Saint Mary's College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Saint Mary's College Observatory clear sky chart from 2005-11-06 to 2024-01-15.

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	8% 5% 2 4 4 2 3 3 5% 7% 58%	74710
	Night	10% 6% 2 5% 5% 3 4 4 5% 7% 51%	75071
Transparency	Day	26% 6% 8% 18% 24% 18%	74429
	Night	31% 7% 10% 17% 20% 15%	74875
Seeing	Day	11% 10% 27% 32% 15% 4	14243
	Night	12% 5% 12% 36% 28% 6%	23678

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	19% 9% 3 6% 5% 3 5% 4 5% 7% 34%	1604
	11th to 20th	Day	14% 5% 3 5% 4 3 4 3 4 6% 47%	1657
	21st to end	Day	16% 6% 2 6% 4 3 5% 4 5% 6% 42%	1782
February	1st to 10th	Day	17% 7% 2 4 4 2 3 4 5% 6% 46%	1620
	11th to 20th	Day	14% 7% 3 6% 4 3 5% 3 5% 7% 44%	1656
	21st to end	Day	15% 6% 3 4 5% 3 4 2 6% 5% 48%	1480
March	1st to 10th	Day	15% 8% 3 7% 5% 3 4 4 6% 8% 36%	1934
	11th to 20th	Day	16% 7% 3 5% 5% 3 4 5% 6% 8% 38%	1980
	21st to end	Day	12% 6% 3 5% 5% 3 5% 4 6% 7% 44%	2212
April	1st to 10th	Day	15% 6% 3 5% 5% 3 3 4 5% 7% 44%	2160
	11th to 20th	Day	10% 5% 2 4 3 2 3 4 5% 6% 56%	2129
	21st to end	Day	6% 5% 1 3 3 2 3 3 5% 9% 58%	2160
May	1st to 10th	Day	5% 5% 2 3 3 2 3 4 4 7% 61%	2231
	11th to 20th	Day	6% 3 2 3 3 2 3 4 5% 8% 62%	2363
	21st to end	Day	5% 4 2 5% 4 2 3 3 5% 8% 59%	2723
June	1st to 10th	Day	5% 4 2 3 3 2 4 4 5% 8% 60%	2520
	11th to 20th	Day	1 2 1 2 2 1 3 3 5% 7% 74%	2520
	21st to end	Day	2 2 1 3 3 2 2 2 4 6% 73%	2520
July	1st to 10th	Day	1 1 1 3 3 1 3 2 4 8% 73%	2520
	11th to 20th	Day	1 2 1 2 3 2 3 3 5% 8% 72%	2373
	21st to end	Day	1 1 1 2 2 1 3 3 4 7% 74%	2546
August	1st to 10th	Day	2 3 2 4 3 2 3 3 4 7% 68%	2328
	11th to 20th	Day	2 1 2 3 2 3 3 4 7% 74%	2187
	21st to end	Day	1 2 1 3 3 1 2 2 3 8% 74%	2376
September	1st to 10th	Day	1 2 2 3 2 1 2 2 3 4 79%	2160
	11th to 20th	Day	3 4 2 4 3 2 3 3 5% 7% 65%	2160
	21st to end	Day	3 3 1 2 2 1 3 2 4 6% 74%	1910
October	1st to 10th	Day	4 3 2 3 2 1 3 3 4 6% 69%	1800
	11th to 20th	Day	6% 3 1 3 3 2 3 3 4 6% 65%	1800
	21st to end	Day	7% 4 1 3 4 2 3 3 4 7% 62%	1980
November	1st to 10th	Day	9% 4 2 4 4 2 3 4 5% 8% 55%	1813
	11th to 20th	Day	11% 5% 3 6% 4 3 4 4 4 7% 49%	1710
	21st to end	Day	12% 6% 2 3 4 2 3 4 5% 6% 53%	1554
December	1st to 10th	Day	14% 8% 3 5% 4 2 5% 4 5% 7% 45%	1520
	11th to 20th	Day	24% 11% 3 6% 5% 3 4 4 5% 5% 31%	1928
	21st to end	Day	18% 9% 3 6% 5% 3 4 5% 5% 7% 34%	2794
January	1st to 10th	Night	17% 8% 3 6% 4 2 4 5% 5% 8% 38%	2556
	11th to 20th	Night	13% 6% 2 5% 5% 3 4 4 6% 7% 44%	2397
	21st to end	Night	16% 7% 2 5% 5% 3 4 4 4 7% 45%	2574
February	1st to 10th	Night	16% 7% 2 4 4 2 3 3 4 4 50%	2340
	11th to 20th	Night	12% 7% 3 4 5% 3 4 4 4 7% 46%	2190
	21st to end	Night	13% 6% 2 4 4 3 3 3 5% 6% 51%	1776
March	1st to 10th	Night	17% 8% 3 5% 5% 2 5% 4 4 6% 40%	2021
