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 2025-02-06.

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%	109049
	Night	15% 6% 2 4 3 3 3 3 4 6% 50%	109440
Transparency	Day	36% 6% 9% 20% 19% 10%	112867
	Night	34% 5% 10% 19% 19% 12%	113749
Seeing	Day	15% 8% 23% 30% 20% 5%	15104
	Night	17% 7% 25% 34% 15% 1	25162

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	16% 8% 2 6% 2 3 2 2 4 5% 50%	4077
	11th to 20th	Day	17% 5% 1 3 2 3 2 2 5% 5% 53%	4080
	21st to end	Day	26% 7% 2 5% 3 2 2 2 3 6% 42%	4833
February	1st to 10th	Day	18% 7% 2 6% 3 3 3 3 3 5% 46%	1950
	11th to 20th	Day	18% 8% 2 4 3 3 3 3 4 7% 45%	1849
	21st to end	Day	18% 5% 2 6% 3 2 3 2 4 5% 49%	1570
March	1st to 10th	Day	15% 8% 3 7% 4 4 3 3 5% 5% 43%	3393
	11th to 20th	Day	21% 10% 3 8% 5% 4 4 3 4 5% 33%	3480
	21st to end	Day	15% 8% 2 5% 4 3 3 3 5% 6% 46%	3814
April	1st to 10th	Day	14% 8% 3 6% 5% 3 4 3 5% 6% 43%	2090
	11th to 20th	Day	20% 7% 3 5% 4 3 3 3 5% 5% 43%	2071
	21st to end	Day	14% 8% 1 5% 5% 4 5% 3 5% 6% 44%	2418
May	1st to 10th	Day	22% 9% 3 7% 5% 4 4 4 5% 6% 31%	4082
	11th to 20th	Day	15% 7% 2 6% 4 4 4 3 5% 6% 43%	4082
	21st to end	Day	12% 6% 2 6% 5% 5% 6% 5% 7% 10% 37%	4477
June	1st to 10th	Day	6% 4 2 5% 4 4 6% 5% 7% 9% 48%	2470
	11th to 20th	Day	8% 5% 2 5% 4 4 5% 4 7% 8% 47%	2470
	21st to end	Day	8% 6% 2 6% 6% 5% 5% 5% 7% 8% 40%	2470
July	1st to 10th	Day	6% 5% 2 5% 5% 5% 5% 4 6% 8% 48%	4120
	11th to 20th	Day	5% 3 2 4 4 5% 5% 5% 7% 11% 50%	4120
	21st to end	Day	10% 6% 2 6% 5% 5% 5% 5% 8% 10% 38%	4465
August	1st to 10th	Day	1 2 2 3 3 3 3 5% 9% 70%	4077
	11th to 20th	Day	4 3 1 3 3 3 4 4 6% 10% 60%	3726
	21st to end	Day	4 3 1 5% 5% 5% 6% 5% 8% 11% 46%	4004
September	1st to 10th	Day	9% 6% 2 6% 6% 5% 5% 4 5% 7% 44%	2090
	11th to 20th	Day	11% 7% 3 7% 5% 4 5% 4 6% 8% 40%	2090
	21st to end	Day	8% 4 2 4 5% 4 5% 4 7% 7% 50%	2090
October	1st to 10th	Day	7% 5% 2 5% 3 3 5% 4 5% 6% 56%	2090
	11th to 20th	Day	12% 5% 2 5% 4 4 3 3 5% 6% 52%	1969
	21st to end	Day	11% 5% 2 4 3 3 3 2 4 5% 58%	2090
November	1st to 10th	Day	14% 8% 2 6% 4 3 3 3 6% 6% 44%	1958
	11th to 20th	Day	12% 5% 2 6% 4 4 3 3 5% 5% 51%	1865
	21st to end	Day	16% 7% 2 6% 4 3 3 2 4 5% 47%	1800
December	1st to 10th	Day	26% 7% 3 6% 3 3 3 2 3 5% 39%	2940
	11th to 20th	Day	17% 7% 2 6% 4 4 3 4 5% 5% 44%	3391
	21st to end	Day	14% 6% 2 3 2 2 3 2 3 4 59%	4488
January	1st to 10th	Night	20% 6% 2 3 2 2 2 2 3 5% 51%	5888
	11th to 20th	Night	13% 3 1 2 2 2 2 3 3 5% 64%	5900
	21st to end	Night	29% 6% 2 5% 3 2 2 2 3 4 40%	6186
February	1st to 10th	Night	18% 7% 2 6% 3 2 2 2 4 5% 48%	2340
	11th to 20th	Night	19% 8% 2 5% 2 3 2 2 4 4 50%	2218
	21st to end	Night	19% 7% 2 4 4 3 3 3 4 5% 45%	1884
March	1st to 10th	Night	17% 8% 2 6% 5% 4 4 3 5% 6% 40%	3462
