MEL Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the MEL Observatory clear sky chart from 2005-11-06 to 2026-06-01.

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% 6% 2 5% 4 4 4 3 5% 7% 47%	132507
	Night	15% 6% 2 4 3 3 3 3 4 6% 50%	130967
Transparency	Day	35% 6% 9% 20% 19% 10%	136286
	Night	34% 5% 10% 19% 19% 12%	135252
Seeing	Day	15% 9% 24% 29% 19% 5%	16860
	Night	17% 8% 25% 34% 15% 1	26836

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% 8% 2 5% 2 3 3 2 4 5% 52%	5289
	11th to 20th	Day	15% 5% 2 4 3 3 3 2 5% 5% 55%	5310
	21st to end	Day	24% 7% 2 5% 3 2 2 2 3 6% 41%	6265
February	1st to 10th	Day	18% 7% 2 6% 3 3 3 3 4 5% 46%	2100
	11th to 20th	Day	18% 7% 2 5% 3 3 3 3 4 7% 45%	2049
	21st to end	Day	18% 5% 2 5% 3 2 3 2 4 5% 51%	1730
March	1st to 10th	Day	13% 6% 2 5% 3 3 3 3 5% 5% 52%	4975
	11th to 20th	Day	14% 7% 2 5% 4 3 3 2 3 5% 51%	5090
	21st to end	Day	16% 8% 2 5% 3 3 3 3 4 6% 47%	5583
April	1st to 10th	Day	14% 8% 2 6% 5% 4 4 3 5% 6% 43%	2310
	11th to 20th	Day	20% 7% 3 5% 4 4 4 3 5% 5% 41%	2291
	21st to end	Day	15% 8% 2 5% 5% 4 5% 3 5% 6% 42%	2672
May	1st to 10th	Day	20% 7% 3 6% 4 4 4 4 6% 7% 35%	5992
	11th to 20th	Day	11% 6% 2 6% 4 4 4 4 5% 8% 46%	5992
	21st to end	Day	14% 6% 2 7% 5% 5% 5% 5% 6% 9% 35%	6577
June	1st to 10th	Day	5% 4 2 5% 4 4 5% 5% 7% 9% 49%	2601
	11th to 20th	Day	8% 5% 2 5% 4 4 5% 4 7% 9% 47%	2600
	21st to end	Day	8% 6% 2 6% 6% 5% 5% 5% 8% 9% 40%	2600
July	1st to 10th	Day	12% 7% 2 7% 5% 5% 5% 4 6% 8% 38%	5900
	11th to 20th	Day	6% 4 2 5% 4 6% 6% 6% 8% 10% 44%	5900
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 10% 43%	6422
August	1st to 10th	Day	1 2 2 3 3 3 3 5% 9% 70%	4206
	11th to 20th	Day	3 2 1 3 3 4 4 4 7% 10% 59%	3846
	21st to end	Day	4 3 1 5% 5% 5% 6% 5% 8% 11% 46%	4131
September	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 4 5% 6% 45%	2200
	11th to 20th	Day	10% 7% 2 7% 5% 4 5% 4 7% 8% 41%	2200
	21st to end	Day	8% 4 2 4 5% 4 5% 4 7% 7% 50%	2200
October	1st to 10th	Day	7% 4 2 5% 3 3 5% 4 5% 6% 57%	2200
	11th to 20th	Day	12% 4 2 4 4 4 3 3 5% 6% 53%	2072
	21st to end	Day	11% 5% 2 4 3 3 3 2 4 5% 58%	2200
November	1st to 10th	Day	13% 7% 2 6% 4 3 3 3 5% 6% 46%	2058
	11th to 20th	Day	12% 5% 2 6% 4 4 3 3 5% 5% 52%	1958
	21st to end	Day	17% 7% 2 6% 4 3 3 2 4 5% 46%	1890
December	1st to 10th	Day	25% 7% 3 6% 3 3 2 2 3 5% 40%	3030
	11th to 20th	Day	17% 7% 2 6% 4 4 3 4 5% 5% 44%	3481
	21st to end	Day	14% 6% 2 3 2 2 3 2 3 4 58%	4587
January	1st to 10th	Night	16% 7% 2 3 2 2 2 2 3 5% 53%	7634
	11th to 20th	Night	10% 4 1 3 2 2 3 3 4 4 64%	7680
	21st to end	Night	29% 5% 2 5% 3 3 2 2 3 4 40%	8004
February	1st to 10th	Night	18% 7% 2 6% 3 2 2 2 4 5% 49%	2520
	11th to 20th	Night	19% 7% 2 5% 3 3 2 2 3 5% 49%	2458
	21st to end	Night	20% 7% 2 4 4 3 3 3 4 5% 47%	2076
