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 2026-06-01.

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	17% 10% 4 9% 5% 5% 5% 4 5% 6% 31%	132241
	Night	18% 12% 4 9% 5% 5% 4 4 5% 6% 28%	130909
Transparency	Day	52% 6% 12% 20% 9% 1	135682
	Night	56% 6% 11% 18% 9% 1	135541
Seeing	Day	25% 17% 29% 19% 8% 2	16485
	Night	26% 15% 28% 23% 8% 1	25575

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	16% 19% 7% 13% 7% 5% 4 3 4 4 18%	4661
	11th to 20th	Day	11% 17% 7% 15% 7% 6% 5% 4 5% 4 19%	4650
	21st to end	Day	11% 16% 7% 13% 9% 7% 5% 4 5% 5% 18%	5163
February	1st to 10th	Day	16% 16% 5% 12% 7% 5% 5% 3 5% 6% 22%	1890
	11th to 20th	Day	15% 12% 4 11% 7% 6% 4 4 6% 5% 25%	1922
	21st to end	Day	19% 15% 5% 10% 6% 5% 4 4 4 5% 23%	1730
March	1st to 10th	Day	18% 12% 5% 11% 6% 5% 6% 3 4 5% 25%	4576
	11th to 20th	Day	12% 12% 5% 11% 6% 5% 5% 3 5% 6% 30%	4902
	21st to end	Day	19% 9% 4 10% 6% 4 4 3 4 4 35%	6026
April	1st to 10th	Day	21% 13% 3 7% 5% 4 3 3 4 5% 32%	2520
	11th to 20th	Day	22% 8% 3 7% 5% 5% 4 3 4 5% 34%	2494
	21st to end	Day	22% 10% 3 7% 5% 4 4 3 4 6% 32%	2640
May	1st to 10th	Day	26% 10% 4 9% 6% 4 5% 3 5% 6% 24%	6481
	11th to 20th	Day	19% 9% 3 9% 5% 5% 4 3 5% 7% 31%	6458
	21st to end	Day	21% 8% 3 7% 4 5% 5% 4 5% 7% 32%	7103
June	1st to 10th	Day	21% 9% 3 7% 4 5% 4 4 5% 7% 31%	2800
	11th to 20th	Day	13% 6% 3 6% 5% 5% 5% 5% 6% 8% 37%	2800
	21st to end	Day	20% 7% 2 7% 6% 4 5% 4 6% 7% 32%	2800
July	1st to 10th	Day	12% 7% 2 7% 6% 6% 6% 5% 7% 8% 35%	6360
	11th to 20th	Day	11% 5% 2 7% 5% 5% 5% 4 7% 8% 41%	6360
	21st to end	Day	12% 5% 1 4 3 4 5% 4 6% 10% 45%	6926
August	1st to 10th	Day	15% 7% 3 7% 5% 5% 5% 4 6% 8% 35%	4563
	11th to 20th	Day	14% 6% 2 7% 4 4 5% 4 6% 8% 40%	4059
	21st to end	Day	16% 6% 2 6% 5% 4 5% 4 6% 8% 39%	4312
September	1st to 10th	Day	13% 6% 2 6% 4 5% 4 3 5% 7% 44%	2400
	11th to 20th	Day	13% 4 2 6% 4 4 4 3 5% 7% 49%	2346
	21st to end	Day	16% 6% 2 7% 4 4 5% 3 5% 7% 39%	2200
October	1st to 10th	Day	19% 8% 2 6% 5% 4 4 4 5% 6% 37%	2019
	11th to 20th	Day	18% 10% 4 9% 5% 5% 6% 3 4 6% 31%	2000
	21st to end	Day	25% 10% 4 8% 4 4 5% 2 5% 5% 28%	1980
November	1st to 10th	Day	16% 11% 4 8% 6% 5% 5% 4 5% 6% 31%	1851
	11th to 20th	Day	16% 12% 4 8% 5% 6% 5% 4 5% 6% 28%	1775
	21st to end	Day	21% 16% 4 10% 6% 5% 5% 3 5% 5% 20%	1680
December	1st to 10th	Day	20% 19% 6% 13% 6% 5% 4 4 4 4 15%	2680
	11th to 20th	Day	19% 16% 5% 11% 5% 5% 5% 3 4 4 22%	3066
	21st to end	Day	23% 15% 4 10% 6% 6% 5% 5% 5% 4 17%	4048
January	1st to 10th	Night	15% 22% 7% 14% 6% 4 3 3 3 5% 17%	8234
	11th to 20th	Night	13% 19% 8% 17% 8% 6% 5% 4 4 4 14%	8235
	21st to end	Night	13% 17% 5% 12% 7% 6% 5% 3 5% 6% 20%	9015
February	1st to 10th	Night	15% 15% 6% 13% 7% 6% 5% 4 4 4 21%	2730
	11th to 20th	Night	15% 13% 5% 12% 7% 5% 4 4 5% 4 27%	2584
