Westminster College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Westminster College Observatory clear sky chart from 2005-11-06 to 2024-03-23.

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	18% 11% 4 7% 6% 3 4 4 5% 6% 29%	78777
	Night	18% 12% 5% 7% 6% 3 4 4 4 6% 30%	81111
Transparency	Day	55% 6% 11% 18% 9% 1	78495
	Night	55% 5% 11% 17% 10% 1	80912
Seeing	Day	23% 15% 27% 20% 11% 4	13738
	Night	24% 11% 27% 26% 11% 1	23570

By Month

Percentage Cloud Cover SE (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	21% 22% 9% 11% 6% 3 3 3 3 4 16%	2804
	11th to 20th	Day	13% 25% 9% 13% 8% 4 3 3 3 4 16%	2840
	21st to end	Day	27% 19% 10% 11% 8% 3 3 3 3 3 10%	3124
February	1st to 10th	Day	20% 17% 7% 11% 8% 4 3 3 3 4 20%	1710
	11th to 20th	Day	17% 14% 8% 13% 9% 4 4 4 3 5% 20%	1663
	21st to end	Day	21% 15% 4 11% 6% 3 4 4 4 5% 23%	1560
March	1st to 10th	Day	18% 13% 6% 8% 6% 3 3 4 5% 6% 28%	2090
	11th to 20th	Day	18% 13% 5% 8% 5% 3 4 4 4 6% 30%	2090
	21st to end	Day	23% 10% 5% 8% 7% 3 5% 3 4 5% 28%	2200
April	1st to 10th	Day	17% 13% 5% 6% 6% 3 4 5% 5% 6% 31%	2067
	11th to 20th	Day	20% 11% 3 6% 4 3 4 4 5% 6% 32%	2196
	21st to end	Day	23% 9% 3 7% 5% 3 3 3 4 6% 34%	2340
May	1st to 10th	Day	21% 9% 4 7% 6% 3 5% 6% 6% 6% 29%	2340
	11th to 20th	Day	22% 7% 3 6% 6% 4 5% 5% 6% 6% 30%	2301
	21st to end	Day	13% 6% 3 5% 6% 4 6% 6% 6% 8% 38%	2686
June	1st to 10th	Day	17% 6% 3 6% 6% 4 5% 5% 7% 9% 33%	2520
	11th to 20th	Day	16% 6% 3 6% 7% 4 5% 5% 4 9% 35%	2520
	21st to end	Day	15% 4 2 5% 6% 4 5% 6% 6% 9% 38%	2520
July	1st to 10th	Day	12% 6% 2 5% 5% 4 5% 5% 6% 9% 41%	2467
	11th to 20th	Day	13% 6% 3 5% 5% 4 6% 5% 5% 8% 40%	2340
	21st to end	Day	13% 6% 2 7% 5% 4 5% 7% 7% 9% 35%	2547
August	1st to 10th	Day	11% 6% 2 6% 5% 4 6% 6% 8% 9% 35%	2327
	11th to 20th	Day	15% 6% 3 6% 7% 3 6% 6% 7% 8% 33%	2303
	21st to end	Day	13% 5% 2 6% 6% 4 6% 7% 7% 9% 36%	2477
September	1st to 10th	Day	18% 6% 3 4 4 4 4 4 5% 8% 41%	2152
	11th to 20th	Day	13% 6% 3 5% 5% 3 5% 4 6% 8% 42%	1980
	21st to end	Day	19% 8% 3 6% 5% 3 4 4 6% 7% 36%	1980
October	1st to 10th	Day	17% 8% 3 5% 5% 3 4 5% 6% 7% 37%	1810
	11th to 20th	Day	23% 8% 3 7% 6% 3 4 5% 5% 5% 31%	1800
	21st to end	Day	27% 10% 3 6% 5% 3 4 4 4 6% 27%	1980
November	1st to 10th	Day	12% 10% 5% 5% 6% 3 4 5% 5% 6% 40%	1744
	11th to 20th	Day	19% 16% 4 7% 5% 3 4 3 4 5% 29%	1577
