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 2024-12-02.

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	7% 5% 2 4 3 3 3 3 5% 6% 59%	105256
	Night	9% 6% 2 7% 5% 4 4 3 5% 6% 50%	103104
Transparency	Day	24% 6% 8% 18% 23% 21%	105014
	Night	30% 7% 11% 16% 19% 17%	102937
Seeing	Day	11% 10% 28% 32% 15% 4	15463
	Night	12% 5% 12% 36% 28% 6%	24811

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	17% 11% 3 6% 4 4 4 4 5% 6% 37%	2676
	11th to 20th	Day	17% 10% 5% 10% 5% 5% 3 3 4 5% 33%	2860
	21st to end	Day	22% 10% 3 7% 4 4 5% 3 5% 6% 30%	3146
February	1st to 10th	Day	17% 8% 2 5% 3 3 4 3 6% 5% 45%	1710
	11th to 20th	Day	14% 8% 3 7% 4 3 5% 3 5% 7% 42%	1750
	21st to end	Day	14% 7% 2 6% 4 3 4 2 6% 5% 48%	1570
March	1st to 10th	Day	14% 13% 3 10% 6% 5% 4 3 6% 6% 29%	3374
	11th to 20th	Day	10% 6% 2 4 3 3 3 3 5% 7% 53%	3490
	21st to end	Day	12% 9% 3 7% 5% 5% 6% 4 7% 7% 35%	3884
April	1st to 10th	Day	14% 7% 2 5% 5% 4 3 3 6% 7% 45%	2280
	11th to 20th	Day	10% 5% 2 4 3 3 3 4 5% 6% 55%	2249
	21st to end	Day	5% 5% 1 3 3 3 3 3 6% 9% 59%	2280
May	1st to 10th	Day	6% 3 1 2 1 2 2 2 3 5% 72%	3923
	11th to 20th	Day	3 2 1 2 2 2 2 3 4 6% 73%	4185
	21st to end	Day	3 3 1 5% 3 2 2 2 4 6% 68%	4813
June	1st to 10th	Day	5% 5% 1 3 3 4 4 3 5% 7% 61%	2660
	11th to 20th	Day	1 2 1 2 2 2 2 3 5% 7% 74%	2660
	21st to end	Day	2 2 1 3 3 2 2 2 4 5% 74%	2660
July	1st to 10th	Day	1 1 1 2 2 1 1 1 3 5% 82%	4420
	11th to 20th	Day	1 1 1 3 2 2 3 2 4 6% 75%	4156
	21st to end	Day	1 1 4 2 2 2 2 4 7% 74%	4455
August	1st to 10th	Day	1 2 1 4 3 3 4 3 6% 9% 64%	4091
	11th to 20th	Day	1 1 3 3 2 3 2 4 6% 75%	3825
	21st to end	Day	1 2 1 3 2 3 2 2 4 6% 73%	4180
September	1st to 10th	Day	1 2 2 3 1 2 2 2 4 5% 78%	2280
	11th to 20th	Day	3 4 2 5% 3 3 3 2 5% 7% 64%	2280
	21st to end	Day	3 3 1 2 2 2 3 1 4 6% 75%	2015
October	1st to 10th	Day	2 3 1 3 2 2 2 2 4 7% 72%	3180
	11th to 20th	Day	4 4 2 5% 3 3 3 2 4 6% 66%	3161
	21st to end	Day	5% 4 2 4 3 3 3 3 5% 7% 59%	3448
November	1st to 10th	Day	8% 5% 2 4 4 2 4 3 5% 7% 56%	1910
	11th to 20th	Day	11% 6% 2 7% 4 4 4 3 4 7% 48%	1800
	21st to end	Day	12% 7% 2 4 3 3 3 4 6% 6% 52%	1635
December	1st to 10th	Day	14% 8% 2 5% 3 3 4 4 5% 7% 45%	1528
	11th to 20th	Day	24% 11% 2 7% 4 3 4 3 5% 5% 31%	1928
	21st to end	Day	18% 10% 3 7% 4 4 5% 3 5% 7% 34%	2794
January	1st to 10th	Night	14% 8% 2 6% 4 3 4 3 5% 6% 44%	4268
	11th to 20th	Night	18% 10% 3 8% 5% 5% 5% 3 5% 5% 32%	4110
	21st to end	Night	22% 14% 3 8% 5% 4 3 2 4 5% 31%	4521
February	1st to 10th	Night	16% 8% 2 4 3 3 3 3 4 4 49%	2470
	11th to 20th	Night	12% 8% 3 5% 5% 4 4 3 4 8% 45%	2316
	21st to end	Night	12% 7% 2 4 4 4 4 2 4 7% 51%	1884
March	1st to 10th	Night	16% 12% 4 8% 4 3 5% 3 4 5% 34%	3495
