Surrey
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Surrey clear sky chart from 2005-11-06 to 2026-08-30.

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	21% 8% 3 7% 4 4 4 3 5% 6% 33%	148233
	Night	26% 9% 3 7% 4 4 4 3 5% 6% 29%	142662
Transparency	Day	34% 36% 11% 14% 6% 1	147942
	Night	37% 36% 9% 12% 5% 1	142535
Seeing	Day	26% 10% 21% 25% 14% 4	17672
	Night	33% 8% 17% 24% 17% 2	25768

By Month

Percentage Cloud Cover NW (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	41% 13% 4 9% 6% 5% 4 2 4 4 8%	4125
	11th to 20th	Day	22% 9% 3 6% 4 5% 4 3 6% 10% 29%	4155
	21st to end	Day	41% 9% 3 7% 4 3 3 2 4 4 23%	5228
February	1st to 10th	Day	33% 11% 3 10% 5% 4 3 3 5% 6% 17%	1890
	11th to 20th	Day	31% 10% 3 8% 6% 5% 4 3 4 5% 22%	1845
	21st to end	Day	28% 13% 4 8% 6% 5% 5% 3 4 5% 20%	1585
March	1st to 10th	Day	27% 14% 4 10% 5% 4 5% 3 5% 6% 16%	4748
	11th to 20th	Day	24% 12% 4 9% 5% 5% 4 3 5% 6% 23%	5110
	21st to end	Day	28% 13% 4 9% 5% 6% 5% 4 5% 6% 16%	5775
April	1st to 10th	Day	27% 10% 3 8% 6% 5% 5% 5% 5% 7% 20%	2520
	11th to 20th	Day	21% 9% 3 9% 5% 6% 6% 4 6% 6% 26%	2579
	21st to end	Day	22% 10% 4 8% 5% 4 5% 4 5% 6% 27%	2799
May	1st to 10th	Day	11% 7% 2 7% 4 5% 5% 4 5% 6% 43%	8195
	11th to 20th	Day	15% 9% 3 7% 5% 5% 6% 4 6% 9% 30%	8160
	21st to end	Day	19% 7% 2 6% 4 4 5% 5% 7% 8% 31%	9089
June	1st to 10th	Day	20% 8% 3 7% 5% 4 5% 4 5% 6% 32%	3150
	11th to 20th	Day	20% 8% 3 9% 5% 4 5% 4 6% 8% 28%	3150
	21st to end	Day	14% 7% 3 8% 5% 5% 6% 3 7% 7% 37%	3150
July	1st to 10th	Day	10% 4 2 5% 4 3 4 3 5% 6% 53%	8820
	11th to 20th	Day	6% 4 1 4 2 2 2 3 4 6% 65%	8820
	21st to end	Day	12% 4 2 4 3 3 3 3 6% 8% 53%	9002
August	1st to 10th	Day	6% 3 1 4 3 2 3 2 4 6% 66%	4657
	11th to 20th	Day	13% 5% 2 6% 4 3 4 4 6% 7% 46%	4286
	21st to end	Day	16% 7% 3 6% 4 4 4 3 5% 7% 41%	4657
September	1st to 10th	Day	14% 7% 2 6% 4 4 3 3 5% 7% 46%	2400
	11th to 20th	Day	18% 7% 3 7% 4 4 4 3 5% 6% 39%	2400
	21st to end	Day	24% 8% 3 6% 4 4 4 3 4 6% 34%	2105
October	1st to 10th	Day	22% 7% 2 8% 5% 5% 5% 4 6% 7% 28%	3280
	11th to 20th	Day	35% 8% 3 6% 3 4 4 3 5% 6% 23%	3261
	21st to end	Day	32% 11% 4 9% 6% 5% 5% 4 5% 6% 15%	3286
November	1st to 10th	Day	38% 8% 3 7% 5% 4 5% 3 5% 6% 15%	1648
	11th to 20th	Day	38% 11% 3 6% 5% 3 4 3 3 4 19%	1680
