MIRA Weaver Student Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the MIRA Weaver Student Observatory clear sky chart from 2005-11-06 to 2024-12-02.

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	6% 7% 2 6% 4 3 4 3 6% 9% 50%	105163
	Night	8% 10% 3 7% 4 3 3 3 4 6% 48%	103230
Transparency	Day	25% 6% 10% 19% 23% 17%	104923
	Night	30% 8% 11% 16% 20% 15%	103060
Seeing	Day	10% 3 9% 26% 36% 16%	15598
	Night	14% 2 8% 27% 35% 15%	25035

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	11% 8% 3 6% 5% 4 4 3 5% 6% 44%	2763
	11th to 20th	Day	13% 7% 3 9% 5% 5% 5% 4 5% 7% 37%	2860
	21st to end	Day	17% 10% 4 8% 5% 5% 4 4 5% 7% 32%	3146
February	1st to 10th	Day	13% 7% 2 4 4 4 4 4 4 6% 47%	1710
	11th to 20th	Day	11% 8% 2 5% 4 4 5% 4 5% 7% 45%	1794
	21st to end	Day	11% 6% 3 6% 4 4 4 3 5% 6% 48%	1570
March	1st to 10th	Day	10% 9% 3 8% 6% 6% 6% 6% 8% 9% 30%	3370
	11th to 20th	Day	8% 5% 2 6% 3 4 3 3 5% 7% 54%	3490
	21st to end	Day	10% 9% 2 7% 5% 6% 5% 5% 6% 8% 37%	3904
April	1st to 10th	Day	8% 7% 2 6% 4 3 4 3 5% 7% 48%	2280
	11th to 20th	Day	7% 8% 2 7% 4 4 3 3 5% 7% 50%	2249
	21st to end	Day	5% 8% 3 5% 3 3 3 4 6% 9% 51%	2280
May	1st to 10th	Day	6% 5% 1 3 2 2 2 2 3 7% 67%	3800
	11th to 20th	Day	3 6% 2 5% 3 2 3 3 5% 11% 57%	4118
	21st to end	Day	5% 12% 3 6% 3 4 3 3 4 8% 48%	4813
June	1st to 10th	Day	4 8% 2 6% 4 4 4 3 5% 9% 49%	2660
	11th to 20th	Day	3 6% 2 4 3 2 3 3 5% 11% 59%	2660
	21st to end	Day	3 7% 2 5% 3 3 4 3 6% 11% 53%	2660
July	1st to 10th	Day	1 5% 2 4 3 2 3 3 6% 11% 60%	4370
	11th to 20th	Day	2 7% 3 6% 4 3 4 3 7% 14% 47%	4110
	21st to end	Day	1 12% 4 9% 3 4 5% 4 7% 14% 39%	4455
August	1st to 10th	Day	2 7% 4 7% 4 4 5% 4 11% 20% 31%	4091
	11th to 20th	Day	1 5% 2 6% 3 3 4 3 6% 13% 53%	3754
	21st to end	Day	1 7% 3 5% 3 3 3 3 6% 13% 52%	4180
September	1st to 10th	Day	2 5% 1 3 3 3 3 3 5% 10% 63%	2280
	11th to 20th	Day	4 9% 2 4 3 3 3 3 6% 11% 53%	2280
	21st to end	Day	2 4 2 3 2 3 2 2 5% 9% 65%	2028
October	1st to 10th	Day	2 3 1 3 2 3 2 3 5% 9% 67%	3180
	11th to 20th	Day	3 8% 2 6% 4 3 3 3 4 7% 57%	3161
	21st to end	Day	3 4 1 4 2 3 4 3 6% 7% 61%	3448
November	1st to 10th	Day	6% 4 1 4 3 3 5% 3 5% 8% 56%	1934
	11th to 20th	Day	7% 6% 2 6% 4 3 4 2 5% 7% 54%	1800
	21st to end	Day	9% 5% 2 4 3 3 3 3 4 8% 56%	1715
December	1st to 10th	Day	9% 5% 3 7% 4 3 4 4 6% 8% 46%	1528
	11th to 20th	Day	18% 9% 3 7% 6% 4 4 3 5% 7% 34%	1928
	21st to end	Day	13% 9% 3 7% 5% 4 4 4 5% 7% 39%	2794
January	1st to 10th	Night	11% 7% 2 6% 5% 3 3 3 4 6% 48%	4186
	11th to 20th	Night	14% 8% 3 8% 5% 4 5% 4 6% 8% 35%	4110
	21st to end	Night	19% 10% 3 8% 4 4 3 3 5% 7% 33%	4521
February	1st to 10th	Night	13% 8% 3 5% 4 3 3 3 4 5% 50%	2470
	11th to 20th	Night	11% 8% 3 7% 4 4 4 4 5% 7% 44%	2274
	21st to end	Night	12% 8% 2 4 3 4 3 3 5% 7% 49%	1884
March	1st to 10th	Night	15% 13% 3 8% 6% 5% 4 4 5% 5% 32%	3500
