Martin Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Martin Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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	13% 7% 3 7% 5% 5% 5% 4 6% 8% 36%	127059
	Night	14% 7% 3 7% 5% 4 5% 4 6% 8% 39%	123647
Transparency	Day	42% 8% 12% 20% 14% 3	130691
	Night	40% 7% 12% 22% 15% 3	128084
Seeing	Day	21% 15% 24% 21% 14% 5%	16677
	Night	19% 13% 22% 28% 16% 2	26751

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% 9% 3 7% 4 4 3 3 5% 7% 40%	4056
	11th to 20th	Day	11% 10% 3 9% 6% 4 5% 4 5% 5% 38%	4060
	21st to end	Day	17% 8% 3 7% 4 3 4 3 4 6% 41%	4466
February	1st to 10th	Day	13% 10% 3 10% 5% 6% 4 3 5% 6% 35%	1800
	11th to 20th	Day	19% 8% 3 6% 5% 5% 4 3 6% 6% 34%	1836
	21st to end	Day	19% 9% 3 8% 5% 4 4 3 5% 6% 35%	1696
March	1st to 10th	Day	17% 7% 3 8% 5% 5% 5% 4 6% 7% 33%	4935
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 3 4 7% 48%	4980
	21st to end	Day	15% 8% 3 7% 5% 5% 5% 3 5% 6% 38%	5465
April	1st to 10th	Day	12% 8% 3 7% 4 4 5% 3 5% 8% 41%	2245
	11th to 20th	Day	14% 7% 2 7% 5% 4 4 4 5% 6% 41%	2374
	21st to end	Day	17% 7% 3 7% 5% 4 5% 4 5% 7% 38%	2584
May	1st to 10th	Day	14% 8% 3 8% 6% 6% 7% 5% 8% 9% 27%	5862
	11th to 20th	Day	21% 7% 3 8% 6% 5% 5% 4 7% 9% 25%	5861
	21st to end	Day	14% 6% 2 7% 5% 5% 6% 5% 7% 10% 33%	6433
June	1st to 10th	Day	12% 6% 2 5% 6% 5% 5% 5% 7% 10% 38%	2600
	11th to 20th	Day	9% 6% 3 7% 6% 6% 5% 6% 7% 10% 35%	2600
	21st to end	Day	8% 5% 2 7% 5% 6% 6% 5% 8% 11% 37%	2600
July	1st to 10th	Day	7% 4 2 8% 5% 6% 7% 6% 8% 10% 37%	5900
	11th to 20th	Day	7% 5% 3 8% 6% 7% 7% 7% 9% 12% 29%	5900
	21st to end	Day	11% 7% 3 9% 7% 7% 7% 6% 9% 10% 24%	6424
August	1st to 10th	Day	13% 7% 2 8% 6% 7% 6% 6% 8% 11% 27%	4234
	11th to 20th	Day	9% 5% 2 6% 5% 7% 7% 6% 8% 11% 32%	4174
	21st to end	Day	6% 5% 2 6% 5% 4 5% 4 7% 10% 47%	4397
September	1st to 10th	Day	12% 5% 2 7% 5% 5% 6% 4 6% 7% 43%	2313
	11th to 20th	Day	15% 6% 3 7% 5% 5% 5% 4 5% 8% 38%	2200
	21st to end	Day	21% 8% 3 7% 5% 5% 5% 4 5% 7% 32%	2200
October	1st to 10th	Day	15% 6% 2 5% 3 3 4 3 4 6% 48%	2040
	11th to 20th	Day	14% 7% 2 5% 3 3 5% 3 6% 6% 46%	2000
	21st to end	Day	15% 7% 2 6% 4 4 4 3 4 6% 46%	2200
November	1st to 10th	Day	11% 6% 2 6% 4 4 4 3 5% 7% 49%	2057
	11th to 20th	Day	11% 9% 2 4 5% 4 4 3 6% 7% 44%	1890
	21st to end	Day	15% 7% 3 6% 4 3 4 3 5% 7% 44%	1890
December	1st to 10th	Day	12% 10% 3 7% 4 4 4 3 5% 5% 42%	2948
	11th to 20th	Day	20% 11% 2 6% 4 4 4 3 5% 6% 34%	3373
	21st to end	Day	19% 10% 3 7% 5% 5% 6% 4 6% 6% 28%	4466
January	1st to 10th	Night	12% 8% 4 7% 5% 5% 5% 4 7% 8% 35%	5889
	11th to 20th	Night	12% 7% 3 9% 6% 4 4 4 5% 5% 40%	5900
	21st to end	Night	18% 8% 2 5% 3 3 4 3 4 6% 44%	6490
February	1st to 10th	Night	14% 9% 3 6% 5% 5% 5% 4 7% 7% 35%	2600
	11th to 20th	Night	20% 8% 3 7% 5% 5% 5% 4 5% 6% 33%	2448
