Keswick Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Keswick Observing Site 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	13% 10% 4 9% 6% 5% 5% 4 6% 7% 32%	133007
	Night	13% 11% 4 10% 6% 5% 5% 4 6% 7% 29%	130107
Transparency	Day	51% 5% 12% 20% 11% 1	136599
	Night	52% 5% 12% 19% 10% 1	134604
Seeing	Day	18% 10% 28% 25% 15% 5%	16716
	Night	19% 9% 30% 30% 12% 1	25765

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% 12% 5% 15% 7% 6% 5% 4 5% 6% 21%	4661
	11th to 20th	Day	7% 16% 8% 19% 10% 9% 6% 4 6% 5% 11%	5022
	21st to end	Day	10% 14% 6% 13% 8% 8% 6% 5% 6% 6% 17%	5780
February	1st to 10th	Day	12% 15% 6% 12% 8% 7% 6% 5% 6% 5% 19%	1890
	11th to 20th	Day	12% 12% 6% 12% 9% 6% 6% 4 5% 6% 22%	1843
	21st to end	Day	15% 13% 6% 11% 7% 6% 5% 4 6% 6% 22%	1600
March	1st to 10th	Day	16% 11% 4 10% 6% 4 4 3 5% 6% 30%	5042
	11th to 20th	Day	9% 12% 5% 11% 5% 6% 6% 4 6% 7% 28%	5090
	21st to end	Day	17% 10% 4 11% 6% 5% 4 4 4 5% 32%	5586
April	1st to 10th	Day	16% 12% 4 9% 6% 5% 5% 4 5% 5% 29%	2483
	11th to 20th	Day	18% 10% 3 6% 5% 3 4 3 5% 6% 35%	2587
	21st to end	Day	18% 9% 3 8% 6% 4 3 3 5% 6% 34%	2730
May	1st to 10th	Day	15% 8% 3 8% 5% 4 4 4 5% 7% 36%	6036
	11th to 20th	Day	13% 9% 3 9% 5% 5% 4 4 6% 7% 36%	6458
	21st to end	Day	17% 8% 3 7% 4 4 4 3 5% 7% 38%	7103
June	1st to 10th	Day	16% 7% 3 6% 5% 4 4 4 7% 7% 37%	2800
	11th to 20th	Day	11% 5% 2 7% 5% 4 5% 5% 7% 9% 41%	2867
	21st to end	Day	14% 7% 2 6% 5% 5% 4 4 6% 8% 39%	3000
July	1st to 10th	Day	10% 6% 3 7% 6% 5% 5% 4 7% 8% 39%	6663
	11th to 20th	Day	8% 5% 2 6% 5% 4 5% 4 6% 9% 46%	6360
	21st to end	Day	7% 5% 2 6% 4 5% 4 4 6% 9% 48%	6547
August	1st to 10th	Day	13% 8% 3 6% 4 5% 5% 4 6% 8% 39%	4234
	11th to 20th	Day	12% 7% 2 8% 4 4 5% 4 7% 10% 37%	4174
	21st to end	Day	9% 6% 2 6% 4 4 5% 4 7% 8% 44%	4512
September	1st to 10th	Day	13% 6% 2 6% 4 5% 5% 4 5% 8% 41%	2400
	11th to 20th	Day	11% 6% 2 6% 4 4 4 4 5% 8% 44%	2285
	21st to end	Day	14% 7% 2 7% 5% 6% 5% 4 6% 7% 36%	2157
October	1st to 10th	Day	17% 9% 2 8% 5% 4 4 4 5% 7% 35%	2000
	11th to 20th	Day	18% 10% 4 10% 6% 5% 4 5% 6% 6% 27%	2000
	21st to end	Day	23% 13% 4 10% 5% 4 5% 3 5% 5% 22%	2200
November	1st to 10th	Day	14% 13% 4 9% 6% 4 4 4 7% 7% 27%	1743
	11th to 20th	Day	13% 16% 5% 11% 6% 6% 5% 4 6% 6% 22%	1680
	21st to end	Day	17% 18% 5% 12% 7% 6% 5% 4 5% 5% 16%	1680
December	1st to 10th	Day	15% 21% 7% 17% 8% 6% 4 4 4 4 9%	2680
	11th to 20th	Day	15% 18% 5% 11% 7% 6% 6% 4 5% 5% 19%	3066
	21st to end	Day	22% 15% 7% 16% 6% 6% 5% 3 4 5% 12%	4048
January	1st to 10th	Night	13% 14% 6% 16% 7% 6% 6% 4 5% 6% 16%	8234
	11th to 20th	Night	10% 14% 6% 17% 11% 8% 7% 4 5% 5% 13%	7861
	21st to end	Night	10% 12% 6% 12% 10% 7% 7% 5% 7% 7% 18%	8401
February	1st to 10th	Night	12% 15% 5% 12% 8% 7% 5% 4 6% 5% 21%	2730
	11th to 20th	Night	12% 14% 5% 11% 7% 6% 5% 5% 6% 6% 22%	2663