	21st to end	Day	20% 17% 6% 7% 5% 4 4 4 5% 5% 22%	1520
December	1st to 10th	Day	23% 20% 8% 9% 6% 3 4 3 4 4 16%	1520
	11th to 20th	Day	20% 18% 9% 12% 5% 2 4 4 5% 5% 16%	1910
	21st to end	Day	28% 25% 7% 8% 6% 2 3 3 3 4 11%	2772
January	1st to 10th	Night	18% 24% 10% 12% 6% 3 4 3 3 4 13%	4137
	11th to 20th	Night	17% 19% 10% 15% 7% 3 5% 4 3 4 13%	4120
	21st to end	Night	29% 17% 10% 9% 8% 3 3 3 2 3 14%	4532
February	1st to 10th	Night	21% 14% 6% 9% 8% 3 4 3 4 5% 22%	2470
	11th to 20th	Night	18% 14% 6% 11% 6% 3 5% 4 5% 4 23%	2403
	21st to end	Night	21% 13% 4 11% 6% 3 4 4 4 5% 24%	1893
March	1st to 10th	Night	18% 12% 5% 7% 7% 4 5% 4 4 5% 29%	2090
	11th to 20th	Night	21% 11% 4 6% 5% 3 4 3 3 7% 33%	2090
	21st to end	Night	19% 12% 6% 7% 6% 2 3 4 5% 4 32%	2201
April	1st to 10th	Night	18% 12% 4 6% 6% 3 4 5% 4 6% 32%	1891
	11th to 20th	Night	17% 10% 4 5% 5% 3 5% 4 4 7% 35%	1728
	21st to end	Night	17% 10% 2 6% 6% 4 4 4 5% 7% 35%	1620
May	1st to 10th	Night	18% 9% 3 7% 5% 3 5% 5% 5% 7% 32%	1620
	11th to 20th	Night	16% 9% 2 6% 5% 3 5% 4 6% 7% 37%	1592
	21st to end	Night	11% 5% 2 5% 5% 3 5% 5% 6% 9% 44%	1588
June	1st to 10th	Night	10% 5% 2 5% 6% 3 6% 5% 6% 7% 44%	1440
	11th to 20th	Night	11% 6% 3 6% 5% 3 4 4 6% 8% 44%	1440
	21st to end	Night	11% 5% 2 6% 5% 2 5% 6% 5% 9% 42%	1440
July	1st to 10th	Night	7% 6% 3 5% 6% 4 4 5% 6% 8% 47%	1488
	11th to 20th	Night	9% 5% 1 4 6% 3 5% 4 6% 9% 47%	1620
	21st to end	Night	10% 5% 3 6% 6% 4 5% 6% 7% 8% 42%	1771
August	1st to 10th	Night	9% 6% 3 5% 5% 2 4 4 6% 8% 48%	1611
	11th to 20th	Night	12% 6% 3 4 4 4 5% 4 5% 8% 44%	1598
	21st to end	Night	11% 4 2 4 5% 3 4 6% 5% 9% 46%	1859
September	1st to 10th	Night	13% 5% 2 4 5% 3 4 5% 6% 7% 47%	1804
	11th to 20th	Night	12% 5% 1 4 4 2 3 4 5% 8% 52%	1980
	21st to end	Night	17% 6% 3 6% 5% 3 4 4 5% 6% 40%	1980
October	1st to 10th	Night	16% 7% 2 4 4 3 4 4 5% 7% 44%	2150
	11th to 20th	Night	19% 8% 3 5% 6% 3 4 5% 6% 7% 34%	2160
	21st to end	Night	26% 9% 3 5% 5% 3 4 4 4 6% 32%	2376
November	1st to 10th	Night	15% 9% 3 4 5% 3 4 3 4 8% 41%	2330
	11th to 20th	Night	19% 15% 4 7% 5% 3 4 4 4 6% 29%	2600
	21st to end	Night	20% 17% 4 6% 5% 2 5% 4 4 5% 27%	2660
December	1st to 10th	Night	23% 18% 8% 10% 5% 2 4 4 4 5% 18%	2660
	11th to 20th	Night	20% 18% 8% 10% 6% 2 4 4 4 5% 20%	3285
	21st to end	Night	27% 21% 7% 8% 7% 3 4 4 4 4 11%	4884