	21st to end	Night	16% 7% 4 7% 5% 5% 4 4 6% 7% 35%	1932
March	1st to 10th	Night	19% 8% 2 6% 4 4 4 3 4 6% 39%	4918
	11th to 20th	Night	12% 5% 2 5% 4 4 4 4 6% 7% 48%	4980
	21st to end	Night	15% 7% 2 7% 5% 4 5% 4 5% 7% 40%	5467
April	1st to 10th	Night	14% 8% 2 6% 5% 5% 4 3 6% 8% 38%	2155
	11th to 20th	Night	14% 5% 2 5% 4 4 4 3 5% 8% 44%	1975
	21st to end	Night	15% 6% 3 6% 4 4 4 4 4 7% 43%	1832
May	1st to 10th	Night	17% 8% 3 7% 4 4 5% 4 6% 9% 33%	4033
	11th to 20th	Night	22% 8% 3 8% 5% 5% 4 4 5% 7% 29%	4032
	21st to end	Night	18% 6% 3 6% 5% 4 5% 4 7% 8% 34%	4421
June	1st to 10th	Night	8% 6% 2 6% 5% 4 5% 5% 9% 10% 40%	1800
	11th to 20th	Night	10% 6% 2 6% 4 4 4 4 6% 10% 43%	1800
	21st to end	Night	9% 4 2 5% 4 5% 5% 5% 7% 11% 44%	1800
July	1st to 10th	Night	8% 5% 2 6% 5% 5% 6% 5% 7% 10% 39%	4058
	11th to 20th	Night	7% 6% 3 7% 5% 6% 6% 6% 8% 11% 35%	4057
	21st to end	Night	9% 7% 3 9% 7% 6% 6% 5% 8% 10% 30%	4427
August	1st to 10th	Night	14% 8% 3 9% 5% 5% 5% 4 6% 9% 33%	2915
	11th to 20th	Night	10% 6% 2 6% 5% 5% 7% 4 7% 9% 38%	2875
	21st to end	Night	7% 4 2 5% 4 4 4 4 6% 9% 51%	3454
September	1st to 10th	Night	10% 6% 2 7% 4 5% 4 3 6% 7% 46%	2082
	11th to 20th	Night	14% 7% 2 6% 4 4 4 3 5% 8% 43%	2200
	21st to end	Night	19% 7% 3 6% 4 4 5% 4 4 5% 39%	2200
October	1st to 10th	Night	14% 6% 2 5% 4 3 3 3 6% 5% 48%	2359
	11th to 20th	Night	13% 6% 2 7% 4 4 4 3 5% 6% 47%	2400
	21st to end	Night	14% 5% 2 6% 3 4 4 3 5% 7% 48%	2640
November	1st to 10th	Night	14% 7% 3 4 4 3 4 4 4 6% 48%	2461
	11th to 20th	Night	13% 6% 2 5% 4 3 4 3 5% 7% 46%	2728
	21st to end	Night	14% 6% 3 5% 5% 4 4 4 5% 8% 43%	2730
December	1st to 10th	Night	12% 7% 3 7% 5% 4 4 3 6% 7% 42%	4277
	11th to 20th	Night	17% 11% 4 8% 5% 4 4 3 5% 7% 31%	4822
	21st to end	Night	19% 8% 5% 7% 4 5% 5% 4 6% 7% 30%	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	43% 4 9% 21% 21% 2	4057
	11th to 20th	Day	44% 5% 8% 16% 22% 5%	4060
	21st to end	Day	44% 6% 8% 22% 18% 3	4466
February	1st to 10th	Day	49% 5% 10% 20% 14% 2	1800
	11th to 20th	Day	50% 6% 10% 19% 13% 3	1845
	21st to end	Day	50% 5% 9% 17% 15% 4	1696
March	1st to 10th	Day	47% 5% 8% 19% 14% 7%	4935
	11th to 20th	Day	35% 5% 8% 21% 22% 8%	4980
	21st to end	Day	46% 5% 9% 21% 16% 3	5465
April	1st to 10th	Day	40% 7% 11% 22% 16% 4	2245
	11th to 20th	Day	41% 6% 11% 23% 15% 4	2374
	21st to end	Day	45% 7% 11% 23% 11% 2	2584
May	1st to 10th	Day	49% 10% 12% 19% 9% 1	5897
	11th to 20th	Day	52% 8% 12% 19% 7% 1	5865
	21st to end	Day	41% 10% 15% 17% 14% 3	6447
June	1st to 10th	Day	40% 11% 15% 21% 14%	2600
	11th to 20th	Day	39% 12% 15% 18% 14% 2	2600
	21st to end	Day	38% 11% 15% 21% 13% 1	2600
July	1st to 10th	Day	37% 12% 17% 20% 10% 3	5835
	11th to 20th	Day	40% 16% 19% 19% 6%	5831
	21st to end	Day	49% 13% 14% 17% 8%	6240
August	1st to 10th	Day	46% 11% 18% 19% 5%	4221
	11th to 20th	Day	40% 12% 15% 23% 10% 1	4213
