McElroy Mountain
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the McElroy Mountain clear sky chart from 2005-11-06 to 2026-01-17.

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	14% 7% 3 6% 4 4 5% 4 6% 7% 40%	127182
	Night	15% 7% 2 6% 4 4 4 4 5% 7% 43%	124528
Transparency	Day	40% 7% 11% 19% 17% 5%	130969
	Night	38% 7% 11% 19% 19% 6%	128808
Seeing	Day	19% 12% 24% 23% 15% 7%	16412
	Night	17% 10% 23% 28% 19% 3	26471

By Month

Percentage Cloud Cover SE (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	19% 10% 4 5% 3 3 3 2 3 4 45%	4146
	11th to 20th	Day	15% 5% 4 6% 3 3 3 2 3 4 52%	4114
	21st to end	Day	22% 11% 3 5% 3 3 2 3 4 5% 40%	4621
February	1st to 10th	Day	19% 9% 2 5% 4 4 3 4 5% 5% 39%	2000
	11th to 20th	Day	26% 7% 2 5% 4 3 3 3 4 5% 38%	1948
	21st to end	Day	23% 7% 3 6% 4 3 3 3 4 4 39%	1650
March	1st to 10th	Day	22% 7% 3 7% 4 4 4 3 5% 5% 35%	4609
	11th to 20th	Day	13% 6% 2 5% 4 3 3 3 5% 6% 50%	4980
	21st to end	Day	16% 6% 3 7% 4 4 4 3 4 6% 43%	5465
April	1st to 10th	Day	14% 6% 2 5% 5% 4 5% 4 6% 7% 41%	2270
	11th to 20th	Day	13% 6% 2 6% 4 4 4 3 6% 6% 46%	2374
	21st to end	Day	13% 4 2 5% 4 5% 5% 4 6% 8% 45%	2569
May	1st to 10th	Day	18% 7% 3 6% 4 5% 5% 4 7% 8% 33%	5862
	11th to 20th	Day	19% 9% 3 8% 5% 5% 6% 4 7% 7% 26%	5861
	21st to end	Day	16% 7% 3 7% 4 4 5% 4 6% 7% 38%	6433
June	1st to 10th	Day	12% 7% 3 7% 5% 6% 6% 4 8% 10% 32%	2600
	11th to 20th	Day	6% 5% 2 7% 6% 5% 7% 6% 8% 11% 37%	2600
	21st to end	Day	8% 5% 2 6% 6% 6% 7% 6% 9% 10% 35%	2600
July	1st to 10th	Day	6% 6% 2 7% 5% 6% 7% 6% 9% 11% 35%	5900
	11th to 20th	Day	7% 5% 2 7% 6% 7% 7% 6% 9% 13% 31%	5900
	21st to end	Day	10% 8% 3 10% 7% 7% 7% 6% 8% 10% 23%	6424
August	1st to 10th	Day	6% 5% 2 7% 5% 5% 6% 6% 8% 10% 38%	4234
	11th to 20th	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 10% 39%	3930
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 5% 6% 7% 47%	4312
September	1st to 10th	Day	10% 7% 3 6% 5% 5% 5% 5% 7% 7% 38%	2215
	11th to 20th	Day	14% 6% 3 7% 4 5% 5% 5% 6% 7% 39%	2200
	21st to end	Day	13% 5% 2 6% 4 5% 4 3 5% 8% 44%	2200
October	1st to 10th	Day	11% 6% 2 5% 3 4 4 3 5% 6% 53%	2200
	11th to 20th	Day	12% 5% 1 4 3 3 3 3 4 6% 57%	2085
	21st to end	Day	15% 5% 2 5% 3 3 3 2 4 5% 54%	2200
November	1st to 10th	Day	13% 5% 2 4 3 3 3 3 4 6% 54%	1852
