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 2024-01-17.

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 7% 7% 4 6% 5% 6% 8% 34%	74534
	Night	8% 9% 4 8% 7% 4 6% 6% 6% 8% 34%	75224
Transparency	Day	46% 9% 17% 20% 7% 1	74214
	Night	47% 8% 15% 20% 9% 1	75061
Seeing	Day	13% 7% 27% 30% 17% 5%	14420
	Night	12% 6% 29% 36% 15% 2	23681

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	9% 11% 5% 10% 12% 5% 7% 5% 6% 7% 22%	1329
	11th to 20th	Day	9% 11% 5% 9% 8% 5% 7% 6% 6% 7% 25%	1302
	21st to end	Day	7% 15% 7% 12% 10% 5% 4 5% 7% 6% 22%	1395
February	1st to 10th	Day	8% 14% 6% 12% 10% 4 7% 5% 7% 6% 23%	1489
	11th to 20th	Day	7% 14% 6% 11% 9% 5% 7% 6% 6% 7% 23%	1575
	21st to end	Day	6% 12% 5% 11% 8% 5% 8% 7% 7% 8% 22%	1332
March	1st to 10th	Day	8% 14% 6% 9% 10% 6% 8% 5% 7% 7% 19%	1810
	11th to 20th	Day	8% 11% 6% 8% 6% 4 4 6% 5% 8% 33%	1979
	21st to end	Day	10% 13% 6% 11% 8% 3 5% 5% 5% 7% 26%	2178
April	1st to 10th	Day	9% 12% 5% 8% 7% 3 6% 5% 6% 7% 32%	2249
	11th to 20th	Day	13% 10% 5% 9% 6% 4 5% 5% 5% 6% 32%	2310
	21st to end	Day	15% 11% 3 6% 5% 3 5% 5% 6% 8% 33%	2391
May	1st to 10th	Day	10% 9% 4 7% 7% 4 7% 6% 6% 7% 33%	2556
	11th to 20th	Day	10% 6% 3 5% 6% 4 5% 5% 7% 8% 40%	2654
	21st to end	Day	16% 7% 3 5% 5% 3 5% 5% 6% 8% 35%	2917
June	1st to 10th	Day	14% 7% 4 7% 6% 4 6% 5% 7% 8% 31%	2700
	11th to 20th	Day	20% 9% 4 8% 7% 5% 6% 6% 7% 7% 22%	2757
	21st to end	Day	12% 6% 3 5% 6% 4 6% 6% 7% 11% 34%	2700
July	1st to 10th	Day	9% 5% 3 5% 7% 4 5% 6% 7% 9% 41%	2700
	11th to 20th	Day	10% 6% 2 6% 6% 3 5% 6% 8% 10% 37%	2700
	21st to end	Day	9% 4 2 5% 5% 3 5% 5% 7% 10% 45%	2938
August	1st to 10th	Day	10% 5% 3 6% 5% 4 6% 6% 7% 8% 41%	2662
	11th to 20th	Day	10% 5% 2 6% 5% 3 4 6% 6% 9% 44%	2299
	21st to end	Day	11% 5% 2 5% 5% 3 5% 5% 6% 9% 44%	2574
September	1st to 10th	Day	11% 6% 3 5% 6% 4 5% 6% 6% 8% 41%	2250
	11th to 20th	Day	16% 8% 3 5% 5% 3 4 5% 5% 7% 40%	2048
	21st to end	Day	12% 7% 2 6% 4 3 4 5% 6% 8% 44%	1976
October	1st to 10th	Day	12% 10% 4 7% 7% 3 5% 4 4 7% 38%	1800
	11th to 20th	Day	11% 8% 4 8% 5% 3 5% 4 4 7% 40%	1754
	21st to end	Day	9% 11% 5% 9% 6% 4 5% 6% 6% 8% 31%	1670
November	1st to 10th	Day	12% 13% 6% 9% 10% 4 5% 5% 5% 7% 25%	1488
	11th to 20th	Day	8% 11% 7% 14% 9% 4 4 5% 7% 7% 25%	1482
	21st to end	Day	5% 11% 7% 13% 10% 4 6% 5% 5% 8% 28%	1330
December	1st to 10th	Day	8% 12% 6% 9% 8% 5% 8% 5% 6% 7% 25%	1285
	11th to 20th	Day	7% 11% 4 11% 10% 5% 8% 5% 7% 8% 24%	1513
	21st to end	Day	4 8% 5% 8% 6% 4 5% 5% 5% 8% 42%	2442
January	1st to 10th	Night	7% 11% 4 11% 10% 6% 8% 7% 7% 7% 23%	2836
	11th to 20th	Night	6% 11% 5% 10% 8% 5% 6% 6% 6% 7% 29%	2803
	21st to end	Night	7% 16% 7% 10% 9% 5% 5% 6% 6% 7% 21%	2970
February	1st to 10th	Night	6% 10% 5% 10% 10% 5% 7% 7% 6% 8% 26%	2474
	11th to 20th	Night	6% 10% 5% 10% 11% 5% 7% 7% 7% 6% 27%	2272