	21st to end	Day	31% 9% 17% 28% 13% 1	4397
September	1st to 10th	Day	38% 9% 13% 18% 18% 4	2313
	11th to 20th	Day	43% 8% 10% 23% 15% 1	2200
	21st to end	Day	51% 6% 12% 16% 14% 1	2200
October	1st to 10th	Day	29% 7% 9% 20% 27% 8%	3326
	11th to 20th	Day	27% 4 10% 24% 19% 16%	3240
	21st to end	Day	29% 5% 10% 26% 17% 13%	3538
November	1st to 10th	Day	34% 5% 11% 22% 23% 4	2057
	11th to 20th	Day	37% 6% 10% 21% 22% 4	1890
	21st to end	Day	40% 5% 12% 22% 19% 2	1890
December	1st to 10th	Day	40% 8% 8% 19% 21% 3	2948
	11th to 20th	Day	50% 4 11% 17% 16% 2	3370
	21st to end	Day	52% 7% 12% 18% 10% 1	4466
January	1st to 10th	Night	44% 6% 11% 17% 18% 4	5889
	11th to 20th	Night	43% 5% 9% 18% 20% 6%	5900
	21st to end	Night	42% 4 8% 25% 17% 3	6490
February	1st to 10th	Night	44% 7% 15% 19% 14% 1	2600
	11th to 20th	Night	50% 7% 10% 18% 13% 4	2450
	21st to end	Night	46% 7% 11% 20% 11% 5%	1932
March	1st to 10th	Night	44% 5% 8% 21% 16% 5%	4918
	11th to 20th	Night	33% 5% 9% 23% 23% 6%	4980
	21st to end	Night	42% 6% 11% 21% 17% 3	5467
April	1st to 10th	Night	42% 7% 15% 19% 15% 3	2155
	11th to 20th	Night	38% 5% 13% 21% 19% 4	1975
	21st to end	Night	40% 6% 14% 25% 13% 1	1832
May	1st to 10th	Night	45% 11% 14% 20% 9% 1	4060
	11th to 20th	Night	52% 9% 13% 20% 6%	4033
	21st to end	Night	43% 11% 17% 21% 6% 3	4429
June	1st to 10th	Night	35% 12% 17% 22% 14% 1	1800
	11th to 20th	Night	35% 10% 18% 24% 12% 1	1800
	21st to end	Night	31% 12% 17% 24% 14% 2	1800
July	1st to 10th	Night	37% 12% 17% 22% 11% 2	4015
	11th to 20th	Night	39% 13% 21% 21% 5% 1	4006
	21st to end	Night	44% 13% 15% 19% 8%	4297
August	1st to 10th	Night	45% 11% 19% 20% 6%	2904
	11th to 20th	Night	38% 10% 17% 25% 10%	2902
	21st to end	Night	28% 8% 15% 34% 13% 1	3454
September	1st to 10th	Night	37% 10% 13% 19% 17% 4	2082
	11th to 20th	Night	39% 7% 13% 22% 18% 1	2200
	21st to end	Night	47% 8% 11% 20% 14% 2	2200
October	1st to 10th	Night	33% 6% 8% 21% 27% 4	3853
	11th to 20th	Night	28% 5% 10% 22% 24% 13%	3899
	21st to end	Night	24% 5% 10% 28% 21% 12%	4252
November	1st to 10th	Night	37% 6% 10% 21% 22% 4	2461
	11th to 20th	Night	36% 6% 10% 23% 20% 5%	2728
	21st to end	Night	39% 5% 10% 25% 17% 4	2730
December	1st to 10th	Night	37% 8% 10% 26% 18% 1	4277
	11th to 20th	Night	52% 5% 12% 18% 12% 2	4824
	21st to end	Night	49% 7% 13% 18% 11%	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	41% 27% 18% 10% 4	300
	11th to 20th	Day	42% 20% 28% 9% 1	300
	21st to end	Day	36% 26% 25% 10% 3	330
February	1st to 10th	Day	30% 27% 28% 12% 3	300
	11th to 20th	Day	48% 25% 20% 6% 1	300
	21st to end	Day	36% 25% 27% 10% 2	295
March	1st to 10th	Day	29% 25% 23% 18% 5%	500
	11th to 20th	Day	30% 25% 25% 15% 3 1	500
	21st to end	Day	29% 22% 29% 16% 5% 1	550
April	1st to 10th	Day	26% 26% 23% 16% 8%	500
	11th to 20th	Day	23% 21% 27% 17% 9% 3	492
	21st to end	Day	24% 18% 28% 17% 11% 2	500
