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 2024-01-16.

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% 10% 5% 9% 9% 5% 7% 7% 7% 8% 23%	74849
	Night	8% 13% 6% 11% 10% 5% 7% 7% 7% 8% 18%	74945
Transparency	Day	56% 10% 15% 15% 5%	74465
	Night	61% 8% 12% 13% 5%	74849
Seeing	Day	14% 5% 17% 27% 25% 12%	16557
	Night	13% 3 13% 33% 29% 9%	22256

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	6% 14% 6% 10% 12% 6% 11% 6% 9% 8% 12%	759
	11th to 20th	Day	5% 14% 7% 15% 16% 7% 9% 5% 6% 6% 10%	803
	21st to end	Day	6% 16% 7% 12% 9% 5% 10% 7% 6% 8% 13%	1064
February	1st to 10th	Day	3 9% 5% 12% 11% 6% 9% 9% 7% 11% 18%	1171
	11th to 20th	Day	3 10% 7% 16% 12% 4 7% 9% 8% 12% 12%	1313
	21st to end	Day	2 9% 4 16% 13% 7% 12% 8% 10% 8% 11%	1265
March	1st to 10th	Day	2 13% 7% 10% 12% 5% 7% 6% 8% 9% 21%	1716
	11th to 20th	Day	2 12% 6% 11% 10% 6% 8% 6% 7% 8% 25%	1880
	21st to end	Day	3 8% 6% 9% 9% 6% 7% 6% 7% 7% 33%	2340
April	1st to 10th	Day	4 12% 6% 10% 9% 5% 6% 7% 7% 8% 26%	2275
	11th to 20th	Day	5% 13% 5% 7% 7% 4 6% 7% 7% 9% 30%	2486
	21st to end	Day	5% 12% 5% 8% 7% 5% 6% 6% 6% 8% 32%	2670
May	1st to 10th	Day	8% 10% 4 8% 9% 5% 8% 7% 8% 8% 27%	2880
	11th to 20th	Day	10% 11% 5% 7% 7% 5% 6% 6% 7% 7% 30%	2997
	21st to end	Day	10% 8% 4 8% 7% 5% 6% 6% 7% 9% 29%	3499
June	1st to 10th	Day	11% 8% 4 8% 8% 5% 7% 8% 9% 10% 23%	3279
	11th to 20th	Day	10% 8% 4 10% 9% 5% 8% 7% 7% 8% 23%	3564
	21st to end	Day	16% 7% 4 7% 9% 6% 7% 7% 7% 9% 22%	3522
July	1st to 10th	Day	11% 7% 3 8% 8% 6% 7% 7% 7% 9% 27%	3361
	11th to 20th	Day	15% 7% 4 8% 8% 5% 8% 7% 9% 9% 21%	3240
	21st to end	Day	11% 7% 3 7% 7% 5% 7% 7% 8% 9% 29%	3380
August	1st to 10th	Day	13% 7% 3 7% 8% 4 6% 5% 8% 10% 29%	2865
	11th to 20th	Day	16% 9% 4 8% 7% 5% 7% 8% 6% 8% 23%	2665
	21st to end	Day	18% 10% 4 9% 9% 5% 6% 7% 6% 7% 19%	2772
September	1st to 10th	Day	14% 11% 5% 8% 9% 4 6% 5% 6% 8% 24%	2291
	11th to 20th	Day	14% 7% 4 8% 7% 5% 5% 6% 7% 7% 29%	2160
	21st to end	Day	14% 12% 3 9% 8% 5% 6% 6% 6% 7% 23%	1910
October	1st to 10th	Day	14% 15% 5% 9% 8% 6% 7% 7% 5% 8% 15%	1787
	11th to 20th	Day	13% 17% 6% 9% 9% 5% 6% 5% 5% 7% 17%	1530
	21st to end	Day	11% 17% 7% 14% 13% 5% 6% 5% 5% 5% 13%	1513
November	1st to 10th	Day	7% 17% 7% 13% 10% 4 7% 7% 9% 6% 12%	1177
	11th to 20th	Day	6% 15% 10% 14% 12% 7% 9% 7% 5% 6% 8%	1034
	21st to end	Day	6% 16% 8% 14% 12% 6% 9% 11% 9% 5% 5%	799
December	1st to 10th	Day	9% 16% 7% 13% 11% 4 6% 8% 8% 10% 9%	760
	11th to 20th	Day	6% 16% 6% 11% 14% 8% 8% 8% 9% 10% 5%	962
	21st to end	Day	8% 14% 6% 14% 11% 5% 6% 8% 8% 11% 9%	1160
January	1st to 10th	Night	7% 13% 6% 12% 12% 5% 8% 7% 7% 9% 15%	3406
	11th to 20th	Night	8% 16% 8% 13% 11% 7% 7% 6% 6% 8% 10%	3296
	21st to end	Night	5% 15% 8% 14% 12% 5% 8% 8% 6% 6% 13%	3291
February	1st to 10th	Night	3 11% 6% 10% 11% 6% 9% 7% 7% 10% 20%	2786
	11th to 20th	Night	4 13% 6% 12% 10% 5% 6% 7% 9% 11% 17%	2534