	11th to 20th	Day	14% 6% 1 4 3 4 3 2 5% 5% 52%	1890
	21st to end	Day	16% 6% 2 5% 4 4 3 3 4 7% 45%	1890
December	1st to 10th	Day	22% 6% 2 4 3 3 3 3 5% 6% 44%	3020
	11th to 20th	Day	17% 10% 4 7% 4 4 4 3 4 4 39%	3463
	21st to end	Day	21% 7% 4 4 3 3 4 4 5% 6% 37%	4565
January	1st to 10th	Night	17% 8% 2 6% 3 3 3 3 4 5% 46%	6019
	11th to 20th	Night	16% 7% 3 6% 3 3 3 3 4 5% 47%	5979
	21st to end	Night	20% 10% 2 5% 3 3 3 2 3 5% 43%	6334
February	1st to 10th	Night	16% 7% 3 5% 4 4 4 3 5% 6% 43%	2400
	11th to 20th	Night	22% 8% 2 6% 4 4 3 3 4 5% 39%	2338
	21st to end	Night	22% 7% 3 6% 4 3 4 3 5% 6% 39%	1980
March	1st to 10th	Night	22% 9% 3 7% 4 3 4 4 4 6% 33%	5242
	11th to 20th	Night	12% 5% 2 4 3 3 4 4 6% 7% 49%	4980
	21st to end	Night	16% 7% 2 6% 5% 3 3 3 4 6% 44%	5467
April	1st to 10th	Night	14% 4 2 5% 3 4 4 4 6% 7% 47%	2145
	11th to 20th	Night	13% 7% 2 5% 3 3 3 4 5% 7% 49%	1975
	21st to end	Night	11% 7% 2 5% 4 3 4 3 6% 6% 49%	1830
May	1st to 10th	Night	15% 7% 3 7% 5% 5% 5% 4 6% 9% 35%	4033
	11th to 20th	Night	22% 9% 2 7% 5% 5% 5% 4 6% 8% 28%	4032
	21st to end	Night	14% 6% 2 6% 4 4 5% 4 6% 9% 40%	4421
June	1st to 10th	Night	9% 5% 3 7% 7% 5% 5% 5% 7% 9% 36%	1800
	11th to 20th	Night	7% 4 2 6% 5% 4 5% 6% 7% 13% 42%	1800
	21st to end	Night	6% 5% 2 6% 5% 5% 6% 4 8% 11% 43%	1800
July	1st to 10th	Night	8% 7% 2 8% 5% 5% 6% 5% 7% 10% 36%	4058
	11th to 20th	Night	6% 5% 3 7% 6% 6% 6% 6% 8% 11% 35%	4057
	21st to end	Night	11% 8% 3 9% 6% 6% 6% 5% 8% 8% 31%	4427
August	1st to 10th	Night	7% 6% 2 7% 6% 5% 6% 4 8% 11% 39%	2915
	11th to 20th	Night	10% 7% 2 8% 5% 5% 3 3 5% 8% 45%	3102
	21st to end	Night	8% 5% 2 5% 3 4 4 3 6% 8% 51%	3586
September	1st to 10th	Night	9% 7% 2 6% 5% 5% 6% 4 6% 8% 41%	2165
	11th to 20th	Night	13% 6% 2 5% 4 5% 5% 4 7% 7% 43%	2200
	21st to end	Night	12% 6% 2 5% 4 3 4 4 5% 7% 49%	2200
October	1st to 10th	Night	13% 4 2 5% 2 3 2 2 4 6% 57%	2200
	11th to 20th	Night	11% 4 2 5% 3 3 3 3 3 4 57%	2314
	21st to end	Night	14% 5% 2 5% 3 3 3 2 4 5% 54%	2640
November	1st to 10th	Night	13% 6% 2 4 4 3 3 2 4 6% 54%	2664
	11th to 20th	Night	14% 5% 2 5% 3 2 3 3 4 6% 53%	2730
	21st to end	Night	17% 6% 2 5% 4 3 4 3 6% 6% 45%	2730
December	1st to 10th	Night	19% 7% 2 5% 2 3 3 3 4 5% 47%	4380
	11th to 20th	Night	19% 8% 3 6% 3 3 3 3 4 5% 43%	4952
	21st to end	Night	21% 7% 3 6% 4 4 4 3 4 6% 38%	6633

