Gairloch Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gairloch Mountain Observatory 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	21% 14% 5% 11% 6% 5% 4 3 5% 5% 22%	120481
	Night	20% 13% 5% 13% 7% 5% 4 3 4 5% 20%	118818
Transparency	Day	62% 6% 10% 15% 6%	123998
	Night	63% 7% 10% 14% 6%	123471
Seeing	Day	33% 21% 26% 14% 5% 1	18654
	Night	32% 17% 24% 20% 6%	26035

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	12% 23% 10% 21% 8% 5% 4 3 3 3 9%	3616
	11th to 20th	Day	14% 17% 11% 22% 9% 5% 4 3 4 3 8%	3591
	21st to end	Day	11% 11% 7% 21% 12% 9% 4 3 4 5% 15%	4097
February	1st to 10th	Day	19% 14% 6% 17% 11% 7% 4 3 4 4 10%	1800
	11th to 20th	Day	16% 15% 8% 15% 12% 7% 5% 3 5% 4 11%	1753
	21st to end	Day	13% 18% 6% 11% 9% 6% 3 4 5% 5% 22%	1633
March	1st to 10th	Day	22% 14% 4 13% 9% 6% 3 2 2 4 21%	4474
	11th to 20th	Day	15% 19% 7% 12% 7% 5% 4 4 4 5% 19%	4980
	21st to end	Day	25% 17% 8% 15% 5% 4 4 3 3 3 12%	5759
April	1st to 10th	Day	22% 19% 6% 11% 5% 3 4 3 5% 4 19%	2400
	11th to 20th	Day	22% 15% 4 7% 4 3 4 2 4 6% 28%	2374
	21st to end	Day	22% 17% 5% 8% 5% 3 3 3 4 5% 26%	2585
May	1st to 10th	Day	26% 17% 5% 8% 4 4 3 3 4 5% 21%	6345
	11th to 20th	Day	19% 13% 3 7% 5% 4 4 3 5% 6% 30%	6318
	21st to end	Day	22% 12% 3 6% 5% 4 4 4 5% 7% 28%	6948
June	1st to 10th	Day	28% 12% 3 8% 5% 4 4 3 5% 5% 22%	2800
	11th to 20th	Day	20% 10% 3 7% 4 4 4 4 5% 7% 33%	2800
	21st to end	Day	30% 11% 3 9% 4 4 4 3 5% 5% 22%	2800
July	1st to 10th	Day	23% 9% 3 8% 6% 5% 5% 4 6% 6% 26%	4580
	11th to 20th	Day	23% 8% 4 7% 5% 4 4 4 7% 7% 27%	4580
	21st to end	Day	19% 8% 2 7% 5% 5% 5% 4 6% 7% 32%	4815
August	1st to 10th	Day	22% 10% 4 9% 5% 5% 6% 5% 6% 7% 22%	4394
	11th to 20th	Day	23% 8% 3 8% 4 4 4 4 7% 8% 28%	4027
	21st to end	Day	14% 8% 3 7% 4 5% 5% 4 6% 9% 35%	4180
September	1st to 10th	Day	21% 8% 2 7% 4 4 4 5% 6% 7% 33%	2280
	11th to 20th	Day	23% 10% 4 7% 4 4 4 4 5% 7% 30%	2280
	21st to end	Day	21% 10% 3 7% 4 4 5% 3 4 7% 32%	2025
October	1st to 10th	Day	22% 10% 4 8% 4 4 4 3 6% 7% 28%	1900
	11th to 20th	Day	23% 14% 4 8% 5% 4 4 4 5% 7% 21%	1900
	21st to end	Day	25% 17% 5% 10% 6% 4 4 3 4 5% 17%	1929
November	1st to 10th	Day	24% 19% 5% 10% 5% 3 4 3 3 5% 17%	1730
	11th to 20th	Day	24% 25% 6% 12% 5% 5% 3 2 3 4 12%	1600
	21st to end	Day	19% 24% 9% 12% 5% 5% 4 3 4 5% 10%	1600
December	1st to 10th	Day	17% 20% 12% 17% 7% 5% 4 3 4 4 9%	2610
	11th to 20th	Day	30% 17% 7% 18% 5% 4 3 2 2 3 9%	2996
	21st to end	Day	20% 17% 9% 18% 7% 5% 4 3 3 3 12%	3982
January	1st to 10th	Night	14% 20% 9% 23% 8% 6% 4 2 3 3 7%	6349
	11th to 20th	Night	17% 15% 8% 26% 8% 5% 5% 3 3 4 7%	6314
	21st to end	Night	9% 13% 6% 19% 16% 10% 5% 3 4 3 12%	6861
February	1st to 10th	Night	18% 14% 5% 16% 11% 8% 6% 4 4 4 12%	2600
	11th to 20th	Night	17% 15% 6% 13% 10% 8% 5% 4 4 5% 14%	2533
