Highlands Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Highlands Observatory clear sky chart from 2005-11-06 to 2026-01-02.

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	16% 8% 3 8% 5% 5% 5% 4 6% 7% 32%	127497
	Night	16% 8% 3 8% 5% 4 5% 4 6% 7% 34%	123863
Transparency	Day	49% 6% 11% 21% 12% 1	131000
	Night	47% 6% 13% 21% 12% 1	128447
Seeing	Day	26% 16% 27% 18% 10% 3	16248
	Night	23% 15% 27% 24% 9% 1	25506

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	10% 11% 6% 18% 9% 6% 5% 3 5% 6% 22%	3605
	11th to 20th	Day	12% 13% 7% 18% 7% 6% 5% 4 4 5% 19%	3570
	21st to end	Day	17% 11% 6% 10% 6% 4 4 3 4 6% 29%	4399
February	1st to 10th	Day	16% 11% 4 11% 8% 5% 6% 5% 6% 5% 24%	1800
	11th to 20th	Day	12% 12% 3 10% 7% 5% 5% 4 5% 8% 28%	1861
	21st to end	Day	18% 12% 3 9% 7% 4 4 3 6% 6% 28%	1650
March	1st to 10th	Day	19% 10% 4 9% 5% 4 4 3 5% 5% 31%	4476
	11th to 20th	Day	12% 8% 4 8% 5% 5% 5% 4 5% 6% 37%	4969
	21st to end	Day	17% 9% 2 6% 5% 5% 6% 4 5% 7% 33%	5802
April	1st to 10th	Day	17% 12% 3 9% 5% 5% 4 4 4 6% 32%	2400
	11th to 20th	Day	16% 11% 3 7% 5% 5% 5% 3 5% 6% 34%	2374
	21st to end	Day	17% 9% 3 8% 6% 5% 5% 3 4 6% 35%	2400
May	1st to 10th	Day	29% 8% 2 7% 5% 5% 4 4 5% 6% 26%	6104
	11th to 20th	Day	21% 8% 3 7% 6% 6% 5% 4 5% 7% 29%	6318
	21st to end	Day	20% 7% 2 5% 5% 5% 6% 6% 8% 9% 29%	6949
June	1st to 10th	Day	15% 6% 3 8% 6% 5% 6% 4 6% 9% 32%	2800
	11th to 20th	Day	17% 7% 3 6% 6% 5% 5% 5% 7% 8% 31%	2800
	21st to end	Day	14% 6% 3 7% 5% 5% 5% 4 6% 10% 35%	2800
July	1st to 10th	Day	8% 6% 3 8% 6% 6% 6% 5% 7% 10% 36%	6360
	11th to 20th	Day	10% 6% 2 7% 6% 5% 7% 6% 9% 11% 31%	6360
	21st to end	Day	13% 6% 2 6% 5% 4 5% 4 6% 8% 39%	6926
August	1st to 10th	Day	20% 6% 2 8% 5% 5% 5% 4 6% 8% 30%	4282
	11th to 20th	Day	15% 6% 2 7% 4 5% 6% 4 7% 10% 33%	3925
	21st to end	Day	17% 5% 2 5% 3 4 5% 4 6% 8% 41%	4312
September	1st to 10th	Day	16% 7% 2 6% 4 4 4 4 6% 8% 39%	2400
	11th to 20th	Day	13% 6% 2 6% 3 4 5% 3 5% 8% 45%	2371
	21st to end	Day	19% 6% 2 7% 4 4 4 4 7% 7% 34%	2190
October	1st to 10th	Day	18% 7% 2 7% 5% 4 5% 4 5% 7% 36%	2000
	11th to 20th	Day	15% 7% 3 7% 5% 6% 5% 3 7% 6% 36%	2000
	21st to end	Day	21% 10% 3 7% 5% 4 4 4 6% 6% 30%	2090
November	1st to 10th	Day	12% 8% 3 8% 4 5% 4 3 5% 6% 41%	1851
	11th to 20th	Day	15% 11% 3 9% 5% 5% 4 4 5% 5% 34%	1879
	21st to end	Day	17% 12% 3 8% 6% 5% 5% 3 6% 7% 28%	1680
December	1st to 10th	Day	16% 10% 5% 10% 7% 6% 6% 5% 6% 6% 24%	2680
	11th to 20th	Day	19% 13% 5% 9% 4 4 4 3 4 5% 30%	3066
	21st to end	Day	17% 13% 6% 12% 5% 5% 5% 3 5% 6% 23%	4048
January	1st to 10th	Night	13% 8% 4 17% 8% 5% 4 4 6% 7% 24%	6364
	11th to 20th	Night	12% 10% 4 13% 6% 6% 6% 5% 6% 8% 24%	6315
	21st to end	Night	15% 10% 4 10% 7% 6% 6% 4 6% 6% 26%	6554
February	1st to 10th	Night	15% 11% 4 8% 6% 6% 4 4 5% 7% 30%	2600
	11th to 20th	Night	13% 9% 3 10% 6% 5% 6% 4 7% 8% 28%	2424