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	42% 4 5% 19% 23% 7%	4147
	11th to 20th	Day	34% 4 6% 18% 28% 10%	4114
	21st to end	Day	47% 3 4 18% 20% 7%	4621
February	1st to 10th	Day	47% 4 8% 18% 19% 4	2000
	11th to 20th	Day	47% 4 7% 18% 18% 6%	1958
	21st to end	Day	47% 4 7% 14% 21% 7%	1650
March	1st to 10th	Day	49% 5% 8% 11% 19% 9%	4609
	11th to 20th	Day	34% 4 6% 14% 23% 17%	4980
	21st to end	Day	43% 4 7% 23% 18% 4	5465
April	1st to 10th	Day	39% 7% 10% 21% 18% 5%	2270
	11th to 20th	Day	38% 6% 8% 18% 23% 7%	2374
	21st to end	Day	34% 7% 13% 26% 17% 3	2569
May	1st to 10th	Day	45% 8% 14% 19% 13% 1	5897
	11th to 20th	Day	52% 10% 13% 18% 7% 1	5865
	21st to end	Day	42% 8% 11% 21% 16% 2	6447
June	1st to 10th	Day	43% 11% 16% 19% 9% 2	2600
	11th to 20th	Day	36% 12% 19% 22% 11% 1	2600
	21st to end	Day	37% 12% 17% 19% 12% 2	2600
July	1st to 10th	Day	37% 13% 19% 24% 7% 1	5835
	11th to 20th	Day	37% 16% 27% 16% 3 1	5831
	21st to end	Day	51% 13% 16% 15% 4	6240
August	1st to 10th	Day	36% 10% 15% 29% 9%	4221
	11th to 20th	Day	35% 10% 20% 20% 13% 3	3968
	21st to end	Day	34% 9% 12% 25% 18% 2	4311
September	1st to 10th	Day	40% 9% 14% 16% 17% 4	2215
	11th to 20th	Day	42% 8% 13% 18% 18% 1	2200
	21st to end	Day	38% 6% 14% 16% 19% 6%	2200
October	1st to 10th	Day	26% 5% 7% 13% 39% 11%	3590
	11th to 20th	Day	20% 3 6% 17% 27% 28%	3377
	21st to end	Day	29% 4 5% 22% 27% 12%	3538
November	1st to 10th	Day	33% 3 7% 16% 31% 9%	1852
	11th to 20th	Day	33% 4 8% 19% 26% 10%	1890
	21st to end	Day	39% 5% 7% 16% 26% 7%	1890
December	1st to 10th	Day	42% 3 6% 16% 25% 8%	3020
	11th to 20th	Day	48% 3 7% 16% 19% 6%	3460
	21st to end	Day	43% 6% 9% 26% 14% 2	4565
January	1st to 10th	Night	38% 4 7% 17% 27% 6%	6019
	11th to 20th	Night	38% 4 6% 19% 23% 9%	5979
	21st to end	Night	44% 3 4 16% 25% 7%	6334
February	1st to 10th	Night	41% 4 8% 20% 20% 6%	2400
	11th to 20th	Night	47% 5% 7% 17% 18% 7%	2339
	21st to end	Night	46% 4 8% 15% 19% 8%	1980
March	1st to 10th	Night	47% 6% 8% 14% 18% 6%	5242
	11th to 20th	Night	30% 5% 8% 16% 25% 15%	4980
	21st to end	Night	41% 4 6% 22% 18% 7%	5467
April	1st to 10th	Night	34% 6% 12% 19% 23% 6%	2145
	11th to 20th	Night	34% 5% 8% 22% 21% 9%	1975
	21st to end	Night	34% 6% 12% 25% 19% 3	1830
May	1st to 10th	Night	43% 10% 16% 20% 11% 1	4060
	11th to 20th	Night	51% 8% 17% 17% 7%	4033
	21st to end	Night	37% 9% 14% 25% 14% 1	4429
June	1st to 10th	Night	40% 11% 17% 19% 11% 2	1800
	11th to 20th	Night	31% 11% 22% 22% 13% 1	1800
	21st to end	Night	31% 12% 19% 21% 13% 4	1800
July	1st to 10th	Night	41% 11% 17% 23% 8%	4015
	11th to 20th	Night	38% 15% 25% 18% 4 1	4006
	21st to end	Night	48% 12% 18% 17% 4 1	4297
August	1st to 10th	Night	35% 10% 19% 28% 7%	2904
	11th to 20th	Night	36% 9% 16% 23% 15% 1	3129
	21st to end	Night	30% 9% 13% 23% 22% 2	3586
September	1st to 10th	Night	38% 10% 15% 17% 15% 5%	2165
	11th to 20th	Night	38% 10% 14% 18% 18% 2	2200
	21st to end	Night	34% 8% 12% 20% 18% 7%	2200
October	1st to 10th	Night	24% 5% 9% 15% 36% 11%	3590
	11th to 20th	Night	19% 3 5% 18% 32% 23%	3761
	21st to end	Night	26% 3 5% 23% 27% 15%	4252
November	1st to 10th	Night	32% 4 8% 18% 26% 10%	2664
	11th to 20th	Night	32% 5% 7% 18% 27% 11%	2730
	21st to end	Night	38% 5% 7% 19% 24% 7%	2730
December	1st to 10th	Night	40% 4 6% 17% 26% 8%	4380
	11th to 20th	Night	43% 5% 8% 16% 22% 6%	4954
	21st to end	Night	44% 6% 8% 27% 13% 2	6633

