Barrhead
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Barrhead clear sky chart from 2005-11-06 to 2026-07-01.

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	10% 9% 3 9% 6% 5% 5% 4 6% 8% 35%	142532
	Night	9% 10% 4 10% 6% 5% 5% 4 6% 8% 33%	138487
Transparency	Day	45% 7% 18% 23% 8%	142223
	Night	48% 6% 16% 21% 9% 1	138379
Seeing	Day	13% 6% 23% 31% 20% 7%	18290
	Night	13% 5% 26% 37% 17% 2	26686

By Month

Percentage Cloud Cover NW (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	9% 15% 4 11% 7% 5% 6% 5% 5% 7% 25%	3740
	11th to 20th	Day	3 9% 3 9% 5% 6% 5% 5% 7% 9% 38%	4119
	21st to end	Day	7% 11% 3 9% 5% 5% 5% 4 6% 7% 36%	4906
February	1st to 10th	Day	8% 13% 5% 10% 7% 6% 7% 6% 8% 8% 23%	1680
	11th to 20th	Day	5% 13% 5% 11% 6% 6% 7% 5% 7% 7% 27%	1737
	21st to end	Day	5% 12% 5% 11% 8% 7% 6% 4 5% 8% 27%	1641
March	1st to 10th	Day	5% 10% 5% 11% 7% 6% 7% 5% 7% 8% 30%	4618
	11th to 20th	Day	6% 12% 4 10% 6% 5% 5% 4 6% 8% 34%	4991
	21st to end	Day	9% 15% 6% 16% 6% 5% 5% 4 6% 7% 22%	5781
April	1st to 10th	Day	8% 12% 4 9% 7% 6% 7% 6% 7% 7% 28%	2601
	11th to 20th	Day	8% 12% 3 9% 6% 6% 5% 5% 7% 8% 32%	2700
	21st to end	Day	11% 11% 4 9% 6% 5% 6% 4 6% 7% 31%	2762
May	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	8630
	11th to 20th	Day	13% 11% 3 9% 6% 6% 6% 5% 6% 8% 27%	8755
	21st to end	Day	11% 6% 3 8% 6% 6% 6% 5% 7% 10% 33%	9649
June	1st to 10th	Day	16% 8% 3 7% 6% 5% 5% 5% 6% 7% 31%	3229
	11th to 20th	Day	18% 9% 3 8% 7% 6% 6% 5% 7% 7% 24%	3360
	21st to end	Day	10% 6% 3 7% 5% 6% 6% 5% 7% 8% 36%	3360
July	1st to 10th	Day	8% 4 2 6% 5% 5% 6% 5% 7% 9% 43%	7241
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 4 7% 8% 43%	6882
	21st to end	Day	11% 5% 2 6% 4 4 5% 4 6% 9% 43%	7376
August	1st to 10th	Day	13% 6% 2 5% 4 4 5% 4 6% 9% 42%	4642
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 4 7% 9% 42%	4358
	21st to end	Day	11% 5% 2 6% 4 5% 5% 4 6% 8% 42%	4664
September	1st to 10th	Day	11% 6% 3 6% 4 5% 5% 4 6% 8% 42%	2521
	11th to 20th	Day	13% 6% 2 6% 4 4 5% 3 6% 8% 43%	2223
	21st to end	Day	9% 7% 2 7% 5% 4 5% 4 5% 9% 42%	2200
October	1st to 10th	Day	9% 8% 4 9% 5% 4 5% 4 6% 8% 39%	3411
	11th to 20th	Day	12% 10% 4 8% 5% 4 4 3 5% 8% 36%	2932
	21st to end	Day	8% 12% 5% 9% 6% 5% 4 3 6% 7% 36%	3178
November	1st to 10th	Day	11% 14% 5% 9% 6% 6% 5% 4 6% 7% 26%	1622
	11th to 20th	Day	7% 11% 6% 10% 7% 5% 4 5% 6% 7% 32%	1470
