Lancaster
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lancaster clear sky chart from 2005-11-06 to 2025-02-06.

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% 10% 3 8% 5% 5% 5% 4 5% 6% 31%	108482
	Night	18% 13% 4 9% 5% 5% 4 4 5% 6% 28%	109751
Transparency	Day	51% 6% 13% 21% 9% 1	111971
	Night	56% 6% 11% 18% 9% 1	114412
Seeing	Day	25% 16% 29% 19% 8% 2	14772
	Night	26% 14% 28% 23% 8% 1	23940

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	17% 20% 7% 13% 6% 5% 3 3 3 3 19%	3597
	11th to 20th	Day	12% 17% 8% 15% 7% 5% 4 4 4 4 20%	3570
	21st to end	Day	13% 18% 7% 12% 5% 5% 4 3 5% 6% 20%	3985
February	1st to 10th	Day	16% 16% 5% 12% 7% 4 5% 3 5% 5% 22%	1755
	11th to 20th	Day	16% 12% 4 11% 7% 6% 4 4 6% 5% 26%	1734
	21st to end	Day	20% 15% 5% 9% 6% 5% 4 4 4 5% 24%	1570
March	1st to 10th	Day	20% 13% 4 10% 6% 5% 4 3 4 5% 27%	3116
	11th to 20th	Day	14% 14% 5% 12% 6% 5% 5% 3 4 5% 27%	3352
	21st to end	Day	16% 8% 3 9% 5% 4 3 3 4 5% 39%	4120
April	1st to 10th	Day	21% 13% 3 7% 5% 4 3 3 4 5% 32%	2280
	11th to 20th	Day	22% 8% 3 7% 5% 5% 4 3 4 6% 35%	2254
	21st to end	Day	22% 10% 3 7% 5% 4 4 3 5% 6% 31%	2388
May	1st to 10th	Day	21% 9% 3 7% 5% 4 5% 4 5% 6% 31%	4421
	11th to 20th	Day	18% 9% 3 8% 5% 4 4 3 5% 7% 33%	4398
	21st to end	Day	15% 7% 2 7% 4 5% 5% 4 6% 8% 37%	4837
June	1st to 10th	Day	22% 9% 3 7% 5% 5% 4 4 5% 7% 30%	2660
	11th to 20th	Day	13% 6% 3 6% 5% 5% 5% 5% 6% 8% 36%	2660
	21st to end	Day	20% 7% 2 7% 6% 4 5% 4 6% 7% 32%	2660
July	1st to 10th	Day	12% 7% 2 6% 5% 5% 5% 5% 6% 8% 38%	4440
	11th to 20th	Day	13% 5% 2 7% 5% 5% 5% 4 7% 9% 38%	4440
	21st to end	Day	12% 6% 2 5% 4 5% 5% 4 7% 9% 42%	4814
August	1st to 10th	Day	15% 8% 3 7% 5% 4 5% 4 6% 8% 34%	4423
	11th to 20th	Day	14% 6% 2 7% 4 4 5% 4 6% 8% 40%	3932
	21st to end	Day	16% 6% 2 6% 5% 5% 5% 4 6% 8% 39%	4180
September	1st to 10th	Day	13% 6% 2 6% 4 5% 5% 3 5% 7% 44%	2280
	11th to 20th	Day	13% 5% 2 6% 4 4 4 3 5% 7% 47%	2229
	21st to end	Day	16% 6% 2 7% 4 4 5% 4 5% 7% 39%	2090
October	1st to 10th	Day	20% 8% 2 7% 5% 4 4 4 5% 6% 35%	1918
	11th to 20th	Day	19% 10% 4 9% 5% 5% 6% 3 4 6% 30%	1900
	21st to end	Day	25% 10% 4 8% 4 4 5% 3 5% 5% 27%	1881
November	1st to 10th	Day	16% 10% 4 9% 6% 5% 5% 4 5% 6% 32%	1761
	11th to 20th	Day	16% 11% 4 8% 5% 6% 5% 4 5% 6% 29%	1691
	21st to end	Day	21% 15% 4 10% 6% 5% 5% 3 5% 6% 20%	1600
December	1st to 10th	Day	20% 20% 6% 13% 7% 4 4 4 4 4 15%	2600
	11th to 20th	Day	19% 16% 5% 11% 5% 5% 5% 3 4 4 23%	2986
	21st to end	Day	23% 16% 4 9% 6% 6% 5% 5% 5% 4 17%	3960
January	1st to 10th	Night	16% 24% 8% 14% 5% 4 3 2 3 5% 17%	6349
	11th to 20th	Night	11% 19% 8% 16% 8% 6% 5% 4 5% 4 15%	6315
	21st to end	Night	15% 19% 6% 12% 6% 5% 4 3 4 5% 19%	6968
February	1st to 10th	Night	16% 15% 6% 12% 7% 6% 5% 4 4 4 21%	2536
	11th to 20th	Night	14% 13% 5% 11% 6% 5% 5% 4 5% 4 27%	2332
