McMurtrey Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the McMurtrey Site clear sky chart from 2005-11-06 to 2026-02-01.

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	21% 9% 3 6% 4 4 4 3 5% 6% 37%	136747
	Night	24% 10% 2 6% 4 3 3 3 4 5% 36%	132597
Transparency	Day	29% 40% 8% 12% 9% 3	131504
	Night	32% 40% 8% 11% 7% 2	128838
Seeing	Day	24% 9% 21% 26% 16% 4	16056
	Night	28% 6% 19% 32% 14% 1	25126

By Month

Percentage Cloud Cover SW (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	44% 17% 3 7% 3 3 3 3 3 4 10%	3719
	11th to 20th	Day	29% 13% 3 6% 4 4 4 3 5% 6% 23%	3720
	21st to end	Day	32% 8% 2 6% 3 4 3 2 4 5% 30%	4439
February	1st to 10th	Day	35% 12% 4 6% 4 4 4 3 5% 5% 17%	1801
	11th to 20th	Day	35% 14% 4 8% 5% 4 3 2 3 4 18%	1755
	21st to end	Day	36% 15% 3 7% 4 3 3 2 3 4 19%	1570
March	1st to 10th	Day	23% 14% 4 8% 5% 5% 5% 4 6% 7% 19%	4694
	11th to 20th	Day	29% 15% 4 7% 4 3 3 2 3 3 26%	5000
	21st to end	Day	31% 13% 4 9% 6% 5% 5% 4 5% 6% 13%	5617
April	1st to 10th	Day	31% 14% 4 8% 5% 4 4 4 5% 6% 17%	2400
	11th to 20th	Day	25% 13% 3 7% 4 4 4 4 4 5% 27%	2369
	21st to end	Day	23% 12% 3 7% 5% 5% 5% 4 5% 6% 25%	2586
May	1st to 10th	Day	17% 8% 2 6% 4 4 4 3 4 6% 42%	6267
	11th to 20th	Day	18% 10% 3 9% 6% 5% 5% 3 5% 7% 29%	6277
	21st to end	Day	21% 8% 3 6% 5% 5% 5% 4 5% 8% 31%	6902
June	1st to 10th	Day	24% 8% 3 6% 5% 4 4 3 5% 6% 33%	2800
	11th to 20th	Day	22% 8% 3 7% 5% 5% 5% 4 5% 7% 28%	2800
	21st to end	Day	11% 6% 2 6% 4 4 5% 4 5% 6% 48%	2800
July	1st to 10th	Day	5% 3 1 3 2 2 3 2 3 5% 71%	6319
	11th to 20th	Day	4 2 1 2 1 2 2 2 3 3 79%	6320
	21st to end	Day	5% 3 1 3 2 2 2 2 4 5% 71%	6880
August	1st to 10th	Day	5% 4 2 4 3 3 3 3 5% 6% 63%	5875
	11th to 20th	Day	8% 4 2 5% 3 4 4 4 6% 7% 52%	5796
	21st to end	Day	10% 6% 2 5% 4 4 4 3 5% 6% 51%	6143
September	1st to 10th	Day	9% 4 2 4 3 3 3 4 5% 6% 57%	2400
	11th to 20th	Day	15% 7% 3 7% 4 4 4 3 5% 6% 44%	2400
	21st to end	Day	18% 6% 2 6% 3 4 3 3 5% 6% 45%	2107
October	1st to 10th	Day	19% 6% 2 4 3 3 3 3 5% 7% 45%	3280
	11th to 20th	Day	23% 7% 3 7% 4 5% 5% 3 6% 7% 30%	3261
	21st to end	Day	32% 9% 3 7% 4 4 4 3 5% 7% 23%	3446
November	1st to 10th	Day	37% 11% 4 8% 4 4 3 3 5% 6% 15%	1744
	11th to 20th	Day	38% 11% 3 6% 3 3 4 3 5% 5% 18%	1680
