Megamark Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Megamark Observatory clear sky chart from 2005-11-06 to 2024-01-15.

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	14% 9% 4 7% 6% 3 5% 5% 5% 7% 36%	74829
	Night	14% 9% 4 6% 5% 3 4 4 5% 7% 39%	74987
Transparency	Day	46% 7% 12% 21% 12% 2	74530
	Night	45% 6% 12% 22% 14% 2	74807
Seeing	Day	17% 11% 29% 25% 14% 5%	14633
	Night	18% 9% 30% 30% 12% 1	24611

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	15% 12% 4 9% 8% 3 6% 3 5% 6% 28%	1517
	11th to 20th	Day	15% 16% 7% 11% 8% 3 4 4 5% 5% 23%	1477
	21st to end	Day	15% 16% 6% 11% 8% 4 4 4 4 5% 23%	1645
February	1st to 10th	Day	18% 13% 5% 8% 7% 5% 6% 5% 5% 5% 25%	1654
	11th to 20th	Day	15% 11% 5% 8% 7% 3 5% 4 4 6% 33%	1748
	21st to end	Day	16% 15% 5% 9% 6% 2 3 4 4 5% 30%	1480
March	1st to 10th	Day	16% 14% 5% 7% 6% 3 4 4 4 6% 31%	1800
	11th to 20th	Day	14% 8% 4 7% 6% 3 5% 5% 6% 7% 34%	1857
	21st to end	Day	20% 11% 5% 7% 7% 3 6% 5% 5% 6% 24%	2376
April	1st to 10th	Day	16% 11% 5% 6% 6% 3 4 4 6% 7% 31%	2160
	11th to 20th	Day	17% 9% 3 6% 5% 3 5% 4 6% 6% 36%	2134
	21st to end	Day	20% 8% 4 7% 6% 3 5% 4 5% 6% 32%	2245
May	1st to 10th	Day	18% 8% 3 6% 5% 4 5% 5% 6% 8% 30%	2340
	11th to 20th	Day	19% 8% 3 6% 6% 3 4 5% 5% 8% 34%	2421
	21st to end	Day	14% 7% 3 6% 6% 4 6% 6% 6% 8% 36%	2725
June	1st to 10th	Day	10% 6% 3 5% 4 4 5% 6% 6% 8% 46%	2520
	11th to 20th	Day	9% 5% 3 5% 5% 4 5% 5% 6% 9% 44%	2520
	21st to end	Day	11% 5% 3 6% 6% 4 6% 6% 7% 9% 37%	2520
July	1st to 10th	Day	7% 4 2 4 6% 3 5% 5% 7% 9% 47%	2520
	11th to 20th	Day	9% 4 2 5% 5% 3 5% 6% 6% 10% 45%	2520
	21st to end	Day	6% 5% 2 6% 7% 3 6% 5% 8% 9% 44%	2743
August	1st to 10th	Day	7% 5% 3 5% 5% 4 5% 6% 6% 11% 43%	2457
	11th to 20th	Day	8% 5% 3 5% 5% 4 5% 6% 7% 10% 42%	2125
	21st to end	Day	8% 4 1 5% 5% 3 5% 5% 7% 11% 47%	2376
September	1st to 10th	Day	11% 6% 2 6% 5% 3 5% 5% 5% 7% 47%	2160
	11th to 20th	Day	11% 4 2 4 4 3 4 4 5% 7% 52%	2052
	21st to end	Day	9% 7% 3 6% 5% 3 4 6% 6% 8% 45%	1980
October	1st to 10th	Day	12% 7% 2 5% 5% 3 4 4 5% 7% 46%	1925
	11th to 20th	Day	18% 7% 3 7% 5% 3 4 5% 5% 7% 35%	1800
	21st to end	Day	20% 8% 3 6% 6% 2 3 3 4 7% 36%	1782
November	1st to 10th	Day	13% 8% 3 5% 4 3 3 5% 5% 6% 46%	1672
	11th to 20th	Day	17% 10% 5% 7% 5% 3 5% 3 5% 7% 33%	1710
	21st to end	Day	19% 10% 3 9% 5% 3 4 3 3 5% 35%	1647
December	1st to 10th	Day	16% 12% 4 8% 6% 4 5% 4 4 6% 31%	1520
	11th to 20th	Day	14% 13% 5% 9% 6% 4 5% 5% 4 6% 29%	1918
	21st to end	Day	22% 21% 9% 11% 7% 3 4 3 2 4 14%	2783
January	1st to 10th	Night	15% 13% 4 8% 7% 3 4 4 4 5% 34%	2648
	11th to 20th	Night	16% 15% 6% 9% 6% 4 4 4 4 6% 27%	2589
	21st to end	Night	15% 12% 7% 10% 6% 3 5% 4 4 6% 26%	2709
February	1st to 10th	Night	16% 15% 6% 8% 7% 3 4 3 3 6% 30%	2304
	11th to 20th	Night	14% 11% 5% 6% 6% 3 4 5% 4 6% 36%	2098
