Rosthern
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rosthern clear sky chart from 2005-11-06 to 2025-02-25.

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	9% 8% 3 8% 6% 5% 5% 4 6% 8% 37%	111544
	Night	7% 9% 4 10% 6% 6% 5% 4 6% 8% 35%	115748
Transparency	Day	43% 8% 18% 23% 8%	111284
	Night	45% 7% 17% 23% 8% 1	115654
Seeing	Day	13% 7% 25% 30% 19% 6%	16102
	Night	12% 5% 28% 36% 16% 2	25589

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	6% 14% 5% 15% 8% 8% 8% 5% 6% 5% 20%	3013
	11th to 20th	Day	4 7% 4 12% 10% 9% 7% 5% 7% 7% 30%	3159
	21st to end	Day	7% 12% 4 13% 7% 6% 5% 4 6% 8% 29%	3724
February	1st to 10th	Day	8% 10% 4 11% 9% 7% 6% 6% 8% 8% 24%	1650
	11th to 20th	Day	6% 11% 5% 12% 6% 6% 6% 4 7% 9% 29%	1755
	21st to end	Day	4 9% 4 13% 8% 8% 8% 6% 7% 8% 25%	1479
March	1st to 10th	Day	6% 13% 5% 11% 8% 6% 8% 5% 7% 7% 25%	3130
	11th to 20th	Day	6% 12% 4 9% 7% 6% 4 4 7% 8% 33%	3422
	21st to end	Day	6% 9% 3 7% 6% 6% 6% 4 7% 7% 38%	4036
April	1st to 10th	Day	7% 10% 4 8% 6% 5% 4 5% 7% 8% 36%	2280
	11th to 20th	Day	12% 12% 3 8% 6% 5% 5% 4 6% 8% 31%	2352
	21st to end	Day	12% 9% 3 8% 6% 5% 5% 4 6% 9% 33%	2660
May	1st to 10th	Day	17% 10% 3 7% 4 4 4 4 5% 7% 34%	4430
	11th to 20th	Day	9% 8% 3 9% 6% 6% 6% 4 6% 8% 36%	4388
	21st to end	Day	14% 10% 3 8% 5% 4 5% 4 6% 8% 32%	5348
June	1st to 10th	Day	13% 7% 3 7% 5% 5% 5% 5% 7% 8% 34%	3040
	11th to 20th	Day	21% 8% 3 8% 5% 6% 4 4 7% 6% 26%	3040
	21st to end	Day	13% 5% 2 6% 5% 5% 6% 4 7% 9% 37%	3040
July	1st to 10th	Day	9% 4 1 5% 4 5% 4 4 7% 10% 47%	5060
	11th to 20th	Day	7% 3 1 5% 3 4 5% 4 7% 11% 49%	4744
	21st to end	Day	5% 3 1 4 4 4 5% 4 7% 11% 51%	5052
August	1st to 10th	Day	12% 4 2 6% 4 5% 5% 4 6% 8% 45%	4412
	11th to 20th	Day	8% 4 2 6% 4 4 4 4 6% 9% 48%	4318
	21st to end	Day	12% 6% 2 5% 4 4 3 3 5% 8% 49%	4411
September	1st to 10th	Day	10% 5% 2 7% 5% 4 4 3 5% 7% 47%	2280
	11th to 20th	Day	12% 7% 3 8% 5% 5% 5% 4 6% 8% 38%	2280
	21st to end	Day	10% 6% 1 5% 4 4 4 4 5% 8% 49%	2024
October	1st to 10th	Day	10% 6% 2 6% 5% 4 4 3 6% 8% 47%	3170
	11th to 20th	Day	6% 7% 2 6% 5% 5% 5% 5% 8% 12% 38%	3058
	21st to end	Day	7% 8% 4 10% 6% 6% 7% 5% 6% 6% 36%	2988
November	1st to 10th	Day	15% 15% 4 9% 7% 4 5% 3 5% 6% 26%	1568
	11th to 20th	Day	9% 14% 6% 13% 7% 6% 5% 5% 5% 6% 24%	1589
	21st to end	Day	7% 15% 6% 12% 7% 7% 4 4 6% 8% 24%	1400
December	1st to 10th	Day	6% 13% 4 11% 8% 7% 8% 4 7% 8% 24%	1993
	11th to 20th	Day	5% 13% 7% 16% 10% 8% 6% 3 5% 6% 22%	2259
	21st to end	Day	3 9% 4 14% 7% 5% 5% 4 5% 6% 37%	2992
January	1st to 10th	Night	4 10% 6% 15% 11% 8% 6% 4 8% 8% 20%	6930
	11th to 20th	Night	3 9% 4 14% 10% 10% 7% 4 6% 7% 28%	6739
	21st to end	Night	5% 10% 4 11% 7% 6% 6% 4 7% 9% 30%	7241
February	1st to 10th	Night	6% 11% 5% 13% 8% 6% 6% 5% 7% 7% 26%	2768
	11th to 20th	Night	4 11% 5% 10% 6% 6% 7% 5% 7% 6% 34%	2532