	21st to end	Night	15% 15% 5% 10% 6% 5% 4 4 4 4 28%	2076
March	1st to 10th	Night	21% 13% 4 9% 6% 5% 4 3 4 4 28%	5484
	11th to 20th	Night	16% 14% 3 8% 4 4 4 4 6% 6% 29%	5262
	21st to end	Night	17% 13% 4 9% 5% 4 4 3 4 5% 31%	5148
April	1st to 10th	Night	23% 12% 3 8% 5% 3 3 3 5% 5% 30%	2100
	11th to 20th	Night	22% 11% 3 7% 4 4 3 3 4 5% 32%	2075
	21st to end	Night	21% 9% 3 7% 5% 3 4 3 5% 5% 35%	1979
May	1st to 10th	Night	26% 10% 3 8% 4 5% 5% 4 4 6% 23%	3724
	11th to 20th	Night	22% 8% 3 8% 5% 4 4 3 4 6% 33%	3655
	21st to end	Night	18% 9% 3 9% 5% 5% 4 4 6% 7% 31%	4014
June	1st to 10th	Night	19% 7% 2 7% 5% 6% 5% 5% 6% 10% 28%	1600
	11th to 20th	Night	17% 7% 2 6% 4 4 4 4 5% 8% 40%	1600
	21st to end	Night	18% 7% 3 7% 5% 4 5% 3 5% 8% 34%	1600
July	1st to 10th	Night	13% 6% 3 7% 4 5% 5% 5% 8% 10% 34%	3600
	11th to 20th	Night	16% 6% 2 6% 4 4 4 4 6% 7% 42%	3600
	21st to end	Night	11% 6% 2 6% 5% 4 4 4 6% 8% 44%	3926
August	1st to 10th	Night	15% 8% 3 7% 3 4 3 3 7% 8% 39%	2586
	11th to 20th	Night	16% 7% 3 7% 4 5% 4 3 5% 6% 40%	2940
	21st to end	Night	15% 6% 2 7% 5% 5% 4 4 5% 8% 40%	3586
September	1st to 10th	Night	11% 6% 3 8% 5% 5% 4 3 5% 7% 43%	2000
	11th to 20th	Night	14% 6% 2 4 3 3 4 3 5% 7% 49%	2053
	21st to end	Night	17% 6% 1 6% 6% 4 4 4 4 7% 41%	2200
October	1st to 10th	Night	19% 7% 3 6% 4 4 4 4 5% 6% 38%	2378
	11th to 20th	Night	19% 8% 4 8% 4 5% 4 4 5% 6% 34%	2400
	21st to end	Night	23% 10% 3 8% 5% 5% 4 3 4 7% 28%	2860
November	1st to 10th	Night	20% 11% 3 7% 5% 4 4 3 5% 6% 32%	2667
	11th to 20th	Night	18% 11% 4 8% 5% 5% 5% 4 5% 6% 31%	2840
	21st to end	Night	21% 14% 5% 9% 5% 5% 5% 4 5% 5% 22%	2940
December	1st to 10th	Night	19% 20% 7% 12% 6% 5% 5% 3 4 4 15%	4720
	11th to 20th	Night	23% 15% 5% 10% 5% 4 5% 3 5% 4 21%	5348
	21st to end	Night	26% 16% 5% 11% 4 4 4 6% 4 3 18%	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	69% 6% 12% 11% 2	4662
	11th to 20th	Day	67% 5% 9% 9% 8% 1	4680
	21st to end	Day	66% 6% 11% 9% 8% 1	5163
February	1st to 10th	Day	63% 6% 10% 17% 4 1	1890
	11th to 20th	Day	60% 5% 12% 17% 6% 1	1931
	21st to end	Day	63% 4 10% 13% 8% 1	1730
March	1st to 10th	Day	58% 6% 10% 15% 8% 3	4579
	11th to 20th	Day	53% 6% 16% 15% 8% 1	4902
	21st to end	Day	52% 4 8% 17% 17% 2	6026
April	1st to 10th	Day	54% 5% 8% 21% 11% 2	2520
	11th to 20th	Day	51% 4 7% 21% 15% 2	2494
	21st to end	Day	52% 4 10% 18% 13% 3	2640
May	1st to 10th	Day	60% 6% 9% 16% 8% 1	6492
	11th to 20th	Day	51% 7% 13% 19% 9% 1	6462
	21st to end	Day	50% 7% 12% 21% 10% 1	7103
June	1st to 10th	Day	51% 6% 11% 19% 12% 1	2800
	11th to 20th	Day	42% 7% 12% 24% 13% 2	2800
	21st to end	Day	48% 8% 13% 20% 11% 1	2800
July	1st to 10th	Day	44% 7% 13% 27% 9%	6290
	11th to 20th	Day	37% 8% 14% 28% 13%	6285
	21st to end	Day	34% 7% 13% 37% 9%	6728
August	1st to 10th	Day	45% 7% 16% 23% 9%	4549
