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 2024-11-04.

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% 13% 5% 10% 6% 5% 4 3 5% 6% 22%	101617
	Night	21% 13% 5% 12% 6% 5% 4 3 4 5% 21%	97504
Transparency	Day	62% 6% 11% 15% 6% 1	105108
	Night	62% 6% 11% 14% 6%	102111
Seeing	Day	33% 21% 27% 14% 5% 1	17661
	Night	32% 17% 24% 20% 6%	25032

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% 25% 9% 6% 4 3 3 3 9%	2526
	11th to 20th	Day	14% 19% 6% 23% 11% 6% 5% 3 3 3 8%	2530
	21st to end	Day	13% 13% 8% 21% 9% 5% 4 3 4 5% 14%	2859
February	1st to 10th	Day	19% 14% 6% 16% 11% 7% 4 3 4 4 10%	1710
	11th to 20th	Day	16% 15% 8% 15% 12% 7% 5% 3 5% 4 11%	1663
	21st to end	Day	12% 18% 6% 10% 9% 6% 3 3 5% 5% 22%	1553
March	1st to 10th	Day	23% 14% 4 9% 5% 7% 3 2 3 4 27%	3114
	11th to 20th	Day	14% 25% 9% 14% 7% 5% 3 3 3 4 14%	3480
	21st to end	Day	25% 19% 8% 12% 5% 4 4 2 3 3 15%	4025
April	1st to 10th	Day	22% 19% 5% 10% 4 3 4 3 5% 4 20%	2280
	11th to 20th	Day	22% 15% 4 8% 4 3 4 2 4 5% 29%	2254
	21st to end	Day	22% 17% 5% 8% 5% 3 3 3 4 5% 27%	2456
May	1st to 10th	Day	25% 16% 4 9% 5% 4 3 4 4 5% 20%	4425
	11th to 20th	Day	19% 11% 4 9% 6% 4 4 3 5% 6% 28%	4398
	21st to end	Day	22% 12% 3 7% 6% 5% 5% 5% 6% 7% 22%	4836
June	1st to 10th	Day	29% 13% 3 8% 5% 4 4 3 5% 5% 21%	2660
	11th to 20th	Day	20% 10% 3 7% 4 4 4 3 5% 7% 32%	2660
	21st to end	Day	30% 11% 3 8% 4 4 4 3 4 5% 21%	2660
July	1st to 10th	Day	23% 8% 3 8% 5% 5% 5% 4 6% 6% 26%	4440
	11th to 20th	Day	24% 8% 4 7% 5% 4 4 4 7% 7% 26%	4440
	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% 18% 5% 10% 6% 4 4 3 3 5% 18%	1669
	11th to 20th	Day	23% 24% 6% 12% 5% 5% 3 3 3 4 12%	1520
	21st to end	Day	19% 23% 9% 13% 5% 5% 4 4 4 5% 10%	1520
December	1st to 10th	Day	25% 22% 10% 13% 7% 5% 3 2 4 3 7%	1520
	11th to 20th	Day	32% 17% 9% 16% 6% 4 4 2 3 3 5%	1906
	21st to end	Day	22% 19% 12% 16% 7% 5% 4 2 3 3 7%	2783
January	1st to 10th	Night	12% 16% 8% 27% 9% 7% 5% 3 4 3 7%	4429
	11th to 20th	Night	21% 15% 5% 26% 9% 5% 4 2 2 3 7%	4440
	21st to end	Night	13% 13% 6% 18% 12% 5% 5% 3 3 4 16%	4791
February	1st to 10th	Night	19% 14% 5% 15% 10% 8% 6% 4 4 4 12%	2470
	11th to 20th	Night	17% 16% 7% 13% 9% 7% 5% 3 4 4 14%	2403
	21st to end	Night	12% 15% 6% 13% 10% 6% 4 4 5% 4 20%	1900
March	1st to 10th	Night	18% 13% 4 10% 8% 5% 4 3 5% 4 25%	3709
	11th to 20th	Night	20% 15% 5% 13% 7% 5% 5% 3 4 4 20%	3479
	21st to end	Night	28% 15% 7% 12% 6% 4 3 3 2 4 17%	3607
April	1st to 10th	Night	23% 14% 5% 7% 6% 5% 4 4 4 5% 22%	1900
	11th to 20th	Night	19% 14% 5% 7% 4 4 4 4 4 5% 30%	1876
	21st to end	Night	24% 13% 4 7% 6% 4 3 3 3 3 31%	1720
May	1st to 10th	Night	24% 16% 3 7% 5% 5% 6% 3 5% 6% 21%	2525
	11th to 20th	Night	17% 11% 4 7% 5% 5% 4 4 5% 5% 33%	2506
	21st to end	Night	19% 11% 4 9% 6% 6% 5% 4 6% 8% 22%	2749
June	1st to 10th	Night	28% 13% 4 8% 6% 4 3 3 4 6% 21%	1520
	11th to 20th	Night	22% 9% 3 6% 4 4 4 3 4 6% 35%	1520
	21st to end	Night	29% 10% 4 9% 6% 4 4 3 5% 5% 22%	1520
July	1st to 10th	Night	22% 9% 4 10% 5% 5% 5% 4 6% 5% 24%	2530
