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 2024-04-10.

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	22% 14% 6% 9% 7% 3 4 4 4 5% 21%	82982
	Night	22% 12% 5% 11% 8% 4 5% 4 4 5% 20%	85810
Transparency	Day	64% 6% 10% 14% 6% 1	82685
	Night	64% 6% 10% 13% 6%	85642
Seeing	Day	33% 21% 26% 14% 5% 1	16769
	Night	32% 17% 24% 20% 6%	24358

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% 15% 9% 21% 13% 5% 4 4 3 4 9%	2526
	11th to 20th	Day	14% 18% 7% 15% 18% 5% 6% 3 3 3 8%	2530
	21st to end	Day	13% 12% 9% 19% 11% 5% 4 4 4 5% 14%	2859
February	1st to 10th	Day	20% 13% 6% 14% 13% 6% 5% 4 4 5% 10%	1710
	11th to 20th	Day	17% 15% 8% 14% 12% 5% 6% 3 5% 4 11%	1663
	21st to end	Day	13% 16% 8% 10% 10% 6% 4 4 5% 5% 22%	1553
March	1st to 10th	Day	23% 13% 5% 7% 7% 5% 5% 2 2 4 27%	3114
	11th to 20th	Day	14% 24% 10% 12% 8% 4 3 3 4 4 14%	3480
	21st to end	Day	26% 18% 8% 11% 6% 3 4 3 3 3 15%	4025
April	1st to 10th	Day	23% 18% 6% 10% 5% 3 4 4 5% 4 20%	2271
	11th to 20th	Day	22% 14% 5% 7% 5% 2 3 3 4 6% 29%	2134
	21st to end	Day	23% 16% 5% 7% 5% 2 3 4 4 5% 26%	2326
May	1st to 10th	Day	27% 14% 3 6% 5% 3 4 4 4 5% 23%	2520
	11th to 20th	Day	22% 12% 4 6% 6% 3 5% 4 4 6% 29%	2478
	21st to end	Day	22% 11% 3 6% 5% 3 4 5% 5% 8% 27%	2724
June	1st to 10th	Day	30% 11% 4 6% 6% 3 5% 4 4 5% 22%	2520
	11th to 20th	Day	21% 10% 3 6% 5% 3 4 4 5% 7% 32%	2520
	21st to end	Day	30% 11% 3 7% 5% 3 5% 4 4 5% 22%	2520
July	1st to 10th	Day	19% 8% 3 7% 6% 4 5% 5% 6% 7% 31%	2520
	11th to 20th	Day	22% 8% 4 6% 5% 3 4 5% 7% 8% 30%	2520
	21st to end	Day	26% 10% 3 7% 6% 3 5% 5% 5% 7% 23%	2745
August	1st to 10th	Day	20% 9% 3 7% 5% 3 6% 6% 6% 8% 28%	2490
	11th to 20th	Day	23% 9% 3 8% 5% 3 4 5% 7% 6% 28%	2300
	21st to end	Day	19% 8% 3 6% 6% 4 5% 5% 5% 9% 29%	2376
September	1st to 10th	Day	22% 7% 2 6% 5% 3 4 5% 6% 7% 33%	2160
	11th to 20th	Day	24% 10% 4 7% 4 3 4 4 4 7% 29%	2160
	21st to end	Day	22% 10% 3 6% 4 3 5% 4 4 7% 34%	1919
October	1st to 10th	Day	22% 9% 4 7% 5% 3 5% 4 6% 8% 28%	1800
	11th to 20th	Day	24% 13% 5% 7% 6% 3 4 5% 5% 7% 20%	1800
	21st to end	Day	27% 16% 5% 9% 7% 3 5% 3 4 5% 16%	1828
November	1st to 10th	Day	25% 17% 6% 9% 7% 2 4 4 3 5% 18%	1642
	11th to 20th	Day	24% 23% 6% 10% 6% 4 4 3 3 4 12%	1520
	21st to end	Day	19% 22% 10% 11% 7% 4 5% 4 4 5% 10%	1520
December	1st to 10th	Day	26% 19% 12% 12% 8% 4 3 3 4 3 7%	1520
	11th to 20th	Day	33% 15% 11% 14% 7% 4 4 3 2 3 5%	1906
	21st to end	Day	22% 18% 13% 15% 8% 4 4 3 3 3 7%	2783
January	1st to 10th	Night	12% 15% 8% 24% 12% 6% 5% 4 4 3 8%	4429
	11th to 20th	Night	21% 14% 6% 22% 13% 4 5% 3 2 3 7%	4440
	21st to end	Night	13% 13% 6% 16% 14% 4 5% 5% 3 4 16%	4791
February	1st to 10th	Night	19% 13% 6% 13% 11% 6% 7% 5% 4 4 13%	2470
	11th to 20th	Night	17% 15% 7% 11% 10% 6% 5% 5% 4 5% 14%	2403