	11th to 20th	Night	22% 11% 3 7% 4 4 4 3 4 5% 34%	3480
	21st to end	Night	18% 7% 2 5% 4 4 3 3 4 5% 44%	3817
April	1st to 10th	Night	14% 6% 2 5% 4 3 3 4 5% 7% 46%	2090
	11th to 20th	Night	19% 7% 2 5% 4 3 4 3 4 5% 45%	2054
	21st to end	Night	19% 7% 2 4 4 3 3 3 4 6% 46%	1760
May	1st to 10th	Night	25% 9% 2 6% 3 3 3 3 4 4 37%	2823
	11th to 20th	Night	12% 6% 2 6% 4 4 3 3 5% 7% 49%	2823
	21st to end	Night	14% 6% 2 5% 3 4 4 4 6% 8% 43%	3090
June	1st to 10th	Night	5% 4 1 4 3 2 3 4 6% 9% 59%	1710
	11th to 20th	Night	5% 3 2 3 4 3 4 4 5% 9% 57%	1710
	21st to end	Night	7% 5% 2 4 4 3 4 4 7% 10% 51%	1710
July	1st to 10th	Night	7% 4 1 4 3 3 4 4 6% 10% 55%	2850
	11th to 20th	Night	3 4 1 4 4 4 5% 4 8% 9% 55%	2850
	21st to end	Night	13% 6% 2 5% 4 4 4 4 6% 9% 41%	3098
August	1st to 10th	Night	1 1 2 1 2 3 3 5% 8% 75%	2858
	11th to 20th	Night	3 2 1 3 2 3 4 4 6% 12% 59%	3113
	21st to end	Night	5% 3 1 4 3 4 4 4 6% 11% 55%	3642
September	1st to 10th	Night	8% 5% 2 5% 4 4 3 4 5% 7% 52%	2090
	11th to 20th	Night	9% 7% 2 5% 5% 4 4 4 6% 7% 46%	2090
	21st to end	Night	9% 5% 2 4 3 4 4 3 5% 8% 52%	2090
October	1st to 10th	Night	6% 4 2 4 3 4 4 3 4 6% 61%	2090
	11th to 20th	Night	14% 5% 2 4 3 3 4 3 3 6% 54%	2190
	21st to end	Night	15% 4 2 4 2 2 3 2 2 4 60%	2508
November	1st to 10th	Night	15% 7% 2 5% 3 3 3 2 3 5% 52%	2340
	11th to 20th	Night	10% 6% 2 5% 3 3 3 3 4 6% 55%	2530
	21st to end	Night	16% 6% 1 6% 4 3 3 2 4 6% 48%	2600
December	1st to 10th	Night	23% 8% 2 5% 3 3 3 3 4 5% 40%	4250
	11th to 20th	Night	22% 9% 2 5% 3 3 2 2 3 3 47%	4816
	21st to end	Night	18% 5% 2 4 2 2 2 2 3 4 57%	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	38% 4 4 20% 22% 12%	4078
	11th to 20th	Day	34% 2 5% 15% 25% 19%	4080
	21st to end	Day	45% 4 4 9% 24% 14%	4833
February	1st to 10th	Day	43% 3 4 12% 21% 16%	1950
	11th to 20th	Day	42% 4 7% 16% 18% 13%	1859
	21st to end	Day	39% 4 6% 17% 18% 16%	1570
March	1st to 10th	Day	42% 6% 6% 14% 21% 11%	3393
	11th to 20th	Day	54% 6% 6% 9% 16% 9%	3480
	21st to end	Day	38% 6% 9% 16% 20% 11%	3814
April	1st to 10th	Day	42% 6% 7% 16% 17% 12%	2090
	11th to 20th	Day	44% 6% 7% 15% 18% 11%	2071
	21st to end	Day	41% 6% 6% 20% 18% 10%	2418
May	1st to 10th	Day	53% 7% 8% 13% 14% 5%	4120
	11th to 20th	Day	40% 7% 8% 18% 21% 6%	4086
	21st to end	Day	39% 9% 13% 28% 11% 1	4489
June	1st to 10th	Day	30% 9% 12% 26% 19% 3	2470
	11th to 20th	Day	34% 7% 13% 24% 17% 5%	2470
	21st to end	Day	40% 8% 11% 23% 15% 3	2470
July	1st to 10th	Day	31% 9% 15% 37% 7% 1	4056
	11th to 20th	Day	27% 9% 17% 30% 16% 1	4051
	21st to end	Day	36% 10% 17% 29% 8%	4325
August	1st to 10th	Day	13% 5% 12% 41% 27% 1	4063
	11th to 20th	Day	19% 6% 23% 38% 12% 2	3762
	21st to end	Day	28% 12% 17% 25% 17% 2	4003
September	1st to 10th	Day	40% 7% 9% 19% 16% 9%	2090