	11th to 20th	Night	15% 8% 3 5% 5% 3 3 4 6% 6% 41%	1980
	21st to end	Night	15% 7% 3 4 4 2 4 3 3 6% 49%	2142
April	1st to 10th	Night	11% 7% 3 6% 4 2 3 4 5% 8% 46%	1800
	11th to 20th	Night	10% 6% 2 4 4 2 3 3 5% 6% 55%	1779
	21st to end	Night	5% 3 2 4 4 2 4 4 6% 8% 58%	1800
May	1st to 10th	Night	5% 6% 2 4 4 3 4 3 5% 7% 57%	1726
	11th to 20th	Night	5% 5% 3 6% 6% 3 4 3 6% 9% 51%	1527
	21st to end	Night	5% 5% 4 7% 6% 3 4 4 5% 7% 51%	1552
June	1st to 10th	Night	4 5% 3 4 5% 3 4 4 7% 7% 54%	1440
	11th to 20th	Night	1 2 1 4 5% 2 3 3 4 7% 68%	1440
	21st to end	Night	2 4 2 5% 5% 3 4 3 3 5% 63%	1440
July	1st to 10th	Night	1 2 2 6% 5% 3 4 3 5% 8% 63%	1440
	11th to 20th	Night	2 1 1 5% 5% 4 5% 3 6% 9% 59%	1582
	21st to end	Night	1 3 2 7% 6% 5% 5% 6% 7% 9% 48%	1773
August	1st to 10th	Night	2 6% 3 9% 8% 4 4 4 6% 9% 45%	1611
	11th to 20th	Night	2 2 5% 7% 4 5% 4 6% 10% 54%	1712
	21st to end	Night	3 2 6% 8% 3 5% 4 5% 8% 56%	1980
September	1st to 10th	Night	1 2 2 4 5% 2 3 3 4 8% 66%	1800
	11th to 20th	Night	3 5% 3 7% 6% 3 4 3 6% 6% 54%	1800
	21st to end	Night	2 3 1 3 4 2 3 2 4 6% 69%	2045
October	1st to 10th	Night	4 5% 1 3 3 2 4 3 4 6% 65%	2160
	11th to 20th	Night	6% 4 2 5% 4 3 3 3 5% 6% 59%	2160
	21st to end	Night	6% 5% 2 4 3 3 3 4 6% 6% 59%	2376
November	1st to 10th	Night	8% 5% 2 4 3 3 3 4 5% 8% 54%	2262
	11th to 20th	Night	10% 6% 2 4 4 3 3 4 5% 7% 51%	2470
	21st to end	Night	10% 4 1 3 4 3 3 4 5% 6% 57%	2622
December	1st to 10th	Night	14% 7% 3 5% 4 3 4 3 4 6% 47%	2660
	11th to 20th	Night	26% 11% 3 5% 4 2 3 4 5% 6% 32%	3276
	21st to end	Night	17% 9% 3 7% 6% 4 5% 4 4 7% 35%	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	47% 7% 13% 19% 10% 4	1604
	11th to 20th	Day	39% 5% 10% 17% 18% 11%	1657
	21st to end	Day	43% 5% 10% 14% 21% 7%	1782
February	1st to 10th	Day	40% 4 8% 21% 20% 8%	1620
	11th to 20th	Day	39% 5% 8% 19% 22% 6%	1665
	21st to end	Day	37% 5% 7% 23% 20% 7%	1480
March	1st to 10th	Day	45% 5% 10% 20% 16% 4	1934
	11th to 20th	Day	44% 6% 8% 21% 16% 5%	1980
	21st to end	Day	38% 7% 14% 20% 18% 4	2212
April	1st to 10th	Day	39% 6% 12% 19% 19% 5%	2160
	11th to 20th	Day	29% 6% 13% 25% 22% 6%	2132
	21st to end	Day	23% 5% 12% 29% 26% 5%	2160
May	1st to 10th	Day	22% 6% 8% 26% 30% 8%	2231
	11th to 20th	Day	20% 7% 7% 29% 27% 9%	2366
	21st to end	Day	23% 8% 11% 29% 24% 5%	2724
June	1st to 10th	Day	22% 8% 9% 25% 26% 10%	2520
	11th to 20th	Day	10% 6% 8% 20% 35% 19%	2520
	21st to end	Day	13% 5% 5% 12% 31% 34%	2520
July	1st to 10th	Day	10% 5% 6% 7% 23% 48%	2451
	11th to 20th	Day	12% 5% 9% 9% 21% 45%	2302
	21st to end	Day	9% 4 6% 7% 26% 48%	2405
August	1st to 10th	Day	15% 5% 7% 11% 27% 34%	2312
	11th to 20th	Day	10% 5% 5% 9% 34% 38%	2187
	21st to end	Day	11% 5% 6% 11% 33% 34%	2376
September	1st to 10th	Day	10% 3 4 7% 31% 44%	2160
	11th to 20th	Day	18% 7% 5% 20% 31% 18%	2160