	11th to 20th	Night	12% 6% 2 7% 3 3 3 2 4 4 52%	3490
	21st to end	Night	14% 14% 4 7% 4 4 4 2 4 6% 37%	3767
April	1st to 10th	Night	10% 8% 3 6% 4 3 3 4 5% 7% 47%	1900
	11th to 20th	Night	9% 7% 2 5% 3 3 3 2 5% 5% 54%	1879
	21st to end	Night	5% 4 2 4 3 3 3 3 6% 8% 58%	1900
May	1st to 10th	Night	4 5% 1 4 3 3 2 1 3 4 69%	3036
	11th to 20th	Night	3 3 2 6% 7% 7% 4 3 5% 7% 54%	2711
	21st to end	Night	3 3 2 10% 4 3 2 2 4 5% 60%	2762
June	1st to 10th	Night	4 6% 2 5% 5% 3 4 3 7% 7% 54%	1520
	11th to 20th	Night	1 2 1 5% 4 2 3 3 4 7% 68%	1520
	21st to end	Night	2 4 2 6% 4 4 3 3 4 5% 64%	1520
July	1st to 10th	Night	1 1 5% 5% 3 3 1 3 5% 72%	2540
	11th to 20th	Night	1 2 1 8% 7% 5% 4 2 5% 7% 58%	2794
	21st to end	Night	1 5% 3 15% 7% 6% 5% 4 6% 7% 43%	3109
August	1st to 10th	Night	3 4 2 9% 13% 14% 7% 4 5% 7% 33%	2848
	11th to 20th	Night	2 1 10% 7% 6% 3 3 4 7% 57%	3019
	21st to end	Night	2 3 2 8% 6% 7% 4 2 3 5% 57%	3498
September	1st to 10th	Night	1 2 1 6% 4 3 3 2 4 8% 66%	1900
	11th to 20th	Night	3 5% 3 8% 5% 4 4 3 5% 6% 53%	1900
	21st to end	Night	2 3 1 5% 4 3 2 2 4 6% 68%	2160
October	1st to 10th	Night	3 3 1 3 3 3 3 2 4 6% 70%	3800
	11th to 20th	Night	4 7% 2 12% 5% 4 3 2 4 4 53%	3777
	21st to end	Night	5% 5% 2 5% 3 4 4 3 5% 5% 59%	4121
November	1st to 10th	Night	8% 5% 2 4 3 3 3 3 5% 8% 55%	2385
	11th to 20th	Night	10% 7% 2 5% 3 4 3 3 6% 7% 51%	2600
	21st to end	Night	11% 5% 1 4 3 3 3 3 5% 7% 55%	2760
December	1st to 10th	Night	14% 7% 3 5% 4 4 4 2 4 6% 47%	2686
	11th to 20th	Night	26% 11% 3 5% 4 3 3 3 5% 6% 32%	3276
	21st to end	Night	17% 9% 3 8% 5% 5% 5% 3 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	44% 7% 14% 26% 7% 2	2676
	11th to 20th	Day	53% 6% 10% 14% 11% 6%	2860
	21st to end	Day	53% 8% 8% 15% 12% 4	3146
February	1st to 10th	Day	40% 4 8% 20% 19% 8%	1710
	11th to 20th	Day	40% 6% 8% 18% 21% 6%	1759
	21st to end	Day	37% 6% 7% 24% 20% 6%	1570
March	1st to 10th	Day	51% 10% 9% 15% 14% 3	3374
	11th to 20th	Day	29% 7% 10% 29% 13% 11%	3490
	21st to end	Day	40% 11% 14% 21% 11% 2	3884
April	1st to 10th	Day	38% 6% 12% 20% 19% 5%	2280
	11th to 20th	Day	29% 6% 13% 25% 22% 6%	2252
	21st to end	Day	22% 5% 12% 29% 26% 5%	2280
May	1st to 10th	Day	16% 5% 9% 30% 25% 16%	3923
	11th to 20th	Day	12% 7% 5% 23% 40% 12%	4188
	21st to end	Day	14% 10% 7% 31% 33% 4	4814
June	1st to 10th	Day	21% 8% 9% 25% 27% 10%	2660
	11th to 20th	Day	10% 6% 8% 20% 37% 19%	2660
	21st to end	Day	12% 5% 5% 11% 31% 36%	2660
July	1st to 10th	Day	6% 3 4 4 13% 70%	4351
	11th to 20th	Day	10% 3 7% 6% 13% 62%	4085
	21st to end	Day	7% 3 8% 4 25% 53%	4315
August	1st to 10th	Day	13% 4 8% 7% 23% 45%	4076
	11th to 20th	Day	7% 5% 4 7% 39% 37%	3863
	21st to end	Day	11% 5% 5% 10% 34% 36%	4179
