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 2025-02-25.

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% 5% 4 5% 6% 29%	108601
	Night	18% 14% 5% 10% 5% 5% 4 3 4 5% 26%	110080
Transparency	Day	55% 4 11% 20% 8% 1	112105
	Night	60% 3 10% 17% 9% 1	114740
Seeing	Day	26% 15% 29% 20% 8% 2	14948
	Night	28% 13% 26% 24% 9% 1	24056

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	14% 22% 9% 18% 6% 5% 3 3 3 3 14%	3597
	11th to 20th	Day	11% 18% 9% 19% 6% 5% 5% 3 4 4 16%	3570
	21st to end	Day	14% 18% 7% 13% 5% 9% 5% 3 4 5% 17%	3960
February	1st to 10th	Day	15% 15% 6% 14% 9% 6% 5% 3 4 5% 18%	1745
	11th to 20th	Day	13% 17% 6% 15% 6% 6% 4 3 4 5% 20%	1905
	21st to end	Day	15% 16% 6% 11% 7% 5% 4 3 4 4 23%	1610
March	1st to 10th	Day	23% 15% 4 9% 6% 4 4 3 4 5% 22%	3116
	11th to 20th	Day	14% 22% 5% 12% 5% 5% 5% 3 3 5% 22%	3266
	21st to end	Day	17% 12% 4 10% 5% 4 3 2 4 5% 33%	4166
April	1st to 10th	Day	22% 13% 4 9% 5% 4 3 3 4 5% 29%	2280
	11th to 20th	Day	20% 11% 2 6% 5% 5% 4 3 4 5% 34%	2254
	21st to end	Day	25% 10% 3 7% 4 4 4 3 4 5% 31%	2447
May	1st to 10th	Day	24% 10% 3 8% 5% 4 4 4 5% 6% 27%	4255
	11th to 20th	Day	18% 8% 3 8% 6% 5% 5% 5% 5% 7% 29%	4398
	21st to end	Day	15% 6% 3 6% 4 4 6% 5% 7% 8% 35%	4837
June	1st to 10th	Day	25% 7% 2 7% 5% 5% 5% 3 6% 8% 28%	2660
	11th to 20th	Day	17% 6% 2 6% 4 4 6% 5% 7% 8% 34%	2660
	21st to end	Day	22% 8% 3 7% 5% 5% 5% 4 6% 8% 29%	2660
July	1st to 10th	Day	13% 5% 3 6% 5% 5% 6% 5% 8% 10% 35%	4440
	11th to 20th	Day	13% 7% 2 6% 5% 5% 5% 5% 7% 10% 34%	4440
	21st to end	Day	14% 7% 2 6% 5% 5% 5% 4 5% 7% 40%	4814
August	1st to 10th	Day	18% 8% 3 8% 4 5% 5% 4 6% 8% 31%	4423
	11th to 20th	Day	16% 7% 3 7% 5% 4 5% 4 6% 9% 34%	3951
	21st to end	Day	15% 6% 2 6% 4 4 5% 3 6% 9% 41%	4180
September	1st to 10th	Day	16% 6% 2 5% 4 4 4 4 6% 6% 44%	2280
	11th to 20th	Day	17% 6% 1 5% 3 3 4 3 4 6% 47%	2171
	21st to end	Day	17% 6% 2 6% 4 5% 5% 3 6% 7% 39%	2090
October	1st to 10th	Day	23% 8% 3 8% 4 4 5% 3 5% 6% 30%	2007
	11th to 20th	Day	22% 10% 2 6% 4 4 6% 5% 4 6% 31%	1841
	21st to end	Day	26% 16% 3 7% 5% 3 3 3 4 4 25%	1881
November	1st to 10th	Day	20% 16% 3 8% 4 4 4 3 5% 6% 29%	1761
	11th to 20th	Day	18% 16% 5% 9% 5% 4 5% 3 4 5% 25%	1790
	21st to end	Day	19% 18% 7% 10% 6% 4 4 4 4 4 20%	1600
December	1st to 10th	Day	17% 19% 8% 19% 6% 5% 4 3 4 4 11%	2600
	11th to 20th	Day	18% 15% 6% 12% 5% 5% 4 3 5% 5% 22%	2986
	21st to end	Day	20% 16% 5% 13% 6% 5% 4 4 5% 5% 16%	3960
January	1st to 10th	Night	14% 22% 11% 19% 5% 4 3 2 2 3 14%	6349
	11th to 20th	Night	11% 21% 9% 20% 7% 5% 5% 3 4 4 13%	6315
	21st to end	Night	14% 19% 6% 13% 8% 7% 5% 3 4 4 17%	6996
February	1st to 10th	Night	16% 17% 6% 15% 8% 5% 4 3 3 5% 17%	2652
	11th to 20th	Night	14% 18% 5% 13% 8% 6% 5% 3 4 3 20%	2379
	21st to end	Night	15% 17% 6% 12% 6% 4 4 3 3 4 25%	1939
