Highland
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Highland clear sky chart from 2005-11-06 to 2025-04-26.

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	6% 7% 2 6% 4 4 4 4 5% 7% 50%	117627
	Night	7% 8% 3 6% 4 4 4 4 5% 6% 50%	119865
Transparency	Day	32% 6% 11% 19% 20% 12%	117392
	Night	33% 6% 12% 21% 19% 10%	119689
Seeing	Day	12% 9% 24% 33% 19% 4	15383
	Night	12% 5% 20% 37% 22% 3	26060

By Month

Percentage Cloud Cover SW (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	7% 13% 5% 11% 6% 5% 5% 5% 7% 6% 30%	4098
	11th to 20th	Day	7% 13% 4 15% 7% 5% 5% 3 5% 5% 31%	4100
	21st to end	Day	6% 13% 4 8% 4 4 4 3 4 5% 45%	4510
February	1st to 10th	Day	11% 14% 4 9% 5% 6% 5% 4 5% 6% 33%	1800
	11th to 20th	Day	9% 14% 3 9% 6% 5% 5% 5% 6% 6% 32%	1829
	21st to end	Day	8% 15% 4 9% 6% 4 4 4 6% 7% 33%	1650
March	1st to 10th	Day	5% 15% 4 8% 5% 5% 5% 4 5% 7% 37%	4840
	11th to 20th	Day	6% 10% 3 7% 5% 6% 7% 5% 6% 7% 37%	5000
	21st to end	Day	7% 16% 5% 12% 7% 5% 5% 4 4 6% 29%	5486
April	1st to 10th	Day	9% 9% 3 8% 4 4 5% 3 7% 8% 40%	2218
	11th to 20th	Day	8% 9% 3 7% 5% 4 5% 4 6% 7% 41%	2518
	21st to end	Day	8% 8% 3 6% 5% 4 4 3 5% 8% 45%	2537
May	1st to 10th	Day	8% 10% 4 7% 5% 5% 5% 4 6% 7% 39%	4110
	11th to 20th	Day	5% 4 1 4 3 4 4 4 6% 9% 55%	4071
	21st to end	Day	8% 6% 2 5% 4 4 5% 4 6% 8% 49%	4477
June	1st to 10th	Day	4 4 2 5% 3 4 3 4 6% 9% 55%	2470
	11th to 20th	Day	5% 4 2 4 4 3 3 3 4 6% 62%	2530
	21st to end	Day	3 2 1 3 2 3 4 3 4 6% 70%	2470
July	1st to 10th	Day	1 2 2 2 2 3 2 4 5% 78%	4110
	11th to 20th	Day	2 3 1 4 4 4 5% 4 7% 8% 57%	4110
	21st to end	Day	3 2 1 3 3 3 4 4 6% 7% 65%	4454
August	1st to 10th	Day	2 3 1 4 3 4 4 4 6% 7% 62%	4092
	11th to 20th	Day	2 2 1 4 4 4 4 3 5% 6% 64%	4028
	21st to end	Day	3 3 1 3 3 3 3 3 4 6% 69%	4473
September	1st to 10th	Day	3 3 1 3 2 3 3 3 5% 6% 69%	2143
	11th to 20th	Day	4 3 1 4 3 3 4 3 6% 6% 64%	2090
	21st to end	Day	6% 3 1 3 2 3 2 3 4 5% 68%	2090
October	1st to 10th	Day	5% 3 1 3 3 2 3 3 4 4 69%	3333
	11th to 20th	Day	6% 5% 2 5% 3 3 3 3 5% 7% 58%	3150
	21st to end	Day	6% 7% 3 6% 3 3 3 3 4 6% 57%	3438
November	1st to 10th	Day	11% 6% 2 5% 5% 4 4 3 5% 6% 48%	1766
	11th to 20th	Day	11% 7% 2 6% 4 5% 4 3 6% 7% 45%	1800
	21st to end	Day	8% 9% 4 8% 5% 6% 5% 3 6% 6% 42%	1680
December	1st to 10th	Day	7% 6% 1 5% 3 2 4 4 4 9% 55%	2620
	11th to 20th	Day	9% 11% 3 8% 5% 5% 5% 4 6% 9% 35%	3026
	21st to end	Day	12% 11% 4 9% 5% 5% 5% 5% 6% 7% 30%	4510
January	1st to 10th	Night	6% 12% 4 8% 5% 5% 5% 6% 6% 7% 35%	5867
	11th to 20th	Night	10% 13% 3 10% 5% 5% 5% 5% 5% 6% 34%	5880
	21st to end	Night	5% 11% 4 7% 4 4 3 3 5% 5% 48%	6468
February	1st to 10th	Night	9% 12% 3 8% 5% 4 5% 5% 6% 7% 36%	2600
	11th to 20th	Night	8% 11% 3 8% 5% 4 4 3 5% 6% 42%	2455
	21st to end	Night	7% 12% 3 9% 5% 5% 4 4 6% 6% 39%	1980
