MIRA Chews Ridge Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the MIRA Chews Ridge Observatory clear sky chart from 2005-11-06 to 2025-09-04.

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 3 1 3 2 3 3 2 4 5% 70%	134288
	Night	3 4 1 4 3 3 3 3 4 6% 65%	131904
Transparency	Day	18% 4 7% 17% 27% 28%	129271
	Night	21% 4 7% 15% 23% 30%	127918
Seeing	Day	8% 9% 25% 33% 20% 5%	16625
	Night	7% 5% 16% 32% 31% 9%	26055

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	6% 5% 2 6% 5% 4 4 3 5% 7% 53%	3961
	11th to 20th	Day	6% 5% 2 5% 5% 5% 4 3 4 5% 57%	4100
	21st to end	Day	7% 7% 3 6% 5% 5% 4 3 6% 7% 46%	4510
February	1st to 10th	Day	10% 8% 2 6% 4 5% 5% 4 5% 6% 45%	1800
	11th to 20th	Day	9% 7% 2 6% 4 4 5% 3 5% 8% 46%	1916
	21st to end	Day	9% 12% 2 5% 4 4 4 4 5% 6% 45%	1650
March	1st to 10th	Day	7% 9% 3 8% 5% 6% 7% 5% 7% 7% 37%	4815
	11th to 20th	Day	6% 5% 2 5% 4 4 6% 4 6% 8% 50%	5000
	21st to end	Day	6% 7% 2 7% 5% 5% 5% 4 6% 9% 43%	5616
April	1st to 10th	Day	7% 8% 2 5% 4 4 4 4 6% 7% 49%	2400
	11th to 20th	Day	5% 6% 1 4 3 3 4 4 5% 7% 57%	2369
	21st to end	Day	4 4 1 3 2 3 4 3 5% 8% 62%	2400
May	1st to 10th	Day	2 2 1 3 2 2 2 1 2 4 79%	5440
	11th to 20th	Day	1 1 1 1 1 2 2 3 5% 83%	5828
	21st to end	Day	1 1 1 1 1 1 1 3 5% 84%	6902
June	1st to 10th	Day	2 2 1 2 2 2 3 3 5% 6% 72%	2800
	11th to 20th	Day	1 1 1 1 1 1 1 2 4 86%	2800
	21st to end	Day	1 1 1 1 1 1 1 2 2 88%	2800
July	1st to 10th	Day	1 1 1 1 1 94%	6243
	11th to 20th	Day	1 1 1 1 1 1 1 1 2 90%	5880
	21st to end	Day	1 1 1 3 4 88%	6402
August	1st to 10th	Day	1 1 1 1 1 1 3 3 87%	5748
	11th to 20th	Day	1 1 1 2 94%	5364
	21st to end	Day	1 1 1 2 1 2 3 89%	5981
September	1st to 10th	Day	1 1 1 1 1 1 1 2 2 90%	2317
	11th to 20th	Day	1 1 1 1 2 2 2 2 3 6% 81%	2280
	21st to end	Day	1 1 1 2 2 2 2 2 3 5% 80%	2041
October	1st to 10th	Day	1 1 1 2 2 2 3 2 4 6% 75%	3180
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 4 79%	3161
	21st to end	Day	2 2 1 3 3 3 4 4 5% 7% 66%	3448
November	1st to 10th	Day	5% 5% 2 4 4 4 4 3 4 7% 58%	1929
	11th to 20th	Day	4 4 2 5% 4 4 3 3 6% 7% 58%	1800
	21st to end	Day	6% 6% 2 5% 3 4 4 4 6% 6% 53%	1755
December	1st to 10th	Day	4 5% 2 5% 5% 4 5% 3 5% 8% 53%	2620
	11th to 20th	Day	11% 9% 2 6% 5% 4 4 4 6% 7% 41%	3028
	21st to end	Day	9% 8% 3 8% 5% 5% 6% 4 7% 9% 36%	4004
January	1st to 10th	Night	5% 6% 2 5% 4 4 4 3 5% 8% 53%	5982
	11th to 20th	Night	4 6% 2 6% 4 5% 4 3 5% 5% 55%	5880
	21st to end	Night	7% 7% 2 7% 4 5% 5% 4 5% 6% 47%	6468
February	1st to 10th	Night	9% 6% 2 6% 5% 5% 4 4 5% 6% 47%	2600
	11th to 20th	Night	6% 6% 2 6% 4 5% 5% 4 6% 8% 47%	2370
	21st to end	Night	6% 9% 2 5% 5% 4 3 3 6% 7% 49%	1980
March	1st to 10th	Night	6% 9% 3 10% 7% 7% 7% 4 6% 6% 35%	5074