	21st to end	Day	34% 12% 4 7% 4 3 3 3 5% 4 21%	1680
December	1st to 10th	Day	27% 10% 2 5% 3 3 4 4 6% 6% 30%	2700
	11th to 20th	Day	38% 13% 3 10% 5% 4 3 3 4 4 12%	3108
	21st to end	Day	36% 16% 4 9% 5% 4 4 2 4 4 11%	4092
January	1st to 10th	Night	42% 14% 3 7% 4 4 3 3 3 4 13%	6446
	11th to 20th	Night	29% 14% 3 6% 3 3 3 2 6% 6% 25%	6460
	21st to end	Night	31% 10% 2 6% 3 3 2 2 3 6% 32%	6769
February	1st to 10th	Night	32% 14% 3 7% 5% 5% 4 3 3 5% 19%	2600
	11th to 20th	Night	34% 13% 2 6% 4 4 3 3 3 4 25%	2532
	21st to end	Night	33% 15% 3 6% 4 3 4 2 4 6% 21%	2059
March	1st to 10th	Night	24% 15% 3 8% 5% 4 5% 4 5% 6% 21%	5183
	11th to 20th	Night	29% 14% 2 7% 4 3 3 2 3 4 29%	5000
	21st to end	Night	36% 14% 3 8% 5% 5% 4 3 4 5% 14%	5358
April	1st to 10th	Night	29% 14% 3 9% 6% 4 4 3 4 4 22%	2000
	11th to 20th	Night	23% 11% 3 7% 3 4 3 3 4 6% 32%	1979
	21st to end	Night	28% 9% 3 6% 5% 3 4 3 4 6% 30%	1810
May	1st to 10th	Night	21% 8% 2 4 2 2 3 2 4 5% 48%	3693
	11th to 20th	Night	17% 8% 3 7% 4 3 4 3 5% 6% 42%	3616
	21st to end	Night	16% 8% 2 6% 5% 4 4 4 7% 8% 36%	3972
June	1st to 10th	Night	20% 8% 3 6% 4 3 3 3 3 6% 40%	1600
	11th to 20th	Night	18% 8% 3 7% 4 4 3 2 5% 6% 40%	1600
	21st to end	Night	9% 6% 2 4 3 3 3 3 4 5% 58%	1600
July	1st to 10th	Night	3 1 2 2 2 2 2 3 4 79%	3640
	11th to 20th	Night	3 2 1 2 2 2 1 1 2 3 82%	3640
	21st to end	Night	4 2 1 2 2 1 2 2 3 5% 76%	3970
August	1st to 10th	Night	4 3 1 3 3 3 3 3 5% 6% 65%	3930
	11th to 20th	Night	13% 5% 2 4 3 3 3 3 4 6% 54%	4052
	21st to end	Night	10% 4 2 4 3 3 3 3 5% 6% 57%	4849
September	1st to 10th	Night	9% 3 1 3 2 3 3 2 4 6% 64%	2000
	11th to 20th	Night	14% 6% 2 6% 4 3 4 2 5% 4 50%	2000
	21st to end	Night	13% 6% 2 5% 3 3 3 3 4 8% 50%	2290
October	1st to 10th	Night	15% 5% 2 4 3 3 4 3 5% 6% 50%	3920
	11th to 20th	Night	20% 8% 2 6% 3 3 4 4 6% 8% 36%	3897
	21st to end	Night	29% 12% 3 7% 4 3 4 3 4 5% 26%	4359
November	1st to 10th	Night	36% 10% 3 7% 4 4 4 3 4 5% 20%	2771
	11th to 20th	Night	41% 9% 3 6% 3 3 3 3 3 4 20%	2940
	21st to end	Night	32% 10% 3 7% 4 4 4 3 4 5% 26%	2940
December	1st to 10th	Night	24% 10% 3 5% 4 4 5% 3 4 7% 32%	4700
	11th to 20th	Night	39% 14% 3 8% 5% 4 3 2 3 5% 13%	5316
	21st to end	Night	37% 15% 4 9% 5% 4 3 3 3 4 14%	7106

