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 2025-06-10.

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	17% 11% 4 9% 5% 4 4 4 5% 6% 29%	119547
	Night	18% 13% 5% 9% 5% 4 4 3 4 6% 29%	118668
Transparency	Day	54% 5% 10% 19% 10% 1	123192
	Night	55% 5% 10% 18% 10% 1	123160
Seeing	Day	24% 15% 28% 19% 10% 4	15350
	Night	24% 12% 27% 26% 11% 1	25099

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	16% 21% 7% 24% 5% 3 3 2 2 3 14%	4010
	11th to 20th	Day	12% 24% 9% 19% 8% 7% 3 2 2 3 12%	4060
	21st to end	Day	21% 20% 8% 13% 8% 5% 3 2 3 3 13%	4466
February	1st to 10th	Day	20% 19% 6% 13% 7% 4 3 2 3 4 20%	1800
	11th to 20th	Day	17% 15% 7% 15% 8% 4 3 3 3 5% 20%	1753
	21st to end	Day	20% 17% 4 12% 5% 4 4 3 5% 6% 23%	1640
March	1st to 10th	Day	17% 13% 6% 11% 4 3 3 3 4 6% 29%	4935
	11th to 20th	Day	13% 11% 6% 9% 5% 5% 4 3 4 5% 35%	4980
	21st to end	Day	22% 11% 4 9% 5% 4 3 3 4 6% 29%	5465
April	1st to 10th	Day	18% 15% 4 7% 5% 4 4 3 5% 6% 30%	2297
	11th to 20th	Day	19% 12% 3 7% 4 4 4 4 5% 6% 32%	2444
	21st to end	Day	21% 10% 3 7% 4 4 3 3 4 5% 35%	2600
May	1st to 10th	Day	23% 9% 3 8% 5% 5% 5% 4 5% 5% 27%	5862
	11th to 20th	Day	20% 9% 3 7% 5% 5% 5% 5% 7% 8% 29%	5861
	21st to end	Day	20% 7% 2 6% 5% 5% 5% 5% 6% 7% 33%	6839
June	1st to 10th	Day	17% 6% 2 7% 5% 5% 4 4 6% 8% 34%	2786
	11th to 20th	Day	15% 6% 3 7% 6% 5% 5% 4 5% 9% 35%	2660
	21st to end	Day	15% 5% 2 6% 5% 5% 5% 4 7% 8% 37%	2660
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 5% 6% 10% 38%	4345
	11th to 20th	Day	11% 6% 2 6% 4 5% 5% 5% 6% 9% 41%	4120
	21st to end	Day	12% 6% 2 8% 5% 5% 6% 5% 7% 8% 37%	4466
August	1st to 10th	Day	16% 8% 3 9% 5% 6% 6% 5% 7% 7% 30%	4104
	11th to 20th	Day	15% 7% 2 7% 5% 5% 6% 5% 7% 8% 34%	4044
	21st to end	Day	10% 5% 2 6% 5% 5% 7% 5% 7% 9% 39%	4351
September	1st to 10th	Day	17% 6% 3 5% 4 4 4 3 5% 7% 43%	2271
	11th to 20th	Day	13% 6% 2 6% 4 4 4 3 6% 8% 43%	2090
	21st to end	Day	20% 9% 2 7% 4 5% 3 3 6% 7% 34%	2090
October	1st to 10th	Day	17% 8% 3 6% 4 4 4 4 6% 7% 38%	1910
	11th to 20th	Day	23% 8% 3 8% 5% 3 5% 3 5% 5% 32%	1900
	21st to end	Day	25% 10% 3 7% 4 4 4 4 4 7% 28%	2090
November	1st to 10th	Day	12% 10% 4 5% 6% 4 4 4 5% 6% 40%	1838
	11th to 20th	Day	19% 17% 4 7% 5% 4 4 3 4 6% 28%	1660
	21st to end	Day	20% 19% 5% 9% 5% 4 5% 3 5% 5% 21%	1600
December	1st to 10th	Day	22% 18% 9% 17% 5% 4 3 2 3 3 14%	2600
	11th to 20th	Day	24% 20% 7% 10% 4 2 3 3 4 4 19%	2990
	21st to end	Day	26% 25% 6% 11% 5% 4 3 2 3 4 11%	3960
January	1st to 10th	Night	15% 20% 9% 25% 4 4 3 2 3 3 12%	5917
	11th to 20th	Night	13% 20% 11% 17% 9% 5% 4 3 3 4 12%	5900
	21st to end	Night	23% 18% 8% 12% 6% 4 5% 2 3 3 15%	6490
February	1st to 10th	Night	21% 16% 6% 12% 7% 5% 3 2 4 5% 21%	2600
	11th to 20th	Night	18% 15% 6% 12% 6% 5% 4 3 5% 4 22%	2533
