St Albans City
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the St Albans City clear sky chart from 2005-11-06 to 2026-07-01.

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 9% 5% 5% 5% 4 5% 6% 28%	137899
	Night	18% 14% 5% 11% 6% 5% 4 3 4 5% 26%	134474
Transparency	Day	55% 4 11% 21% 8% 1	141357
	Night	61% 3 10% 17% 8% 1	139105
Seeing	Day	27% 15% 29% 19% 8% 2	16718
	Night	28% 13% 26% 24% 9% 1	25677

By Month

Percentage Cloud Cover NE (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	13% 21% 8% 18% 7% 6% 4 3 3 4 13%	4661
	11th to 20th	Day	10% 17% 8% 19% 9% 6% 5% 3 4 4 15%	4650
	21st to end	Day	11% 15% 6% 13% 10% 11% 5% 3 4 5% 16%	5124
February	1st to 10th	Day	15% 14% 5% 15% 9% 6% 4 3 4 5% 18%	1832
	11th to 20th	Day	13% 17% 6% 15% 6% 6% 4 3 4 5% 20%	2003
	21st to end	Day	14% 17% 7% 12% 7% 6% 3 3 4 5% 22%	1730
March	1st to 10th	Day	20% 15% 5% 11% 6% 5% 5% 3 4 5% 21%	4576
	11th to 20th	Day	13% 17% 4 10% 5% 5% 5% 3 5% 6% 27%	4771
	21st to end	Day	19% 13% 5% 10% 5% 4 4 3 4 5% 28%	6102
April	1st to 10th	Day	22% 14% 5% 9% 5% 4 3 3 4 5% 28%	2520
	11th to 20th	Day	20% 12% 3 7% 5% 5% 4 3 4 5% 33%	2494
	21st to end	Day	24% 10% 3 7% 4 4 4 3 4 6% 31%	2705
May	1st to 10th	Day	27% 11% 3 8% 5% 5% 5% 4 5% 6% 22%	7912
	11th to 20th	Day	20% 8% 2 8% 6% 5% 5% 4 6% 7% 30%	8221
	21st to end	Day	21% 9% 3 7% 4 4 5% 4 6% 6% 31%	9060
June	1st to 10th	Day	24% 7% 2 7% 5% 4 5% 3 6% 8% 29%	2940
	11th to 20th	Day	17% 6% 2 6% 4 5% 6% 5% 7% 8% 34%	2940
	21st to end	Day	22% 7% 3 7% 5% 5% 5% 4 6% 8% 30%	2940
July	1st to 10th	Day	14% 6% 3 7% 5% 6% 6% 5% 8% 10% 31%	6360
	11th to 20th	Day	12% 6% 2 6% 4 5% 5% 5% 7% 10% 37%	6360
	21st to end	Day	14% 6% 2 5% 4 4 5% 4 5% 8% 42%	6926
August	1st to 10th	Day	17% 8% 3 7% 4 5% 5% 4 7% 8% 32%	4563
	11th to 20th	Day	16% 7% 3 7% 5% 4 5% 4 6% 8% 35%	4078
	21st to end	Day	15% 6% 2 6% 4 4 5% 3 6% 9% 41%	4312
September	1st to 10th	Day	16% 6% 2 5% 4 4 4 4 6% 6% 45%	2400
	11th to 20th	Day	16% 6% 1 5% 3 3 4 3 4 6% 48%	2285
	21st to end	Day	17% 6% 2 6% 4 5% 5% 3 6% 7% 40%	2200
October	1st to 10th	Day	22% 7% 3 7% 4 4 5% 3 5% 6% 33%	2112
	11th to 20th	Day	21% 10% 2 6% 4 4 6% 4 4 6% 31%	1938
	21st to end	Day	26% 15% 4 7% 5% 4 3 3 4 4 25%	1980
November	1st to 10th	Day	20% 17% 3 8% 4 4 4 3 5% 5% 28%	1851
	11th to 20th	Day	18% 17% 5% 9% 5% 4 5% 3 4 5% 24%	1879
	21st to end	Day	19% 18% 8% 10% 6% 4 4 4 4 4 19%	1680
December	1st to 10th	Day	16% 19% 8% 19% 6% 6% 4 3 4 4 11%	2680
	11th to 20th	Day	17% 15% 6% 13% 5% 5% 4 3 5% 5% 22%	3066
	21st to end	Day	20% 16% 5% 13% 7% 5% 4 4 5% 5% 16%	4048
January	1st to 10th	Night	13% 22% 10% 18% 6% 4 3 2 3 3 15%	8234
	11th to 20th	Night	12% 21% 9% 20% 7% 6% 5% 3 4 4 11%	8235
	21st to end	Night	12% 16% 5% 13% 12% 8% 5% 3 4 4 17%	9057
February	1st to 10th	Night	16% 17% 6% 16% 8% 5% 4 3 3 5% 18%	2784