September	1st to 10th	Day	10% 3 4 7% 31% 45%	2280
	11th to 20th	Day	19% 7% 5% 20% 32% 18%	2280
	21st to end	Day	12% 5% 4 20% 34% 26%	2015
October	1st to 10th	Day	14% 3 4 12% 27% 41%	3180
	11th to 20th	Day	18% 7% 4 21% 30% 19%	3161
	21st to end	Day	23% 5% 6% 21% 31% 15%	3448
November	1st to 10th	Day	26% 5% 9% 26% 21% 12%	1910
	11th to 20th	Day	36% 5% 8% 23% 22% 6%	1800
	21st to end	Day	32% 4 9% 18% 25% 11%	1635
December	1st to 10th	Day	37% 7% 9% 21% 17% 8%	1528
	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	38% 8% 17% 24% 10% 3	4268
	11th to 20th	Night	52% 7% 12% 12% 10% 7%	4110
	21st to end	Night	54% 8% 8% 13% 12% 6%	4521
February	1st to 10th	Night	38% 4 9% 19% 20% 11%	2470
	11th to 20th	Night	38% 6% 9% 18% 21% 8%	2318
	21st to end	Night	33% 6% 10% 28% 19% 4	1884
March	1st to 10th	Night	46% 13% 11% 13% 14% 3	3495
	11th to 20th	Night	31% 9% 9% 22% 18% 11%	3490
	21st to end	Night	41% 13% 13% 20% 11% 3	3767
April	1st to 10th	Night	33% 8% 13% 21% 19% 5%	1900
	11th to 20th	Night	29% 8% 11% 25% 19% 8%	1879
	21st to end	Night	22% 8% 11% 30% 24% 6%	1900
May	1st to 10th	Night	16% 8% 9% 25% 28% 14%	3036
	11th to 20th	Night	15% 13% 16% 15% 27% 14%	2712
	21st to end	Night	16% 15% 9% 27% 29% 3	2762
June	1st to 10th	Night	23% 10% 10% 21% 25% 11%	1520
	11th to 20th	Night	14% 7% 9% 16% 32% 22%	1520
	21st to end	Night	15% 7% 11% 9% 25% 33%	1520
July	1st to 10th	Night	8% 3 12% 3 12% 63%	2500
	11th to 20th	Night	17% 3 17% 4 9% 50%	2743
	21st to end	Night	23% 4 23% 3 15% 32%	3011
August	1st to 10th	Night	21% 7% 32% 4 14% 22%	2839
	11th to 20th	Night	16% 7% 14% 4 30% 30%	3047
	21st to end	Night	19% 6% 16% 6% 24% 29%	3498
September	1st to 10th	Night	14% 5% 9% 5% 23% 45%	1900
	11th to 20th	Night	25% 8% 9% 16% 24% 18%	1900
	21st to end	Night	14% 6% 6% 18% 29% 27%	2160
October	1st to 10th	Night	15% 4 6% 10% 18% 47%	3800
	11th to 20th	Night	29% 8% 6% 16% 19% 22%	3777
	21st to end	Night	23% 7% 4 19% 24% 22%	4121
November	1st to 10th	Night	27% 7% 10% 24% 18% 15%	2385
	11th to 20th	Night	32% 5% 9% 25% 21% 8%	2600
	21st to end	Night	30% 5% 9% 19% 25% 13%	2760
December	1st to 10th	Night	38% 6% 8% 19% 17% 12%	2686
	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	20% 10% 23% 31% 15% 2	370
	21st to end	Day	19% 8% 28% 27% 16% 3	407
February	1st to 10th	Day	22% 10% 23% 26% 16% 3	370
	11th to 20th	Day	22% 16% 26% 24% 11% 1	405
	21st to end	Day	17% 13% 29% 28% 11% 2	382
March	1st to 10th	Day	27% 13% 29% 20% 9% 2	460
	11th to 20th	Day	23% 14% 25% 21% 13% 4	460
	21st to end	Day	19% 18% 31% 23% 8% 1	506
April	1st to 10th	Day	19% 16% 31% 22% 10% 3	460
	11th to 20th	Day	17% 17% 31% 23% 8% 4	452
	21st to end	Day	8% 20% 30% 27% 10% 5%	460
May	1st to 10th	Day	12% 14% 34% 27% 10% 2	463