	11th to 20th	Night	5% 6% 2 6% 5% 5% 5% 4 5% 7% 51%	5000
	21st to end	Night	5% 8% 3 8% 6% 6% 5% 4 7% 7% 42%	5358
April	1st to 10th	Night	5% 6% 2 5% 5% 5% 4 4 5% 8% 52%	2000
	11th to 20th	Night	3 6% 2 4 3 4 4 3 5% 7% 59%	1979
	21st to end	Night	2 2 1 4 3 3 3 3 6% 10% 62%	2000
May	1st to 10th	Night	1 2 2 2 2 2 2 3 4 80%	4560
	11th to 20th	Night	1 1 1 1 1 2 2 4 5% 82%	4078
	21st to end	Night	1 1 1 1 1 1 2 2 4 87%	3972
June	1st to 10th	Night	2 1 1 2 2 3 3 2 4 7% 74%	1600
	11th to 20th	Night	1 1 2 1 1 1 2 3 3 84%	1600
	21st to end	Night	1 1 1 1 1 2 3 90%	1600
July	1st to 10th	Night	1 1 2 96%	3700
	11th to 20th	Night	1 1 1 1 2 1 2 2 88%	4100
	21st to end	Night	1 1 1 1 3 4 88%	4472
August	1st to 10th	Night	1 1 1 1 1 1 1 2 3 89%	4053
	11th to 20th	Night	1 1 1 1 95%	4506
	21st to end	Night	1 1 1 1 1 2 3 89%	5015
September	1st to 10th	Night	1 1 1 1 2 2 3 89%	1930
	11th to 20th	Night	1 1 1 2 2 2 2 1 3 4 83%	1900
	21st to end	Night	1 1 1 2 1 1 2 1 2 4 85%	2132
October	1st to 10th	Night	1 1 1 1 2 2 2 2 4 5% 78%	3800
	11th to 20th	Night	1 2 1 2 2 3 3 2 3 4 77%	3777
	21st to end	Night	2 1 1 3 3 2 3 2 5% 7% 71%	4121
November	1st to 10th	Night	3 3 1 4 4 3 4 4 5% 9% 61%	2369
	11th to 20th	Night	4 4 1 5% 4 3 4 3 5% 8% 57%	2600
	21st to end	Night	5% 4 2 5% 4 4 4 5% 6% 7% 54%	2640
December	1st to 10th	Night	4 4 1 4 4 4 4 5% 5% 7% 57%	4560
	11th to 20th	Night	9% 8% 3 7% 5% 5% 5% 5% 7% 9% 37%	5176
	21st to end	Night	6% 7% 3 9% 8% 7% 7% 5% 7% 8% 33%	6952

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	28% 6% 12% 26% 18% 10%	3961
	11th to 20th	Day	29% 5% 7% 13% 18% 28%	4100
	21st to end	Day	36% 7% 6% 14% 17% 21%	4510
February	1st to 10th	Day	38% 4 7% 14% 22% 14%	1800
	11th to 20th	Day	36% 5% 8% 18% 22% 11%	1926
	21st to end	Day	40% 5% 8% 20% 19% 8%	1650
March	1st to 10th	Day	40% 10% 10% 13% 22% 6%	4815
	11th to 20th	Day	31% 8% 8% 27% 17% 9%	5000
	21st to end	Day	34% 10% 12% 26% 16% 2	5616
April	1st to 10th	Day	33% 4 11% 23% 21% 8%	2400
	11th to 20th	Day	24% 5% 13% 25% 24% 9%	2372
	21st to end	Day	20% 5% 11% 30% 28% 6%	2400
May	1st to 10th	Day	11% 2 6% 24% 40% 17%	5440
	11th to 20th	Day	7% 3 11% 26% 39% 14%	5831
	21st to end	Day	7% 3 6% 26% 40% 18%	6904
June	1st to 10th	Day	12% 4 9% 22% 35% 17%	2800
	11th to 20th	Day	6% 1 7% 17% 35% 34%	2800
	21st to end	Day	6% 1 2 10% 29% 51%	2800
July	1st to 10th	Day	2 1 2 3 17% 76%	6176
	11th to 20th	Day	4 1 2 4 17% 72%	5811
	21st to end	Day	3 2 2 12% 33% 48%	6217
August	1st to 10th	Day	7% 2 3 7% 26% 55%	4188
	11th to 20th	Day	3 1 2 6% 27% 61%	3880
	21st to end	Day	4 1 2 10% 34% 48%	4311
September	1st to 10th	Day	5% 1 3 7% 28% 55%	2317
	11th to 20th	Day	8% 3 5% 16% 38% 31%	2280
	21st to end	Day	9% 1 4 16% 41% 28%	2041