	11th to 20th	Night	8% 7% 3 6% 4 4 4 3 4 7% 52%	3490
	21st to end	Night	14% 13% 3 7% 5% 4 4 4 5% 6% 35%	3747
April	1st to 10th	Night	9% 8% 3 5% 4 4 3 4 5% 7% 48%	1900
	11th to 20th	Night	9% 9% 2 5% 3 2 3 3 5% 6% 52%	1879
	21st to end	Night	5% 7% 3 4 3 3 3 3 5% 9% 55%	1900
May	1st to 10th	Night	4 7% 3 6% 3 2 2 2 3 5% 63%	3166
	11th to 20th	Night	3 16% 8% 9% 4 3 3 2 4 7% 43%	2777
	21st to end	Night	5% 16% 3 6% 3 2 2 1 3 4 56%	2762
June	1st to 10th	Night	5% 11% 3 6% 4 3 3 3 4 6% 51%	1520
	11th to 20th	Night	3 10% 3 7% 3 3 2 2 4 7% 58%	1520
	21st to end	Night	3 10% 2 7% 3 4 3 2 4 7% 54%	1520
July	1st to 10th	Night	1 9% 3 6% 3 3 3 2 4 6% 60%	2587
	11th to 20th	Night	1 15% 3 8% 5% 4 3 2 4 6% 48%	2860
	21st to end	Night	1 29% 3 12% 4 4 3 2 4 6% 31%	3109
August	1st to 10th	Night	2 13% 10% 17% 5% 4 4 3 6% 6% 30%	2848
	11th to 20th	Night	2 12% 3 8% 3 3 3 2 3 5% 56%	3092
	21st to end	Night	2 11% 4 7% 4 3 4 2 4 6% 52%	3498
September	1st to 10th	Night	3 7% 2 4 3 4 3 3 5% 7% 58%	1900
	11th to 20th	Night	4 11% 3 6% 3 4 3 2 4 8% 53%	1900
	21st to end	Night	3 7% 2 4 3 2 3 2 4 7% 63%	2146
October	1st to 10th	Night	3 4 2 3 2 2 2 2 3 5% 71%	3800
	11th to 20th	Night	4 15% 2 7% 3 3 3 2 3 5% 52%	3777
	21st to end	Night	4 7% 2 4 3 3 3 2 4 6% 61%	4121
November	1st to 10th	Night	7% 7% 2 5% 3 4 3 3 5% 7% 56%	2362
	11th to 20th	Night	8% 6% 2 5% 3 3 3 3 5% 8% 53%	2600
	21st to end	Night	8% 5% 2 4 3 3 3 3 5% 6% 57%	2680
December	1st to 10th	Night	12% 7% 2 6% 3 3 4 3 5% 7% 49%	2686
	11th to 20th	Night	17% 9% 3 8% 5% 4 4 4 6% 7% 34%	3276
	21st to end	Night	15% 7% 3 7% 5% 5% 5% 4 6% 6% 38%	4862

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	40% 9% 10% 28% 9% 3	2763
	11th to 20th	Day	46% 8% 12% 15% 11% 8%	2860
	21st to end	Day	49% 9% 8% 15% 13% 5%	3146
February	1st to 10th	Day	38% 6% 9% 17% 20% 11%	1710
	11th to 20th	Day	38% 8% 9% 18% 19% 8%	1804
	21st to end	Day	38% 8% 9% 22% 19% 5%	1570
March	1st to 10th	Day	45% 15% 12% 12% 13% 3	3370
	11th to 20th	Day	29% 7% 12% 26% 16% 11%	3490
	21st to end	Day	41% 13% 14% 20% 10% 2	3904
April	1st to 10th	Day	36% 8% 13% 24% 16% 4	2280
	11th to 20th	Day	33% 7% 12% 25% 17% 6%	2252
	21st to end	Day	28% 8% 14% 26% 20% 4	2280
May	1st to 10th	Day	19% 5% 11% 28% 29% 9%	3800
	11th to 20th	Day	14% 5% 9% 30% 30% 11%	4121
	21st to end	Day	24% 7% 11% 25% 27% 5%	4814
June	1st to 10th	Day	25% 7% 12% 24% 25% 7%	2660
	11th to 20th	Day	15% 4 11% 24% 32% 15%	2660
	21st to end	Day	15% 4 5% 18% 31% 27%	2660
July	1st to 10th	Day	8% 2 5% 10% 16% 59%	4302
	11th to 20th	Day	13% 5% 8% 9% 20% 45%	4041
	21st to end	Day	14% 6% 10% 16% 32% 22%	4315
August	1st to 10th	Day	14% 4 11% 14% 32% 26%	4076
	11th to 20th	Day	8% 3 8% 15% 40% 26%	3790
	21st to end	Day	12% 4 8% 13% 35% 29%	4179
September	1st to 10th	Day	11% 2 3 13% 28% 42%	2280