March	1st to 10th	Night	17% 7% 2 4 4 4 3 3 5% 6% 45%	5088
	11th to 20th	Night	15% 8% 2 5% 3 3 3 2 3 5% 51%	5090
	21st to end	Night	21% 8% 2 4 3 3 3 2 4 5% 44%	5588
April	1st to 10th	Night	16% 6% 2 5% 4 3 3 4 4 7% 45%	2310
	11th to 20th	Night	20% 7% 2 5% 3 3 4 3 4 5% 44%	2272
	21st to end	Night	21% 8% 2 5% 4 3 3 2 3 6% 44%	1945
May	1st to 10th	Night	24% 9% 3 6% 3 3 3 3 4 5% 37%	4143
	11th to 20th	Night	10% 5% 2 6% 4 4 4 3 5% 8% 49%	4143
	21st to end	Night	15% 7% 2 6% 4 4 5% 4 6% 8% 37%	4542
June	1st to 10th	Night	6% 4 1 4 3 3 3 4 5% 9% 58%	1800
	11th to 20th	Night	6% 4 2 3 4 3 4 4 5% 9% 56%	1800
	21st to end	Night	7% 5% 2 4 4 3 4 4 7% 9% 51%	1800
July	1st to 10th	Night	12% 6% 2 6% 4 4 4 4 6% 10% 42%	4078
	11th to 20th	Night	5% 4 2 4 4 4 5% 4 8% 10% 50%	4077
	21st to end	Night	10% 5% 2 5% 4 4 4 4 7% 10% 45%	4449
August	1st to 10th	Night	1 1 2 1 2 3 3 5% 8% 75%	2949
	11th to 20th	Night	3 2 1 3 2 3 4 4 6% 12% 59%	3213
	21st to end	Night	5% 3 1 4 3 4 4 4 6% 11% 55%	3757
September	1st to 10th	Night	8% 5% 2 5% 4 4 3 4 6% 8% 51%	2200
	11th to 20th	Night	9% 7% 2 5% 5% 4 4 4 6% 7% 47%	2200
	21st to end	Night	9% 5% 2 5% 3 4 4 3 5% 8% 52%	2200
October	1st to 10th	Night	6% 4 2 4 3 4 4 3 4 6% 62%	2200
	11th to 20th	Night	13% 4 2 4 3 3 4 3 3 6% 55%	2305
	21st to end	Night	15% 4 2 4 2 2 3 2 3 4 60%	2640
November	1st to 10th	Night	14% 7% 2 5% 3 3 3 2 3 5% 54%	2460
	11th to 20th	Night	11% 6% 2 5% 3 2 2 3 4 6% 56%	2657
	21st to end	Night	17% 6% 1 6% 3 3 3 2 4 6% 47%	2730
December	1st to 10th	Night	23% 8% 2 5% 3 3 3 3 4 5% 41%	4380
	11th to 20th	Night	22% 9% 2 5% 3 3 2 2 3 3 47%	4946
	21st to end	Night	18% 6% 2 4 2 2 2 2 3 4 57%	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	36% 4 5% 21% 24% 10%	5290
	11th to 20th	Day	33% 2 4 14% 25% 21%	5310
	21st to end	Day	45% 4 5% 12% 23% 11%	6265
February	1st to 10th	Day	43% 3 4 12% 21% 16%	2100
	11th to 20th	Day	42% 4 7% 17% 18% 12%	2059
	21st to end	Day	38% 4 6% 17% 21% 15%	1730
March	1st to 10th	Day	34% 4 5% 18% 17% 21%	4975
	11th to 20th	Day	38% 4 5% 8% 16% 29%	5090
	21st to end	Day	39% 5% 8% 18% 20% 10%	5585
April	1st to 10th	Day	43% 6% 7% 16% 17% 12%	2310
	11th to 20th	Day	45% 6% 7% 14% 17% 11%	2291
	21st to end	Day	43% 6% 7% 19% 17% 9%	2672
May	1st to 10th	Day	48% 7% 8% 19% 15% 4	6030
	11th to 20th	Day	35% 6% 11% 18% 21% 8%	5996
	21st to end	Day	42% 8% 11% 25% 13% 1	6590
June	1st to 10th	Day	29% 9% 12% 28% 18% 3	2601
	11th to 20th	Day	34% 7% 12% 25% 18% 5%	2600
	21st to end	Day	38% 9% 12% 23% 14% 3	2600
July	1st to 10th	Day	42% 9% 16% 28% 5% 1	5836
	11th to 20th	Day	30% 10% 17% 29% 13% 1	5831
	21st to end	Day	33% 9% 15% 25% 15% 2	6240
August	1st to 10th	Day	13% 5% 12% 42% 27% 1	4192
	11th to 20th	Day	19% 6% 22% 38% 11% 2	3882
	21st to end	Day	28% 11% 17% 25% 17% 2	4130
September	1st to 10th	Day	40% 7% 9% 19% 16% 9%	2200
