Westminster
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Westminster clear sky chart from 2005-11-06 to 2025-09-19.

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	5% 5% 2 5% 4 4 4 4 6% 8% 54%	135100
	Night	5% 6% 2 5% 3 3 4 3 5% 7% 57%	131420
Transparency	Day	26% 6% 10% 23% 24% 11%	130050
	Night	25% 6% 10% 24% 25% 10%	127475
Seeing	Day	8% 8% 27% 32% 19% 6%	16832
	Night	7% 4 31% 42% 15% 1	26766

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	2 7% 2 6% 3 3 3 3 5% 7% 59%	3638
	11th to 20th	Day	1 4 3 6% 5% 4 4 3 5% 7% 57%	3748
	21st to end	Day	1 5% 3 6% 3 3 3 3 5% 7% 61%	4510
February	1st to 10th	Day	2 6% 4 7% 3 3 4 4 6% 7% 54%	1800
	11th to 20th	Day	1 5% 2 7% 5% 4 4 4 5% 9% 55%	1924
	21st to end	Day	3 7% 3 6% 4 3 3 3 5% 6% 58%	1650
March	1st to 10th	Day	4 9% 2 5% 3 3 3 2 5% 6% 58%	4500
	11th to 20th	Day	5% 8% 2 6% 4 4 5% 4 6% 8% 48%	4942
	21st to end	Day	4 7% 2 5% 3 3 4 4 6% 8% 53%	5827
April	1st to 10th	Day	6% 6% 2 5% 3 3 3 3 6% 8% 55%	2400
	11th to 20th	Day	9% 9% 2 5% 3 3 4 4 5% 7% 48%	2371
	21st to end	Day	10% 7% 2 5% 3 3 4 4 7% 8% 47%	2400
May	1st to 10th	Day	10% 6% 2 6% 4 4 4 4 6% 8% 45%	5752
	11th to 20th	Day	8% 6% 2 5% 4 5% 5% 4 7% 8% 45%	6290
	21st to end	Day	7% 5% 2 6% 4 4 5% 4 7% 9% 46%	6925
June	1st to 10th	Day	5% 4 1 5% 4 4 5% 5% 8% 10% 49%	2800
	11th to 20th	Day	5% 3 1 4 3 4 6% 5% 7% 10% 51%	2800
	21st to end	Day	4 3 1 5% 4 5% 6% 5% 8% 10% 49%	2800
July	1st to 10th	Day	4 3 1 4 4 4 4 5% 6% 8% 58%	6339
	11th to 20th	Day	3 2 1 4 5% 5% 6% 6% 11% 11% 45%	6340
	21st to end	Day	5% 4 2 6% 4 5% 6% 5% 7% 8% 48%	6798
August	1st to 10th	Day	7% 4 1 6% 5% 6% 7% 5% 9% 10% 40%	5779
	11th to 20th	Day	3 2 1 4 4 4 6% 5% 8% 9% 55%	5367
	21st to end	Day	6% 5% 2 6% 4 5% 6% 5% 7% 9% 45%	5984
September	1st to 10th	Day	5% 3 1 4 3 4 4 3 5% 7% 61%	2400
	11th to 20th	Day	5% 4 1 5% 3 2 4 5% 6% 7% 59%	2353
	21st to end	Day	6% 4 2 3 2 3 3 3 3 6% 65%	2090
October	1st to 10th	Day	4 2 2 2 2 2 2 3 5% 77%	3201
	11th to 20th	Day	4 4 2 4 3 3 3 3 5% 7% 62%	3150
	21st to end	Day	5% 6% 1 3 2 2 2 3 3 6% 66%	3314
November	1st to 10th	Day	3 5% 1 4 3 2 3 3 5% 7% 64%	1763
	11th to 20th	Day	2 5% 1 4 2 2 3 3 6% 8% 64%	1800
	21st to end	Day	3 6% 2 5% 3 3 2 3 5% 7% 60%	1695
December	1st to 10th	Day	1 4 1 4 3 3 3 2 4 6% 70%	2620
	11th to 20th	Day	2 4 1 3 2 2 3 2 5% 7% 69%	3026
	21st to end	Day	2 7% 3 6% 4 4 4 4 5% 8% 53%	4004
January	1st to 10th	Night	2 9% 3 7% 3 3 3 2 4 7% 57%	6327
	11th to 20th	Night	1 5% 2 6% 4 4 4 3 5% 7% 60%	6276
	21st to end	Night	3 6% 3 7% 4 3 3 3 4 5% 60%	6468
February	1st to 10th	Night	2 8% 3 6% 3 4 4 3 5% 7% 56%	2600
	11th to 20th	Night	2 8% 3 7% 6% 4 3 3 4 7% 52%	2361
	21st to end	Night	3 8% 2 6% 4 3 2 2 4 6% 59%	1980
March	1st to 10th	Night	5% 10% 3 6% 3 3 3 3 4 5% 55%	5374
	11th to 20th	Night	8% 9% 2 5% 3 3 3 3 4 6% 55%	5054