	21st to end	Night	4 7% 5% 11% 9% 4 6% 7% 7% 10% 29%	1924
March	1st to 10th	Night	8% 11% 4 10% 9% 5% 7% 6% 6% 7% 27%	2146
	11th to 20th	Night	7% 10% 5% 7% 8% 4 5% 6% 7% 8% 33%	1980
	21st to end	Night	9% 13% 6% 9% 7% 4 4 5% 6% 8% 29%	2178
April	1st to 10th	Night	8% 8% 4 5% 6% 4 6% 6% 7% 8% 39%	1709
	11th to 20th	Night	10% 8% 4 9% 8% 4 6% 6% 7% 8% 30%	1601
	21st to end	Night	13% 10% 5% 7% 4 3 5% 5% 5% 7% 35%	1568
May	1st to 10th	Night	10% 8% 4 7% 8% 3 6% 6% 6% 8% 33%	1403
	11th to 20th	Night	10% 6% 3 6% 6% 3 5% 6% 7% 7% 41%	1239
	21st to end	Night	16% 7% 3 7% 6% 3 5% 6% 7% 7% 34%	1358
June	1st to 10th	Night	13% 7% 4 7% 7% 5% 6% 6% 7% 7% 31%	1260
	11th to 20th	Night	18% 9% 4 6% 5% 4 5% 6% 7% 9% 27%	1159
	21st to end	Night	10% 4 3 6% 6% 4 8% 7% 7% 7% 39%	1260
July	1st to 10th	Night	10% 5% 2 4 5% 2 5% 5% 7% 9% 45%	1260
	11th to 20th	Night	10% 4 2 5% 5% 3 6% 6% 7% 10% 42%	1260
	21st to end	Night	7% 4 2 5% 4 2 5% 5% 7% 8% 50%	1379
August	1st to 10th	Night	8% 3 2 5% 5% 3 5% 7% 8% 11% 43%	1273
	11th to 20th	Night	11% 6% 2 5% 4 3 5% 4 7% 10% 42%	1602
	21st to end	Night	9% 5% 3 5% 6% 3 4 4 5% 8% 48%	1782
September	1st to 10th	Night	11% 6% 2 5% 6% 3 4 3 5% 7% 47%	1710
	11th to 20th	Night	12% 5% 1 5% 5% 2 4 5% 6% 7% 48%	1910
	21st to end	Night	6% 6% 3 5% 4 3 4 6% 6% 9% 49%	1984
October	1st to 10th	Night	10% 8% 3 8% 6% 3 5% 5% 5% 7% 39%	2160
	11th to 20th	Night	9% 10% 3 7% 6% 3 4 5% 6% 7% 40%	2204
	21st to end	Night	10% 12% 4 8% 6% 3 5% 6% 6% 9% 33%	2668
November	1st to 10th	Night	14% 13% 6% 7% 6% 4 6% 6% 4 6% 27%	2590
	11th to 20th	Night	8% 11% 5% 13% 10% 5% 6% 4 6% 7% 25%	2696
	21st to end	Night	5% 10% 6% 11% 10% 4 6% 5% 6% 7% 29%	2850
December	1st to 10th	Night	7% 11% 6% 10% 9% 5% 6% 6% 6% 8% 25%	2887
	11th to 20th	Night	7% 12% 5% 14% 12% 5% 6% 6% 5% 7% 21%	3655
	21st to end	Night	3 7% 3 7% 7% 4 6% 5% 6% 6% 45%	5214

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	58% 7% 12% 14% 7% 1	1329
	11th to 20th	Day	57% 6% 14% 15% 8% 1	1302
	21st to end	Day	61% 7% 9% 14% 8%	1395
February	1st to 10th	Day	59% 6% 13% 16% 6%	1489
	11th to 20th	Day	59% 8% 12% 14% 7%	1584
	21st to end	Day	56% 7% 14% 15% 7%	1332
March	1st to 10th	Day	61% 9% 14% 13% 3	1810
	11th to 20th	Day	47% 8% 17% 20% 8%	1980
	21st to end	Day	56% 8% 14% 18% 4	2178
April	1st to 10th	Day	50% 8% 15% 19% 8%	2249
	11th to 20th	Day	51% 7% 14% 19% 8% 1	2310
	21st to end	Day	47% 9% 14% 20% 8% 2	2391
May	1st to 10th	Day	47% 9% 13% 20% 10% 1	2556
	11th to 20th	Day	39% 9% 14% 25% 12% 1	2658
	21st to end	Day	45% 9% 15% 21% 10% 1	2918
June	1st to 10th	Day	48% 9% 16% 19% 7% 1	2700
	11th to 20th	Day	57% 11% 12% 15% 4 1	2757
	21st to end	Day	40% 13% 21% 20% 5% 1	2700
July	1st to 10th	Day	36% 12% 22% 25% 5% 1	2626
	11th to 20th	Day	40% 12% 26% 20% 2	2619
	21st to end	Day	33% 10% 27% 26% 4	2775
August	1st to 10th	Day	37% 13% 24% 22% 4	2644