	11th to 20th	Night	14% 18% 6% 13% 8% 6% 5% 3 4 3 21%	2501
	21st to end	Night	15% 17% 7% 11% 6% 4 4 3 3 4 26%	2076
March	1st to 10th	Night	19% 16% 6% 10% 7% 4 3 2 4 4 24%	5484
	11th to 20th	Night	16% 17% 4 8% 6% 4 4 3 5% 6% 27%	5405
	21st to end	Night	17% 18% 5% 12% 5% 4 3 2 3 3 26%	5072
April	1st to 10th	Night	23% 13% 4 8% 4 3 4 3 5% 5% 27%	2100
	11th to 20th	Night	26% 12% 3 8% 5% 3 4 3 3 4 29%	2075
	21st to end	Night	22% 13% 3 6% 4 4 3 3 4 5% 33%	1914
May	1st to 10th	Night	25% 12% 3 8% 5% 5% 4 3 5% 7% 23%	5029
	11th to 20th	Night	20% 9% 3 8% 5% 4 4 3 6% 7% 29%	4639
	21st to end	Night	20% 10% 3 7% 5% 4 4 4 5% 7% 31%	5114
June	1st to 10th	Night	26% 7% 2 7% 4 4 4 3 6% 7% 30%	1680
	11th to 20th	Night	19% 6% 3 6% 4 4 5% 4 6% 8% 35%	1680
	21st to end	Night	21% 8% 3 7% 4 4 4 5% 6% 7% 32%	1680
July	1st to 10th	Night	14% 6% 2 7% 6% 5% 5% 5% 7% 8% 34%	3600
	11th to 20th	Night	16% 7% 3 6% 4 4 5% 4 6% 8% 39%	3600
	21st to end	Night	14% 7% 3 7% 5% 5% 5% 4 6% 7% 39%	3926
August	1st to 10th	Night	20% 8% 3 7% 4 4 4 3 5% 7% 35%	2586
	11th to 20th	Night	14% 8% 3 9% 6% 5% 5% 4 5% 6% 35%	2935
	21st to end	Night	17% 6% 2 5% 4 5% 4 4 6% 7% 41%	3586
September	1st to 10th	Night	16% 5% 2 6% 4 5% 4 3 6% 8% 43%	2000
	11th to 20th	Night	15% 6% 2 5% 4 3 4 3 5% 6% 48%	2114
	21st to end	Night	20% 6% 2 5% 3 5% 3 4 5% 5% 42%	2200
October	1st to 10th	Night	23% 8% 2 6% 4 5% 4 3 5% 6% 35%	2285
	11th to 20th	Night	23% 10% 3 7% 5% 4 4 3 4 5% 31%	2461
	21st to end	Night	27% 12% 3 7% 5% 4 5% 3 4 4 26%	2860
November	1st to 10th	Night	22% 13% 3 7% 4 5% 4 3 4 6% 29%	2667
	11th to 20th	Night	20% 15% 5% 9% 5% 4 4 3 4 5% 27%	2737
	21st to end	Night	21% 18% 5% 10% 5% 4 4 3 5% 4 21%	2940
December	1st to 10th	Night	16% 27% 10% 14% 5% 4 4 3 3 3 11%	4720
	11th to 20th	Night	22% 19% 5% 13% 5% 5% 3 2 3 3 19%	5348
	21st to end	Night	19% 18% 6% 12% 5% 5% 4 3 3 4 19%	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	75% 4 10% 10% 1	4662
	11th to 20th	Day	74% 3 8% 10% 5% 1	4680
	21st to end	Day	69% 4 9% 11% 7%	5124
February	1st to 10th	Day	68% 5% 6% 15% 6% 1	1832
	11th to 20th	Day	66% 3 9% 15% 6% 1	2012
	21st to end	Day	65% 3 8% 14% 7% 2	1730
March	1st to 10th	Day	65% 3 8% 14% 6% 3	4579
	11th to 20th	Day	58% 4 13% 15% 9% 1	4771
	21st to end	Day	59% 4 5% 15% 15% 1	6102
April	1st to 10th	Day	60% 3 6% 20% 9% 2	2520
	11th to 20th	Day	55% 3 7% 19% 15% 1	2494
	21st to end	Day	55% 3 9% 19% 11% 3	2705
May	1st to 10th	Day	63% 4 7% 18% 8%	7924
	11th to 20th	Day	53% 4 13% 22% 8% 1	8225
	21st to end	Day	52% 4 11% 24% 9% 1	9061
June	1st to 10th	Day	53% 4 11% 21% 10% 1	2940
	11th to 20th	Day	46% 5% 11% 24% 13% 2	2940
	21st to end	Day	52% 4 11% 22% 9% 1	2940
July	1st to 10th	Day	47% 4 15% 24% 9%	6290
	11th to 20th	Day	41% 5% 13% 31% 10%	6285
	21st to end	Day	40% 5% 12% 35% 8%	6728