March	1st to 10th	Night	23% 14% 4 7% 6% 4 3 2 4 5% 28%	3724
	11th to 20th	Night	18% 21% 5% 9% 5% 4 3 2 3 4 25%	3690
	21st to end	Night	17% 17% 5% 11% 5% 3 3 2 3 3 30%	3466
April	1st to 10th	Night	23% 13% 4 9% 4 4 4 3 5% 5% 27%	1900
	11th to 20th	Night	25% 12% 3 8% 5% 4 4 3 3 5% 30%	1875
	21st to end	Night	22% 13% 3 6% 4 4 3 3 4 4 32%	1732
May	1st to 10th	Night	25% 11% 3 8% 5% 4 4 3 5% 6% 26%	2715
	11th to 20th	Night	21% 9% 3 9% 4 4 3 3 4 6% 33%	2495
	21st to end	Night	16% 6% 2 7% 5% 5% 4 5% 6% 8% 37%	2738
June	1st to 10th	Night	27% 7% 2 7% 4 4 4 3 6% 7% 29%	1520
	11th to 20th	Night	18% 6% 3 5% 4 4 5% 4 6% 9% 35%	1520
	21st to end	Night	21% 8% 3 7% 4 4 4 5% 6% 7% 31%	1520
July	1st to 10th	Night	14% 7% 2 8% 6% 5% 4 5% 7% 7% 35%	2520
	11th to 20th	Night	14% 7% 3 6% 4 5% 5% 4 6% 8% 38%	2520
	21st to end	Night	15% 7% 2 7% 5% 5% 5% 4 6% 6% 37%	2739
August	1st to 10th	Night	21% 9% 3 7% 4 4 4 3 5% 7% 34%	2506
	11th to 20th	Night	15% 8% 3 9% 6% 5% 5% 4 5% 6% 34%	2843
	21st to end	Night	17% 6% 2 5% 4 5% 4 4 5% 7% 42%	3476
September	1st to 10th	Night	16% 5% 2 6% 4 5% 4 3 6% 8% 42%	1900
	11th to 20th	Night	15% 6% 2 5% 3 3 4 3 5% 6% 48%	2008
	21st to end	Night	20% 6% 1 4 3 5% 3 4 6% 5% 42%	2090
October	1st to 10th	Night	24% 8% 2 6% 4 5% 4 3 5% 6% 33%	2171
	11th to 20th	Night	23% 10% 3 8% 5% 4 4 3 4 5% 31%	2338
	21st to end	Night	27% 12% 3 7% 5% 4 4 3 4 4 26%	2717
November	1st to 10th	Night	21% 13% 4 7% 4 5% 4 3 5% 6% 29%	2537
	11th to 20th	Night	20% 14% 4 9% 5% 4 4 3 4 5% 28%	2606
	21st to end	Night	22% 18% 5% 9% 5% 4 4 3 5% 5% 21%	2800
December	1st to 10th	Night	16% 27% 10% 14% 5% 4 4 3 3 3 11%	4580
	11th to 20th	Night	23% 19% 5% 13% 5% 5% 3 2 3 3 19%	5208
	21st to end	Night	19% 18% 6% 12% 5% 5% 4 3 4 4 19%	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	76% 3 10% 10% 1	3598
	11th to 20th	Day	73% 3 9% 9% 5% 1	3600
	21st to end	Day	69% 5% 9% 11% 6% 1	3960
February	1st to 10th	Day	68% 5% 6% 14% 5% 1	1745
	11th to 20th	Day	67% 4 9% 14% 6% 1	1914
	21st to end	Day	65% 3 8% 14% 7% 2	1610
March	1st to 10th	Day	64% 4 9% 16% 5% 1	3119
	11th to 20th	Day	65% 4 9% 11% 9% 1	3266
	21st to end	Day	54% 4 6% 18% 17% 2	4166
April	1st to 10th	Day	59% 3 6% 20% 9% 2	2280
	11th to 20th	Day	53% 3 8% 19% 15% 2	2254
	21st to end	Day	56% 3 9% 18% 11% 3	2447
May	1st to 10th	Day	57% 4 9% 20% 9% 1	4269
	11th to 20th	Day	52% 5% 14% 19% 9% 1	4402
	21st to end	Day	44% 5% 13% 26% 11% 1	4837
June	1st to 10th	Day	54% 4 11% 19% 10% 1	2660
	11th to 20th	Day	45% 5% 11% 24% 13% 2	2660
	21st to end	Day	53% 4 11% 22% 9% 1	2660
July	1st to 10th	Day	42% 4 15% 28% 10%	4370
	11th to 20th	Day	43% 4 14% 32% 6%	4365
	21st to end	Day	43% 5% 12% 30% 9%	4661
August	1st to 10th	Day	50% 4 16% 22% 8%	4409
	11th to 20th	Day	47% 4 17% 25% 6%	3991