	21st to end	Night	13% 12% 4 12% 7% 6% 5% 4 6% 6% 25%	2198
March	1st to 10th	Night	14% 10% 4 10% 7% 6% 5% 4 5% 6% 28%	5018
	11th to 20th	Night	9% 9% 4 9% 5% 5% 5% 5% 7% 9% 32%	5089
	21st to end	Night	18% 8% 4 10% 6% 5% 5% 4 5% 6% 30%	5588
April	1st to 10th	Night	18% 12% 3 9% 5% 5% 5% 4 4 6% 30%	2136
	11th to 20th	Night	17% 8% 3 7% 5% 4 4 4 5% 7% 35%	1998
	21st to end	Night	15% 9% 4 9% 5% 4 4 4 5% 6% 37%	1890
May	1st to 10th	Night	18% 8% 3 7% 5% 4 4 3 5% 8% 35%	4125
	11th to 20th	Night	11% 7% 3 7% 5% 4 5% 4 6% 7% 41%	3655
	21st to end	Night	17% 6% 3 6% 4 4 4 3 5% 7% 42%	4014
June	1st to 10th	Night	13% 6% 2 6% 5% 5% 5% 4 6% 8% 40%	1600
	11th to 20th	Night	9% 4 2 7% 5% 6% 6% 4 7% 8% 42%	1525
	21st to end	Night	10% 6% 3 7% 5% 5% 4 5% 5% 9% 42%	1400
July	1st to 10th	Night	8% 6% 3 6% 5% 4 5% 4 6% 8% 45%	3292
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 5% 8% 50%	3600
	21st to end	Night	8% 5% 2 5% 3 4 4 4 6% 8% 50%	4288
August	1st to 10th	Night	9% 6% 3 5% 3 5% 4 4 6% 9% 46%	2915
	11th to 20th	Night	13% 5% 2 6% 3 4 4 3 6% 8% 46%	2875
	21st to end	Night	9% 6% 2 6% 4 4 5% 4 6% 8% 45%	3351
September	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 6% 8% 46%	2000
	11th to 20th	Night	11% 5% 2 6% 3 3 3 3 6% 8% 51%	2111
	21st to end	Night	16% 7% 2 6% 5% 4 3 3 5% 5% 42%	2242
October	1st to 10th	Night	16% 7% 3 7% 4 4 5% 4 5% 7% 38%	2400
	11th to 20th	Night	17% 10% 4 8% 6% 5% 5% 3 5% 6% 31%	2400
	21st to end	Night	20% 11% 4 8% 6% 5% 4 4 6% 6% 26%	2640
November	1st to 10th	Night	17% 10% 4 9% 5% 4 5% 4 5% 7% 31%	2770
	11th to 20th	Night	15% 16% 5% 10% 6% 5% 4 4 5% 6% 23%	2940
	21st to end	Night	18% 17% 5% 10% 5% 6% 5% 4 6% 6% 19%	2940
December	1st to 10th	Night	14% 19% 7% 15% 8% 6% 6% 4 5% 4 13%	4720
	11th to 20th	Night	13% 15% 6% 14% 7% 6% 6% 5% 6% 6% 17%	5348
	21st to end	Night	21% 18% 6% 16% 6% 5% 5% 3 4 5% 11%	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	62% 5% 12% 14% 7%	4662
	11th to 20th	Day	72% 6% 8% 9% 4	5056
	21st to end	Day	64% 5% 11% 11% 9%	5780
February	1st to 10th	Day	66% 5% 8% 15% 5%	1890
	11th to 20th	Day	62% 4 10% 16% 6% 1	1852
	21st to end	Day	62% 4 11% 17% 7%	1600
March	1st to 10th	Day	55% 4 13% 15% 13%	5045
	11th to 20th	Day	55% 4 13% 18% 7% 2	5090
	21st to end	Day	55% 4 7% 19% 14% 1	5586
April	1st to 10th	Day	57% 4 8% 19% 10% 1	2483
	11th to 20th	Day	49% 4 10% 22% 14% 1	2587
	21st to end	Day	51% 4 11% 19% 12% 3	2730
May	1st to 10th	Day	48% 4 12% 20% 11% 5%	6045
	11th to 20th	Day	48% 4 11% 18% 18% 1	6462
	21st to end	Day	46% 3 12% 22% 14% 3	7103
June	1st to 10th	Day	45% 4 12% 23% 15% 1	2800
	11th to 20th	Day	39% 5% 14% 26% 15% 1	2867
	21st to end	Day	43% 5% 13% 25% 13% 1	3000
July	1st to 10th	Day	41% 6% 15% 24% 14% 1	6593
	11th to 20th	Day	33% 6% 13% 31% 16% 1	6285
	21st to end	Day	32% 6% 13% 33% 15% 1	6360
August	1st to 10th	Day	42% 5% 15% 25% 12% 1	4221