March	1st to 10th	Night	6% 14% 5% 9% 5% 5% 5% 4 5% 6% 37%	5053
	11th to 20th	Night	6% 9% 3 8% 5% 4 5% 4 6% 8% 40%	5000
	21st to end	Night	9% 13% 4 8% 4 4 4 4 5% 6% 40%	5489
April	1st to 10th	Night	11% 10% 2 6% 3 3 3 3 5% 7% 46%	2180
	11th to 20th	Night	6% 7% 2 6% 5% 4 6% 5% 5% 7% 47%	1826
	21st to end	Night	9% 6% 3 6% 5% 4 4 3 6% 6% 47%	1759
May	1st to 10th	Night	9% 10% 3 6% 5% 5% 5% 4 6% 7% 41%	2860
	11th to 20th	Night	5% 4 1 4 4 3 3 3 5% 7% 60%	2834
	21st to end	Night	9% 5% 2 5% 3 3 4 3 5% 8% 54%	3110
June	1st to 10th	Night	6% 5% 1 4 3 3 5% 4 6% 8% 55%	1708
	11th to 20th	Night	5% 4 1 4 2 3 2 3 4 6% 66%	1581
	21st to end	Night	3 3 1 4 2 2 3 3 5% 7% 67%	1710
July	1st to 10th	Night	4 2 1 3 3 3 2 3 4 4 71%	2860
	11th to 20th	Night	3 3 2 5% 4 5% 6% 5% 8% 8% 52%	2860
	21st to end	Night	3 5% 1 6% 3 4 4 4 5% 6% 59%	3109
August	1st to 10th	Night	3 2 1 4 4 4 5% 5% 6% 8% 59%	2847
	11th to 20th	Night	2 3 1 5% 4 4 4 4 6% 6% 60%	2811
	21st to end	Night	5% 3 1 3 2 3 3 3 4 5% 69%	3194
September	1st to 10th	Night	3 3 1 4 3 3 4 3 3 6% 66%	2033
	11th to 20th	Night	6% 3 1 3 3 3 4 3 5% 7% 63%	2090
	21st to end	Night	6% 3 2 4 2 2 2 2 3 5% 70%	2090
October	1st to 10th	Night	5% 3 1 3 3 2 3 2 3 5% 71%	3641
	11th to 20th	Night	8% 5% 2 4 3 2 3 3 4 5% 61%	3778
	21st to end	Night	5% 5% 2 5% 3 3 4 4 5% 7% 58%	4120
November	1st to 10th	Night	10% 7% 2 4 3 3 3 4 6% 6% 52%	2530
	11th to 20th	Night	9% 5% 1 5% 4 3 4 3 6% 8% 51%	2600
	21st to end	Night	8% 8% 3 6% 4 3 4 4 5% 7% 48%	2695
December	1st to 10th	Night	6% 6% 2 4 2 2 4 3 5% 6% 61%	4570
	11th to 20th	Night	10% 12% 3 5% 3 3 4 3 5% 6% 44%	5181
	21st to end	Night	9% 10% 4 9% 5% 6% 6% 5% 6% 7% 33%	6496

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	47% 13% 17% 14% 8% 1	4099
	11th to 20th	Day	50% 9% 15% 15% 9% 2	4100
	21st to end	Day	40% 10% 13% 15% 19% 2	4510
February	1st to 10th	Day	51% 10% 14% 14% 10% 1	1800
	11th to 20th	Day	50% 6% 15% 18% 10% 1	1838
	21st to end	Day	47% 5% 15% 20% 11% 2	1650
March	1st to 10th	Day	41% 8% 14% 21% 12% 5%	4840
	11th to 20th	Day	36% 11% 13% 28% 11% 1	5000
	21st to end	Day	53% 4 19% 16% 6% 1	5486
April	1st to 10th	Day	41% 6% 13% 20% 16% 5%	2218
	11th to 20th	Day	40% 4 14% 22% 17% 3	2521
	21st to end	Day	36% 4 12% 25% 20% 2	2537
May	1st to 10th	Day	38% 6% 11% 23% 18% 5%	4110
	11th to 20th	Day	22% 5% 8% 31% 29% 5%	4074
	21st to end	Day	29% 6% 10% 25% 26% 4	4477
June	1st to 10th	Day	24% 4 9% 21% 30% 12%	2470
	11th to 20th	Day	23% 3 7% 17% 29% 22%	2530
	21st to end	Day	16% 2 3 12% 27% 40%	2470
July	1st to 10th	Day	11% 2 4 7% 16% 60%	4046
	11th to 20th	Day	20% 5% 7% 12% 25% 31%	4041
	21st to end	Day	17% 4 8% 11% 25% 35%	4314
August	1st to 10th	Day	19% 4 7% 21% 25% 24%	4078
	11th to 20th	Day	20% 4 6% 17% 28% 25%	4067
	21st to end	Day	16% 3 5% 15% 31% 29%	4472