	21st to end	Night	12% 15% 6% 13% 9% 6% 5% 4 5% 4 20%	1996
March	1st to 10th	Night	21% 13% 4 12% 11% 6% 3 3 4 4 19%	5349
	11th to 20th	Night	18% 13% 5% 13% 7% 4 5% 3 5% 5% 23%	4979
	21st to end	Night	27% 16% 6% 15% 6% 4 3 2 2 3 16%	5173
April	1st to 10th	Night	24% 14% 5% 7% 6% 4 4 4 4 5% 22%	2000
	11th to 20th	Night	19% 14% 5% 8% 4 4 4 4 5% 5% 29%	1976
	21st to end	Night	24% 13% 4 7% 5% 4 3 3 3 3 31%	1810
May	1st to 10th	Night	25% 16% 4 7% 4 5% 5% 3 4 5% 21%	3615
	11th to 20th	Night	16% 14% 4 7% 4 4 4 4 5% 6% 30%	3596
	21st to end	Night	16% 12% 4 9% 5% 5% 5% 4 5% 7% 28%	3948
June	1st to 10th	Night	27% 12% 4 8% 6% 4 3 3 4 6% 23%	1600
	11th to 20th	Night	22% 9% 3 6% 3 4 4 3 4 6% 35%	1600
	21st to end	Night	29% 9% 3 9% 6% 4 4 3 5% 5% 22%	1600
July	1st to 10th	Night	22% 9% 4 10% 5% 5% 5% 4 6% 6% 24%	2610
	11th to 20th	Night	26% 8% 3 7% 4 3 4 4 6% 6% 28%	2610
	21st to end	Night	22% 8% 3 7% 4 4 5% 4 5% 6% 33%	2751
August	1st to 10th	Night	21% 9% 3 8% 6% 5% 5% 4 6% 6% 26%	2541
	11th to 20th	Night	24% 8% 3 7% 5% 4 4 4 6% 6% 30%	2799
	21st to end	Night	16% 6% 3 6% 4 3 4 3 6% 7% 41%	3476
September	1st to 10th	Night	19% 6% 2 6% 5% 5% 4 4 5% 7% 35%	1900
	11th to 20th	Night	23% 9% 2 6% 4 4 3 3 4 6% 36%	1900
	21st to end	Night	21% 9% 2 6% 4 4 5% 4 5% 6% 34%	2151
October	1st to 10th	Night	20% 9% 3 7% 4 4 5% 4 5% 6% 33%	2280
	11th to 20th	Night	24% 10% 3 8% 6% 5% 5% 3 5% 6% 26%	2280
	21st to end	Night	26% 14% 4 9% 5% 5% 4 3 4 6% 19%	2668
November	1st to 10th	Night	23% 15% 4 9% 6% 5% 5% 4 4 6% 19%	2564
	11th to 20th	Night	25% 18% 4 11% 6% 5% 5% 4 4 4 14%	2800
	21st to end	Night	24% 18% 8% 12% 7% 5% 5% 3 5% 4 9%	2800
December	1st to 10th	Night	19% 14% 7% 16% 6% 5% 5% 5% 5% 5% 13%	4580
	11th to 20th	Night	28% 16% 5% 17% 6% 4 4 2 3 3 11%	5213
	21st to end	Night	23% 15% 6% 17% 6% 4 3 3 4 4 15%	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	78% 5% 8% 6% 2	3617
	11th to 20th	Day	78% 6% 9% 6% 1	3620
	21st to end	Day	70% 6% 7% 10% 7% 1	4097
February	1st to 10th	Day	75% 4 7% 10% 4	1800
	11th to 20th	Day	73% 6% 10% 8% 3 1	1761
	21st to end	Day	63% 5% 9% 12% 8% 2	1633
March	1st to 10th	Day	70% 3 7% 11% 8% 1	4478
	11th to 20th	Day	63% 9% 13% 10% 4	4980
	21st to end	Day	75% 6% 7% 8% 4 1	5759
April	1st to 10th	Day	66% 5% 8% 11% 8% 2	2400
	11th to 20th	Day	57% 5% 8% 17% 12% 1	2374
	21st to end	Day	60% 7% 9% 14% 10% 1	2585
May	1st to 10th	Day	65% 7% 8% 14% 6% 1	6357
	11th to 20th	Day	53% 7% 13% 18% 8%	6322
	21st to end	Day	53% 7% 11% 22% 7%	6948
June	1st to 10th	Day	62% 5% 10% 15% 7% 1	2800
	11th to 20th	Day	50% 7% 12% 20% 10% 1	2800
	21st to end	Day	63% 6% 8% 14% 8% 1	2800
July	1st to 10th	Day	57% 7% 12% 15% 8% 1	4510
	11th to 20th	Day	53% 7% 14% 20% 5%	4504
	21st to end	Day	47% 8% 17% 21% 6%	4662