	11th to 20th	Day	40% 6% 18% 26% 10%	4213
	21st to end	Day	35% 6% 14% 33% 12% 1	4512
September	1st to 10th	Day	40% 6% 12% 29% 11% 1	2400
	11th to 20th	Day	37% 5% 13% 31% 13% 1	2285
	21st to end	Day	46% 5% 13% 26% 9%	2157
October	1st to 10th	Day	42% 6% 12% 26% 13% 1	3260
	11th to 20th	Day	46% 6% 14% 22% 12%	3240
	21st to end	Day	57% 6% 15% 17% 5%	3538
November	1st to 10th	Day	54% 5% 13% 19% 7% 1	1743
	11th to 20th	Day	61% 5% 13% 13% 7% 1	1680
	21st to end	Day	69% 6% 10% 13% 2	1680
December	1st to 10th	Day	78% 6% 8% 7% 1	2680
	11th to 20th	Day	66% 7% 9% 10% 7%	3066
	21st to end	Day	75% 5% 8% 8% 4	4048
January	1st to 10th	Night	66% 6% 11% 13% 4	8234
	11th to 20th	Night	70% 6% 9% 9% 5% 1	7902
	21st to end	Night	61% 7% 11% 11% 10% 1	8401
February	1st to 10th	Night	63% 5% 9% 18% 5%	2730
	11th to 20th	Night	60% 5% 10% 18% 6%	2665
	21st to end	Night	59% 4 11% 18% 8% 1	2198
March	1st to 10th	Night	55% 3 10% 18% 11% 2	5028
	11th to 20th	Night	46% 5% 18% 21% 9% 1	5090
	21st to end	Night	53% 5% 9% 18% 14% 1	5588
April	1st to 10th	Night	56% 4 9% 19% 11% 1	2136
	11th to 20th	Night	48% 5% 8% 22% 15% 2	1998
	21st to end	Night	49% 4 10% 20% 15% 2	1890
May	1st to 10th	Night	49% 3 12% 16% 15% 6%	4132
	11th to 20th	Night	41% 5% 13% 23% 16% 2	3656
	21st to end	Night	43% 4 13% 25% 14% 2	4015
June	1st to 10th	Night	41% 4 11% 26% 16% 2	1600
	11th to 20th	Night	38% 6% 12% 23% 20% 1	1525
	21st to end	Night	40% 5% 13% 26% 15% 1	1400
July	1st to 10th	Night	36% 5% 15% 26% 16% 2	3254
	11th to 20th	Night	33% 5% 12% 31% 18% 1	3552
	21st to end	Night	31% 6% 11% 34% 17% 1	4161
August	1st to 10th	Night	35% 4 16% 29% 16% 1	2904
	11th to 20th	Night	35% 5% 18% 28% 13% 1	2902
	21st to end	Night	34% 6% 15% 32% 12% 1	3351
September	1st to 10th	Night	36% 5% 15% 30% 12% 1	2000
	11th to 20th	Night	31% 5% 13% 34% 17%	2111
	21st to end	Night	44% 4 13% 27% 11%	2242
October	1st to 10th	Night	35% 5% 11% 27% 20% 1	3920
	11th to 20th	Night	48% 4 15% 22% 11%	3899
	21st to end	Night	46% 7% 15% 24% 8%	4252
November	1st to 10th	Night	52% 5% 12% 21% 9% 1	2770
	11th to 20th	Night	60% 5% 12% 14% 7% 1	2940
	21st to end	Night	65% 5% 14% 13% 2	2940
December	1st to 10th	Night	72% 6% 11% 9% 2	4720
	11th to 20th	Night	66% 7% 10% 11% 5%	5348
	21st to end	Night	76% 5% 9% 8% 3	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	16% 14% 40% 20% 9%	220
	11th to 20th	Day	19% 13% 42% 19% 6% 1	289
	21st to end	Day	21% 13% 34% 25% 6% 1	363
February	1st to 10th	Day	19% 14% 43% 18% 6% 1	330
	11th to 20th	Day	19% 19% 39% 21% 2	330
	21st to end	Day	24% 20% 33% 22% 1	285
March	1st to 10th	Day	19% 12% 37% 24% 8% 1	540
	11th to 20th	Day	21% 16% 34% 23% 7% 1	540
	21st to end	Day	26% 11% 32% 21% 9% 1	594
April	1st to 10th	Day	22% 14% 32% 21% 9% 2	540
	11th to 20th	Day	27% 11% 28% 18% 11% 4	532
	21st to end	Day	23% 10% 27% 26% 11% 4	540
