Donald C. Martin Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Donald C. Martin Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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	15% 9% 3 7% 5% 5% 5% 4 6% 7% 36%	126578
	Night	15% 9% 3 8% 5% 4 4 3 5% 7% 36%	123136
Transparency	Day	47% 4 10% 24% 14% 2	130263
	Night	47% 3 9% 22% 16% 2	127520
Seeing	Day	21% 12% 25% 22% 14% 6%	16232
	Night	20% 9% 23% 29% 16% 2	26266

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	16% 19% 6% 14% 6% 4 3 3 4 5% 20%	4056
	11th to 20th	Day	15% 18% 8% 12% 6% 3 3 2 3 4 27%	4060
	21st to end	Day	20% 13% 4 8% 5% 4 4 3 5% 5% 28%	4466
February	1st to 10th	Day	21% 13% 5% 9% 5% 4 3 2 4 4 30%	1800
	11th to 20th	Day	23% 11% 4 10% 5% 3 4 3 5% 5% 28%	1937
	21st to end	Day	21% 13% 3 6% 4 4 3 3 4 6% 32%	1650
March	1st to 10th	Day	18% 14% 4 7% 4 5% 4 3 4 5% 32%	4792
	11th to 20th	Day	13% 7% 2 6% 4 4 4 3 5% 7% 43%	4980
	21st to end	Day	17% 8% 2 8% 5% 5% 4 4 6% 7% 34%	5465
April	1st to 10th	Day	16% 9% 3 7% 4 4 5% 3 4 7% 37%	2200
	11th to 20th	Day	16% 8% 2 7% 4 5% 4 3 5% 7% 41%	2499
	21st to end	Day	17% 8% 2 7% 5% 4 4 3 5% 7% 37%	2600
May	1st to 10th	Day	17% 9% 3 8% 5% 6% 5% 5% 7% 8% 27%	5862
	11th to 20th	Day	18% 9% 3 7% 5% 5% 5% 4 6% 7% 31%	5861
	21st to end	Day	14% 6% 3 7% 4 6% 5% 5% 7% 9% 34%	6433
June	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 8% 9% 41%	2600
	11th to 20th	Day	9% 5% 2 7% 5% 5% 5% 4 6% 10% 41%	2600
	21st to end	Day	9% 5% 2 5% 5% 6% 5% 5% 7% 9% 41%	2600
July	1st to 10th	Day	9% 6% 2 6% 5% 5% 5% 4 6% 9% 44%	5900
	11th to 20th	Day	10% 6% 3 7% 5% 5% 6% 5% 8% 11% 34%	5900
	21st to end	Day	14% 8% 3 8% 6% 6% 6% 5% 6% 8% 31%	6424
August	1st to 10th	Day	9% 6% 3 7% 5% 5% 6% 5% 7% 11% 37%	4234
	11th to 20th	Day	10% 6% 2 7% 5% 5% 5% 5% 6% 10% 39%	4174
	21st to end	Day	5% 3 1 4 3 4 4 3 6% 9% 56%	4530
September	1st to 10th	Day	13% 6% 2 5% 4 3 4 4 5% 7% 48%	2200
	11th to 20th	Day	11% 5% 2 6% 4 4 4 4 5% 8% 47%	2200
	21st to end	Day	18% 7% 2 6% 4 4 4 3 5% 5% 41%	2200
October	1st to 10th	Day	13% 5% 1 5% 3 4 4 3 5% 7% 50%	2130
	11th to 20th	Day	14% 7% 2 5% 3 3 4 3 5% 6% 49%	1980
	21st to end	Day	20% 9% 2 6% 3 3 3 2 4 6% 42%	2090
November	1st to 10th	Day	13% 6% 2 5% 4 2 3 3 6% 5% 51%	1786
	11th to 20th	Day	15% 11% 3 6% 4 4 4 2 5% 6% 40%	1800
	21st to end	Day	16% 12% 3 6% 4 4 4 3 5% 5% 39%	1800
December	1st to 10th	Day	20% 11% 3 9% 4 4 4 3 4 5% 33%	2930
	11th to 20th	Day	24% 13% 5% 7% 4 4 4 4 4 5% 26%	3373
	21st to end	Day	18% 16% 5% 11% 6% 6% 5% 4 5% 5% 21%	4466
January	1st to 10th	Night	17% 16% 6% 16% 5% 4 4 3 4 5% 21%	5889
	11th to 20th	Night	20% 13% 6% 11% 5% 4 3 3 4 5% 27%	5900
	21st to end	Night	20% 11% 4 8% 4 4 4 3 4 5% 33%	6490
February	1st to 10th	Night	20% 12% 4 8% 5% 4 4 3 5% 5% 29%	2600
	11th to 20th	Night	23% 11% 4 8% 5% 4 4 4 4 5% 28%	2347