	21st to end	Day	41% 4 18% 22% 15% 1	4180
September	1st to 10th	Day	40% 5% 11% 30% 13% 1	2280
	11th to 20th	Day	39% 4 10% 33% 14% 1	2171
	21st to end	Day	43% 4 15% 26% 10% 1	2090
October	1st to 10th	Day	47% 3 12% 29% 9%	3332
	11th to 20th	Day	50% 3 15% 24% 8%	3031
	21st to end	Day	52% 3 14% 25% 5% 1	3081
November	1st to 10th	Day	56% 4 13% 19% 7% 1	1761
	11th to 20th	Day	62% 4 10% 15% 8% 1	1790
	21st to end	Day	68% 4 12% 12% 4	1600
December	1st to 10th	Day	79% 2 9% 9% 2	2600
	11th to 20th	Day	65% 3 9% 18% 5%	2986
	21st to end	Day	69% 4 8% 12% 7%	3960
January	1st to 10th	Night	78% 2 9% 8% 2	6349
	11th to 20th	Night	75% 4 9% 9% 3	6360
	21st to end	Night	72% 3 8% 9% 8% 1	6996
February	1st to 10th	Night	71% 3 7% 12% 5% 1	2652
	11th to 20th	Night	67% 4 8% 14% 6%	2381
	21st to end	Night	64% 3 8% 15% 9% 1	1939
March	1st to 10th	Night	60% 3 11% 16% 9% 1	3734
	11th to 20th	Night	64% 4 7% 15% 9% 1	3691
	21st to end	Night	58% 5% 5% 17% 14% 2	3466
April	1st to 10th	Night	59% 3 8% 17% 11% 1	1900
	11th to 20th	Night	59% 3 7% 18% 13% 1	1875
	21st to end	Night	55% 3 8% 16% 14% 3	1732
May	1st to 10th	Night	60% 3 9% 17% 9% 2	2721
	11th to 20th	Night	53% 3 11% 21% 10% 1	2496
	21st to end	Night	45% 4 12% 21% 16% 1	2739
June	1st to 10th	Night	54% 4 10% 22% 9% 1	1520
	11th to 20th	Night	45% 4 12% 23% 13% 2	1520
	21st to end	Night	51% 5% 12% 20% 11%	1520
July	1st to 10th	Night	48% 4 14% 22% 12% 1	2480
	11th to 20th	Night	43% 4 14% 30% 9%	2472
	21st to end	Night	47% 5% 13% 22% 13%	2651
August	1st to 10th	Night	51% 4 13% 24% 8%	2496
	11th to 20th	Night	51% 4 16% 20% 9%	2869
	21st to end	Night	42% 3 15% 25% 14% 1	3476
September	1st to 10th	Night	40% 5% 13% 29% 12% 1	1900
	11th to 20th	Night	37% 3 14% 33% 12% 1	2008
	21st to end	Night	42% 5% 12% 26% 14% 1	2090
October	1st to 10th	Night	54% 3 10% 25% 8%	3626
	11th to 20th	Night	52% 2 10% 24% 11%	3887
	21st to end	Night	53% 3 13% 21% 10%	4467
November	1st to 10th	Night	57% 3 11% 19% 8% 1	2537
	11th to 20th	Night	59% 4 9% 17% 9% 1	2606
	21st to end	Night	66% 4 11% 14% 4	2800
December	1st to 10th	Night	81% 2 8% 7% 2	4580
	11th to 20th	Night	71% 3 7% 12% 7%	5208
	21st to end	Night	70% 3 8% 12% 6%	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	33% 17% 29% 16% 6% 1	200
	11th to 20th	Day	34% 22% 31% 11% 3 1	200
	21st to end	Day	29% 20% 30% 15% 5%	220
February	1st to 10th	Day	34% 25% 29% 9% 2	273
	11th to 20th	Day	28% 27% 31% 11% 2	453
	21st to end	Day	29% 22% 32% 15% 3	398
March	1st to 10th	Day	35% 15% 29% 18% 3	460
	11th to 20th	Day	29% 22% 28% 15% 5% 1	460
	21st to end	Day	34% 20% 24% 16% 5% 1	506
April	1st to 10th	Day	33% 19% 24% 19% 3 1	460
	11th to 20th	Day	32% 20% 25% 14% 7% 2	452
	21st to end	Day	34% 16% 29% 14% 5% 2	460
May	1st to 10th	Day	32% 15% 27% 17% 6% 2	463
