ME Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ME Observatory clear sky chart from 2005-11-06 to 2025-05-11.

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	12% 6% 2 6% 4 4 4 4 6% 8% 43%	114175
	Night	13% 6% 2 6% 4 4 4 4 5% 7% 45%	114778
Transparency	Day	37% 7% 11% 20% 19% 6%	117995
	Night	36% 6% 10% 19% 21% 7%	119097
Seeing	Day	15% 8% 21% 24% 20% 11%	15544
	Night	15% 8% 23% 29% 21% 4	25643

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	17% 8% 3 5% 3 3 3 3 4 5% 46%	4056
	11th to 20th	Day	11% 7% 2 5% 3 3 3 3 4 6% 53%	4060
	21st to end	Day	23% 8% 3 5% 3 3 3 2 4 6% 40%	4622
February	1st to 10th	Day	15% 7% 3 7% 4 5% 5% 4 4 6% 40%	2000
	11th to 20th	Day	22% 7% 2 5% 3 4 4 3 4 6% 41%	1948
	21st to end	Day	19% 6% 3 6% 5% 3 4 4 4 5% 41%	1650
March	1st to 10th	Day	21% 10% 4 7% 4 4 3 3 5% 6% 35%	4656
	11th to 20th	Day	10% 6% 2 6% 4 4 4 3 5% 7% 50%	4980
	21st to end	Day	16% 6% 2 6% 4 4 4 3 4 5% 45%	5465
April	1st to 10th	Day	12% 5% 2 6% 4 4 4 3 6% 7% 46%	2230
	11th to 20th	Day	11% 4 2 6% 5% 4 4 3 5% 8% 48%	2374
	21st to end	Day	10% 5% 2 4 3 3 5% 4 6% 9% 50%	2573
May	1st to 10th	Day	10% 7% 3 6% 4 4 4 4 6% 10% 43%	4212
	11th to 20th	Day	16% 8% 2 7% 4 4 4 4 6% 8% 37%	4081
	21st to end	Day	8% 5% 2 5% 4 5% 5% 4 8% 10% 45%	4476
June	1st to 10th	Day	9% 6% 2 6% 5% 6% 5% 5% 7% 11% 37%	2470
	11th to 20th	Day	5% 4 1 4 5% 4 5% 5% 10% 12% 45%	2470
	21st to end	Day	6% 4 2 7% 6% 5% 6% 5% 8% 10% 41%	2470
July	1st to 10th	Day	7% 6% 3 8% 6% 7% 8% 6% 10% 11% 27%	4120
	11th to 20th	Day	7% 5% 3 7% 5% 6% 7% 6% 8% 12% 34%	4120
	21st to end	Day	9% 8% 4 11% 8% 7% 8% 6% 9% 10% 20%	4466
August	1st to 10th	Day	7% 6% 2 6% 5% 5% 6% 5% 9% 12% 37%	4104
	11th to 20th	Day	7% 5% 2 6% 4 4 5% 4 6% 10% 45%	3823
	21st to end	Day	7% 5% 2 5% 3 4 5% 4 6% 9% 49%	4159
September	1st to 10th	Day	9% 7% 2 8% 5% 5% 4 5% 7% 9% 40%	2090
	11th to 20th	Day	14% 7% 2 6% 3 4 5% 5% 6% 8% 39%	2090
	21st to end	Day	13% 5% 2 6% 4 3 4 3 5% 6% 49%	2090
October	1st to 10th	Day	11% 5% 2 3 3 3 4 3 4 5% 55%	2090
	11th to 20th	Day	10% 4 1 4 3 3 2 3 5% 7% 57%	1971
	21st to end	Day	13% 4 1 5% 3 3 2 3 5% 5% 55%	2090
November	1st to 10th	Day	12% 6% 3 4 3 3 3 3 4 5% 55%	1800
	11th to 20th	Day	13% 5% 2 4 3 3 3 4 5% 7% 51%	1800
	21st to end	Day	14% 6% 2 4 4 4 4 3 5% 6% 48%	1800
December	1st to 10th	Day	18% 6% 2 5% 3 3 4 3 5% 5% 46%	2930
	11th to 20th	Day	15% 9% 3 7% 4 4 4 4 4 5% 41%	3373
	21st to end	Day	22% 6% 2 5% 3 4 3 3 5% 7% 40%	4466
January	1st to 10th	Night	15% 5% 2 5% 3 3 3 3 4 6% 50%	5889
	11th to 20th	Night	12% 7% 3 6% 4 3 4 3 4 5% 49%	5900
	21st to end	Night	19% 7% 2 6% 4 4 3 3 4 5% 43%	6330
February	1st to 10th	Night	16% 7% 2 5% 4 3 3 4 5% 6% 45%	2400
	11th to 20th	Night	21% 7% 3 6% 3 3 3 3 4 4 42%	2338
	21st to end	Night	20% 6% 2 5% 4 4 3 3 5% 6% 43%	1980