	21st to end	Night	23% 14% 3 7% 4 3 4 4 4 5% 29%	1980
March	1st to 10th	Night	19% 11% 3 7% 4 4 4 3 4 6% 37%	5061
	11th to 20th	Night	12% 8% 2 5% 4 3 4 3 5% 7% 45%	4980
	21st to end	Night	15% 9% 3 8% 6% 5% 5% 4 6% 6% 33%	5467
April	1st to 10th	Night	16% 8% 3 7% 5% 4 4 4 5% 7% 38%	2200
	11th to 20th	Night	14% 8% 3 6% 4 3 4 4 6% 8% 40%	1864
	21st to end	Night	16% 6% 3 6% 4 5% 4 3 7% 7% 39%	1800
May	1st to 10th	Night	20% 9% 3 8% 5% 4 5% 4 6% 8% 29%	4033
	11th to 20th	Night	16% 7% 2 7% 5% 5% 5% 4 6% 8% 33%	4032
	21st to end	Night	14% 8% 3 8% 4 4 5% 4 6% 8% 35%	4421
June	1st to 10th	Night	10% 5% 2 5% 4 5% 5% 4 7% 9% 44%	1800
	11th to 20th	Night	7% 4 3 7% 5% 5% 5% 4 6% 9% 44%	1800
	21st to end	Night	10% 5% 2 4 4 4 5% 5% 8% 11% 45%	1800
July	1st to 10th	Night	8% 6% 3 7% 4 5% 5% 5% 7% 9% 41%	4058
	11th to 20th	Night	10% 7% 2 7% 5% 5% 5% 4 7% 9% 38%	4057
	21st to end	Night	10% 8% 3 8% 6% 6% 5% 4 7% 8% 35%	4427
August	1st to 10th	Night	10% 8% 3 8% 5% 4 5% 5% 6% 9% 37%	2915
	11th to 20th	Night	10% 6% 3 7% 5% 5% 5% 4 6% 7% 43%	2875
	21st to end	Night	5% 3 1 4 4 4 4 4 6% 9% 57%	3316
September	1st to 10th	Night	11% 4 2 5% 4 3 5% 4 6% 8% 50%	2200
	11th to 20th	Night	10% 5% 2 5% 4 4 4 3 5% 7% 53%	2200
	21st to end	Night	14% 7% 2 6% 4 4 5% 4 6% 6% 43%	2200
October	1st to 10th	Night	11% 4 2 6% 4 3 4 3 4 7% 52%	2270
	11th to 20th	Night	14% 6% 2 6% 4 3 4 3 6% 5% 47%	2377
	21st to end	Night	19% 6% 2 5% 4 4 4 2 5% 5% 43%	2508
November	1st to 10th	Night	12% 6% 2 5% 4 4 4 3 4 5% 50%	2507
	11th to 20th	Night	17% 8% 2 6% 4 3 4 3 5% 7% 41%	2600
	21st to end	Night	17% 11% 3 6% 4 4 4 3 4 5% 39%	2600
December	1st to 10th	Night	17% 11% 4 9% 5% 4 4 3 5% 5% 33%	4250
	11th to 20th	Night	19% 14% 6% 9% 4 4 4 3 4 5% 30%	4822
	21st to end	Night	18% 16% 4 11% 6% 5% 5% 4 5% 6% 22%	6490

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	69% 2 8% 15% 6%	4057
	11th to 20th	Day	65% 2 5% 16% 11% 1	4060
	21st to end	Day	59% 2 7% 23% 7% 1	4466
February	1st to 10th	Day	59% 3 9% 18% 11% 1	1800
	11th to 20th	Day	60% 3 6% 19% 10% 2	1947
	21st to end	Day	55% 2 8% 21% 12% 2	1650
March	1st to 10th	Day	56% 1 5% 21% 12% 4	4792
	11th to 20th	Day	42% 2 7% 26% 21% 3	4980
	21st to end	Day	50% 2 8% 24% 15% 2	5465
April	1st to 10th	Day	48% 2 10% 22% 14% 4	2200
	11th to 20th	Day	45% 3 8% 23% 18% 4	2499
	21st to end	Day	48% 3 11% 23% 12% 3	2600
May	1st to 10th	Day	54% 4 11% 18% 11% 2	5897
	11th to 20th	Day	52% 4 9% 24% 10% 1	5865
	21st to end	Day	46% 4 12% 22% 14% 3	6447
June	1st to 10th	Day	37% 5% 14% 26% 17% 1	2600
	11th to 20th	Day	38% 6% 15% 23% 17% 2	2600
	21st to end	Day	37% 6% 14% 24% 15% 3	2600
July	1st to 10th	Day	37% 6% 12% 27% 16% 1	5835
	11th to 20th	Day	41% 8% 14% 26% 9% 1	5831
	21st to end	Day	49% 6% 11% 20% 13% 1	6240
August	1st to 10th	Day	39% 6% 14% 30% 11%	4221
	11th to 20th	Day	38% 5% 10% 31% 16% 1	4213
