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 2024-03-01.

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	7% 7% 3 6% 5% 3 5% 5% 6% 7% 47%	78455
	Night	8% 8% 3 6% 5% 3 4 5% 5% 7% 47%	80952
Transparency	Day	35% 6% 12% 19% 18% 11%	78180
	Night	36% 6% 13% 20% 16% 9%	80748
Seeing	Day	12% 7% 23% 34% 21% 4	13831
	Night	12% 4 19% 38% 24% 3	24514

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	9% 13% 6% 10% 8% 4 6% 6% 6% 5% 28%	2858
	11th to 20th	Day	11% 17% 6% 13% 6% 3 4 3 5% 5% 27%	2860
	21st to end	Day	9% 15% 5% 8% 6% 3 4 5% 5% 6% 34%	3146
February	1st to 10th	Day	11% 12% 4 8% 5% 5% 6% 5% 5% 6% 33%	1710
	11th to 20th	Day	9% 13% 4 7% 7% 3 5% 6% 6% 6% 33%	1735
	21st to end	Day	9% 14% 4 8% 7% 4 4 4 6% 8% 32%	1570
March	1st to 10th	Day	8% 11% 4 6% 5% 3 6% 7% 7% 8% 35%	1929
	11th to 20th	Day	7% 10% 4 6% 5% 3 5% 6% 7% 8% 39%	1980
	21st to end	Day	10% 12% 4 9% 7% 4 5% 6% 5% 8% 31%	2178
April	1st to 10th	Day	10% 9% 3 8% 5% 3 5% 4 7% 8% 39%	1996
	11th to 20th	Day	9% 9% 3 6% 6% 3 6% 5% 6% 7% 40%	2262
	21st to end	Day	8% 8% 3 6% 5% 3 4 4 5% 8% 46%	2340
May	1st to 10th	Day	7% 6% 4 5% 5% 3 6% 7% 6% 8% 42%	2340
	11th to 20th	Day	6% 5% 2 5% 5% 4 5% 6% 7% 10% 46%	2301
	21st to end	Day	8% 5% 3 6% 5% 4 6% 7% 7% 9% 41%	2530
June	1st to 10th	Day	4 4 2 4 4 3 4 4 6% 10% 55%	2340
	11th to 20th	Day	6% 4 2 3 4 2 3 4 4 6% 62%	2398
	21st to end	Day	3 2 1 3 3 2 4 4 4 6% 69%	2340
July	1st to 10th	Day	2 2 1 3 4 3 4 4 6% 8% 64%	2340
	11th to 20th	Day	4 3 2 4 4 3 5% 5% 6% 8% 57%	2340
	21st to end	Day	4 3 2 3 5% 4 5% 6% 7% 8% 54%	2545
August	1st to 10th	Day	3 3 2 4 4 3 5% 6% 7% 8% 56%	2328
	11th to 20th	Day	3 3 1 3 4 2 5% 5% 6% 7% 62%	2299
	21st to end	Day	4 4 1 4 3 2 4 4 5% 8% 61%	2552
September	1st to 10th	Day	3 3 1 2 3 2 3 4 5% 6% 69%	2031
	11th to 20th	Day	5% 3 1 3 3 2 4 3 6% 6% 64%	1980
	21st to end	Day	6% 3 1 3 3 2 3 3 4 5% 67%	1980
October	1st to 10th	Day	10% 5% 2 4 4 3 3 5% 5% 6% 54%	1893
	11th to 20th	Day	6% 5% 2 5% 3 2 3 3 4 7% 60%	1800
	21st to end	Day	6% 5% 2 4 4 3 4 5% 5% 6% 55%	1980
November	1st to 10th	Day	11% 6% 2 4 6% 3 4 5% 5% 6% 48%	1676
	11th to 20th	Day	11% 7% 3 5% 5% 4 5% 4 6% 7% 45%	1710
	21st to end	Day	8% 8% 4 7% 6% 5% 5% 4 5% 6% 42%	1596
December	1st to 10th	Day	12% 10% 3 7% 5% 2 4 5% 6% 8% 38%	1520
	11th to 20th	Day	13% 10% 3 9% 6% 4 6% 6% 6% 7% 30%	1926
	21st to end	Day	8% 11% 4 8% 7% 4 5% 6% 6% 8% 32%	3146
January	1st to 10th	Night	8% 11% 5% 7% 6% 4 6% 8% 7% 7% 32%	4097
	11th to 20th	Night	13% 17% 4 11% 7% 4 4 4 4 5% 28%	4110
	21st to end	Night	8% 14% 4 7% 5% 4 5% 5% 6% 7% 35%	4521
February	1st to 10th	Night	9% 11% 3 6% 6% 4 5% 6% 6% 7% 37%	2470
	11th to 20th	Night	8% 10% 4 7% 6% 3 5% 4 5% 6% 42%	2329
	21st to end	Night	8% 12% 4 7% 6% 4 4 5% 6% 7% 38%	1884
March	1st to 10th	Night	9% 11% 5% 7% 6% 4 5% 6% 5% 6% 36%	2036