	21st to end	Night	8% 9% 2 5% 3 3 4 3 5% 7% 51%	5131
April	1st to 10th	Night	6% 7% 2 4 3 3 4 3 5% 7% 56%	2000
	11th to 20th	Night	11% 8% 2 5% 3 4 3 3 4 7% 51%	1977
	21st to end	Night	13% 7% 3 5% 3 3 4 3 5% 6% 48%	2000
May	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 6% 7% 45%	4227
	11th to 20th	Night	9% 7% 2 6% 4 3 4 4 5% 8% 47%	3621
	21st to end	Night	10% 6% 2 5% 3 3 3 3 5% 5% 55%	3972
June	1st to 10th	Night	6% 3 1 3 3 2 5% 4 6% 10% 57%	1600
	11th to 20th	Night	5% 3 1 3 2 3 4 3 6% 8% 62%	1600
	21st to end	Night	5% 3 1 3 3 3 5% 4 6% 8% 59%	1600
July	1st to 10th	Night	3 3 1 3 4 3 4 4 5% 8% 63%	3640
	11th to 20th	Night	4 3 2 3 3 4 5% 4 8% 11% 53%	3640
	21st to end	Night	4 4 2 4 4 4 5% 4 8% 9% 52%	4059
August	1st to 10th	Night	6% 4 2 5% 3 4 5% 4 7% 8% 51%	4033
	11th to 20th	Night	3 2 1 3 4 4 6% 4 7% 11% 56%	4479
	21st to end	Night	6% 5% 2 6% 5% 5% 6% 5% 7% 9% 45%	4993
September	1st to 10th	Night	6% 4 1 4 3 3 4 3 5% 7% 60%	2000
	11th to 20th	Night	7% 3 1 3 2 3 2 3 5% 7% 64%	2003
	21st to end	Night	7% 3 1 4 3 2 3 3 3 6% 65%	2090
October	1st to 10th	Night	5% 3 1 2 1 1 2 2 4 4 74%	3765
	11th to 20th	Night	6% 4 1 4 3 3 3 3 5% 7% 63%	3778
	21st to end	Night	6% 6% 1 4 2 2 3 2 3 6% 65%	4212
November	1st to 10th	Night	4 4 1 4 3 2 3 3 5% 7% 65%	2535
	11th to 20th	Night	3 5% 1 4 3 2 3 4 4 6% 65%	2600
	21st to end	Night	4 6% 2 5% 3 2 3 3 4 6% 62%	2700
December	1st to 10th	Night	1 4 2 3 2 3 2 2 3 5% 73%	4570
	11th to 20th	Night	1 4 2 4 3 2 3 2 5% 8% 66%	5181
	21st to end	Night	4 9% 3 6% 4 3 4 3 5% 6% 53%	6974

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	23% 5% 9% 34% 21% 9%	3639
	11th to 20th	Day	25% 6% 10% 23% 26% 10%	3748
	21st to end	Day	23% 4 10% 26% 25% 11%	4510
February	1st to 10th	Day	29% 5% 12% 26% 22% 8%	1800
	11th to 20th	Day	26% 6% 9% 28% 26% 6%	1934
	21st to end	Day	25% 5% 8% 21% 28% 12%	1650
March	1st to 10th	Day	27% 4 7% 18% 29% 16%	4500
	11th to 20th	Day	30% 8% 11% 21% 26% 5%	4942
	21st to end	Day	26% 7% 13% 35% 17% 3	5827
April	1st to 10th	Day	26% 6% 12% 25% 22% 9%	2400
	11th to 20th	Day	33% 7% 11% 21% 19% 9%	2374
	21st to end	Day	31% 7% 13% 24% 21% 2	2400
May	1st to 10th	Day	33% 8% 12% 26% 16% 5%	5752
	11th to 20th	Day	31% 9% 12% 19% 20% 9%	6294
	21st to end	Day	30% 9% 14% 23% 19% 5%	6926
June	1st to 10th	Day	26% 9% 15% 23% 17% 8%	2800
	11th to 20th	Day	25% 8% 11% 19% 23% 14%	2800
	21st to end	Day	25% 7% 12% 20% 23% 12%	2800
July	1st to 10th	Day	21% 6% 8% 16% 28% 21%	6271
	11th to 20th	Day	26% 9% 14% 22% 19% 11%	6265
	21st to end	Day	31% 6% 11% 18% 18% 16%	6602
August	1st to 10th	Day	36% 9% 14% 20% 16% 6%	4208
	11th to 20th	Day	25% 7% 14% 17% 21% 15%	3883
	21st to end	Day	27% 6% 11% 19% 24% 13%	4311
September	1st to 10th	Day	22% 5% 8% 21% 26% 18%	2400
