Thurso
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Thurso clear sky chart from 2005-11-06 to 2025-07-21.

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	17% 10% 4 8% 5% 5% 4 4 5% 6% 31%	120356
	Night	17% 12% 4 9% 5% 5% 4 3 5% 6% 30%	118739
Transparency	Day	51% 7% 12% 20% 10% 1	123853
	Night	53% 6% 12% 18% 10% 1	123403
Seeing	Day	22% 15% 30% 21% 10% 3	15336
	Night	22% 10% 29% 28% 10% 1	24377

By Month

Percentage Cloud Cover NE (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	14% 19% 6% 16% 7% 6% 4 2 4 4 19%	3597
	11th to 20th	Day	12% 15% 7% 14% 6% 5% 4 4 6% 5% 22%	3570
	21st to end	Day	11% 18% 7% 12% 6% 6% 5% 3 5% 5% 21%	4034
February	1st to 10th	Day	12% 16% 5% 12% 7% 5% 3 4 5% 5% 25%	1800
	11th to 20th	Day	13% 16% 5% 12% 7% 4 4 4 4 5% 27%	1781
	21st to end	Day	19% 13% 5% 10% 6% 5% 4 4 6% 4 25%	1650
March	1st to 10th	Day	20% 13% 5% 11% 6% 5% 4 3 4 5% 25%	4476
	11th to 20th	Day	13% 12% 4 9% 5% 5% 5% 4 6% 7% 30%	4904
	21st to end	Day	19% 10% 3 8% 5% 4 4 3 4 5% 35%	5826
April	1st to 10th	Day	21% 12% 4 7% 4 4 4 2 4 5% 32%	2400
	11th to 20th	Day	21% 10% 3 7% 4 4 4 3 3 6% 35%	2374
	21st to end	Day	22% 10% 3 8% 5% 5% 4 4 4 5% 31%	2547
May	1st to 10th	Day	23% 10% 4 9% 5% 5% 5% 4 5% 6% 25%	6346
	11th to 20th	Day	19% 8% 3 9% 5% 5% 4 3 5% 7% 31%	6318
	21st to end	Day	22% 9% 3 7% 4 4 4 3 5% 7% 32%	6949
June	1st to 10th	Day	20% 9% 3 7% 6% 4 5% 4 6% 6% 30%	2800
	11th to 20th	Day	14% 6% 3 6% 4 6% 5% 5% 6% 8% 38%	2800
	21st to end	Day	20% 6% 2 7% 4 5% 6% 5% 7% 7% 31%	2800
July	1st to 10th	Day	12% 7% 3 7% 6% 5% 5% 5% 7% 9% 34%	4580
	11th to 20th	Day	13% 6% 2 7% 5% 5% 5% 4 7% 8% 38%	4580
	21st to end	Day	11% 6% 2 5% 4 4 5% 4 6% 8% 46%	4814
August	1st to 10th	Day	14% 6% 3 8% 5% 6% 5% 4 6% 8% 35%	4423
	11th to 20th	Day	15% 7% 3 6% 3 4 4 4 7% 9% 38%	3964
	21st to end	Day	15% 5% 2 6% 4 4 5% 4 6% 10% 39%	4180
September	1st to 10th	Day	14% 5% 2 6% 4 5% 4 3 5% 8% 42%	2280
	11th to 20th	Day	13% 5% 2 5% 4 4 4 3 5% 8% 48%	2259
	21st to end	Day	19% 5% 2 5% 5% 4 3 3 5% 7% 41%	2061
October	1st to 10th	Day	21% 8% 3 6% 5% 3 5% 3 5% 7% 35%	1900
	11th to 20th	Day	23% 10% 3 7% 5% 5% 5% 3 5% 6% 28%	1900
	21st to end	Day	25% 9% 3 8% 5% 4 4 3 5% 5% 28%	1906
November	1st to 10th	Day	17% 10% 3 6% 5% 5% 4 4 6% 8% 34%	1761
	11th to 20th	Day	19% 12% 4 8% 6% 5% 5% 3 4 6% 28%	1630
	21st to end	Day	23% 14% 4 9% 6% 5% 4 3 5% 6% 21%	1600
December	1st to 10th	Day	21% 21% 5% 10% 5% 4 5% 3 5% 6% 15%	2600
	11th to 20th	Day	18% 16% 5% 10% 5% 5% 4 3 3 4 26%	2986
	21st to end	Day	23% 17% 5% 9% 5% 4 4 3 4 4 22%	3960
January	1st to 10th	Night	15% 21% 7% 14% 5% 4 3 2 4 4 21%	6349
	11th to 20th	Night	11% 16% 6% 15% 8% 6% 5% 4 5% 4 21%	6315
	21st to end	Night	12% 17% 7% 12% 6% 6% 5% 3 5% 5% 21%	6904
February	1st to 10th	Night	14% 14% 5% 12% 7% 5% 5% 4 5% 5% 25%	2600
