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 2024-11-01.

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	12% 6% 2 6% 4 4 5% 4 6% 8% 42%	100946
	Night	13% 6% 2 6% 4 4 4 4 6% 8% 43%	97951
Transparency	Day	38% 8% 12% 20% 17% 4	100759
	Night	37% 7% 11% 19% 19% 6%	97808
Seeing	Day	15% 8% 20% 24% 21% 12%	14904
	Night	15% 7% 22% 29% 22% 4	24911

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% 9% 3 6% 4 4 5% 4 5% 6% 37%	2836
	11th to 20th	Day	13% 5% 2 5% 3 4 4 4 4 5% 51%	2840
	21st to end	Day	26% 7% 2 5% 3 2 3 2 4 6% 40%	3224
February	1st to 10th	Day	16% 7% 3 8% 4 4 4 4 4 6% 39%	1900
	11th to 20th	Day	22% 7% 2 5% 3 4 4 3 4 6% 42%	1848
	21st to end	Day	20% 7% 3 6% 5% 3 4 4 4 5% 39%	1570
March	1st to 10th	Day	24% 11% 3 6% 3 3 2 3 4 6% 34%	3240
	11th to 20th	Day	12% 5% 2 6% 5% 4 4 4 5% 7% 47%	3480
	21st to end	Day	19% 6% 2 6% 4 4 4 4 5% 6% 41%	3815
April	1st to 10th	Day	12% 6% 2 6% 5% 4 4 3 6% 7% 46%	2118
	11th to 20th	Day	12% 5% 2 6% 5% 4 4 3 5% 8% 47%	2254
	21st to end	Day	10% 5% 2 4 3 3 5% 4 6% 8% 51%	2444
May	1st to 10th	Day	10% 7% 3 6% 4 4 4 4 6% 10% 43%	4082
	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	11% 5% 2 4 3 2 4 3 4 5% 57%	1708
	11th to 20th	Day	13% 5% 2 4 3 3 3 4 5% 7% 51%	1710
	21st to end	Day	14% 5% 2 4 4 4 4 3 5% 6% 47%	1710
December	1st to 10th	Day	20% 7% 3 6% 4 4 5% 4 5% 5% 38%	1710
	11th to 20th	Day	17% 7% 3 6% 4 4 3 4 4 5% 44%	2153
	21st to end	Day	22% 7% 3 6% 4 4 3 4 5% 8% 36%	3124
January	1st to 10th	Night	18% 7% 2 5% 4 4 4 3 5% 7% 41%	4109
	11th to 20th	Night	12% 7% 3 6% 4 4 4 3 4 5% 46%	4120
	21st to end	Night	24% 8% 3 6% 3 3 3 2 4 4 40%	4428
February	1st to 10th	Night	16% 7% 2 5% 5% 3 3 4 5% 6% 44%	2280
	11th to 20th	Night	20% 7% 2 6% 3 3 3 3 4 4 43%	2218
	21st to end	Night	21% 6% 2 6% 4 4 3 3 5% 5% 41%	1884
March	1st to 10th	Night	23% 8% 2 6% 3 4 4 3 5% 5% 37%	3611
	11th to 20th	Night	11% 5% 2 5% 3 3 3 4 6% 8% 49%	3480
	21st to end	Night	17% 6% 2 6% 4 4 4 3 5% 6% 42%	3817
April	1st to 10th	Night	11% 5% 1 4 4 4 4 5% 6% 7% 48%	2076
	11th to 20th	Night	13% 5% 2 4 3 3 4 4 6% 7% 50%	1875
	21st to end	Night	8% 5% 2 5% 3 3 4 3 5% 7% 53%	1734
May	1st to 10th	Night	9% 6% 3 7% 6% 5% 5% 4 7% 8% 41%	2813
	11th to 20th	Night	15% 6% 2 6% 5% 5% 4 4 5% 8% 41%	2812
	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 6% 58%	2508
November	1st to 10th	Night	12% 6% 1 3 3 3 2 3 5% 5% 57%	2372
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 4 6% 54%	2470
	21st to end	Night	16% 6% 1 4 3 3 4 4 6% 6% 47%	2470
December	1st to 10th	Night	17% 6% 3 5% 4 4 4 3 6% 6% 41%	2470
	11th to 20th	Night	17% 6% 2 4 3 3 3 2 5% 6% 49%	3042
	21st to end	Night	19% 9% 2 7% 5% 5% 4 4 5% 7% 35%	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	44% 6% 8% 15% 20% 6%	2837
	11th to 20th	Day	33% 4 8% 25% 24% 7%	2840
	21st to end	Day	47% 3 5% 17% 21% 7%	3224
February	1st to 10th	Day	45% 5% 8% 17% 20% 5%	1900
	11th to 20th	Day	44% 4 7% 18% 20% 6%	1858
	21st to end	Day	46% 5% 8% 15% 19% 8%	1570
March	1st to 10th	Day	51% 5% 9% 12% 13% 9%	3240
	11th to 20th	Day	36% 6% 9% 16% 25% 8%	3480
	21st to end	Day	42% 5% 9% 20% 19% 4	3815
April	1st to 10th	Day	37% 6% 9% 22% 21% 5%	2118
	11th to 20th	Day	36% 6% 9% 19% 23% 8%	2254
	21st to end	Day	29% 6% 12% 29% 22% 3	2444
May	1st to 10th	Day	35% 8% 15% 22% 18% 2	4120
	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	31% 5% 10% 17% 30% 7%	2090
	11th to 20th	Day	27% 4 9% 23% 25% 11%	1971