May	1st to 10th	Day	21% 20% 31% 16% 8% 5%	503
	11th to 20th	Day	23% 11% 26% 21% 14% 4	527
	21st to end	Day	18% 9% 24% 25% 16% 9%	573
June	1st to 10th	Day	12% 9% 30% 25% 17% 8%	530
	11th to 20th	Day	10% 10% 25% 27% 20% 8%	530
	21st to end	Day	9% 9% 28% 27% 18% 8%	530
July	1st to 10th	Day	11% 4 18% 31% 24% 12%	525
	11th to 20th	Day	8% 4 18% 32% 25% 13%	524
	21st to end	Day	12% 5% 19% 29% 24% 12%	576
August	1st to 10th	Day	19% 6% 20% 21% 22% 11%	526
	11th to 20th	Day	12% 5% 18% 25% 24% 15%	519
	21st to end	Day	10% 4 16% 30% 20% 19%	583
September	1st to 10th	Day	12% 4 20% 26% 25% 12%	530
	11th to 20th	Day	15% 4 16% 28% 25% 12%	530
	21st to end	Day	24% 11% 22% 26% 14% 3	530
October	1st to 10th	Day	19% 9% 20% 25% 22% 5%	530
	11th to 20th	Day	17% 17% 22% 25% 17% 2	530
	21st to end	Day	22% 16% 26% 22% 12% 3	583
November	1st to 10th	Day	19% 16% 26% 25% 11% 2	535
	11th to 20th	Day	28% 20% 26% 20% 6% 1	330
	21st to end	Day	28% 22% 24% 21% 5% 1	330
December	1st to 10th	Day	29% 27% 28% 13% 3	306
	11th to 20th	Day	29% 22% 27% 16% 6%	300
	21st to end	Day	32% 18% 26% 15% 9%	330
January	1st to 10th	Night	28% 25% 26% 18% 3	800
	11th to 20th	Night	27% 22% 29% 20% 2	800
	21st to end	Night	23% 26% 32% 18% 1	880
February	1st to 10th	Night	23% 21% 30% 24% 2	800
	11th to 20th	Night	31% 25% 28% 15% 1	783
	21st to end	Night	25% 22% 33% 18% 1	660
March	1st to 10th	Night	23% 20% 33% 22% 3	800
	11th to 20th	Night	25% 20% 27% 22% 7%	800
	21st to end	Night	22% 16% 33% 23% 5%	880
April	1st to 10th	Night	23% 20% 25% 26% 5%	800
	11th to 20th	Night	19% 17% 27% 26% 11%	791
	21st to end	Night	16% 15% 26% 33% 10%	632
May	1st to 10th	Night	16% 12% 28% 30% 13% 1	600
	11th to 20th	Night	16% 10% 22% 35% 17% 1	591
	21st to end	Night	16% 4 17% 30% 30% 4	650
June	1st to 10th	Night	8% 3 17% 44% 25% 3	600
	11th to 20th	Night	10% 4 16% 34% 32% 4	600
	21st to end	Night	9% 3 14% 39% 32% 4	600
July	1st to 10th	Night	11% 2 8% 38% 35% 6%	585
	11th to 20th	Night	7% 2 8% 33% 45% 5%	582
	21st to end	Night	7% 2 9% 38% 39% 5%	657
August	1st to 10th	Night	14% 3 10% 36% 33% 4	597
	11th to 20th	Night	14% 1 9% 31% 39% 5%	594
	21st to end	Night	12% 3 10% 31% 38% 7%	800
September	1st to 10th	Night	12% 4 12% 34% 34% 5%	800
	11th to 20th	Night	15% 4 12% 30% 33% 5%	800
	21st to end	Night	23% 6% 18% 30% 21% 1	800
October	1st to 10th	Night	18% 9% 16% 30% 25% 3	800
	11th to 20th	Night	17% 11% 24% 30% 17% 1	800
	21st to end	Night	20% 12% 22% 33% 13% 1	880
November	1st to 10th	Night	18% 10% 24% 33% 13% 1	820
	11th to 20th	Night	20% 15% 24% 31% 10%	840
	21st to end	Night	21% 16% 25% 29% 9% 1	840
December	1st to 10th	Night	25% 20% 28% 21% 5%	809
	11th to 20th	Night	20% 18% 31% 26% 6%	800
	21st to end	Night	26% 17% 26% 28% 4	880

Page generated at 2025-11-30 01:00:50. Version ac07

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