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 2024-04-10.

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	16% 9% 3 6% 5% 3 5% 4 5% 7% 36%	83503
	Night	16% 9% 3 6% 5% 3 4 4 5% 7% 37%	85290
Transparency	Day	49% 3 10% 23% 14% 2	83226
	Night	48% 3 9% 22% 15% 2	85086
Seeing	Day	21% 12% 25% 22% 14% 6%	14127
	Night	20% 9% 23% 29% 16% 2	24321

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	18% 17% 5% 10% 6% 3 3 4 5% 7% 23%	2836
	11th to 20th	Day	17% 18% 8% 11% 8% 3 3 3 3 4 22%	2840
	21st to end	Day	25% 18% 5% 6% 5% 2 4 4 4 5% 23%	3124
February	1st to 10th	Day	22% 12% 5% 8% 5% 3 4 3 4 5% 30%	1710
	11th to 20th	Day	22% 10% 4 8% 6% 3 4 4 5% 5% 29%	1837
	21st to end	Day	22% 13% 4 5% 4 3 4 4 3 6% 31%	1570
March	1st to 10th	Day	22% 14% 4 6% 5% 3 4 4 3 5% 29%	3334
	11th to 20th	Day	16% 7% 3 5% 4 3 4 4 5% 6% 42%	3480
	21st to end	Day	18% 7% 3 6% 6% 3 4 4 6% 8% 35%	3815
April	1st to 10th	Day	15% 8% 4 6% 5% 3 5% 4 4 7% 38%	2080
	11th to 20th	Day	17% 8% 2 6% 5% 3 4 4 4 7% 41%	2245
	21st to end	Day	18% 7% 3 5% 6% 3 5% 4 5% 7% 37%	2340
May	1st to 10th	Day	19% 7% 3 6% 6% 3 5% 5% 6% 7% 34%	2340
	11th to 20th	Day	20% 7% 3 6% 6% 3 5% 5% 5% 5% 34%	2301
	21st to end	Day	10% 5% 3 6% 5% 5% 6% 7% 7% 9% 38%	2531
June	1st to 10th	Day	12% 4 3 4 5% 4 4 6% 7% 10% 42%	2340
	11th to 20th	Day	9% 5% 2 6% 6% 4 6% 5% 6% 10% 41%	2340
	21st to end	Day	10% 5% 2 5% 5% 4 6% 6% 7% 9% 40%	2340
July	1st to 10th	Day	13% 5% 3 5% 6% 4 5% 6% 6% 10% 39%	2340
	11th to 20th	Day	10% 4 3 6% 6% 4 7% 7% 8% 9% 37%	2340
	21st to end	Day	13% 5% 2 5% 5% 5% 6% 6% 6% 9% 38%	2547
August	1st to 10th	Day	12% 5% 3 5% 6% 3 6% 6% 7% 12% 35%	2327
	11th to 20th	Day	12% 5% 3 6% 7% 3 6% 6% 6% 10% 36%	2303
	21st to end	Day	9% 5% 2 5% 5% 3 5% 5% 7% 8% 45%	2503
September	1st to 10th	Day	14% 5% 2 4 4 2 4 5% 5% 7% 47%	1980
	11th to 20th	Day	11% 5% 2 6% 5% 3 4 4 5% 8% 47%	1980
	21st to end	Day	17% 6% 2 5% 5% 3 4 4 5% 5% 44%	1980
October	1st to 10th	Day	14% 5% 2 4 4 2 5% 4 5% 7% 48%	1918
	11th to 20th	Day	16% 7% 2 4 4 2 5% 4 4 6% 47%	1800
	21st to end	Day	21% 8% 2 5% 5% 2 3 3 4 5% 41%	1980
November	1st to 10th	Day	13% 5% 2 5% 4 2 3 4 5% 5% 51%	1695
	11th to 20th	Day	16% 9% 3 6% 5% 3 4 3 5% 6% 41%	1710
	21st to end	Day	18% 9% 3 6% 4 3 4 4 5% 5% 40%	1710
December	1st to 10th	Day	26% 11% 4 7% 5% 3 4 5% 4 6% 26%	1710
	11th to 20th	Day	20% 12% 5% 7% 5% 3 4 4 4 6% 31%	2153
	21st to end	Day	20% 17% 4 8% 6% 4 5% 5% 5% 6% 19%	3124
January	1st to 10th	Night	17% 18% 6% 9% 7% 3 4 4 4 5% 23%	4109
	11th to 20th	Night	23% 13% 6% 9% 5% 2 3 3 3 5% 28%	4120
	21st to end	Night	25% 14% 4 7% 5% 3 4 4 4 5% 26%	4532
February	1st to 10th	Night	20% 11% 4 8% 6% 3 4 4 5% 5% 30%	2470
	11th to 20th	Night	24% 10% 4 7% 6% 3 4 4 4 5% 29%	2227
	21st to end	Night	24% 14% 4 6% 5% 2 4 4 4 5% 28%	1884