	11th to 20th	Day	24% 12% 30% 22% 10% 1	487
	21st to end	Day	20% 15% 32% 25% 7% 1	529
June	1st to 10th	Day	28% 11% 32% 22% 7% 1	490
	11th to 20th	Day	21% 14% 31% 22% 10% 2	490
	21st to end	Day	27% 10% 32% 21% 8% 2	490
July	1st to 10th	Day	17% 5% 32% 32% 11% 2	485
	11th to 20th	Day	17% 7% 37% 24% 12% 2	484
	21st to end	Day	20% 5% 32% 32% 10% 2	532
August	1st to 10th	Day	16% 8% 30% 29% 14% 2	486
	11th to 20th	Day	18% 5% 27% 30% 16% 4	479
	21st to end	Day	17% 8% 31% 24% 13% 6%	539
September	1st to 10th	Day	19% 11% 29% 23% 11% 8%	490
	11th to 20th	Day	18% 10% 22% 22% 21% 7%	490
	21st to end	Day	16% 11% 29% 24% 14% 6%	490
October	1st to 10th	Day	26% 13% 25% 17% 15% 3	490
	11th to 20th	Day	32% 17% 23% 17% 10% 1	458
	21st to end	Day	37% 14% 26% 18% 5%	429
November	1st to 10th	Day	35% 16% 28% 14% 6% 1	395
	11th to 20th	Day	32% 21% 25% 14% 7% 1	390
	21st to end	Day	38% 22% 24% 14% 3	200
December	1st to 10th	Day	29% 22% 28% 17% 5% 1	200
	11th to 20th	Day	31% 23% 35% 9% 3	200
	21st to end	Day	31% 24% 24% 17% 4 1	220
January	1st to 10th	Night	32% 18% 29% 18% 4	800
	11th to 20th	Night	31% 19% 25% 19% 6%	800
	21st to end	Night	25% 15% 30% 24% 5% 1	880
February	1st to 10th	Night	27% 17% 31% 22% 3	800
	11th to 20th	Night	26% 18% 32% 20% 3	783
	21st to end	Night	26% 16% 30% 22% 6%	647
March	1st to 10th	Night	30% 14% 29% 23% 4 1	760
	11th to 20th	Night	33% 17% 23% 21% 6%	760
	21st to end	Night	30% 15% 23% 23% 8% 1	836
April	1st to 10th	Night	31% 13% 28% 23% 5%	760
	11th to 20th	Night	33% 15% 22% 21% 8% 1	751
	21st to end	Night	32% 12% 29% 20% 6% 1	592
May	1st to 10th	Night	30% 10% 26% 25% 8% 1	503
	11th to 20th	Night	25% 7% 30% 28% 9% 2	374
	21st to end	Night	20% 7% 31% 32% 9% 1	411
June	1st to 10th	Night	29% 7% 24% 29% 10% 1	380
	11th to 20th	Night	22% 5% 29% 31% 13% 1	380
	21st to end	Night	23% 5% 27% 33% 11% 1	380
July	1st to 10th	Night	18% 6% 28% 35% 14% 1	370
	11th to 20th	Night	12% 2 24% 44% 16% 2	368
	21st to end	Night	19% 3 19% 46% 13%	416
August	1st to 10th	Night	18% 2 18% 43% 18% 1	378
	11th to 20th	Night	17% 4 20% 37% 21% 2	595
	21st to end	Night	18% 4 23% 33% 20% 2	836
September	1st to 10th	Night	19% 7% 25% 32% 16% 2	760
	11th to 20th	Night	19% 6% 21% 28% 23% 3	760
	21st to end	Night	23% 8% 25% 23% 18% 3	760
October	1st to 10th	Night	27% 10% 25% 23% 14% 1	760
	11th to 20th	Night	29% 12% 25% 24% 9% 1	760
	21st to end	Night	33% 14% 27% 21% 5%	836
November	1st to 10th	Night	32% 15% 29% 18% 5% 1	780
	11th to 20th	Night	32% 16% 25% 18% 8% 1	800
	21st to end	Night	35% 18% 24% 17% 6% 1	800
December	1st to 10th	Night	36% 18% 25% 17% 5%	800
	11th to 20th	Night	32% 18% 27% 18% 5% 1	800
	21st to end	Night	36% 18% 22% 20% 4	880

Page generated at 2025-02-22 06:51:33. Version ac07

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