	11th to 20th	Night	13% 13% 5% 11% 7% 6% 4 3 4 6% 28%	2504
	21st to end	Night	15% 15% 4 9% 6% 4 5% 3 5% 6% 28%	1980
March	1st to 10th	Night	22% 14% 4 8% 5% 4 4 3 4 4 28%	5364
	11th to 20th	Night	13% 13% 4 8% 5% 5% 5% 4 6% 6% 32%	5053
	21st to end	Night	18% 10% 3 9% 5% 4 4 3 5% 5% 33%	5105
April	1st to 10th	Night	21% 10% 3 8% 5% 4 4 3 4 6% 31%	2000
	11th to 20th	Night	19% 9% 3 7% 5% 4 3 4 5% 5% 35%	1975
	21st to end	Night	22% 8% 3 7% 4 3 4 3 5% 6% 35%	1869
May	1st to 10th	Night	21% 9% 4 10% 5% 6% 5% 4 6% 6% 25%	3594
	11th to 20th	Night	17% 7% 3 8% 4 4 5% 4 5% 7% 36%	3575
	21st to end	Night	18% 10% 3 8% 5% 5% 5% 4 5% 6% 31%	3926
June	1st to 10th	Night	17% 7% 3 6% 4 5% 5% 4 7% 9% 33%	1600
	11th to 20th	Night	14% 7% 3 7% 4 4 5% 4 5% 7% 42%	1600
	21st to end	Night	18% 6% 3 7% 5% 4 5% 4 6% 8% 35%	1600
July	1st to 10th	Night	13% 6% 3 8% 6% 5% 6% 4 7% 7% 35%	2600
	11th to 20th	Night	14% 7% 3 6% 3 4 4 4 7% 8% 41%	2600
	21st to end	Night	11% 6% 2 6% 5% 5% 5% 4 6% 7% 44%	2739
August	1st to 10th	Night	12% 8% 3 7% 4 5% 5% 4 7% 7% 38%	2506
	11th to 20th	Night	16% 6% 2 6% 4 4 4 4 6% 7% 42%	2830
	21st to end	Night	14% 5% 2 7% 5% 4 5% 4 6% 7% 41%	3476
September	1st to 10th	Night	11% 5% 2 6% 5% 4 4 4 5% 8% 45%	1900
	11th to 20th	Night	15% 5% 2 4 3 3 4 3 5% 7% 49%	1920
	21st to end	Night	17% 6% 2 6% 4 4 3 3 4 6% 45%	2118
October	1st to 10th	Night	19% 7% 3 7% 4 4 4 5% 5% 5% 38%	2280
	11th to 20th	Night	20% 8% 3 8% 4 5% 4 4 4 5% 33%	2280
	21st to end	Night	23% 10% 3 8% 5% 4 4 3 4 6% 30%	2690
November	1st to 10th	Night	20% 10% 3 7% 5% 4 4 3 6% 5% 33%	2537
	11th to 20th	Night	18% 11% 4 9% 5% 5% 4 3 5% 5% 30%	2766
	21st to end	Night	20% 14% 5% 9% 6% 5% 5% 3 5% 6% 23%	2800
December	1st to 10th	Night	20% 16% 5% 11% 6% 5% 5% 3 5% 6% 18%	4580
	11th to 20th	Night	24% 14% 4 9% 5% 4 3 2 4 4 27%	5208
	21st to end	Night	27% 17% 4 9% 4 4 3 3 4 4 22%	6996

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	68% 7% 6% 14% 4 1	3598
	11th to 20th	Day	62% 6% 11% 13% 8% 1	3600
	21st to end	Day	63% 7% 10% 11% 8% 1	4034
February	1st to 10th	Day	61% 5% 9% 18% 6% 2	1800
	11th to 20th	Day	60% 6% 10% 15% 9% 1	1790
	21st to end	Day	60% 5% 10% 13% 10% 1	1650
March	1st to 10th	Day	61% 5% 8% 12% 11% 3	4479
	11th to 20th	Day	51% 7% 16% 15% 10% 1	4904
	21st to end	Day	51% 4 7% 17% 18% 2	5826
April	1st to 10th	Day	54% 4 8% 18% 13% 3	2400
	11th to 20th	Day	51% 4 7% 21% 15% 2	2374
	21st to end	Day	55% 6% 8% 15% 13% 4	2547
May	1st to 10th	Day	58% 6% 8% 18% 8% 2	6357
	11th to 20th	Day	50% 8% 13% 18% 10% 1	6322
	21st to end	Day	51% 7% 11% 19% 12% 1	6949
June	1st to 10th	Day	52% 7% 11% 18% 11% 1	2800
	11th to 20th	Day	41% 8% 12% 24% 12% 2	2800
	21st to end	Day	48% 9% 13% 19% 10% 1	2800
July	1st to 10th	Day	42% 8% 15% 24% 10%	4510
	11th to 20th	Day	40% 8% 15% 29% 8%	4505
	21st to end	Day	35% 8% 14% 33% 11%	4661