	21st to end	Night	15% 15% 5% 9% 6% 5% 4 4 4 4 29%	1884
March	1st to 10th	Night	21% 11% 3 7% 6% 4 4 3 4 5% 31%	3724
	11th to 20th	Night	19% 18% 4 9% 5% 4 3 3 4 4 27%	3592
	21st to end	Night	14% 13% 3 8% 5% 4 4 3 4 5% 38%	3512
April	1st to 10th	Night	22% 12% 3 8% 5% 4 4 3 5% 5% 30%	1900
	11th to 20th	Night	22% 10% 3 7% 4 4 4 4 5% 5% 33%	1875
	21st to end	Night	21% 10% 3 7% 5% 4 3 3 5% 5% 34%	1791
May	1st to 10th	Night	23% 9% 3 8% 4 5% 5% 4 5% 7% 28%	2549
	11th to 20th	Night	23% 8% 3 7% 4 3 4 3 4 5% 35%	2495
	21st to end	Night	15% 7% 3 7% 4 4 4 4 6% 8% 38%	2738
June	1st to 10th	Night	18% 7% 2 6% 5% 6% 5% 5% 7% 10% 28%	1520
	11th to 20th	Night	17% 7% 2 6% 4 4 4 4 5% 8% 40%	1520
	21st to end	Night	18% 8% 3 7% 5% 4 5% 3 5% 8% 34%	1520
July	1st to 10th	Night	14% 7% 3 8% 4 5% 5% 5% 7% 8% 34%	2520
	11th to 20th	Night	15% 5% 2 6% 4 4 4 4 6% 7% 41%	2520
	21st to end	Night	13% 7% 2 6% 5% 4 4 4 6% 8% 41%	2739
August	1st to 10th	Night	15% 8% 3 7% 4 4 3 3 6% 8% 38%	2506
	11th to 20th	Night	16% 7% 3 7% 4 5% 4 3 5% 7% 39%	2848
	21st to end	Night	15% 6% 2 7% 5% 5% 4 4 5% 8% 40%	3476
September	1st to 10th	Night	12% 6% 3 8% 5% 5% 4 3 5% 7% 43%	1900
	11th to 20th	Night	15% 6% 2 4 3 3 4 3 5% 7% 49%	1950
	21st to end	Night	17% 6% 1 6% 6% 4 4 4 4 7% 41%	2090
October	1st to 10th	Night	20% 7% 3 6% 4 5% 5% 4 5% 6% 36%	2259
	11th to 20th	Night	20% 8% 4 8% 4 5% 4 4 5% 5% 33%	2280
	21st to end	Night	23% 10% 3 8% 5% 5% 4 3 4 7% 28%	2717
November	1st to 10th	Night	19% 11% 3 7% 5% 4 4 3 5% 6% 32%	2537
	11th to 20th	Night	18% 10% 4 8% 5% 5% 5% 4 5% 6% 31%	2705
	21st to end	Night	22% 13% 5% 9% 5% 5% 4 4 5% 5% 23%	2800
December	1st to 10th	Night	19% 20% 7% 12% 6% 5% 5% 3 4 4 15%	4580
	11th to 20th	Night	24% 15% 5% 10% 5% 4 5% 3 5% 4 21%	5208
	21st to end	Night	26% 16% 5% 11% 4 4 4 6% 4 4 18%	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	70% 6% 11% 11% 2	3598
	11th to 20th	Day	66% 6% 11% 10% 6%	3600
	21st to end	Day	63% 6% 13% 10% 7% 1	3985
February	1st to 10th	Day	63% 6% 10% 17% 3 1	1755
	11th to 20th	Day	59% 5% 12% 17% 6% 1	1743
	21st to end	Day	61% 4 11% 13% 8% 1	1570
March	1st to 10th	Day	58% 6% 10% 18% 6% 1	3119
	11th to 20th	Day	58% 6% 13% 14% 8% 1	3352
	21st to end	Day	46% 5% 10% 20% 17% 2	4120
April	1st to 10th	Day	54% 5% 8% 21% 11% 2	2280
	11th to 20th	Day	50% 4 8% 21% 15% 2	2254
	21st to end	Day	53% 4 10% 16% 13% 3	2388
May	1st to 10th	Day	52% 7% 11% 19% 10% 1	4435
	11th to 20th	Day	49% 8% 14% 19% 9% 1	4402
	21st to end	Day	41% 8% 13% 24% 12% 2	4837
June	1st to 10th	Day	51% 6% 12% 18% 12% 1	2660
	11th to 20th	Day	43% 7% 12% 24% 13% 2	2660
	21st to end	Day	49% 8% 13% 19% 10% 1	2660
July	1st to 10th	Day	41% 7% 13% 28% 10% 1	4370
	11th to 20th	Day	39% 8% 16% 30% 7%	4365
	21st to end	Day	36% 8% 14% 32% 9%	4661
August	1st to 10th	Day	46% 7% 15% 23% 9%	4409
