Tzec Maun Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tzec Maun Observatory clear sky chart from 2005-11-06 to 2025-01-07.

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	3 4 2 5% 4 4 4 4 6% 7% 56%	108312
	Night	2 4 2 5% 4 4 4 4 6% 7% 60%	109483
Transparency	Day	22% 9% 9% 17% 26% 18%	108077
	Night	18% 9% 9% 18% 28% 19%	109308
Seeing	Day	13% 19% 27% 24% 14% 3	14921
	Night	9% 16% 26% 30% 16% 3	25220

By Month

Percentage Cloud Cover SW (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	1 5% 3 9% 6% 7% 4 3 4 4 52%	2912
	11th to 20th	Day	1 4 3 4 2 3 3 2 4 5% 68%	2860
	21st to end	Day	3 11% 5% 8% 4 4 4 4 8% 5% 44%	3146
February	1st to 10th	Day	1 6% 2 5% 3 3 3 3 5% 7% 61%	1860
	11th to 20th	Day	2 6% 3 5% 4 3 2 2 5% 7% 61%	1849
	21st to end	Day	2 4 2 4 4 5% 3 3 4 6% 64%	1570
March	1st to 10th	Day	1 6% 3 5% 5% 3 4 3 5% 6% 58%	3290
	11th to 20th	Day	2 4 2 6% 7% 4 5% 4 5% 6% 54%	3490
	21st to end	Day	1 6% 3 5% 4 5% 4 5% 7% 9% 53%	3969
April	1st to 10th	Day	1 4 1 4 3 3 3 3 6% 7% 65%	2280
	11th to 20th	Day	2 4 1 4 3 3 3 2 4 6% 68%	2251
	21st to end	Day	2 4 1 4 3 3 3 3 3 6% 70%	2280
May	1st to 10th	Day	1 2 1 3 2 2 2 3 3 4 78%	3800
	11th to 20th	Day	2 2 1 3 3 4 5% 4 5% 6% 65%	3765
	21st to end	Day	1 1 2 2 2 2 2 4 5% 78%	4429
June	1st to 10th	Day	2 3 1 4 4 4 4 4 7% 9% 59%	2660
	11th to 20th	Day	2 2 1 2 2 3 3 4 6% 8% 66%	2660
	21st to end	Day	5% 5% 2 6% 5% 6% 6% 5% 8% 8% 43%	2609
July	1st to 10th	Day	5% 4 1 7% 6% 6% 7% 6% 10% 10% 38%	4110
	11th to 20th	Day	4 4 2 6% 6% 6% 7% 6% 8% 10% 40%	4110
	21st to end	Day	9% 5% 2 8% 7% 7% 8% 7% 11% 11% 26%	4434
August	1st to 10th	Day	5% 4 2 7% 5% 6% 8% 6% 8% 11% 38%	3784
	11th to 20th	Day	5% 5% 2 7% 5% 7% 8% 6% 8% 9% 39%	3726
	21st to end	Day	5% 5% 2 7% 6% 7% 8% 7% 10% 11% 32%	4180
September	1st to 10th	Day	9% 7% 3 9% 5% 4 5% 4 6% 8% 39%	2280
	11th to 20th	Day	7% 5% 2 6% 4 5% 5% 4 5% 7% 50%	2280
	21st to end	Day	4 3 2 5% 3 4 4 3 5% 6% 62%	2090
October	1st to 10th	Day	4 3 1 5% 3 3 2 3 3 6% 67%	3180
	11th to 20th	Day	3 3 1 4 3 4 4 3 5% 7% 64%	3150
	21st to end	Day	1 1 2 2 1 1 2 2 3 83%	3438
November	1st to 10th	Day	1 2 1 5% 4 2 3 2 5% 7% 68%	1959
	11th to 20th	Day	1 3 1 4 3 4 3 2 5% 7% 66%	1826