August	1st to 10th	Day	45% 8% 16% 22% 9%	4409
	11th to 20th	Day	40% 8% 17% 28% 7%	4003
	21st to end	Day	39% 8% 18% 23% 12% 1	4180
September	1st to 10th	Day	40% 7% 14% 28% 12%	2280
	11th to 20th	Day	34% 8% 13% 31% 13% 1	2259
	21st to end	Day	42% 7% 14% 27% 10%	2061
October	1st to 10th	Day	47% 6% 12% 27% 8%	3160
	11th to 20th	Day	47% 7% 14% 24% 7%	3140
	21st to end	Day	48% 5% 19% 21% 6%	3118
November	1st to 10th	Day	48% 7% 14% 23% 6% 1	1761
	11th to 20th	Day	57% 6% 12% 15% 10%	1630
	21st to end	Day	64% 7% 12% 11% 6% 1	1600
December	1st to 10th	Day	69% 6% 13% 11% 2	2600
	11th to 20th	Day	63% 5% 10% 15% 7%	2986
	21st to end	Day	65% 5% 8% 16% 5%	3960
January	1st to 10th	Night	68% 5% 11% 12% 4	6349
	11th to 20th	Night	64% 7% 11% 12% 5%	6360
	21st to end	Night	64% 5% 9% 9% 11% 1	6904
February	1st to 10th	Night	59% 5% 11% 17% 7% 1	2600
	11th to 20th	Night	57% 5% 10% 19% 8%	2506
	21st to end	Night	55% 6% 9% 18% 10% 1	1980
March	1st to 10th	Night	59% 4 7% 14% 12% 3	5374
	11th to 20th	Night	51% 7% 15% 16% 8% 3	5054
	21st to end	Night	51% 6% 8% 22% 12% 2	5105
April	1st to 10th	Night	54% 5% 9% 18% 13% 2	2000
	11th to 20th	Night	49% 5% 8% 21% 15% 1	1975
	21st to end	Night	49% 5% 10% 18% 15% 3	1869
May	1st to 10th	Night	56% 8% 9% 18% 7% 2	3600
	11th to 20th	Night	46% 9% 13% 18% 13% 2	3576
	21st to end	Night	52% 7% 10% 17% 13% 1	3927
June	1st to 10th	Night	45% 8% 12% 23% 11% 1	1600
	11th to 20th	Night	41% 7% 12% 23% 16% 2	1600
	21st to end	Night	45% 9% 12% 19% 14% 1	1600
July	1st to 10th	Night	44% 8% 16% 19% 12% 1	2560
	11th to 20th	Night	39% 7% 15% 31% 8%	2552
	21st to end	Night	36% 8% 16% 25% 14%	2651
August	1st to 10th	Night	42% 8% 16% 25% 8%	2496
	11th to 20th	Night	41% 6% 17% 29% 7%	2857
	21st to end	Night	39% 7% 16% 23% 12% 2	3476
September	1st to 10th	Night	37% 6% 12% 28% 14% 1	1900
	11th to 20th	Night	34% 7% 14% 34% 10% 1	1920
	21st to end	Night	41% 6% 12% 30% 11%	2118
October	1st to 10th	Night	41% 8% 14% 26% 11%	3800
	11th to 20th	Night	44% 8% 14% 25% 9%	3779
	21st to end	Night	46% 6% 18% 19% 10%	4428
November	1st to 10th	Night	51% 6% 13% 21% 9%	2537
	11th to 20th	Night	54% 6% 12% 17% 10% 1	2766
	21st to end	Night	60% 8% 12% 16% 4	2800
December	1st to 10th	Night	66% 6% 13% 12% 3	4580
	11th to 20th	Night	62% 5% 8% 15% 10% 1	5208
	21st to end	Night	67% 4 7% 13% 10%	6996

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	26% 14% 35% 23% 3	200
	11th to 20th	Day	23% 18% 39% 16% 5% 1	200
	21st to end	Day	19% 17% 37% 23% 4	237
February	1st to 10th	Day	26% 26% 35% 11% 2	300
	11th to 20th	Day	25% 26% 35% 13% 1	328
	21st to end	Day	28% 17% 34% 19% 2	414
March	1st to 10th	Day	26% 17% 31% 20% 5%	500
	11th to 20th	Day	23% 19% 35% 18% 4 1	500
	21st to end	Day	29% 18% 30% 14% 8% 2	550
April	1st to 10th	Day	28% 21% 30% 14% 7% 1	500
	11th to 20th	Day	30% 21% 21% 18% 8% 2	492
