Mayo
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mayo clear sky chart from 2005-11-06 to 2026-04-02.

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	8% 10% 4 12% 8% 7% 7% 5% 7% 8% 23%	135399
	Night	6% 13% 5% 14% 9% 8% 7% 5% 7% 8% 18%	135064
Transparency	Day	53% 10% 15% 16% 5%	135024
	Night	59% 8% 12% 13% 7% 1	135020
Seeing	Day	13% 6% 17% 27% 25% 12%	19495
	Night	13% 3 13% 33% 28% 9%	25185

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	4 15% 6% 14% 12% 9% 5% 3 5% 7% 20%	2331
	11th to 20th	Day	3 13% 4 12% 9% 8% 7% 5% 6% 10% 22%	2545
	21st to end	Day	3 11% 4 11% 10% 11% 11% 9% 10% 8% 12%	3471
February	1st to 10th	Day	3 10% 5% 15% 9% 8% 8% 7% 8% 11% 17%	1367
	11th to 20th	Day	2 10% 6% 17% 9% 6% 8% 7% 8% 12% 14%	1538
	21st to end	Day	2 9% 4 19% 12% 11% 9% 6% 10% 7% 11%	1480
March	1st to 10th	Day	1 14% 6% 19% 12% 7% 6% 4 6% 7% 19%	4399
	11th to 20th	Day	3 11% 6% 15% 9% 9% 7% 5% 6% 9% 20%	4854
	21st to end	Day	1 9% 6% 16% 9% 8% 8% 5% 6% 7% 25%	6007
April	1st to 10th	Day	3 12% 5% 12% 8% 6% 6% 6% 7% 8% 26%	2542
	11th to 20th	Day	4 15% 4 8% 7% 5% 5% 5% 7% 8% 31%	2766
	21st to end	Day	5% 14% 4 9% 6% 6% 6% 5% 7% 7% 31%	2968
May	1st to 10th	Day	9% 12% 5% 12% 9% 8% 8% 6% 7% 8% 18%	7240
	11th to 20th	Day	7% 12% 4 10% 7% 6% 6% 5% 7% 8% 27%	7637
	21st to end	Day	10% 10% 3 10% 7% 7% 7% 6% 8% 9% 24%	8911
June	1st to 10th	Day	10% 9% 4 9% 7% 7% 7% 7% 9% 10% 22%	3644
	11th to 20th	Day	9% 8% 3 11% 8% 7% 7% 5% 8% 8% 25%	3960
	21st to end	Day	14% 8% 3 8% 8% 7% 7% 6% 7% 9% 22%	3912
July	1st to 10th	Day	9% 7% 3 10% 7% 8% 8% 7% 10% 10% 22%	8449
	11th to 20th	Day	11% 9% 4 10% 7% 7% 8% 6% 8% 9% 21%	8160
	21st to end	Day	8% 7% 3 9% 7% 7% 7% 6% 8% 10% 28%	8482
August	1st to 10th	Day	10% 7% 3 8% 5% 5% 5% 3 6% 7% 41%	5197
	11th to 20th	Day	13% 8% 3 9% 6% 6% 7% 6% 7% 8% 28%	4811
	21st to end	Day	13% 11% 4 10% 8% 7% 6% 5% 7% 9% 20%	5027
September	1st to 10th	Day	13% 11% 4 10% 7% 6% 6% 4 6% 8% 24%	2543
	11th to 20th	Day	13% 8% 4 10% 7% 7% 5% 4 7% 7% 28%	2400
	21st to end	Day	14% 12% 3 11% 7% 6% 6% 5% 6% 7% 23%	2121
October	1st to 10th	Day	9% 12% 4 10% 5% 7% 6% 4 5% 8% 30%	3243
	11th to 20th	Day	9% 14% 5% 11% 6% 6% 6% 3 5% 7% 28%	2770
	21st to end	Day	8% 15% 8% 21% 11% 6% 6% 4 4 5% 13%	2714
November	1st to 10th	Day	7% 18% 6% 16% 9% 6% 8% 5% 9% 6% 11%	1303