	21st to end	Night	17% 9% 3 9% 6% 5% 6% 5% 5% 6% 29%	1980
March	1st to 10th	Night	18% 10% 3 8% 6% 4 4 4 5% 6% 32%	5364
	11th to 20th	Night	12% 8% 4 7% 6% 5% 5% 4 5% 7% 37%	4988
	21st to end	Night	15% 10% 3 8% 5% 4 5% 4 6% 6% 33%	5128
April	1st to 10th	Night	16% 11% 3 8% 6% 5% 6% 4 5% 6% 30%	2000
	11th to 20th	Night	17% 10% 3 6% 5% 4 4 3 5% 7% 35%	1975
	21st to end	Night	18% 8% 2 7% 5% 4 5% 4 5% 7% 37%	2000
May	1st to 10th	Night	32% 7% 3 6% 4 4 3 4 5% 6% 28%	3875
	11th to 20th	Night	21% 9% 3 7% 5% 4 4 4 5% 6% 33%	3575
	21st to end	Night	20% 8% 2 6% 4 4 5% 4 6% 7% 33%	3926
June	1st to 10th	Night	17% 6% 2 6% 5% 4 5% 4 5% 8% 40%	1600
	11th to 20th	Night	15% 6% 2 6% 5% 4 3 3 7% 9% 41%	1600
	21st to end	Night	10% 4 2 5% 5% 4 5% 5% 7% 10% 44%	1600
July	1st to 10th	Night	10% 7% 3 7% 5% 4 5% 4 6% 8% 41%	3600
	11th to 20th	Night	13% 7% 3 6% 5% 5% 4 5% 7% 9% 37%	3600
	21st to end	Night	12% 6% 2 5% 4 3 4 4 6% 7% 48%	3926
August	1st to 10th	Night	20% 5% 2 6% 5% 4 5% 4 7% 8% 33%	2849
	11th to 20th	Night	17% 6% 3 6% 4 3 4 3 5% 7% 41%	3089
	21st to end	Night	14% 4 2 4 4 3 4 3 5% 9% 48%	3586
September	1st to 10th	Night	16% 5% 2 6% 4 3 4 3 5% 7% 45%	2000
	11th to 20th	Night	13% 5% 1 4 2 4 3 3 5% 7% 51%	2027
	21st to end	Night	17% 6% 2 7% 5% 5% 4 3 5% 6% 39%	2208
October	1st to 10th	Night	19% 7% 2 5% 3 4 4 3 6% 7% 41%	2400
	11th to 20th	Night	18% 8% 2 5% 3 4 3 3 5% 6% 42%	2400
	21st to end	Night	20% 10% 2 6% 3 4 4 4 5% 9% 33%	2750
November	1st to 10th	Night	12% 8% 2 7% 4 4 4 4 5% 6% 43%	2667
	11th to 20th	Night	18% 8% 2 6% 4 4 4 4 7% 7% 35%	2735
	21st to end	Night	14% 9% 3 8% 5% 5% 5% 4 7% 7% 33%	2940
December	1st to 10th	Night	14% 10% 4 9% 5% 4 5% 4 6% 8% 29%	4720
	11th to 20th	Night	22% 10% 4 7% 4 4 4 3 5% 5% 33%	5348
	21st to end	Night	20% 13% 6% 9% 5% 4 4 4 5% 6% 23%	7150

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	63% 5% 8% 19% 5% 1	3606
	11th to 20th	Day	65% 6% 7% 13% 7% 2	3600
	21st to end	Day	56% 4 8% 21% 10%	4399
February	1st to 10th	Day	58% 8% 10% 15% 9%	1800
	11th to 20th	Day	53% 6% 9% 21% 11% 1	1870
	21st to end	Day	55% 5% 10% 20% 9% 1	1650
March	1st to 10th	Day	54% 4 11% 16% 11% 4	4479
	11th to 20th	Day	45% 5% 15% 24% 7% 4	4969
	21st to end	Day	49% 5% 9% 13% 20% 4	5802
April	1st to 10th	Day	54% 4 7% 21% 10% 3	2400
	11th to 20th	Day	49% 6% 11% 19% 14% 2	2374
	21st to end	Day	49% 6% 9% 20% 12% 4	2400
May	1st to 10th	Day	58% 5% 11% 14% 9% 2	6115
	11th to 20th	Day	54% 7% 9% 20% 10% 1	6322
	21st to end	Day	48% 7% 12% 20% 11% 1	6949
June	1st to 10th	Day	47% 7% 12% 21% 13%	2800
	11th to 20th	Day	48% 7% 12% 22% 10% 1	2800
	21st to end	Day	44% 7% 13% 25% 11% 1	2800
July	1st to 10th	Day	44% 7% 13% 23% 12% 1	6290
	11th to 20th	Day	43% 8% 16% 23% 10%	6285
	21st to end	Day	41% 5% 13% 24% 17% 1	6728
August	1st to 10th	Day	51% 6% 13% 19% 10%	4268