	21st to end	Night	12% 14% 6% 13% 10% 5% 5% 5% 5% 5% 20%	1900
March	1st to 10th	Night	18% 13% 4 9% 9% 4 4 4 4 4 25%	3709
	11th to 20th	Night	20% 15% 6% 11% 8% 3 5% 4 4 4 20%	3479
	21st to end	Night	28% 14% 7% 11% 7% 3 4 3 2 4 17%	3607
April	1st to 10th	Night	24% 13% 5% 7% 6% 4 4 5% 4 5% 22%	1899
	11th to 20th	Night	20% 13% 5% 6% 5% 2 3 5% 4 5% 30%	1776
	21st to end	Night	24% 13% 4 6% 6% 2 3 3 3 3 31%	1631
May	1st to 10th	Night	28% 11% 3 6% 4 2 4 3 4 6% 29%	1440
	11th to 20th	Night	21% 10% 4 6% 6% 3 4 4 5% 4 33%	1416
	21st to end	Night	19% 9% 3 6% 7% 4 6% 6% 6% 7% 27%	1550
June	1st to 10th	Night	28% 11% 4 7% 7% 4 3 4 4 6% 22%	1440
	11th to 20th	Night	23% 8% 4 5% 4 3 5% 4 4 6% 34%	1440
	21st to end	Night	29% 9% 4 8% 7% 3 4 4 5% 5% 23%	1440
July	1st to 10th	Night	19% 8% 4 7% 6% 4 6% 5% 6% 7% 30%	1440
	11th to 20th	Night	22% 8% 3 7% 5% 3 5% 6% 6% 8% 28%	1440
	21st to end	Night	24% 10% 4 8% 5% 3 4 5% 6% 7% 24%	1573
August	1st to 10th	Night	16% 6% 3 6% 7% 4 5% 6% 6% 7% 33%	1445
	11th to 20th	Night	23% 8% 3 6% 6% 3 6% 4 6% 6% 29%	1599
	21st to end	Night	16% 6% 3 6% 7% 3 5% 5% 7% 8% 34%	1980
September	1st to 10th	Night	19% 6% 3 5% 6% 3 5% 5% 5% 8% 35%	1800
	11th to 20th	Night	24% 9% 2 5% 5% 3 4 4 4 6% 34%	1800
	21st to end	Night	21% 8% 2 5% 5% 3 5% 5% 5% 6% 34%	2037
October	1st to 10th	Night	20% 9% 3 6% 5% 3 5% 5% 5% 7% 33%	2160
	11th to 20th	Night	25% 10% 3 6% 7% 3 5% 4 5% 6% 25%	2160
	21st to end	Night	26% 13% 4 9% 6% 4 5% 4 4 6% 19%	2527
November	1st to 10th	Night	23% 14% 5% 9% 6% 3 5% 5% 4 7% 19%	2432
	11th to 20th	Night	24% 17% 4 10% 8% 4 5% 5% 4 5% 14%	2660
	21st to end	Night	23% 17% 8% 11% 9% 4 5% 4 5% 4 10%	2660
December	1st to 10th	Night	25% 16% 9% 12% 8% 3 5% 5% 4 4 9%	2660
	11th to 20th	Night	33% 15% 7% 14% 9% 3 4 3 3 3 8%	3293
	21st to end	Night	28% 15% 8% 15% 8% 3 3 2 3 3 11%	4884

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	77% 5% 10% 7% 1	2527
	11th to 20th	Day	78% 6% 8% 7% 2	2530
	21st to end	Day	70% 5% 7% 7% 8% 2	2859
February	1st to 10th	Day	75% 4 7% 10% 4	1710
	11th to 20th	Day	73% 5% 10% 8% 3 1	1671
	21st to end	Day	63% 5% 8% 12% 8% 2	1553
March	1st to 10th	Day	64% 3 8% 14% 10% 1	3118
	11th to 20th	Day	72% 7% 7% 9% 5% 1	3480
	21st to end	Day	75% 4 8% 8% 5% 1	4025
April	1st to 10th	Day	66% 5% 8% 12% 8% 2	2271
	11th to 20th	Day	56% 5% 8% 17% 12% 1	2134
	21st to end	Day	60% 7% 9% 14% 9% 1	2326
May	1st to 10th	Day	63% 5% 8% 15% 8% 1	2520
	11th to 20th	Day	57% 6% 8% 19% 10% 1	2482
	21st to end	Day	55% 7% 11% 19% 8%	2724
June	1st to 10th	Day	63% 5% 10% 15% 6%	2520
	11th to 20th	Day	52% 7% 11% 20% 10% 1	2520
	21st to end	Day	63% 7% 8% 14% 8% 1	2520
July	1st to 10th	Day	51% 7% 14% 21% 7%	2450
	11th to 20th	Day	50% 7% 12% 21% 8% 1	2444
	21st to end	Day	60% 7% 12% 14% 6%	2592