	11th to 20th	Day	18% 4 9% 23% 30% 16%	2280
	21st to end	Day	13% 4 7% 23% 35% 19%	2028
October	1st to 10th	Day	13% 3 6% 13% 28% 37%	3180
	11th to 20th	Day	20% 5% 10% 21% 27% 17%	3161
	21st to end	Day	19% 4 8% 23% 29% 17%	3448
November	1st to 10th	Day	25% 6% 11% 21% 24% 12%	1934
	11th to 20th	Day	29% 6% 12% 19% 23% 10%	1800
	21st to end	Day	29% 6% 10% 15% 27% 13%	1715
December	1st to 10th	Day	34% 7% 11% 17% 18% 13%	1528
	11th to 20th	Day	50% 8% 10% 14% 13% 6%	1928
	21st to end	Day	44% 8% 12% 22% 11% 4	2794
January	1st to 10th	Night	37% 11% 14% 22% 13% 3	4186
	11th to 20th	Night	45% 10% 15% 12% 9% 8%	4110
	21st to end	Night	50% 9% 8% 14% 11% 7%	4521
February	1st to 10th	Night	38% 6% 9% 15% 19% 13%	2470
	11th to 20th	Night	37% 8% 10% 17% 21% 7%	2275
	21st to end	Night	36% 8% 11% 24% 15% 6%	1884
March	1st to 10th	Night	52% 10% 11% 11% 13% 3	3500
	11th to 20th	Night	30% 9% 10% 22% 19% 10%	3490
	21st to end	Night	45% 14% 12% 16% 10% 3	3747
April	1st to 10th	Night	34% 10% 14% 22% 14% 5%	1900
	11th to 20th	Night	33% 8% 12% 24% 16% 6%	1879
	21st to end	Night	27% 10% 14% 25% 19% 5%	1900
May	1st to 10th	Night	22% 13% 10% 19% 23% 13%	3166
	11th to 20th	Night	20% 7% 19% 20% 23% 12%	2778
	21st to end	Night	28% 10% 10% 20% 26% 6%	2762
June	1st to 10th	Night	29% 8% 10% 20% 23% 10%	1520
	11th to 20th	Night	21% 7% 11% 14% 29% 19%	1520
	21st to end	Night	16% 4 6% 14% 30% 30%	1520
July	1st to 10th	Night	11% 3 8% 6% 17% 54%	2546
	11th to 20th	Night	19% 6% 9% 8% 17% 41%	2806
	21st to end	Night	31% 9% 10% 11% 19% 20%	3011
August	1st to 10th	Night	19% 6% 25% 10% 20% 21%	2839
	11th to 20th	Night	15% 3 9% 8% 38% 28%	3122
	21st to end	Night	18% 4 9% 12% 31% 26%	3498
September	1st to 10th	Night	17% 2 5% 11% 25% 40%	1900
	11th to 20th	Night	20% 6% 8% 18% 29% 19%	1900
	21st to end	Night	15% 4 7% 17% 35% 21%	2146
October	1st to 10th	Night	13% 3 6% 13% 25% 39%	3800
	11th to 20th	Night	25% 11% 6% 18% 25% 16%	3777
	21st to end	Night	20% 4 6% 24% 28% 18%	4121
November	1st to 10th	Night	27% 6% 9% 19% 23% 16%	2362
	11th to 20th	Night	29% 6% 12% 20% 23% 10%	2600
	21st to end	Night	28% 7% 9% 17% 24% 15%	2680
December	1st to 10th	Night	34% 7% 10% 17% 18% 14%	2686
	11th to 20th	Night	50% 9% 10% 13% 14% 5%	3276
	21st to end	Night	44% 13% 13% 15% 11% 5%	4862

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% 3 10% 28% 26% 10%	313
	11th to 20th	Day	16% 2 10% 27% 33% 12%	370
	21st to end	Day	14% 2 9% 30% 31% 13%	407
February	1st to 10th	Day	14% 2 15% 26% 29% 14%	370
	11th to 20th	Day	13% 5% 14% 34% 27% 6%	428
	21st to end	Day	10% 5% 15% 37% 25% 9%	382
March	1st to 10th	Day	18% 4 16% 33% 24% 5%	460
	11th to 20th	Day	15% 5% 12% 31% 26% 11%	460
	21st to end	Day	12% 5% 19% 35% 21% 8%	506
April	1st to 10th	Day	13% 4 17% 30% 27% 9%	460
	11th to 20th	Day	10% 6% 19% 27% 27% 11%	452