March	1st to 10th	Night	21% 8% 2 6% 3 4 4 3 5% 6% 37%	5195
	11th to 20th	Night	10% 5% 2 5% 3 3 3 3 6% 8% 52%	4980
	21st to end	Night	15% 6% 2 5% 4 4 4 3 5% 6% 45%	5467
April	1st to 10th	Night	11% 5% 2 4 4 4 5% 5% 6% 7% 47%	2185
	11th to 20th	Night	12% 5% 2 4 3 3 4 4 5% 7% 51%	1975
	21st to end	Night	8% 6% 2 5% 4 4 4 3 5% 7% 52%	1825
May	1st to 10th	Night	9% 6% 3 7% 6% 5% 5% 4 7% 9% 41%	2903
	11th to 20th	Night	15% 6% 2 6% 5% 5% 4 4 5% 7% 41%	2818
	21st to end	Night	7% 4 2 4 4 5% 4 5% 6% 11% 48%	3079
June	1st to 10th	Night	7% 5% 2 7% 5% 6% 7% 6% 7% 11% 36%	1710
	11th to 20th	Night	4 3 2 5% 5% 5% 5% 5% 8% 13% 44%	1710
	21st to end	Night	6% 4 1 5% 4 5% 7% 6% 9% 11% 41%	1710
July	1st to 10th	Night	8% 8% 3 8% 6% 5% 6% 5% 7% 12% 31%	2840
	11th to 20th	Night	5% 5% 2 7% 6% 6% 7% 7% 10% 14% 31%	2840
	21st to end	Night	10% 9% 4 10% 7% 6% 7% 6% 8% 11% 22%	3087
August	1st to 10th	Night	7% 5% 2 7% 5% 5% 6% 5% 9% 13% 35%	2825
	11th to 20th	Night	7% 5% 2 7% 5% 4 5% 4 7% 9% 45%	3000
	21st to end	Night	7% 5% 2 6% 4 4 4 4 5% 8% 51%	3476
September	1st to 10th	Night	8% 5% 2 7% 5% 5% 5% 4 7% 9% 42%	2077
	11th to 20th	Night	12% 7% 2 6% 3 4 4 5% 6% 7% 43%	2090
	21st to end	Night	12% 5% 2 6% 4 4 5% 4 5% 8% 46%	2090
October	1st to 10th	Night	11% 5% 2 4 3 3 3 3 5% 7% 53%	2090
	11th to 20th	Night	9% 4 2 5% 4 3 3 3 3 4 59%	2206
	21st to end	Night	12% 4 1 4 3 2 3 3 4 5% 58%	2508
November	1st to 10th	Night	12% 6% 1 4 3 3 2 3 4 5% 55%	2493
	11th to 20th	Night	15% 5% 2 4 3 3 3 3 4 6% 54%	2600
	21st to end	Night	15% 6% 1 4 3 3 4 4 6% 6% 49%	2600
December	1st to 10th	Night	15% 6% 2 5% 3 3 3 3 4 5% 51%	4250
	11th to 20th	Night	18% 6% 2 5% 3 3 3 2 5% 6% 47%	4822
	21st to end	Night	21% 7% 2 5% 4 4 3 3 4 6% 41%	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	39% 4 6% 20% 23% 7%	4057
	11th to 20th	Day	33% 3 6% 22% 28% 8%	4060
	21st to end	Day	46% 3 5% 16% 21% 8%	4622
February	1st to 10th	Day	44% 5% 8% 18% 20% 5%	2000
	11th to 20th	Day	45% 4 7% 18% 20% 6%	1958
	21st to end	Day	44% 5% 8% 15% 20% 8%	1650
March	1st to 10th	Day	51% 4 8% 11% 20% 7%	4656
	11th to 20th	Day	33% 5% 8% 14% 24% 17%	4980
	21st to end	Day	42% 4 7% 22% 21% 4	5465
April	1st to 10th	Day	37% 6% 9% 23% 20% 5%	2230
	11th to 20th	Day	35% 6% 9% 20% 24% 8%	2374
	21st to end	Day	30% 6% 13% 28% 21% 2	2573
May	1st to 10th	Day	36% 8% 15% 22% 18% 2	4250
	11th to 20th	Day	42% 8% 13% 24% 12% 1	4085
	21st to end	Day	30% 9% 16% 24% 20% 1	4489
June	1st to 10th	Day	39% 10% 18% 23% 9% 2	2470
	11th to 20th	Day	27% 10% 23% 25% 14% 1	2470
	21st to end	Day	34% 11% 17% 23% 13% 3	2470
July	1st to 10th	Day	43% 12% 22% 18% 4 1	4055
	11th to 20th	Day	40% 13% 21% 19% 6% 2	4051
	21st to end	Day	53% 15% 15% 13% 4	4324
August	1st to 10th	Day	36% 10% 16% 31% 7%	4091
	11th to 20th	Day	31% 10% 18% 25% 14% 2	3862
	21st to end	Day	29% 9% 15% 24% 20% 3	4159