	21st to end	Night	2 10% 5% 11% 11% 6% 10% 8% 10% 10% 17%	1988
March	1st to 10th	Night	2 10% 5% 12% 10% 5% 6% 7% 8% 10% 26%	2244
	11th to 20th	Night	2 9% 7% 11% 10% 6% 8% 7% 6% 8% 25%	2077
	21st to end	Night	3 12% 6% 8% 7% 3 5% 6% 4 6% 39%	2016
April	1st to 10th	Night	3 11% 6% 10% 8% 4 8% 7% 8% 8% 28%	1684
	11th to 20th	Night	5% 12% 5% 9% 8% 4 7% 7% 7% 9% 27%	1423
	21st to end	Night	5% 10% 4 8% 9% 5% 7% 8% 7% 7% 30%	1288
May	1st to 10th	Night	6% 7% 5% 10% 8% 6% 8% 7% 7% 9% 27%	1080
	11th to 20th	Night	12% 8% 4 8% 6% 4 5% 4 5% 7% 36%	897
	21st to end	Night	9% 8% 3 6% 7% 2 7% 7% 7% 9% 36%	776
June	1st to 10th	Night	12% 7% 4 9% 6% 4 7% 5% 7% 12% 27%	680
	11th to 20th	Night	9% 6% 3 10% 7% 8% 8% 8% 8% 9% 25%	396
	21st to end	Night	15% 8% 4 9% 8% 3 7% 6% 6% 9% 26%	434
July	1st to 10th	Night	12% 6% 3 7% 8% 6% 9% 6% 7% 10% 28%	598
	11th to 20th	Night	14% 7% 3 9% 8% 6% 9% 6% 8% 8% 22%	720
	21st to end	Night	9% 7% 4 8% 9% 4 7% 6% 6% 8% 32%	935
August	1st to 10th	Night	12% 7% 4 5% 7% 4 5% 9% 8% 9% 29%	1074
	11th to 20th	Night	17% 8% 4 10% 9% 5% 6% 3 6% 8% 25%	1234
	21st to end	Night	14% 10% 4 10% 8% 5% 6% 8% 7% 8% 19%	1584
September	1st to 10th	Night	13% 10% 4 8% 8% 5% 6% 6% 6% 7% 28%	1666
	11th to 20th	Night	12% 8% 4 9% 8% 4 7% 6% 6% 9% 29%	1800
	21st to end	Night	16% 11% 4 6% 8% 4 7% 7% 5% 6% 25%	2048
October	1st to 10th	Night	16% 13% 4 8% 8% 4 6% 5% 6% 6% 23%	2173
	11th to 20th	Night	13% 18% 5% 12% 10% 4 5% 5% 6% 6% 17%	2429
	21st to end	Night	10% 18% 7% 15% 9% 6% 7% 6% 5% 7% 11%	2843
November	1st to 10th	Night	10% 18% 7% 14% 9% 4 6% 5% 8% 7% 12%	2898
	11th to 20th	Night	6% 15% 7% 14% 13% 7% 9% 7% 7% 7% 8%	3126
	21st to end	Night	7% 16% 8% 14% 11% 6% 8% 9% 8% 6% 6%	3378
December	1st to 10th	Night	7% 18% 8% 14% 11% 5% 6% 7% 7% 7% 8%	3420
	11th to 20th	Night	6% 15% 8% 14% 13% 7% 8% 8% 8% 7% 6%	4230
	21st to end	Night	8% 18% 6% 11% 11% 6% 7% 7% 7% 9% 11%	6493

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	64% 9% 7% 13% 6% 1	759
	11th to 20th	Day	73% 5% 7% 10% 4	803
	21st to end	Day	64% 7% 9% 11% 8% 1	1064
February	1st to 10th	Day	57% 5% 10% 14% 12% 2	1171
	11th to 20th	Day	61% 9% 11% 12% 6%	1320
	21st to end	Day	62% 10% 12% 11% 4	1265
March	1st to 10th	Day	57% 6% 8% 18% 11%	1716
	11th to 20th	Day	54% 7% 11% 17% 9% 1	1880
	21st to end	Day	46% 8% 16% 19% 10% 1	2340
April	1st to 10th	Day	54% 9% 17% 14% 6% 1	2275
	11th to 20th	Day	48% 9% 19% 19% 6%	2486
	21st to end	Day	48% 9% 16% 20% 6%	2670
May	1st to 10th	Day	51% 10% 17% 18% 3	2880
	11th to 20th	Day	51% 8% 12% 21% 7%	3001
	21st to end	Day	48% 9% 17% 20% 5%	3500
June	1st to 10th	Day	51% 12% 15% 17% 5%	3279
	11th to 20th	Day	56% 10% 13% 16% 5%	3564
	21st to end	Day	56% 11% 14% 15% 4	3522
July	1st to 10th	Day	50% 13% 16% 17% 4	3270
	11th to 20th	Day	56% 12% 17% 12% 3	3141
	21st to end	Day	48% 13% 22% 15% 3	3191