March	1st to 10th	Night	23% 11% 3 6% 5% 3 5% 4 4 6% 32%	3519
	11th to 20th	Night	14% 9% 3 5% 5% 2 5% 4 5% 7% 41%	3480
	21st to end	Night	14% 7% 3 6% 5% 3 5% 5% 6% 7% 39%	3817
April	1st to 10th	Night	15% 7% 3 6% 5% 3 4 5% 5% 7% 39%	2090
	11th to 20th	Night	14% 7% 3 5% 5% 2 4 5% 6% 8% 41%	1676
	21st to end	Night	17% 6% 3 5% 5% 4 5% 4 6% 6% 39%	1620
May	1st to 10th	Night	18% 6% 3 6% 6% 3 5% 5% 5% 7% 36%	1620
	11th to 20th	Night	18% 7% 3 5% 6% 3 6% 4 6% 8% 35%	1592
	21st to end	Night	9% 5% 3 5% 5% 3 4 5% 7% 12% 41%	1744
June	1st to 10th	Night	10% 4 2 4 4 3 6% 6% 6% 10% 44%	1620
	11th to 20th	Night	7% 4 3 6% 5% 4 5% 5% 6% 10% 45%	1620
	21st to end	Night	10% 4 2 3 4 3 5% 5% 7% 11% 45%	1620
July	1st to 10th	Night	10% 6% 2 5% 5% 3 4 6% 6% 9% 42%	1620
	11th to 20th	Night	7% 4 2 5% 6% 3 5% 4 7% 8% 48%	1620
	21st to end	Night	9% 5% 2 5% 6% 4 5% 5% 8% 9% 42%	1771
August	1st to 10th	Night	10% 5% 2 6% 5% 3 5% 6% 7% 9% 42%	1611
	11th to 20th	Night	10% 5% 2 6% 5% 4 5% 6% 6% 7% 44%	1598
	21st to end	Night	9% 4 2 4 5% 3 4 5% 6% 9% 49%	1830
September	1st to 10th	Night	12% 4 1 4 4 2 4 6% 6% 8% 49%	1980
	11th to 20th	Night	10% 4 2 4 5% 3 4 4 5% 7% 53%	1980
	21st to end	Night	13% 6% 2 5% 5% 2 4 5% 6% 6% 46%	1980
October	1st to 10th	Night	12% 4 2 5% 5% 3 4 4 4 7% 51%	2042
	11th to 20th	Night	16% 6% 3 4 5% 2 3 4 6% 5% 45%	2160
	21st to end	Night	21% 6% 2 5% 5% 4 4 4 4 5% 42%	2376
November	1st to 10th	Night	12% 5% 3 4 4 2 5% 4 4 5% 52%	2378
	11th to 20th	Night	17% 7% 3 5% 4 3 4 3 5% 7% 43%	2470
	21st to end	Night	17% 10% 3 5% 5% 3 4 4 4 5% 40%	2470
December	1st to 10th	Night	23% 11% 4 8% 5% 3 4 4 4 5% 29%	2470
	11th to 20th	Night	18% 13% 6% 7% 5% 3 4 4 4 6% 31%	3042
	21st to end	Night	20% 15% 4 9% 8% 3 5% 5% 5% 5% 22%	4532

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	64% 2 10% 16% 8% 1	2837
	11th to 20th	Day	70% 1 6% 18% 5% 1	2840
	21st to end	Day	66% 1 6% 21% 5% 1	3124
February	1st to 10th	Day	59% 3 9% 18% 11% 1	1710
	11th to 20th	Day	59% 3 6% 20% 11% 2	1847
	21st to end	Day	56% 2 7% 21% 11% 2	1570
March	1st to 10th	Day	61% 1 6% 21% 9% 2	3334
	11th to 20th	Day	45% 1 8% 27% 17% 1	3480
	21st to end	Day	50% 2 7% 26% 14% 1	3815
April	1st to 10th	Day	47% 2 10% 23% 14% 4	2080
	11th to 20th	Day	45% 3 8% 22% 18% 4	2245
	21st to end	Day	48% 3 11% 24% 11% 3	2340
May	1st to 10th	Day	50% 4 11% 23% 11% 2	2340
	11th to 20th	Day	53% 4 12% 20% 10% 2	2305
	21st to end	Day	41% 6% 14% 25% 13% 1	2531
June	1st to 10th	Day	37% 5% 14% 27% 16% 1	2340
	11th to 20th	Day	39% 5% 15% 23% 16% 2	2340
	21st to end	Day	39% 6% 14% 24% 14% 3	2340
July	1st to 10th	Day	40% 6% 15% 21% 17% 2	2275
	11th to 20th	Day	40% 6% 13% 22% 16% 3	2271
	21st to end	Day	41% 6% 12% 26% 14% 2	2405
August	1st to 10th	Day	41% 5% 15% 26% 12% 1	2314
	11th to 20th	Day	43% 5% 11% 27% 12% 1	2303
