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 2024-11-03.

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 8% 5% 4 4 4 5% 7% 31%	101146
	Night	18% 12% 4 8% 5% 4 4 3 5% 6% 31%	97789
Transparency	Day	53% 6% 11% 19% 10% 1	100955
	Night	54% 5% 11% 19% 10% 1	97650
Seeing	Day	23% 15% 27% 20% 11% 4	14638
	Night	23% 11% 27% 26% 11% 1	24293

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	20% 23% 8% 12% 4 3 3 2 3 4 16%	2804
	11th to 20th	Day	13% 26% 8% 14% 7% 5% 2 2 3 3 16%	2840
	21st to end	Day	27% 21% 9% 14% 6% 3 3 2 3 3 10%	3124
February	1st to 10th	Day	20% 19% 6% 12% 7% 4 3 2 2 4 20%	1710
	11th to 20th	Day	17% 15% 7% 14% 8% 4 4 3 3 5% 20%	1663
	21st to end	Day	20% 17% 4 11% 5% 4 4 3 4 5% 23%	1560
March	1st to 10th	Day	21% 13% 5% 9% 5% 3 4 3 5% 6% 26%	3435
	11th to 20th	Day	13% 13% 5% 10% 5% 5% 3 3 4 5% 32%	3480
	21st to end	Day	22% 10% 4 8% 4 4 4 3 5% 6% 30%	3815
April	1st to 10th	Day	17% 15% 3 6% 5% 4 4 3 5% 6% 30%	2182
	11th to 20th	Day	20% 12% 3 6% 4 4 4 4 5% 6% 32%	2320
	21st to end	Day	22% 10% 3 7% 4 4 3 3 4 6% 34%	2470
May	1st to 10th	Day	24% 9% 3 7% 5% 4 5% 4 5% 6% 28%	4082
	11th to 20th	Day	21% 8% 3 7% 5% 5% 5% 4 7% 7% 30%	4081
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 5% 6% 8% 39%	4756
June	1st to 10th	Day	17% 6% 2 7% 5% 5% 4 4 7% 9% 33%	2660
	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 6% 28%	2090
November	1st to 10th	Day	12% 11% 4 5% 6% 4 4 4 5% 6% 40%	1764
	11th to 20th	Day	19% 17% 4 7% 5% 4 3 3 4 5% 29%	1577
	21st to end	Day	20% 18% 5% 8% 5% 4 5% 3 5% 5% 22%	1520
December	1st to 10th	Day	22% 23% 6% 10% 5% 4 3 3 4 4 16%	1520
	11th to 20th	Day	19% 19% 8% 13% 4 3 4 4 5% 5% 16%	1910
	21st to end	Day	27% 26% 7% 9% 4 3 3 2 3 4 11%	2772
January	1st to 10th	Night	17% 24% 10% 14% 5% 4 3 2 3 4 13%	4137
	11th to 20th	Night	17% 20% 9% 16% 6% 5% 5% 3 3 4 13%	4120
	21st to end	Night	29% 19% 9% 12% 5% 4 3 2 2 3 14%	4532
February	1st to 10th	Night	21% 16% 5% 11% 7% 5% 3 2 4 5% 21%	2470
	11th to 20th	Night	18% 15% 6% 12% 5% 5% 4 3 5% 4 22%	2403
	21st to end	Night	20% 14% 4 12% 6% 4 4 3 4 5% 24%	1893
March	1st to 10th	Night	24% 13% 4 6% 4 4 3 2 4 5% 31%	3418
	11th to 20th	Night	20% 14% 6% 6% 4 3 3 3 4 6% 32%	3480
	21st to end	Night	19% 11% 3 7% 4 3 3 3 4 5% 37%	3817
April	1st to 10th	Night	18% 14% 3 6% 5% 4 4 4 4 6% 31%	1996
	11th to 20th	Night	18% 10% 3 5% 5% 4 5% 3 5% 6% 35%	1825
	21st to end	Night	17% 10% 2 7% 5% 4 4 3 6% 7% 35%	1710
May	1st to 10th	Night	27% 10% 2 6% 4 4 4 3 5% 7% 28%	2813
	11th to 20th	Night	17% 10% 3 7% 4 4 4 3 5% 7% 37%	2812
	21st to end	Night	12% 6% 2 5% 4 4 3 3 7% 11% 43%	2798
June	1st to 10th	Night	10% 6% 1 6% 6% 5% 5% 4 6% 7% 43%	1520
	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% 3 5% 4 4 3 3 4 7% 41%	2355
