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 2025-04-26.

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	23% 9% 3 7% 4 4 4 3 5% 6% 32%	117170
	Night	27% 10% 3 7% 4 4 4 3 5% 5% 28%	120154
Transparency	Day	31% 44% 8% 11% 5% 1	116920
	Night	35% 42% 8% 10% 5% 1	120050
Seeing	Day	27% 10% 21% 24% 14% 4	15820
	Night	33% 8% 17% 23% 17% 2	24232

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	39% 13% 3 9% 6% 5% 4 3 4 5% 9%	3179
	11th to 20th	Day	21% 10% 3 7% 5% 5% 4 3 4 7% 31%	3181
	21st to end	Day	44% 9% 3 5% 3 2 2 2 3 3 23%	4032
February	1st to 10th	Day	33% 11% 3 10% 5% 4 3 3 5% 5% 17%	1800
	11th to 20th	Day	32% 10% 3 8% 6% 4 4 3 4 5% 21%	1755
	21st to end	Day	28% 13% 4 7% 6% 5% 5% 3 4 5% 20%	1512
March	1st to 10th	Day	27% 14% 4 10% 5% 4 5% 3 5% 7% 16%	4645
	11th to 20th	Day	24% 12% 4 9% 5% 5% 4 3 5% 6% 23%	5000
	21st to end	Day	29% 13% 4 9% 5% 5% 5% 4 5% 6% 16%	5651
April	1st to 10th	Day	27% 10% 4 8% 6% 4 5% 5% 5% 7% 19%	2400
	11th to 20th	Day	21% 9% 3 9% 5% 5% 5% 4 6% 6% 27%	2455
	21st to end	Day	23% 10% 4 8% 6% 4 5% 4 5% 6% 26%	2600
May	1st to 10th	Day	16% 8% 2 7% 4 4 4 4 5% 5% 42%	4420
	11th to 20th	Day	12% 7% 2 6% 4 5% 6% 5% 7% 9% 37%	4377
	21st to end	Day	27% 10% 3 8% 5% 4 5% 4 6% 6% 23%	4867
June	1st to 10th	Day	21% 9% 3 7% 5% 5% 5% 4 5% 6% 30%	2850
	11th to 20th	Day	21% 9% 3 9% 5% 4 5% 4 6% 8% 26%	2850
	21st to end	Day	13% 7% 3 7% 4 5% 6% 3 6% 7% 37%	2850
July	1st to 10th	Day	8% 3 1 4 3 2 3 2 4 5% 63%	4740
	11th to 20th	Day	6% 3 1 3 2 2 2 2 4 5% 70%	4740
	21st to end	Day	13% 4 2 4 3 3 3 3 5% 6% 55%	4835
August	1st to 10th	Day	6% 3 1 4 3 2 3 2 4 5% 67%	4379
	11th to 20th	Day	13% 5% 2 5% 4 3 4 4 6% 7% 45%	4026
	21st to end	Day	17% 7% 3 6% 4 4 4 3 5% 7% 41%	4402
September	1st to 10th	Day	14% 8% 2 6% 4 4 3 3 5% 7% 45%	2280
	11th to 20th	Day	18% 8% 3 7% 4 3 4 3 5% 6% 39%	2280
	21st to end	Day	25% 7% 3 6% 4 4 4 3 4 6% 35%	2000
October	1st to 10th	Day	22% 7% 2 8% 5% 5% 5% 4 5% 7% 28%	3180
	11th to 20th	Day	35% 7% 3 6% 3 4 4 3 5% 6% 22%	3161
	21st to end	Day	31% 11% 4 9% 6% 5% 5% 4 5% 6% 15%	3184
November	1st to 10th	Day	39% 8% 3 7% 5% 4 5% 3 5% 5% 15%	1568
	11th to 20th	Day	37% 12% 3 6% 5% 3 4 3 3 4 20%	1600
	21st to end	Day	38% 11% 3 7% 5% 4 3 4 4 5% 16%	1600
December	1st to 10th	Day	25% 10% 4 8% 5% 5% 5% 5% 6% 8% 19%	2620
	11th to 20th	Day	41% 14% 4 8% 5% 5% 3 3 4 3 10%	2653
	21st to end	Day	39% 15% 5% 10% 4 5% 3 3 3 4 9%	3498
January	1st to 10th	Night	39% 13% 4 9% 5% 4 4 3 4 4 12%	6766
	11th to 20th	Night	24% 7% 2 6% 5% 4 5% 4 6% 8% 29%	6715
	21st to end	Night	39% 10% 3 6% 3 3 2 3 4 4 24%	6908
February	1st to 10th	Night	34% 12% 3 7% 4 4 4 2 5% 5% 20%	2600
	11th to 20th	Night	29% 13% 3 8% 5% 3 3 3 4 5% 24%	2532
	21st to end	Night	31% 12% 2 6% 4 3 3 3 4 5% 25%	2117