August	1st to 10th	Day	51% 9% 13% 20% 6%	2476
	11th to 20th	Day	54% 8% 13% 20% 5% 1	2300
	21st to end	Day	51% 7% 13% 21% 7%	2376
September	1st to 10th	Day	48% 7% 12% 21% 10% 1	2160
	11th to 20th	Day	55% 6% 10% 20% 9% 1	2160
	21st to end	Day	49% 6% 12% 22% 10% 1	1919
October	1st to 10th	Day	53% 6% 10% 21% 9%	1800
	11th to 20th	Day	62% 6% 12% 15% 5%	1800
	21st to end	Day	69% 5% 9% 12% 4	1828
November	1st to 10th	Day	68% 5% 10% 14% 3	1642
	11th to 20th	Day	75% 5% 8% 9% 3	1520
	21st to end	Day	74% 5% 12% 6% 2	1520
December	1st to 10th	Day	82% 5% 7% 5% 1	1520
	11th to 20th	Day	85% 4 5% 4 1	1905
	21st to end	Day	82% 5% 8% 4 2	2783
January	1st to 10th	Night	75% 6% 12% 6% 1	4429
	11th to 20th	Night	81% 6% 7% 5% 2	4440
	21st to end	Night	68% 5% 9% 8% 9%	4791
February	1st to 10th	Night	72% 5% 8% 9% 5% 1	2470
	11th to 20th	Night	69% 5% 9% 13% 4 1	2406
	21st to end	Night	63% 5% 9% 13% 9% 1	1900
March	1st to 10th	Night	60% 4 9% 13% 13% 1	3718
	11th to 20th	Night	63% 7% 9% 14% 6%	3480
	21st to end	Night	72% 4 6% 11% 5% 1	3607
April	1st to 10th	Night	60% 5% 9% 14% 10%	1899
	11th to 20th	Night	54% 5% 10% 18% 12% 1	1776
	21st to end	Night	58% 4 8% 18% 11% 1	1631
May	1st to 10th	Night	56% 5% 10% 18% 10% 1	1440
	11th to 20th	Night	51% 4 11% 21% 11% 2	1417
	21st to end	Night	54% 7% 12% 20% 8%	1551
June	1st to 10th	Night	64% 5% 9% 15% 5% 1	1440
	11th to 20th	Night	51% 5% 12% 20% 12% 1	1440
	21st to end	Night	63% 6% 9% 14% 8%	1440
July	1st to 10th	Night	51% 7% 17% 18% 6%	1401
	11th to 20th	Night	52% 8% 11% 20% 8% 1	1392
	21st to end	Night	57% 8% 14% 16% 5%	1486
August	1st to 10th	Night	48% 8% 16% 20% 7%	1434
	11th to 20th	Night	52% 9% 14% 18% 6%	1599
	21st to end	Night	45% 8% 16% 23% 8%	1980
September	1st to 10th	Night	45% 8% 13% 24% 10%	1800
	11th to 20th	Night	49% 6% 10% 23% 10% 1	1800
	21st to end	Night	48% 6% 10% 25% 10% 1	2037
October	1st to 10th	Night	50% 5% 13% 20% 11% 1	2160
	11th to 20th	Night	58% 5% 12% 21% 3	2160
	21st to end	Night	65% 6% 9% 15% 5%	2527
November	1st to 10th	Night	63% 7% 14% 12% 4	2432
	11th to 20th	Night	69% 7% 11% 9% 3	2660
	21st to end	Night	72% 7% 11% 7% 3	2660
December	1st to 10th	Night	76% 6% 9% 7% 2	2660
	11th to 20th	Night	81% 4 7% 6% 2	3295
	21st to end	Night	78% 4 9% 7% 2	4884

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% 24% 10% 2	370
	11th to 20th	Day	40% 22% 24% 12% 2	370
	21st to end	Day	27% 29% 30% 11% 3	407
February	1st to 10th	Day	38% 26% 23% 11% 3	370
	11th to 20th	Day	41% 28% 21% 9% 1	366
	21st to end	Day	29% 32% 22% 14% 3	307
March	1st to 10th	Day	30% 28% 27% 11% 4	370
	11th to 20th	Day	31% 27% 27% 10% 5% 1	460
	21st to end	Day	37% 28% 22% 8% 3 1	506
April	1st to 10th	Day	41% 28% 21% 8% 2	457
	11th to 20th	Day	34% 32% 20% 11% 3	412
	21st to end	Day	32% 24% 28% 12% 2 1	478
