MAC-Johnson Site
Clear Sky Chart
History

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

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	12% 6% 2 6% 4 4 5% 4 6% 8% 42%	127279
	Night	12% 6% 2 6% 4 4 4 4 6% 8% 44%	124087
Transparency	Day	38% 5% 12% 19% 18% 6%	130951
	Night	38% 5% 11% 18% 20% 8%	128484
Seeing	Day	14% 7% 22% 25% 21% 12%	16879
	Night	14% 7% 23% 33% 21% 3	26964

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	14% 5% 2 4 3 2 4 3 5% 6% 53%	4065
	11th to 20th	Day	12% 5% 1 5% 3 3 2 2 4 6% 57%	4060
	21st to end	Day	15% 8% 2 6% 4 4 4 3 4 6% 45%	4466
February	1st to 10th	Day	14% 8% 3 6% 4 3 4 4 6% 7% 42%	1840
	11th to 20th	Day	20% 7% 2 6% 3 3 2 3 4 5% 44%	1948
	21st to end	Day	17% 7% 2 5% 4 3 4 4 4 6% 45%	1708
March	1st to 10th	Day	20% 8% 3 6% 4 4 4 3 4 5% 39%	4935
	11th to 20th	Day	8% 4 2 5% 4 4 4 3 5% 7% 55%	4980
	21st to end	Day	14% 7% 2 6% 4 4 4 3 4 4 47%	5465
April	1st to 10th	Day	9% 5% 2 6% 4 4 4 3 6% 7% 51%	2200
	11th to 20th	Day	10% 6% 2 5% 5% 4 4 4 5% 7% 49%	2330
	21st to end	Day	7% 4 1 5% 4 3 5% 3 6% 7% 53%	2520
May	1st to 10th	Day	10% 6% 2 6% 5% 4 4 4 7% 10% 41%	5862
	11th to 20th	Day	17% 7% 3 7% 4 4 5% 4 7% 9% 33%	5861
	21st to end	Day	10% 6% 2 6% 4 5% 5% 4 6% 8% 45%	6433
June	1st to 10th	Day	10% 6% 3 7% 5% 5% 6% 5% 8% 10% 36%	2600
	11th to 20th	Day	6% 5% 2 5% 5% 4 5% 5% 8% 11% 43%	2600
	21st to end	Day	5% 4 2 6% 6% 6% 6% 6% 9% 11% 40%	2600
July	1st to 10th	Day	12% 6% 3 9% 6% 8% 7% 5% 8% 10% 25%	5900
	11th to 20th	Day	4 4 2 7% 6% 7% 8% 6% 10% 15% 31%	5900
	21st to end	Day	8% 6% 3 8% 6% 6% 7% 6% 9% 13% 28%	6424
August	1st to 10th	Day	21% 7% 3 7% 5% 6% 6% 5% 7% 10% 22%	4234
	11th to 20th	Day	9% 5% 2 7% 6% 6% 7% 5% 8% 11% 35%	4174
	21st to end	Day	7% 5% 2 6% 4 5% 5% 5% 8% 11% 44%	4287
September	1st to 10th	Day	7% 4 3 7% 5% 5% 6% 5% 8% 9% 41%	2200
	11th to 20th	Day	12% 7% 2 7% 4 5% 4 5% 6% 8% 40%	2200
	21st to end	Day	16% 5% 2 6% 4 5% 4 4 5% 7% 42%	2200
October	1st to 10th	Day	12% 6% 2 5% 5% 4 4 3 5% 5% 49%	2102
	11th to 20th	Day	10% 4 2 4 3 3 2 2 4 6% 59%	2000
	21st to end	Day	11% 5% 2 4 3 3 4 3 4 4 57%	2200
November	1st to 10th	Day	12% 5% 2 4 4 3 3 3 4 5% 54%	2057
	11th to 20th	Day	13% 7% 2 4 2 2 3 3 4 5% 55%	1990
	21st to end	Day	13% 7% 2 5% 3 3 4 3 5% 7% 49%	1890
December	1st to 10th	Day	11% 5% 2 6% 4 4 4 3 6% 7% 50%	3020
	11th to 20th	Day	14% 6% 2 5% 3 3 3 3 5% 6% 48%	3463
	21st to end	Day	20% 7% 2 5% 3 3 3 3 4 5% 45%	4565
January	1st to 10th	Night	11% 5% 2 4 3 3 3 3 4 5% 57%	5903
	11th to 20th	Night	11% 6% 2 5% 3 4 4 3 6% 8% 47%	5900
	21st to end	Night	14% 8% 3 6% 4 4 3 3 4 5% 46%	6490
February	1st to 10th	Night	15% 6% 2 5% 5% 4 4 4 5% 7% 45%	2555
	11th to 20th	Night	19% 7% 2 6% 3 3 4 2 4 6% 44%	2338
	21st to end	Night	14% 5% 2 7% 3 3 5% 3 5% 7% 45%	1920
March	1st to 10th	Night	19% 8% 2 6% 4 4 4 3 5% 6% 39%	4918