August	1st to 10th	Day	50% 11% 21% 14% 3	2846
	11th to 20th	Day	57% 12% 18% 12% 1	2665
	21st to end	Day	62% 12% 17% 9% 1	2772
September	1st to 10th	Day	56% 10% 18% 14% 1	2291
	11th to 20th	Day	51% 11% 21% 14% 3	2160
	21st to end	Day	57% 10% 16% 15% 2	1910
October	1st to 10th	Day	66% 10% 14% 9% 2	1787
	11th to 20th	Day	66% 10% 15% 7% 1	1530
	21st to end	Day	73% 9% 8% 9% 1	1513
November	1st to 10th	Day	66% 10% 10% 10% 3	1177
	11th to 20th	Day	75% 7% 8% 9% 1	1034
	21st to end	Day	71% 10% 10% 8% 2	799
December	1st to 10th	Day	68% 9% 10% 7% 6%	760
	11th to 20th	Day	68% 7% 7% 9% 9%	964
	21st to end	Day	64% 7% 8% 14% 6%	1160
January	1st to 10th	Night	61% 9% 10% 12% 7% 1	3406
	11th to 20th	Night	69% 7% 8% 10% 5% 1	3296
	21st to end	Night	67% 7% 8% 10% 7% 1	3291
February	1st to 10th	Night	57% 6% 9% 16% 12% 1	2786
	11th to 20th	Night	57% 9% 9% 16% 8%	2538
	21st to end	Night	54% 10% 14% 15% 8%	1988
March	1st to 10th	Night	51% 7% 9% 23% 10%	2244
	11th to 20th	Night	53% 8% 10% 17% 10% 1	2079
	21st to end	Night	45% 8% 15% 18% 12% 1	2016
April	1st to 10th	Night	49% 10% 19% 14% 6% 1	1684
	11th to 20th	Night	50% 10% 15% 18% 7%	1423
	21st to end	Night	47% 11% 15% 20% 6% 1	1288
May	1st to 10th	Night	49% 8% 19% 21% 3	1080
	11th to 20th	Night	45% 9% 13% 26% 7%	897
	21st to end	Night	42% 10% 20% 22% 6%	776
June	1st to 10th	Night	49% 12% 16% 18% 5%	680
	11th to 20th	Night	50% 14% 11% 19% 5%	396
	21st to end	Night	53% 12% 17% 15% 3	434
July	1st to 10th	Night	50% 14% 16% 15% 6%	580
	11th to 20th	Night	55% 14% 16% 12% 2	696
	21st to end	Night	47% 11% 23% 17% 2	884
August	1st to 10th	Night	47% 15% 19% 15% 3	1068
	11th to 20th	Night	58% 8% 18% 14% 1	1234
	21st to end	Night	61% 12% 18% 9% 1	1584
September	1st to 10th	Night	53% 10% 20% 16% 2	1666
	11th to 20th	Night	51% 10% 19% 17% 3	1800
	21st to end	Night	59% 8% 11% 19% 3	2048
October	1st to 10th	Night	59% 10% 15% 13% 2 1	2173
	11th to 20th	Night	66% 9% 14% 8% 2	2429
	21st to end	Night	70% 9% 12% 7% 1	2843
November	1st to 10th	Night	68% 8% 12% 10% 3	2898
	11th to 20th	Night	73% 7% 11% 8% 1	3126
	21st to end	Night	70% 8% 9% 11% 2	3378
December	1st to 10th	Night	72% 7% 9% 9% 4	3420
	11th to 20th	Night	71% 7% 9% 8% 5%	4227
	21st to end	Night	66% 6% 8% 12% 6% 2	6493

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	7% 3 9% 28% 41% 12%	90
	11th to 20th	Day	9% 11% 36% 26% 18%	85
	21st to end	Day	16% 2 7% 29% 30% 16%	122
February	1st to 10th	Day	4 4 7% 35% 32% 17%	251
	11th to 20th	Day	6% 3 17% 33% 22% 19%	335
	21st to end	Day	5% 4 13% 35% 27% 16%	280
March	1st to 10th	Day	5% 6% 16% 27% 24% 23%	340
	11th to 20th	Day	7% 4 17% 29% 25% 18%	340
	21st to end	Day	4 6% 15% 32% 27% 15%	446
April	1st to 10th	Day	10% 6% 19% 28% 26% 10%	505
	11th to 20th	Day	8% 5% 17% 32% 27% 10%	590
	21st to end	Day	11% 9% 22% 31% 20% 7%	600
May	1st to 10th	Day	12% 13% 26% 26% 16% 7%	603