	11th to 20th	Night	26% 8% 3 7% 4 3 4 4 6% 6% 28%	2530
	21st to end	Night	22% 8% 3 7% 4 4 5% 4 5% 6% 33%	2750
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% 14% 4 9% 5% 5% 5% 4 4 6% 19%	2478
	11th to 20th	Night	24% 18% 4 12% 6% 5% 5% 4 4 4 14%	2660
	21st to end	Night	23% 17% 8% 13% 7% 5% 5% 3 5% 4 10%	2660
December	1st to 10th	Night	25% 18% 8% 14% 6% 5% 5% 3 4 4 9%	2660
	11th to 20th	Night	32% 17% 5% 15% 7% 4 4 2 3 3 8%	3293
	21st to end	Night	27% 16% 7% 17% 6% 4 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	2280
	11th to 20th	Day	57% 5% 8% 17% 13% 1	2254
	21st to end	Day	59% 6% 9% 14% 11% 1	2456
May	1st to 10th	Day	64% 7% 10% 13% 5% 1	4440
	11th to 20th	Day	54% 8% 11% 19% 7%	4402
	21st to end	Day	56% 8% 13% 16% 6%	4836
June	1st to 10th	Day	64% 5% 10% 15% 6%	2660
	11th to 20th	Day	51% 7% 12% 20% 10% 1	2660
	21st to end	Day	64% 6% 8% 14% 8% 1	2660
July	1st to 10th	Day	57% 7% 12% 15% 9% 1	4370
	11th to 20th	Day	54% 7% 14% 20% 5%	4364
	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	1669
	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% 11%	1900
	11th to 20th	Night	54% 5% 10% 18% 12% 1	1876
	21st to end	Night	57% 4 8% 18% 11% 2	1720
May	1st to 10th	Night	58% 10% 12% 14% 6% 1	2530
	11th to 20th	Night	50% 6% 14% 22% 7% 1	2507
	21st to end	Night	55% 8% 13% 18% 5%	2750
June	1st to 10th	Night	65% 5% 9% 15% 5% 1	1520
	11th to 20th	Night	50% 5% 12% 21% 12% 1	1520
	21st to end	Night	63% 6% 9% 14% 7%	1520
July	1st to 10th	Night	59% 7% 13% 13% 6% 1	2491
	11th to 20th	Night	56% 7% 12% 17% 8% 1	2482
	21st to end	Night	50% 8% 15% 19% 9%	2663
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% 7% 14% 12% 4	2478
	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% 29% 20% 8% 2	460
	11th to 20th	Day	35% 31% 20% 11% 3	452
	21st to end	Day	32% 25% 28% 12% 2 1	522
May	1st to 10th	Day	40% 22% 21% 11% 4 2	653
	11th to 20th	Day	29% 21% 25% 16% 7% 2	674
	21st to end	Day	30% 22% 29% 15% 3 1	735
June	1st to 10th	Day	41% 15% 24% 15% 4 1	680
	11th to 20th	Day	27% 19% 33% 14% 6% 1	680
	21st to end	Day	37% 14% 26% 16% 5% 1	680
July	1st to 10th	Day	27% 15% 30% 21% 6% 1	670
	11th to 20th	Day	25% 12% 33% 22% 7% 1	668
	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	37% 22% 21% 13% 5% 2	374
	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% 20% 23% 20% 3	570
	11th to 20th	Night	27% 18% 26% 24% 3 1	564
	21st to end	Night	33% 15% 25% 21% 6%	570
May	1st to 10th	Night	34% 14% 25% 18% 8% 1	570
	11th to 20th	Night	28% 12% 27% 22% 10% 1	562
	21st to end	Night	26% 17% 26% 26% 5%	616
June	1st to 10th	Night	37% 11% 25% 22% 5%	570
	11th to 20th	Night	28% 9% 28% 29% 6% 1	570
	21st to end	Night	33% 11% 27% 23% 6%	570
July	1st to 10th	Night	25% 8% 23% 34% 10% 1	556
	11th to 20th	Night	27% 7% 26% 32% 8%	552
	21st to end	Night	30% 5% 22% 34% 9%	623
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	36% 18% 23% 20% 3 1	848
	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 2024-11-01 23:47:43. Version ac07

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