	11th to 20th	Day	34% 11% 26% 22% 7%	2299
	21st to end	Day	34% 10% 22% 27% 6% 1	2574
September	1st to 10th	Day	39% 8% 19% 23% 8% 2	2250
	11th to 20th	Day	42% 7% 17% 27% 6%	2048
	21st to end	Day	36% 9% 17% 27% 9% 2	1976
October	1st to 10th	Day	46% 7% 17% 23% 7%	1800
	11th to 20th	Day	43% 7% 15% 25% 9%	1754
	21st to end	Day	48% 11% 14% 19% 8%	1670
November	1st to 10th	Day	57% 7% 13% 17% 5%	1488
	11th to 20th	Day	56% 10% 16% 14% 4	1482
	21st to end	Day	54% 8% 16% 17% 4 1	1330
December	1st to 10th	Day	56% 7% 12% 17% 6% 1	1285
	11th to 20th	Day	55% 8% 16% 16% 5%	1514
	21st to end	Day	40% 5% 12% 28% 14% 2	2442
January	1st to 10th	Night	57% 9% 13% 15% 6% 1	2836
	11th to 20th	Night	51% 8% 17% 17% 6% 1	2803
	21st to end	Night	60% 8% 13% 13% 6%	2970
February	1st to 10th	Night	53% 8% 13% 17% 8%	2474
	11th to 20th	Night	53% 8% 13% 17% 9%	2273
	21st to end	Night	47% 9% 13% 21% 9% 1	1924
March	1st to 10th	Night	53% 8% 16% 16% 6%	2146
	11th to 20th	Night	46% 8% 12% 23% 10% 1	1980
	21st to end	Night	51% 8% 15% 18% 8%	2178
April	1st to 10th	Night	41% 8% 16% 20% 14% 1	1709
	11th to 20th	Night	49% 8% 13% 18% 9% 2	1601
	21st to end	Night	45% 8% 14% 20% 11% 2	1568
May	1st to 10th	Night	46% 8% 12% 23% 11% 1	1403
	11th to 20th	Night	38% 10% 11% 26% 13% 2	1239
	21st to end	Night	46% 8% 13% 21% 9% 1	1358
June	1st to 10th	Night	48% 9% 14% 20% 8%	1260
	11th to 20th	Night	50% 12% 13% 17% 6%	1159
	21st to end	Night	38% 12% 21% 22% 5% 1	1260
July	1st to 10th	Night	33% 10% 22% 25% 8% 1	1225
	11th to 20th	Night	36% 11% 24% 22% 7%	1218
	21st to end	Night	28% 9% 26% 30% 7%	1302
August	1st to 10th	Night	32% 12% 26% 26% 5%	1266
	11th to 20th	Night	35% 10% 21% 27% 7%	1602
	21st to end	Night	33% 8% 16% 32% 11%	1782
September	1st to 10th	Night	36% 8% 19% 24% 9% 3	1710
	11th to 20th	Night	34% 7% 18% 30% 10% 1	1910
	21st to end	Night	31% 8% 15% 31% 12% 3	1984
October	1st to 10th	Night	42% 8% 15% 23% 10% 1	2160
	11th to 20th	Night	40% 7% 15% 28% 9%	2204
	21st to end	Night	48% 8% 14% 22% 7% 1	2668
November	1st to 10th	Night	56% 8% 13% 16% 6% 1	2590
	11th to 20th	Night	59% 8% 14% 14% 5% 1	2696
	21st to end	Night	53% 9% 18% 16% 5%	2850
December	1st to 10th	Night	55% 7% 13% 17% 7% 2	2887
	11th to 20th	Night	61% 8% 14% 11% 6%	3652
	21st to end	Night	38% 7% 15% 21% 17% 3	5214

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	8% 7% 26% 35% 20% 4	180
	11th to 20th	Day	13% 10% 21% 28% 25% 2	172
	21st to end	Day	15% 12% 29% 34% 8% 2	176
February	1st to 10th	Day	8% 10% 36% 31% 12% 3	182
	11th to 20th	Day	10% 7% 38% 27% 15% 4	240
	21st to end	Day	10% 4 27% 32% 20% 7%	198
March	1st to 10th	Day	12% 7% 31% 33% 15% 2	370
	11th to 20th	Day	11% 6% 33% 29% 16% 5%	420
	21st to end	Day	14% 7% 28% 29% 19% 3	462
April	1st to 10th	Day	13% 9% 32% 33% 10% 4	420
	11th to 20th	Day	17% 8% 27% 30% 14% 4	412