May	1st to 10th	Day	16% 9% 28% 29% 13% 4	543
	11th to 20th	Day	16% 8% 24% 29% 18% 6%	567
	21st to end	Day	17% 7% 24% 28% 18% 6%	617
June	1st to 10th	Day	16% 7% 26% 28% 19% 3	530
	11th to 20th	Day	12% 7% 25% 31% 21% 5%	597
	21st to end	Day	16% 8% 25% 31% 16% 4	730
July	1st to 10th	Day	10% 5% 22% 31% 27% 7%	659
	11th to 20th	Day	12% 3 21% 28% 27% 10%	524
	21st to end	Day	10% 4 20% 36% 22% 9%	576
August	1st to 10th	Day	11% 3 17% 30% 28% 12%	526
	11th to 20th	Day	11% 4 16% 29% 27% 13%	519
	21st to end	Day	12% 4 23% 28% 22% 10%	583
September	1st to 10th	Day	13% 7% 20% 24% 24% 12%	530
	11th to 20th	Day	9% 4 16% 26% 26% 19%	530
	21st to end	Day	15% 7% 26% 25% 20% 8%	530
October	1st to 10th	Day	19% 7% 28% 22% 17% 7%	530
	11th to 20th	Day	22% 11% 35% 23% 8% 2	530
	21st to end	Day	28% 10% 32% 23% 6% 1	583
November	1st to 10th	Day	23% 17% 30% 18% 10% 1	287
	11th to 20th	Day	20% 20% 33% 19% 5% 2	220
	21st to end	Day	26% 17% 35% 18% 3	220
December	1st to 10th	Day	20% 17% 38% 20% 5%	220
	11th to 20th	Day	21% 22% 35% 16% 5% 1	220
	21st to end	Day	28% 12% 38% 18% 4	242
January	1st to 10th	Night	20% 13% 40% 19% 7% 1	840
	11th to 20th	Night	23% 12% 34% 25% 6%	840
	21st to end	Night	19% 9% 33% 31% 7% 1	924
February	1st to 10th	Night	19% 10% 41% 26% 4	840
	11th to 20th	Night	20% 11% 37% 28% 4	823
	21st to end	Night	20% 11% 37% 27% 5%	692
March	1st to 10th	Night	18% 11% 36% 29% 5%	840
	11th to 20th	Night	18% 12% 37% 26% 6% 1	840
	21st to end	Night	21% 9% 33% 28% 10%	924
April	1st to 10th	Night	23% 10% 32% 28% 5%	840
	11th to 20th	Night	20% 8% 32% 29% 9% 2	754
	21st to end	Night	20% 8% 30% 31% 12%	630
May	1st to 10th	Night	17% 5% 25% 36% 16% 1	625
	11th to 20th	Night	17% 5% 26% 38% 13% 1	414
	21st to end	Night	13% 5% 23% 39% 19% 1	455
June	1st to 10th	Night	14% 6% 28% 38% 14% 1	400
	11th to 20th	Night	9% 8% 23% 45% 16% 1	400
	21st to end	Night	14% 4 22% 41% 19% 1	400
July	1st to 10th	Night	8% 3 18% 47% 23% 1	390
	11th to 20th	Night	11% 4 19% 42% 22% 2	388
	21st to end	Night	10% 4 18% 44% 24% 1	595
August	1st to 10th	Night	11% 2 13% 43% 30% 2	597
	11th to 20th	Night	8% 3 15% 38% 33% 4	594
	21st to end	Night	11% 4 21% 35% 26% 3	736
September	1st to 10th	Night	11% 6% 22% 34% 22% 6%	800
	11th to 20th	Night	13% 3 18% 31% 26% 8%	800
	21st to end	Night	19% 6% 20% 31% 20% 5%	800
October	1st to 10th	Night	18% 6% 28% 28% 19% 2	800
	11th to 20th	Night	22% 12% 29% 27% 8% 1	800
	21st to end	Night	25% 11% 31% 26% 6%	880
November	1st to 10th	Night	24% 12% 28% 25% 11%	820
	11th to 20th	Night	23% 15% 32% 21% 8% 1	840
	21st to end	Night	28% 13% 33% 22% 4	840
December	1st to 10th	Night	26% 14% 38% 18% 4	840
	11th to 20th	Night	22% 15% 38% 20% 5% 1	840
	21st to end	Night	28% 11% 35% 22% 3	924

Page generated at 2026-05-29 01:53:28. Version ac07

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