	11th to 20th	Night	8% 7% 3 5% 5% 3 5% 5% 7% 8% 44%	1980
	21st to end	Night	8% 10% 4 7% 5% 3 5% 6% 6% 8% 38%	2178
April	1st to 10th	Night	12% 10% 3 6% 3 2 4 4 4 7% 45%	1962
	11th to 20th	Night	7% 7% 3 5% 6% 3 6% 6% 5% 7% 46%	1642
	21st to end	Night	9% 5% 3 6% 6% 3 4 4 6% 6% 49%	1620
May	1st to 10th	Night	7% 6% 2 5% 6% 4 6% 6% 6% 7% 44%	1620
	11th to 20th	Night	7% 5% 2 4 6% 3 4 4 5% 8% 52%	1594
	21st to end	Night	9% 5% 2 6% 5% 2 4 4 7% 8% 47%	1746
June	1st to 10th	Night	6% 4 2 3 4 2 5% 5% 5% 8% 56%	1618
	11th to 20th	Night	5% 4 1 3 2 2 3 3 4 6% 66%	1497
	21st to end	Night	3 3 1 3 3 2 3 4 5% 7% 67%	1620
July	1st to 10th	Night	3 3 2 5% 4 3 5% 5% 6% 7% 57%	1620
	11th to 20th	Night	4 3 2 5% 5% 3 5% 6% 6% 7% 54%	1620
	21st to end	Night	4 5% 2 5% 5% 2 5% 5% 7% 9% 53%	1773
August	1st to 10th	Night	4 3 2 4 4 3 4 6% 6% 8% 56%	1611
	11th to 20th	Night	3 3 2 5% 4 2 4 5% 7% 7% 59%	1602
	21st to end	Night	3 3 2 3 4 3 4 6% 6% 7% 60%	1804
September	1st to 10th	Night	3 3 1 3 3 2 4 4 3 6% 67%	1926
	11th to 20th	Night	6% 3 1 3 3 2 4 3 4 7% 63%	1980
	21st to end	Night	6% 3 2 3 3 1 2 3 3 5% 69%	1980
October	1st to 10th	Night	8% 4 2 4 4 2 4 4 4 7% 58%	2066
	11th to 20th	Night	7% 5% 2 4 3 2 4 3 4 5% 62%	2160
	21st to end	Night	6% 5% 2 4 3 2 4 5% 4 6% 60%	2376
November	1st to 10th	Night	10% 7% 2 4 3 2 4 4 6% 6% 52%	2400
	11th to 20th	Night	10% 5% 2 5% 5% 2 3 4 5% 8% 52%	2470
	21st to end	Night	9% 7% 4 5% 5% 2 4 5% 5% 7% 47%	2560
December	1st to 10th	Night	11% 9% 3 5% 5% 2 4 4 6% 7% 45%	2660
	11th to 20th	Night	14% 11% 3 6% 5% 3 5% 6% 5% 7% 35%	3271
	21st to end	Night	10% 10% 4 8% 7% 5% 6% 6% 5% 7% 33%	4549

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	51% 12% 17% 15% 4	2859
	11th to 20th	Day	60% 7% 16% 11% 5% 1	2860
	21st to end	Day	48% 13% 18% 13% 7% 1	3146
February	1st to 10th	Day	50% 10% 15% 15% 9% 1	1710
	11th to 20th	Day	49% 5% 16% 19% 11% 1	1744
	21st to end	Day	49% 5% 16% 20% 10% 1	1570
March	1st to 10th	Day	43% 8% 16% 22% 10% 2	1929
	11th to 20th	Day	39% 6% 16% 23% 13% 2	1980
	21st to end	Day	49% 5% 18% 17% 8% 3	2178
April	1st to 10th	Day	44% 5% 12% 18% 16% 5%	1996
	11th to 20th	Day	41% 4 14% 22% 16% 3	2265
	21st to end	Day	36% 4 12% 25% 20% 3	2340
May	1st to 10th	Day	35% 5% 12% 24% 19% 5%	2340
	11th to 20th	Day	31% 4 10% 25% 23% 7%	2304
	21st to end	Day	35% 5% 13% 23% 19% 4	2530
June	1st to 10th	Day	24% 4 9% 21% 30% 12%	2340
	11th to 20th	Day	23% 2 7% 17% 28% 22%	2398
	21st to end	Day	16% 2 3 11% 28% 39%	2340
July	1st to 10th	Day	18% 3 7% 12% 25% 35%	2276
	11th to 20th	Day	22% 4 7% 13% 24% 30%	2271
	21st to end	Day	24% 4 11% 13% 22% 26%	2404
August	1st to 10th	Day	24% 4 8% 18% 20% 27%	2313
	11th to 20th	Day	21% 3 6% 12% 28% 29%	2299
	21st to end	Day	21% 3 5% 19% 30% 22%	2552
September	1st to 10th	Day	16% 3 6% 16% 28% 31%	2031
