Grey Highland Skies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grey Highland Skies Observatory clear sky chart from 2005-11-06 to 2026-08-30.

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	13% 11% 4 10% 6% 5% 5% 4 5% 7% 29%	145097
	Night	12% 11% 5% 11% 6% 5% 5% 4 5% 6% 29%	136877
Transparency	Day	53% 5% 11% 20% 10% 1	148675
	Night	53% 5% 12% 19% 10% 1	141388
Seeing	Day	22% 21% 31% 17% 7% 2	17304
	Night	20% 22% 31% 19% 7% 1	26041

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	13% 13% 8% 21% 9% 7% 6% 3 4 4 11%	4428
	11th to 20th	Day	7% 18% 10% 24% 11% 11% 5% 4 4 3 4	5168
	21st to end	Day	10% 16% 7% 18% 10% 9% 9% 4 4 4 9%	5780
February	1st to 10th	Day	11% 16% 7% 16% 12% 7% 5% 3 4 4 14%	1890
	11th to 20th	Day	11% 13% 7% 18% 11% 6% 5% 5% 6% 5% 15%	1843
	21st to end	Day	14% 17% 6% 11% 8% 7% 6% 4 5% 4 19%	1636
March	1st to 10th	Day	14% 13% 5% 13% 8% 5% 4 3 4 5% 26%	4997
	11th to 20th	Day	9% 14% 7% 13% 6% 4 5% 4 5% 7% 26%	5090
	21st to end	Day	15% 16% 6% 10% 5% 4 3 3 4 5% 29%	5586
April	1st to 10th	Day	15% 16% 5% 10% 5% 4 4 3 4 6% 26%	2450
	11th to 20th	Day	16% 13% 4 7% 4 4 4 4 5% 6% 33%	2636
	21st to end	Day	18% 12% 4 8% 4 4 4 3 5% 6% 32%	2730
May	1st to 10th	Day	14% 10% 3 8% 5% 5% 4 3 6% 7% 34%	7642
	11th to 20th	Day	14% 11% 4 9% 5% 5% 5% 4 6% 7% 34%	8093
	21st to end	Day	17% 7% 2 6% 4 4 4 3 5% 7% 42%	9060
June	1st to 10th	Day	14% 7% 2 7% 5% 4 5% 5% 6% 9% 36%	2940
	11th to 20th	Day	11% 7% 3 7% 5% 4 5% 5% 6% 9% 39%	3130
	21st to end	Day	14% 7% 2 7% 5% 6% 4 4 7% 8% 36%	3150
July	1st to 10th	Day	10% 8% 3 8% 6% 5% 6% 4 7% 9% 35%	8879
	11th to 20th	Day	7% 5% 2 6% 5% 5% 5% 5% 6% 9% 45%	8387
	21st to end	Day	8% 6% 3 7% 5% 5% 5% 4 6% 8% 43%	8340
August	1st to 10th	Day	13% 7% 3 7% 6% 5% 6% 5% 6% 8% 35%	4364
	11th to 20th	Day	11% 8% 3 8% 5% 6% 6% 5% 8% 9% 30%	4304
	21st to end	Day	9% 7% 3 7% 5% 5% 6% 5% 7% 9% 38%	4674
September	1st to 10th	Day	15% 8% 3 7% 4 5% 5% 4 5% 7% 37%	2400
	11th to 20th	Day	12% 7% 2 6% 4 4 5% 4 5% 8% 43%	2234
	21st to end	Day	15% 9% 4 8% 6% 5% 5% 4 6% 7% 32%	2190
October	1st to 10th	Day	17% 10% 4 9% 5% 4 5% 3 5% 7% 32%	2000
	11th to 20th	Day	17% 15% 4 9% 6% 5% 4 3 6% 6% 24%	2000
	21st to end	Day	22% 19% 5% 9% 4 4 5% 3 5% 5% 19%	2190
November	1st to 10th	Day	13% 18% 6% 10% 6% 5% 4 3 5% 5% 26%	1762
	11th to 20th	Day	12% 18% 6% 11% 7% 6% 5% 4 5% 7% 20%	1680
	21st to end	Day	14% 19% 7% 16% 6% 5% 5% 5% 5% 5% 13%	1680
December	1st to 10th	Day	14% 22% 10% 18% 7% 5% 4 3 4 4 8%	2680
	11th to 20th	Day	17% 21% 7% 15% 6% 5% 4 3 4 4 14%	3066
	21st to end	Day	20% 20% 7% 18% 8% 6% 4 2 3 3 9%	4018
January	1st to 10th	Night	12% 12% 7% 18% 10% 7% 6% 4 5% 6% 11%	8250
	11th to 20th	Night	8% 14% 9% 22% 14% 10% 6% 3 4 3 7%	7701
	21st to end	Night	10% 11% 7% 16% 11% 9% 8% 5% 6% 5% 12%	8401
February	1st to 10th	Night	10% 15% 6% 14% 9% 7% 6% 5% 6% 5% 17%	2730