	21st to end	Day	3 4 2 5% 4 4 4 3 6% 8% 57%	1800
December	1st to 10th	Day	3 5% 1 4 2 2 3 3 5% 7% 63%	2940
	11th to 20th	Day	1 4 2 6% 4 4 3 2 4 8% 62%	3090
	21st to end	Day	1 3 2 4 3 3 3 2 4 9% 67%	4255
January	1st to 10th	Night	1 3 3 10% 5% 4 4 3 5% 5% 56%	4175
	11th to 20th	Night	4 2 4 4 4 3 2 4 5% 68%	4110
	21st to end	Night	3 8% 4 7% 3 4 6% 6% 8% 7% 44%	4521
February	1st to 10th	Night	1 5% 2 6% 3 4 4 4 5% 7% 61%	2318
	11th to 20th	Night	2 6% 2 4 3 2 4 3 5% 7% 61%	2218
	21st to end	Night	1 3 2 5% 4 3 4 3 5% 7% 61%	1884
March	1st to 10th	Night	1 5% 2 5% 4 4 4 3 4 6% 62%	3562
	11th to 20th	Night	2 5% 3 8% 7% 4 5% 3 4 6% 52%	3490
	21st to end	Night	1 5% 4 5% 3 3 4 4 6% 9% 56%	3679
April	1st to 10th	Night	1 3 1 3 3 3 4 3 6% 8% 65%	1900
	11th to 20th	Night	2 4 1 3 3 3 3 4 5% 8% 65%	1877
	21st to end	Night	2 3 1 3 3 3 3 3 4 6% 68%	1900
May	1st to 10th	Night	1 1 2 2 2 2 2 3 4 79%	3170
	11th to 20th	Night	1 2 1 3 3 4 3 3 4 6% 69%	3140
	21st to end	Night	1 1 2 2 2 2 2 3 4 81%	3148
June	1st to 10th	Night	2 3 2 5% 4 4 4 5% 8% 8% 54%	1520
	11th to 20th	Night	2 2 1 2 2 3 3 3 5% 7% 68%	1520
	21st to end	Night	4 5% 3 6% 6% 6% 6% 5% 7% 8% 44%	1563
July	1st to 10th	Night	4 4 2 6% 5% 5% 5% 6% 9% 11% 42%	2860
	11th to 20th	Night	2 2 1 5% 5% 5% 6% 5% 8% 10% 51%	2860
	21st to end	Night	5% 5% 2 6% 5% 5% 6% 7% 11% 13% 35%	3125
August	1st to 10th	Night	2 3 1 4 4 4 5% 5% 10% 14% 47%	3155
	11th to 20th	Night	3 3 1 4 3 4 4 4 8% 11% 55%	3113
	21st to end	Night	3 4 2 6% 5% 5% 6% 6% 9% 13% 41%	3487
September	1st to 10th	Night	5% 4 2 6% 4 5% 5% 6% 7% 8% 48%	1900
	11th to 20th	Night	5% 3 1 4 4 4 4 4 6% 7% 57%	1900
	21st to end	Night	2 2 2 4 3 3 4 4 6% 6% 65%	2090
October	1st to 10th	Night	2 1 1 2 2 3 3 2 4 5% 74%	3786
	11th to 20th	Night	2 2 1 3 3 3 3 3 5% 6% 71%	3778
	21st to end	Night	2 2 1 3 2 2 2 2 3 6% 76%	4120
November	1st to 10th	Night	2 3 1 3 3 4 3 3 5% 6% 67%	2340
	11th to 20th	Night	1 3 1 3 3 3 3 4 5% 8% 67%	2550
	21st to end	Night	2 5% 2 6% 4 3 4 3 6% 7% 58%	2600
December	1st to 10th	Night	2 4 1 4 3 3 3 4 7% 7% 64%	4250
	11th to 20th	Night	1 2 1 5% 3 4 5% 4 5% 7% 62%	5112
	21st to end	Night	1 5% 2 5% 3 3 2 3 5% 8% 63%	6762