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	33% 25% 32% 10% 1	330
	11th to 20th	Day	33% 22% 28% 13% 3	318
	21st to end	Day	30% 28% 31% 9% 2 1	330
February	1st to 10th	Day	27% 22% 38% 12% 2	300
	11th to 20th	Day	45% 23% 25% 7%	300
	21st to end	Day	39% 22% 30% 7% 2	249
March	1st to 10th	Day	32% 19% 29% 15% 5%	351
	11th to 20th	Day	27% 20% 29% 19% 5% 1	500
	21st to end	Day	25% 20% 34% 17% 5%	550
April	1st to 10th	Day	26% 14% 29% 22% 8% 1	500
	11th to 20th	Day	16% 18% 34% 21% 10% 1	492
	21st to end	Day	20% 12% 28% 23% 14% 3	500
May	1st to 10th	Day	18% 14% 29% 22% 12% 5%	503
	11th to 20th	Day	16% 12% 25% 24% 16% 8%	527
	21st to end	Day	18% 8% 19% 28% 17% 10%	573
June	1st to 10th	Day	15% 6% 23% 28% 19% 10%	530
	11th to 20th	Day	7% 5% 19% 28% 23% 18%	530
	21st to end	Day	9% 5% 19% 29% 24% 14%	530
July	1st to 10th	Day	11% 3 9% 29% 28% 19%	525
	11th to 20th	Day	8% 3 10% 25% 34% 20%	524
	21st to end	Day	8% 2 11% 28% 31% 20%	576
August	1st to 10th	Day	14% 5% 16% 27% 21% 17%	526
	11th to 20th	Day	13% 4 13% 27% 29% 14%	519
	21st to end	Day	15% 3 13% 25% 32% 13%	583
September	1st to 10th	Day	9% 5% 18% 32% 25% 11%	530
	11th to 20th	Day	15% 5% 15% 26% 29% 10%	530
	21st to end	Day	14% 5% 21% 27% 25% 8%	530
October	1st to 10th	Day	15% 6% 25% 31% 15% 7%	530
	11th to 20th	Day	15% 10% 24% 29% 18% 3	530
	21st to end	Day	20% 11% 27% 27% 13% 2	583
November	1st to 10th	Day	22% 15% 27% 25% 9% 2	330
	11th to 20th	Day	19% 21% 36% 19% 4 1	330
	21st to end	Day	19% 21% 35% 20% 4 1	330
December	1st to 10th	Day	35% 22% 28% 14% 2 1	330
	11th to 20th	Day	28% 18% 32% 18% 5%	330
	21st to end	Day	34% 15% 32% 15% 3 1	363
January	1st to 10th	Night	28% 19% 27% 22% 4	840
	11th to 20th	Night	24% 19% 29% 22% 6%	825
	21st to end	Night	22% 18% 34% 23% 2	880
February	1st to 10th	Night	21% 15% 31% 28% 5%	800
	11th to 20th	Night	29% 18% 31% 20% 2	783
	21st to end	Night	24% 20% 35% 20% 2	660
March	1st to 10th	Night	21% 19% 31% 25% 4	800
	11th to 20th	Night	19% 16% 33% 25% 7% 1	800
	21st to end	Night	20% 16% 28% 28% 8%	880
April	1st to 10th	Night	17% 16% 29% 28% 10%	745
	11th to 20th	Night	16% 14% 26% 31% 12% 1	593
	21st to end	Night	14% 7% 25% 35% 18% 1	600
May	1st to 10th	Night	12% 10% 25% 32% 21% 1	600
	11th to 20th	Night	17% 6% 20% 31% 21% 4	591
	21st to end	Night	14% 5% 14% 31% 31% 5%	650
June	1st to 10th	Night	10% 2 15% 37% 31% 5%	600
	11th to 20th	Night	7% 3 14% 29% 35% 13%	600
	21st to end	Night	6% 2 10% 36% 38% 8%	600
July	1st to 10th	Night	9% 3 8% 26% 43% 11%	585
	11th to 20th	Night	9% 2 9% 23% 45% 13%	582
	21st to end	Night	10% 1 8% 27% 43% 12%	657
August	1st to 10th	Night	13% 2 9% 25% 40% 10%	597
	11th to 20th	Night	12% 3 13% 26% 38% 9%	594
	21st to end	Night	15% 3 12% 25% 36% 9%	660
September	1st to 10th	Night	11% 3 14% 29% 35% 8%	765
	11th to 20th	Night	14% 3 16% 29% 35% 2	800
	21st to end	Night	14% 5% 15% 35% 29% 2	800
October	1st to 10th	Night	14% 5% 23% 32% 21% 6%	800
	11th to 20th	Night	12% 9% 26% 33% 18% 3	800
	21st to end	Night	16% 10% 23% 33% 16% 2	880
November	1st to 10th	Night	18% 8% 25% 30% 18% 1	820
	11th to 20th	Night	16% 14% 27% 30% 11% 1	840
	21st to end	Night	21% 15% 25% 27% 11%	840
December	1st to 10th	Night	25% 15% 27% 25% 7%	840
	11th to 20th	Night	23% 13% 29% 28% 7% 1	840
	21st to end	Night	29% 12% 27% 26% 5%	924

Page generated at 2026-01-14 01:14:28. Version ac07

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