Virden
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Virden clear sky chart from 2005-11-06 to 2025-02-21.

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% 8% 3 9% 6% 5% 5% 4 6% 8% 36%	112187
	Night	8% 9% 3 10% 6% 5% 5% 4 6% 7% 35%	115151
Transparency	Day	46% 5% 16% 25% 8% 1	111946
	Night	47% 5% 15% 24% 9% 1	115039
Seeing	Day	14% 8% 29% 30% 15% 4	16561
	Night	14% 7% 30% 35% 13% 2	26704

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	4 13% 7% 18% 10% 8% 7% 6% 7% 8% 13%	3193
	11th to 20th	Day	4 8% 4 15% 10% 8% 9% 10% 8% 7% 16%	3616
	21st to end	Day	6% 12% 7% 15% 7% 7% 5% 3 5% 6% 29%	4004
February	1st to 10th	Day	9% 13% 4 14% 7% 7% 5% 4 8% 7% 23%	1600
	11th to 20th	Day	6% 12% 4 11% 7% 7% 7% 5% 7% 8% 27%	1781
	21st to end	Day	6% 13% 4 12% 7% 6% 6% 8% 6% 7% 25%	1570
March	1st to 10th	Day	9% 14% 6% 11% 8% 7% 6% 4 7% 8% 21%	3130
	11th to 20th	Day	8% 10% 6% 13% 9% 7% 5% 4 6% 7% 26%	3340
	21st to end	Day	8% 10% 5% 14% 8% 6% 5% 5% 7% 8% 25%	4022
April	1st to 10th	Day	8% 11% 3 8% 6% 4 5% 4 6% 8% 38%	2280
	11th to 20th	Day	14% 13% 4 9% 5% 5% 5% 4 5% 8% 29%	2408
	21st to end	Day	12% 10% 3 8% 6% 5% 4 4 5% 8% 35%	2470
May	1st to 10th	Day	20% 8% 2 6% 5% 4 3 3 5% 7% 36%	4354
	11th to 20th	Day	17% 9% 3 7% 5% 5% 5% 4 6% 7% 32%	4510
	21st to end	Day	17% 8% 2 6% 4 4 5% 4 5% 8% 37%	5173
June	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 4 7% 9% 39%	2850
	11th to 20th	Day	16% 6% 3 7% 5% 4 5% 5% 7% 8% 34%	2850
	21st to end	Day	12% 5% 2 6% 5% 5% 5% 4 7% 9% 39%	2850
July	1st to 10th	Day	7% 3 2 5% 5% 5% 6% 5% 8% 10% 44%	4750
	11th to 20th	Day	5% 4 1 5% 4 4 4 4 6% 10% 52%	4664
	21st to end	Day	4 3 1 4 3 4 5% 4 7% 11% 52%	4802
August	1st to 10th	Day	9% 5% 2 6% 3 4 4 4 6% 9% 46%	4412
	11th to 20th	Day	9% 4 2 5% 3 4 4 4 6% 10% 48%	4158
	21st to end	Day	10% 5% 2 5% 4 4 4 4 6% 8% 48%	4478
September	1st to 10th	Day	10% 5% 2 5% 4 4 4 4 4 7% 52%	2280
	11th to 20th	Day	13% 6% 2 6% 5% 4 4 3 5% 7% 45%	2134
	21st to end	Day	14% 7% 2 6% 4 4 3 3 5% 6% 47%	2090
October	1st to 10th	Day	10% 7% 3 7% 4 5% 4 3 4 7% 46%	3396
	11th to 20th	Day	8% 6% 2 6% 4 4 4 4 5% 9% 48%	2832
	21st to end	Day	9% 10% 4 9% 5% 4 4 3 4 7% 39%	3091
November	1st to 10th	Day	17% 12% 5% 10% 6% 4 5% 3 4 6% 28%	1763
	11th to 20th	Day	9% 16% 5% 12% 7% 5% 4 4 6% 5% 27%	1509
	21st to end	Day	8% 15% 6% 11% 7% 6% 5% 4 5% 6% 27%	1400
December	1st to 10th	Day	6% 13% 6% 19% 9% 10% 7% 4 4 5% 17%	2290
	11th to 20th	Day	9% 12% 6% 13% 10% 8% 6% 4 5% 7% 19%	2639
	21st to end	Day	4 10% 3 13% 9% 7% 6% 4 5% 6% 33%	3498
January	1st to 10th	Night	5% 11% 5% 18% 10% 8% 7% 4 6% 7% 19%	6769
	11th to 20th	Night	5% 8% 4 14% 9% 7% 9% 8% 10% 9% 18%	6302
	21st to end	Night	6% 12% 5% 17% 7% 7% 5% 4 6% 6% 25%	6974
February	1st to 10th	Night	8% 13% 6% 11% 5% 6% 5% 5% 7% 8% 26%	2800
	11th to 20th	Night	5% 10% 3 9% 7% 6% 7% 5% 7% 7% 35%	2523