	21st to end	Night	19% 10% 4 8% 5% 3 3 4 4 5% 35%	1776
March	1st to 10th	Night	16% 10% 4 8% 6% 3 5% 3 5% 5% 34%	2160
	11th to 20th	Night	13% 8% 3 5% 5% 3 4 4 6% 7% 41%	2116
	21st to end	Night	21% 11% 4 7% 4 3 5% 4 5% 6% 30%	1980
April	1st to 10th	Night	16% 9% 4 6% 4 3 4 4 6% 6% 39%	1800
	11th to 20th	Night	20% 7% 3 5% 5% 2 3 4 5% 6% 39%	1775
	21st to end	Night	18% 8% 3 6% 6% 3 5% 5% 6% 7% 34%	1715
May	1st to 10th	Night	16% 7% 2 6% 6% 3 5% 5% 6% 8% 36%	1620
	11th to 20th	Night	14% 7% 2 4 6% 4 5% 5% 5% 8% 41%	1470
	21st to end	Night	12% 6% 3 5% 5% 3 5% 5% 6% 10% 40%	1551
June	1st to 10th	Night	9% 4 2 5% 5% 3 4 4 6% 6% 51%	1440
	11th to 20th	Night	8% 5% 2 4 5% 3 5% 5% 7% 9% 47%	1440
	21st to end	Night	10% 5% 2 6% 5% 3 6% 5% 6% 8% 43%	1440
July	1st to 10th	Night	6% 4 3 6% 5% 2 5% 5% 6% 10% 49%	1440
	11th to 20th	Night	6% 3 2 5% 5% 2 4 5% 7% 8% 54%	1440
	21st to end	Night	4 3 2 3 6% 3 5% 5% 7% 9% 52%	1575
August	1st to 10th	Night	5% 4 2 5% 5% 4 4 5% 7% 9% 50%	1478
	11th to 20th	Night	7% 3 2 5% 4 2 4 5% 6% 9% 53%	1776
	21st to end	Night	8% 3 1 3 4 3 4 4 5% 9% 54%	1980
September	1st to 10th	Night	10% 5% 2 4 4 2 4 3 4 7% 53%	1800
	11th to 20th	Night	9% 3 2 4 4 3 4 3 5% 8% 55%	1890
	21st to end	Night	10% 4 2 6% 5% 3 4 5% 6% 7% 47%	1980
October	1st to 10th	Night	11% 5% 2 5% 4 3 4 4 5% 6% 51%	2033
	11th to 20th	Night	19% 7% 2 4 4 2 4 3 4 7% 45%	2160
	21st to end	Night	21% 9% 4 5% 4 2 4 3 4 6% 38%	2574
November	1st to 10th	Night	13% 7% 3 5% 5% 2 3 4 4 6% 49%	2406
	11th to 20th	Night	15% 10% 3 7% 6% 2 5% 3 5% 7% 38%	2470
	21st to end	Night	17% 9% 4 7% 5% 3 4 3 4 6% 37%	2531
December	1st to 10th	Night	17% 10% 4 9% 6% 3 4 4 4 6% 32%	2660
	11th to 20th	Night	16% 13% 5% 9% 5% 3 5% 4 4 5% 32%	3279
	21st to end	Night	24% 17% 7% 9% 6% 4 4 4 4 5% 16%	4884

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	57% 5% 12% 16% 10%	1518
	11th to 20th	Day	64% 5% 13% 13% 5% 1	1477
	21st to end	Day	64% 5% 10% 13% 8%	1645
February	1st to 10th	Day	61% 6% 11% 13% 7% 2	1654
	11th to 20th	Day	54% 4 11% 21% 9% 2	1758
	21st to end	Day	57% 6% 9% 19% 8% 2	1480
March	1st to 10th	Day	56% 5% 10% 21% 7% 1	1800
	11th to 20th	Day	48% 6% 13% 21% 10% 2	1857
	21st to end	Day	60% 7% 10% 17% 5% 1	2376
April	1st to 10th	Day	52% 7% 11% 20% 11%	2160
	11th to 20th	Day	47% 7% 11% 20% 11% 4	2134
	21st to end	Day	51% 6% 13% 16% 12% 2	2245
May	1st to 10th	Day	50% 8% 13% 21% 8%	2340
	11th to 20th	Day	48% 7% 11% 22% 11% 1	2425
	21st to end	Day	44% 8% 12% 19% 16% 2	2725
June	1st to 10th	Day	35% 8% 12% 27% 17% 2	2520
	11th to 20th	Day	36% 7% 11% 23% 20% 3	2520
	21st to end	Day	41% 10% 13% 21% 13% 2	2520
July	1st to 10th	Day	32% 8% 15% 27% 15% 3	2451
	11th to 20th	Day	33% 8% 15% 26% 16% 2	2445
	21st to end	Day	34% 9% 17% 24% 15% 3	2591
August	1st to 10th	Day	33% 9% 17% 28% 11% 2	2442