	21st to end	Day	39% 11% 3 7% 5% 4 3 4 4 5% 16%	1680
December	1st to 10th	Day	26% 11% 4 8% 5% 5% 5% 5% 6% 7% 19%	2700
	11th to 20th	Day	41% 14% 4 8% 5% 5% 4 3 4 3 10%	2723
	21st to end	Day	39% 15% 5% 10% 4 5% 3 3 3 4 9%	3575
January	1st to 10th	Night	39% 13% 4 9% 5% 4 4 3 4 4 11%	8770
	11th to 20th	Night	25% 7% 2 6% 5% 4 4 4 7% 9% 27%	8741
	21st to end	Night	37% 9% 3 6% 3 3 3 3 5% 5% 23%	8937
February	1st to 10th	Night	34% 12% 3 7% 4 4 4 2 5% 4 20%	2730
	11th to 20th	Night	29% 13% 3 8% 4 3 3 3 4 5% 24%	2662
	21st to end	Night	31% 12% 3 7% 4 3 4 3 3 6% 25%	2220
March	1st to 10th	Night	26% 14% 4 8% 4 4 5% 4 5% 6% 20%	5364
	11th to 20th	Night	27% 11% 3 7% 4 4 4 5% 6% 7% 22%	5109
	21st to end	Night	36% 13% 4 8% 5% 3 3 3 4 4 16%	5442
April	1st to 10th	Night	27% 12% 3 8% 6% 4 4 3 5% 5% 23%	2100
	11th to 20th	Night	21% 10% 3 9% 6% 5% 4 4 5% 6% 28%	1987
	21st to end	Night	23% 9% 3 7% 5% 5% 5% 4 4 6% 29%	1820
May	1st to 10th	Night	13% 7% 2 5% 4 4 4 3 4 5% 49%	4724
	11th to 20th	Night	15% 9% 3 8% 5% 5% 5% 4 7% 7% 34%	4700
	21st to end	Night	16% 9% 3 8% 5% 5% 5% 4 5% 8% 32%	5064
June	1st to 10th	Night	21% 8% 2 7% 5% 4 4 4 5% 7% 32%	1470
	11th to 20th	Night	16% 10% 4 8% 6% 5% 4 4 6% 7% 31%	1470
	21st to end	Night	14% 7% 3 6% 5% 4 4 3 6% 6% 42%	1470
July	1st to 10th	Night	9% 5% 2 4 3 3 3 2 5% 7% 57%	4140
	11th to 20th	Night	5% 3 1 3 2 2 2 2 3 5% 72%	4140
	21st to end	Night	8% 4 2 5% 3 3 3 3 5% 7% 56%	5052
August	1st to 10th	Night	6% 2 1 2 2 2 2 3 4 6% 71%	2712
	11th to 20th	Night	11% 6% 2 6% 4 4 4 3 4 8% 49%	2992
	21st to end	Night	13% 5% 2 6% 5% 4 4 3 6% 8% 42%	3454
September	1st to 10th	Night	9% 4 2 4 3 4 4 4 5% 6% 55%	2000
	11th to 20th	Night	18% 8% 2 5% 5% 4 4 3 5% 6% 42%	2000
	21st to end	Night	22% 6% 2 8% 4 4 4 2 4 6% 37%	2291
October	1st to 10th	Night	19% 8% 3 8% 7% 5% 5% 4 5% 6% 30%	3920
	11th to 20th	Night	30% 8% 3 7% 4 4 4 3 5% 5% 27%	3897
	21st to end	Night	31% 10% 4 8% 4 4 4 3 4 5% 22%	4511
November	1st to 10th	Night	38% 10% 3 7% 4 5% 4 3 4 5% 17%	2870
	11th to 20th	Night	38% 10% 3 6% 3 3 4 3 4 5% 21%	2940
	21st to end	Night	37% 11% 4 7% 6% 5% 4 3 3 4 18%	2940
December	1st to 10th	Night	27% 10% 3 8% 6% 7% 6% 5% 6% 6% 16%	4700
	11th to 20th	Night	43% 14% 3 8% 5% 3 4 3 3 3 11%	5700
	21st to end	Night	39% 15% 4 8% 4 4 3 2 3 4 14%	7623