	11th to 20th	Day	23% 6% 7% 21% 27% 16%	2353
	21st to end	Day	21% 4 7% 19% 36% 13%	2090
October	1st to 10th	Day	14% 3 6% 15% 37% 26%	3201
	11th to 20th	Day	23% 4 8% 26% 30% 9%	3150
	21st to end	Day	21% 3 5% 23% 37% 11%	3314
November	1st to 10th	Day	19% 3 9% 25% 28% 15%	1763
	11th to 20th	Day	19% 5% 7% 26% 34% 10%	1800
	21st to end	Day	24% 3 7% 23% 30% 13%	1695
December	1st to 10th	Day	18% 3 7% 33% 28% 10%	2620
	11th to 20th	Day	15% 5% 6% 31% 34% 9%	3024
	21st to end	Day	26% 6% 7% 25% 26% 9%	4004
January	1st to 10th	Night	27% 5% 7% 30% 24% 7%	6327
	11th to 20th	Night	21% 7% 10% 25% 24% 14%	6276
	21st to end	Night	26% 5% 10% 23% 25% 11%	6468
February	1st to 10th	Night	27% 5% 9% 28% 24% 6%	2600
	11th to 20th	Night	31% 6% 7% 28% 22% 6%	2361
	21st to end	Night	26% 4 5% 23% 27% 14%	1980
March	1st to 10th	Night	29% 4 6% 17% 33% 10%	5374
	11th to 20th	Night	30% 6% 9% 15% 28% 13%	5054
	21st to end	Night	31% 6% 9% 34% 18% 2	5131
April	1st to 10th	Night	27% 6% 10% 24% 23% 10%	2000
	11th to 20th	Night	32% 7% 10% 23% 20% 8%	1977
	21st to end	Night	34% 7% 10% 25% 20% 4	2000
May	1st to 10th	Night	35% 8% 13% 26% 12% 6%	4224
	11th to 20th	Night	32% 8% 15% 21% 17% 7%	3622
	21st to end	Night	27% 9% 18% 25% 18% 4	3972
June	1st to 10th	Night	21% 6% 19% 31% 14% 8%	1600
	11th to 20th	Night	20% 5% 14% 21% 24% 16%	1600
	21st to end	Night	20% 7% 11% 24% 28% 10%	1600
July	1st to 10th	Night	19% 5% 10% 18% 35% 12%	3600
	11th to 20th	Night	22% 7% 17% 27% 20% 7%	3592
	21st to end	Night	25% 6% 16% 22% 20% 12%	3944
August	1st to 10th	Night	33% 7% 15% 23% 17% 5%	2928
	11th to 20th	Night	20% 9% 20% 21% 21% 10%	3239
	21st to end	Night	25% 6% 14% 22% 27% 6%	3597
September	1st to 10th	Night	23% 6% 9% 22% 24% 16%	2000
	11th to 20th	Night	19% 5% 10% 23% 27% 15%	2003
	21st to end	Night	22% 4 8% 21% 33% 12%	2090
October	1st to 10th	Night	15% 4 6% 19% 32% 24%	3765
	11th to 20th	Night	21% 3 9% 26% 30% 10%	3778
	21st to end	Night	23% 3 5% 23% 34% 13%	4212
November	1st to 10th	Night	19% 5% 8% 25% 28% 15%	2535
	11th to 20th	Night	20% 4 7% 26% 34% 9%	2600
	21st to end	Night	24% 4 8% 22% 32% 10%	2700
December	1st to 10th	Night	16% 3 7% 33% 33% 8%	4570
	11th to 20th	Night	18% 4 6% 33% 34% 5%	5182
	21st to end	Night	29% 7% 8% 22% 26% 8%	6974

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	5% 10% 28% 43% 11% 4	300
	11th to 20th	Day	7% 8% 37% 35% 13% 1	300
	21st to end	Day	4 8% 41% 32% 13% 2	330
February	1st to 10th	Day	5% 15% 43% 29% 6% 2	300
	11th to 20th	Day	4 7% 46% 35% 8% 1	300
	21st to end	Day	6% 13% 39% 35% 7%	249
March	1st to 10th	Day	5% 10% 34% 38% 12% 2	300
	11th to 20th	Day	13% 15% 30% 29% 12% 2	468
	21st to end	Day	13% 20% 31% 28% 7% 2	550
April	1st to 10th	Day	12% 17% 31% 29% 10% 2	500
	11th to 20th	Day	15% 19% 34% 23% 7% 2	492
	21st to end	Day	15% 14% 31% 28% 12% 1	500