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	60% 29% 4 4 4	3719
	11th to 20th	Day	43% 30% 7% 12% 7% 1	3720
	21st to end	Day	42% 29% 4 10% 12% 3	4439
February	1st to 10th	Day	29% 63% 3 3 1	1801
	11th to 20th	Day	32% 62% 2 2 2 1	1764
	21st to end	Day	27% 63% 3 3 4	1570
March	1st to 10th	Day	42% 34% 11% 9% 4	4694
	11th to 20th	Day	44% 32% 4 16% 4	5000
	21st to end	Day	50% 35% 8% 5% 2	5617
April	1st to 10th	Day	25% 65% 6% 4 1	2400
	11th to 20th	Day	20% 64% 4 6% 5%	2372
	21st to end	Day	23% 64% 4 6% 3	2586
May	1st to 10th	Day	27% 32% 8% 17% 15%	6267
	11th to 20th	Day	36% 36% 12% 12% 4	6280
	21st to end	Day	33% 36% 16% 12% 3	6904
June	1st to 10th	Day	14% 63% 8% 11% 4	2800
	11th to 20th	Day	18% 64% 5% 9% 4	2800
	21st to end	Day	14% 62% 7% 12% 4 1	2800
July	1st to 10th	Day	10% 26% 9% 21% 15% 19%	6251
	11th to 20th	Day	6% 26% 6% 16% 34% 13%	6245
	21st to end	Day	12% 27% 16% 26% 17% 2	6681
August	1st to 10th	Day	11% 36% 4 17% 22% 10%	4279
	11th to 20th	Day	20% 42% 13% 16% 5% 3	4195
	21st to end	Day	22% 38% 7% 14% 12% 7%	4422
September	1st to 10th	Day	15% 59% 6% 10% 6% 3	2400
	11th to 20th	Day	18% 59% 5% 8% 7% 3	2400
	21st to end	Day	17% 58% 7% 8% 8% 1	2107
October	1st to 10th	Day	21% 37% 5% 20% 16% 1	3280
	11th to 20th	Day	35% 39% 8% 10% 7%	3261
	21st to end	Day	39% 39% 5% 12% 4	3446
November	1st to 10th	Day	34% 56% 5% 5% 1	1744
	11th to 20th	Day	31% 57% 4 4 3 1	1680
	21st to end	Day	28% 56% 3 4 8% 1	1680
December	1st to 10th	Day	32% 38% 7% 11% 11%	2700
	11th to 20th	Day	54% 34% 6% 5% 1	3108
	21st to end	Day	57% 32% 6% 4 1	4092
January	1st to 10th	Night	58% 29% 5% 4 3 1	6446
	11th to 20th	Night	41% 32% 7% 14% 6% 1	6460
	21st to end	Night	40% 29% 5% 10% 13% 3	6769
February	1st to 10th	Night	29% 64% 3 3 1	2600
	11th to 20th	Night	30% 62% 2 4 2 1	2534
	21st to end	Night	24% 63% 5% 5% 2 1	2059
March	1st to 10th	Night	43% 34% 9% 10% 4	5183
	11th to 20th	Night	41% 33% 7% 14% 5%	5000
	21st to end	Night	52% 34% 7% 7%	5358
April	1st to 10th	Night	25% 63% 7% 4 1	2000
	11th to 20th	Night	19% 64% 5% 6% 6%	1979
	21st to end	Night	20% 63% 7% 7% 2 1	1810
May	1st to 10th	Night	25% 31% 8% 23% 13%	3690
	11th to 20th	Night	29% 32% 18% 18% 4	3616
	21st to end	Night	28% 37% 21% 12% 3	3972
June	1st to 10th	Night	14% 62% 7% 12% 4	1600
	11th to 20th	Night	14% 63% 7% 14% 4	1600
	21st to end	Night	11% 62% 8% 14% 4 1	1600
July	1st to 10th	Night	5% 25% 11% 24% 16% 19%	3600
	11th to 20th	Night	7% 24% 6% 16% 35% 12%	3592
	21st to end	Night	11% 26% 17% 25% 19% 2	3857
August	1st to 10th	Night	8% 35% 5% 21% 25% 6%	2851
	11th to 20th	Night	20% 40% 18% 15% 4 3	2929
	21st to end	Night	20% 37% 10% 14% 12% 6%	3494
September	1st to 10th	Night	12% 59% 9% 11% 6% 3	2000
	11th to 20th	Night	18% 59% 6% 7% 6% 4	2000
	21st to end	Night	15% 59% 8% 9% 7% 2	2290
October	1st to 10th	Night	18% 38% 8% 21% 14%	3920
	11th to 20th	Night	31% 39% 11% 12% 7%	3897
	21st to end	Night	40% 40% 5% 11% 4 1	4359
November	1st to 10th	Night	32% 55% 4 6% 2	2771
	11th to 20th	Night	32% 56% 4 4 3 1	2940
	21st to end	Night	27% 56% 3 4 9% 1	2940
December	1st to 10th	Night	30% 39% 9% 14% 7%	4700
	11th to 20th	Night	55% 34% 5% 4 2	5316
	21st to end	Night	56% 33% 6% 4 1	7106