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	64% 24% 5% 5% 1	4125
	11th to 20th	Day	37% 23% 7% 22% 10% 2	4176
	21st to end	Day	53% 22% 4 15% 5% 2	5229
February	1st to 10th	Day	30% 59% 3 3 4 2	1890
	11th to 20th	Day	28% 59% 4 5% 3 1	1854
	21st to end	Day	28% 59% 4 5% 3 1	1585
March	1st to 10th	Day	46% 31% 9% 11% 3	4748
	11th to 20th	Day	45% 31% 7% 15% 2	5110
	21st to end	Day	50% 32% 10% 6% 2	5775
April	1st to 10th	Day	26% 61% 6% 7% 1	2520
	11th to 20th	Day	25% 61% 5% 6% 2 1	2579
	21st to end	Day	24% 60% 5% 7% 4	2799
May	1st to 10th	Day	30% 25% 14% 21% 9%	8195
	11th to 20th	Day	41% 27% 18% 12% 2	8163
	21st to end	Day	36% 27% 19% 15% 2	9090
June	1st to 10th	Day	20% 60% 8% 9% 3	3150
	11th to 20th	Day	21% 60% 6% 10% 4	3150
	21st to end	Day	20% 59% 8% 9% 3	3150
July	1st to 10th	Day	26% 22% 15% 18% 14% 5%	8746
	11th to 20th	Day	15% 21% 14% 29% 18% 2	8739
	21st to end	Day	26% 22% 23% 22% 7%	8807
August	1st to 10th	Day	11% 36% 8% 29% 15% 1	4640
	11th to 20th	Day	30% 38% 17% 11% 3 1	4325
	21st to end	Day	30% 37% 11% 12% 9% 1	4656
September	1st to 10th	Day	20% 59% 7% 10% 4 1	2400
	11th to 20th	Day	23% 58% 5% 8% 6%	2400
	21st to end	Day	22% 58% 8% 8% 4	2105
October	1st to 10th	Day	36% 38% 12% 13% 2	3280
	11th to 20th	Day	43% 36% 7% 8% 6%	3261
	21st to end	Day	46% 38% 5% 7% 3	3286
November	1st to 10th	Day	32% 55% 6% 6% 1	1648
	11th to 20th	Day	30% 56% 5% 5% 3 2	1680
	21st to end	Day	29% 55% 5% 5% 5% 1	1680
December	1st to 10th	Day	38% 36% 7% 13% 5%	2700
	11th to 20th	Day	55% 33% 6% 4 1	2726
	21st to end	Day	59% 31% 6% 3 1	3575
January	1st to 10th	Night	62% 23% 6% 7% 2	8770
	11th to 20th	Night	35% 25% 10% 16% 13% 1	8782
	21st to end	Night	49% 23% 6% 15% 7%	8937
February	1st to 10th	Night	28% 59% 3 3 4 3	2730
	11th to 20th	Night	29% 58% 3 5% 3 1	2664
	21st to end	Night	24% 60% 4 8% 3 1	2220
March	1st to 10th	Night	45% 32% 9% 10% 5%	5364
	11th to 20th	Night	41% 32% 11% 15% 2	5110
	21st to end	Night	53% 31% 10% 5% 1	5442
April	1st to 10th	Night	26% 60% 5% 8% 1	2100
	11th to 20th	Night	24% 60% 6% 5% 3 1	1987
	21st to end	Night	22% 60% 7% 7% 4	1820
May	1st to 10th	Night	31% 24% 9% 27% 9%	4721
	11th to 20th	Night	41% 27% 18% 12% 2	4700
	21st to end	Night	39% 26% 18% 15% 2	5068
June	1st to 10th	Night	19% 60% 8% 8% 4 1	1470
	11th to 20th	Night	19% 60% 5% 12% 4	1470
	21st to end	Night	18% 58% 9% 10% 4	1470
July	1st to 10th	Night	22% 21% 15% 24% 13% 5%	4105
	11th to 20th	Night	14% 20% 12% 28% 23% 3	4098
	21st to end	Night	24% 21% 19% 24% 11% 1	4942
August	1st to 10th	Night	8% 34% 10% 29% 19% 1	2704
	11th to 20th	Night	27% 37% 18% 13% 4 1	3019
	21st to end	Night	28% 39% 11% 13% 8% 1	3454
September	1st to 10th	Night	16% 60% 8% 9% 5% 1	2000
	11th to 20th	Night	21% 59% 6% 7% 6% 1	2000
	21st to end	Night	22% 58% 6% 10% 4	2291
October	1st to 10th	Night	39% 38% 12% 9% 2	3920
	11th to 20th	Night	40% 37% 7% 11% 5%	3897
	21st to end	Night	44% 38% 7% 8% 3	4511
November	1st to 10th	Night	33% 54% 6% 5% 1	2870
	11th to 20th	Night	31% 55% 4 5% 3 2	2940
	21st to end	Night	32% 56% 3 3 5% 1	2940
December	1st to 10th	Night	40% 38% 9% 9% 3	4700
	11th to 20th	Night	55% 33% 5% 5% 1	5696
	21st to end	Night	57% 29% 6% 7% 1	7623