May	1st to 10th	Day	39% 22% 22% 11% 4 2	603
	11th to 20th	Day	31% 21% 25% 14% 7% 2	624
	21st to end	Day	31% 21% 28% 15% 3 1	680
June	1st to 10th	Day	42% 13% 24% 16% 4 1	630
	11th to 20th	Day	27% 18% 32% 14% 7% 1	630
	21st to end	Day	36% 13% 27% 17% 5% 1	630
July	1st to 10th	Day	27% 15% 30% 21% 6% 1	620
	11th to 20th	Day	25% 11% 33% 22% 7% 1	618
	21st to end	Day	34% 14% 26% 18% 8%	685
August	1st to 10th	Day	22% 13% 32% 22% 10% 1	609
	11th to 20th	Day	27% 14% 31% 18% 9% 1	439
	21st to end	Day	22% 18% 35% 18% 5% 2	495
September	1st to 10th	Day	24% 19% 34% 16% 5% 1	450
	11th to 20th	Day	35% 17% 28% 14% 5% 2	450
	21st to end	Day	29% 22% 26% 15% 7% 2	365
October	1st to 10th	Day	30% 16% 29% 16% 7% 2	360
	11th to 20th	Day	32% 18% 27% 14% 6% 3	360
	21st to end	Day	40% 19% 27% 9% 6% 1	396
November	1st to 10th	Day	38% 21% 21% 13% 5% 2	365
	11th to 20th	Day	43% 20% 22% 10% 3 1	370
	21st to end	Day	40% 26% 22% 9% 3 1	370
December	1st to 10th	Day	43% 21% 22% 11% 3	370
	11th to 20th	Day	42% 25% 22% 9% 1	370
	21st to end	Day	35% 29% 21% 12% 2	407
January	1st to 10th	Night	35% 24% 24% 13% 4	850
	11th to 20th	Night	35% 25% 24% 14% 3	850
	21st to end	Night	30% 25% 25% 16% 4	909
February	1st to 10th	Night	32% 21% 27% 16% 4	760
	11th to 20th	Night	34% 28% 23% 12% 3	743
	21st to end	Night	25% 24% 28% 18% 5%	628
March	1st to 10th	Night	25% 24% 31% 17% 3	760
	11th to 20th	Night	29% 19% 25% 18% 8% 1	760
	21st to end	Night	32% 20% 26% 17% 4 1	717
April	1st to 10th	Night	34% 19% 24% 20% 3	570
	11th to 20th	Night	28% 18% 26% 25% 3 1	534
	21st to end	Night	33% 15% 24% 21% 6%	540
May	1st to 10th	Night	34% 14% 25% 18% 8% 1	540
	11th to 20th	Night	29% 12% 27% 23% 9% 1	532
	21st to end	Night	25% 17% 26% 26% 5%	583
June	1st to 10th	Night	37% 10% 25% 22% 5%	540
	11th to 20th	Night	28% 9% 27% 29% 6% 1	540
	21st to end	Night	33% 10% 27% 24% 6%	540
July	1st to 10th	Night	24% 8% 23% 34% 10% 1	526
	11th to 20th	Night	27% 6% 27% 33% 8%	522
	21st to end	Night	30% 5% 21% 34% 9%	590
August	1st to 10th	Night	20% 8% 23% 36% 12% 1	538
	11th to 20th	Night	29% 7% 20% 29% 14% 1	532
	21st to end	Night	21% 7% 22% 36% 13%	594
September	1st to 10th	Night	22% 9% 27% 31% 11%	540
	11th to 20th	Night	30% 13% 23% 21% 11% 1	540
	21st to end	Night	29% 13% 28% 23% 6% 1	607
October	1st to 10th	Night	25% 15% 23% 27% 9% 1	720
	11th to 20th	Night	32% 15% 23% 20% 9% 1	720
	21st to end	Night	37% 19% 23% 16% 5%	868
November	1st to 10th	Night	36% 17% 23% 20% 3 1	830
	11th to 20th	Night	38% 20% 21% 15% 5%	850
	21st to end	Night	36% 24% 22% 12% 4	850
December	1st to 10th	Night	41% 21% 23% 11% 4	850
	11th to 20th	Night	42% 23% 20% 11% 3	850
	21st to end	Night	39% 22% 24% 12% 3 1	935

Page generated at 2024-04-07 10:28:09. Version ac07

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