	30% 58% 5% 5% 2	1663
December	1st to 10th	Day	47% 38% 9% 5% 1	2600
	11th to 20th	Day	51% 34% 8% 7% 1	2986
	21st to end	Day	58% 29% 5% 5% 3	4374
January	1st to 10th	Night	57% 29% 5% 7% 3	5889
	11th to 20th	Night	59% 28% 6% 4 2	5900
	21st to end	Night	51% 30% 9% 5% 5%	6490
February	1st to 10th	Night	26% 63% 4 5% 2	2536
	11th to 20th	Night	21% 65% 3 8% 3	2304
	21st to end	Night	22% 65% 3 7% 3	1884
March	1st to 10th	Night	33% 41% 8% 12% 5%	3555
	11th to 20th	Night	24% 45% 13% 11% 6%	3480
	21st to end	Night	28% 44% 9% 13% 6%	3817
April	1st to 10th	Night	21% 65% 4 7% 3	2075
	11th to 20th	Night	19% 66% 4 5% 6% 1	1687
	21st to end	Night	16% 66% 4 8% 5% 1	1710
May	1st to 10th	Night	31% 43% 6% 12% 7% 1	2840
	11th to 20th	Night	27% 45% 9% 15% 5%	2813
	21st to end	Night	18% 43% 8% 15% 14% 2	3087
June	1st to 10th	Night	15% 66% 3 11% 6%	1620
	11th to 20th	Night	16% 65% 3 9% 6% 1	1382
	21st to end	Night	13% 64% 5% 10% 6% 1	1475
July	1st to 10th	Night	22% 38% 9% 17% 10% 4	2490
	11th to 20th	Night	20% 38% 9% 18% 11% 3	2743
	21st to end	Night	21% 38% 10% 14% 14% 2	2988
August	1st to 10th	Night	28% 39% 8% 16% 9% 1	2814
	11th to 20th	Night	25% 40% 6% 21% 8%	2812
	21st to end	Night	24% 40% 8% 19% 8% 2	3124
September	1st to 10th	Night	14% 61% 7% 11% 6% 1	2038
	11th to 20th	Night	12% 61% 4 11% 10% 1	2090
	21st to end	Night	17% 61% 4 10% 7% 1	2090
October	1st to 10th	Night	15% 37% 6% 22% 20%	3635
	11th to 20th	Night	28% 38% 6% 15% 14%	3779
	21st to end	Night	22% 38% 7% 20% 13% 1	4213
November	1st to 10th	Night	27% 56% 5% 8% 3 1	2536
	11th to 20th	Night	25% 56% 4 8% 6% 1	2600
	21st to end	Night	30% 57% 5% 6% 2	2733
December	1st to 10th	Night	45% 37% 9% 6% 2	4580
	11th to 20th	Night	48% 33% 9% 8% 1	5208
	21st to end	Night	54% 29% 8% 7% 1	6564