	21st to end	Day	6% 12% 6% 13% 7% 5% 5% 5% 7% 7% 28%	1470
December	1st to 10th	Day	11% 15% 5% 12% 8% 6% 5% 4 5% 6% 23%	2360
	11th to 20th	Day	9% 15% 5% 11% 7% 7% 6% 5% 7% 6% 21%	2665
	21st to end	Day	4 8% 3 10% 6% 6% 5% 4 6% 9% 39%	3058
January	1st to 10th	Night	7% 14% 5% 15% 8% 6% 5% 4 6% 6% 22%	9149
	11th to 20th	Night	3 8% 3 9% 7% 6% 5% 4 6% 8% 40%	8779
	21st to end	Night	7% 13% 4 11% 6% 4 5% 4 6% 7% 32%	9321
February	1st to 10th	Night	6% 12% 5% 13% 8% 6% 6% 5% 7% 6% 26%	2940
	11th to 20th	Night	5% 12% 4 11% 7% 7% 6% 5% 6% 8% 28%	2768
	21st to end	Night	5% 13% 4 11% 7% 6% 7% 5% 6% 7% 29%	2165
March	1st to 10th	Night	4 7% 5% 13% 7% 6% 6% 4 7% 9% 32%	5496
	11th to 20th	Night	6% 10% 4 12% 7% 6% 6% 5% 6% 9% 30%	5228
	21st to end	Night	11% 14% 6% 14% 6% 6% 5% 3 6% 6% 25%	5432
April	1st to 10th	Night	8% 10% 4 10% 6% 7% 7% 5% 7% 8% 29%	2019
	11th to 20th	Night	10% 10% 5% 9% 6% 6% 6% 5% 6% 8% 29%	1871
	21st to end	Night	11% 10% 5% 7% 6% 5% 6% 4 6% 8% 34%	1854
May	1st to 10th	Night	13% 7% 3 7% 5% 5% 4 3 5% 8% 41%	4288
	11th to 20th	Night	12% 10% 4 11% 7% 6% 7% 5% 6% 7% 24%	4105
	21st to end	Night	11% 7% 2 8% 6% 5% 5% 4 6% 10% 37%	4522
June	1st to 10th	Night	17% 9% 4 7% 5% 5% 5% 4 6% 7% 31%	1387
	11th to 20th	Night	15% 10% 3 10% 7% 5% 6% 7% 9% 9% 18%	1260
	21st to end	Night	11% 6% 2 7% 5% 6% 6% 5% 7% 8% 36%	1260
July	1st to 10th	Night	10% 5% 2 6% 4 5% 4 5% 6% 9% 45%	2720
	11th to 20th	Night	11% 6% 2 7% 5% 5% 5% 5% 7% 9% 37%	3078
	21st to end	Night	12% 6% 2 5% 5% 4 5% 4 6% 8% 42%	3467
August	1st to 10th	Night	11% 7% 2 6% 5% 5% 5% 4 6% 8% 40%	2505
	11th to 20th	Night	14% 7% 3 7% 5% 5% 5% 4 6% 9% 36%	2692
	21st to end	Night	12% 5% 2 7% 4 3 4 3 7% 9% 44%	3245
September	1st to 10th	Night	11% 5% 3 6% 4 4 4 3 6% 8% 46%	1876
	11th to 20th	Night	12% 7% 2 5% 4 3 4 4 6% 8% 44%	2176
	21st to end	Night	10% 9% 2 6% 4 5% 5% 4 7% 8% 42%	2200
October	1st to 10th	Night	8% 8% 3 8% 5% 6% 6% 4 7% 9% 36%	3782
	11th to 20th	Night	10% 7% 3 8% 5% 5% 5% 4 6% 7% 40%	4216
	21st to end	Night	9% 9% 4 8% 5% 6% 5% 5% 7% 9% 34%	4600
November	1st to 10th	Night	13% 15% 5% 10% 6% 4 4 4 5% 6% 27%	2881
	11th to 20th	Night	8% 14% 5% 12% 6% 6% 6% 5% 5% 7% 28%	3150
	21st to end	Night	6% 14% 5% 12% 7% 6% 5% 4 5% 8% 27%	3150
December	1st to 10th	Night	10% 17% 5% 12% 6% 5% 5% 4 5% 6% 25%	5040
	11th to 20th	Night	11% 15% 6% 11% 7% 6% 6% 5% 6% 7% 20%	5725
	21st to end	Night	3 7% 3 11% 6% 5% 5% 4 6% 8% 41%	8140