	21st to end	Day	28% 18% 32% 14% 6% 2	500
May	1st to 10th	Day	23% 14% 35% 19% 6% 3	503
	11th to 20th	Day	19% 16% 33% 20% 10% 2	527
	21st to end	Day	20% 15% 32% 22% 9% 2	573
June	1st to 10th	Day	24% 13% 29% 25% 9% 1	530
	11th to 20th	Day	15% 14% 32% 24% 13% 2	530
	21st to end	Day	22% 13% 29% 23% 11% 2	530
July	1st to 10th	Day	13% 10% 30% 29% 14% 4	525
	11th to 20th	Day	11% 10% 34% 25% 17% 3	524
	21st to end	Day	14% 7% 29% 32% 16% 4	532
August	1st to 10th	Day	14% 7% 26% 34% 15% 3	486
	11th to 20th	Day	14% 7% 21% 32% 18% 8%	479
	21st to end	Day	13% 10% 27% 27% 14% 8%	539
September	1st to 10th	Day	16% 9% 29% 21% 17% 8%	490
	11th to 20th	Day	12% 10% 21% 20% 22% 15%	490
	21st to end	Day	20% 12% 26% 22% 15% 6%	490
October	1st to 10th	Day	24% 14% 22% 22% 13% 3	490
	11th to 20th	Day	26% 19% 28% 17% 9% 1	490
	21st to end	Day	29% 15% 31% 20% 5%	442
November	1st to 10th	Day	27% 15% 27% 22% 8% 1	395
	11th to 20th	Day	25% 20% 35% 14% 3 2	230
	21st to end	Day	31% 19% 31% 15% 5% 1	200
December	1st to 10th	Day	32% 13% 29% 22% 5%	200
	11th to 20th	Day	26% 19% 40% 12% 3 1	200
	21st to end	Day	32% 17% 35% 10% 5% 1	220
January	1st to 10th	Night	27% 12% 36% 20% 5%	800
	11th to 20th	Night	28% 13% 31% 22% 6% 1	800
	21st to end	Night	19% 10% 35% 29% 6% 1	880
February	1st to 10th	Night	19% 12% 40% 25% 4	800
	11th to 20th	Night	21% 15% 38% 22% 4 1	783
	21st to end	Night	24% 13% 30% 30% 3	660
March	1st to 10th	Night	24% 13% 32% 25% 5%	800
	11th to 20th	Night	25% 13% 34% 23% 5%	800
	21st to end	Night	23% 12% 33% 24% 8%	880
April	1st to 10th	Night	26% 14% 30% 27% 5%	800
	11th to 20th	Night	23% 13% 32% 24% 7% 1	791
	21st to end	Night	26% 11% 32% 24% 7% 1	669
May	1st to 10th	Night	24% 10% 28% 28% 9% 2	400
	11th to 20th	Night	23% 9% 26% 32% 9% 1	394
	21st to end	Night	16% 7% 35% 28% 13%	433
June	1st to 10th	Night	16% 6% 28% 40% 10%	400
	11th to 20th	Night	16% 5% 28% 37% 13% 1	400
	21st to end	Night	18% 6% 27% 34% 14% 2	400
July	1st to 10th	Night	10% 3 27% 42% 18%	390
	11th to 20th	Night	12% 6% 17% 45% 19% 1	388
	21st to end	Night	11% 4 22% 43% 18% 2	416
August	1st to 10th	Night	12% 3 17% 46% 20% 1	378
	11th to 20th	Night	13% 3 19% 35% 27% 3	583
	21st to end	Night	13% 6% 23% 36% 20% 2	836
September	1st to 10th	Night	14% 6% 26% 35% 19% 1	760
	11th to 20th	Night	16% 6% 19% 30% 25% 3	760
	21st to end	Night	19% 8% 24% 29% 19% 2	760
October	1st to 10th	Night	21% 8% 26% 29% 15% 1	760
	11th to 20th	Night	25% 9% 28% 29% 9% 1	760
	21st to end	Night	26% 11% 30% 27% 5%	836
November	1st to 10th	Night	26% 11% 26% 28% 8% 1	780
	11th to 20th	Night	25% 13% 32% 20% 10% 1	800
	21st to end	Night	27% 16% 28% 23% 6%	800
December	1st to 10th	Night	28% 13% 32% 23% 4	800
	11th to 20th	Night	29% 14% 30% 22% 5% 1	800
	21st to end	Night	34% 12% 30% 20% 4	880

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