March	1st to 10th	Night	26% 14% 4 8% 4 4 5% 4 5% 6% 20%	5247
	11th to 20th	Night	26% 11% 3 7% 4 4 4 5% 6% 7% 22%	4999
	21st to end	Night	37% 13% 4 8% 5% 3 3 3 4 4 16%	5324
April	1st to 10th	Night	28% 12% 3 7% 6% 4 5% 3 5% 5% 22%	2000
	11th to 20th	Night	21% 10% 3 8% 5% 4 4 4 5% 6% 29%	1891
	21st to end	Night	24% 10% 3 7% 5% 5% 5% 4 4 6% 27%	1695
May	1st to 10th	Night	18% 8% 3 5% 3 3 3 3 3 4 46%	2540
	11th to 20th	Night	11% 6% 2 5% 3 4 4 3 7% 7% 46%	2516
	21st to end	Night	23% 11% 3 8% 5% 5% 5% 3 4 6% 27%	2704
June	1st to 10th	Night	22% 9% 2 7% 5% 4 4 4 5% 7% 32%	1330
	11th to 20th	Night	17% 11% 5% 8% 5% 5% 4 5% 5% 7% 28%	1330
	21st to end	Night	13% 7% 3 6% 5% 4 4 3 6% 6% 42%	1330
July	1st to 10th	Night	6% 3 1 3 3 2 2 2 4 6% 68%	2220
	11th to 20th	Night	4 2 1 2 2 2 2 2 3 4 76%	2220
	21st to end	Night	8% 4 2 6% 3 3 3 3 4 6% 58%	2717
August	1st to 10th	Night	5% 2 1 2 2 2 2 3 4 6% 71%	2551
	11th to 20th	Night	11% 6% 2 6% 4 4 4 3 4 8% 49%	2812
	21st to end	Night	13% 5% 2 6% 5% 4 5% 3 6% 9% 41%	3268
September	1st to 10th	Night	9% 4 2 4 3 4 4 4 5% 6% 55%	1900
	11th to 20th	Night	18% 7% 2 5% 4 4 4 2 4 6% 42%	1900
	21st to end	Night	21% 6% 2 8% 4 4 4 2 5% 6% 38%	2176
October	1st to 10th	Night	19% 8% 3 8% 7% 5% 5% 4 5% 6% 29%	3800
	11th to 20th	Night	31% 8% 3 7% 4 3 4 3 5% 6% 27%	3777
	21st to end	Night	32% 10% 4 8% 4 4 4 3 4 5% 22%	4371
November	1st to 10th	Night	39% 10% 3 7% 4 5% 4 3 4 5% 17%	2730
	11th to 20th	Night	38% 10% 3 6% 3 3 4 2 4 5% 21%	2800
	21st to end	Night	36% 11% 4 7% 6% 5% 4 3 3 4 18%	2800
December	1st to 10th	Night	27% 10% 3 8% 6% 7% 6% 5% 6% 6% 17%	4560
	11th to 20th	Night	43% 13% 3 8% 5% 3 4 3 3 3 11%	5550
	21st to end	Night	39% 15% 4 8% 4 4 3 2 3 3 14%	7458

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	57% 30% 6% 6% 1 1	3179
	11th to 20th	Day	36% 28% 8% 19% 7% 2	3202
	21st to end	Day	50% 28% 4 13% 5%	4032
February	1st to 10th	Day	28% 61% 2 3 3 2	1800
	11th to 20th	Day	27% 62% 3 5% 3 1	1764
	21st to end	Day	26% 62% 4 5% 2 1	1512
March	1st to 10th	Day	46% 32% 9% 11% 3	4645
	11th to 20th	Day	44% 31% 7% 16% 2	5000
	21st to end	Day	50% 33% 10% 6% 2	5651
April	1st to 10th	Day	25% 64% 5% 5% 1	2400
	11th to 20th	Day	23% 63% 4 7% 2 1	2455
	21st to end	Day	22% 64% 4 7% 3	2600
May	1st to 10th	Day	22% 42% 12% 15% 9%	4420
	11th to 20th	Day	21% 45% 19% 13% 2	4380
	21st to end	Day	36% 45% 13% 6% 1	4868
June	1st to 10th	Day	18% 66% 7% 6% 2	2850
	11th to 20th	Day	19% 66% 6% 6% 3	2850
	21st to end	Day	17% 65% 7% 8% 3	2850
July	1st to 10th	Day	13% 36% 5% 16% 20% 10%	4666
	11th to 20th	Day	8% 36% 7% 28% 20% 2	4659
	21st to end	Day	22% 37% 16% 17% 6% 1	4682
August	1st to 10th	Day	10% 38% 7% 30% 14% 1	4362
	11th to 20th	Day	29% 40% 17% 11% 3	4065
	21st to end	Day	30% 39% 10% 11% 9%	4401