	11th to 20th	Day	6% 17% 8% 18% 12% 10% 7% 4 5% 6% 7%	1142
	21st to end	Day	5% 16% 8% 17% 10% 8% 7% 10% 8% 5% 6%	883
December	1st to 10th	Day	6% 15% 8% 21% 9% 5% 4 4 6% 8% 12%	1340
	11th to 20th	Day	4 12% 5% 13% 12% 10% 7% 7% 8% 11% 11%	1542
	21st to end	Day	6% 17% 7% 15% 10% 8% 7% 5% 7% 11% 8%	1700
January	1st to 10th	Night	4 11% 6% 16% 11% 7% 6% 5% 6% 8% 20%	10562
	11th to 20th	Night	4 12% 4 12% 8% 9% 8% 5% 6% 9% 22%	10411
	21st to end	Night	5% 11% 5% 14% 11% 11% 11% 7% 8% 7% 10%	10708
February	1st to 10th	Night	3 11% 6% 12% 10% 9% 7% 7% 7% 10% 18%	3250
	11th to 20th	Night	3 13% 6% 13% 8% 7% 6% 6% 9% 11% 18%	2969
	21st to end	Night	2 11% 5% 13% 10% 9% 9% 6% 10% 8% 16%	2323
March	1st to 10th	Night	3 13% 5% 19% 10% 9% 7% 5% 6% 7% 18%	5695
	11th to 20th	Night	3 11% 7% 15% 9% 8% 8% 6% 7% 8% 20%	5332
	21st to end	Night	3 14% 7% 17% 7% 6% 6% 5% 5% 4 27%	5184
April	1st to 10th	Night	2 11% 6% 12% 8% 6% 7% 5% 8% 7% 28%	1880
	11th to 20th	Night	4 14% 4 10% 6% 6% 6% 5% 7% 8% 28%	1583
	21st to end	Night	5% 11% 4 8% 8% 8% 6% 6% 7% 7% 29%	1430
May	1st to 10th	Night	9% 9% 3 11% 7% 8% 8% 6% 8% 9% 22%	2720
	11th to 20th	Night	8% 11% 4 12% 6% 6% 5% 5% 5% 7% 32%	2257
	21st to end	Night	10% 10% 3 9% 7% 5% 7% 6% 8% 9% 24%	1964
June	1st to 10th	Night	11% 9% 4 9% 5% 6% 6% 5% 8% 11% 27%	755
	11th to 20th	Night	8% 7% 2 10% 7% 10% 7% 6% 10% 8% 26%	440
	21st to end	Night	14% 8% 4 9% 7% 5% 7% 4 7% 9% 26%	483
July	1st to 10th	Night	12% 9% 2 9% 6% 8% 8% 6% 7% 9% 24%	1509
	11th to 20th	Night	11% 9% 3 12% 6% 8% 7% 6% 9% 7% 21%	1800
	21st to end	Night	9% 8% 4 10% 7% 6% 7% 4 6% 7% 30%	2359
August	1st to 10th	Night	8% 5% 2 5% 5% 5% 6% 5% 7% 8% 43%	1953
	11th to 20th	Night	13% 7% 4 9% 6% 6% 6% 4 6% 9% 30%	2243
	21st to end	Night	13% 10% 4 11% 7% 7% 7% 6% 8% 9% 20%	2882
September	1st to 10th	Night	13% 10% 4 8% 7% 6% 6% 5% 7% 7% 28%	1854
	11th to 20th	Night	12% 8% 3 11% 7% 6% 7% 4 5% 8% 28%	2000
	21st to end	Night	16% 12% 4 8% 6% 7% 7% 5% 6% 6% 24%	2277
October	1st to 10th	Night	11% 13% 4 9% 6% 5% 5% 4 6% 6% 31%	3947
	11th to 20th	Night	10% 13% 5% 11% 7% 5% 5% 4 7% 8% 24%	4377
	21st to end	Night	7% 18% 8% 17% 9% 7% 6% 4 6% 7% 11%	5083
November	1st to 10th	Night	10% 19% 6% 15% 8% 6% 5% 5% 7% 7% 12%	3212
	11th to 20th	Night	6% 17% 7% 18% 11% 9% 8% 5% 7% 6% 7%	3456