	21st to end	Night	20% 14% 4 13% 6% 4 4 3 4 4 24%	1988
March	1st to 10th	Night	22% 13% 5% 8% 5% 3 3 2 3 5% 29%	4918
	11th to 20th	Night	16% 12% 5% 6% 4 3 3 3 4 6% 38%	4980
	21st to end	Night	20% 12% 4 8% 5% 4 3 3 4 4 33%	5467
April	1st to 10th	Night	19% 14% 4 6% 5% 4 4 4 4 6% 30%	2101
	11th to 20th	Night	17% 11% 3 6% 5% 4 5% 3 5% 6% 34%	1922
	21st to end	Night	17% 10% 2 7% 5% 4 4 3 6% 7% 35%	1800
May	1st to 10th	Night	28% 11% 3 7% 4 3 4 3 4 6% 26%	4033
	11th to 20th	Night	17% 10% 3 8% 4 3 4 3 5% 6% 37%	4032
	21st to end	Night	20% 7% 3 5% 4 4 4 3 6% 9% 37%	4014
June	1st to 10th	Night	11% 6% 1 6% 6% 4 5% 4 6% 7% 43%	1598
	11th to 20th	Night	11% 6% 3 6% 5% 3 4 4 6% 7% 45%	1520
	21st to end	Night	11% 6% 2 7% 5% 3 6% 5% 5% 9% 41%	1520
July	1st to 10th	Night	12% 8% 3 7% 5% 5% 4 4 5% 8% 40%	2610
	11th to 20th	Night	11% 4 2 6% 5% 4 4 3 6% 9% 46%	2840
	21st to end	Night	10% 7% 2 8% 5% 4 5% 4 6% 7% 43%	3087
August	1st to 10th	Night	13% 8% 3 8% 5% 4 5% 3 6% 6% 38%	2825
	11th to 20th	Night	16% 7% 3 5% 4 4 4 3 5% 7% 42%	2785
	21st to end	Night	9% 4 2 6% 5% 5% 6% 4 6% 8% 44%	3271
September	1st to 10th	Night	13% 5% 2 5% 4 3 4 4 5% 7% 48%	1905
	11th to 20th	Night	11% 5% 1 5% 3 3 4 3 5% 8% 52%	2090
	21st to end	Night	18% 8% 3 7% 4 4 4 3 5% 6% 39%	2090
October	1st to 10th	Night	16% 6% 2 5% 3 4 4 3 5% 7% 45%	2270
	11th to 20th	Night	18% 8% 2 6% 5% 4 4 4 6% 7% 35%	2280
	21st to end	Night	25% 9% 2 5% 4 3 4 3 5% 6% 33%	2508
November	1st to 10th	Night	15% 10% 2 5% 4 5% 3 3 4 7% 41%	2456
	11th to 20th	Night	19% 17% 3 8% 4 4 4 4 4 6% 28%	2737
	21st to end	Night	21% 17% 4 7% 4 3 4 3 4 5% 26%	2800
December	1st to 10th	Night	18% 19% 10% 15% 5% 4 4 3 3 4 15%	4580
	11th to 20th	Night	22% 20% 8% 11% 4 3 3 2 4 4 18%	5205
	21st to end	Night	25% 20% 6% 10% 5% 4 4 3 4 5% 14%	6996

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	78% 3 6% 8% 4 1	4011
	11th to 20th	Day	80% 3 6% 8% 2 1	4060
	21st to end	Day	77% 3 7% 9% 3	4466
February	1st to 10th	Day	70% 3 8% 13% 6%	1800
	11th to 20th	Day	69% 4 7% 13% 6%	1762
	21st to end	Day	63% 5% 10% 15% 6% 1	1640
March	1st to 10th	Day	57% 5% 11% 20% 7%	4935
	11th to 20th	Day	51% 4 14% 22% 8% 1	4980
	21st to end	Day	57% 4 7% 19% 12% 1	5465
April	1st to 10th	Day	54% 6% 11% 16% 12% 1	2297
	11th to 20th	Day	52% 5% 10% 18% 12% 2	2444
	21st to end	Day	52% 4 11% 20% 11% 2	2600
May	1st to 10th	Day	56% 6% 8% 17% 9% 4	5897
	11th to 20th	Day	52% 7% 9% 21% 10% 1	5865
	21st to end	Day	48% 7% 11% 20% 11% 3	6854
June	1st to 10th	Day	45% 8% 12% 23% 11% 1	2786
	11th to 20th	Day	45% 7% 12% 21% 13% 1	2660
	21st to end	Day	42% 7% 11% 25% 13% 1	2660
July	1st to 10th	Day	41% 6% 12% 24% 14% 3	4277
	11th to 20th	Day	38% 6% 11% 27% 17% 1	4051
	21st to end	Day	42% 7% 13% 20% 17% 1	4324
August	1st to 10th	Day	50% 7% 12% 21% 9%	4091
	11th to 20th	Day	45% 7% 11% 28% 9% 1	4083