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	40% 16% 18% 14% 9% 4	220
	11th to 20th	Day	41% 10% 13% 21% 10% 4	220
	21st to end	Day	42% 12% 15% 14% 13% 3	258
February	1st to 10th	Day	41% 11% 19% 19% 7% 2	430
	11th to 20th	Day	36% 13% 20% 20% 10% 1	425
	21st to end	Day	36% 17% 24% 14% 8% 1	371
March	1st to 10th	Day	35% 15% 20% 19% 10% 1	540
	11th to 20th	Day	38% 13% 17% 17% 13% 2	540
	21st to end	Day	37% 14% 20% 20% 8% 1	594
April	1st to 10th	Day	31% 13% 23% 20% 12% 2	540
	11th to 20th	Day	25% 12% 26% 22% 12% 3	532
	21st to end	Day	26% 14% 27% 18% 13% 3	540
May	1st to 10th	Day	19% 11% 24% 30% 13% 3	543
	11th to 20th	Day	24% 10% 22% 27% 13% 4	568
	21st to end	Day	21% 13% 27% 23% 12% 4	648
June	1st to 10th	Day	21% 12% 25% 29% 11% 2	780
	11th to 20th	Day	22% 12% 28% 27% 9% 2	780
	21st to end	Day	19% 10% 22% 29% 17% 3	780
July	1st to 10th	Day	14% 8% 26% 33% 16% 4	771
	11th to 20th	Day	10% 7% 23% 40% 16% 4	768
	21st to end	Day	11% 4 19% 36% 22% 8%	654
August	1st to 10th	Day	13% 6% 17% 33% 22% 9%	567
	11th to 20th	Day	11% 4 16% 36% 24% 10%	557
	21st to end	Day	14% 10% 22% 29% 18% 8%	620
September	1st to 10th	Day	17% 8% 21% 30% 16% 8%	530
	11th to 20th	Day	22% 8% 18% 27% 21% 4	530
	21st to end	Day	24% 10% 21% 24% 16% 5%	530
October	1st to 10th	Day	29% 6% 20% 22% 18% 5%	530
	11th to 20th	Day	38% 9% 16% 18% 15% 5%	530
	21st to end	Day	40% 12% 17% 17% 11% 3	434
November	1st to 10th	Day	40% 15% 16% 20% 10%	220
	11th to 20th	Day	47% 10% 14% 12% 12% 5%	220
	21st to end	Day	46% 8% 21% 8% 10% 7%	220
December	1st to 10th	Day	44% 13% 12% 12% 12% 7%	220
	11th to 20th	Day	54% 11% 14% 13% 6% 1	220
	21st to end	Day	49% 11% 16% 17% 7% 1	242
January	1st to 10th	Night	48% 12% 16% 16% 7% 1	950
	11th to 20th	Night	43% 8% 14% 24% 10% 1	943
	21st to end	Night	41% 10% 15% 23% 11% 1	924
February	1st to 10th	Night	41% 11% 22% 19% 7%	840
	11th to 20th	Night	37% 10% 20% 24% 8%	824
	21st to end	Night	37% 10% 23% 21% 8% 1	692
March	1st to 10th	Night	35% 11% 19% 21% 13% 1	840
	11th to 20th	Night	41% 9% 18% 21% 10%	840
	21st to end	Night	39% 9% 18% 23% 10% 1	924
April	1st to 10th	Night	33% 9% 18% 25% 13% 2	840
	11th to 20th	Night	26% 9% 24% 26% 12% 3	739
	21st to end	Night	31% 6% 19% 28% 13% 2	560
May	1st to 10th	Night	19% 7% 16% 32% 25% 2	420
	11th to 20th	Night	21% 5% 10% 32% 29% 3	414
	21st to end	Night	20% 5% 14% 33% 27% 2	455
June	1st to 10th	Night	26% 5% 14% 30% 23% 2	420
	11th to 20th	Night	21% 7% 18% 30% 20% 3	420
	21st to end	Night	16% 5% 14% 30% 32% 3	420
July	1st to 10th	Night	12% 2 10% 33% 39% 3	410
	11th to 20th	Night	11% 3 8% 35% 37% 6%	408
	21st to end	Night	6% 1 9% 28% 48% 8%	460
August	1st to 10th	Night	13% 1 5% 23% 48% 10%	428
	11th to 20th	Night	9% 2 6% 27% 45% 12%	624
	21st to end	Night	13% 4 11% 29% 38% 6%	763
September	1st to 10th	Night	14% 2 10% 36% 33% 6%	800
	11th to 20th	Night	22% 3 14% 23% 33% 6%	800
	21st to end	Night	28% 5% 15% 28% 23% 2	800
October	1st to 10th	Night	26% 5% 16% 27% 24% 3	800
	11th to 20th	Night	36% 6% 15% 24% 16% 2	800
	21st to end	Night	39% 8% 20% 24% 8% 2	880
November	1st to 10th	Night	46% 9% 19% 22% 4	820
	11th to 20th	Night	47% 8% 22% 15% 8% 1	840
	21st to end	Night	43% 10% 18% 18% 10% 1	840
December	1st to 10th	Night	38% 9% 19% 22% 11% 1	840
	11th to 20th	Night	52% 11% 19% 14% 3	945
	21st to end	Night	48% 11% 22% 15% 4	1045

Page generated at 2026-08-27 01:07:13. Version ac07

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