	21st to end	Day	12% 5% 4 21% 34% 24%	1910
October	1st to 10th	Day	17% 5% 6% 17% 30% 25%	1800
	11th to 20th	Day	20% 7% 7% 19% 24% 24%	1800
	21st to end	Day	23% 5% 8% 20% 25% 19%	1980
November	1st to 10th	Day	27% 5% 9% 25% 20% 12%	1813
	11th to 20th	Day	36% 5% 8% 23% 23% 6%	1710
	21st to end	Day	31% 4 10% 18% 26% 11%	1554
December	1st to 10th	Day	37% 7% 9% 21% 17% 8%	1520
	11th to 20th	Day	55% 4 9% 13% 15% 4	1928
	21st to end	Day	47% 9% 12% 19% 11% 3	2794
January	1st to 10th	Night	43% 7% 11% 21% 12% 5%	2556
	11th to 20th	Night	39% 6% 11% 17% 15% 13%	2397
	21st to end	Night	41% 5% 10% 14% 20% 10%	2574
February	1st to 10th	Night	38% 3 8% 19% 21% 11%	2340
	11th to 20th	Night	37% 5% 8% 19% 22% 9%	2192
	21st to end	Night	34% 5% 10% 27% 19% 5%	1776
March	1st to 10th	Night	44% 7% 11% 17% 16% 5%	2021
	11th to 20th	Night	42% 5% 7% 22% 19% 6%	1980
	21st to end	Night	37% 7% 12% 20% 18% 5%	2142
April	1st to 10th	Night	34% 8% 13% 20% 19% 6%	1800
	11th to 20th	Night	29% 7% 11% 24% 20% 9%	1779
	21st to end	Night	22% 7% 11% 29% 24% 6%	1800
May	1st to 10th	Night	25% 9% 7% 21% 27% 11%	1726
	11th to 20th	Night	27% 9% 9% 22% 24% 8%	1528
	21st to end	Night	27% 13% 10% 21% 23% 5%	1552
June	1st to 10th	Night	24% 10% 10% 20% 25% 11%	1440
	11th to 20th	Night	15% 7% 9% 16% 31% 22%	1440
	21st to end	Night	15% 8% 11% 10% 25% 31%	1440
July	1st to 10th	Night	12% 6% 13% 5% 21% 44%	1400
	11th to 20th	Night	16% 5% 16% 7% 15% 42%	1531
	21st to end	Night	21% 5% 16% 5% 16% 35%	1674
August	1st to 10th	Night	27% 6% 15% 7% 17% 27%	1602
	11th to 20th	Night	18% 7% 12% 6% 22% 35%	1712
	21st to end	Night	19% 5% 15% 7% 23% 30%	1980
September	1st to 10th	Night	14% 5% 9% 5% 23% 43%	1800
	11th to 20th	Night	24% 8% 10% 16% 23% 19%	1800
	21st to end	Night	14% 6% 5% 19% 29% 27%	2045
October	1st to 10th	Night	18% 7% 9% 16% 24% 26%	2160
	11th to 20th	Night	23% 7% 8% 17% 21% 25%	2160
	21st to end	Night	23% 7% 6% 19% 23% 23%	2376
November	1st to 10th	Night	27% 8% 10% 24% 17% 15%	2262
	11th to 20th	Night	32% 5% 9% 25% 21% 8%	2470
	21st to end	Night	28% 5% 9% 19% 26% 13%	2622
December	1st to 10th	Night	38% 6% 8% 19% 17% 12%	2660
	11th to 20th	Night	53% 5% 9% 14% 16% 4	3276
	21st to end	Night	47% 10% 12% 16% 12% 4	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	27% 9% 28% 21% 12% 2	265
	11th to 20th	Day	19% 10% 24% 30% 15% 2	353
	21st to end	Day	18% 8% 26% 28% 17% 3	374
February	1st to 10th	Day	22% 9% 23% 26% 17% 3	340
	11th to 20th	Day	21% 16% 25% 25% 12% 1	371
	21st to end	Day	17% 13% 29% 28% 12% 1	346
March	1st to 10th	Day	26% 13% 30% 20% 9% 2	420
	11th to 20th	Day	23% 14% 26% 21% 12% 4	420
	21st to end	Day	17% 18% 31% 24% 8% 1	462
April	1st to 10th	Day	20% 16% 31% 21% 10% 2	420
	11th to 20th	Day	17% 18% 32% 23% 7% 3	412
	21st to end	Day	8% 21% 29% 27% 10% 5%	420