	21st to end	Night	6% 11% 4 10% 7% 5% 5% 5% 6% 7% 34%	1884
March	1st to 10th	Night	10% 13% 4 9% 6% 6% 6% 5% 7% 9% 27%	3734
	11th to 20th	Night	7% 9% 3 8% 5% 4 4 4 6% 8% 41%	3622
	21st to end	Night	7% 9% 5% 13% 8% 5% 5% 4 6% 7% 31%	3615
April	1st to 10th	Night	4 9% 3 7% 5% 4 4 5% 8% 8% 43%	1900
	11th to 20th	Night	14% 12% 4 9% 5% 5% 4 4 5% 7% 32%	1722
	21st to end	Night	15% 8% 2 6% 4 4 6% 5% 7% 6% 37%	1710
May	1st to 10th	Night	16% 9% 2 7% 4 5% 5% 4 6% 7% 36%	2613
	11th to 20th	Night	13% 8% 2 7% 5% 4 4 4 5% 8% 39%	2396
	21st to end	Night	15% 7% 3 6% 4 4 4 4 5% 7% 41%	2414
June	1st to 10th	Night	14% 5% 2 4 3 5% 4 4 7% 8% 44%	1330
	11th to 20th	Night	14% 6% 2 6% 4 4 5% 4 6% 10% 41%	1330
	21st to end	Night	11% 4 2 5% 4 4 4 5% 7% 11% 42%	1330
July	1st to 10th	Night	6% 5% 2 5% 4 4 5% 5% 8% 10% 47%	2220
	11th to 20th	Night	6% 3 1 5% 2 3 4 3 6% 8% 59%	2303
	21st to end	Night	4 4 1 4 3 3 4 4 7% 9% 56%	2762
August	1st to 10th	Night	6% 4 1 5% 3 4 4 4 6% 8% 54%	2528
	11th to 20th	Night	5% 4 2 6% 5% 4 4 4 6% 9% 52%	2681
	21st to end	Night	8% 5% 2 5% 3 4 4 3 5% 9% 52%	3187
September	1st to 10th	Night	9% 4 1 4 3 4 4 4 6% 7% 55%	1900
	11th to 20th	Night	11% 6% 2 5% 3 3 4 4 6% 8% 48%	2045
	21st to end	Night	12% 8% 3 6% 4 4 3 4 6% 7% 44%	2090
October	1st to 10th	Night	11% 8% 2 6% 4 3 3 3 5% 7% 49%	3530
	11th to 20th	Night	8% 6% 2 6% 5% 5% 5% 5% 7% 8% 44%	4097
	21st to end	Night	7% 8% 3 8% 5% 4 5% 3 6% 7% 44%	4467
November	1st to 10th	Night	13% 11% 3 8% 5% 5% 4 4 6% 7% 32%	2535
	11th to 20th	Night	10% 13% 4 11% 7% 5% 5% 4 5% 6% 30%	2890
	21st to end	Night	9% 12% 4 9% 7% 5% 5% 4 7% 7% 32%	3000
December	1st to 10th	Night	7% 13% 4 13% 10% 8% 8% 6% 6% 7% 18%	4900
	11th to 20th	Night	9% 14% 5% 12% 8% 6% 7% 5% 5% 7% 22%	5568
	21st to end	Night	5% 8% 4 14% 8% 7% 7% 5% 6% 7% 29%	7480

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	66% 5% 9% 15% 5%	3193
	11th to 20th	Day	62% 5% 10% 16% 7%	3640
	21st to end	Day	57% 3 16% 19% 4	4004
February	1st to 10th	Day	59% 4 12% 18% 7% 1	1600
	11th to 20th	Day	53% 5% 12% 20% 9%	1790
	21st to end	Day	54% 7% 12% 18% 8%	1570
March	1st to 10th	Day	58% 6% 14% 17% 4	3130
	11th to 20th	Day	56% 7% 15% 17% 5%	3342
	21st to end	Day	55% 6% 13% 18% 8%	4022
April	1st to 10th	Day	42% 5% 16% 25% 11% 1	2280
	11th to 20th	Day	53% 5% 11% 19% 11% 1	2408
	21st to end	Day	47% 5% 16% 23% 8% 1	2470
May	1st to 10th	Day	46% 5% 15% 24% 9% 1	4354
	11th to 20th	Day	49% 6% 11% 21% 13% 1	4514
	21st to end	Day	43% 6% 13% 29% 8%	5173
June	1st to 10th	Day	40% 6% 19% 23% 10% 2	2850
	11th to 20th	Day	46% 6% 16% 21% 10% 1	2850
	21st to end	Day	41% 6% 19% 26% 7% 1	2850
July	1st to 10th	Day	33% 5% 28% 28% 5%	4676
	11th to 20th	Day	28% 4 19% 41% 8% 1	4583
	21st to end	Day	25% 4 24% 37% 10% 1	4651
August	1st to 10th	Day	34% 3 18% 33% 11%	4396