	21st to end	Night	3 9% 4 10% 7% 5% 6% 5% 8% 8% 35%	2069
March	1st to 10th	Night	6% 12% 5% 11% 7% 6% 8% 5% 7% 8% 26%	3734
	11th to 20th	Night	8% 10% 3 8% 5% 6% 6% 4 7% 9% 32%	3552
	21st to end	Night	7% 7% 3 7% 5% 5% 6% 4 8% 11% 37%	3610
April	1st to 10th	Night	5% 10% 3 7% 6% 5% 5% 4 7% 8% 40%	1900
	11th to 20th	Night	10% 12% 3 9% 5% 5% 6% 4 7% 7% 32%	1776
	21st to end	Night	13% 7% 3 10% 6% 4 5% 4 5% 7% 36%	1520
May	1st to 10th	Night	17% 10% 3 7% 6% 5% 6% 4 4 7% 32%	2540
	11th to 20th	Night	11% 7% 3 7% 6% 4 5% 4 7% 8% 38%	2518
	21st to end	Night	15% 7% 3 7% 6% 5% 6% 5% 6% 8% 31%	2211
June	1st to 10th	Night	12% 6% 2 6% 5% 4 4 4 6% 9% 40%	1140
	11th to 20th	Night	18% 7% 3 7% 5% 5% 5% 3 7% 7% 33%	1140
	21st to end	Night	9% 6% 2 5% 5% 4 5% 5% 7% 7% 45%	1140
July	1st to 10th	Night	8% 4 2 6% 3 5% 5% 6% 7% 11% 43%	1900
	11th to 20th	Night	7% 3 1 4 4 4 4 4 5% 8% 56%	2212
	21st to end	Night	3 3 1 3 3 3 4 3 6% 8% 63%	2502
August	1st to 10th	Night	7% 3 2 5% 4 4 5% 4 6% 7% 53%	2528
	11th to 20th	Night	7% 3 1 5% 4 5% 6% 5% 7% 10% 47%	2518
	21st to end	Night	10% 4 2 5% 4 3 4 3 5% 8% 52%	3256
September	1st to 10th	Night	9% 4 2 6% 4 3 4 4 6% 7% 50%	1900
	11th to 20th	Night	13% 5% 3 6% 4 4 4 3 5% 9% 44%	1900
	21st to end	Night	8% 6% 2 5% 3 4 5% 3 6% 9% 49%	2154
October	1st to 10th	Night	10% 7% 3 6% 3 4 4 3 6% 8% 48%	3800
	11th to 20th	Night	6% 6% 3 7% 5% 5% 5% 3 5% 6% 49%	3838
	21st to end	Night	8% 9% 4 8% 5% 5% 5% 4 6% 8% 40%	4557
November	1st to 10th	Night	15% 15% 4 8% 6% 5% 4 4 5% 6% 27%	2730
	11th to 20th	Night	8% 12% 6% 13% 7% 7% 5% 4 6% 6% 25%	2810
	21st to end	Night	5% 13% 6% 12% 7% 6% 4 4 6% 8% 28%	3000
December	1st to 10th	Night	7% 14% 6% 13% 9% 7% 6% 4 5% 7% 22%	5186
	11th to 20th	Night	5% 12% 6% 15% 7% 7% 7% 5% 6% 8% 23%	5933
	21st to end	Night	3 8% 4 14% 8% 5% 5% 3 5% 6% 40%	7964

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% 8% 8% 16% 6%	3013
	11th to 20th	Day	49% 8% 15% 18% 10%	3180
	21st to end	Day	53% 8% 20% 15% 4	3724
February	1st to 10th	Day	54% 9% 14% 15% 9%	1650
	11th to 20th	Day	50% 6% 15% 18% 11% 1	1764
	21st to end	Day	52% 10% 13% 17% 7%	1479
March	1st to 10th	Day	55% 9% 16% 12% 8%	3130
	11th to 20th	Day	46% 12% 18% 18% 5%	3423
	21st to end	Day	40% 10% 13% 21% 16%	4036
April	1st to 10th	Day	43% 9% 16% 21% 11%	2280
	11th to 20th	Day	50% 7% 14% 19% 9% 1	2352
	21st to end	Day	46% 8% 17% 21% 7% 1	2660
May	1st to 10th	Day	47% 7% 15% 21% 9%	4430
	11th to 20th	Day	39% 12% 14% 24% 10% 1	4392
	21st to end	Day	45% 13% 15% 18% 10%	5348
June	1st to 10th	Day	44% 9% 18% 20% 8% 1	3040
	11th to 20th	Day	54% 8% 14% 16% 7% 1	3040
	21st to end	Day	41% 10% 20% 22% 8%	3040
July	1st to 10th	Day	31% 6% 29% 28% 5%	4981
	11th to 20th	Day	27% 8% 18% 41% 6%	4662
	21st to end	Day	26% 7% 22% 38% 7%	4892
August	1st to 10th	Day	36% 7% 22% 28% 6%	4396