August	1st to 10th	Day	49% 4 16% 23% 8%	4549
	11th to 20th	Day	47% 4 17% 25% 7%	4118
	21st to end	Day	41% 4 18% 22% 15%	4312
September	1st to 10th	Day	39% 5% 11% 32% 12% 1	2400
	11th to 20th	Day	38% 3 9% 34% 15% 1	2285
	21st to end	Day	43% 4 15% 26% 11% 1	2200
October	1st to 10th	Day	46% 3 12% 30% 9%	3437
	11th to 20th	Day	50% 3 15% 24% 8%	3128
	21st to end	Day	52% 3 14% 24% 6% 1	3180
November	1st to 10th	Day	57% 4 12% 19% 7% 1	1851
	11th to 20th	Day	63% 4 10% 14% 8% 1	1879
	21st to end	Day	69% 4 12% 11% 4	1680
December	1st to 10th	Day	79% 2 8% 9% 2	2680
	11th to 20th	Day	65% 3 9% 18% 5%	3066
	21st to end	Day	69% 4 8% 12% 7%	4048
January	1st to 10th	Night	76% 2 10% 9% 3	8234
	11th to 20th	Night	76% 4 7% 9% 4	8280
	21st to end	Night	71% 3 7% 10% 8%	9057
February	1st to 10th	Night	70% 3 7% 13% 6% 1	2784
	11th to 20th	Night	67% 4 8% 15% 6%	2503
	21st to end	Night	63% 3 8% 15% 10% 1	2076
March	1st to 10th	Night	64% 3 8% 13% 11% 1	5494
	11th to 20th	Night	59% 4 12% 15% 9% 1	5406
	21st to end	Night	63% 4 4 17% 11% 1	5072
April	1st to 10th	Night	59% 3 8% 17% 11% 2	2100
	11th to 20th	Night	60% 2 7% 17% 12% 1	2075
	21st to end	Night	54% 3 8% 17% 15% 3	1914
May	1st to 10th	Night	62% 3 10% 17% 7% 1	5035
	11th to 20th	Night	55% 3 10% 23% 8% 1	4640
	21st to end	Night	53% 3 8% 21% 13% 1	5115
June	1st to 10th	Night	53% 4 9% 22% 11% 1	1680
	11th to 20th	Night	47% 4 12% 23% 13% 2	1680
	21st to end	Night	51% 5% 12% 21% 11%	1680
July	1st to 10th	Night	47% 3 17% 23% 9%	3560
	11th to 20th	Night	43% 4 13% 28% 11%	3552
	21st to end	Night	44% 4 13% 26% 12%	3809
August	1st to 10th	Night	49% 4 14% 25% 8%	2576
	11th to 20th	Night	50% 4 16% 21% 9% 1	2961
	21st to end	Night	42% 3 15% 25% 14% 1	3586
September	1st to 10th	Night	40% 5% 13% 30% 12% 1	2000
	11th to 20th	Night	37% 3 14% 33% 12% 1	2114
	21st to end	Night	43% 5% 11% 26% 14% 1	2200
October	1st to 10th	Night	53% 3 10% 25% 9%	3740
	11th to 20th	Night	52% 2 10% 24% 11%	4010
	21st to end	Night	53% 3 13% 21% 10%	4610
November	1st to 10th	Night	58% 3 11% 19% 8% 1	2667
	11th to 20th	Night	60% 4 9% 16% 9% 1	2737
	21st to end	Night	67% 4 11% 14% 4	2940
December	1st to 10th	Night	80% 2 8% 8% 2	4720
	11th to 20th	Night	72% 3 7% 12% 7%	5348
	21st to end	Night	70% 3 8% 12% 6%	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	33% 16% 30% 16% 5%	220
	11th to 20th	Day	34% 23% 30% 10% 2 1	220
	21st to end	Day	29% 20% 30% 16% 5%	242
February	1st to 10th	Day	32% 24% 31% 11% 3	300
	11th to 20th	Day	27% 27% 32% 12% 2	491
	21st to end	Day	29% 23% 33% 13% 2	446
March	1st to 10th	Day	34% 16% 30% 16% 3	540
	11th to 20th	Day	32% 22% 28% 14% 4 1	540
	21st to end	Day	35% 21% 24% 14% 5% 1	594
April	1st to 10th	Day	35% 20% 24% 17% 3 1	540
	11th to 20th	Day	34% 18% 26% 14% 6% 2	532
	21st to end	Day	33% 17% 30% 14% 5% 2	540