	11th to 20th	Day	14% 8% 26% 26% 18% 8%	711
	21st to end	Day	15% 7% 21% 29% 23% 5%	874
June	1st to 10th	Day	14% 6% 17% 26% 27% 10%	849
	11th to 20th	Day	13% 4 16% 26% 30% 11%	990
	21st to end	Day	16% 4 13% 25% 29% 13%	990
July	1st to 10th	Day	12% 3 14% 25% 32% 14%	915
	11th to 20th	Day	17% 4 11% 24% 27% 17%	792
	21st to end	Day	15% 3 13% 20% 30% 19%	742
August	1st to 10th	Day	16% 6% 16% 24% 25% 14%	626
	11th to 20th	Day	18% 7% 19% 30% 19% 7%	615
	21st to end	Day	19% 5% 14% 25% 28% 10%	693
September	1st to 10th	Day	16% 7% 21% 18% 23% 14%	535
	11th to 20th	Day	14% 7% 18% 24% 25% 12%	450
	21st to end	Day	20% 9% 18% 24% 18% 9%	396
October	1st to 10th	Day	19% 7% 23% 27% 15% 9%	360
	11th to 20th	Day	21% 7% 17% 23% 19% 12%	360
	21st to end	Day	20% 2 11% 31% 20% 14%	325
November	1st to 10th	Day	13% 3 17% 32% 26% 10%	180
	11th to 20th	Day	13% 2 10% 33% 22% 20%	180
	21st to end	Day	14% 2 12% 40% 18% 15%	108
December	1st to 10th	Day	17% 2 12% 40% 22% 7%	90
	11th to 20th	Day	12% 3 19% 26% 30% 10%	90
	21st to end	Day	8% 2 11% 35% 28% 15%	99
January	1st to 10th	Night	11% 2 10% 35% 29% 13%	1040
	11th to 20th	Night	15% 3 12% 41% 21% 8%	968
	21st to end	Night	12% 3 15% 36% 27% 7%	880
February	1st to 10th	Night	8% 2 13% 37% 31% 9%	800
	11th to 20th	Night	9% 2 16% 38% 27% 8%	784
	21st to end	Night	6% 1 12% 41% 30% 9%	658
March	1st to 10th	Night	6% 2 13% 36% 31% 12%	756
	11th to 20th	Night	7% 2 11% 36% 36% 8%	639
	21st to end	Night	7% 5% 11% 34% 33% 10%	594
April	1st to 10th	Night	6% 4 12% 33% 36% 8%	540
	11th to 20th	Night	9% 1 16% 32% 34% 8%	533
	21st to end	Night	9% 5% 14% 32% 35% 6%	468
May	1st to 10th	Night	8% 7% 15% 37% 29% 4	360
	11th to 20th	Night	16% 4 19% 35% 24% 2	275
	21st to end	Night	11% 3 11% 46% 27% 3	195
June	1st to 10th	Night	17% 4 10% 23% 38% 7%	180
	11th to 20th	Night	9% 3 12% 29% 43% 4	180
	21st to end	Night	18% 4 8% 26% 38% 7%	180
July	1st to 10th	Night	15% 1 9% 31% 36% 8%	175
	11th to 20th	Night	20% 3 9% 18% 43% 7%	174
	21st to end	Night	11% 1 6% 21% 49% 13%	207
August	1st to 10th	Night	15% 3 13% 21% 42% 6%	358
	11th to 20th	Night	19% 3 12% 28% 29% 9%	486
	21st to end	Night	19% 3 10% 24% 34% 10%	594
September	1st to 10th	Night	20% 2 12% 26% 31% 9%	540
	11th to 20th	Night	17% 4 10% 29% 33% 8%	540
	21st to end	Night	23% 4 12% 29% 23% 9%	684
October	1st to 10th	Night	23% 4 14% 31% 23% 6%	720
	11th to 20th	Night	21% 3 14% 29% 26% 7%	765
	21st to end	Night	19% 3 12% 31% 27% 7%	891
November	1st to 10th	Night	17% 3 14% 33% 23% 9%	830
	11th to 20th	Night	12% 3 10% 32% 29% 14%	936
	21st to end	Night	15% 3 15% 30% 25% 12%	1040
December	1st to 10th	Night	14% 4 14% 35% 23% 9%	1040
	11th to 20th	Night	13% 4 16% 33% 25% 10%	1040
	21st to end	Night	11% 4 12% 33% 25% 15%	1206

Page generated at 2024-01-13 06:37:52. Version ac07

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