September	1st to 10th	Day	16% 3 6% 17% 28% 30%	2143
	11th to 20th	Day	19% 3 9% 19% 27% 22%	2090
	21st to end	Day	18% 4 5% 20% 33% 20%	2090
October	1st to 10th	Day	20% 3 5% 15% 38% 18%	3333
	11th to 20th	Day	26% 6% 8% 28% 27% 6%	3150
	21st to end	Day	28% 5% 12% 27% 22% 5%	3438
November	1st to 10th	Day	37% 5% 14% 22% 18% 4	1766
	11th to 20th	Day	37% 6% 14% 27% 15% 1	1800
	21st to end	Day	41% 8% 15% 21% 13% 2	1680
December	1st to 10th	Day	26% 6% 14% 39% 13% 1	2620
	11th to 20th	Day	44% 10% 17% 19% 10% 1	3024
	21st to end	Day	48% 11% 15% 15% 10%	4510
January	1st to 10th	Night	42% 11% 19% 16% 10% 2	5867
	11th to 20th	Night	45% 9% 14% 19% 12% 1	5880
	21st to end	Night	36% 9% 16% 17% 21% 1	6468
February	1st to 10th	Night	45% 9% 18% 18% 8% 2	2600
	11th to 20th	Night	43% 6% 14% 22% 14% 2	2456
	21st to end	Night	42% 5% 15% 23% 12% 3	1980
March	1st to 10th	Night	44% 6% 11% 23% 12% 4	5053
	11th to 20th	Night	37% 8% 11% 27% 15% 2	5000
	21st to end	Night	43% 6% 17% 19% 14% 1	5489
April	1st to 10th	Night	37% 4 12% 22% 18% 6%	2180
	11th to 20th	Night	34% 4 13% 21% 22% 5%	1826
	21st to end	Night	35% 5% 13% 23% 21% 2	1759
May	1st to 10th	Night	36% 5% 11% 23% 22% 3	2860
	11th to 20th	Night	23% 4 8% 29% 28% 8%	2835
	21st to end	Night	27% 5% 9% 28% 27% 4	3110
June	1st to 10th	Night	25% 5% 9% 21% 28% 12%	1708
	11th to 20th	Night	20% 3 7% 17% 30% 23%	1581
	21st to end	Night	17% 2 5% 9% 27% 39%	1710
July	1st to 10th	Night	16% 3 4 8% 11% 59%	2815
	11th to 20th	Night	25% 5% 7% 11% 23% 29%	2806
	21st to end	Night	25% 4 7% 13% 21% 31%	3011
August	1st to 10th	Night	21% 5% 9% 19% 23% 24%	2838
	11th to 20th	Night	24% 5% 7% 13% 26% 25%	2838
	21st to end	Night	18% 3 6% 12% 29% 32%	3194
September	1st to 10th	Night	20% 4 6% 14% 26% 32%	2033
	11th to 20th	Night	21% 3 7% 19% 26% 24%	2090
	21st to end	Night	20% 3 5% 16% 33% 24%	2090
October	1st to 10th	Night	18% 3 6% 16% 36% 22%	3641
	11th to 20th	Night	27% 3 6% 29% 27% 8%	3778
	21st to end	Night	25% 5% 10% 30% 25% 4	4120
November	1st to 10th	Night	31% 5% 11% 25% 21% 6%	2530
	11th to 20th	Night	31% 5% 16% 29% 16% 3	2600
	21st to end	Night	35% 6% 15% 27% 14% 3	2695
December	1st to 10th	Night	24% 5% 13% 37% 20% 1	4570
	11th to 20th	Night	41% 7% 21% 23% 9%	5182
	21st to end	Night	46% 9% 19% 17% 9%	6496

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% 10% 23% 22% 14% 5%	300
	11th to 20th	Day	14% 12% 26% 29% 17% 2	300
	21st to end	Day	15% 6% 29% 32% 15% 3	330
February	1st to 10th	Day	26% 9% 30% 24% 9% 2	300
	11th to 20th	Day	19% 10% 30% 28% 11% 2	374
	21st to end	Day	15% 11% 33% 28% 11% 1	414
March	1st to 10th	Day	17% 14% 28% 27% 13% 2	500
	11th to 20th	Day	17% 11% 26% 30% 16% 1	500
	21st to end	Day	20% 15% 29% 26% 10%	550
April	1st to 10th	Day	16% 15% 24% 29% 14% 2	500
	11th to 20th	Day	15% 17% 30% 26% 10% 2	492
	21st to end	Day	15% 15% 27% 25% 15% 3	481