	11th to 20th	Day	33% 6% 23% 33% 5%	4360
	21st to end	Day	34% 4 22% 33% 6%	4410
September	1st to 10th	Day	35% 7% 18% 26% 12% 2	2280
	11th to 20th	Day	43% 8% 18% 23% 7% 1	2280
	21st to end	Day	32% 7% 17% 31% 11% 2	2024
October	1st to 10th	Day	36% 4 24% 26% 9%	3170
	11th to 20th	Day	38% 4 22% 30% 6%	3058
	21st to end	Day	47% 6% 20% 24% 3	2988
November	1st to 10th	Day	58% 7% 13% 15% 6%	1568
	11th to 20th	Day	61% 9% 14% 13% 3	1589
	21st to end	Day	57% 9% 15% 15% 4	1400
December	1st to 10th	Day	58% 5% 17% 13% 6%	1993
	11th to 20th	Day	64% 5% 16% 10% 4	2260
	21st to end	Day	47% 5% 13% 20% 14% 2	2992
January	1st to 10th	Night	57% 7% 13% 18% 5%	6930
	11th to 20th	Night	55% 5% 14% 15% 10%	6780
	21st to end	Night	48% 9% 22% 17% 4	7241
February	1st to 10th	Night	53% 8% 12% 18% 9%	2768
	11th to 20th	Night	46% 8% 14% 22% 10%	2533
	21st to end	Night	41% 8% 18% 24% 9%	2069
March	1st to 10th	Night	51% 10% 17% 18% 4	3734
	11th to 20th	Night	44% 9% 21% 18% 7%	3552
	21st to end	Night	37% 8% 13% 26% 15%	3610
April	1st to 10th	Night	38% 8% 16% 23% 14% 1	1900
	11th to 20th	Night	48% 7% 14% 18% 12% 1	1776
	21st to end	Night	45% 7% 14% 22% 11% 1	1520
May	1st to 10th	Night	48% 10% 12% 21% 8% 1	2540
	11th to 20th	Night	38% 12% 12% 21% 16% 1	2518
	21st to end	Night	41% 15% 15% 21% 8% 1	2211
June	1st to 10th	Night	37% 9% 16% 26% 10% 2	1140
	11th to 20th	Night	48% 9% 15% 17% 11% 1	1140
	21st to end	Night	33% 10% 21% 25% 11%	1140
July	1st to 10th	Night	32% 8% 27% 29% 5%	1870
	11th to 20th	Night	26% 6% 16% 43% 11%	2171
	21st to end	Night	19% 6% 22% 42% 10% 1	2423
August	1st to 10th	Night	30% 6% 22% 32% 9%	2520
	11th to 20th	Night	30% 6% 24% 30% 10%	2542
	21st to end	Night	31% 4 21% 34% 10%	3256
September	1st to 10th	Night	30% 9% 17% 28% 12% 4	1900
	11th to 20th	Night	37% 8% 15% 29% 10% 1	1900
	21st to end	Night	31% 6% 15% 30% 15% 2	2154
October	1st to 10th	Night	35% 4 20% 34% 7%	3800
	11th to 20th	Night	36% 4 14% 32% 13%	3838
	21st to end	Night	42% 5% 21% 26% 6%	4557
November	1st to 10th	Night	57% 8% 13% 16% 5% 1	2730
	11th to 20th	Night	57% 10% 15% 14% 4	2810
	21st to end	Night	53% 9% 17% 17% 4	3000
December	1st to 10th	Night	62% 6% 14% 13% 4	5186
	11th to 20th	Night	58% 6% 19% 13% 4	5931
	21st to end	Night	45% 4 11% 24% 13% 2	7964

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	15% 7% 22% 32% 19% 7%	200
	11th to 20th	Day	10% 7% 30% 34% 18% 2	200
	21st to end	Day	13% 7% 29% 31% 18% 2	268
February	1st to 10th	Day	11% 6% 31% 31% 19% 2	300
	11th to 20th	Day	10% 2 25% 35% 21% 7%	300
	21st to end	Day	6% 7% 25% 38% 19% 5%	237
March	1st to 10th	Day	13% 7% 24% 36% 17% 3	270
	11th to 20th	Day	13% 8% 27% 33% 17% 3	419
	21st to end	Day	14% 6% 25% 34% 19% 3	506
April	1st to 10th	Day	10% 9% 35% 27% 13% 6%	460
	11th to 20th	Day	14% 13% 26% 31% 14% 3	461
	21st to end	Day	13% 10% 28% 31% 15% 3	650
