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 2026-10-14.

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% 5% 4 4 5% 7% 30%	149173
	Night	17% 12% 5% 9% 5% 4 4 3 4 6% 30%	142555
Transparency	Day	53% 5% 10% 20% 10% 1	152776
	Night	55% 4 10% 18% 11% 1	147022
Seeing	Day	24% 15% 28% 20% 10% 4	17149
	Night	24% 12% 27% 26% 11% 1	26778

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	18% 20% 7% 24% 6% 3 3 2 3 3 11%	5199
	11th to 20th	Day	11% 22% 9% 21% 9% 6% 3 2 3 3 12%	5280
	21st to end	Day	19% 18% 7% 13% 10% 7% 3 2 4 4 14%	5780
February	1st to 10th	Day	19% 18% 6% 12% 7% 4 3 2 3 5% 21%	1890
	11th to 20th	Day	17% 14% 8% 15% 8% 4 3 3 3 5% 20%	1843
	21st to end	Day	19% 17% 5% 12% 5% 4 4 3 4 5% 23%	1719
March	1st to 10th	Day	17% 13% 6% 11% 4 3 3 3 4 6% 29%	5045
	11th to 20th	Day	13% 11% 5% 10% 5% 5% 4 3 4 5% 35%	5090
	21st to end	Day	22% 11% 4 9% 5% 4 4 3 4 6% 29%	5586
April	1st to 10th	Day	18% 14% 3 7% 5% 4 4 3 5% 6% 30%	2412
	11th to 20th	Day	19% 12% 3 7% 4 4 4 4 5% 6% 32%	2568
	21st to end	Day	21% 10% 3 7% 4 4 3 3 4 5% 35%	2730
May	1st to 10th	Day	22% 9% 3 8% 5% 5% 5% 4 5% 6% 27%	7616
	11th to 20th	Day	19% 8% 3 7% 5% 5% 5% 4 6% 8% 31%	7626
	21st to end	Day	21% 8% 3 6% 4 4 4 4 5% 6% 35%	8923
June	1st to 10th	Day	16% 6% 2 7% 5% 5% 4 4 6% 8% 35%	2940
	11th to 20th	Day	16% 7% 3 7% 6% 5% 5% 4 5% 9% 35%	2940
	21st to end	Day	15% 5% 2 6% 5% 5% 5% 4 6% 9% 37%	2940
July	1st to 10th	Day	11% 6% 3 7% 5% 5% 5% 4 6% 9% 39%	8101
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 10% 39%	7680
	21st to end	Day	13% 6% 2 7% 5% 4 5% 5% 7% 8% 37%	8340
August	1st to 10th	Day	19% 9% 3 8% 5% 5% 5% 4 6% 6% 29%	6014
	11th to 20th	Day	19% 7% 3 7% 4 4 5% 5% 7% 8% 31%	5951
	21st to end	Day	9% 4 2 7% 5% 5% 7% 5% 8% 11% 37%	6337
September	1st to 10th	Day	16% 6% 3 5% 4 5% 4 3 5% 8% 43%	2510
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 6% 8% 44%	2310
	21st to end	Day	20% 9% 3 7% 4 4 4 3 6% 7% 34%	2310
October	1st to 10th	Day	16% 7% 2 5% 4 3 4 4 5% 6% 42%	2110
	11th to 20th	Day	22% 8% 3 8% 5% 3 5% 3 5% 6% 33%	2030
	21st to end	Day	25% 11% 3 8% 4 4 4 4 4 6% 28%	2200
November	1st to 10th	Day	12% 10% 4 5% 5% 4 4 4 5% 6% 39%	1932
	11th to 20th	Day	19% 17% 4 8% 5% 4 4 3 4 6% 28%	1743
	21st to end	Day	20% 19% 6% 9% 5% 4 5% 3 5% 5% 21%	1680
December	1st to 10th	Day	22% 18% 9% 17% 5% 4 3 2 3 3 14%	2680
	11th to 20th	Day	23% 20% 7% 10% 4 2 3 3 4 4 19%	3070
	21st to end	Day	26% 25% 6% 11% 5% 4 3 2 3 4 11%	4048
January	1st to 10th	Night	16% 21% 8% 23% 5% 4 3 2 3 3 11%	7676
	11th to 20th	Night	13% 19% 10% 18% 10% 5% 4 2 3 3 11%	7680
	21st to end	Night	19% 17% 7% 12% 7% 6% 5% 2 3 4 17%	8401
February	1st to 10th	Night	20% 16% 6% 12% 7% 5% 3 2 4 5% 22%	2730
	11th to 20th	Night	18% 15% 6% 13% 5% 5% 4 3 5% 4 21%	2663