	11th to 20th	Night	11% 4 1 3 3 3 3 4 5% 8% 54%	4980
	21st to end	Night	13% 5% 2 5% 4 4 4 3 5% 6% 49%	5467
April	1st to 10th	Night	11% 5% 2 5% 4 4 4 4 5% 8% 49%	2200
	11th to 20th	Night	12% 5% 2 4 3 4 3 3 4 7% 52%	2016
	21st to end	Night	8% 4 2 6% 4 3 4 4 5% 7% 54%	1895
May	1st to 10th	Night	9% 6% 3 7% 5% 4 5% 4 7% 9% 41%	4033
	11th to 20th	Night	17% 6% 2 6% 4 4 5% 4 7% 9% 35%	4032
	21st to end	Night	10% 5% 2 5% 4 4 5% 4 7% 9% 45%	4421
June	1st to 10th	Night	12% 6% 2 7% 3 5% 6% 5% 10% 10% 35%	1800
	11th to 20th	Night	6% 6% 3 6% 5% 5% 5% 5% 9% 11% 40%	1800
	21st to end	Night	5% 6% 2 7% 6% 5% 7% 5% 9% 12% 35%	1800
July	1st to 10th	Night	13% 8% 4 8% 6% 6% 7% 6% 7% 10% 25%	4058
	11th to 20th	Night	6% 5% 2 7% 5% 6% 7% 6% 11% 16% 30%	4057
	21st to end	Night	10% 8% 4 10% 6% 6% 7% 5% 8% 10% 25%	4427
August	1st to 10th	Night	19% 8% 3 9% 6% 6% 6% 5% 8% 9% 23%	2915
	11th to 20th	Night	8% 6% 3 8% 6% 5% 6% 6% 8% 11% 34%	2875
	21st to end	Night	7% 4 2 5% 4 4 4 4 7% 12% 48%	3570
September	1st to 10th	Night	8% 6% 3 6% 4 6% 5% 6% 7% 10% 41%	2200
	11th to 20th	Night	11% 7% 3 8% 6% 5% 5% 3 5% 7% 41%	2200
	21st to end	Night	14% 7% 2 7% 5% 5% 4 4 5% 7% 40%	2200
October	1st to 10th	Night	12% 5% 2 6% 4 4 3 3 5% 6% 51%	2296
	11th to 20th	Night	13% 4 1 4 2 2 2 2 5% 6% 58%	2400
	21st to end	Night	10% 5% 2 5% 3 3 3 3 4 6% 57%	2640
November	1st to 10th	Night	11% 5% 2 4 3 3 3 4 4 5% 56%	2461
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 5% 6% 53%	2625
	21st to end	Night	12% 6% 2 5% 4 3 3 3 6% 7% 49%	2730
December	1st to 10th	Night	13% 6% 2 5% 3 3 4 3 4 5% 54%	4380
	11th to 20th	Night	16% 7% 3 7% 3 3 3 2 4 5% 47%	4952
	21st to end	Night	19% 7% 2 5% 3 3 4 3 5% 6% 43%	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	32% 3 5% 22% 30% 7%	4066
	11th to 20th	Day	31% 3 4 17% 35% 10%	4060
	21st to end	Day	42% 3 4 17% 23% 12%	4466
February	1st to 10th	Day	41% 4 10% 21% 18% 7%	1840
	11th to 20th	Day	44% 3 5% 18% 25% 6%	1958
	21st to end	Day	41% 5% 6% 16% 21% 10%	1708
March	1st to 10th	Day	48% 3 6% 12% 22% 9%	4935
	11th to 20th	Day	30% 3 8% 16% 24% 18%	4980
	21st to end	Day	42% 3 6% 18% 22% 10%	5465
April	1st to 10th	Day	32% 5% 10% 21% 25% 7%	2200
	11th to 20th	Day	35% 5% 9% 20% 20% 11%	2330
	21st to end	Day	29% 6% 11% 27% 23% 4	2520
May	1st to 10th	Day	38% 5% 17% 27% 12% 1	5897
	11th to 20th	Day	48% 4 18% 19% 11% 1	5865
	21st to end	Day	37% 5% 14% 20% 22% 2	6447
June	1st to 10th	Day	40% 9% 17% 22% 11% 1	2600
	11th to 20th	Day	31% 9% 20% 25% 14% 1	2600
	21st to end	Day	34% 10% 20% 23% 12% 2	2600
July	1st to 10th	Day	51% 7% 18% 18% 5% 1	5835
	11th to 20th	Day	39% 9% 35% 14% 3 1	5831
	21st to end	Day	45% 10% 21% 19% 6%	6240
August	1st to 10th	Day	55% 7% 21% 14% 3	4221
	11th to 20th	Day	40% 8% 17% 22% 12%	4213
	21st to end	Day	32% 8% 13% 33% 13% 1	4287