September	1st to 10th	Day	19% 62% 5% 9% 4 1	2280
	11th to 20th	Day	22% 61% 4 6% 5%	2280
	21st to end	Day	20% 61% 7% 8% 4	2000
October	1st to 10th	Day	36% 39% 12% 12% 2	3180
	11th to 20th	Day	42% 37% 7% 8% 6%	3161
	21st to end	Day	45% 39% 5% 7% 3	3184
November	1st to 10th	Day	31% 57% 6% 5% 1	1568
	11th to 20th	Day	27% 58% 5% 5% 3 2	1600
	21st to end	Day	27% 58% 5% 5% 4 1	1600
December	1st to 10th	Day	36% 37% 7% 14% 5%	2620
	11th to 20th	Day	54% 34% 6% 4 1	2656
	21st to end	Day	58% 31% 6% 3 1	3498
January	1st to 10th	Night	56% 29% 5% 7% 2	6766
	11th to 20th	Night	32% 31% 12% 16% 7% 2	6756
	21st to end	Night	46% 29% 7% 13% 5%	6908
February	1st to 10th	Night	27% 62% 2 3 4 3	2600
	11th to 20th	Night	28% 61% 3 5% 2 1	2534
	21st to end	Night	22% 62% 4 8% 3 1	2117
March	1st to 10th	Night	44% 32% 9% 10% 5%	5247
	11th to 20th	Night	39% 32% 11% 15% 2	5000
	21st to end	Night	53% 32% 10% 4 1	5324
April	1st to 10th	Night	25% 63% 4 7% 1	2000
	11th to 20th	Night	22% 63% 5% 6% 3 1	1891
	21st to end	Night	21% 64% 5% 6% 4	1695
May	1st to 10th	Night	25% 41% 6% 20% 8% 1	2540
	11th to 20th	Night	19% 44% 20% 13% 4	2516
	21st to end	Night	38% 43% 11% 7% 1	2704
June	1st to 10th	Night	17% 66% 7% 8% 2 1	1330
	11th to 20th	Night	19% 66% 5% 7% 3	1330
	21st to end	Night	15% 64% 8% 9% 3	1330
July	1st to 10th	Night	9% 35% 4 23% 19% 10%	2185
	11th to 20th	Night	7% 35% 5% 27% 23% 2	2178
	21st to end	Night	22% 36% 9% 19% 13% 1	2631
August	1st to 10th	Night	7% 36% 8% 29% 19% 1	2543
	11th to 20th	Night	27% 39% 17% 13% 4	2839
	21st to end	Night	28% 41% 11% 12% 7%	3268
September	1st to 10th	Night	15% 63% 7% 9% 5% 1	1900
	11th to 20th	Night	21% 62% 5% 6% 6% 1	1900
	21st to end	Night	20% 61% 6% 9% 4	2176
October	1st to 10th	Night	39% 39% 12% 8% 2	3800
	11th to 20th	Night	39% 38% 7% 11% 5%	3777
	21st to end	Night	44% 39% 6% 8% 3	4371
November	1st to 10th	Night	32% 57% 6% 4 1	2730
	11th to 20th	Night	29% 58% 4 5% 3 2	2800
	21st to end	Night	30% 58% 3 3 5% 1	2800
December	1st to 10th	Night	39% 39% 9% 10% 4	4560
	11th to 20th	Night	54% 34% 5% 5% 1 1	5546
	21st to end	Night	57% 30% 5% 7% 1	7458

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% 17% 18% 14% 8% 4	200
	11th to 20th	Day	43% 12% 15% 19% 10% 3	200
	21st to end	Day	43% 13% 15% 15% 11% 3	235
February	1st to 10th	Day	42% 12% 20% 18% 7% 2	400
	11th to 20th	Day	37% 12% 20% 21% 9% 1	395
	21st to end	Day	36% 16% 25% 14% 8% 1	346
March	1st to 10th	Day	34% 14% 21% 20% 9% 1	500
	11th to 20th	Day	36% 13% 17% 18% 14% 2	500
	21st to end	Day	38% 15% 19% 19% 8% 1	550
April	1st to 10th	Day	33% 13% 22% 20% 11% 1	500
	11th to 20th	Day	25% 13% 28% 21% 11% 3	492
	21st to end	Day	27% 13% 26% 18% 12% 3	481
May	1st to 10th	Day	20% 11% 24% 30% 11% 3	463
	11th to 20th	Day	23% 7% 22% 29% 15% 5%	488