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	18% 20% 5% 13% 30% 15%	2913
	11th to 20th	Day	14% 10% 6% 20% 35% 16%	2860
	21st to end	Day	27% 17% 6% 14% 24% 12%	3146
February	1st to 10th	Day	20% 6% 7% 17% 28% 22%	1860
	11th to 20th	Day	22% 7% 5% 18% 26% 22%	1859
	21st to end	Day	18% 9% 4 10% 29% 30%	1570
March	1st to 10th	Day	23% 7% 6% 10% 33% 22%	3290
	11th to 20th	Day	20% 10% 11% 18% 29% 12%	3490
	21st to end	Day	17% 17% 6% 18% 30% 12%	3969
April	1st to 10th	Day	16% 6% 6% 17% 35% 21%	2280
	11th to 20th	Day	18% 5% 4 13% 31% 30%	2254
	21st to end	Day	15% 5% 5% 17% 35% 22%	2280
May	1st to 10th	Day	10% 5% 3 10% 37% 35%	3800
	11th to 20th	Day	17% 7% 7% 14% 35% 21%	3768
	21st to end	Day	9% 3 5% 11% 22% 50%	4429
June	1st to 10th	Day	20% 7% 12% 20% 23% 18%	2660
	11th to 20th	Day	14% 5% 7% 16% 25% 33%	2660
	21st to end	Day	32% 11% 11% 17% 18% 12%	2609
July	1st to 10th	Day	32% 14% 19% 13% 14% 8%	4046
	11th to 20th	Day	31% 12% 15% 21% 19% 3	4041
	21st to end	Day	39% 17% 18% 19% 7%	4294
August	1st to 10th	Day	32% 12% 15% 29% 11% 1	3772
	11th to 20th	Day	32% 12% 16% 22% 17% 1	3762
	21st to end	Day	37% 15% 18% 22% 8% 1	4179
September	1st to 10th	Day	40% 8% 10% 16% 18% 7%	2280
	11th to 20th	Day	31% 8% 10% 19% 25% 7%	2280
	21st to end	Day	22% 6% 10% 21% 31% 11%	2090
October	1st to 10th	Day	19% 6% 6% 11% 33% 25%	3180
	11th to 20th	Day	19% 6% 5% 21% 30% 19%	3150
	21st to end	Day	8% 3 4 15% 38% 31%	3438
November	1st to 10th	Day	15% 4 7% 21% 33% 20%	1959
	11th to 20th	Day	17% 6% 6% 18% 29% 25%	1826
	21st to end	Day	21% 7% 7% 15% 29% 20%	1800
December	1st to 10th	Day	18% 6% 6% 17% 33% 21%	2940
	11th to 20th	Day	20% 5% 6% 11% 36% 22%	3088
	21st to end	Day	13% 7% 5% 16% 34% 26%	4255
January	1st to 10th	Night	18% 16% 5% 14% 30% 18%	4175
	11th to 20th	Night	15% 9% 5% 21% 36% 14%	4110
	21st to end	Night	25% 17% 7% 13% 25% 12%	4521
February	1st to 10th	Night	18% 7% 8% 17% 25% 25%	2318
	11th to 20th	Night	20% 8% 6% 15% 28% 24%	2218
	21st to end	Night	18% 9% 5% 12% 27% 29%	1884
March	1st to 10th	Night	17% 9% 6% 13% 36% 19%	3562
	11th to 20th	Night	24% 11% 12% 14% 28% 12%	3490
	21st to end	Night	12% 19% 7% 20% 29% 12%	3679
April	1st to 10th	Night	15% 6% 7% 16% 36% 20%	1900
	11th to 20th	Night	14% 9% 7% 15% 29% 26%	1877
	21st to end	Night	15% 5% 7% 19% 35% 19%	1900
May	1st to 10th	Night	9% 4 5% 8% 39% 36%	3170
	11th to 20th	Night	13% 8% 8% 15% 35% 21%	3141
	21st to end	Night	9% 2 5% 11% 20% 52%	3149
June	1st to 10th	Night	22% 8% 11% 18% 21% 20%	1520
	11th to 20th	Night	15% 4 6% 17% 21% 38%	1520
	21st to end	Night	33% 9% 11% 18% 14% 15%	1563
July	1st to 10th	Night	28% 12% 20% 18% 15% 7%	2815
	11th to 20th	Night	24% 9% 17% 28% 18% 4	2806
	21st to end	Night	30% 15% 23% 22% 10% 1	3027
August	1st to 10th	Night	22% 9% 19% 32% 16% 2	3144
	11th to 20th	Night	20% 9% 16% 34% 20% 1	3143
	21st to end	Night	28% 12% 21% 26% 12% 1	3487
September	1st to 10th	Night	29% 12% 12% 17% 20% 10%	1900
	11th to 20th	Night	23% 10% 10% 19% 27% 11%	1900
	21st to end	Night	18% 7% 10% 22% 32% 11%	2090
October	1st to 10th	Night	12% 5% 6% 13% 34% 29%	3786
	11th to 20th	Night	13% 5% 8% 19% 35% 20%	3778
	21st to end	Night	10% 3 5% 17% 33% 33%	4120
November	1st to 10th	Night	15% 5% 6% 19% 32% 22%	2340
	11th to 20th	Night	13% 7% 7% 18% 30% 25%	2550
	21st to end	Night	23% 7% 6% 16% 29% 20%	2600
December	1st to 10th	Night	15% 9% 6% 16% 36% 18%	4250
	11th to 20th	Night	17% 7% 7% 14% 27% 28%	5113
	21st to end	Night	16% 7% 5% 17% 31% 25%	6762