	11th to 20th	Day	39% 9% 11% 18% 17% 5%	2090
	21st to end	Day	32% 7% 11% 20% 18% 13%	2090
October	1st to 10th	Day	18% 4 7% 12% 24% 36%	3480
	11th to 20th	Day	23% 5% 8% 18% 24% 21%	3248
	21st to end	Day	27% 4 5% 18% 32% 14%	3428
November	1st to 10th	Day	40% 5% 8% 18% 18% 11%	1958
	11th to 20th	Day	35% 5% 6% 17% 21% 15%	1865
	21st to end	Day	41% 4 6% 16% 19% 14%	1800
December	1st to 10th	Day	50% 3 5% 18% 15% 9%	2940
	11th to 20th	Day	41% 7% 7% 11% 21% 13%	3389
	21st to end	Day	31% 3 4 15% 28% 18%	4488
January	1st to 10th	Night	37% 3 5% 18% 23% 14%	5888
	11th to 20th	Night	25% 2 4 18% 30% 21%	5900
	21st to end	Night	48% 3 4 9% 20% 16%	6186
February	1st to 10th	Night	40% 3 5% 13% 20% 18%	2340
	11th to 20th	Night	41% 3 5% 16% 19% 16%	2218
	21st to end	Night	42% 4 6% 14% 17% 17%	1884
March	1st to 10th	Night	44% 6% 7% 16% 18% 9%	3462
	11th to 20th	Night	52% 7% 9% 10% 13% 9%	3480
	21st to end	Night	41% 6% 9% 15% 19% 10%	3817
April	1st to 10th	Night	39% 6% 8% 17% 17% 14%	2090
	11th to 20th	Night	42% 5% 8% 13% 17% 15%	2054
	21st to end	Night	42% 5% 8% 20% 17% 9%	1760
May	1st to 10th	Night	50% 6% 8% 16% 13% 6%	2850
	11th to 20th	Night	34% 7% 11% 22% 18% 8%	2824
	21st to end	Night	37% 8% 14% 27% 11% 2	3099
June	1st to 10th	Night	23% 7% 16% 30% 22% 3	1710
	11th to 20th	Night	24% 7% 15% 30% 20% 5%	1710
	21st to end	Night	29% 9% 17% 28% 15% 3	1710
July	1st to 10th	Night	24% 8% 26% 36% 6% 1	2805
	11th to 20th	Night	24% 9% 20% 31% 15% 1	2796
	21st to end	Night	38% 10% 21% 24% 7%	2999
August	1st to 10th	Night	10% 5% 16% 46% 22% 1	2848
	11th to 20th	Night	18% 9% 31% 32% 9% 2	3143
	21st to end	Night	23% 12% 21% 26% 16% 2	3642
September	1st to 10th	Night	32% 7% 15% 22% 17% 8%	2090
	11th to 20th	Night	36% 9% 13% 22% 15% 5%	2090
	21st to end	Night	29% 7% 14% 20% 17% 12%	2090
October	1st to 10th	Night	16% 4 7% 15% 21% 37%	3480
	11th to 20th	Night	22% 4 9% 16% 29% 20%	3637
	21st to end	Night	26% 3 5% 18% 32% 16%	4120
November	1st to 10th	Night	37% 4 8% 17% 21% 13%	2340
	11th to 20th	Night	32% 4 8% 19% 20% 17%	2530
	21st to end	Night	39% 5% 8% 16% 19% 15%	2600
December	1st to 10th	Night	48% 4 6% 16% 16% 11%	4250
	11th to 20th	Night	44% 3 5% 9% 21% 17%	4817
	21st to end	Night	34% 3 3 13% 29% 17%	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	24% 16% 35% 21% 4	300
	11th to 20th	Day	28% 19% 27% 20% 7%	300
	21st to end	Day	23% 18% 35% 21% 4	330
February	1st to 10th	Day	30% 14% 29% 21% 5%	285
	11th to 20th	Day	28% 18% 33% 17% 3	270
	21st to end	Day	27% 18% 42% 13%	225
March	1st to 10th	Day	24% 16% 41% 16% 3	270
	11th to 20th	Day	21% 17% 40% 16% 5%	270
	21st to end	Day	22% 17% 37% 19% 4 1	297
April	1st to 10th	Day	23% 16% 39% 19% 3	270
	11th to 20th	Day	24% 21% 30% 19% 6%	269
	21st to end	Day	20% 13% 39% 22% 6%	598