	21st to end	Day	7% 3 20% 32% 27% 13%	460
May	1st to 10th	Day	8% 6% 14% 30% 32% 10%	463
	11th to 20th	Day	10% 2 14% 29% 32% 13%	488
	21st to end	Day	11% 4 12% 25% 34% 15%	529
June	1st to 10th	Day	8% 3 9% 29% 36% 15%	490
	11th to 20th	Day	6% 3 12% 29% 38% 12%	490
	21st to end	Day	8% 1 6% 25% 49% 13%	490
July	1st to 10th	Day	9% 1 3 20% 46% 20%	486
	11th to 20th	Day	6% 1 1 14% 55% 23%	484
	21st to end	Day	6% 2 1 17% 53% 21%	531
August	1st to 10th	Day	8% 1 2 23% 48% 17%	487
	11th to 20th	Day	5% 1 1 23% 48% 22%	477
	21st to end	Day	8% 1 2 16% 47% 25%	539
September	1st to 10th	Day	4 2 4 16% 50% 24%	490
	11th to 20th	Day	8% 2 5% 28% 39% 18%	490
	21st to end	Day	5% 2 7% 19% 41% 26%	490
October	1st to 10th	Day	4 1 8% 26% 37% 24%	490
	11th to 20th	Day	8% 2 9% 22% 36% 24%	490
	21st to end	Day	8% 2 5% 27% 37% 21%	539
November	1st to 10th	Day	7% 2 7% 29% 34% 21%	470
	11th to 20th	Day	8% 3 6% 31% 34% 18%	300
	21st to end	Day	10% 2 6% 33% 35% 15%	257
December	1st to 10th	Day	15% 2 5% 27% 30% 20%	182
	11th to 20th	Day	12% 2 9% 27% 34% 15%	180
	21st to end	Day	16% 2 12% 30% 26% 14%	198
January	1st to 10th	Night	18% 2 10% 36% 26% 7%	760
	11th to 20th	Night	14% 1 11% 33% 31% 9%	760
	21st to end	Night	16% 2 9% 36% 32% 6%	836
February	1st to 10th	Night	18% 2 10% 31% 29% 9%	760
	11th to 20th	Night	15% 4 14% 29% 31% 7%	744
	21st to end	Night	14% 4 12% 36% 29% 5%	628
March	1st to 10th	Night	20% 4 13% 31% 27% 6%	760
	11th to 20th	Night	15% 2 10% 33% 31% 9%	760
	21st to end	Night	16% 3 14% 31% 31% 6%	836
April	1st to 10th	Night	14% 3 12% 32% 32% 7%	760
	11th to 20th	Night	13% 4 13% 30% 29% 11%	751
	21st to end	Night	9% 1 13% 32% 31% 14%	760
May	1st to 10th	Night	10% 1 11% 33% 35% 12%	759
	11th to 20th	Night	13% 1 7% 24% 35% 19%	531
	21st to end	Night	15% 3 9% 25% 32% 17%	411
June	1st to 10th	Night	12% 1 8% 26% 39% 15%	380
	11th to 20th	Night	11% 2 8% 23% 45% 12%	380
	21st to end	Night	9% 2 1 21% 43% 23%	380
July	1st to 10th	Night	13% 4 1 13% 41% 27%	390
	11th to 20th	Night	13% 3 6% 39% 38%	552
	21st to end	Night	22% 5% 2 7% 36% 28%	624
August	1st to 10th	Night	20% 4 1 11% 41% 24%	567
	11th to 20th	Night	12% 4 2 14% 41% 27%	730
	21st to end	Night	16% 3 2 8% 40% 30%	836
September	1st to 10th	Night	7% 1 1 13% 46% 31%	760
	11th to 20th	Night	11% 3 3 19% 43% 21%	760
	21st to end	Night	6% 1 5% 18% 43% 27%	760
October	1st to 10th	Night	9% 2 4 25% 38% 22%	760
	11th to 20th	Night	10% 2 7% 21% 38% 22%	760
	21st to end	Night	11% 1 5% 25% 35% 23%	836
November	1st to 10th	Night	10% 2 8% 28% 38% 13%	780
	11th to 20th	Night	12% 2 6% 29% 42% 10%	800
	21st to end	Night	9% 1 9% 38% 35% 8%	800
December	1st to 10th	Night	15% 2 9% 37% 30% 6%	768
	11th to 20th	Night	20% 1 12% 36% 25% 6%	760
	21st to end	Night	18% 3 14% 38% 23% 4	836

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