May	1st to 10th	Day	11% 14% 29% 29% 13% 3	463
	11th to 20th	Day	11% 7% 25% 34% 18% 5%	488
	21st to end	Day	9% 10% 23% 30% 23% 5%	529
June	1st to 10th	Day	6% 8% 21% 37% 24% 4	490
	11th to 20th	Day	9% 9% 27% 31% 22% 2	550
	21st to end	Day	4 8% 26% 39% 19% 4	490
July	1st to 10th	Day	3 5% 20% 42% 25% 5%	486
	11th to 20th	Day	5% 2 19% 46% 24% 3	484
	21st to end	Day	3 3 13% 46% 28% 7%	531
August	1st to 10th	Day	7% 2 19% 42% 26% 3	487
	11th to 20th	Day	5% 4 21% 35% 28% 6%	477
	21st to end	Day	5% 5% 17% 39% 28% 5%	539
September	1st to 10th	Day	4 4 18% 37% 30% 7%	490
	11th to 20th	Day	9% 7% 22% 31% 26% 5%	490
	21st to end	Day	8% 8% 19% 33% 27% 6%	490
October	1st to 10th	Day	11% 8% 23% 29% 24% 5%	398
	11th to 20th	Day	10% 10% 21% 28% 24% 8%	300
	21st to end	Day	12% 8% 29% 33% 17% 2	330
November	1st to 10th	Day	19% 10% 24% 25% 18% 4	300
	11th to 20th	Day	13% 11% 22% 35% 13% 6%	300
	21st to end	Day	16% 8% 20% 29% 23% 4	300
December	1st to 10th	Day	15% 8% 21% 32% 19% 5%	300
	11th to 20th	Day	16% 9% 24% 36% 13% 2	300
	21st to end	Day	25% 9% 20% 33% 11% 1	330
January	1st to 10th	Night	18% 5% 22% 32% 19% 4	800
	11th to 20th	Night	16% 5% 22% 34% 20% 4	800
	21st to end	Night	15% 5% 21% 38% 18% 3	880
February	1st to 10th	Night	19% 5% 22% 38% 13% 3	800
	11th to 20th	Night	14% 6% 28% 36% 15% 1	784
	21st to end	Night	13% 7% 27% 39% 13% 1	660
March	1st to 10th	Night	18% 7% 25% 35% 13% 3	800
	11th to 20th	Night	12% 6% 23% 39% 19% 2	800
	21st to end	Night	15% 9% 26% 35% 12% 2	880
April	1st to 10th	Night	18% 8% 22% 36% 14% 1	780
	11th to 20th	Night	10% 7% 30% 39% 13% 1	593
	21st to end	Night	13% 6% 24% 35% 19% 2	587
May	1st to 10th	Night	10% 8% 23% 41% 17% 1	570
	11th to 20th	Night	10% 4 18% 36% 29% 2	561
	21st to end	Night	12% 6% 18% 36% 25% 3	617
June	1st to 10th	Night	8% 6% 17% 38% 29% 2	570
	11th to 20th	Night	7% 5% 23% 42% 21% 2	570
	21st to end	Night	5% 2 18% 41% 31% 3	570
July	1st to 10th	Night	6% 3 17% 31% 39% 5%	555
	11th to 20th	Night	5% 1 11% 47% 31% 4	552
	21st to end	Night	6% 1 9% 33% 49% 2	624
August	1st to 10th	Night	5% 1 13% 44% 35% 2	567
	11th to 20th	Night	4 2 12% 42% 36% 3	564
	21st to end	Night	6% 3 9% 43% 37% 2	627
September	1st to 10th	Night	5% 3 13% 41% 35% 4	703
	11th to 20th	Night	8% 3 17% 34% 34% 5%	760
	21st to end	Night	8% 3 16% 32% 33% 7%	760
October	1st to 10th	Night	11% 4 20% 36% 25% 4	760
	11th to 20th	Night	10% 4 18% 37% 27% 4	760
	21st to end	Night	11% 4 21% 38% 23% 3	836
November	1st to 10th	Night	13% 6% 20% 35% 22% 4	780
	11th to 20th	Night	14% 5% 21% 39% 17% 4	800
	21st to end	Night	13% 6% 22% 35% 20% 4	895
December	1st to 10th	Night	16% 4 24% 36% 17% 2	1000
	11th to 20th	Night	19% 7% 21% 39% 13% 2	1000
	21st to end	Night	20% 7% 23% 38% 11% 1	895

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