	11th to 20th	Day	41% 7% 17% 28% 7%	3972
	21st to end	Day	41% 7% 17% 23% 12%	4180
September	1st to 10th	Day	39% 5% 14% 29% 13%	2280
	11th to 20th	Day	36% 5% 14% 31% 14% 1	2229
	21st to end	Day	41% 8% 14% 27% 10% 1	2090
October	1st to 10th	Day	47% 5% 12% 28% 8%	3178
	11th to 20th	Day	46% 6% 16% 25% 7%	3140
	21st to end	Day	48% 4 19% 23% 6%	3081
November	1st to 10th	Day	51% 7% 13% 21% 7% 1	1761
	11th to 20th	Day	55% 6% 12% 16% 10% 1	1691
	21st to end	Day	65% 7% 13% 12% 3	1600
December	1st to 10th	Day	73% 6% 10% 10% 1	2600
	11th to 20th	Day	64% 6% 10% 13% 6%	2986
	21st to end	Day	68% 6% 9% 14% 3	3960
January	1st to 10th	Night	72% 5% 10% 9% 3	6349
	11th to 20th	Night	73% 6% 9% 9% 4	6360
	21st to end	Night	67% 5% 10% 9% 9% 1	6968
February	1st to 10th	Night	67% 6% 9% 13% 5% 1	2536
	11th to 20th	Night	60% 4 11% 17% 7%	2334
	21st to end	Night	58% 5% 9% 17% 10% 1	1884
March	1st to 10th	Night	54% 6% 9% 21% 8% 2	3734
	11th to 20th	Night	61% 6% 10% 14% 7% 2	3593
	21st to end	Night	50% 4 7% 22% 14% 2	3512
April	1st to 10th	Night	56% 4 9% 18% 11% 2	1900
	11th to 20th	Night	53% 5% 10% 19% 12% 1	1875
	21st to end	Night	52% 5% 8% 17% 14% 4	1791
May	1st to 10th	Night	53% 7% 11% 17% 9% 2	2555
	11th to 20th	Night	49% 7% 12% 20% 11% 2	2496
	21st to end	Night	41% 8% 11% 21% 17% 1	2739
June	1st to 10th	Night	49% 9% 13% 17% 11% 1	1520
	11th to 20th	Night	42% 7% 13% 21% 16% 2	1520
	21st to end	Night	48% 7% 13% 22% 11%	1520
July	1st to 10th	Night	46% 6% 13% 21% 13%	2480
	11th to 20th	Night	39% 6% 17% 30% 8%	2472
	21st to end	Night	40% 8% 14% 24% 13%	2651
August	1st to 10th	Night	44% 7% 15% 26% 8%	2496
	11th to 20th	Night	45% 7% 17% 24% 7%	2873
	21st to end	Night	40% 7% 16% 23% 13% 1	3476
September	1st to 10th	Night	41% 6% 11% 27% 14% 1	1900
	11th to 20th	Night	36% 5% 13% 33% 12% 1	1950
	21st to end	Night	42% 6% 12% 26% 14% 1	2090
October	1st to 10th	Night	45% 6% 13% 28% 8%	3766
	11th to 20th	Night	46% 6% 13% 26% 9%	3779
	21st to end	Night	49% 5% 16% 20% 9% 1	4467
November	1st to 10th	Night	54% 6% 12% 19% 8% 1	2537
	11th to 20th	Night	54% 5% 12% 18% 9% 2	2705
	21st to end	Night	64% 6% 11% 14% 4	2800
December	1st to 10th	Night	73% 5% 10% 9% 2	4580
	11th to 20th	Night	66% 5% 8% 13% 8%	5208
	21st to end	Night	67% 5% 8% 12% 8%	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	30% 19% 32% 15% 5%	200
	11th to 20th	Day	31% 25% 34% 9% 3 1	200
	21st to end	Day	23% 22% 34% 17% 4	227
February	1st to 10th	Day	34% 27% 30% 7% 1	285
	11th to 20th	Day	28% 29% 35% 7% 1	341
	21st to end	Day	34% 21% 30% 13% 1	382
March	1st to 10th	Day	32% 22% 27% 15% 4	460
	11th to 20th	Day	31% 23% 31% 11% 3 1	460
	21st to end	Day	34% 22% 23% 13% 7%	506
April	1st to 10th	Day	34% 18% 30% 14% 3 1	460
	11th to 20th	Day	34% 21% 23% 14% 7% 1	452
	21st to end	Day	34% 16% 29% 13% 7% 1	460