	11th to 20th	Day	31% 4 22% 35% 8%	4200
	21st to end	Day	34% 3 18% 36% 9%	4477
September	1st to 10th	Day	33% 4 16% 33% 13% 1	2280
	11th to 20th	Day	39% 4 18% 27% 10% 1	2134
	21st to end	Day	38% 4 13% 32% 13%	2090
October	1st to 10th	Day	40% 2 14% 35% 9%	3396
	11th to 20th	Day	33% 3 16% 37% 10%	2832
	21st to end	Day	46% 3 11% 30% 9%	3091
November	1st to 10th	Day	58% 4 12% 18% 7% 1	1763
	11th to 20th	Day	58% 3 16% 15% 6% 1	1509
	21st to end	Day	58% 5% 15% 17% 5%	1400
December	1st to 10th	Day	70% 4 13% 10% 3	2290
	11th to 20th	Day	63% 4 14% 16% 2	2640
	21st to end	Day	52% 4 9% 23% 11% 1	3498
January	1st to 10th	Night	64% 4 9% 19% 4	6769
	11th to 20th	Night	57% 5% 14% 17% 7%	6340
	21st to end	Night	60% 5% 15% 17% 4	6974
February	1st to 10th	Night	54% 4 14% 18% 10% 1	2800
	11th to 20th	Night	46% 4 13% 23% 13%	2524
	21st to end	Night	48% 4 14% 23% 11%	1884
March	1st to 10th	Night	50% 7% 16% 21% 5%	3734
	11th to 20th	Night	40% 5% 20% 26% 9%	3622
	21st to end	Night	50% 5% 13% 25% 7%	3615
April	1st to 10th	Night	35% 6% 13% 28% 17% 2	1900
	11th to 20th	Night	52% 3 10% 22% 13% 1	1722
	21st to end	Night	45% 5% 15% 22% 12% 1	1710
May	1st to 10th	Night	46% 6% 11% 23% 14%	2613
	11th to 20th	Night	42% 6% 13% 25% 11% 4	2396
	21st to end	Night	42% 5% 14% 30% 9%	2414
June	1st to 10th	Night	35% 6% 17% 25% 14% 3	1330
	11th to 20th	Night	39% 7% 18% 22% 13% 2	1330
	21st to end	Night	34% 8% 19% 31% 7% 1	1330
July	1st to 10th	Night	31% 8% 26% 26% 9% 1	2185
	11th to 20th	Night	25% 4 19% 42% 8% 1	2261
	21st to end	Night	25% 4 22% 33% 15% 2	2674
August	1st to 10th	Night	28% 4 20% 34% 14% 1	2520
	11th to 20th	Night	29% 4 21% 37% 9% 1	2705
	21st to end	Night	30% 3 19% 35% 11% 1	3187
September	1st to 10th	Night	27% 5% 14% 35% 18% 2	1900
	11th to 20th	Night	33% 5% 18% 29% 14% 1	2045
	21st to end	Night	39% 4 12% 31% 12% 2	2090
October	1st to 10th	Night	36% 3 13% 32% 13% 4	3530
	11th to 20th	Night	37% 3 11% 36% 11% 1	4097
	21st to end	Night	40% 4 14% 33% 10%	4467
November	1st to 10th	Night	50% 5% 15% 21% 8% 2	2535
	11th to 20th	Night	54% 6% 15% 18% 6% 1	2890
	21st to end	Night	51% 6% 17% 19% 6%	3000
December	1st to 10th	Night	64% 4 15% 12% 4	4900
	11th to 20th	Night	62% 4 14% 16% 4	5566
	21st to end	Night	54% 4 10% 20% 11% 1	7480

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	11% 8% 30% 31% 16% 4	300
	11th to 20th	Day	9% 10% 32% 38% 10% 2	300
	21st to end	Day	16% 8% 30% 32% 12% 2	330
February	1st to 10th	Day	14% 10% 36% 31% 8% 1	300
	11th to 20th	Day	10% 6% 33% 32% 15% 4	300
	21st to end	Day	5% 6% 31% 38% 18% 3	225
March	1st to 10th	Day	13% 10% 34% 26% 14% 3	270
	11th to 20th	Day	11% 10% 33% 28% 16% 2	270
	21st to end	Day	17% 9% 29% 28% 13% 3	415
April	1st to 10th	Day	13% 12% 35% 30% 8% 2	460
	11th to 20th	Day	21% 11% 27% 27% 12%	606
	21st to end	Day	16% 9% 31% 29% 14% 1	650