	21st to end	Day	36% 5% 15% 25% 17% 3	2503
September	1st to 10th	Day	37% 4 10% 24% 19% 6%	1980
	11th to 20th	Day	37% 3 10% 27% 22% 1	1980
	21st to end	Day	43% 3 8% 23% 21% 2	1980
October	1st to 10th	Day	37% 3 11% 26% 21% 2	1918
	11th to 20th	Day	40% 2 9% 28% 20% 2	1800
	21st to end	Day	47% 2 9% 24% 17% 2	1980
November	1st to 10th	Day	35% 3 8% 27% 22% 4	1695
	11th to 20th	Day	47% 1 7% 22% 20% 3	1710
	21st to end	Day	47% 2 9% 23% 17% 1	1710
December	1st to 10th	Day	62% 3 8% 21% 6% 1	1710
	11th to 20th	Day	57% 2 9% 18% 13% 1	2150
	21st to end	Day	68% 2 8% 18% 5%	3124
January	1st to 10th	Night	66% 1 7% 17% 8% 1	4109
	11th to 20th	Night	62% 1 6% 21% 9% 1	4120
	21st to end	Night	64% 1 7% 20% 6% 1	4532
February	1st to 10th	Night	58% 2 8% 20% 11% 1	2470
	11th to 20th	Night	58% 3 6% 19% 13% 1	2228
	21st to end	Night	61% 2 8% 16% 10% 4	1884
March	1st to 10th	Night	57% 2 4 22% 14% 1	3519
	11th to 20th	Night	45% 2 7% 28% 17% 1	3480
	21st to end	Night	46% 2 8% 26% 17% 1	3817
April	1st to 10th	Night	45% 4 8% 22% 19% 3	2090
	11th to 20th	Night	42% 2 10% 21% 17% 8%	1676
	21st to end	Night	45% 4 11% 21% 17% 2	1620
May	1st to 10th	Night	47% 4 13% 20% 13% 3	1620
	11th to 20th	Night	47% 6% 11% 20% 13% 3	1593
	21st to end	Night	36% 6% 16% 25% 16% 1	1745
June	1st to 10th	Night	35% 6% 13% 25% 18% 4	1620
	11th to 20th	Night	35% 5% 16% 19% 21% 4	1620
	21st to end	Night	33% 7% 14% 25% 17% 3	1620
July	1st to 10th	Night	37% 6% 15% 20% 18% 4	1575
	11th to 20th	Night	33% 6% 15% 24% 18% 4	1566
	21st to end	Night	37% 6% 14% 26% 16% 1	1672
August	1st to 10th	Night	37% 6% 14% 26% 15% 2	1600
	11th to 20th	Night	37% 6% 12% 28% 16% 1	1598
	21st to end	Night	33% 4 10% 28% 20% 4	1830
September	1st to 10th	Night	33% 5% 14% 23% 18% 8%	1980
	11th to 20th	Night	33% 3 10% 26% 25% 3	1980
	21st to end	Night	39% 3 10% 21% 25% 2	1980
October	1st to 10th	Night	35% 3 10% 26% 21% 5%	2042
	11th to 20th	Night	41% 3 8% 24% 21% 3	2160
	21st to end	Night	47% 2 6% 24% 18% 3	2376
November	1st to 10th	Night	37% 2 8% 25% 24% 5%	2378
	11th to 20th	Night	44% 2 7% 22% 21% 4	2470
	21st to end	Night	48% 2 9% 24% 16% 2	2470
December	1st to 10th	Night	60% 3 8% 20% 8% 1	2470
	11th to 20th	Night	57% 2 8% 22% 11% 1	3044
	21st to end	Night	66% 2 7% 18% 7%	4532

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	27% 21% 36% 10% 4 1	270
	11th to 20th	Day	36% 21% 31% 9% 2	270
	21st to end	Day	32% 26% 27% 12% 4	297
February	1st to 10th	Day	31% 22% 31% 10% 6%	270
	11th to 20th	Day	45% 21% 26% 4 2 1	270
	21st to end	Day	41% 16% 34% 8% 1	225
March	1st to 10th	Day	30% 18% 30% 17% 5% 1	393
	11th to 20th	Day	27% 21% 31% 15% 5%	460
	21st to end	Day	27% 17% 33% 19% 4	506
April	1st to 10th	Day	26% 18% 29% 19% 7% 1	457
	11th to 20th	Day	23% 17% 30% 17% 10% 3	412
	21st to end	Day	26% 15% 32% 18% 7% 2	420