	11th to 20th	Day	44% 7% 16% 26% 7%	3963
	21st to end	Day	39% 6% 11% 32% 11% 1	4312
September	1st to 10th	Day	43% 6% 11% 25% 14% 1	2400
	11th to 20th	Day	37% 5% 11% 24% 21% 1	2371
	21st to end	Day	46% 6% 12% 20% 15% 2	2190
October	1st to 10th	Day	44% 7% 7% 23% 17% 2	3260
	11th to 20th	Day	40% 5% 8% 26% 16% 5%	3240
	21st to end	Day	36% 4 16% 27% 14% 2	3354
November	1st to 10th	Day	43% 6% 12% 23% 15% 2	1851
	11th to 20th	Day	49% 6% 11% 20% 12% 2	1879
	21st to end	Day	54% 6% 14% 19% 6% 1	1680
December	1st to 10th	Day	58% 6% 13% 17% 5%	2680
	11th to 20th	Day	57% 5% 11% 16% 10% 1	3066
	21st to end	Day	60% 5% 10% 14% 10% 1	4048
January	1st to 10th	Night	57% 6% 10% 17% 9% 1	6364
	11th to 20th	Night	55% 5% 15% 16% 6% 2	6360
	21st to end	Night	55% 6% 9% 13% 14% 2	6554
February	1st to 10th	Night	52% 6% 13% 19% 9% 1	2600
	11th to 20th	Night	51% 7% 12% 18% 10% 2	2426
	21st to end	Night	52% 5% 10% 20% 10% 2	1980
March	1st to 10th	Night	51% 5% 13% 16% 13% 1	5374
	11th to 20th	Night	45% 5% 18% 20% 7% 4	4989
	21st to end	Night	49% 5% 8% 19% 17% 2	5128
April	1st to 10th	Night	53% 5% 10% 18% 11% 2	2000
	11th to 20th	Night	48% 5% 10% 20% 15%	1975
	21st to end	Night	46% 6% 10% 21% 15% 3	2000
May	1st to 10th	Night	57% 5% 11% 17% 9% 2	3881
	11th to 20th	Night	51% 6% 12% 19% 11% 1	3576
	21st to end	Night	49% 6% 12% 21% 10% 1	3927
June	1st to 10th	Night	43% 6% 11% 24% 16% 1	1600
	11th to 20th	Night	40% 6% 17% 26% 10% 1	1600
	21st to end	Night	33% 10% 17% 26% 14% 1	1600
July	1st to 10th	Night	41% 6% 16% 21% 15% 1	3560
	11th to 20th	Night	43% 6% 15% 23% 12% 1	3552
	21st to end	Night	35% 4 13% 29% 17% 1	3809
August	1st to 10th	Night	48% 7% 16% 21% 7% 1	2839
	11th to 20th	Night	42% 6% 15% 26% 10%	3117
	21st to end	Night	34% 5% 13% 34% 14%	3586
September	1st to 10th	Night	37% 7% 12% 27% 17% 1	2000
	11th to 20th	Night	33% 5% 12% 28% 22% 1	2027
	21st to end	Night	45% 5% 9% 22% 17% 1	2208
October	1st to 10th	Night	43% 6% 10% 24% 15% 1	3920
	11th to 20th	Night	35% 5% 12% 24% 19% 5%	3899
	21st to end	Night	36% 4 17% 27% 15% 1	4436
November	1st to 10th	Night	40% 5% 12% 22% 18% 2	2667
	11th to 20th	Night	46% 5% 13% 21% 13% 3	2735
	21st to end	Night	47% 6% 15% 23% 8% 1	2940
December	1st to 10th	Night	50% 7% 14% 25% 5%	4720
	11th to 20th	Night	54% 4 13% 16% 12% 1	5348
	21st to end	Night	59% 5% 10% 14% 11% 1	7150

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	35% 23% 27% 13% 1	202
	11th to 20th	Day	41% 25% 24% 8% 2	200
	21st to end	Day	29% 29% 29% 9% 3 1	317
February	1st to 10th	Day	34% 28% 30% 6% 2	300
	11th to 20th	Day	40% 25% 29% 5% 1	409
	21st to end	Day	37% 22% 27% 11% 2	414
March	1st to 10th	Day	35% 20% 28% 15% 3	500
	11th to 20th	Day	32% 23% 28% 13% 4 1	500
	21st to end	Day	34% 24% 27% 10% 5% 1	550
April	1st to 10th	Day	35% 22% 27% 11% 4 1	500
	11th to 20th	Day	32% 23% 25% 11% 8% 2	492