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	57% 4 12% 13% 13%	3740
	11th to 20th	Day	42% 5% 17% 17% 18% 1	4140
	21st to end	Day	46% 5% 23% 20% 5%	4907
February	1st to 10th	Day	54% 7% 15% 15% 9%	1680
	11th to 20th	Day	52% 8% 15% 17% 9%	1745
	21st to end	Day	52% 7% 16% 17% 7%	1641
March	1st to 10th	Day	50% 6% 15% 21% 8%	4618
	11th to 20th	Day	46% 6% 16% 23% 9%	4992
	21st to end	Day	58% 6% 12% 18% 6%	5781
April	1st to 10th	Day	50% 9% 17% 16% 7%	2601
	11th to 20th	Day	47% 7% 15% 21% 10% 1	2700
	21st to end	Day	50% 6% 14% 22% 7% 1	2762
May	1st to 10th	Day	40% 5% 11% 29% 14% 1	8630
	11th to 20th	Day	51% 7% 11% 26% 5%	8759
	21st to end	Day	43% 7% 21% 22% 7% 1	9650
June	1st to 10th	Day	49% 8% 14% 20% 7% 1	3229
	11th to 20th	Day	56% 9% 15% 16% 4	3360
	21st to end	Day	42% 9% 21% 22% 6% 1	3360
July	1st to 10th	Day	35% 7% 20% 27% 10% 1	7162
	11th to 20th	Day	36% 7% 16% 31% 10%	6798
	21st to end	Day	36% 7% 23% 25% 8%	7169
August	1st to 10th	Day	38% 7% 22% 31% 2	4624
	11th to 20th	Day	36% 8% 23% 25% 8%	4400
	21st to end	Day	38% 7% 28% 23% 5%	4663
September	1st to 10th	Day	39% 8% 21% 25% 7% 1	2521
	11th to 20th	Day	39% 7% 22% 25% 8%	2223
	21st to end	Day	39% 8% 17% 27% 8% 2	2200
October	1st to 10th	Day	43% 5% 25% 21% 6%	3411
	11th to 20th	Day	47% 5% 15% 26% 7%	2932
	21st to end	Day	48% 5% 19% 25% 3	3178
November	1st to 10th	Day	55% 7% 15% 15% 7% 1	1622
	11th to 20th	Day	50% 6% 18% 19% 7%	1470
	21st to end	Day	53% 8% 15% 17% 5% 1	1470
December	1st to 10th	Day	61% 6% 16% 11% 4 1	2360
	11th to 20th	Day	59% 7% 16% 13% 4	2667
	21st to end	Day	41% 6% 14% 27% 12% 1	3058
January	1st to 10th	Night	60% 4 11% 14% 10% 1	9150
	11th to 20th	Night	41% 5% 17% 18% 18% 1	8820
	21st to end	Night	50% 5% 21% 18% 6%	9321
February	1st to 10th	Night	55% 6% 13% 16% 9% 1	2940
	11th to 20th	Night	52% 7% 14% 15% 11% 1	2770
	21st to end	Night	50% 7% 13% 20% 8% 1	2165
March	1st to 10th	Night	46% 6% 20% 21% 8%	5496
	11th to 20th	Night	47% 6% 17% 20% 9%	5228
	21st to end	Night	57% 6% 11% 16% 9%	5432
April	1st to 10th	Night	47% 9% 14% 19% 10% 1	2019
	11th to 20th	Night	49% 7% 14% 17% 11% 2	1871
	21st to end	Night	47% 7% 9% 23% 13% 1	1854
May	1st to 10th	Night	41% 5% 11% 26% 15% 2	4285
	11th to 20th	Night	53% 8% 12% 19% 7%	4105
	21st to end	Night	42% 6% 22% 23% 7%	4526
June	1st to 10th	Night	49% 5% 14% 20% 10%	1387
	11th to 20th	Night	56% 9% 15% 16% 3	1260
	21st to end	Night	41% 10% 22% 22% 6%	1260
July	1st to 10th	Night	35% 7% 21% 23% 14% 1	2690
	11th to 20th	Night	40% 7% 14% 29% 9%	3039
	21st to end	Night	38% 6% 23% 26% 6%	3369
August	1st to 10th	Night	40% 8% 21% 29% 1	2498
	11th to 20th	Night	43% 7% 19% 20% 10%	2716
	21st to end	Night	35% 6% 27% 26% 6%	3245
September	1st to 10th	Night	35% 6% 19% 30% 8% 2	1876
	11th to 20th	Night	36% 6% 18% 29% 11% 1	2176
	21st to end	Night	39% 7% 16% 26% 11% 2	2200
October	1st to 10th	Night	43% 6% 21% 23% 7%	3782
	11th to 20th	Night	42% 5% 16% 29% 8%	4216
	21st to end	Night	45% 6% 24% 21% 4	4600
November	1st to 10th	Night	57% 6% 12% 19% 6%	2881
	11th to 20th	Night	54% 8% 13% 18% 7% 1	3150
	21st to end	Night	54% 7% 15% 17% 5% 1	3150
December	1st to 10th	Night	60% 6% 14% 15% 5% 1	5040
	11th to 20th	Night	62% 6% 16% 12% 4	5722
	21st to end	Night	40% 5% 15% 28% 11% 1	8140