September	1st to 10th	Day	38% 9% 17% 16% 17% 4	2090
	11th to 20th	Day	39% 9% 11% 17% 22% 2	2090
	21st to end	Day	35% 5% 12% 20% 22% 6%	2090
October	1st to 10th	Day	25% 3 7% 12% 40% 12%	3480
	11th to 20th	Day	19% 3 6% 16% 28% 28%	3250
	21st to end	Day	26% 3 6% 22% 31% 12%	3428
November	1st to 10th	Day	32% 4 8% 15% 30% 10%	1800
	11th to 20th	Day	32% 5% 6% 19% 27% 10%	1800
	21st to end	Day	35% 6% 7% 19% 27% 6%	1800
December	1st to 10th	Day	40% 4 5% 18% 26% 7%	2930
	11th to 20th	Day	45% 5% 8% 17% 20% 6%	3370
	21st to end	Day	42% 5% 10% 24% 17% 2	4466
January	1st to 10th	Night	34% 4 7% 20% 29% 7%	5889
	11th to 20th	Night	37% 4 5% 19% 27% 8%	5900
	21st to end	Night	43% 3 6% 14% 28% 6%	6330
February	1st to 10th	Night	41% 4 8% 18% 20% 9%	2400
	11th to 20th	Night	45% 4 5% 17% 20% 8%	2339
	21st to end	Night	43% 4 8% 16% 19% 11%	1980
March	1st to 10th	Night	47% 4 9% 13% 19% 8%	5195
	11th to 20th	Night	29% 4 9% 14% 26% 18%	4980
	21st to end	Night	38% 4 7% 20% 21% 9%	5467
April	1st to 10th	Night	33% 6% 11% 19% 23% 8%	2185
	11th to 20th	Night	33% 4 8% 21% 24% 10%	1975
	21st to end	Night	31% 6% 11% 26% 21% 5%	1825
May	1st to 10th	Night	36% 10% 15% 20% 16% 2	2930
	11th to 20th	Night	40% 8% 13% 24% 13% 1	2819
	21st to end	Night	26% 10% 16% 25% 19% 3	3087
June	1st to 10th	Night	38% 12% 16% 21% 11% 2	1710
	11th to 20th	Night	28% 9% 21% 27% 13% 2	1710
	21st to end	Night	31% 12% 21% 19% 13% 4	1710
July	1st to 10th	Night	43% 13% 21% 18% 5% 1	2795
	11th to 20th	Night	36% 15% 23% 18% 7% 1	2786
	21st to end	Night	51% 15% 16% 13% 4 1	2988
August	1st to 10th	Night	37% 10% 17% 29% 6%	2814
	11th to 20th	Night	33% 9% 17% 25% 15% 1	3027
	21st to end	Night	30% 8% 14% 21% 24% 3	3476
September	1st to 10th	Night	35% 11% 16% 17% 17% 4	2077
	11th to 20th	Night	38% 10% 11% 20% 18% 3	2090
	21st to end	Night	36% 7% 11% 18% 21% 8%	2090
October	1st to 10th	Night	26% 4 10% 12% 37% 12%	3480
	11th to 20th	Night	18% 3 4 17% 31% 27%	3666
	21st to end	Night	23% 3 4 22% 30% 18%	4120
November	1st to 10th	Night	32% 4 8% 18% 26% 13%	2493
	11th to 20th	Night	33% 3 5% 18% 29% 12%	2600
	21st to end	Night	35% 5% 8% 19% 25% 8%	2600
December	1st to 10th	Night	36% 4 5% 18% 28% 8%	4250
	11th to 20th	Night	39% 4 11% 16% 24% 7%	4824
	21st to end	Night	44% 4 7% 26% 17% 2	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	22% 20% 40% 16% 1	300
	11th to 20th	Day	19% 27% 27% 19% 7% 2	300
	21st to end	Day	22% 21% 36% 16% 4 1	330
February	1st to 10th	Day	19% 16% 35% 24% 6%	300
	11th to 20th	Day	37% 17% 29% 16% 2	300
	21st to end	Day	23% 20% 35% 18% 3	249
March	1st to 10th	Day	23% 15% 29% 23% 9% 1	372
	11th to 20th	Day	19% 16% 32% 25% 7% 1	500
	21st to end	Day	21% 14% 31% 26% 9% 1	550
April	1st to 10th	Day	17% 16% 30% 25% 12% 1	500
	11th to 20th	Day	12% 12% 31% 26% 17% 2	492
	21st to end	Day	15% 8% 22% 29% 21% 5%	500