	21st to end	Night	7% 17% 7% 16% 10% 8% 8% 8% 8% 6% 6%	3734
December	1st to 10th	Night	7% 18% 8% 18% 11% 7% 6% 5% 6% 6% 9%	6060
	11th to 20th	Night	4 14% 6% 18% 10% 9% 8% 6% 8% 7% 10%	6870
	21st to end	Night	6% 19% 7% 13% 10% 8% 7% 6% 7% 8% 9%	9502

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	63% 5% 3 11% 18% 1	2331
	11th to 20th	Day	54% 10% 15% 11% 8% 2	2557
	21st to end	Day	58% 10% 13% 9% 9%	3472
February	1st to 10th	Day	57% 6% 10% 15% 11% 2	1367
	11th to 20th	Day	58% 9% 11% 13% 9%	1545
	21st to end	Day	63% 9% 11% 11% 5%	1480
March	1st to 10th	Day	63% 4 10% 16% 8%	4399
	11th to 20th	Day	58% 6% 8% 20% 8%	4854
	21st to end	Day	50% 11% 15% 15% 10%	6007
April	1st to 10th	Day	53% 9% 16% 15% 6% 1	2542
	11th to 20th	Day	47% 8% 18% 20% 7%	2766
	21st to end	Day	47% 10% 16% 21% 5%	2968
May	1st to 10th	Day	57% 10% 12% 14% 6% 1	7240
	11th to 20th	Day	48% 9% 13% 21% 8% 1	7641
	21st to end	Day	49% 10% 12% 20% 8%	8912
June	1st to 10th	Day	50% 12% 15% 17% 5%	3644
	11th to 20th	Day	53% 9% 13% 19% 6%	3960
	21st to end	Day	53% 11% 16% 15% 4	3912
July	1st to 10th	Day	49% 14% 21% 13% 3	8358
	11th to 20th	Day	54% 12% 21% 12% 1	8061
	21st to end	Day	46% 11% 24% 17% 2	8243
August	1st to 10th	Day	41% 9% 22% 17% 10% 1	5178
	11th to 20th	Day	50% 11% 19% 17% 3	4858
	21st to end	Day	57% 10% 21% 11% 1	5026
September	1st to 10th	Day	56% 10% 18% 14% 1	2543
	11th to 20th	Day	52% 10% 21% 15% 2	2400
	21st to end	Day	58% 10% 16% 14% 2	2121
October	1st to 10th	Day	50% 9% 18% 20% 3	3243
	11th to 20th	Day	54% 9% 11% 24% 1	2770
	21st to end	Day	72% 9% 9% 10%	2714
November	1st to 10th	Day	67% 10% 10% 9% 3	1303
	11th to 20th	Day	75% 6% 8% 9% 2	1142
	21st to end	Day	70% 10% 10% 8% 2	883
December	1st to 10th	Day	69% 10% 8% 9% 5%	1340
	11th to 20th	Day	62% 7% 13% 12% 7%	1544
	21st to end	Day	65% 7% 10% 12% 6%	1700
January	1st to 10th	Night	60% 5% 7% 10% 17% 1	10563
	11th to 20th	Night	56% 8% 9% 13% 11% 3	10461
	21st to end	Night	65% 9% 11% 8% 6% 1	10708
February	1st to 10th	Night	56% 7% 9% 15% 12% 1	3250
	11th to 20th	Night	55% 9% 9% 16% 10% 1	2973
	21st to end	Night	56% 9% 13% 14% 8%	2323
March	1st to 10th	Night	60% 7% 10% 14% 9%	5695
	11th to 20th	Night	57% 6% 10% 17% 11%	5334
	21st to end	Night	53% 11% 11% 14% 11%	5184
April	1st to 10th	Night	50% 10% 18% 15% 6% 1	1880