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	49% 10% 15% 17% 8% 1	220
	11th to 20th	Day	39% 8% 16% 21% 10% 5%	220
	21st to end	Day	37% 9% 18% 20% 12% 4	394
February	1st to 10th	Day	42% 11% 16% 20% 9% 2	401
	11th to 20th	Day	42% 11% 23% 14% 9% 1	395
	21st to end	Day	39% 13% 19% 20% 7% 1	374
March	1st to 10th	Day	35% 12% 22% 22% 8% 1	500
	11th to 20th	Day	31% 14% 22% 19% 9% 4	500
	21st to end	Day	35% 14% 23% 18% 9% 1	550
April	1st to 10th	Day	33% 13% 20% 21% 9% 4	500
	11th to 20th	Day	26% 16% 24% 20% 10% 3	492
	21st to end	Day	25% 10% 28% 20% 15% 1	500
May	1st to 10th	Day	17% 15% 25% 25% 14% 3	503
	11th to 20th	Day	27% 13% 22% 21% 14% 4	528
	21st to end	Day	21% 11% 22% 24% 17% 5%	573
June	1st to 10th	Day	23% 8% 20% 27% 19% 3	530
	11th to 20th	Day	17% 11% 31% 28% 11% 2	530
	21st to end	Day	8% 7% 23% 34% 23% 4	530
July	1st to 10th	Day	10% 6% 22% 37% 21% 5%	526
	11th to 20th	Day	5% 5% 23% 40% 20% 6%	524
	21st to end	Day	6% 3 17% 39% 30% 5%	575
August	1st to 10th	Day	7% 4 17% 41% 24% 7%	527
	11th to 20th	Day	4 3 16% 36% 32% 8%	517
	21st to end	Day	10% 6% 23% 33% 22% 7%	583
September	1st to 10th	Day	8% 4 22% 40% 20% 5%	530
	11th to 20th	Day	15% 6% 18% 32% 22% 5%	530
	21st to end	Day	17% 8% 24% 30% 18% 3	530
October	1st to 10th	Day	22% 8% 20% 28% 19% 4	530
	11th to 20th	Day	26% 7% 20% 25% 17% 4	530
	21st to end	Day	34% 11% 22% 20% 10% 3	520
November	1st to 10th	Day	36% 11% 22% 18% 10% 3	272
	11th to 20th	Day	36% 13% 21% 16% 10% 3	220
	21st to end	Day	39% 11% 12% 24% 13% 1	220
December	1st to 10th	Day	40% 8% 18% 15% 15% 3	220
	11th to 20th	Day	48% 13% 19% 15% 6%	220
	21st to end	Day	40% 13% 16% 20% 10% 1	242
January	1st to 10th	Night	44% 10% 15% 22% 8%	840
	11th to 20th	Night	38% 6% 15% 29% 10%	840
	21st to end	Night	36% 6% 16% 28% 12% 1	924
February	1st to 10th	Night	36% 7% 20% 28% 8% 1	800
	11th to 20th	Night	39% 7% 17% 25% 11% 1	784
	21st to end	Night	39% 8% 16% 28% 8%	660
March	1st to 10th	Night	33% 9% 18% 27% 12% 1	800
	11th to 20th	Night	32% 8% 20% 28% 11% 1	800
	21st to end	Night	36% 8% 21% 24% 10% 1	880
April	1st to 10th	Night	33% 8% 19% 26% 13% 1	800
	11th to 20th	Night	26% 7% 25% 28% 13% 1	791
	21st to end	Night	30% 4 23% 29% 12%	610
May	1st to 10th	Night	18% 6% 28% 35% 13%	427
	11th to 20th	Night	21% 4 22% 37% 16% 1	394
	21st to end	Night	19% 7% 21% 34% 18% 1	433
June	1st to 10th	Night	22% 4 22% 38% 14% 1	400
	11th to 20th	Night	19% 4 29% 39% 9%	400
	21st to end	Night	10% 2 28% 47% 14%	400
July	1st to 10th	Night	6% 2 22% 54% 16% 1	390
	11th to 20th	Night	5% 3 27% 48% 16%	388
	21st to end	Night	3 2 22% 56% 16% 1	438
August	1st to 10th	Night	5% 2 17% 45% 30% 2	544
	11th to 20th	Night	6% 1 12% 44% 34% 4	594
	21st to end	Night	8% 4 19% 45% 22% 2	805
September	1st to 10th	Night	10% 3 18% 46% 22% 2	800
	11th to 20th	Night	15% 4 15% 35% 29% 3	800
	21st to end	Night	14% 3 19% 42% 20% 2	800
October	1st to 10th	Night	21% 4 20% 34% 20% 3	800
	11th to 20th	Night	25% 4 16% 31% 21% 2	800
	21st to end	Night	31% 4 21% 30% 11% 2	880
November	1st to 10th	Night	38% 6% 17% 30% 9%	820
	11th to 20th	Night	44% 7% 15% 25% 8% 1	840
	21st to end	Night	33% 7% 18% 32% 11%	840
December	1st to 10th	Night	35% 6% 19% 27% 11% 1	840
	11th to 20th	Night	45% 7% 20% 21% 7%	840
	21st to end	Night	45% 8% 22% 18% 7%	924

Page generated at 2026-01-28 22:21:42. Version ac07

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