	11th to 20th	Day	34% 8% 15% 26% 16% 1	2125
	21st to end	Day	30% 8% 13% 26% 19% 3	2376
September	1st to 10th	Day	36% 7% 12% 24% 17% 5%	2160
	11th to 20th	Day	31% 6% 12% 29% 20% 3	2052
	21st to end	Day	35% 8% 9% 26% 20% 2	1980
October	1st to 10th	Day	39% 6% 12% 25% 17% 2	1925
	11th to 20th	Day	48% 5% 12% 21% 14% 1	1800
	21st to end	Day	48% 6% 11% 23% 10% 2	1782
November	1st to 10th	Day	39% 6% 14% 23% 16% 2	1672
	11th to 20th	Day	51% 5% 11% 21% 11% 1	1710
	21st to end	Day	53% 4 10% 23% 9%	1647
December	1st to 10th	Day	54% 6% 13% 18% 8% 1	1520
	11th to 20th	Day	55% 6% 13% 18% 7% 1	1915
	21st to end	Day	78% 3 8% 9% 3	2783
January	1st to 10th	Night	53% 5% 10% 19% 12% 1	2648
	11th to 20th	Night	59% 5% 14% 13% 8%	2589
	21st to end	Night	59% 6% 10% 16% 9% 1	2709
February	1st to 10th	Night	58% 4 11% 17% 9% 2	2304
	11th to 20th	Night	50% 4 11% 21% 13% 1	2099
	21st to end	Night	52% 4 10% 19% 12% 3	1776
March	1st to 10th	Night	52% 5% 12% 21% 9% 1	2160
	11th to 20th	Night	42% 6% 12% 20% 16% 3	2116
	21st to end	Night	55% 6% 11% 18% 10% 1	1980
April	1st to 10th	Night	44% 6% 12% 23% 12% 2	1800
	11th to 20th	Night	46% 5% 8% 21% 15% 4	1775
	21st to end	Night	47% 7% 12% 20% 12% 3	1715
May	1st to 10th	Night	45% 7% 12% 24% 11%	1620
	11th to 20th	Night	41% 9% 13% 22% 15% 1	1471
	21st to end	Night	38% 9% 15% 19% 16% 3	1552
June	1st to 10th	Night	32% 7% 12% 25% 20% 4	1440
	11th to 20th	Night	32% 7% 13% 24% 22% 3	1440
	21st to end	Night	38% 9% 15% 20% 16% 3	1440
July	1st to 10th	Night	30% 8% 15% 26% 17% 5%	1400
	11th to 20th	Night	27% 7% 14% 28% 21% 4	1392
	21st to end	Night	25% 7% 18% 26% 18% 5%	1487
August	1st to 10th	Night	27% 8% 19% 28% 16% 2	1469
	11th to 20th	Night	26% 7% 17% 27% 21% 2	1776
	21st to end	Night	27% 6% 13% 28% 20% 5%	1980
September	1st to 10th	Night	32% 6% 11% 25% 23% 3	1800
	11th to 20th	Night	28% 5% 9% 30% 25% 2	1890
	21st to end	Night	34% 6% 12% 28% 17% 3	1980
October	1st to 10th	Night	34% 6% 9% 27% 21% 3	2033
	11th to 20th	Night	39% 6% 10% 24% 20% 2	2160
	21st to end	Night	48% 4 10% 24% 13% 1	2574
November	1st to 10th	Night	37% 6% 9% 26% 19% 4	2406
	11th to 20th	Night	47% 4 13% 22% 13% 2	2470
	21st to end	Night	49% 4 12% 23% 12%	2531
December	1st to 10th	Night	53% 5% 11% 21% 8% 1	2660
	11th to 20th	Night	54% 5% 13% 18% 9% 1	3281
	21st to end	Night	70% 6% 9% 11% 4	4884

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	23% 14% 34% 23% 6% 1	270
	11th to 20th	Day	24% 17% 36% 18% 4 1	254
	21st to end	Day	22% 22% 31% 20% 5%	264
February	1st to 10th	Day	25% 22% 37% 13% 3	240
	11th to 20th	Day	25% 20% 33% 18% 3	240
	21st to end	Day	26% 22% 37% 13% 2 1	198
March	1st to 10th	Day	27% 22% 34% 16% 1	240
	11th to 20th	Day	28% 16% 34% 16% 5% 1	256
	21st to end	Day	29% 15% 29% 18% 6% 2	462
April	1st to 10th	Day	23% 15% 31% 24% 6% 1	420
	11th to 20th	Day	22% 21% 25% 18% 12% 2	412