	21st to end	Night	20% 15% 4 13% 6% 4 4 3 4 4 24%	2085
March	1st to 10th	Night	23% 13% 5% 8% 5% 3 3 2 3 5% 29%	5028
	11th to 20th	Night	16% 12% 5% 6% 4 3 3 3 4 6% 37%	5090
	21st to end	Night	20% 12% 4 8% 5% 4 3 3 4 4 33%	5588
April	1st to 10th	Night	20% 14% 4 6% 5% 4 4 3 4 6% 30%	2206
	11th to 20th	Night	17% 11% 3 6% 5% 4 5% 3 5% 6% 33%	2019
	21st to end	Night	16% 11% 2 7% 5% 4 4 3 5% 7% 35%	1890
May	1st to 10th	Night	27% 10% 3 7% 4 4 4 3 4 6% 29%	5237
	11th to 20th	Night	16% 10% 3 7% 4 4 5% 3 5% 7% 37%	5234
	21st to end	Night	23% 7% 2 5% 4 4 4 3 5% 7% 36%	5230
June	1st to 10th	Night	10% 6% 2 6% 6% 5% 5% 4 5% 7% 44%	1680
	11th to 20th	Night	12% 6% 3 7% 4 3 4 4 6% 7% 44%	1680
	21st to end	Night	12% 6% 2 7% 5% 4 6% 4 5% 9% 41%	1680
July	1st to 10th	Night	12% 8% 3 8% 5% 5% 4 4 5% 8% 39%	4852
	11th to 20th	Night	11% 6% 2 6% 4 4 5% 4 7% 9% 41%	5277
	21st to end	Night	12% 6% 2 7% 4 4 4 4 5% 6% 45%	5736
August	1st to 10th	Night	17% 10% 4 9% 5% 4 5% 3 5% 6% 34%	4135
	11th to 20th	Night	19% 8% 3 6% 4 4 4 3 4 7% 38%	4095
	21st to end	Night	10% 5% 2 6% 5% 5% 5% 4 6% 9% 42%	4740
September	1st to 10th	Night	13% 5% 2 5% 4 3 4 4 5% 7% 48%	2106
	11th to 20th	Night	12% 5% 1 5% 3 3 3 3 5% 7% 52%	2310
	21st to end	Night	18% 8% 3 7% 4 5% 4 3 5% 6% 37%	2310
October	1st to 10th	Night	15% 6% 2 5% 3 4 3 3 5% 7% 47%	2509
	11th to 20th	Night	18% 8% 2 6% 5% 4 4 4 6% 7% 36%	2437
	21st to end	Night	25% 9% 2 5% 4 3 4 3 5% 6% 34%	2640
November	1st to 10th	Night	15% 10% 2 5% 4 5% 4 3 4 7% 41%	2582
	11th to 20th	Night	19% 17% 3 8% 4 4 4 4 4 6% 28%	2874
	21st to end	Night	21% 18% 5% 7% 4 3 4 3 4 5% 26%	2940
December	1st to 10th	Night	18% 20% 10% 15% 5% 4 4 3 3 4 15%	4720
	11th to 20th	Night	22% 20% 8% 11% 4 3 3 2 4 4 19%	5345
	21st to end	Night	25% 20% 6% 11% 5% 4 4 3 4 5% 14%	7150

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	80% 3 7% 6% 3	5200
	11th to 20th	Day	80% 2 6% 8% 3	5280
	21st to end	Day	76% 2 7% 11% 3	5780
February	1st to 10th	Day	69% 3 8% 14% 6%	1890
	11th to 20th	Day	69% 4 8% 13% 6%	1852
	21st to end	Day	63% 5% 10% 15% 6% 1	1719
March	1st to 10th	Day	57% 5% 11% 21% 7%	5045
	11th to 20th	Day	51% 4 14% 22% 7% 1	5090
	21st to end	Day	57% 4 7% 19% 12% 1	5586
April	1st to 10th	Day	54% 6% 11% 16% 12% 1	2412
	11th to 20th	Day	53% 5% 10% 18% 12% 2	2568
	21st to end	Day	52% 4 11% 21% 11% 2	2730
May	1st to 10th	Day	56% 5% 9% 19% 8% 3	7651
	11th to 20th	Day	51% 6% 10% 23% 10% 1	7630
	21st to end	Day	49% 5% 10% 18% 16% 3	8938
June	1st to 10th	Day	44% 8% 11% 23% 13% 1	2940
	11th to 20th	Day	47% 6% 12% 21% 13% 1	2940
	21st to end	Day	43% 7% 12% 24% 13% 1	2940
July	1st to 10th	Day	42% 5% 11% 27% 14% 2	8033
	11th to 20th	Day	38% 5% 11% 32% 13%	7611
	21st to end	Day	42% 5% 10% 22% 19% 1	8156
August	1st to 10th	Day	54% 6% 11% 18% 9% 3	6001
	11th to 20th	Day	49% 6% 12% 26% 7% 1	5990