August	1st to 10th	Day	58% 8% 12% 17% 5%	4380
	11th to 20th	Day	51% 8% 14% 22% 4	4066
	21st to end	Day	44% 7% 12% 25% 11%	4180
September	1st to 10th	Day	48% 7% 12% 21% 11% 1	2280
	11th to 20th	Day	53% 6% 12% 21% 8% 1	2280
	21st to end	Day	50% 6% 12% 22% 10% 1	2025
October	1st to 10th	Day	56% 7% 10% 19% 9%	3160
	11th to 20th	Day	54% 7% 11% 20% 7%	3140
	21st to end	Day	60% 7% 14% 16% 3	3163
November	1st to 10th	Day	68% 6% 10% 13% 3	1730
	11th to 20th	Day	77% 5% 8% 8% 2	1600
	21st to end	Day	75% 5% 12% 6% 3	1600
December	1st to 10th	Day	76% 8% 7% 9% 1	2610
	11th to 20th	Day	81% 4 5% 6% 3	2995
	21st to end	Day	76% 5% 8% 8% 3	3982
January	1st to 10th	Night	79% 5% 9% 6% 1	6349
	11th to 20th	Night	79% 8% 7% 5% 2	6360
	21st to end	Night	72% 6% 8% 7% 7%	6861
February	1st to 10th	Night	73% 5% 8% 8% 5% 1	2600
	11th to 20th	Night	69% 5% 9% 13% 4 1	2536
	21st to end	Night	63% 6% 9% 13% 9% 1	1996
March	1st to 10th	Night	65% 6% 8% 10% 11% 1	5358
	11th to 20th	Night	58% 8% 13% 13% 8%	4980
	21st to end	Night	74% 4 5% 11% 5% 1	5173
April	1st to 10th	Night	61% 6% 9% 13% 11%	2000
	11th to 20th	Night	54% 5% 10% 18% 12% 1	1976
	21st to end	Night	57% 4 8% 19% 11% 2	1810
May	1st to 10th	Night	61% 8% 9% 13% 8% 1	3620
	11th to 20th	Night	51% 6% 14% 21% 7% 1	3597
	21st to end	Night	53% 8% 11% 22% 7%	3949
June	1st to 10th	Night	63% 5% 9% 16% 6% 1	1600
	11th to 20th	Night	49% 5% 12% 21% 13% 1	1600
	21st to end	Night	63% 6% 9% 14% 8%	1600
July	1st to 10th	Night	59% 7% 13% 14% 6% 1	2571
	11th to 20th	Night	55% 7% 12% 17% 8% 1	2562
	21st to end	Night	50% 8% 15% 19% 9%	2664
August	1st to 10th	Night	55% 9% 15% 17% 4	2530
	11th to 20th	Night	52% 9% 16% 19% 3	2826
	21st to end	Night	39% 7% 15% 29% 10%	3475
September	1st to 10th	Night	45% 8% 13% 24% 10%	1900
	11th to 20th	Night	47% 6% 11% 24% 10% 1	1900
	21st to end	Night	48% 6% 10% 26% 9% 1	2151
October	1st to 10th	Night	51% 7% 13% 19% 10%	3800
	11th to 20th	Night	50% 8% 12% 24% 6%	3779
	21st to end	Night	57% 7% 13% 18% 5%	4393
November	1st to 10th	Night	62% 8% 14% 12% 4	2564
	11th to 20th	Night	70% 7% 11% 9% 3	2800
	21st to end	Night	73% 7% 11% 7% 3	2800
December	1st to 10th	Night	66% 9% 10% 13% 2	4580
	11th to 20th	Night	74% 7% 8% 9% 2	5215
	21st to end	Night	71% 6% 9% 11% 4	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	36% 28% 25% 10% 2	400
	11th to 20th	Day	39% 21% 24% 14% 2 1	400
	21st to end	Day	29% 28% 30% 10% 3	440
February	1st to 10th	Day	39% 26% 23% 10% 3	400
	11th to 20th	Day	43% 27% 20% 8% 1	396
	21st to end	Day	29% 31% 23% 13% 3	331
March	1st to 10th	Day	32% 28% 27% 10% 4	400
	11th to 20th	Day	30% 27% 28% 9% 4 1	500
	21st to end	Day	39% 28% 21% 8% 3 1	550
April	1st to 10th	Day	43% 29% 19% 8% 2	500
	11th to 20th	Day	35% 31% 20% 10% 3	492
	21st to end	Day	33% 25% 28% 11% 2 1	565