May	1st to 10th	Day	12% 8% 23% 34% 18% 6%	653
	11th to 20th	Day	12% 9% 21% 32% 19% 7%	675
	21st to end	Day	17% 9% 22% 25% 19% 8%	734
June	1st to 10th	Day	14% 9% 30% 29% 15% 4	680
	11th to 20th	Day	23% 10% 25% 22% 15% 6%	680
	21st to end	Day	13% 7% 23% 26% 22% 10%	680
July	1st to 10th	Day	10% 4 19% 31% 26% 9%	671
	11th to 20th	Day	10% 6% 20% 30% 25% 9%	668
	21st to end	Day	6% 6% 21% 29% 27% 10%	739
August	1st to 10th	Day	10% 5% 19% 32% 24% 9%	676
	11th to 20th	Day	9% 4 20% 34% 26% 7%	653
	21st to end	Day	15% 7% 23% 29% 19% 6%	539
September	1st to 10th	Day	13% 10% 28% 29% 17% 4	490
	11th to 20th	Day	15% 5% 29% 32% 16% 3	490
	21st to end	Day	13% 8% 27% 30% 20% 2	343
October	1st to 10th	Day	17% 4 26% 28% 19% 5%	300
	11th to 20th	Day	12% 11% 29% 28% 16% 4	300
	21st to end	Day	13% 9% 36% 24% 13% 4	330
November	1st to 10th	Day	20% 5% 32% 29% 12% 2	300
	11th to 20th	Day	14% 9% 36% 30% 10% 1	300
	21st to end	Day	13% 5% 33% 35% 12% 2	300
December	1st to 10th	Day	9% 6% 28% 32% 19% 7%	210
	11th to 20th	Day	10% 7% 33% 34% 17%	200
	21st to end	Day	10% 3 25% 35% 18% 9%	220
January	1st to 10th	Night	12% 5% 31% 36% 15% 2	1000
	11th to 20th	Night	11% 7% 33% 37% 11% 1	1000
	21st to end	Night	12% 7% 35% 35% 9% 1	1100
February	1st to 10th	Night	11% 4 27% 37% 18% 2	968
	11th to 20th	Night	8% 6% 26% 39% 17% 3	784
	21st to end	Night	6% 3 24% 39% 25% 4	646
March	1st to 10th	Night	12% 4 28% 36% 17% 3	760
	11th to 20th	Night	11% 7% 30% 37% 14% 1	760
	21st to end	Night	13% 3 25% 38% 18% 3	708
April	1st to 10th	Night	8% 7% 31% 34% 17% 3	570
	11th to 20th	Night	15% 8% 29% 34% 13% 1	563
	21st to end	Night	17% 7% 28% 34% 13% 1	570
May	1st to 10th	Night	15% 5% 28% 36% 15% 1	570
	11th to 20th	Night	13% 7% 28% 34% 18% 1	561
	21st to end	Night	17% 6% 23% 32% 19% 3	433
June	1st to 10th	Night	15% 5% 27% 35% 17% 2	380
	11th to 20th	Night	22% 6% 22% 27% 21% 2	380
	21st to end	Night	12% 4 19% 41% 22% 2	380
July	1st to 10th	Night	7% 4 19% 41% 28% 1	370
	11th to 20th	Night	11% 2 20% 45% 19% 2	368
	21st to end	Night	4 3 24% 34% 31% 3	468
August	1st to 10th	Night	6% 3 21% 39% 27% 4	567
	11th to 20th	Night	8% 2 16% 46% 24% 3	564
	21st to end	Night	11% 4 25% 36% 23% 2	627
September	1st to 10th	Night	11% 4 27% 39% 18% 2	570
	11th to 20th	Night	17% 5% 20% 41% 17% 1	570
	21st to end	Night	10% 5% 34% 39% 12% 1	677
October	1st to 10th	Night	17% 6% 25% 38% 13% 1	760
	11th to 20th	Night	12% 5% 31% 36% 14% 2	795
	21st to end	Night	16% 7% 33% 31% 12% 1	1045
November	1st to 10th	Night	21% 5% 29% 35% 8% 1	975
	11th to 20th	Night	14% 6% 32% 34% 12% 3	1000
	21st to end	Night	12% 6% 35% 35% 10% 3	1000
December	1st to 10th	Night	10% 5% 30% 37% 16% 3	1000
	11th to 20th	Night	13% 6% 32% 33% 14% 2	1000
	21st to end	Night	10% 2 26% 36% 21% 5%	1100

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