	11th to 20th	Day	38% 9% 11% 19% 18% 5%	2200
	21st to end	Day	32% 7% 10% 20% 18% 13%	2200
October	1st to 10th	Day	17% 4 7% 12% 24% 35%	3590
	11th to 20th	Day	23% 5% 8% 19% 25% 21%	3351
	21st to end	Day	27% 4 5% 18% 31% 14%	3538
November	1st to 10th	Day	38% 4 7% 18% 20% 12%	2058
	11th to 20th	Day	35% 5% 6% 18% 21% 15%	1958
	21st to end	Day	42% 4 6% 16% 19% 13%	1890
December	1st to 10th	Day	49% 3 5% 18% 15% 10%	3030
	11th to 20th	Day	41% 6% 7% 11% 21% 14%	3479
	21st to end	Day	32% 3 5% 15% 28% 18%	4587
January	1st to 10th	Night	35% 3 6% 18% 24% 14%	7635
	11th to 20th	Night	25% 2 4 17% 30% 22%	7680
	21st to end	Night	48% 3 4 11% 21% 13%	8004
February	1st to 10th	Night	39% 3 6% 13% 21% 18%	2520
	11th to 20th	Night	41% 3 5% 17% 18% 15%	2458
	21st to end	Night	41% 4 6% 14% 19% 17%	2076
March	1st to 10th	Night	40% 5% 6% 17% 16% 16%	5088
	11th to 20th	Night	37% 5% 7% 10% 14% 26%	5090
	21st to end	Night	42% 5% 7% 18% 18% 10%	5588
April	1st to 10th	Night	40% 5% 8% 17% 16% 14%	2310
	11th to 20th	Night	43% 5% 8% 13% 16% 14%	2272
	21st to end	Night	44% 5% 8% 18% 16% 9%	1945
May	1st to 10th	Night	50% 6% 8% 16% 14% 6%	4170
	11th to 20th	Night	33% 7% 11% 20% 19% 10%	4144
	21st to end	Night	41% 9% 13% 26% 10% 2	4551
June	1st to 10th	Night	23% 7% 16% 30% 21% 2	1800
	11th to 20th	Night	25% 7% 14% 30% 19% 5%	1800
	21st to end	Night	29% 9% 18% 27% 15% 3	1800
July	1st to 10th	Night	37% 9% 23% 26% 4	4035
	11th to 20th	Night	27% 10% 22% 30% 11% 1	4026
	21st to end	Night	33% 9% 20% 23% 12% 3	4320
August	1st to 10th	Night	10% 5% 16% 47% 22% 1	2939
	11th to 20th	Night	18% 9% 31% 32% 9% 2	3243
	21st to end	Night	23% 12% 21% 26% 16% 2	3757
September	1st to 10th	Night	32% 7% 15% 21% 17% 8%	2200
	11th to 20th	Night	35% 9% 13% 23% 14% 5%	2200
	21st to end	Night	29% 7% 14% 21% 17% 12%	2200
October	1st to 10th	Night	16% 4 8% 15% 21% 36%	3590
	11th to 20th	Night	21% 4 9% 17% 29% 20%	3752
	21st to end	Night	26% 3 5% 18% 32% 16%	4252
November	1st to 10th	Night	35% 4 8% 17% 23% 13%	2460
	11th to 20th	Night	31% 4 8% 19% 20% 17%	2657
	21st to end	Night	39% 4 8% 16% 18% 14%	2730
December	1st to 10th	Night	48% 4 6% 15% 16% 11%	4380
	11th to 20th	Night	44% 3 5% 9% 21% 17%	4947
	21st to end	Night	34% 3 3 14% 29% 17%	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	23% 16% 34% 21% 5%	330
	11th to 20th	Day	26% 19% 28% 20% 7%	330
	21st to end	Day	23% 19% 35% 20% 4	363
February	1st to 10th	Day	29% 14% 31% 21% 5%	330
	11th to 20th	Day	28% 19% 34% 17% 3	330
	21st to end	Day	25% 15% 45% 13% 2	273
March	1st to 10th	Day	25% 17% 40% 15% 2	330
	11th to 20th	Day	20% 19% 40% 16% 4 1	330
	21st to end	Day	20% 16% 38% 21% 5% 1	363
April	1st to 10th	Day	22% 16% 39% 20% 3	330
	11th to 20th	Day	23% 20% 32% 20% 5%	329
	21st to end	Day	22% 13% 39% 20% 5%	692