May	1st to 10th	Day	26% 21% 28% 17% 6% 2	463
	11th to 20th	Day	20% 18% 31% 19% 11% 1	487
	21st to end	Day	21% 14% 32% 23% 8% 2	529
June	1st to 10th	Day	28% 13% 30% 20% 7% 1	490
	11th to 20th	Day	17% 13% 34% 25% 8% 2	490
	21st to end	Day	27% 12% 32% 18% 8% 3	490
July	1st to 10th	Day	15% 8% 35% 26% 14% 2	485
	11th to 20th	Day	16% 10% 32% 28% 11% 2	484
	21st to end	Day	16% 10% 32% 31% 10% 2	532
August	1st to 10th	Day	17% 10% 28% 29% 14% 3	486
	11th to 20th	Day	15% 9% 27% 32% 13% 5%	479
	21st to end	Day	17% 14% 29% 23% 13% 6%	539
September	1st to 10th	Day	16% 10% 31% 24% 12% 7%	490
	11th to 20th	Day	16% 8% 23% 25% 17% 10%	490
	21st to end	Day	19% 11% 30% 22% 14% 4	490
October	1st to 10th	Day	25% 13% 27% 17% 13% 4	490
	11th to 20th	Day	29% 20% 24% 18% 7% 2	490
	21st to end	Day	35% 15% 24% 18% 7%	429
November	1st to 10th	Day	32% 17% 25% 16% 9% 1	395
	11th to 20th	Day	33% 22% 26% 13% 5% 1	291
	21st to end	Day	36% 20% 26% 14% 5%	200
December	1st to 10th	Day	32% 23% 24% 18% 4	200
	11th to 20th	Day	33% 27% 26% 12% 3 1	200
	21st to end	Day	35% 24% 22% 15% 4 1	220
January	1st to 10th	Night	31% 17% 34% 16% 3	800
	11th to 20th	Night	30% 20% 27% 17% 4 1	800
	21st to end	Night	23% 18% 31% 23% 5%	880
February	1st to 10th	Night	26% 22% 31% 19% 3	781
	11th to 20th	Night	26% 20% 34% 17% 3	743
	21st to end	Night	27% 18% 31% 21% 3	628
March	1st to 10th	Night	30% 17% 28% 19% 5%	760
	11th to 20th	Night	31% 19% 28% 19% 3	760
	21st to end	Night	29% 15% 28% 22% 6%	836
April	1st to 10th	Night	31% 16% 27% 22% 4	760
	11th to 20th	Night	30% 17% 27% 20% 6%	751
	21st to end	Night	29% 14% 28% 22% 7%	651
May	1st to 10th	Night	31% 9% 26% 25% 8% 1	402
	11th to 20th	Night	23% 11% 30% 29% 7%	374
	21st to end	Night	18% 10% 32% 30% 10%	411
June	1st to 10th	Night	22% 10% 29% 33% 7% 1	380
	11th to 20th	Night	20% 9% 28% 31% 11% 1	380
	21st to end	Night	20% 10% 31% 28% 11%	380
July	1st to 10th	Night	12% 8% 32% 36% 12%	370
	11th to 20th	Night	13% 6% 30% 37% 13% 1	368
	21st to end	Night	15% 6% 32% 36% 10% 1	416
August	1st to 10th	Night	14% 6% 25% 37% 17% 1	378
	11th to 20th	Night	17% 5% 21% 35% 19% 3	599
	21st to end	Night	16% 7% 30% 30% 16% 1	836
September	1st to 10th	Night	17% 9% 26% 33% 13% 1	760
	11th to 20th	Night	17% 8% 21% 32% 19% 4	760
	21st to end	Night	20% 10% 26% 24% 18% 2	760
October	1st to 10th	Night	26% 11% 27% 22% 14% 1	760
	11th to 20th	Night	26% 13% 29% 22% 9%	760
	21st to end	Night	33% 16% 25% 23% 3	836
November	1st to 10th	Night	31% 17% 24% 21% 7% 1	780
	11th to 20th	Night	27% 19% 27% 18% 8% 1	800
	21st to end	Night	32% 20% 23% 18% 7%	800
December	1st to 10th	Night	34% 19% 26% 17% 3	800
	11th to 20th	Night	35% 18% 27% 16% 4 1	800
	21st to end	Night	37% 16% 26% 18% 4	880

Page generated at 2025-02-03 03:05:49. Version ac07

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