	11th to 20th	Night	49% 9% 14% 19% 8%	1583
	21st to end	Night	48% 10% 15% 20% 6% 1	1430
May	1st to 10th	Night	50% 10% 16% 20% 4	2717
	11th to 20th	Night	49% 8% 10% 25% 8%	2257
	21st to end	Night	48% 11% 14% 17% 9%	1964
June	1st to 10th	Night	48% 12% 16% 19% 6%	755
	11th to 20th	Night	48% 14% 12% 21% 5%	440
	21st to end	Night	51% 12% 19% 16% 3	483
July	1st to 10th	Night	54% 13% 20% 10% 3	1491
	11th to 20th	Night	56% 11% 19% 13% 1	1776
	21st to end	Night	48% 10% 20% 20% 1	2293
August	1st to 10th	Night	36% 10% 24% 18% 12% 1	1947
	11th to 20th	Night	50% 9% 19% 20% 2	2262
	21st to end	Night	57% 11% 18% 13%	2882
September	1st to 10th	Night	52% 10% 21% 16% 2	1854
	11th to 20th	Night	52% 9% 18% 17% 3	2000
	21st to end	Night	59% 8% 11% 19% 3	2277
October	1st to 10th	Night	50% 8% 13% 25% 3	3947
	11th to 20th	Night	54% 10% 13% 19% 4	4377
	21st to end	Night	69% 8% 11% 11% 1	5083
November	1st to 10th	Night	67% 8% 12% 10% 3	3212
	11th to 20th	Night	73% 7% 10% 8% 1	3456
	21st to end	Night	69% 8% 10% 11% 2	3734
December	1st to 10th	Night	74% 7% 7% 8% 4	6060
	11th to 20th	Night	67% 8% 10% 10% 4	6867
	21st to end	Night	67% 9% 8% 10% 4 1	9502

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	6% 5% 7% 28% 39% 14%	110
	11th to 20th	Day	11% 3 9% 38% 25% 14%	110
	21st to end	Day	14% 2 10% 32% 28% 15%	168
February	1st to 10th	Day	5% 4 10% 34% 32% 15%	327
	11th to 20th	Day	5% 3 18% 33% 25% 16%	425
	21st to end	Day	5% 4 12% 34% 28% 16%	355
March	1st to 10th	Day	5% 6% 14% 27% 26% 22%	430
	11th to 20th	Day	6% 3 16% 29% 26% 19%	430
	21st to end	Day	4 5% 16% 32% 27% 17%	572
April	1st to 10th	Day	10% 6% 19% 30% 27% 9%	600
	11th to 20th	Day	8% 7% 18% 32% 26% 10%	690
	21st to end	Day	10% 9% 21% 30% 21% 9%	700
May	1st to 10th	Day	13% 14% 26% 25% 16% 6%	703
	11th to 20th	Day	14% 9% 25% 26% 18% 8%	821
	21st to end	Day	16% 7% 21% 29% 23% 5%	1006
June	1st to 10th	Day	13% 6% 18% 27% 26% 10%	974
	11th to 20th	Day	12% 4 14% 26% 31% 13%	1130
	21st to end	Day	14% 4 12% 24% 30% 16%	1130
July	1st to 10th	Day	12% 4 15% 26% 31% 12%	1047
	11th to 20th	Day	16% 5% 12% 24% 28% 17%	912
	21st to end	Day	14% 3 13% 21% 30% 19%	858
August	1st to 10th	Day	15% 6% 14% 24% 26% 15%	726
	11th to 20th	Day	17% 7% 17% 29% 21% 8%	715
	21st to end	Day	17% 5% 16% 26% 27% 9%	803
September	1st to 10th	Day	15% 8% 20% 20% 24% 13%	623