October	1st to 10th	Day	11% 3 5% 10% 25% 45%	3180
	11th to 20th	Day	11% 3 5% 19% 35% 27%	3161
	21st to end	Day	18% 3 5% 15% 36% 21%	3448
November	1st to 10th	Day	24% 5% 7% 19% 27% 18%	1929
	11th to 20th	Day	24% 3 11% 20% 26% 16%	1800
	21st to end	Day	30% 5% 7% 17% 26% 17%	1755
December	1st to 10th	Day	29% 4 6% 15% 29% 17%	2620
	11th to 20th	Day	41% 8% 9% 14% 16% 12%	3028
	21st to end	Day	42% 8% 11% 20% 13% 6%	4004
January	1st to 10th	Night	26% 5% 12% 25% 21% 10%	5982
	11th to 20th	Night	30% 4 7% 14% 16% 29%	5880
	21st to end	Night	34% 6% 7% 11% 17% 24%	6468
February	1st to 10th	Night	37% 4 9% 15% 20% 16%	2600
	11th to 20th	Night	31% 4 10% 19% 26% 10%	2371
	21st to end	Night	34% 4 9% 21% 22% 10%	1980
March	1st to 10th	Night	44% 9% 8% 12% 19% 8%	5074
	11th to 20th	Night	29% 8% 8% 23% 19% 11%	5000
	21st to end	Night	36% 8% 11% 21% 20% 5%	5358
April	1st to 10th	Night	30% 5% 11% 22% 22% 10%	2000
	11th to 20th	Night	25% 3 10% 25% 24% 13%	1979
	21st to end	Night	17% 5% 12% 26% 30% 11%	2000
May	1st to 10th	Night	9% 3 6% 20% 39% 23%	4557
	11th to 20th	Night	8% 2 8% 18% 37% 27%	4079
	21st to end	Night	5% 2 5% 19% 34% 35%	3972
June	1st to 10th	Night	10% 3 11% 16% 34% 26%	1600
	11th to 20th	Night	7% 2 6% 15% 28% 42%	1600
	21st to end	Night	3 1 2 8% 26% 60%	1600
July	1st to 10th	Night	1 1 3 12% 84%	3659
	11th to 20th	Night	3 1 1 6% 13% 76%	4046
	21st to end	Night	3 2 2 9% 24% 59%	4345
August	1st to 10th	Night	7% 1 2 5% 18% 68%	2947
	11th to 20th	Night	3 1 1 4 17% 75%	3256
	21st to end	Night	4 1 2 7% 25% 60%	3608
September	1st to 10th	Night	4 1 3 6% 20% 66%	1930
	11th to 20th	Night	8% 2 3 13% 30% 44%	1900
	21st to end	Night	8% 1 3 15% 37% 36%	2132
October	1st to 10th	Night	10% 3 4 10% 20% 53%	3800
	11th to 20th	Night	12% 3 4 15% 37% 29%	3777
	21st to end	Night	15% 3 4 13% 34% 30%	4121
November	1st to 10th	Night	20% 4 7% 19% 28% 22%	2369
	11th to 20th	Night	23% 4 11% 18% 26% 18%	2600
	21st to end	Night	29% 4 8% 15% 25% 19%	2640
December	1st to 10th	Night	26% 4 6% 14% 28% 23%	4560
	11th to 20th	Night	41% 8% 10% 12% 18% 11%	5176
	21st to end	Night	45% 9% 9% 18% 15% 5%	6952

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	23% 15% 25% 23% 12% 1	340
	11th to 20th	Day	11% 13% 28% 27% 18% 3	400
	21st to end	Day	13% 12% 29% 24% 18% 3	440
February	1st to 10th	Day	16% 20% 26% 17% 16% 5%	400
	11th to 20th	Day	18% 15% 29% 25% 12% 1	479
	21st to end	Day	14% 15% 31% 26% 13%	414
March	1st to 10th	Day	20% 17% 31% 25% 7% 1	500
	11th to 20th	Day	18% 16% 29% 22% 13% 2	500
	21st to end	Day	14% 17% 36% 23% 7% 1	550
April	1st to 10th	Day	13% 16% 31% 25% 14% 2	500
	11th to 20th	Day	11% 15% 32% 27% 12% 2	492
	21st to end	Day	8% 14% 32% 29% 12% 5%	500
May	1st to 10th	Day	8% 12% 34% 29% 13% 4	503