May	1st to 10th	Day	13% 8% 28% 26% 19% 6%	503
	11th to 20th	Day	14% 3 19% 26% 24% 13%	487
	21st to end	Day	14% 2 14% 27% 27% 16%	529
June	1st to 10th	Day	12% 2 11% 27% 30% 17%	490
	11th to 20th	Day	7% 2 13% 26% 24% 28%	490
	21st to end	Day	9% 4 9% 28% 30% 20%	490
July	1st to 10th	Day	9% 2 5% 22% 30% 32%	485
	11th to 20th	Day	8% 1 7% 20% 30% 34%	484
	21st to end	Day	7% 2 7% 20% 39% 25%	532
August	1st to 10th	Day	12% 2 12% 23% 27% 25%	486
	11th to 20th	Day	11% 1 9% 21% 36% 23%	479
	21st to end	Day	15% 2 7% 22% 29% 25%	539
September	1st to 10th	Day	10% 3 12% 24% 31% 19%	490
	11th to 20th	Day	16% 4 10% 22% 33% 14%	490
	21st to end	Day	15% 3 14% 23% 33% 11%	490
October	1st to 10th	Day	14% 4 18% 31% 22% 11%	490
	11th to 20th	Day	11% 8% 25% 24% 25% 7%	490
	21st to end	Day	17% 7% 20% 29% 23% 4	539
November	1st to 10th	Day	19% 7% 25% 25% 18% 5%	338
	11th to 20th	Day	16% 13% 35% 26% 9% 2	300
	21st to end	Day	11% 17% 32% 27% 12% 2	300
December	1st to 10th	Day	27% 15% 34% 17% 6% 1	300
	11th to 20th	Day	22% 14% 31% 25% 7% 1	300
	21st to end	Day	28% 16% 29% 18% 5% 2	330
January	1st to 10th	Night	24% 15% 33% 22% 5%	800
	11th to 20th	Night	19% 15% 33% 24% 8%	800
	21st to end	Night	20% 15% 37% 25% 3	880
February	1st to 10th	Night	18% 13% 32% 32% 6%	800
	11th to 20th	Night	23% 15% 34% 25% 3	783
	21st to end	Night	21% 15% 35% 26% 3	660
March	1st to 10th	Night	17% 12% 33% 32% 5%	800
	11th to 20th	Night	16% 12% 36% 27% 10%	800
	21st to end	Night	17% 12% 29% 32% 10%	880
April	1st to 10th	Night	13% 13% 31% 31% 12% 1	785
	11th to 20th	Night	14% 8% 29% 33% 15% 1	593
	21st to end	Night	11% 7% 24% 37% 21% 2	600
May	1st to 10th	Night	10% 6% 25% 37% 21% 1	600
	11th to 20th	Night	11% 4 22% 33% 25% 5%	563
	21st to end	Night	12% 1 12% 34% 35% 6%	617
June	1st to 10th	Night	9% 2 9% 39% 35% 7%	570
	11th to 20th	Night	5% 1 14% 30% 36% 13%	570
	21st to end	Night	8% 1 8% 31% 41% 11%	570
July	1st to 10th	Night	10% 2 6% 23% 44% 15%	555
	11th to 20th	Night	6% 2 7% 18% 52% 16%	552
	21st to end	Night	8% 2 6% 25% 49% 11%	624
August	1st to 10th	Night	9% 2 6% 28% 43% 12%	567
	11th to 20th	Night	10% 1 8% 27% 44% 9%	564
	21st to end	Night	13% 2 7% 26% 41% 9%	627
September	1st to 10th	Night	10% 2 9% 27% 42% 11%	747
	11th to 20th	Night	15% 3 10% 27% 41% 5%	760
	21st to end	Night	14% 3 12% 32% 36% 3	760
October	1st to 10th	Night	13% 3 17% 34% 27% 7%	760
	11th to 20th	Night	12% 5% 24% 34% 22% 3	760
	21st to end	Night	14% 6% 22% 35% 21% 3	836
November	1st to 10th	Night	15% 7% 21% 32% 22% 1	780
	11th to 20th	Night	17% 8% 31% 28% 16% 1	800
	21st to end	Night	18% 9% 28% 30% 15% 1	800
December	1st to 10th	Night	18% 12% 32% 29% 9% 1	800
	11th to 20th	Night	22% 10% 30% 28% 10% 1	800
	21st to end	Night	25% 12% 28% 29% 7%	880

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