	11th to 20th	Night	10% 12% 6% 16% 9% 7% 6% 4 6% 5% 20%	2663
	21st to end	Night	11% 13% 5% 13% 8% 7% 6% 4 5% 5% 22%	2165
March	1st to 10th	Night	12% 11% 5% 12% 6% 6% 6% 5% 6% 7% 25%	5063
	11th to 20th	Night	7% 10% 6% 10% 6% 6% 5% 4 6% 7% 31%	5089
	21st to end	Night	16% 10% 4 10% 5% 5% 4 4 5% 5% 32%	5588
April	1st to 10th	Night	17% 13% 5% 8% 7% 4 4 4 5% 7% 28%	2169
	11th to 20th	Night	15% 10% 3 8% 5% 5% 4 3 4 7% 36%	1951
	21st to end	Night	13% 11% 4 10% 5% 3 5% 4 5% 6% 34%	1890
May	1st to 10th	Night	16% 9% 2 6% 4 4 4 4 5% 8% 39%	5257
	11th to 20th	Night	13% 8% 2 7% 5% 4 5% 3 5% 7% 41%	4764
	21st to end	Night	16% 7% 1 5% 3 3 3 3 4 7% 47%	5114
June	1st to 10th	Night	13% 7% 2 6% 4 4 5% 4 6% 8% 42%	1680
	11th to 20th	Night	9% 5% 2 6% 3 6% 4 5% 8% 10% 41%	1484
	21st to end	Night	10% 7% 2 7% 4 4 4 4 5% 9% 44%	1470
July	1st to 10th	Night	6% 8% 3 7% 4 4 5% 4 5% 8% 46%	4110
	11th to 20th	Night	8% 5% 2 6% 4 4 4 3 5% 8% 50%	4571
	21st to end	Night	9% 5% 2 6% 5% 4 4 3 6% 7% 48%	5737
August	1st to 10th	Night	10% 6% 2 7% 4 4 4 4 5% 7% 48%	3005
	11th to 20th	Night	12% 6% 1 6% 4 3 5% 4 5% 8% 46%	2965
	21st to end	Night	10% 6% 2 7% 4 4 5% 3 6% 7% 46%	3389
September	1st to 10th	Night	9% 7% 2 6% 3 4 4 4 7% 8% 47%	2000
	11th to 20th	Night	11% 4 2 4 3 4 3 3 6% 7% 53%	2163
	21st to end	Night	15% 9% 3 7% 4 4 4 3 5% 7% 38%	2210
October	1st to 10th	Night	15% 8% 3 7% 5% 5% 4 3 6% 8% 37%	2400
	11th to 20th	Night	16% 14% 4 8% 4 4 5% 4 4 7% 30%	2400
	21st to end	Night	17% 15% 4 8% 6% 5% 5% 4 5% 7% 23%	2645
November	1st to 10th	Night	14% 14% 4 9% 6% 5% 6% 4 5% 5% 28%	2755
	11th to 20th	Night	13% 16% 5% 10% 5% 6% 5% 4 6% 7% 22%	2940
	21st to end	Night	15% 17% 7% 12% 6% 6% 5% 4 5% 7% 16%	2940
December	1st to 10th	Night	13% 17% 8% 19% 7% 7% 6% 4 5% 5% 10%	4720
	11th to 20th	Night	10% 20% 8% 18% 8% 6% 5% 3 4 4 14%	5348
	21st to end	Night	18% 20% 9% 19% 7% 6% 4 3 3 4 7%	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	72% 5% 9% 10% 3	4429
	11th to 20th	Day	82% 3 6% 5% 3	5202
	21st to end	Day	76% 5% 8% 8% 3	5780
February	1st to 10th	Day	72% 5% 8% 12% 3 1	1890
	11th to 20th	Day	68% 5% 10% 12% 5% 1	1852
	21st to end	Day	66% 5% 10% 14% 6% 1	1636
March	1st to 10th	Day	60% 4 12% 13% 10%	5000
	11th to 20th	Day	57% 3 12% 19% 6% 2	5090
	21st to end	Day	58% 3 9% 17% 11% 2	5586
April	1st to 10th	Day	59% 5% 8% 17% 10% 1	2450
	11th to 20th	Day	50% 5% 8% 22% 14% 1	2636
	21st to end	Day	52% 5% 11% 19% 11% 2	2730
May	1st to 10th	Day	48% 4 11% 21% 12% 4	7651
	11th to 20th	Day	48% 6% 10% 19% 16% 1	8097
	21st to end	Day	42% 4 9% 27% 16% 3	9061
June	1st to 10th	Day	43% 6% 12% 21% 16% 1	2940
	11th to 20th	Day	40% 6% 12% 25% 15% 1	3130
	21st to end	Day	44% 6% 13% 24% 13% 1	3150
July	1st to 10th	Day	45% 4 14% 23% 13% 1	8806
	11th to 20th	Day	34% 4 11% 35% 15% 1	8309
	21st to end	Day	38% 4 13% 29% 16% 1	8156