May	1st to 10th	Day	17% 11% 26% 31% 14% 2	653
	11th to 20th	Day	17% 10% 28% 25% 16% 3	675
	21st to end	Day	21% 8% 24% 28% 15% 3	734
June	1st to 10th	Day	14% 10% 28% 28% 16% 4	680
	11th to 20th	Day	18% 9% 33% 26% 11% 3	680
	21st to end	Day	14% 8% 23% 33% 19% 4	680
July	1st to 10th	Day	9% 5% 21% 36% 20% 8%	671
	11th to 20th	Day	11% 6% 29% 29% 18% 6%	668
	21st to end	Day	6% 6% 24% 35% 22% 8%	739
August	1st to 10th	Day	11% 6% 25% 32% 20% 7%	676
	11th to 20th	Day	12% 5% 23% 28% 26% 7%	665
	21st to end	Day	12% 6% 25% 34% 18% 4	720
September	1st to 10th	Day	11% 9% 36% 25% 15% 4	490
	11th to 20th	Day	15% 12% 26% 33% 12% 2	344
	21st to end	Day	19% 11% 30% 24% 15% 1	300
October	1st to 10th	Day	23% 9% 23% 26% 15% 4	300
	11th to 20th	Day	17% 12% 32% 28% 9% 4	300
	21st to end	Day	19% 9% 39% 24% 6% 3	330
November	1st to 10th	Day	23% 10% 27% 26% 12% 1	300
	11th to 20th	Day	19% 11% 36% 24% 10% 1	300
	21st to end	Day	15% 13% 33% 31% 7% 2	300
December	1st to 10th	Day	12% 9% 34% 29% 13% 3	300
	11th to 20th	Day	18% 6% 40% 28% 7% 1	300
	21st to end	Day	12% 6% 30% 31% 17% 3	330
January	1st to 10th	Night	13% 8% 31% 36% 11% 1	1000
	11th to 20th	Night	15% 9% 32% 34% 9% 1	1000
	21st to end	Night	12% 10% 36% 32% 11% 1	1100
February	1st to 10th	Night	13% 9% 32% 32% 13% 2	1000
	11th to 20th	Night	10% 7% 29% 40% 12% 3	905
	21st to end	Night	11% 5% 28% 37% 17% 3	628
March	1st to 10th	Night	17% 7% 27% 32% 14% 4	760
	11th to 20th	Night	10% 6% 34% 37% 12% 2	660
	21st to end	Night	16% 4 27% 38% 13% 3	627
April	1st to 10th	Night	8% 7% 35% 36% 13%	570
	11th to 20th	Night	18% 7% 31% 33% 10% 1	563
	21st to end	Night	18% 7% 29% 34% 12%	570
May	1st to 10th	Night	15% 7% 29% 36% 12% 1	570
	11th to 20th	Night	16% 5% 29% 36% 14%	561
	21st to end	Night	23% 5% 24% 34% 14%	617
June	1st to 10th	Night	17% 5% 32% 31% 15% 1	570
	11th to 20th	Night	16% 6% 33% 32% 12%	570
	21st to end	Night	14% 3 26% 39% 17% 1	570
July	1st to 10th	Night	8% 3 22% 42% 23% 2	555
	11th to 20th	Night	8% 4 26% 43% 17% 2	552
	21st to end	Night	6% 2 22% 49% 18% 3	624
August	1st to 10th	Night	7% 4 22% 46% 20% 1	567
	11th to 20th	Night	9% 4 19% 41% 25% 3	564
	21st to end	Night	12% 4 25% 41% 16% 2	627
September	1st to 10th	Night	11% 4 26% 36% 20% 3	570
	11th to 20th	Night	14% 7% 31% 35% 12% 1	570
	21st to end	Night	16% 8% 29% 34% 12% 1	570
October	1st to 10th	Night	18% 8% 31% 29% 13% 1	594
	11th to 20th	Night	17% 9% 26% 35% 12% 1	950
	21st to end	Night	15% 10% 34% 32% 9% 1	1045
November	1st to 10th	Night	19% 10% 32% 30% 9% 1	975
	11th to 20th	Night	16% 9% 36% 29% 9% 2	1000
	21st to end	Night	14% 13% 33% 32% 8% 1	1000
December	1st to 10th	Night	11% 8% 33% 32% 13% 3	1000
	11th to 20th	Night	17% 9% 30% 33% 9% 2	1000
	21st to end	Night	14% 6% 30% 34% 13% 3	1100

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