May	1st to 10th	Day	19% 12% 38% 24% 6%	753
	11th to 20th	Day	17% 12% 30% 30% 9% 1	775
	21st to end	Day	16% 8% 28% 32% 15% 1	844
June	1st to 10th	Day	8% 3 20% 39% 26% 5%	731
	11th to 20th	Day	9% 4 20% 43% 22% 2	730
	21st to end	Day	8% 2 17% 38% 28% 8%	730
July	1st to 10th	Day	10% 3 11% 37% 31% 8%	721
	11th to 20th	Day	7% 2 9% 36% 40% 7%	718
	21st to end	Day	6% 2 7% 35% 42% 9%	794
August	1st to 10th	Day	3 1 5% 38% 41% 13%	711
	11th to 20th	Day	6% 5% 40% 39% 10%	517
	21st to end	Day	5% 1 6% 33% 37% 19%	480
September	1st to 10th	Day	9% 3 11% 31% 34% 12%	330
	11th to 20th	Day	7% 1 8% 30% 42% 13%	330
	21st to end	Day	9% 2 11% 29% 38% 11%	330
October	1st to 10th	Day	5% 3 16% 38% 29% 9%	330
	11th to 20th	Day	11% 6% 22% 38% 19% 5%	330
	21st to end	Day	19% 12% 25% 26% 15% 3	363
November	1st to 10th	Day	18% 7% 28% 31% 14% 3	330
	11th to 20th	Day	19% 13% 28% 28% 10% 2	330
	21st to end	Day	25% 16% 32% 16% 9% 2	330
December	1st to 10th	Day	20% 12% 35% 24% 7% 3	330
	11th to 20th	Day	22% 16% 31% 22% 10% 1	330
	21st to end	Day	23% 12% 35% 23% 6%	363
January	1st to 10th	Night	20% 11% 30% 31% 8%	1050
	11th to 20th	Night	20% 13% 31% 29% 7%	1050
	21st to end	Night	24% 11% 29% 28% 7%	1013
February	1st to 10th	Night	24% 12% 32% 27% 5%	840
	11th to 20th	Night	28% 12% 33% 25% 3	824
	21st to end	Night	25% 12% 40% 21% 2	692
March	1st to 10th	Night	26% 12% 34% 25% 3	665
	11th to 20th	Night	23% 11% 34% 27% 5%	630
	21st to end	Night	20% 10% 40% 27% 3	693
April	1st to 10th	Night	19% 13% 36% 28% 4	630
	11th to 20th	Night	25% 12% 34% 25% 4	623
	21st to end	Night	23% 11% 37% 22% 7%	630
May	1st to 10th	Night	20% 11% 36% 28% 4	630
	11th to 20th	Night	16% 10% 31% 34% 9%	621
	21st to end	Night	15% 7% 31% 34% 12%	683
June	1st to 10th	Night	7% 3 23% 44% 22% 1	600
	11th to 20th	Night	5% 3 34% 44% 14% 1	600
	21st to end	Night	7% 3 19% 45% 23% 4	600
July	1st to 10th	Night	10% 3 14% 41% 28% 3	585
	11th to 20th	Night	3 2 12% 51% 30% 2	582
	21st to end	Night	6% 2 10% 48% 31% 3	657
August	1st to 10th	Night	1 11% 49% 35% 3	597
	11th to 20th	Night	5% 1 10% 49% 31% 5%	594
	21st to end	Night	7% 7% 38% 42% 6%	660
September	1st to 10th	Night	9% 2 9% 38% 37% 5%	600
	11th to 20th	Night	11% 1 8% 40% 37% 3	600
	21st to end	Night	12% 2 14% 35% 35% 4	600
October	1st to 10th	Night	8% 2 12% 46% 28% 4	600
	11th to 20th	Night	14% 4 21% 38% 21% 2	705
	21st to end	Night	17% 9% 21% 32% 20% 2	880
November	1st to 10th	Night	14% 8% 20% 37% 19% 2	820
	11th to 20th	Night	15% 7% 26% 36% 14% 1	977
	21st to end	Night	20% 12% 27% 29% 10% 1	1050
December	1st to 10th	Night	22% 8% 29% 31% 9% 1	1050
	11th to 20th	Night	26% 9% 26% 31% 8%	1050
	21st to end	Night	22% 8% 30% 33% 8%	1155

Page generated at 2026-05-29 00:53:28. Version ac07

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