August	1st to 10th	Day	45% 6% 13% 24% 12% 1	4351
	11th to 20th	Day	46% 6% 15% 25% 8%	4343
	21st to end	Day	40% 5% 13% 31% 10% 1	4674
September	1st to 10th	Day	44% 6% 13% 25% 10% 2	2400
	11th to 20th	Day	38% 6% 13% 29% 14%	2234
	21st to end	Day	49% 7% 12% 23% 8% 1	2190
October	1st to 10th	Day	47% 5% 11% 25% 12% 1	3260
	11th to 20th	Day	52% 4 11% 20% 13% 1	3240
	21st to end	Day	63% 3 11% 16% 6%	3516
November	1st to 10th	Day	59% 5% 11% 18% 7%	1762
	11th to 20th	Day	63% 5% 11% 14% 6% 1	1680
	21st to end	Day	70% 6% 11% 10% 2	1680
December	1st to 10th	Day	80% 3 9% 6% 2	2680
	11th to 20th	Day	73% 5% 7% 10% 5%	3066
	21st to end	Day	79% 6% 5% 6% 4	4018
January	1st to 10th	Night	69% 6% 12% 10% 4	8250
	11th to 20th	Night	79% 3 8% 7% 3	7742
	21st to end	Night	68% 6% 10% 8% 7%	8401
February	1st to 10th	Night	64% 5% 12% 14% 4	2730
	11th to 20th	Night	63% 5% 12% 14% 6%	2665
	21st to end	Night	61% 5% 12% 14% 7%	2165
March	1st to 10th	Night	54% 7% 13% 16% 9% 2	5073
	11th to 20th	Night	50% 4 17% 20% 8% 2	5090
	21st to end	Night	52% 4 11% 17% 14% 2	5588
April	1st to 10th	Night	55% 4 9% 18% 13% 1	2169
	11th to 20th	Night	48% 5% 8% 21% 16% 1	1951
	21st to end	Night	50% 5% 11% 17% 15% 2	1890
May	1st to 10th	Night	44% 4 11% 21% 15% 6%	5264
	11th to 20th	Night	41% 7% 11% 24% 16% 2	4765
	21st to end	Night	37% 4 10% 29% 17% 2	5115
June	1st to 10th	Night	39% 5% 11% 23% 19% 2	1680
	11th to 20th	Night	35% 7% 13% 27% 18% 1	1484
	21st to end	Night	37% 6% 14% 27% 15% 1	1470
July	1st to 10th	Night	35% 3 18% 27% 14% 2	4075
	11th to 20th	Night	32% 3 13% 33% 19% 1	4526
	21st to end	Night	34% 3 12% 32% 17% 1	5606
August	1st to 10th	Night	36% 4 14% 28% 17% 2	2994
	11th to 20th	Night	35% 5% 16% 29% 14% 1	2992
	21st to end	Night	36% 4 14% 32% 13% 1	3389
September	1st to 10th	Night	33% 8% 15% 31% 13% 1	2000
	11th to 20th	Night	29% 5% 14% 32% 18% 1	2163
	21st to end	Night	46% 5% 13% 25% 10% 1	2210
October	1st to 10th	Night	38% 5% 9% 27% 21% 1	3920
	11th to 20th	Night	48% 4 12% 20% 14% 2	3899
	21st to end	Night	51% 5% 15% 22% 7%	4269
November	1st to 10th	Night	53% 6% 13% 19% 8% 1	2755
	11th to 20th	Night	58% 6% 14% 15% 6% 1	2940
	21st to end	Night	65% 7% 14% 11% 2 1	2940
December	1st to 10th	Night	73% 4 14% 8% 2	4720
	11th to 20th	Night	72% 4 8% 11% 5%	5348
	21st to end	Night	80% 5% 5% 7% 2	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	25% 33% 28% 12% 1	220
	11th to 20th	Day	25% 37% 28% 8% 2	316
	21st to end	Day	28% 31% 32% 10% 1	363
February	1st to 10th	Day	29% 32% 30% 7% 2	330
	11th to 20th	Day	28% 33% 34% 4 1	330
	21st to end	Day	35% 34% 27% 4	273
March	1st to 10th	Day	26% 29% 31% 11% 3	519
	11th to 20th	Day	29% 25% 32% 11% 3 1	540
	21st to end	Day	35% 23% 28% 11% 3	594
April	1st to 10th	Day	29% 26% 33% 9% 4	540
	11th to 20th	Day	34% 22% 23% 12% 6% 2	532