	21st to end	Day	39% 7% 12% 28% 12% 2	4351
September	1st to 10th	Day	41% 6% 11% 22% 17% 3	2271
	11th to 20th	Day	38% 6% 14% 24% 16% 1	2090
	21st to end	Day	50% 6% 11% 22% 11%	2090
October	1st to 10th	Day	35% 5% 12% 28% 19% 1	3170
	11th to 20th	Day	46% 4 11% 21% 13% 4	3140
	21st to end	Day	42% 4 12% 27% 14%	3428
November	1st to 10th	Day	44% 5% 12% 21% 16% 2	1838
	11th to 20th	Day	59% 4 10% 17% 8% 3	1660
	21st to end	Day	65% 6% 9% 14% 5% 1	1600
December	1st to 10th	Day	76% 4 9% 8% 3	2600
	11th to 20th	Day	70% 4 9% 13% 4	2986
	21st to end	Day	79% 3 8% 8% 2	3960
January	1st to 10th	Night	78% 3 7% 8% 3	5917
	11th to 20th	Night	79% 3 8% 8% 2	5900
	21st to end	Night	76% 2 8% 9% 4 1	6490
February	1st to 10th	Night	68% 4 8% 15% 5%	2600
	11th to 20th	Night	66% 5% 9% 15% 5%	2535
	21st to end	Night	63% 5% 8% 14% 9% 1	1988
March	1st to 10th	Night	60% 3 11% 18% 8%	4918
	11th to 20th	Night	48% 3 14% 24% 8% 2	4980
	21st to end	Night	54% 4 9% 18% 14% 1	5467
April	1st to 10th	Night	56% 5% 9% 17% 12% 1	2101
	11th to 20th	Night	51% 6% 10% 17% 14% 2	1922
	21st to end	Night	48% 5% 11% 21% 13% 3	1800
May	1st to 10th	Night	59% 5% 8% 15% 10% 4	4060
	11th to 20th	Night	48% 5% 10% 24% 12% 1	4033
	21st to end	Night	44% 6% 11% 23% 13% 3	4021
June	1st to 10th	Night	38% 8% 11% 25% 15% 2	1598
	11th to 20th	Night	37% 7% 15% 23% 16% 2	1520
	21st to end	Night	39% 8% 12% 23% 17% 1	1520
July	1st to 10th	Night	42% 6% 11% 21% 19% 2	2568
	11th to 20th	Night	34% 6% 14% 29% 16% 1	2786
	21st to end	Night	39% 7% 12% 17% 24% 1	2988
August	1st to 10th	Night	46% 6% 11% 23% 13% 1	2814
	11th to 20th	Night	42% 6% 9% 31% 12% 1	2812
	21st to end	Night	35% 7% 13% 29% 13% 2	3271
September	1st to 10th	Night	35% 6% 11% 25% 18% 4	1905
	11th to 20th	Night	30% 7% 13% 27% 22% 2	2090
	21st to end	Night	46% 5% 12% 20% 16% 1	2090
October	1st to 10th	Night	36% 5% 9% 27% 22% 1	3790
	11th to 20th	Night	42% 5% 11% 23% 13% 6%	3779
	21st to end	Night	38% 4 12% 28% 17% 1	4120
November	1st to 10th	Night	43% 4 12% 23% 14% 3	2456
	11th to 20th	Night	58% 4 9% 17% 8% 3	2737
	21st to end	Night	61% 5% 12% 17% 5% 1	2800
December	1st to 10th	Night	74% 4 10% 9% 3	4580
	11th to 20th	Night	70% 4 10% 13% 2	5208
	21st to end	Night	73% 5% 9% 11% 1	6996

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	31% 31% 27% 8% 3	290
	11th to 20th	Day	35% 28% 27% 6% 3	300
	21st to end	Day	28% 31% 31% 8% 2	330
February	1st to 10th	Day	33% 26% 34% 6% 1	300
	11th to 20th	Day	36% 27% 28% 8% 1	300
	21st to end	Day	37% 25% 30% 7%	259
March	1st to 10th	Day	31% 22% 30% 14% 3	500
	11th to 20th	Day	30% 21% 33% 12% 4 1	500
	21st to end	Day	36% 21% 25% 16% 3	550
April	1st to 10th	Day	29% 22% 27% 14% 6% 1	500
	11th to 20th	Day	29% 18% 25% 18% 7% 3	492
	21st to end	Day	28% 18% 31% 16% 6% 2	500
May	1st to 10th	Day	29% 18% 27% 17% 6% 3	503