	11th to 20th	Day	8% 12% 30% 29% 16% 5%	528
	21st to end	Day	6% 10% 25% 34% 19% 5%	573
June	1st to 10th	Day	4 5% 29% 35% 19% 8%	530
	11th to 20th	Day	2 9% 28% 40% 15% 5%	530
	21st to end	Day	2 3 23% 41% 22% 9%	530
July	1st to 10th	Day	1 21% 46% 25% 7%	526
	11th to 20th	Day	1 1 10% 46% 30% 12%	524
	21st to end	Day	1 13% 47% 26% 13%	575
August	1st to 10th	Day	2 3 20% 46% 22% 8%	527
	11th to 20th	Day	1 20% 44% 25% 9%	517
	21st to end	Day	2 14% 45% 29% 10%	583
September	1st to 10th	Day	1 4 17% 44% 28% 6%	503
	11th to 20th	Day	2 6% 26% 37% 24% 5%	490
	21st to end	Day	3 6% 17% 40% 29% 6%	490
October	1st to 10th	Day	2 6% 25% 36% 26% 5%	490
	11th to 20th	Day	4 7% 21% 32% 28% 7%	490
	21st to end	Day	6% 7% 25% 32% 25% 6%	539
November	1st to 10th	Day	8% 11% 20% 29% 28% 5%	465
	11th to 20th	Day	7% 9% 23% 30% 23% 7%	300
	21st to end	Day	11% 9% 26% 31% 19% 4	277
December	1st to 10th	Day	11% 13% 26% 30% 18% 4	200
	11th to 20th	Day	12% 13% 25% 27% 17% 7%	200
	21st to end	Day	15% 21% 23% 25% 13% 4	220
January	1st to 10th	Night	13% 6% 20% 31% 22% 8%	800
	11th to 20th	Night	10% 7% 18% 32% 27% 7%	800
	21st to end	Night	13% 5% 18% 35% 24% 4	880
February	1st to 10th	Night	13% 8% 20% 32% 22% 5%	800
	11th to 20th	Night	12% 10% 19% 31% 22% 6%	784
	21st to end	Night	13% 7% 22% 37% 18% 3	660
March	1st to 10th	Night	15% 9% 21% 32% 19% 4	800
	11th to 20th	Night	11% 7% 21% 33% 21% 6%	800
	21st to end	Night	10% 8% 24% 33% 21% 3	880
April	1st to 10th	Night	9% 6% 26% 34% 23% 3	800
	11th to 20th	Night	7% 10% 23% 32% 22% 6%	791
	21st to end	Night	3 7% 29% 29% 24% 8%	800
May	1st to 10th	Night	2 6% 26% 34% 26% 7%	800
	11th to 20th	Night	3 4 19% 36% 30% 8%	596
	21st to end	Night	3 4 19% 34% 32% 8%	433
June	1st to 10th	Night	2 4 20% 35% 32% 7%	400
	11th to 20th	Night	1 3 19% 35% 37% 6%	400
	21st to end	Night	3 10% 35% 45% 9%	400
July	1st to 10th	Night	1 6% 32% 50% 11%	410
	11th to 20th	Night	1 2 22% 54% 21%	582
	21st to end	Night	2 22% 56% 20%	657
August	1st to 10th	Night	1 6% 33% 46% 14%	602
	11th to 20th	Night	1 1 10% 32% 43% 14%	792
	21st to end	Night	1 5% 25% 55% 14%	880
September	1st to 10th	Night	1 5% 32% 48% 14%	772
	11th to 20th	Night	1 2 13% 34% 41% 10%	760
	21st to end	Night	2 2 9% 29% 44% 14%	760
October	1st to 10th	Night	4 3 10% 36% 36% 10%	760
	11th to 20th	Night	4 4 12% 33% 33% 13%	760
	21st to end	Night	5% 2 13% 33% 32% 15%	836
November	1st to 10th	Night	7% 5% 14% 33% 31% 11%	780
	11th to 20th	Night	7% 5% 15% 34% 32% 7%	800
	21st to end	Night	8% 4 18% 38% 26% 7%	800
December	1st to 10th	Night	10% 6% 18% 35% 24% 7%	800
	11th to 20th	Night	16% 6% 20% 33% 22% 4	800
	21st to end	Night	13% 12% 22% 31% 18% 4	880

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