	21st to end	Day	38% 7% 16% 28% 11% 1	6337
September	1st to 10th	Day	40% 6% 12% 23% 17% 3	2510
	11th to 20th	Day	37% 6% 14% 25% 17% 1	2310
	21st to end	Day	50% 5% 10% 21% 12% 1	2310
October	1st to 10th	Day	34% 4 11% 29% 20% 1	3370
	11th to 20th	Day	46% 4 11% 21% 13% 4	3270
	21st to end	Day	43% 4 12% 26% 14% 1	3538
November	1st to 10th	Day	45% 5% 12% 21% 15% 2	1932
	11th to 20th	Day	59% 4 10% 17% 8% 2	1743
	21st to end	Day	65% 6% 9% 14% 5% 1	1680
December	1st to 10th	Day	76% 4 9% 8% 3	2680
	11th to 20th	Day	70% 4 9% 13% 4	3066
	21st to end	Day	79% 3 8% 8% 2	4048
January	1st to 10th	Night	80% 3 8% 7% 3	7677
	11th to 20th	Night	80% 3 7% 7% 3	7680
	21st to end	Night	72% 2 8% 12% 5% 1	8401
February	1st to 10th	Night	67% 4 8% 16% 5%	2730
	11th to 20th	Night	66% 4 9% 14% 5%	2665
	21st to end	Night	63% 5% 8% 15% 9% 1	2085
March	1st to 10th	Night	60% 3 11% 18% 8%	5028
	11th to 20th	Night	48% 3 15% 24% 8% 2	5090
	21st to end	Night	54% 4 9% 18% 14% 1	5588
April	1st to 10th	Night	56% 5% 9% 17% 12% 1	2206
	11th to 20th	Night	51% 6% 10% 17% 13% 2	2019
	21st to end	Night	48% 5% 11% 21% 12% 2	1890
May	1st to 10th	Night	57% 5% 7% 17% 11% 3	5264
	11th to 20th	Night	47% 5% 10% 26% 11% 1	5235
	21st to end	Night	47% 5% 10% 22% 14% 2	5237
June	1st to 10th	Night	38% 8% 11% 25% 17% 2	1680
	11th to 20th	Night	38% 7% 14% 22% 16% 2	1680
	21st to end	Night	40% 8% 12% 23% 17% 1	1680
July	1st to 10th	Night	45% 5% 9% 23% 17% 1	4812
	11th to 20th	Night	39% 5% 14% 29% 12% 1	5226
	21st to end	Night	39% 5% 10% 17% 27% 1	5606
August	1st to 10th	Night	52% 5% 10% 18% 12% 2	4124
	11th to 20th	Night	46% 5% 13% 25% 10% 1	4122
	21st to end	Night	37% 6% 15% 29% 12% 2	4740
September	1st to 10th	Night	35% 6% 11% 26% 19% 4	2106
	11th to 20th	Night	31% 6% 13% 28% 21% 2	2310
	21st to end	Night	47% 5% 11% 19% 16% 1	2310
October	1st to 10th	Night	35% 5% 8% 28% 23% 1	4029
	11th to 20th	Night	41% 5% 11% 23% 14% 6%	3936
	21st to end	Night	38% 4 11% 28% 17% 1	4252
November	1st to 10th	Night	43% 4 12% 23% 14% 3	2582
	11th to 20th	Night	59% 4 9% 17% 8% 3	2874
	21st to end	Night	62% 4 12% 17% 5% 1	2940
December	1st to 10th	Night	74% 4 10% 9% 3	4720
	11th to 20th	Night	70% 4 10% 14% 2	5348
	21st to end	Night	73% 5% 9% 11% 1	7150

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	32% 29% 29% 7% 3	319
	11th to 20th	Day	38% 28% 25% 6% 3	330
	21st to end	Day	30% 31% 29% 8% 2	363
February	1st to 10th	Day	32% 26% 34% 7% 2	330
	11th to 20th	Day	36% 27% 28% 8% 1	330
	21st to end	Day	35% 27% 32% 7%	283
March	1st to 10th	Day	32% 22% 30% 13% 3	540
	11th to 20th	Day	31% 21% 31% 11% 4 1	540
	21st to end	Day	36% 21% 25% 15% 3	594
April	1st to 10th	Day	31% 22% 27% 14% 6% 1	540
	11th to 20th	Day	29% 19% 26% 17% 6% 3	532
	21st to end	Day	28% 17% 31% 16% 6% 2	540