May	1st to 10th	Day	15% 10% 28% 31% 13% 2	633
	11th to 20th	Day	18% 8% 22% 30% 17% 5%	725
	21st to end	Day	9% 10% 25% 31% 19% 6%	789
June	1st to 10th	Day	6% 5% 21% 32% 26% 10%	730
	11th to 20th	Day	6% 8% 26% 34% 21% 5%	730
	21st to end	Day	6% 5% 25% 33% 24% 7%	730
July	1st to 10th	Day	7% 3 17% 30% 33% 10%	721
	11th to 20th	Day	5% 3 15% 31% 36% 10%	718
	21st to end	Day	5% 3 15% 28% 38% 11%	751
August	1st to 10th	Day	6% 5% 25% 29% 24% 11%	527
	11th to 20th	Day	4 4 22% 32% 27% 10%	517
	21st to end	Day	8% 3 19% 38% 24% 8%	583
September	1st to 10th	Day	5% 3 20% 35% 26% 10%	530
	11th to 20th	Day	5% 6% 24% 36% 22% 8%	499
	21st to end	Day	6% 6% 22% 34% 22% 9%	300
October	1st to 10th	Day	9% 5% 22% 30% 25% 10%	300
	11th to 20th	Day	4 8% 27% 39% 17% 5%	300
	21st to end	Day	12% 10% 26% 34% 15% 2	330
November	1st to 10th	Day	6% 7% 32% 36% 14% 5%	300
	11th to 20th	Day	8% 9% 29% 39% 12% 3	300
	21st to end	Day	7% 8% 31% 38% 14% 2	300
December	1st to 10th	Day	7% 9% 31% 35% 15% 3	300
	11th to 20th	Day	5% 8% 39% 41% 6% 1	300
	21st to end	Day	7% 9% 38% 37% 7% 2	330
January	1st to 10th	Night	4 5% 41% 39% 10% 2	1000
	11th to 20th	Night	3 6% 48% 33% 8% 1	975
	21st to end	Night	5% 4 42% 40% 8% 1	880
February	1st to 10th	Night	6% 4 50% 35% 6%	800
	11th to 20th	Night	4 5% 46% 38% 6%	784
	21st to end	Night	7% 7% 45% 38% 3	660
March	1st to 10th	Night	4 5% 43% 39% 9%	800
	11th to 20th	Night	10% 5% 33% 42% 11%	800
	21st to end	Night	9% 8% 39% 38% 6%	729
April	1st to 10th	Night	10% 9% 39% 38% 5%	600
	11th to 20th	Night	13% 8% 33% 37% 9%	593
	21st to end	Night	15% 8% 30% 36% 11% 1	600
May	1st to 10th	Night	14% 7% 23% 42% 13% 1	600
	11th to 20th	Night	15% 4 18% 46% 16% 2	591
	21st to end	Night	10% 5% 24% 39% 22% 1	650
June	1st to 10th	Night	6% 4 16% 45% 26% 3	600
	11th to 20th	Night	6% 2 23% 51% 17% 1	600
	21st to end	Night	5% 2 16% 47% 28% 2	600
July	1st to 10th	Night	6% 3 13% 40% 36% 2	585
	11th to 20th	Night	3 1 10% 46% 37% 2	582
	21st to end	Night	5% 2 7% 44% 39% 3	657
August	1st to 10th	Night	7% 2 10% 44% 35% 2	597
	11th to 20th	Night	4 2 14% 46% 32% 2	594
	21st to end	Night	4 1 12% 47% 33% 3	660
September	1st to 10th	Night	6% 3 12% 47% 27% 5%	600
	11th to 20th	Night	7% 3 14% 50% 23% 3	594
	21st to end	Night	7% 2 21% 46% 22% 3	570
October	1st to 10th	Night	11% 4 22% 42% 19% 3	738
	11th to 20th	Night	6% 2 29% 46% 15% 1	760
	21st to end	Night	10% 5% 30% 41% 13% 2	892
November	1st to 10th	Night	5% 4 35% 44% 12% 2	975
	11th to 20th	Night	6% 6% 38% 44% 7% 1	1000
	21st to end	Night	7% 4 35% 44% 10% 1	1000
December	1st to 10th	Night	4 5% 40% 42% 9%	1000
	11th to 20th	Night	5% 6% 44% 38% 7% 1	1000
	21st to end	Night	7% 6% 42% 37% 7% 1	1100

Page generated at 2025-09-15 23:49:16. Version ac07

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