	21st to end	Day	22% 16% 30% 23% 8% 1	420
May	1st to 10th	Day	23% 13% 29% 24% 9% 2	423
	11th to 20th	Day	21% 10% 31% 24% 10% 4	567
	21st to end	Day	15% 10% 30% 23% 16% 6%	679
June	1st to 10th	Day	10% 8% 29% 30% 17% 6%	630
	11th to 20th	Day	12% 7% 30% 29% 18% 4	630
	21st to end	Day	13% 8% 32% 30% 13% 4	630
July	1st to 10th	Day	8% 4 22% 29% 23% 15%	621
	11th to 20th	Day	12% 5% 24% 31% 18% 10%	618
	21st to end	Day	6% 4 31% 33% 21% 6%	684
August	1st to 10th	Day	7% 2 20% 37% 22% 12%	582
	11th to 20th	Day	9% 5% 18% 28% 27% 13%	439
	21st to end	Day	6% 5% 20% 32% 26% 11%	495
September	1st to 10th	Day	13% 6% 22% 32% 22% 6%	450
	11th to 20th	Day	11% 4 19% 31% 26% 10%	450
	21st to end	Day	11% 12% 24% 26% 20% 6%	450
October	1st to 10th	Day	15% 9% 28% 29% 15% 4	395
	11th to 20th	Day	19% 26% 33% 16% 7%	270
	21st to end	Day	31% 15% 32% 16% 5%	297
November	1st to 10th	Day	17% 16% 34% 21% 8% 4	270
	11th to 20th	Day	26% 18% 33% 16% 6% 1	270
	21st to end	Day	25% 12% 37% 20% 6%	270
December	1st to 10th	Day	19% 20% 38% 14% 9%	270
	11th to 20th	Day	20% 16% 41% 18% 5% 1	270
	21st to end	Day	26% 21% 37% 15% 2	297
January	1st to 10th	Night	21% 14% 39% 22% 5%	950
	11th to 20th	Night	24% 14% 34% 22% 5% 1	925
	21st to end	Night	20% 12% 36% 28% 3	990
February	1st to 10th	Night	24% 13% 36% 24% 3	864
	11th to 20th	Night	22% 13% 35% 25% 4	703
	21st to end	Night	24% 14% 36% 22% 4	592
March	1st to 10th	Night	21% 14% 35% 24% 6%	720
	11th to 20th	Night	17% 12% 36% 27% 7% 1	694
	21st to end	Night	24% 11% 34% 24% 6% 1	594
April	1st to 10th	Night	20% 13% 34% 26% 7%	540
	11th to 20th	Night	22% 11% 33% 26% 8%	533
	21st to end	Night	21% 10% 33% 28% 8%	540
May	1st to 10th	Night	19% 7% 37% 28% 8% 1	540
	11th to 20th	Night	19% 6% 30% 33% 12%	531
	21st to end	Night	13% 4 28% 36% 17% 1	584
June	1st to 10th	Night	11% 3 28% 43% 15%	540
	11th to 20th	Night	10% 3 30% 44% 12% 1	540
	21st to end	Night	10% 3 31% 38% 17% 1	540
July	1st to 10th	Night	8% 2 16% 43% 28% 3	525
	11th to 20th	Night	7% 3 19% 41% 28% 2	522
	21st to end	Night	5% 2 19% 47% 26% 2	591
August	1st to 10th	Night	6% 2 13% 47% 29% 4	537
	11th to 20th	Night	6% 2 13% 43% 31% 4	534
	21st to end	Night	8% 2 18% 39% 31% 3	594
September	1st to 10th	Night	13% 2 19% 38% 26% 3	540
	11th to 20th	Night	10% 4 18% 37% 27% 3	630
	21st to end	Night	12% 7% 24% 33% 22% 3	720
October	1st to 10th	Night	13% 6% 27% 34% 19% 2	720
	11th to 20th	Night	23% 11% 30% 26% 10% 1	720
	21st to end	Night	26% 12% 30% 24% 8%	792
November	1st to 10th	Night	15% 11% 38% 24% 10% 1	740
	11th to 20th	Night	19% 15% 34% 26% 6%	760
	21st to end	Night	20% 12% 37% 24% 6%	821
December	1st to 10th	Night	23% 12% 35% 25% 5%	950
	11th to 20th	Night	23% 11% 34% 26% 5%	950
	21st to end	Night	29% 15% 33% 21% 3	1045

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