May	1st to 10th	Day	30% 19% 27% 16% 6% 3	543
	11th to 20th	Day	24% 14% 30% 19% 10% 3	567
	21st to end	Day	21% 11% 23% 26% 13% 6%	617
June	1st to 10th	Day	15% 13% 34% 24% 10% 4	570
	11th to 20th	Day	21% 12% 28% 24% 13% 2	570
	21st to end	Day	15% 10% 33% 26% 13% 4	570
July	1st to 10th	Day	13% 5% 21% 28% 24% 8%	565
	11th to 20th	Day	12% 7% 27% 28% 18% 8%	564
	21st to end	Day	14% 5% 27% 28% 18% 8%	620
August	1st to 10th	Day	13% 6% 25% 28% 21% 7%	566
	11th to 20th	Day	20% 6% 21% 28% 16% 9%	559
	21st to end	Day	11% 6% 22% 31% 19% 9%	627
September	1st to 10th	Day	16% 6% 24% 25% 21% 8%	570
	11th to 20th	Day	13% 5% 20% 30% 22% 10%	570
	21st to end	Day	25% 9% 26% 22% 12% 5%	570
October	1st to 10th	Day	16% 9% 28% 25% 17% 4	570
	11th to 20th	Day	25% 16% 30% 19% 8% 2	542
	21st to end	Day	32% 15% 29% 19% 6%	583
November	1st to 10th	Day	20% 20% 35% 15% 9% 1	410
	11th to 20th	Day	37% 19% 27% 12% 4 1	253
	21st to end	Day	31% 21% 37% 10% 1	220
December	1st to 10th	Day	39% 25% 27% 6% 2 2	220
	11th to 20th	Day	36% 21% 31% 10% 1	220
	21st to end	Day	38% 25% 26% 10% 2	242
January	1st to 10th	Night	31% 20% 33% 13% 3	840
	11th to 20th	Night	33% 19% 31% 14% 3	840
	21st to end	Night	29% 22% 29% 18% 2	924
February	1st to 10th	Night	31% 16% 29% 22% 2	840
	11th to 20th	Night	29% 18% 35% 17% 1	823
	21st to end	Night	32% 16% 33% 17% 2	692
March	1st to 10th	Night	28% 16% 33% 20% 3	840
	11th to 20th	Night	29% 16% 28% 21% 6%	840
	21st to end	Night	26% 16% 32% 21% 4	924
April	1st to 10th	Night	28% 15% 30% 21% 5%	840
	11th to 20th	Night	26% 13% 31% 22% 7% 1	775
	21st to end	Night	23% 11% 31% 30% 5%	630
May	1st to 10th	Night	25% 7% 26% 32% 10%	630
	11th to 20th	Night	21% 8% 28% 34% 9% 1	621
	21st to end	Night	16% 4 25% 35% 18% 2	500
June	1st to 10th	Night	11% 5% 28% 41% 15%	420
	11th to 20th	Night	14% 6% 27% 39% 14%	420
	21st to end	Night	13% 4 27% 34% 21% 1	420
July	1st to 10th	Night	10% 3 15% 44% 23% 5%	465
	11th to 20th	Night	10% 2 17% 39% 27% 4	612
	21st to end	Night	12% 3 13% 43% 27% 1	690
August	1st to 10th	Night	12% 3 15% 43% 24% 3	627
	11th to 20th	Night	17% 3 11% 37% 28% 4	624
	21st to end	Night	13% 3 20% 34% 27% 4	784
September	1st to 10th	Night	15% 3 21% 35% 23% 3	840
	11th to 20th	Night	13% 4 15% 36% 28% 4	840
	21st to end	Night	20% 9% 22% 27% 18% 4	840
October	1st to 10th	Night	17% 7% 24% 31% 19% 2	840
	11th to 20th	Night	22% 10% 32% 25% 9% 1	813
	21st to end	Night	29% 11% 29% 25% 6%	880
November	1st to 10th	Night	21% 11% 31% 27% 10%	820
	11th to 20th	Night	31% 14% 29% 19% 6% 1	840
	21st to end	Night	31% 15% 30% 18% 5%	840
December	1st to 10th	Night	33% 18% 28% 17% 3	840
	11th to 20th	Night	30% 20% 28% 18% 3	840
	21st to end	Night	34% 16% 30% 18% 2	924

Page generated at 2026-10-11 05:06:27. Version ac07

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