	21st to end	Day	25% 4 14% 33% 22% 2	4530
September	1st to 10th	Day	36% 4 9% 23% 22% 6%	2200
	11th to 20th	Day	36% 3 9% 27% 23% 2	2200
	21st to end	Day	45% 3 9% 22% 20% 2	2200
October	1st to 10th	Day	28% 3 7% 27% 34% 2	3468
	11th to 20th	Day	27% 2 10% 31% 24% 5%	3220
	21st to end	Day	33% 2 10% 30% 23% 3	3428
November	1st to 10th	Day	36% 3 8% 28% 22% 3	1786
	11th to 20th	Day	48% 1 7% 22% 20% 3	1800
	21st to end	Day	49% 2 9% 23% 16% 1	1800
December	1st to 10th	Day	55% 2 6% 25% 11%	2930
	11th to 20th	Day	62% 3 8% 18% 9% 1	3370
	21st to end	Day	66% 2 8% 18% 5%	4466
January	1st to 10th	Night	68% 1 8% 15% 6% 1	5889
	11th to 20th	Night	62% 2 6% 18% 10% 3	5900
	21st to end	Night	56% 2 8% 23% 10% 1	6490
February	1st to 10th	Night	58% 2 8% 19% 11% 1	2600
	11th to 20th	Night	59% 3 6% 19% 12% 1	2348
	21st to end	Night	59% 2 8% 16% 11% 3	1980
March	1st to 10th	Night	51% 1 4 26% 16% 3	5061
	11th to 20th	Night	39% 2 6% 28% 23% 2	4980
	21st to end	Night	52% 2 7% 22% 15% 1	5467
April	1st to 10th	Night	47% 4 8% 21% 18% 3	2200
	11th to 20th	Night	43% 2 9% 23% 16% 7%	1864
	21st to end	Night	45% 4 11% 21% 18% 2	1800
May	1st to 10th	Night	54% 5% 12% 18% 10% 2	4060
	11th to 20th	Night	49% 7% 12% 21% 11% 1	4033
	21st to end	Night	47% 5% 12% 21% 12% 3	4429
June	1st to 10th	Night	36% 6% 12% 24% 19% 4	1800
	11th to 20th	Night	36% 5% 15% 19% 21% 4	1800
	21st to end	Night	32% 7% 15% 24% 18% 3	1800
July	1st to 10th	Night	37% 7% 12% 22% 19% 3	4015
	11th to 20th	Night	41% 8% 15% 23% 12% 2	4006
	21st to end	Night	45% 6% 11% 22% 13% 3	4297
August	1st to 10th	Night	41% 6% 14% 26% 12% 1	2904
	11th to 20th	Night	39% 5% 9% 26% 20% 1	2902
	21st to end	Night	24% 4 13% 32% 24% 3	3316
September	1st to 10th	Night	33% 5% 13% 22% 19% 8%	2200
	11th to 20th	Night	32% 3 9% 25% 26% 4	2200
	21st to end	Night	42% 3 10% 20% 23% 2	2200
October	1st to 10th	Night	28% 2 7% 26% 33% 4	3712
	11th to 20th	Night	29% 2 8% 27% 25% 9%	3876
	21st to end	Night	32% 2 6% 26% 28% 6%	4120
November	1st to 10th	Night	37% 2 8% 25% 24% 5%	2507
	11th to 20th	Night	45% 2 7% 22% 20% 4	2600
	21st to end	Night	49% 2 9% 24% 15% 2	2600
December	1st to 10th	Night	54% 3 11% 21% 10% 1	4250
	11th to 20th	Night	58% 2 10% 20% 8% 1	4824
	21st to end	Night	64% 2 9% 18% 7%	6490

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	28% 23% 34% 10% 4 1	300
	11th to 20th	Day	36% 23% 29% 9% 2	300
	21st to end	Day	30% 26% 28% 12% 4	330
February	1st to 10th	Day	30% 23% 32% 9% 5%	300
	11th to 20th	Day	47% 21% 25% 4 2 1	300
	21st to end	Day	38% 17% 34% 9% 1	249
March	1st to 10th	Day	30% 18% 29% 17% 5%	430
	11th to 20th	Day	27% 21% 31% 16% 5%	500
	21st to end	Day	28% 17% 33% 18% 4	550
April	1st to 10th	Day	27% 18% 29% 18% 7% 1	500
	11th to 20th	Day	21% 20% 31% 16% 9% 3	492
	21st to end	Day	23% 16% 32% 20% 7% 2	500
May	1st to 10th	Day	24% 16% 33% 16% 8% 4	503