	11th to 20th	Day	41% 7% 17% 28% 7%	4099
	21st to end	Day	41% 7% 17% 24% 11%	4312
September	1st to 10th	Day	38% 5% 14% 30% 13%	2400
	11th to 20th	Day	35% 5% 14% 32% 14% 1	2346
	21st to end	Day	42% 8% 14% 26% 11% 1	2200
October	1st to 10th	Day	46% 5% 12% 28% 8%	3279
	11th to 20th	Day	45% 6% 16% 25% 7% 1	3240
	21st to end	Day	48% 5% 18% 23% 6%	3180
November	1st to 10th	Day	53% 6% 12% 21% 8% 1	1851
	11th to 20th	Day	56% 6% 12% 15% 9% 1	1775
	21st to end	Day	66% 7% 12% 12% 4	1680
December	1st to 10th	Day	73% 6% 10% 10% 1	2680
	11th to 20th	Day	65% 6% 10% 13% 6%	3066
	21st to end	Day	68% 6% 9% 14% 3	4048
January	1st to 10th	Night	71% 5% 10% 10% 4	8234
	11th to 20th	Night	74% 5% 8% 8% 5%	8280
	21st to end	Night	65% 5% 9% 12% 9%	9015
February	1st to 10th	Night	66% 6% 9% 13% 5% 1	2730
	11th to 20th	Night	61% 4 11% 17% 6%	2586
	21st to end	Night	58% 6% 9% 17% 10% 1	2076
March	1st to 10th	Night	58% 4 8% 16% 11% 2	5494
	11th to 20th	Night	54% 6% 14% 15% 8% 2	5263
	21st to end	Night	55% 4 6% 21% 11% 2	5148
April	1st to 10th	Night	57% 4 9% 18% 11% 2	2100
	11th to 20th	Night	54% 5% 9% 19% 12% 1	2075
	21st to end	Night	51% 5% 8% 17% 15% 4	1979
May	1st to 10th	Night	60% 7% 9% 15% 7% 2	3730
	11th to 20th	Night	51% 7% 10% 19% 11% 1	3656
	21st to end	Night	51% 7% 9% 19% 12% 1	4015
June	1st to 10th	Night	49% 9% 12% 17% 11% 1	1600
	11th to 20th	Night	42% 7% 12% 21% 16% 2	1600
	21st to end	Night	48% 6% 12% 21% 11%	1600
July	1st to 10th	Night	43% 7% 17% 22% 11%	3560
	11th to 20th	Night	40% 5% 15% 28% 12%	3552
	21st to end	Night	38% 7% 13% 30% 12%	3809
August	1st to 10th	Night	43% 7% 15% 26% 8%	2576
	11th to 20th	Night	45% 7% 16% 24% 8%	2965
	21st to end	Night	41% 7% 15% 23% 13% 1	3586
September	1st to 10th	Night	40% 6% 11% 28% 14% 1	2000
	11th to 20th	Night	35% 5% 13% 34% 12% 1	2053
	21st to end	Night	42% 6% 12% 25% 14% 1	2200
October	1st to 10th	Night	44% 6% 13% 28% 9% 1	3885
	11th to 20th	Night	46% 6% 13% 25% 9%	3899
	21st to end	Night	49% 5% 16% 20% 9% 1	4610
November	1st to 10th	Night	54% 6% 12% 19% 8% 1	2667
	11th to 20th	Night	55% 6% 12% 18% 8% 2	2840
	21st to end	Night	64% 6% 11% 14% 4	2940
December	1st to 10th	Night	73% 5% 10% 10% 2	4720
	11th to 20th	Night	66% 5% 8% 12% 8%	5348
	21st to end	Night	67% 5% 8% 12% 7%	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	28% 20% 33% 15% 5%	220
	11th to 20th	Day	32% 26% 31% 8% 2	220
	21st to end	Day	22% 24% 34% 17% 4	250
February	1st to 10th	Day	31% 27% 31% 9% 2	330
	11th to 20th	Day	29% 29% 33% 8% 1	409
	21st to end	Day	33% 22% 32% 12% 1	446
March	1st to 10th	Day	31% 25% 27% 14% 3	540
	11th to 20th	Day	32% 22% 31% 11% 3 1	540
	21st to end	Day	35% 23% 24% 12% 6%	594
April	1st to 10th	Day	36% 20% 28% 13% 3 1	540
	11th to 20th	Day	36% 20% 22% 15% 7% 1	532
	21st to end	Day	33% 17% 30% 12% 6% 1	540