May	1st to 10th	Day	19% 10% 38% 26% 7%	653
	11th to 20th	Day	19% 9% 29% 33% 9% 2	675
	21st to end	Day	17% 8% 26% 33% 15% 1	734
June	1st to 10th	Day	8% 3 18% 39% 28% 5%	680
	11th to 20th	Day	9% 3 17% 44% 24% 3	680
	21st to end	Day	9% 2 17% 38% 28% 8%	680
July	1st to 10th	Day	8% 3 11% 37% 32% 9%	671
	11th to 20th	Day	5% 1 8% 36% 42% 7%	668
	21st to end	Day	6% 1 6% 35% 43% 9%	739
August	1st to 10th	Day	3 1 5% 37% 42% 13%	662
	11th to 20th	Day	6% 5% 40% 39% 10%	477
	21st to end	Day	5% 1 6% 32% 36% 19%	441
September	1st to 10th	Day	9% 2 10% 29% 36% 14%	300
	11th to 20th	Day	7% 1 8% 29% 41% 14%	300
	21st to end	Day	10% 2 11% 28% 39% 11%	300
October	1st to 10th	Day	5% 3 17% 38% 27% 9%	300
	11th to 20th	Day	12% 5% 21% 39% 18% 5%	300
	21st to end	Day	21% 12% 25% 25% 14% 3	330
November	1st to 10th	Day	19% 6% 28% 30% 13% 3	300
	11th to 20th	Day	20% 13% 27% 26% 11% 3	300
	21st to end	Day	24% 16% 33% 16% 9% 2	300
December	1st to 10th	Day	20% 11% 34% 24% 7% 3	300
	11th to 20th	Day	22% 17% 29% 23% 9%	300
	21st to end	Day	23% 12% 36% 22% 7%	330
January	1st to 10th	Night	21% 11% 30% 31% 8%	1000
	11th to 20th	Night	21% 13% 30% 30% 7%	1000
	21st to end	Night	23% 11% 30% 28% 7%	965
February	1st to 10th	Night	24% 12% 31% 26% 6%	780
	11th to 20th	Night	27% 11% 33% 25% 3	744
	21st to end	Night	26% 13% 41% 19% 2	628
March	1st to 10th	Night	24% 10% 36% 26% 4	602
	11th to 20th	Night	24% 9% 34% 28% 5%	570
	21st to end	Night	19% 10% 40% 27% 3	627
April	1st to 10th	Night	17% 13% 36% 29% 4	570
	11th to 20th	Night	23% 11% 35% 27% 4	563
	21st to end	Night	21% 11% 38% 22% 8%	570
May	1st to 10th	Night	20% 10% 36% 28% 5%	570
	11th to 20th	Night	15% 9% 32% 36% 9% 1	561
	21st to end	Night	16% 7% 30% 35% 12%	617
June	1st to 10th	Night	7% 3 21% 46% 22% 1	570
	11th to 20th	Night	5% 2 33% 45% 14% 1	570
	21st to end	Night	7% 3 18% 44% 23% 4	570
July	1st to 10th	Night	9% 3 14% 42% 29% 4	555
	11th to 20th	Night	3 1 11% 51% 31% 2	552
	21st to end	Night	6% 2 9% 48% 31% 3	624
August	1st to 10th	Night	1 10% 49% 35% 3	567
	11th to 20th	Night	5% 1 9% 50% 31% 5%	564
	21st to end	Night	7% 7% 38% 41% 6%	627
September	1st to 10th	Night	9% 2 10% 38% 37% 5%	570
	11th to 20th	Night	12% 1 8% 39% 38% 3	570
	21st to end	Night	12% 2 13% 34% 36% 4	570
October	1st to 10th	Night	8% 2 13% 45% 28% 5%	570
	11th to 20th	Night	15% 4 22% 37% 20% 2	670
	21st to end	Night	17% 8% 21% 31% 20% 2	836
November	1st to 10th	Night	15% 7% 20% 37% 19% 2	780
	11th to 20th	Night	15% 7% 26% 36% 14% 1	930
	21st to end	Night	20% 13% 27% 29% 11% 1	1000
December	1st to 10th	Night	22% 8% 28% 32% 9% 1	1000
	11th to 20th	Night	27% 9% 26% 31% 8%	1000
	21st to end	Night	22% 8% 30% 32% 7%	1100

Page generated at 2025-02-03 02:05:52. Version ac07

Copyright © 2025 Attilla Danko.