	21st to end	Day	19% 8% 28% 30% 13% 2	471
May	1st to 10th	Day	14% 10% 24% 30% 16% 5%	603
	11th to 20th	Day	11% 7% 26% 32% 16% 8%	625
	21st to end	Day	17% 8% 19% 32% 18% 7%	679
June	1st to 10th	Day	16% 7% 25% 36% 13% 4	630
	11th to 20th	Day	20% 8% 26% 25% 16% 6%	630
	21st to end	Day	11% 5% 22% 30% 23% 9%	630
July	1st to 10th	Day	12% 5% 18% 31% 27% 7%	621
	11th to 20th	Day	12% 6% 23% 34% 19% 7%	618
	21st to end	Day	11% 4 22% 31% 25% 8%	684
August	1st to 10th	Day	13% 5% 17% 31% 25% 10%	622
	11th to 20th	Day	10% 7% 22% 34% 21% 6%	437
	21st to end	Day	11% 6% 21% 31% 22% 9%	495
September	1st to 10th	Day	13% 7% 35% 27% 14% 4	450
	11th to 20th	Day	21% 8% 26% 28% 14% 2	450
	21st to end	Day	14% 13% 28% 27% 15% 3	446
October	1st to 10th	Day	14% 7% 31% 27% 17% 3	270
	11th to 20th	Day	17% 6% 32% 30% 12% 3	270
	21st to end	Day	15% 12% 33% 26% 11% 3	297
November	1st to 10th	Day	18% 8% 29% 31% 13% 3	270
	11th to 20th	Day	12% 9% 40% 29% 10% 1	252
	21st to end	Day	5% 7% 46% 27% 11% 4	180
December	1st to 10th	Day	11% 6% 32% 33% 14% 4	180
	11th to 20th	Day	12% 13% 33% 28% 13% 2	180
	21st to end	Day	8% 4 31% 34% 18% 5%	198
January	1st to 10th	Night	11% 7% 31% 37% 13% 1	950
	11th to 20th	Night	11% 7% 33% 35% 13% 1	935
	21st to end	Night	16% 9% 35% 30% 9% 2	990
February	1st to 10th	Night	8% 7% 29% 36% 16% 3	724
	11th to 20th	Night	10% 4 33% 34% 16% 2	704
	21st to end	Night	7% 3 25% 40% 21% 5%	592
March	1st to 10th	Night	11% 5% 29% 39% 13% 2	720
	11th to 20th	Night	11% 5% 30% 36% 15% 2	720
	21st to end	Night	13% 7% 26% 35% 17% 3	792
April	1st to 10th	Night	9% 7% 27% 38% 15% 3	552
	11th to 20th	Night	13% 5% 31% 37% 13% 2	533
	21st to end	Night	16% 7% 23% 38% 15%	540
May	1st to 10th	Night	12% 6% 28% 35% 16% 2	503
	11th to 20th	Night	11% 5% 22% 43% 16% 2	354
	21st to end	Night	19% 4 22% 32% 22% 2	389
June	1st to 10th	Night	14% 4 26% 37% 17% 2	360
	11th to 20th	Night	23% 6% 18% 31% 21% 1	360
	21st to end	Night	10% 4 20% 37% 26% 3	360
July	1st to 10th	Night	11% 5% 16% 41% 27% 1	350
	11th to 20th	Night	11% 4 19% 46% 18% 2	348
	21st to end	Night	7% 4 19% 43% 24% 4	394
August	1st to 10th	Night	11% 1 17% 37% 29% 5%	377
	11th to 20th	Night	11% 5% 20% 39% 22% 3	534
	21st to end	Night	10% 3 22% 41% 23% 1	594
September	1st to 10th	Night	15% 4 23% 36% 20% 1	540
	11th to 20th	Night	14% 4 21% 43% 17% 1	650
	21st to end	Night	9% 7% 29% 37% 17% 1	720
October	1st to 10th	Night	13% 5% 29% 37% 13% 3	720
	11th to 20th	Night	13% 5% 32% 36% 13% 2	720
	21st to end	Night	15% 8% 34% 32% 10% 1	886
November	1st to 10th	Night	20% 6% 30% 33% 10% 2	925
	11th to 20th	Night	13% 6% 33% 32% 12% 3	950
	21st to end	Night	9% 6% 38% 35% 10% 2	950
December	1st to 10th	Night	12% 6% 34% 28% 15% 5%	950
	11th to 20th	Night	13% 7% 34% 32% 12% 1	950
	21st to end	Night	10% 5% 30% 36% 16% 4	1045

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