	11th to 20th	Day	19% 3 9% 19% 25% 23%	1980
	21st to end	Day	19% 4 6% 20% 33% 19%	1980
October	1st to 10th	Day	31% 5% 8% 21% 26% 10%	1893
	11th to 20th	Day	26% 4 9% 29% 26% 6%	1800
	21st to end	Day	27% 6% 16% 27% 20% 3	1980
November	1st to 10th	Day	37% 5% 14% 22% 18% 4	1676
	11th to 20th	Day	38% 5% 14% 26% 15% 1	1710
	21st to end	Day	42% 8% 15% 21% 13% 2	1596
December	1st to 10th	Day	45% 8% 19% 19% 8% 2	1520
	11th to 20th	Day	52% 8% 14% 20% 6%	1924
	21st to end	Day	45% 10% 19% 15% 10%	3146
January	1st to 10th	Night	45% 11% 20% 18% 5% 1	4097
	11th to 20th	Night	58% 10% 15% 12% 5% 1	4110
	21st to end	Night	45% 12% 22% 14% 7% 1	4521
February	1st to 10th	Night	45% 9% 18% 19% 8% 1	2470
	11th to 20th	Night	43% 5% 14% 22% 14% 1	2330
	21st to end	Night	43% 5% 15% 24% 11% 2	1884
March	1st to 10th	Night	46% 6% 13% 23% 11%	2036
	11th to 20th	Night	34% 5% 14% 27% 16% 3	1980
	21st to end	Night	41% 6% 15% 24% 10% 3	2178
April	1st to 10th	Night	40% 4 12% 21% 17% 7%	1962
	11th to 20th	Night	35% 4 14% 22% 22% 3	1642
	21st to end	Night	35% 4 13% 23% 23% 2	1620
May	1st to 10th	Night	35% 5% 12% 23% 21% 4	1620
	11th to 20th	Night	30% 4 10% 27% 22% 8%	1595
	21st to end	Night	33% 5% 13% 27% 18% 4	1746
June	1st to 10th	Night	25% 4 8% 21% 28% 12%	1618
	11th to 20th	Night	19% 3 7% 18% 29% 24%	1497
	21st to end	Night	17% 2 5% 10% 27% 39%	1620
July	1st to 10th	Night	24% 4 6% 13% 18% 35%	1575
	11th to 20th	Night	27% 4 8% 13% 19% 29%	1566
	21st to end	Night	27% 4 10% 15% 19% 25%	1674
August	1st to 10th	Night	24% 4 10% 17% 19% 25%	1602
	11th to 20th	Night	22% 4 8% 12% 25% 29%	1602
	21st to end	Night	21% 4 7% 16% 28% 25%	1804
September	1st to 10th	Night	19% 4 6% 14% 25% 32%	1926
	11th to 20th	Night	21% 3 7% 18% 26% 25%	1980
	21st to end	Night	20% 3 5% 16% 33% 23%	1980
October	1st to 10th	Night	27% 3 9% 22% 28% 10%	2066
	11th to 20th	Night	26% 3 8% 26% 29% 8%	2160
	21st to end	Night	26% 4 12% 29% 24% 5%	2376
November	1st to 10th	Night	31% 5% 11% 26% 21% 5%	2400
	11th to 20th	Night	32% 5% 16% 28% 16% 3	2470
	21st to end	Night	36% 6% 16% 27% 13% 3	2560
December	1st to 10th	Night	39% 7% 19% 24% 10% 2	2660
	11th to 20th	Night	47% 8% 17% 21% 8%	3272
	21st to end	Night	48% 8% 20% 15% 8%	4549

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	27% 7% 21% 24% 15% 5%	270
	11th to 20th	Day	15% 9% 27% 30% 17% 2	270
	21st to end	Day	16% 3 27% 35% 16% 3	297
February	1st to 10th	Day	21% 9% 30% 27% 10% 2	270
	11th to 20th	Day	18% 8% 31% 30% 12% 2	340
	21st to end	Day	15% 11% 33% 27% 11% 2	382
March	1st to 10th	Day	17% 11% 29% 28% 13% 2	421
	11th to 20th	Day	17% 7% 26% 32% 17% 1	420
	21st to end	Day	19% 12% 31% 26% 11%	462
April	1st to 10th	Day	18% 13% 23% 30% 14% 2	420
	11th to 20th	Day	15% 17% 31% 24% 10% 2	412
	21st to end	Day	15% 15% 27% 23% 16% 3	420