May	1st to 10th	Day	34% 16% 26% 16% 5% 2	543
	11th to 20th	Day	26% 13% 30% 21% 9% 1	567
	21st to end	Day	22% 15% 32% 23% 6% 1	617
June	1st to 10th	Day	28% 10% 32% 22% 7% 1	570
	11th to 20th	Day	22% 13% 34% 20% 8% 2	570
	21st to end	Day	26% 10% 34% 21% 8% 1	570
July	1st to 10th	Day	17% 6% 33% 32% 10% 2	525
	11th to 20th	Day	17% 7% 36% 26% 12% 2	524
	21st to end	Day	20% 6% 33% 30% 9% 2	576
August	1st to 10th	Day	15% 7% 29% 32% 15% 2	526
	11th to 20th	Day	18% 5% 27% 30% 16% 4	519
	21st to end	Day	17% 9% 31% 24% 13% 6%	583
September	1st to 10th	Day	18% 11% 29% 24% 11% 7%	530
	11th to 20th	Day	17% 9% 22% 24% 21% 7%	530
	21st to end	Day	17% 11% 29% 23% 14% 6%	530
October	1st to 10th	Day	25% 13% 26% 18% 15% 3	530
	11th to 20th	Day	31% 17% 24% 18% 9% 1	495
	21st to end	Day	37% 14% 26% 18% 6%	462
November	1st to 10th	Day	38% 16% 27% 13% 6%	425
	11th to 20th	Day	32% 21% 25% 15% 7% 1	419
	21st to end	Day	38% 23% 24% 12% 3	220
December	1st to 10th	Day	29% 22% 29% 16% 4	220
	11th to 20th	Day	31% 23% 36% 8% 2	220
	21st to end	Day	31% 24% 24% 17% 3 1	242
January	1st to 10th	Night	32% 17% 30% 18% 4	840
	11th to 20th	Night	32% 20% 25% 18% 5%	840
	21st to end	Night	25% 16% 30% 24% 5% 1	924
February	1st to 10th	Night	25% 16% 31% 23% 4	840
	11th to 20th	Night	26% 17% 33% 21% 3	823
	21st to end	Night	26% 16% 30% 23% 5%	692
March	1st to 10th	Night	30% 15% 29% 22% 3	840
	11th to 20th	Night	33% 18% 23% 20% 5%	840
	21st to end	Night	31% 16% 24% 22% 7% 1	924
April	1st to 10th	Night	32% 13% 28% 22% 5%	840
	11th to 20th	Night	34% 15% 23% 21% 7% 1	831
	21st to end	Night	31% 13% 29% 20% 7%	654
May	1st to 10th	Night	30% 10% 27% 25% 7% 1	555
	11th to 20th	Night	25% 7% 31% 27% 8% 1	414
	21st to end	Night	21% 9% 30% 31% 8% 1	455
June	1st to 10th	Night	29% 6% 25% 30% 9%	420
	11th to 20th	Night	23% 5% 30% 30% 12% 1	420
	21st to end	Night	23% 5% 27% 34% 10%	420
July	1st to 10th	Night	17% 6% 28% 35% 14% 1	390
	11th to 20th	Night	12% 3 25% 43% 15% 2	388
	21st to end	Night	18% 4 20% 45% 13%	438
August	1st to 10th	Night	17% 2 19% 43% 18% 1	398
	11th to 20th	Night	17% 4 20% 37% 20% 2	627
	21st to end	Night	18% 5% 23% 33% 19% 2	880
September	1st to 10th	Night	19% 6% 25% 32% 16% 2	800
	11th to 20th	Night	19% 6% 20% 29% 23% 3	800
	21st to end	Night	24% 8% 25% 23% 18% 2	800
October	1st to 10th	Night	27% 9% 26% 24% 13% 1	800
	11th to 20th	Night	29% 11% 26% 25% 8% 1	800
	21st to end	Night	33% 14% 27% 22% 5%	880
November	1st to 10th	Night	33% 15% 29% 18% 5% 1	820
	11th to 20th	Night	32% 16% 25% 18% 8% 1	840
	21st to end	Night	35% 18% 25% 16% 5% 1	840
December	1st to 10th	Night	36% 19% 25% 16% 4	840
	11th to 20th	Night	32% 18% 27% 17% 5% 1	840
	21st to end	Night	37% 18% 22% 19% 3	924

Page generated at 2026-06-28 02:10:45. Version ac07

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