	11th to 20th	Day	24% 14% 30% 18% 10% 3	527
	21st to end	Day	20% 11% 23% 26% 13% 6%	573
June	1st to 10th	Day	17% 12% 34% 25% 10% 4	526
	11th to 20th	Day	18% 10% 27% 27% 15% 3	490
	21st to end	Day	17% 10% 33% 24% 12% 4	490
July	1st to 10th	Day	13% 5% 21% 27% 24% 9%	485
	11th to 20th	Day	13% 7% 28% 27% 18% 8%	484
	21st to end	Day	14% 5% 27% 27% 19% 8%	532
August	1st to 10th	Day	13% 6% 24% 28% 22% 8%	486
	11th to 20th	Day	19% 6% 22% 27% 16% 10%	479
	21st to end	Day	12% 6% 22% 31% 20% 10%	539
September	1st to 10th	Day	16% 6% 24% 26% 20% 8%	490
	11th to 20th	Day	13% 4 20% 31% 22% 10%	490
	21st to end	Day	25% 10% 25% 21% 13% 5%	490
October	1st to 10th	Day	17% 9% 29% 24% 17% 4	490
	11th to 20th	Day	25% 15% 29% 19% 9% 2	490
	21st to end	Day	32% 14% 29% 19% 6%	539
November	1st to 10th	Day	19% 20% 35% 16% 9% 1	376
	11th to 20th	Day	33% 20% 29% 12% 4 1	230
	21st to end	Day	29% 22% 38% 10% 1	200
December	1st to 10th	Day	38% 25% 27% 7% 2 2	200
	11th to 20th	Day	36% 21% 32% 11% 1	200
	21st to end	Day	36% 25% 27% 10% 2	220
January	1st to 10th	Night	30% 20% 34% 13% 3 1	800
	11th to 20th	Night	33% 18% 32% 14% 3	800
	21st to end	Night	29% 21% 29% 18% 2	880
February	1st to 10th	Night	32% 17% 29% 20% 2	800
	11th to 20th	Night	29% 18% 35% 16% 1	783
	21st to end	Night	32% 16% 32% 17% 2	660
March	1st to 10th	Night	27% 16% 34% 20% 3	800
	11th to 20th	Night	28% 16% 28% 22% 6%	800
	21st to end	Night	26% 15% 33% 21% 5%	880
April	1st to 10th	Night	28% 15% 30% 22% 6%	800
	11th to 20th	Night	26% 13% 30% 22% 8% 1	738
	21st to end	Night	22% 11% 32% 29% 5%	600
May	1st to 10th	Night	24% 7% 25% 33% 10% 1	600
	11th to 20th	Night	21% 8% 27% 34% 10% 1	591
	21st to end	Night	14% 4 25% 35% 19% 2	476
June	1st to 10th	Night	12% 4 27% 41% 15%	400
	11th to 20th	Night	12% 6% 26% 40% 15%	380
	21st to end	Night	12% 4 27% 35% 21% 1	380
July	1st to 10th	Night	10% 2 16% 44% 22% 5%	419
	11th to 20th	Night	11% 3 17% 39% 27% 4	552
	21st to end	Night	12% 3 13% 43% 28% 1	624
August	1st to 10th	Night	12% 3 14% 42% 25% 3	567
	11th to 20th	Night	16% 2 11% 37% 29% 4	564
	21st to end	Night	13% 3 19% 32% 28% 5%	709
September	1st to 10th	Night	16% 3 20% 33% 25% 3	760
	11th to 20th	Night	13% 4 16% 35% 28% 4	760
	21st to end	Night	20% 8% 23% 26% 19% 4	760
October	1st to 10th	Night	18% 7% 24% 30% 19% 3	760
	11th to 20th	Night	23% 10% 31% 26% 9% 1	760
	21st to end	Night	29% 11% 29% 25% 6%	836
November	1st to 10th	Night	21% 11% 30% 27% 11% 1	780
	11th to 20th	Night	31% 14% 30% 18% 7% 1	800
	21st to end	Night	30% 15% 30% 19% 5%	800
December	1st to 10th	Night	33% 18% 28% 17% 4	800
	11th to 20th	Night	31% 20% 28% 19% 4	800
	21st to end	Night	33% 16% 30% 19% 2	880

Page generated at 2025-06-07 10:28:16. Version ac07

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