	21st to end	Day	29% 20% 31% 12% 7% 2	500
May	1st to 10th	Day	38% 15% 25% 13% 8% 2	503
	11th to 20th	Day	25% 18% 28% 19% 7% 3	527
	21st to end	Day	22% 12% 28% 21% 13% 4	573
June	1st to 10th	Day	22% 12% 32% 20% 11% 3	530
	11th to 20th	Day	22% 12% 26% 23% 13% 2	530
	21st to end	Day	18% 13% 32% 25% 9% 3	530
July	1st to 10th	Day	12% 7% 30% 29% 17% 5%	525
	11th to 20th	Day	10% 9% 26% 32% 18% 4	524
	21st to end	Day	15% 8% 35% 26% 12% 4	576
August	1st to 10th	Day	14% 7% 25% 26% 19% 8%	526
	11th to 20th	Day	15% 8% 23% 30% 18% 6%	519
	21st to end	Day	16% 10% 26% 26% 12% 9%	583
September	1st to 10th	Day	22% 11% 20% 22% 18% 7%	530
	11th to 20th	Day	14% 8% 25% 23% 19% 11%	530
	21st to end	Day	24% 10% 27% 22% 13% 4	530
October	1st to 10th	Day	25% 12% 21% 21% 14% 7%	530
	11th to 20th	Day	22% 18% 29% 22% 8% 2	530
	21st to end	Day	29% 18% 27% 18% 7% 1	522
November	1st to 10th	Day	25% 19% 23% 19% 11% 2	425
	11th to 20th	Day	30% 23% 25% 16% 5%	419
	21st to end	Day	37% 22% 28% 11% 2	220
December	1st to 10th	Day	33% 25% 29% 8% 5%	220
	11th to 20th	Day	40% 19% 28% 10% 3	220
	21st to end	Day	33% 23% 27% 12% 4	242
January	1st to 10th	Night	26% 25% 31% 15% 3	805
	11th to 20th	Night	29% 20% 30% 19% 3	800
	21st to end	Night	22% 23% 36% 16% 3	880
February	1st to 10th	Night	29% 19% 34% 17% 1	800
	11th to 20th	Night	29% 24% 33% 13% 1	783
	21st to end	Night	29% 22% 32% 14% 2	660
March	1st to 10th	Night	27% 23% 31% 16% 3	800
	11th to 20th	Night	28% 21% 29% 20% 3	800
	21st to end	Night	24% 23% 32% 17% 4 1	880
April	1st to 10th	Night	28% 20% 31% 18% 3	800
	11th to 20th	Night	24% 17% 34% 19% 5% 1	791
	21st to end	Night	24% 17% 30% 22% 6%	800
May	1st to 10th	Night	30% 13% 24% 24% 7% 1	525
	11th to 20th	Night	22% 10% 31% 28% 9% 1	394
	21st to end	Night	19% 7% 24% 35% 14% 1	433
June	1st to 10th	Night	18% 6% 25% 35% 15% 1	400
	11th to 20th	Night	17% 9% 26% 33% 16%	400
	21st to end	Night	13% 5% 21% 46% 15%	400
July	1st to 10th	Night	13% 5% 20% 40% 21% 2	390
	11th to 20th	Night	13% 4 18% 45% 18% 1	388
	21st to end	Night	16% 5% 20% 39% 20%	438
August	1st to 10th	Night	14% 2 19% 39% 23% 3	557
	11th to 20th	Night	16% 4 13% 40% 23% 3	718
	21st to end	Night	18% 6% 17% 33% 25% 2	880
September	1st to 10th	Night	17% 8% 23% 28% 22% 2	800
	11th to 20th	Night	16% 6% 21% 29% 24% 5%	800
	21st to end	Night	21% 8% 22% 31% 16% 2	800
October	1st to 10th	Night	24% 11% 20% 24% 19% 1	800
	11th to 20th	Night	25% 13% 25% 29% 8%	800
	21st to end	Night	27% 16% 28% 22% 6%	880
November	1st to 10th	Night	19% 15% 30% 25% 9% 1	820
	11th to 20th	Night	28% 18% 25% 22% 6% 1	840
	21st to end	Night	22% 20% 31% 21% 5%	840
December	1st to 10th	Night	28% 21% 30% 18% 3	840
	11th to 20th	Night	28% 21% 28% 20% 3	840
	21st to end	Night	29% 22% 29% 18% 3	924

Page generated at 2025-12-30 01:14:10. Version ac07

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