	11th to 20th	Day	10% 14% 33% 31% 11% 1	488
	21st to end	Day	11% 13% 31% 30% 13% 2	529
June	1st to 10th	Day	8% 10% 33% 34% 12% 3	490
	11th to 20th	Day	4 11% 36% 37% 10% 2	490
	21st to end	Day	2 9% 33% 40% 13% 3	490
July	1st to 10th	Day	3 5% 29% 44% 16% 2	486
	11th to 20th	Day	3 27% 50% 17% 3	484
	21st to end	Day	2 2 28% 46% 17% 6%	531
August	1st to 10th	Day	2 7% 28% 42% 17% 4	487
	11th to 20th	Day	2 5% 30% 40% 17% 7%	477
	21st to end	Day	1 4 25% 45% 20% 5%	539
September	1st to 10th	Day	3 5% 21% 43% 20% 7%	490
	11th to 20th	Day	6% 12% 26% 35% 15% 6%	490
	21st to end	Day	3 8% 24% 34% 25% 5%	490
October	1st to 10th	Day	5% 8% 29% 33% 18% 9%	490
	11th to 20th	Day	8% 9% 23% 32% 21% 7%	490
	21st to end	Day	10% 8% 25% 29% 21% 6%	539
November	1st to 10th	Day	12% 11% 20% 27% 24% 7%	446
	11th to 20th	Day	10% 9% 19% 35% 21% 7%	300
	21st to end	Day	15% 8% 28% 29% 20% 1	217
December	1st to 10th	Day	20% 12% 20% 27% 17% 4	182
	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% 11% 38% 24% 5%	760
	21st to end	Night	21% 4 11% 36% 24% 4	836
February	1st to 10th	Night	21% 6% 12% 34% 23% 4	760
	11th to 20th	Night	15% 9% 15% 35% 21% 5%	744
	21st to end	Night	16% 7% 17% 36% 21% 3	628
March	1st to 10th	Night	23% 6% 14% 31% 22% 3	760
	11th to 20th	Night	21% 6% 13% 33% 24% 3	760
	21st to end	Night	17% 8% 19% 36% 19% 2	836
April	1st to 10th	Night	14% 9% 17% 38% 20% 2	760
	11th to 20th	Night	14% 10% 18% 33% 22% 3	751
	21st to end	Night	8% 7% 21% 35% 23% 5%	760
May	1st to 10th	Night	10% 5% 19% 40% 24% 4	681
	11th to 20th	Night	8% 6% 11% 42% 29% 5%	491
	21st to end	Night	10% 11% 14% 39% 22% 4	411
June	1st to 10th	Night	8% 6% 17% 42% 26% 2	380
	11th to 20th	Night	2 4 14% 56% 22% 2	380
	21st to end	Night	5% 4 12% 44% 33% 2	380
July	1st to 10th	Night	5% 2 9% 42% 39% 3	370
	11th to 20th	Night	3 2 7% 43% 36% 8%	515
	21st to end	Night	4 4 7% 34% 43% 9%	624
August	1st to 10th	Night	9% 6% 9% 35% 34% 7%	567
	11th to 20th	Night	2 6% 10% 37% 37% 7%	681
	21st to end	Night	3 3 9% 39% 39% 8%	836
September	1st to 10th	Night	3 2 8% 37% 41% 10%	760
	11th to 20th	Night	9% 3 10% 36% 36% 6%	760
	21st to end	Night	4 3 9% 34% 39% 10%	760
October	1st to 10th	Night	8% 3 8% 39% 34% 8%	760
	11th to 20th	Night	9% 6% 8% 32% 36% 8%	760
	21st to end	Night	10% 3 10% 33% 32% 12%	836
November	1st to 10th	Night	10% 4 11% 34% 32% 8%	780
	11th to 20th	Night	14% 3 12% 36% 28% 7%	800
	21st to end	Night	14% 4 12% 37% 26% 6%	800
December	1st to 10th	Night	18% 5% 11% 37% 23% 6%	768
	11th to 20th	Night	25% 5% 10% 35% 21% 4	760
	21st to end	Night	22% 7% 18% 32% 20% 2	836

Page generated at 2024-11-29 07:43:11. Version ac07

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