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	25% 21% 33% 15% 6% 1	300
	11th to 20th	Day	26% 20% 40% 10% 5%	300
	21st to end	Day	24% 22% 33% 17% 3 1	330
February	1st to 10th	Day	24% 22% 35% 17% 2	285
	11th to 20th	Day	26% 22% 35% 14% 3	270
	21st to end	Day	22% 30% 36% 10% 2	225
March	1st to 10th	Day	21% 20% 32% 22% 5%	368
	11th to 20th	Day	20% 21% 31% 22% 6% 1	460
	21st to end	Day	27% 17% 31% 21% 4 1	506
April	1st to 10th	Day	17% 16% 35% 24% 8% 1	460
	11th to 20th	Day	21% 17% 30% 20% 8% 4	452
	21st to end	Day	23% 15% 31% 23% 7% 2	460
May	1st to 10th	Day	22% 11% 29% 26% 10% 2	463
	11th to 20th	Day	18% 7% 29% 28% 14% 5%	487
	21st to end	Day	12% 6% 24% 30% 22% 6%	529
June	1st to 10th	Day	14% 5% 24% 38% 16% 3	490
	11th to 20th	Day	10% 5% 23% 36% 22% 3	620
	21st to end	Day	11% 7% 23% 35% 21% 3	680
July	1st to 10th	Day	8% 3 18% 35% 26% 10%	671
	11th to 20th	Day	10% 5% 17% 34% 26% 9%	510
	21st to end	Day	9% 3 20% 33% 28% 7%	532
August	1st to 10th	Day	11% 3 18% 30% 29% 9%	486
	11th to 20th	Day	10% 5% 16% 32% 28% 8%	479
	21st to end	Day	10% 4 17% 30% 29% 9%	539
September	1st to 10th	Day	17% 3 16% 27% 26% 11%	490
	11th to 20th	Day	11% 3 14% 29% 30% 12%	490
	21st to end	Day	20% 10% 20% 26% 17% 7%	490
October	1st to 10th	Day	16% 6% 22% 28% 22% 7%	490
	11th to 20th	Day	21% 14% 29% 24% 9% 3	490
	21st to end	Day	27% 17% 27% 21% 7% 1	477
November	1st to 10th	Day	15% 19% 29% 23% 12% 2	300
	11th to 20th	Day	28% 20% 29% 16% 6% 1	300
	21st to end	Day	32% 21% 32% 14% 1	231
December	1st to 10th	Day	22% 24% 32% 17% 5% 1	200
	11th to 20th	Day	24% 26% 33% 17% 1	200
	21st to end	Day	27% 19% 34% 17% 3	310
January	1st to 10th	Night	28% 17% 33% 18% 5%	800
	11th to 20th	Night	30% 16% 33% 17% 4	800
	21st to end	Night	25% 15% 34% 22% 4	880
February	1st to 10th	Night	27% 13% 32% 23% 5%	781
	11th to 20th	Night	23% 16% 34% 23% 4	743
	21st to end	Night	26% 16% 34% 20% 3	628
March	1st to 10th	Night	23% 13% 31% 26% 7%	760
	11th to 20th	Night	19% 15% 31% 26% 7% 1	760
	21st to end	Night	23% 11% 29% 27% 10% 1	836
April	1st to 10th	Night	20% 13% 31% 29% 6%	746
	11th to 20th	Night	21% 9% 33% 25% 10% 2	563
	21st to end	Night	19% 8% 30% 32% 10% 1	570
May	1st to 10th	Night	20% 9% 22% 34% 14% 1	570
	11th to 20th	Night	21% 5% 23% 31% 18% 2	561
	21st to end	Night	11% 4 18% 35% 27% 5%	617
June	1st to 10th	Night	14% 4 22% 39% 20% 1	480
	11th to 20th	Night	11% 2 21% 42% 23% 1	380
	21st to end	Night	11% 4 22% 36% 24% 2	525
July	1st to 10th	Night	9% 3 13% 40% 31% 5%	555
	11th to 20th	Night	11% 3 17% 33% 29% 6%	552
	21st to end	Night	8% 3 14% 40% 31% 3	624
August	1st to 10th	Night	12% 3 13% 38% 29% 6%	567
	11th to 20th	Night	13% 3 13% 37% 28% 6%	564
	21st to end	Night	11% 5% 14% 33% 31% 6%	627
September	1st to 10th	Night	13% 6% 13% 34% 27% 6%	741
	11th to 20th	Night	13% 4 12% 34% 31% 6%	760
	21st to end	Night	19% 8% 18% 28% 23% 4	760
October	1st to 10th	Night	18% 8% 21% 28% 22% 4	760
	11th to 20th	Night	26% 12% 27% 23% 11% 2	760
	21st to end	Night	30% 14% 25% 23% 7%	836
November	1st to 10th	Night	23% 15% 26% 24% 11% 1	780
	11th to 20th	Night	28% 15% 31% 19% 6% 2	800
	21st to end	Night	32% 16% 28% 18% 5%	800
December	1st to 10th	Night	29% 20% 27% 19% 5%	800
	11th to 20th	Night	27% 18% 34% 18% 4 1	800
	21st to end	Night	33% 14% 30% 21% 3	880

Page generated at 2025-02-03 03:05:49. Version ac07

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