May	1st to 10th	Day	11% 12% 30% 30% 14% 3	423
	11th to 20th	Day	12% 6% 25% 34% 18% 5%	448
	21st to end	Day	9% 9% 23% 31% 22% 6%	485
June	1st to 10th	Day	5% 8% 22% 36% 24% 5%	450
	11th to 20th	Day	8% 6% 26% 33% 23% 3	508
	21st to end	Day	4 5% 25% 42% 21% 4	450
July	1st to 10th	Day	2 3 18% 45% 27% 5%	446
	11th to 20th	Day	5% 2 18% 45% 25% 4	444
	21st to end	Day	3 1 10% 48% 30% 7%	487
August	1st to 10th	Day	7% 1 18% 43% 28% 4	447
	11th to 20th	Day	5% 3 17% 38% 30% 7%	437
	21st to end	Day	5% 3 16% 41% 30% 6%	495
September	1st to 10th	Day	4 4 17% 35% 32% 7%	450
	11th to 20th	Day	8% 5% 21% 33% 28% 6%	450
	21st to end	Day	8% 8% 16% 33% 29% 6%	450
October	1st to 10th	Day	12% 8% 22% 28% 25% 5%	363
	11th to 20th	Day	9% 10% 18% 29% 26% 9%	270
	21st to end	Day	10% 6% 29% 35% 17% 2	297
November	1st to 10th	Day	20% 7% 21% 27% 20% 4	270
	11th to 20th	Day	12% 8% 20% 39% 14% 7%	270
	21st to end	Day	15% 5% 18% 31% 26% 5%	270
December	1st to 10th	Day	16% 7% 20% 32% 20% 5%	270
	11th to 20th	Day	15% 7% 23% 38% 14% 3	270
	21st to end	Day	23% 7% 20% 36% 12% 1	297
January	1st to 10th	Night	18% 4 20% 34% 20% 4	760
	11th to 20th	Night	16% 4 20% 35% 20% 4	760
	21st to end	Night	15% 5% 19% 39% 19% 3	836
February	1st to 10th	Night	18% 4 22% 40% 13% 3	760
	11th to 20th	Night	14% 6% 26% 37% 16% 1	744
	21st to end	Night	13% 7% 26% 39% 14% 1	628
March	1st to 10th	Night	18% 6% 23% 38% 13% 3	722
	11th to 20th	Night	12% 4 21% 40% 20% 3	720
	21st to end	Night	15% 8% 25% 37% 13% 2	792
April	1st to 10th	Night	19% 8% 21% 36% 15% 2	702
	11th to 20th	Night	10% 7% 28% 40% 14% 1	533
	21st to end	Night	13% 6% 22% 36% 21% 2	540
May	1st to 10th	Night	10% 7% 23% 41% 18% 1	540
	11th to 20th	Night	10% 4 17% 36% 31% 3	531
	21st to end	Night	12% 5% 18% 36% 25% 4	584
June	1st to 10th	Night	9% 7% 17% 37% 29% 2	540
	11th to 20th	Night	7% 4 22% 43% 22% 2	540
	21st to end	Night	5% 2 16% 41% 33% 3	540
July	1st to 10th	Night	6% 2 14% 32% 41% 5%	525
	11th to 20th	Night	6% 1 12% 45% 33% 4	522
	21st to end	Night	6% 1 7% 32% 52% 2	591
August	1st to 10th	Night	5% 1 12% 44% 36% 2	537
	11th to 20th	Night	4 1 10% 44% 38% 3	534
	21st to end	Night	5% 2 9% 43% 39% 2	594
September	1st to 10th	Night	5% 2 12% 40% 36% 5%	666
	11th to 20th	Night	7% 2 15% 34% 36% 6%	720
	21st to end	Night	8% 3 14% 32% 35% 8%	720
October	1st to 10th	Night	12% 3 19% 35% 27% 4	720
	11th to 20th	Night	10% 4 16% 38% 28% 5%	720
	21st to end	Night	10% 4 20% 38% 24% 4	792
November	1st to 10th	Night	13% 5% 19% 36% 23% 4	740
	11th to 20th	Night	14% 4 20% 41% 18% 4	760
	21st to end	Night	13% 5% 20% 36% 21% 4	850
December	1st to 10th	Night	17% 4 23% 36% 17% 2	950
	11th to 20th	Night	20% 6% 19% 40% 13% 2	950
	21st to end	Night	20% 6% 23% 39% 12% 1	851

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