September	1st to 10th	Day	36% 9% 16% 17% 17% 5%	2200
	11th to 20th	Day	41% 7% 13% 17% 19% 4	2200
	21st to end	Day	42% 6% 10% 20% 17% 6%	2200
October	1st to 10th	Day	36% 5% 5% 16% 28% 10%	3427
	11th to 20th	Day	17% 3 5% 15% 30% 30%	3240
	21st to end	Day	26% 3 4 18% 34% 15%	3538
November	1st to 10th	Day	33% 4 7% 17% 25% 14%	2057
	11th to 20th	Day	33% 3 6% 20% 26% 11%	1990
	21st to end	Day	37% 4 9% 20% 22% 9%	1890
December	1st to 10th	Day	35% 3 7% 19% 27% 9%	3020
	11th to 20th	Day	38% 3 9% 22% 22% 7%	3460
	21st to end	Day	42% 3 7% 22% 24% 3	4565
January	1st to 10th	Night	31% 4 4 19% 36% 7%	5903
	11th to 20th	Night	35% 4 5% 18% 30% 8%	5900
	21st to end	Night	41% 3 5% 12% 27% 11%	6490
February	1st to 10th	Night	39% 4 8% 20% 20% 9%	2555
	11th to 20th	Night	43% 3 6% 18% 21% 9%	2339
	21st to end	Night	38% 4 9% 16% 20% 13%	1920
March	1st to 10th	Night	47% 4 7% 12% 22% 9%	4918
	11th to 20th	Night	29% 3 7% 12% 29% 20%	4980
	21st to end	Night	36% 3 8% 19% 21% 12%	5467
April	1st to 10th	Night	34% 5% 10% 21% 21% 10%	2200
	11th to 20th	Night	32% 4 8% 22% 22% 11%	2016
	21st to end	Night	30% 5% 12% 26% 22% 5%	1895
May	1st to 10th	Night	39% 5% 16% 25% 13% 2	4060
	11th to 20th	Night	44% 6% 20% 18% 11%	4033
	21st to end	Night	35% 6% 13% 22% 21% 2	4429
June	1st to 10th	Night	38% 11% 18% 20% 12% 1	1800
	11th to 20th	Night	35% 10% 19% 25% 10% 2	1800
	21st to end	Night	36% 12% 21% 20% 10% 2	1800
July	1st to 10th	Night	52% 8% 21% 13% 4 1	4015
	11th to 20th	Night	40% 12% 33% 12% 3 1	4006
	21st to end	Night	53% 9% 19% 14% 5%	4297
August	1st to 10th	Night	56% 8% 20% 12% 3	2904
	11th to 20th	Night	42% 8% 21% 21% 7% 1	2902
	21st to end	Night	29% 6% 17% 34% 13% 1	3570
September	1st to 10th	Night	36% 10% 16% 21% 12% 6%	2200
	11th to 20th	Night	42% 7% 11% 19% 17% 3	2200
	21st to end	Night	43% 7% 12% 18% 15% 5%	2200
October	1st to 10th	Night	35% 3 8% 18% 29% 8%	3751
	11th to 20th	Night	19% 2 5% 15% 25% 34%	3899
	21st to end	Night	24% 3 5% 19% 30% 18%	4252
November	1st to 10th	Night	31% 3 10% 15% 26% 15%	2461
	11th to 20th	Night	33% 4 7% 16% 27% 13%	2625
	21st to end	Night	35% 3 8% 19% 23% 11%	2730
December	1st to 10th	Night	34% 3 7% 15% 32% 9%	4380
	11th to 20th	Night	41% 2 9% 18% 20% 9%	4954
	21st to end	Night	43% 3 5% 22% 23% 3	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	18% 13% 45% 20% 4	303
	11th to 20th	Day	19% 15% 36% 23% 6% 1	300
	21st to end	Day	23% 17% 39% 18% 4	330
February	1st to 10th	Day	15% 15% 43% 20% 7%	300
	11th to 20th	Day	29% 14% 36% 17% 3	300
	21st to end	Day	18% 18% 37% 21% 6% 1	307
March	1st to 10th	Day	17% 16% 29% 24% 13% 1	500
	11th to 20th	Day	14% 11% 40% 26% 7% 1	500
	21st to end	Day	16% 11% 38% 25% 9% 1	550
April	1st to 10th	Day	12% 14% 32% 28% 12% 2	500
	11th to 20th	Day	10% 11% 26% 28% 19% 6%	492