	21st to end	Day	29% 24% 30% 12% 4 2	540
May	1st to 10th	Day	22% 26% 30% 15% 5% 1	543
	11th to 20th	Day	19% 20% 32% 19% 7% 3	567
	21st to end	Day	21% 18% 31% 20% 8% 3	617
June	1st to 10th	Day	17% 20% 34% 21% 7% 1	570
	11th to 20th	Day	16% 20% 34% 22% 7% 1	760
	21st to end	Day	18% 16% 38% 19% 6% 2	780
July	1st to 10th	Day	14% 12% 33% 27% 10% 3	771
	11th to 20th	Day	15% 14% 34% 22% 10% 5%	608
	21st to end	Day	11% 12% 40% 24% 9% 4	620
August	1st to 10th	Day	12% 12% 31% 29% 11% 5%	566
	11th to 20th	Day	12% 11% 32% 25% 14% 6%	559
	21st to end	Day	14% 13% 33% 24% 11% 4	619
September	1st to 10th	Day	17% 16% 27% 25% 10% 5%	530
	11th to 20th	Day	11% 11% 26% 26% 19% 7%	530
	21st to end	Day	18% 17% 28% 21% 13% 3	530
October	1st to 10th	Day	22% 21% 26% 17% 11% 2	530
	11th to 20th	Day	23% 29% 32% 12% 3	530
	21st to end	Day	31% 27% 28% 12% 2	573
November	1st to 10th	Day	30% 30% 25% 11% 3	282
	11th to 20th	Day	27% 29% 32% 8% 2 1	220
	21st to end	Day	31% 26% 36% 7%	220
December	1st to 10th	Day	27% 36% 28% 9%	220
	11th to 20th	Day	34% 29% 28% 8% 1	220
	21st to end	Day	29% 34% 28% 7% 2	242
January	1st to 10th	Night	21% 35% 31% 11% 2	856
	11th to 20th	Night	25% 30% 30% 13% 2	840
	21st to end	Night	18% 26% 36% 16% 4	924
February	1st to 10th	Night	20% 36% 31% 12% 1	840
	11th to 20th	Night	22% 29% 34% 13% 2	823
	21st to end	Night	24% 32% 31% 10% 2	692
March	1st to 10th	Night	23% 28% 33% 14% 3	840
	11th to 20th	Night	23% 28% 32% 14% 3	840
	21st to end	Night	23% 22% 32% 18% 4	924
April	1st to 10th	Night	25% 25% 34% 15% 2	840
	11th to 20th	Night	22% 23% 29% 19% 6% 1	707
	21st to end	Night	22% 20% 31% 20% 7%	630
May	1st to 10th	Night	17% 17% 32% 26% 8% 1	630
	11th to 20th	Night	18% 13% 33% 25% 9%	463
	21st to end	Night	14% 15% 31% 31% 10%	455
June	1st to 10th	Night	16% 12% 38% 27% 7% 1	420
	11th to 20th	Night	14% 14% 28% 34% 10%	420
	21st to end	Night	14% 12% 33% 28% 12% 1	420
July	1st to 10th	Night	9% 6% 33% 36% 16%	410
	11th to 20th	Night	9% 11% 35% 29% 15% 1	409
	21st to end	Night	10% 8% 34% 33% 14% 1	690
August	1st to 10th	Night	10% 7% 30% 34% 17% 1	627
	11th to 20th	Night	8% 8% 27% 34% 21% 2	624
	21st to end	Night	14% 9% 32% 27% 17% 2	733
September	1st to 10th	Night	13% 13% 28% 28% 15% 4	800
	11th to 20th	Night	12% 10% 28% 28% 18% 5%	800
	21st to end	Night	19% 14% 24% 27% 14% 3	800
October	1st to 10th	Night	20% 16% 33% 18% 13% 2	800
	11th to 20th	Night	26% 22% 28% 17% 6% 1	800
	21st to end	Night	26% 25% 31% 15% 3	880
November	1st to 10th	Night	26% 23% 30% 16% 5%	820
	11th to 20th	Night	23% 32% 26% 14% 5% 1	840
	21st to end	Night	26% 28% 30% 13% 2	840
December	1st to 10th	Night	26% 30% 32% 9% 3	840
	11th to 20th	Night	22% 34% 32% 10% 2	840
	21st to end	Night	29% 31% 27% 12% 1	924

Page generated at 2026-08-27 04:07:15. Version ac07

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