May	1st to 10th	Day	13% 14% 34% 28% 10% 2	423
	11th to 20th	Day	10% 15% 34% 29% 11% 1	448
	21st to end	Day	11% 14% 31% 30% 12% 2	485
June	1st to 10th	Day	9% 10% 32% 34% 12% 4	450
	11th to 20th	Day	4 11% 35% 37% 10% 2	450
	21st to end	Day	2 10% 31% 40% 14% 3	450
July	1st to 10th	Day	4 5% 30% 44% 15% 2	446
	11th to 20th	Day	2 26% 50% 18% 3	444
	21st to end	Day	2 1 26% 47% 17% 6%	487
August	1st to 10th	Day	3 7% 28% 42% 17% 4	447
	11th to 20th	Day	1 4 27% 42% 19% 8%	437
	21st to end	Day	1 4 24% 46% 21% 5%	495
September	1st to 10th	Day	4 4 20% 44% 21% 8%	450
	11th to 20th	Day	5% 10% 25% 38% 16% 6%	450
	21st to end	Day	4 8% 24% 34% 25% 6%	450
October	1st to 10th	Day	5% 8% 29% 32% 17% 8%	450
	11th to 20th	Day	9% 8% 22% 33% 21% 7%	450
	21st to end	Day	11% 7% 25% 29% 22% 6%	495
November	1st to 10th	Day	12% 9% 19% 27% 25% 7%	409
	11th to 20th	Day	9% 9% 16% 37% 23% 7%	270
	21st to end	Day	12% 7% 28% 30% 21% 2	196
December	1st to 10th	Day	21% 12% 20% 26% 17% 4	180
	11th to 20th	Day	18% 12% 25% 23% 21% 2	180
	21st to end	Day	24% 10% 28% 23% 15% 1	198
January	1st to 10th	Night	21% 4 13% 33% 23% 7%	760
	11th to 20th	Night	17% 5% 12% 38% 24% 5%	738
	21st to end	Night	20% 5% 11% 36% 25% 4	792
February	1st to 10th	Night	20% 6% 11% 34% 25% 4	720
	11th to 20th	Night	14% 9% 15% 35% 21% 6%	704
	21st to end	Night	16% 7% 18% 35% 21% 2	592
March	1st to 10th	Night	23% 6% 14% 31% 23% 3	720
	11th to 20th	Night	20% 6% 13% 33% 25% 3	720
	21st to end	Night	17% 8% 18% 36% 19% 2	792
April	1st to 10th	Night	14% 9% 16% 38% 20% 2	720
	11th to 20th	Night	14% 11% 19% 32% 22% 3	711
	21st to end	Night	8% 8% 21% 35% 23% 6%	720
May	1st to 10th	Night	10% 4 18% 40% 24% 4	646
	11th to 20th	Night	8% 6% 11% 41% 30% 5%	465
	21st to end	Night	10% 12% 14% 38% 22% 4	389
June	1st to 10th	Night	7% 7% 15% 42% 26% 2	360
	11th to 20th	Night	2 4 13% 56% 24% 2	360
	21st to end	Night	5% 4 12% 43% 34% 3	360
July	1st to 10th	Night	5% 2 9% 41% 40% 3	350
	11th to 20th	Night	3 2 7% 43% 37% 9%	487
	21st to end	Night	4 4 6% 34% 44% 9%	591
August	1st to 10th	Night	9% 6% 9% 34% 35% 8%	537
	11th to 20th	Night	2 5% 9% 37% 39% 8%	644
	21st to end	Night	3 3 8% 39% 39% 8%	792
September	1st to 10th	Night	3 2 7% 37% 41% 10%	720
	11th to 20th	Night	9% 3 9% 36% 37% 6%	720
	21st to end	Night	5% 3 9% 34% 39% 11%	720
October	1st to 10th	Night	8% 3 8% 38% 34% 8%	720
	11th to 20th	Night	9% 6% 8% 32% 36% 8%	720
	21st to end	Night	10% 3 9% 32% 33% 13%	792
November	1st to 10th	Night	10% 4 10% 34% 32% 9%	740
	11th to 20th	Night	13% 3 11% 36% 29% 8%	760
	21st to end	Night	13% 4 12% 38% 27% 6%	760
December	1st to 10th	Night	19% 5% 11% 37% 23% 6%	760
	11th to 20th	Night	25% 5% 10% 35% 21% 4	760
	21st to end	Night	22% 7% 18% 32% 20% 2	836

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