	21st to end	Day	23% 13% 26% 23% 11% 4	557
June	1st to 10th	Day	23% 11% 26% 27% 11% 2	680
	11th to 20th	Day	24% 12% 27% 26% 9% 2	680
	21st to end	Day	18% 10% 23% 29% 17% 3	680
July	1st to 10th	Day	14% 7% 26% 32% 18% 4	671
	11th to 20th	Day	9% 8% 22% 39% 17% 5%	668
	21st to end	Day	11% 4 19% 36% 22% 7%	563
August	1st to 10th	Day	13% 6% 17% 34% 22% 9%	487
	11th to 20th	Day	10% 3 17% 37% 25% 8%	477
	21st to end	Day	15% 10% 22% 29% 17% 6%	539
September	1st to 10th	Day	18% 8% 23% 29% 16% 6%	490
	11th to 20th	Day	23% 9% 18% 28% 19% 3	490
	21st to end	Day	25% 10% 20% 24% 16% 5%	490
October	1st to 10th	Day	30% 6% 19% 21% 18% 4	490
	11th to 20th	Day	39% 7% 16% 19% 16% 4	490
	21st to end	Day	39% 12% 18% 17% 11% 3	398
November	1st to 10th	Day	40% 14% 16% 19% 12%	200
	11th to 20th	Day	46% 11% 14% 12% 13% 6%	200
	21st to end	Day	46% 8% 21% 8% 10% 8%	200
December	1st to 10th	Day	41% 13% 12% 13% 14% 8%	200
	11th to 20th	Day	53% 11% 15% 14% 7% 1	200
	21st to end	Day	50% 10% 16% 18% 5%	220
January	1st to 10th	Night	48% 12% 16% 16% 8% 1	900
	11th to 20th	Night	43% 8% 15% 24% 9% 1	894
	21st to end	Night	41% 10% 16% 22% 10% 1	880
February	1st to 10th	Night	41% 12% 22% 18% 7%	800
	11th to 20th	Night	37% 10% 21% 23% 8%	784
	21st to end	Night	37% 10% 23% 21% 8% 1	660
March	1st to 10th	Night	34% 11% 20% 21% 13% 1	800
	11th to 20th	Night	40% 9% 18% 22% 10% 1	800
	21st to end	Night	41% 9% 18% 22% 10% 1	880
April	1st to 10th	Night	34% 9% 18% 26% 12% 2	800
	11th to 20th	Night	26% 9% 25% 26% 12% 2	703
	21st to end	Night	32% 7% 20% 27% 13% 2	523
May	1st to 10th	Night	20% 7% 17% 30% 24% 2	380
	11th to 20th	Night	20% 4 9% 33% 30% 4	374
	21st to end	Night	21% 5% 13% 33% 26% 2	411
June	1st to 10th	Night	27% 5% 13% 29% 23% 2	380
	11th to 20th	Night	23% 8% 19% 28% 19% 3	380
	21st to end	Night	15% 5% 15% 30% 32% 3	380
July	1st to 10th	Night	12% 2 11% 32% 39% 4	370
	11th to 20th	Night	11% 4 8% 33% 38% 6%	368
	21st to end	Night	6% 1 9% 27% 49% 7%	416
August	1st to 10th	Night	13% 1 5% 22% 49% 10%	387
	11th to 20th	Night	8% 2 6% 27% 45% 11%	564
	21st to end	Night	14% 4 12% 29% 37% 5%	697
September	1st to 10th	Night	14% 2 11% 37% 32% 5%	760
	11th to 20th	Night	22% 4 14% 22% 33% 6%	760
	21st to end	Night	28% 5% 14% 28% 23% 2	760
October	1st to 10th	Night	26% 5% 16% 27% 24% 3	760
	11th to 20th	Night	37% 6% 15% 24% 17% 2	760
	21st to end	Night	39% 8% 20% 23% 8% 2	836
November	1st to 10th	Night	46% 9% 19% 22% 4	780
	11th to 20th	Night	47% 8% 21% 15% 8% 1	800
	21st to end	Night	43% 11% 17% 17% 10% 2	800
December	1st to 10th	Night	36% 8% 20% 23% 11% 1	800
	11th to 20th	Night	51% 11% 20% 14% 4	895
	21st to end	Night	49% 12% 22% 14% 3	990

Page generated at 2025-04-23 06:25:05. Version ac07

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