	11th to 20th	Day	22% 11% 27% 23% 13% 5%	527
	21st to end	Day	16% 10% 22% 28% 16% 9%	573
June	1st to 10th	Day	13% 7% 24% 29% 19% 7%	530
	11th to 20th	Day	13% 5% 21% 30% 21% 9%	530
	21st to end	Day	10% 7% 30% 23% 21% 10%	530
July	1st to 10th	Day	13% 3 17% 27% 24% 16%	525
	11th to 20th	Day	9% 3 22% 33% 21% 13%	524
	21st to end	Day	13% 5% 18% 28% 25% 12%	576
August	1st to 10th	Day	13% 3 17% 30% 26% 11%	526
	11th to 20th	Day	15% 3 15% 30% 26% 12%	519
	21st to end	Day	8% 4 15% 32% 27% 14%	583
September	1st to 10th	Day	13% 2 16% 29% 27% 12%	530
	11th to 20th	Day	13% 3 10% 28% 31% 15%	530
	21st to end	Day	23% 8% 19% 26% 17% 7%	530
October	1st to 10th	Day	14% 7% 22% 27% 22% 7%	530
	11th to 20th	Day	15% 12% 29% 25% 15% 4	522
	21st to end	Day	24% 11% 28% 25% 11% 1	539
November	1st to 10th	Day	19% 13% 32% 25% 8% 3	324
	11th to 20th	Day	27% 16% 29% 19% 9% 1	300
	21st to end	Day	25% 18% 32% 18% 5% 1	300
December	1st to 10th	Day	30% 21% 29% 14% 6% 1	300
	11th to 20th	Day	28% 19% 31% 16% 5% 1	300
	21st to end	Day	32% 19% 31% 14% 3 1	330
January	1st to 10th	Night	28% 18% 29% 20% 5%	800
	11th to 20th	Night	32% 16% 29% 19% 4	800
	21st to end	Night	25% 16% 32% 23% 3	880
February	1st to 10th	Night	27% 13% 32% 26% 2	800
	11th to 20th	Night	32% 17% 30% 19% 2	783
	21st to end	Night	31% 16% 32% 20% 2	660
March	1st to 10th	Night	26% 15% 28% 27% 5%	800
	11th to 20th	Night	25% 15% 31% 22% 7% 1	800
	21st to end	Night	23% 13% 33% 24% 7%	880
April	1st to 10th	Night	22% 14% 32% 25% 8%	800
	11th to 20th	Night	17% 12% 31% 27% 12% 1	638
	21st to end	Night	20% 9% 35% 28% 7% 1	600
May	1st to 10th	Night	18% 8% 28% 31% 13% 2	600
	11th to 20th	Night	20% 8% 23% 32% 15% 2	591
	21st to end	Night	13% 4 20% 34% 26% 4	650
June	1st to 10th	Night	11% 3 18% 40% 25% 3	600
	11th to 20th	Night	8% 2 16% 42% 27% 5%	600
	21st to end	Night	11% 4 15% 37% 32% 2	600
July	1st to 10th	Night	12% 2 11% 33% 34% 7%	585
	11th to 20th	Night	6% 2 14% 37% 37% 5%	582
	21st to end	Night	12% 2 11% 35% 34% 6%	657
August	1st to 10th	Night	12% 2 10% 38% 33% 5%	597
	11th to 20th	Night	11% 3 10% 35% 34% 8%	594
	21st to end	Night	9% 2 11% 34% 36% 8%	680
September	1st to 10th	Night	12% 1 12% 33% 36% 6%	800
	11th to 20th	Night	11% 3 12% 32% 37% 7%	800
	21st to end	Night	17% 5% 18% 32% 25% 4	800
October	1st to 10th	Night	13% 4 18% 32% 30% 3	800
	11th to 20th	Night	17% 8% 24% 32% 17% 2	793
	21st to end	Night	22% 8% 23% 30% 15% 1	836
November	1st to 10th	Night	15% 7% 27% 32% 17% 1	780
	11th to 20th	Night	22% 12% 27% 28% 11%	800
	21st to end	Night	24% 10% 23% 33% 8% 1	800
December	1st to 10th	Night	31% 12% 27% 25% 5%	800
	11th to 20th	Night	24% 15% 28% 27% 6%	800
	21st to end	Night	30% 14% 25% 28% 4	880

Page generated at 2025-10-16 01:50:25. Version ac07

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