	21st to end	Day	31% 4 6% 18% 30% 11%	2090
November	1st to 10th	Day	30% 4 8% 15% 31% 11%	1708
	11th to 20th	Day	32% 5% 7% 19% 27% 10%	1710
	21st to end	Day	36% 6% 7% 19% 25% 7%	1710
December	1st to 10th	Day	47% 6% 6% 19% 16% 6%	1710
	11th to 20th	Day	43% 4 6% 15% 24% 8%	2150
	21st to end	Day	43% 6% 11% 18% 19% 3	3124
January	1st to 10th	Night	39% 6% 8% 16% 25% 6%	4109
	11th to 20th	Night	38% 5% 6% 22% 20% 8%	4120
	21st to end	Night	48% 4 5% 12% 25% 6%	4428
February	1st to 10th	Night	42% 4 8% 17% 20% 9%	2280
	11th to 20th	Night	44% 4 5% 17% 21% 9%	2219
	21st to end	Night	45% 4 8% 15% 18% 10%	1884
March	1st to 10th	Night	47% 5% 10% 16% 14% 7%	3611
	11th to 20th	Night	30% 6% 10% 16% 26% 12%	3480
	21st to end	Night	41% 5% 8% 18% 19% 9%	3817
April	1st to 10th	Night	33% 6% 11% 19% 23% 8%	2076
	11th to 20th	Night	34% 4 8% 20% 23% 11%	1875
	21st to end	Night	30% 6% 11% 27% 22% 5%	1734
May	1st to 10th	Night	36% 10% 15% 20% 17% 2	2840
	11th to 20th	Night	40% 8% 13% 24% 13% 1	2813
	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	30% 6% 11% 17% 28% 8%	2090
	11th to 20th	Night	29% 5% 7% 20% 27% 12%	2206
	21st to end	Night	28% 4 6% 19% 30% 13%	2508
November	1st to 10th	Night	30% 4 7% 18% 27% 13%	2372
	11th to 20th	Night	33% 3 6% 18% 27% 13%	2470
	21st to end	Night	36% 5% 8% 18% 25% 8%	2470
December	1st to 10th	Night	42% 6% 8% 20% 19% 6%	2470
	11th to 20th	Night	37% 4 8% 16% 28% 7%	3044
	21st to end	Night	47% 6% 8% 20% 17% 3	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	22% 19% 39% 18% 1	270
	11th to 20th	Day	19% 25% 27% 19% 8% 1	270
	21st to end	Day	22% 21% 37% 15% 4 1	297
February	1st to 10th	Day	21% 18% 33% 22% 5%	270
	11th to 20th	Day	35% 17% 29% 17% 2	270
	21st to end	Day	25% 21% 35% 16% 2	225
March	1st to 10th	Day	22% 14% 29% 25% 9% 1	338
	11th to 20th	Day	20% 15% 33% 24% 7% 1	460
	21st to end	Day	21% 13% 31% 25% 9% 1	506
April	1st to 10th	Day	17% 16% 29% 24% 13% 2	460
	11th to 20th	Day	12% 13% 30% 25% 17% 2	452
	21st to end	Day	14% 9% 23% 29% 20% 5%	460
May	1st to 10th	Day	12% 8% 27% 27% 20% 6%	463
	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	16% 7% 20% 29% 24% 4	539
November	1st to 10th	Day	18% 7% 25% 26% 18% 5%	306
	11th to 20th	Day	15% 13% 36% 25% 9% 1	270
	21st to end	Day	11% 15% 32% 27% 13% 2	270
December	1st to 10th	Day	28% 15% 31% 19% 6% 1	270
	11th to 20th	Day	22% 15% 31% 25% 6% 1	270
	21st to end	Day	28% 16% 30% 19% 5% 2	297
January	1st to 10th	Night	24% 14% 33% 23% 5%	760
	11th to 20th	Night	19% 15% 32% 25% 9%	760
	21st to end	Night	20% 16% 36% 25% 3	836
February	1st to 10th	Night	18% 13% 32% 31% 6%	760
	11th to 20th	Night	22% 14% 35% 25% 3	743
	21st to end	Night	22% 15% 35% 25% 3	628
March	1st to 10th	Night	17% 12% 33% 32% 6%	760
	11th to 20th	Night	16% 11% 35% 27% 10%	760
	21st to end	Night	17% 12% 28% 32% 11%	836
April	1st to 10th	Night	13% 13% 31% 31% 13% 1	746
	11th to 20th	Night	14% 8% 29% 33% 15% 1	563
	21st to end	Night	11% 7% 24% 36% 21% 2	570
May	1st to 10th	Night	10% 7% 25% 36% 22% 1	570
	11th to 20th	Night	11% 4 22% 33% 25% 5%	561
	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% 34% 21% 3	836
November	1st to 10th	Night	15% 7% 22% 32% 23% 1	743
	11th to 20th	Night	17% 9% 31% 28% 15% 1	760
	21st to end	Night	18% 8% 28% 31% 15% 1	760
December	1st to 10th	Night	18% 12% 31% 29% 10% 1	760
	11th to 20th	Night	22% 10% 30% 28% 10% 1	760
	21st to end	Night	25% 12% 28% 28% 7%	836

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