	11th to 20th	Day	13% 8% 18% 24% 25% 12%	530
	21st to end	Day	20% 8% 18% 25% 19% 10%	464
October	1st to 10th	Day	17% 8% 25% 27% 14% 10%	420
	11th to 20th	Day	20% 7% 16% 23% 21% 12%	420
	21st to end	Day	19% 4 13% 30% 20% 14%	383
November	1st to 10th	Day	13% 5% 16% 31% 25% 10%	220
	11th to 20th	Day	10% 3 9% 32% 23% 22%	220
	21st to end	Day	12% 2 11% 37% 20% 17%	132
December	1st to 10th	Day	14% 3 13% 38% 23% 10%	110
	11th to 20th	Day	10% 4 15% 30% 28% 13%	110
	21st to end	Day	8% 2 10% 37% 27% 16%	121
January	1st to 10th	Night	11% 2 11% 34% 29% 13%	1160
	11th to 20th	Night	14% 3 14% 42% 21% 7%	1105
	21st to end	Night	11% 3 15% 37% 25% 7%	1045
February	1st to 10th	Night	8% 2 13% 39% 30% 8%	950
	11th to 20th	Night	8% 2 16% 39% 26% 8%	934
	21st to end	Night	6% 3 12% 41% 30% 9%	783
March	1st to 10th	Night	6% 3 12% 37% 31% 11%	889
	11th to 20th	Night	6% 2 12% 33% 38% 8%	744
	21st to end	Night	6% 5% 11% 34% 32% 11%	693
April	1st to 10th	Night	6% 4 13% 35% 35% 7%	603
	11th to 20th	Night	9% 2 16% 32% 33% 8%	593
	21st to end	Night	8% 5% 13% 32% 35% 6%	520
May	1st to 10th	Night	8% 8% 17% 35% 28% 4	400
	11th to 20th	Night	15% 6% 20% 34% 23% 2	305
	21st to end	Night	12% 3 12% 45% 26% 2	217
June	1st to 10th	Night	16% 5% 12% 25% 37% 7%	200
	11th to 20th	Night	8% 4 11% 29% 45% 4	200
	21st to end	Night	17% 4 8% 26% 39% 8%	200
July	1st to 10th	Night	15% 2 9% 33% 33% 7%	195
	11th to 20th	Night	19% 4 9% 21% 42% 6%	194
	21st to end	Night	10% 2 5% 22% 50% 11%	231
August	1st to 10th	Night	14% 3 13% 23% 42% 5%	398
	11th to 20th	Night	18% 4 11% 30% 29% 9%	541
	21st to end	Night	18% 3 11% 25% 33% 10%	660
September	1st to 10th	Night	19% 3 13% 28% 30% 8%	600
	11th to 20th	Night	16% 4 11% 30% 32% 8%	600
	21st to end	Night	22% 4 12% 29% 23% 10%	761
October	1st to 10th	Night	22% 4 15% 32% 21% 6%	800
	11th to 20th	Night	20% 4 14% 28% 27% 7%	855
	21st to end	Night	18% 3 13% 32% 26% 7%	1001
November	1st to 10th	Night	17% 3 15% 34% 22% 9%	930
	11th to 20th	Night	12% 3 9% 32% 29% 15%	1046
	21st to end	Night	13% 4 14% 30% 26% 13%	1160
December	1st to 10th	Night	14% 4 14% 35% 24% 9%	1160
	11th to 20th	Night	12% 3 15% 34% 25% 11%	1160
	21st to end	Night	11% 4 12% 34% 26% 14%	1352

Page generated at 2026-03-29 22:18:42. Version ac07

Copyright © 2026 Attilla Danko.