	21st to end	Day	9% 8% 22% 30% 20% 11%	500
May	1st to 10th	Day	8% 9% 27% 30% 19% 8%	503
	11th to 20th	Day	15% 3 18% 23% 27% 14%	527
	21st to end	Day	13% 3 15% 27% 21% 22%	573
June	1st to 10th	Day	10% 3 12% 27% 29% 19%	530
	11th to 20th	Day	8% 3 12% 25% 28% 23%	530
	21st to end	Day	5% 3 9% 28% 30% 25%	530
July	1st to 10th	Day	6% 2 8% 22% 28% 34%	525
	11th to 20th	Day	7% 2 4 15% 40% 31%	524
	21st to end	Day	7% 2 6% 22% 36% 27%	576
August	1st to 10th	Day	16% 2 10% 20% 31% 21%	526
	11th to 20th	Day	10% 2 8% 19% 31% 29%	519
	21st to end	Day	13% 2 7% 22% 30% 25%	583
September	1st to 10th	Day	8% 2 10% 28% 30% 21%	530
	11th to 20th	Day	16% 4 10% 24% 35% 12%	530
	21st to end	Day	19% 1 12% 26% 29% 12%	530
October	1st to 10th	Day	14% 4 20% 31% 23% 8%	530
	11th to 20th	Day	10% 5% 19% 32% 24% 9%	530
	21st to end	Day	14% 6% 22% 27% 24% 7%	583
November	1st to 10th	Day	17% 4 24% 30% 20% 5%	535
	11th to 20th	Day	17% 8% 34% 28% 11% 2	430
	21st to end	Day	11% 13% 32% 30% 11% 2	330
December	1st to 10th	Day	22% 10% 35% 25% 7% 1	330
	11th to 20th	Day	18% 12% 35% 24% 9% 2	330
	21st to end	Day	24% 9% 36% 21% 9% 2	363
January	1st to 10th	Night	19% 12% 34% 28% 7%	805
	11th to 20th	Night	15% 12% 34% 32% 8%	800
	21st to end	Night	17% 12% 38% 30% 3	880
February	1st to 10th	Night	18% 11% 31% 34% 6%	800
	11th to 20th	Night	22% 11% 33% 30% 3	783
	21st to end	Night	16% 14% 33% 32% 4	660
March	1st to 10th	Night	16% 9% 35% 35% 6%	800
	11th to 20th	Night	15% 10% 37% 30% 8%	800
	21st to end	Night	14% 10% 37% 29% 10%	880
April	1st to 10th	Night	13% 13% 32% 31% 11%	800
	11th to 20th	Night	14% 9% 27% 35% 15% 1	791
	21st to end	Night	10% 6% 26% 43% 14%	695
May	1st to 10th	Night	11% 4 29% 42% 14% 1	600
	11th to 20th	Night	12% 4 24% 37% 21% 3	591
	21st to end	Night	10% 2 12% 40% 32% 4	650
June	1st to 10th	Night	14% 2 14% 39% 26% 5%	600
	11th to 20th	Night	10% 2 11% 34% 33% 10%	600
	21st to end	Night	6% 2 11% 36% 39% 7%	600
July	1st to 10th	Night	10% 3 7% 31% 41% 9%	585
	11th to 20th	Night	8% 1 3 31% 47% 9%	582
	21st to end	Night	8% 2 6% 32% 45% 7%	657
August	1st to 10th	Night	13% 1 8% 27% 42% 9%	597
	11th to 20th	Night	12% 2 6% 31% 42% 7%	594
	21st to end	Night	14% 1 9% 27% 41% 8%	830
September	1st to 10th	Night	9% 2 9% 31% 43% 6%	800
	11th to 20th	Night	13% 3 10% 32% 38% 5%	800
	21st to end	Night	17% 2 13% 33% 31% 5%	800
October	1st to 10th	Night	14% 3 15% 37% 25% 4	800
	11th to 20th	Night	15% 4 21% 32% 25% 3	800
	21st to end	Night	12% 5% 22% 35% 23% 2	880
November	1st to 10th	Night	13% 6% 23% 36% 19% 3	820
	11th to 20th	Night	15% 9% 27% 36% 12% 1	840
	21st to end	Night	13% 8% 29% 34% 16% 1	840
December	1st to 10th	Night	20% 9% 27% 33% 11% 1	840
	11th to 20th	Night	18% 7% 29% 33% 12% 1	840
	21st to end	Night	23% 10% 26% 32% 8% 1	924

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