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	73% 4 8% 10% 4	2805
	11th to 20th	Day	75% 3 8% 12% 2	2840
	21st to end	Day	81% 3 7% 6% 2	3124
February	1st to 10th	Day	70% 3 8% 13% 6%	1710
	11th to 20th	Day	68% 4 7% 14% 6%	1672
	21st to end	Day	63% 5% 10% 14% 6% 2	1560
March	1st to 10th	Day	57% 5% 12% 19% 6%	2090
	11th to 20th	Day	57% 5% 10% 18% 10% 1	2090
	21st to end	Day	60% 3 8% 17% 11% 1	2200
April	1st to 10th	Day	52% 6% 12% 16% 13% 1	2067
	11th to 20th	Day	52% 5% 10% 17% 13% 3	2196
	21st to end	Day	53% 4 11% 20% 10% 2	2340
May	1st to 10th	Day	54% 7% 11% 18% 9% 2	2340
	11th to 20th	Day	53% 8% 9% 16% 12% 2	2305
	21st to end	Day	42% 9% 14% 23% 11% 2	2686
June	1st to 10th	Day	46% 8% 12% 22% 11% 1	2520
	11th to 20th	Day	47% 7% 13% 21% 12% 1	2520
	21st to end	Day	42% 8% 11% 25% 13% 1	2520
July	1st to 10th	Day	40% 8% 12% 24% 13% 3	2399
	11th to 20th	Day	41% 8% 12% 24% 14% 1	2271
	21st to end	Day	41% 10% 13% 20% 15% 1	2405
August	1st to 10th	Day	41% 9% 14% 27% 9% 1	2314
	11th to 20th	Day	45% 9% 14% 22% 9% 1	2303
	21st to end	Day	41% 9% 12% 21% 14% 3	2477
September	1st to 10th	Day	42% 6% 12% 21% 15% 3	2152
	11th to 20th	Day	39% 7% 14% 24% 15% 1	1980
	21st to end	Day	47% 6% 11% 23% 12%	1980
October	1st to 10th	Day	45% 7% 13% 19% 15% 1	1810
	11th to 20th	Day	54% 5% 12% 20% 9% 1	1800
	21st to end	Day	59% 5% 11% 15% 9% 1	1980
November	1st to 10th	Day	45% 5% 13% 20% 16% 2	1744
	11th to 20th	Day	58% 4 10% 17% 8% 3	1577
	21st to end	Day	64% 6% 9% 15% 6% 1	1520
December	1st to 10th	Day	72% 5% 9% 10% 3	1520
	11th to 20th	Day	70% 5% 9% 13% 3	1906
	21st to end	Day	79% 3 9% 8% 1	2772
January	1st to 10th	Night	74% 4 10% 9% 3	4137
	11th to 20th	Night	76% 4 9% 9% 2	4120
	21st to end	Night	79% 3 6% 9% 3	4532
February	1st to 10th	Night	67% 4 8% 15% 5%	2470
	11th to 20th	Night	65% 5% 9% 15% 6%	2405
	21st to end	Night	62% 5% 8% 15% 9% 1	1893
March	1st to 10th	Night	57% 4 10% 18% 10%	2090
	11th to 20th	Night	52% 4 13% 20% 9% 1	2090
	21st to end	Night	55% 4 8% 18% 13% 2	2201
April	1st to 10th	Night	53% 5% 9% 18% 12% 2	1891
	11th to 20th	Night	50% 5% 11% 17% 15% 3	1728
	21st to end	Night	49% 5% 11% 20% 12% 3	1620
May	1st to 10th	Night	50% 8% 8% 18% 12% 4	1620
	11th to 20th	Night	47% 7% 11% 19% 15% 3	1593
	21st to end	Night	35% 7% 14% 26% 15% 2	1589
June	1st to 10th	Night	38% 8% 12% 26% 15% 2	1440
	11th to 20th	Night	38% 8% 14% 22% 17% 2	1440
	21st to end	Night	38% 8% 12% 24% 17% 1	1440
July	1st to 10th	Night	36% 8% 11% 22% 19% 3	1446
	11th to 20th	Night	32% 8% 15% 24% 19% 3	1566
	21st to end	Night	37% 9% 15% 21% 16% 2	1672
August	1st to 10th	Night	33% 8% 13% 28% 18% 1	1600
	11th to 20th	Night	36% 7% 14% 26% 15% 1	1598
	21st to end	Night	33% 9% 13% 25% 16% 4	1859
September	1st to 10th	Night	35% 6% 12% 24% 18% 4	1804
	11th to 20th	Night	30% 7% 13% 27% 21% 2	1980
	21st to end	Night	44% 6% 12% 20% 17% 1	1980
October	1st to 10th	Night	40% 6% 13% 23% 16% 2	2150
	11th to 20th	Night	47% 6% 13% 22% 11% 1	2160
	21st to end	Night	54% 5% 11% 18% 10% 2	2376
November	1st to 10th	Night	43% 5% 12% 23% 14% 3	2330
	11th to 20th	Night	58% 4 9% 18% 8% 3	2600
	21st to end	Night	60% 5% 12% 17% 5% 1	2660
December	1st to 10th	Night	69% 5% 11% 10% 4	2660
	11th to 20th	Night	68% 5% 11% 13% 3	3288
	21st to end	Night	77% 6% 9% 8% 1	4884