May	1st to 10th	Day	41% 21% 21% 10% 4 2	703
	11th to 20th	Day	31% 22% 24% 15% 7% 2	724
	21st to end	Day	30% 21% 30% 15% 3 1	790
June	1st to 10th	Day	40% 15% 24% 16% 4 1	730
	11th to 20th	Day	26% 20% 33% 14% 6% 1	730
	21st to end	Day	38% 15% 26% 15% 5% 1	730
July	1st to 10th	Day	28% 15% 30% 21% 5% 1	720
	11th to 20th	Day	25% 12% 33% 22% 7% 1	718
	21st to end	Day	32% 14% 27% 18% 8%	740
August	1st to 10th	Day	22% 14% 32% 21% 10% 1	658
	11th to 20th	Day	27% 15% 31% 18% 8% 1	479
	21st to end	Day	22% 19% 34% 18% 5% 1	539
September	1st to 10th	Day	24% 20% 34% 16% 5% 1	490
	11th to 20th	Day	32% 16% 30% 15% 5% 2	490
	21st to end	Day	28% 22% 26% 17% 6% 2	395
October	1st to 10th	Day	31% 16% 28% 16% 7% 2	390
	11th to 20th	Day	34% 18% 26% 13% 5% 3	390
	21st to end	Day	40% 19% 27% 8% 6%	429
November	1st to 10th	Day	38% 23% 20% 12% 5% 2	395
	11th to 20th	Day	44% 21% 22% 10% 3 1	400
	21st to end	Day	41% 26% 22% 8% 3 1	400
December	1st to 10th	Day	41% 21% 23% 11% 3	400
	11th to 20th	Day	43% 25% 23% 9% 2	400
	21st to end	Day	35% 28% 22% 12% 2	440
January	1st to 10th	Night	35% 24% 24% 12% 3	900
	11th to 20th	Night	34% 24% 23% 14% 4	900
	21st to end	Night	31% 25% 25% 15% 4	961
February	1st to 10th	Night	32% 22% 26% 16% 4	800
	11th to 20th	Night	35% 28% 23% 11% 3	783
	21st to end	Night	25% 24% 28% 18% 5%	660
March	1st to 10th	Night	27% 24% 31% 16% 3	800
	11th to 20th	Night	28% 19% 27% 18% 8% 1	800
	21st to end	Night	34% 20% 26% 16% 4 1	755
April	1st to 10th	Night	35% 20% 23% 19% 3	600
	11th to 20th	Night	28% 18% 26% 24% 3 1	594
	21st to end	Night	33% 16% 25% 20% 6%	600
May	1st to 10th	Night	35% 14% 25% 18% 8% 1	600
	11th to 20th	Night	28% 13% 28% 22% 10% 1	592
	21st to end	Night	25% 17% 27% 27% 5%	649
June	1st to 10th	Night	37% 11% 26% 22% 5%	600
	11th to 20th	Night	28% 10% 28% 28% 6% 1	600
	21st to end	Night	33% 11% 28% 23% 6%	600
July	1st to 10th	Night	24% 8% 24% 34% 10% 1	586
	11th to 20th	Night	26% 7% 25% 33% 8%	582
	21st to end	Night	30% 5% 21% 34% 9%	624
August	1st to 10th	Night	20% 9% 23% 35% 12% 1	568
	11th to 20th	Night	29% 7% 20% 29% 15% 1	562
	21st to end	Night	21% 7% 22% 36% 14%	627
September	1st to 10th	Night	23% 9% 27% 31% 10%	570
	11th to 20th	Night	29% 13% 24% 23% 11% 1	570
	21st to end	Night	29% 13% 27% 23% 6% 1	641
October	1st to 10th	Night	25% 15% 22% 27% 9% 1	760
	11th to 20th	Night	33% 15% 23% 20% 8% 1	760
	21st to end	Night	37% 20% 22% 16% 5%	921
November	1st to 10th	Night	37% 18% 23% 19% 3 1	880
	11th to 20th	Night	39% 20% 21% 14% 5%	900
	21st to end	Night	38% 24% 22% 12% 4	900
December	1st to 10th	Night	40% 21% 23% 11% 4	900
	11th to 20th	Night	42% 23% 20% 11% 3	900
	21st to end	Night	38% 22% 25% 12% 3 1	990

Page generated at 2025-07-18 02:10:04. Version ac07

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