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	19% 34% 25% 19% 3	288
	11th to 20th	Day	23% 23% 27% 21% 4 1	270
	21st to end	Day	13% 31% 30% 16% 9%	297
February	1st to 10th	Day	20% 26% 27% 21% 6%	270
	11th to 20th	Day	21% 34% 27% 15% 3	270
	21st to end	Day	33% 32% 21% 10% 3	225
March	1st to 10th	Day	19% 38% 28% 12% 3	365
	11th to 20th	Day	23% 29% 31% 11% 6% 1	460
	21st to end	Day	27% 33% 30% 7% 2	506
April	1st to 10th	Day	15% 37% 34% 11% 2 1	460
	11th to 20th	Day	18% 37% 33% 10% 2	452
	21st to end	Day	17% 34% 33% 12% 3	460
May	1st to 10th	Day	7% 33% 45% 12% 3	463
	11th to 20th	Day	8% 24% 42% 19% 6% 1	488
	21st to end	Day	7% 23% 42% 20% 6% 1	614
June	1st to 10th	Day	4 8% 32% 37% 17% 2	680
	11th to 20th	Day	3 12% 33% 33% 16% 2	680
	21st to end	Day	6% 5% 26% 34% 25% 4	629
July	1st to 10th	Day	7% 3 20% 36% 27% 7%	486
	11th to 20th	Day	7% 1 11% 37% 35% 9%	484
	21st to end	Day	6% 2 9% 32% 41% 10%	531
August	1st to 10th	Day	6% 2 12% 35% 36% 10%	487
	11th to 20th	Day	9% 4 17% 36% 27% 8%	477
	21st to end	Day	10% 3 12% 37% 31% 7%	539
September	1st to 10th	Day	12% 5% 22% 38% 19% 4	490
	11th to 20th	Day	6% 6% 31% 35% 17% 4	490
	21st to end	Day	6% 11% 35% 29% 14% 5%	300
October	1st to 10th	Day	12% 12% 32% 28% 12% 4	300
	11th to 20th	Day	10% 15% 33% 26% 13% 3	300
	21st to end	Day	13% 24% 26% 21% 12% 4	330
November	1st to 10th	Day	17% 29% 28% 16% 8% 2	300
	11th to 20th	Day	15% 17% 30% 22% 14% 2	300
	21st to end	Day	21% 29% 24% 19% 7% 1	300
December	1st to 10th	Day	22% 24% 26% 22% 6% 1	300
	11th to 20th	Day	19% 28% 27% 21% 5%	300
	21st to end	Day	28% 27% 28% 14% 3	330
January	1st to 10th	Night	8% 22% 28% 27% 11% 3	784
	11th to 20th	Night	10% 20% 30% 26% 11% 3	760
	21st to end	Night	13% 19% 28% 26% 12% 1	836
February	1st to 10th	Night	10% 21% 34% 23% 11% 2	760
	11th to 20th	Night	12% 24% 37% 22% 5% 1	744
	21st to end	Night	16% 31% 32% 17% 3 1	628
March	1st to 10th	Night	14% 27% 27% 24% 7% 1	760
	11th to 20th	Night	14% 20% 26% 28% 11% 1	760
	21st to end	Night	12% 27% 33% 21% 7%	751
April	1st to 10th	Night	10% 26% 38% 18% 7%	570
	11th to 20th	Night	12% 27% 33% 22% 6% 1	563
	21st to end	Night	11% 26% 32% 26% 5%	570
May	1st to 10th	Night	5% 23% 41% 26% 5%	570
	11th to 20th	Night	5% 17% 29% 35% 12% 1	561
	21st to end	Night	5% 17% 35% 35% 7% 1	617
June	1st to 10th	Night	3 8% 28% 45% 16% 1	570
	11th to 20th	Night	4 8% 32% 39% 16% 2	570
	21st to end	Night	6% 4 19% 44% 24% 2	570
July	1st to 10th	Night	6% 2 16% 41% 32% 3	555
	11th to 20th	Night	4 11% 38% 41% 7%	552
	21st to end	Night	5% 2 10% 35% 37% 11%	624
August	1st to 10th	Night	4 2 6% 32% 42% 13%	567
	11th to 20th	Night	6% 2 11% 31% 40% 10%	564
	21st to end	Night	4 2 12% 34% 41% 7%	627
September	1st to 10th	Night	7% 3 14% 36% 35% 6%	570
	11th to 20th	Night	7% 3 16% 40% 30% 4	570
	21st to end	Night	4 6% 22% 39% 25% 3	570
October	1st to 10th	Night	3 11% 28% 37% 19% 3	746
	11th to 20th	Night	6% 10% 29% 36% 17% 3	760
	21st to end	Night	8% 15% 22% 27% 20% 7%	836
November	1st to 10th	Night	9% 16% 23% 29% 17% 5%	780
	11th to 20th	Night	8% 13% 24% 36% 15% 3	950
	21st to end	Night	12% 20% 25% 25% 15% 3	1000
December	1st to 10th	Night	13% 21% 24% 30% 11% 2	1000
	11th to 20th	Night	12% 23% 31% 25% 8% 1	1000
	21st to end	Night	15% 22% 28% 26% 7% 1	1005

Page generated at 2025-01-04 01:24:00. Version ac07

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