	11th to 20th	Night	19% 17% 3 8% 4 4 4 4 4 6% 29%	2600
	21st to end	Night	20% 17% 4 6% 4 3 4 3 4 5% 27%	2660
December	1st to 10th	Night	22% 20% 6% 11% 4 3 3 3 4 5% 18%	2660
	11th to 20th	Night	20% 19% 7% 11% 5% 3 4 3 4 5% 19%	3285
	21st to end	Night	27% 22% 7% 9% 5% 4 4 3 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	58% 6% 10% 22% 5%	3435
	11th to 20th	Day	54% 5% 14% 20% 6% 1	3480
	21st to end	Day	55% 5% 9% 19% 12% 1	3815
April	1st to 10th	Day	53% 6% 11% 16% 12% 1	2182
	11th to 20th	Day	52% 5% 10% 18% 12% 2	2320
	21st to end	Day	52% 4 11% 20% 10% 2	2470
May	1st to 10th	Day	55% 6% 9% 20% 9% 1	4120
	11th to 20th	Day	51% 8% 10% 20% 9% 1	4085
	21st to end	Day	41% 8% 13% 22% 13% 3	4770
June	1st to 10th	Day	46% 8% 12% 23% 11% 1	2660
	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	44% 6% 13% 20% 16% 1	1910
	11th to 20th	Day	53% 5% 12% 20% 9% 1	1900
	21st to end	Day	57% 5% 11% 17% 9% 1	2090
November	1st to 10th	Day	45% 5% 12% 20% 16% 2	1764
	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% 20% 8%	3418
	11th to 20th	Night	54% 4 16% 19% 7% 1	3480
	21st to end	Night	49% 5% 10% 22% 13% 1	3817
April	1st to 10th	Night	54% 5% 9% 17% 12% 2	1996
	11th to 20th	Night	50% 6% 11% 17% 14% 3	1825
	21st to end	Night	48% 5% 11% 20% 13% 3	1710
May	1st to 10th	Night	55% 6% 8% 19% 10% 2	2840
	11th to 20th	Night	46% 6% 12% 23% 10% 2	2813
	21st to end	Night	33% 8% 14% 26% 16% 4	2805
June	1st to 10th	Night	38% 8% 12% 26% 15% 2	1520
	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	40% 6% 12% 24% 16% 2	2270
	11th to 20th	Night	46% 6% 13% 22% 11% 2	2280
	21st to end	Night	52% 5% 10% 20% 11% 2	2508
November	1st to 10th	Night	43% 5% 12% 23% 14% 3	2355
	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	35% 21% 24% 17% 3	506
April	1st to 10th	Day	27% 23% 28% 14% 7% 1	460
	11th to 20th	Day	28% 17% 25% 19% 7% 3	452
	21st to end	Day	28% 18% 31% 15% 6% 2	460
May	1st to 10th	Day	29% 19% 26% 17% 6% 3	463
	11th to 20th	Day	24% 13% 31% 18% 11% 3	487
	21st to end	Day	17% 11% 23% 28% 15% 7%	529
June	1st to 10th	Day	16% 12% 33% 25% 10% 4	490
	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% 33% 17% 10% 1	350
	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%	836
April	1st to 10th	Night	26% 14% 31% 22% 6%	760
	11th to 20th	Night	25% 12% 31% 23% 8% 1	701
	21st to end	Night	22% 10% 33% 29% 5%	570
May	1st to 10th	Night	24% 7% 25% 33% 10% 1	570
	11th to 20th	Night	21% 6% 28% 34% 10% 1	561
	21st to end	Night	13% 4 25% 36% 20% 2	452
June	1st to 10th	Night	11% 4 27% 42% 16%	380
	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% 10% 29% 28% 11% 1	749
	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

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