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	30% 30% 28% 8% 3	261
	11th to 20th	Day	35% 26% 28% 7% 3	270
	21st to end	Day	28% 31% 30% 8% 2	297
February	1st to 10th	Day	32% 24% 37% 7% 1	270
	11th to 20th	Day	33% 30% 29% 8% 1	270
	21st to end	Day	38% 26% 28% 8%	234
March	1st to 10th	Day	31% 21% 29% 14% 4	460
	11th to 20th	Day	30% 20% 33% 12% 4 1	460
	21st to end	Day	34% 20% 25% 17% 3	470
April	1st to 10th	Day	27% 22% 28% 15% 7% 1	420
	11th to 20th	Day	27% 17% 25% 19% 8% 4	412
	21st to end	Day	29% 18% 31% 15% 6% 2	420
May	1st to 10th	Day	28% 18% 27% 17% 7% 3	423
	11th to 20th	Day	24% 13% 31% 19% 10% 3	447
	21st to end	Day	17% 10% 23% 27% 15% 7%	485
June	1st to 10th	Day	16% 10% 34% 26% 10% 4	450
	11th to 20th	Day	20% 11% 25% 26% 15% 3	450
	21st to end	Day	16% 9% 33% 26% 12% 4	450
July	1st to 10th	Day	13% 5% 20% 27% 24% 10%	445
	11th to 20th	Day	13% 6% 27% 27% 18% 8%	444
	21st to end	Day	14% 6% 28% 26% 18% 8%	488
August	1st to 10th	Day	11% 6% 24% 28% 23% 9%	446
	11th to 20th	Day	19% 5% 21% 27% 16% 11%	439
	21st to end	Day	13% 6% 22% 31% 19% 10%	495
September	1st to 10th	Day	17% 6% 23% 26% 20% 8%	450
	11th to 20th	Day	13% 5% 20% 30% 22% 10%	450
	21st to end	Day	22% 11% 25% 22% 14% 6%	450
October	1st to 10th	Day	17% 8% 29% 24% 17% 5%	450
	11th to 20th	Day	26% 16% 31% 19% 8% 1	450
	21st to end	Day	34% 14% 28% 19% 5%	495
November	1st to 10th	Day	19% 20% 33% 17% 10% 1	342
	11th to 20th	Day	34% 21% 26% 13% 5% 1	207
	21st to end	Day	27% 23% 39% 11% 1	180
December	1st to 10th	Day	38% 25% 25% 7% 2 2	180
	11th to 20th	Day	34% 22% 33% 10% 1	180
	21st to end	Day	35% 28% 26% 10% 2	198
January	1st to 10th	Night	31% 20% 33% 13% 3 1	760
	11th to 20th	Night	33% 17% 31% 15% 3	760
	21st to end	Night	29% 20% 30% 19% 3	836
February	1st to 10th	Night	31% 16% 29% 21% 2	760
	11th to 20th	Night	29% 18% 36% 16% 1	743
	21st to end	Night	32% 16% 32% 17% 2	628
March	1st to 10th	Night	27% 16% 33% 21% 3	760
	11th to 20th	Night	29% 16% 28% 21% 6%	760
	21st to end	Night	25% 15% 32% 22% 5%	801
April	1st to 10th	Night	26% 14% 31% 23% 6%	720
	11th to 20th	Night	25% 12% 31% 23% 9% 1	664
	21st to end	Night	22% 11% 34% 28% 5%	540
May	1st to 10th	Night	23% 7% 25% 33% 11% 1	540
	11th to 20th	Night	21% 6% 28% 34% 9% 1	531
	21st to end	Night	13% 4 26% 34% 21% 2	428
June	1st to 10th	Night	11% 4 26% 43% 16%	360
	11th to 20th	Night	13% 6% 26% 39% 16%	360
	21st to end	Night	11% 4 27% 37% 20% 1	360
July	1st to 10th	Night	9% 2 16% 43% 24% 6%	396
	11th to 20th	Night	11% 2 17% 38% 28% 4	522
	21st to end	Night	12% 3 13% 42% 28% 2	591
August	1st to 10th	Night	11% 3 14% 43% 26% 3	537
	11th to 20th	Night	15% 2 11% 38% 30% 4	534
	21st to end	Night	13% 3 20% 32% 27% 5%	671
September	1st to 10th	Night	16% 3 20% 33% 25% 3	720
	11th to 20th	Night	13% 4 16% 35% 27% 5%	720
	21st to end	Night	19% 8% 22% 27% 20% 4	720
October	1st to 10th	Night	18% 8% 23% 30% 18% 3	720
	11th to 20th	Night	23% 10% 31% 26% 8% 1	720
	21st to end	Night	31% 11% 28% 24% 6%	792
November	1st to 10th	Night	21% 10% 29% 28% 11% 1	740
	11th to 20th	Night	31% 14% 29% 19% 7% 1	760
	21st to end	Night	29% 15% 30% 19% 5%	760
December	1st to 10th	Night	33% 18% 28% 17% 4	760
	11th to 20th	Night	29% 20% 28% 19% 4	760
	21st to end	Night	34% 16% 30% 19% 2	836

Page generated at 2024-03-19 23:31:02. Version ac07

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