May	1st to 10th	Day	27% 21% 29% 15% 6% 2	543
	11th to 20th	Day	21% 18% 32% 18% 10% 1	567
	21st to end	Day	23% 15% 32% 22% 7% 1	617
June	1st to 10th	Day	27% 14% 32% 20% 6% 1	530
	11th to 20th	Day	17% 14% 35% 24% 8% 2	530
	21st to end	Day	26% 12% 33% 18% 7% 2	530
July	1st to 10th	Day	14% 8% 35% 26% 14% 3	525
	11th to 20th	Day	15% 10% 33% 29% 10% 2	524
	21st to end	Day	15% 11% 32% 29% 10% 2	576
August	1st to 10th	Day	16% 9% 29% 29% 15% 3	526
	11th to 20th	Day	15% 9% 27% 32% 13% 5%	519
	21st to end	Day	17% 14% 29% 22% 13% 5%	583
September	1st to 10th	Day	16% 10% 30% 24% 14% 6%	530
	11th to 20th	Day	15% 8% 23% 27% 18% 10%	530
	21st to end	Day	19% 11% 31% 22% 14% 3	530
October	1st to 10th	Day	24% 13% 28% 18% 13% 4	530
	11th to 20th	Day	27% 20% 25% 19% 7% 2	530
	21st to end	Day	35% 16% 24% 18% 7%	462
November	1st to 10th	Day	34% 17% 25% 15% 8%	425
	11th to 20th	Day	33% 22% 26% 13% 5% 2	315
	21st to end	Day	35% 23% 24% 13% 5%	220
December	1st to 10th	Day	32% 23% 25% 17% 4	220
	11th to 20th	Day	32% 26% 27% 11% 3	220
	21st to end	Day	35% 22% 24% 14% 3 1	242
January	1st to 10th	Night	30% 18% 34% 16% 3	840
	11th to 20th	Night	32% 20% 27% 17% 4	840
	21st to end	Night	23% 18% 30% 22% 6%	924
February	1st to 10th	Night	25% 21% 31% 20% 3	840
	11th to 20th	Night	26% 21% 34% 17% 3	823
	21st to end	Night	27% 18% 31% 21% 3	692
March	1st to 10th	Night	30% 18% 28% 18% 5%	840
	11th to 20th	Night	31% 19% 28% 19% 3	840
	21st to end	Night	30% 16% 27% 22% 5%	924
April	1st to 10th	Night	32% 16% 27% 21% 3	840
	11th to 20th	Night	31% 17% 26% 20% 6%	831
	21st to end	Night	28% 14% 29% 22% 6%	719
May	1st to 10th	Night	31% 10% 26% 24% 8% 1	444
	11th to 20th	Night	24% 12% 30% 28% 7%	414
	21st to end	Night	19% 11% 32% 29% 9%	455
June	1st to 10th	Night	22% 10% 29% 32% 7% 1	400
	11th to 20th	Night	21% 9% 29% 31% 10% 1	400
	21st to end	Night	20% 11% 31% 28% 11%	400
July	1st to 10th	Night	12% 9% 31% 36% 13%	390
	11th to 20th	Night	14% 6% 30% 38% 12% 1	388
	21st to end	Night	15% 6% 32% 37% 9% 1	438
August	1st to 10th	Night	13% 6% 24% 38% 18% 1	398
	11th to 20th	Night	17% 5% 21% 36% 19% 2	631
	21st to end	Night	17% 7% 30% 30% 16% 1	880
September	1st to 10th	Night	17% 10% 25% 33% 14% 1	800
	11th to 20th	Night	16% 7% 21% 33% 20% 4	800
	21st to end	Night	21% 10% 26% 24% 18% 2	800
October	1st to 10th	Night	25% 10% 27% 22% 14% 1	800
	11th to 20th	Night	26% 13% 30% 23% 9%	800
	21st to end	Night	32% 15% 25% 24% 3	880
November	1st to 10th	Night	32% 17% 24% 21% 6% 1	820
	11th to 20th	Night	27% 20% 27% 18% 8% 1	840
	21st to end	Night	32% 20% 23% 18% 6%	840
December	1st to 10th	Night	34% 19% 26% 18% 3	840
	11th to 20th	Night	34% 19% 27% 16% 4 1	840
	21st to end	Night	37% 16% 26% 17% 3	924

Page generated at 2026-05-29 01:53:26. Version ac07

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