May	1st to 10th	Day	26% 14% 31% 17% 8% 4	423
	11th to 20th	Day	22% 10% 27% 24% 12% 5%	447
	21st to end	Day	14% 9% 20% 29% 18% 10%	485
June	1st to 10th	Day	13% 6% 23% 29% 20% 8%	450
	11th to 20th	Day	14% 6% 22% 29% 19% 10%	450
	21st to end	Day	11% 8% 30% 22% 20% 9%	450
July	1st to 10th	Day	14% 2 17% 26% 23% 18%	445
	11th to 20th	Day	9% 2 21% 33% 22% 13%	444
	21st to end	Day	13% 4 18% 28% 25% 13%	488
August	1st to 10th	Day	14% 3 16% 30% 25% 12%	446
	11th to 20th	Day	15% 4 14% 30% 25% 12%	439
	21st to end	Day	9% 4 16% 31% 27% 13%	495
September	1st to 10th	Day	14% 2 15% 29% 27% 13%	450
	11th to 20th	Day	13% 3 11% 30% 29% 14%	450
	21st to end	Day	21% 8% 18% 27% 18% 8%	450
October	1st to 10th	Day	15% 8% 23% 28% 20% 7%	450
	11th to 20th	Day	17% 13% 30% 24% 13% 3	450
	21st to end	Day	26% 11% 28% 23% 10% 1	495
November	1st to 10th	Day	18% 13% 31% 26% 9% 4	293
	11th to 20th	Day	27% 17% 27% 19% 9% 1	270
	21st to end	Day	24% 16% 33% 20% 6% 1	270
December	1st to 10th	Day	31% 20% 27% 14% 6% 1	270
	11th to 20th	Day	27% 19% 33% 14% 5% 1	270
	21st to end	Day	33% 19% 31% 13% 3 1	297
January	1st to 10th	Night	28% 18% 29% 19% 5%	760
	11th to 20th	Night	33% 16% 28% 19% 4	760
	21st to end	Night	26% 16% 32% 22% 3	836
February	1st to 10th	Night	27% 13% 31% 27% 2	760
	11th to 20th	Night	31% 17% 30% 19% 2	743
	21st to end	Night	32% 15% 33% 18% 2	628
March	1st to 10th	Night	26% 15% 28% 26% 5%	760
	11th to 20th	Night	25% 15% 31% 21% 7% 1	760
	21st to end	Night	24% 12% 32% 25% 7%	836
April	1st to 10th	Night	21% 14% 31% 26% 8%	760
	11th to 20th	Night	17% 11% 30% 28% 13% 2	574
	21st to end	Night	21% 8% 36% 28% 7% 1	540
May	1st to 10th	Night	18% 8% 28% 31% 13% 2	540
	11th to 20th	Night	20% 7% 24% 32% 15% 2	531
	21st to end	Night	12% 4 18% 33% 28% 4	584
June	1st to 10th	Night	11% 2 18% 40% 26% 3	540
	11th to 20th	Night	9% 2 16% 42% 26% 5%	540
	21st to end	Night	11% 4 15% 38% 29% 2	540
July	1st to 10th	Night	13% 2 10% 33% 34% 8%	525
	11th to 20th	Night	6% 2 14% 36% 38% 5%	522
	21st to end	Night	11% 2 12% 36% 34% 5%	591
August	1st to 10th	Night	12% 2 9% 38% 34% 5%	537
	11th to 20th	Night	11% 2 10% 34% 35% 8%	534
	21st to end	Night	10% 2 10% 34% 36% 8%	612
September	1st to 10th	Night	13% 1 12% 32% 36% 7%	720
	11th to 20th	Night	11% 3 12% 31% 37% 6%	720
	21st to end	Night	16% 4 18% 33% 26% 4	720
October	1st to 10th	Night	13% 4 19% 32% 28% 3	720
	11th to 20th	Night	18% 8% 25% 31% 16% 1	720
	21st to end	Night	23% 8% 24% 29% 14% 1	792
November	1st to 10th	Night	15% 7% 27% 32% 18% 1	740
	11th to 20th	Night	21% 13% 26% 28% 11%	760
	21st to end	Night	23% 11% 23% 34% 8% 1	760
December	1st to 10th	Night	32% 12% 26% 25% 5%	760
	11th to 20th	Night	24% 15% 29% 26% 6%	760
	21st to end	Night	30% 14% 25% 27% 4	836

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