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	13% 5% 20% 35% 19% 7%	220
	11th to 20th	Day	10% 10% 30% 28% 16% 5%	220
	21st to end	Day	11% 9% 29% 30% 16% 5%	242
February	1st to 10th	Day	13% 7% 24% 30% 21% 5%	220
	11th to 20th	Day	10% 6% 30% 32% 15% 7%	272
	21st to end	Day	6% 7% 30% 36% 17% 4	357
March	1st to 10th	Day	11% 10% 25% 33% 19% 3	540
	11th to 20th	Day	11% 5% 25% 36% 17% 6%	540
	21st to end	Day	17% 8% 26% 29% 16% 4	594
April	1st to 10th	Day	11% 9% 27% 34% 14% 4	540
	11th to 20th	Day	14% 7% 24% 31% 19% 5%	532
	21st to end	Day	15% 9% 27% 28% 16% 6%	540
May	1st to 10th	Day	13% 10% 24% 30% 18% 4	694
	11th to 20th	Day	12% 5% 23% 33% 21% 6%	775
	21st to end	Day	15% 5% 20% 31% 21% 7%	844
June	1st to 10th	Day	18% 9% 24% 29% 15% 4	859
	11th to 20th	Day	19% 6% 18% 28% 22% 7%	990
	21st to end	Day	12% 4 18% 27% 29% 10%	990
July	1st to 10th	Day	12% 3 17% 35% 24% 8%	917
	11th to 20th	Day	11% 5% 19% 30% 24% 10%	765
	21st to end	Day	11% 3 16% 30% 27% 12%	794
August	1st to 10th	Day	11% 4 17% 29% 21% 18%	589
	11th to 20th	Day	9% 5% 23% 35% 21% 8%	517
	21st to end	Day	13% 6% 17% 28% 24% 12%	583
September	1st to 10th	Day	14% 7% 25% 24% 21% 9%	530
	11th to 20th	Day	17% 5% 20% 30% 21% 8%	530
	21st to end	Day	12% 9% 30% 29% 16% 4	530
October	1st to 10th	Day	14% 8% 23% 31% 18% 5%	435
	11th to 20th	Day	16% 5% 29% 31% 15% 4	330
	21st to end	Day	14% 9% 36% 26% 11% 3	363
November	1st to 10th	Day	18% 8% 23% 31% 17% 2	316
	11th to 20th	Day	10% 3 32% 36% 13% 5%	220
	21st to end	Day	9% 4 31% 35% 13% 8%	220
December	1st to 10th	Day	9% 7% 25% 42% 11% 6%	220
	11th to 20th	Day	11% 7% 27% 35% 17% 2	220
	21st to end	Day	10% 3 24% 28% 29% 7%	242
January	1st to 10th	Night	12% 4 30% 37% 14% 2	1121
	11th to 20th	Night	10% 8% 30% 37% 13% 2	1050
	21st to end	Night	14% 8% 33% 32% 10% 2	1044
February	1st to 10th	Night	10% 7% 24% 37% 18% 3	840
	11th to 20th	Night	10% 5% 29% 37% 18% 1	824
	21st to end	Night	8% 5% 26% 40% 18% 4	692
March	1st to 10th	Night	9% 4 29% 38% 18% 2	840
	11th to 20th	Night	12% 4 28% 39% 15% 3	840
	21st to end	Night	15% 5% 23% 38% 17% 3	924
April	1st to 10th	Night	11% 6% 25% 40% 17% 2	759
	11th to 20th	Night	15% 5% 28% 36% 14% 2	623
	21st to end	Night	16% 7% 25% 37% 15% 1	594
May	1st to 10th	Night	14% 6% 25% 38% 15% 2	420
	11th to 20th	Night	13% 4 25% 37% 20% 2	414
	21st to end	Night	19% 4 21% 35% 21% 1	455
June	1st to 10th	Night	20% 6% 20% 39% 14% 1	420
	11th to 20th	Night	18% 5% 18% 33% 23% 3	420
	21st to end	Night	13% 2 15% 39% 27% 4	420
July	1st to 10th	Night	14% 3 15% 38% 29% 1	390
	11th to 20th	Night	12% 4 16% 40% 26% 2	388
	21st to end	Night	12% 4 14% 41% 26% 3	438
August	1st to 10th	Night	12% 4 15% 33% 32% 5%	398
	11th to 20th	Night	13% 3 17% 40% 26% 2	459
	21st to end	Night	14% 3 17% 40% 25% 1	660
September	1st to 10th	Night	14% 3 17% 39% 24% 2	676
	11th to 20th	Night	14% 3 17% 42% 23% 1	800
	21st to end	Night	13% 4 31% 36% 14% 2	800
October	1st to 10th	Night	16% 4 25% 38% 16% 1	800
	11th to 20th	Night	14% 5% 29% 37% 14% 2	800
	21st to end	Night	14% 7% 32% 35% 11% 1	880
November	1st to 10th	Night	20% 5% 29% 31% 13% 1	1014
	11th to 20th	Night	13% 5% 36% 33% 11% 2	1050
	21st to end	Night	10% 4 33% 41% 10% 2	1050
December	1st to 10th	Night	13% 6% 32% 33% 14% 3	1050
	11th to 20th	Night	14% 5% 30% 35% 14% 2	1057
	21st to end	Night	11% 3 25% 40% 17% 3	1276

Page generated at 2026-06-28 00:10:45. Version ac07

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