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 2024-10-25.

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	22% 9% 3 7% 4 4 4 3 5% 6% 34%	101603
	Night	27% 9% 3 6% 4 4 4 3 4 5% 31%	97269
Transparency	Day	26% 50% 8% 10% 5% 1	101353
	Night	29% 50% 7% 9% 5% 1	97165
Seeing	Day	27% 10% 21% 25% 14% 4	15307
	Night	33% 8% 17% 23% 17% 2	23473

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	42% 14% 3 7% 5% 4 4 2 4 5% 10%	2219
	11th to 20th	Day	30% 13% 4 7% 4 4 3 2 3 6% 25%	2207
	21st to end	Day	56% 11% 3 6% 3 3 2 2 3 3 9%	2808
February	1st to 10th	Day	35% 11% 3 9% 5% 4 3 3 5% 5% 16%	1710
	11th to 20th	Day	32% 9% 3 8% 6% 4 4 3 4 5% 21%	1665
	21st to end	Day	27% 13% 3 7% 6% 5% 5% 3 5% 5% 21%	1438
March	1st to 10th	Day	25% 15% 4 9% 5% 4 5% 4 5% 7% 17%	3226
	11th to 20th	Day	22% 9% 3 7% 5% 4 3 3 5% 7% 31%	3490
	21st to end	Day	27% 13% 4 8% 5% 5% 4 4 4 6% 21%	3938
April	1st to 10th	Day	27% 10% 4 8% 6% 4 5% 5% 5% 6% 19%	2280
	11th to 20th	Day	22% 9% 3 9% 6% 5% 5% 4 6% 5% 25%	2330
	21st to end	Day	23% 10% 4 8% 6% 4 5% 4 5% 6% 25%	2532
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	25% 8% 2 7% 3 3 5% 4 5% 7% 32%	1900
	11th to 20th	Day	32% 9% 3 8% 3 4 4 3 5% 7% 22%	1900
	21st to end	Day	35% 9% 3 6% 5% 4 4 3 4 6% 22%	1881
November	1st to 10th	Day	40% 8% 3 7% 5% 4 4 3 5% 5% 16%	1488
	11th to 20th	Day	37% 11% 3 6% 5% 3 3 3 3 4 21%	1520
	21st to end	Day	38% 11% 3 7% 4 3 3 4 4 5% 17%	1520
December	1st to 10th	Day	33% 12% 4 6% 4 4 3 4 3 4 21%	1520
	11th to 20th	Day	36% 15% 4 9% 5% 6% 4 4 4 3 11%	1693
	21st to end	Day	39% 16% 5% 10% 4 5% 3 2 3 4 10%	2442
January	1st to 10th	Night	40% 13% 4 8% 4 4 4 3 4 4 12%	4726
	11th to 20th	Night	34% 8% 2 5% 3 3 4 3 5% 8% 25%	4689
	21st to end	Night	52% 12% 3 6% 3 2 2 2 3 3 12%	4831
February	1st to 10th	Night	35% 12% 3 7% 4 4 4 2 5% 5% 20%	2470
	11th to 20th	Night	29% 13% 3 8% 5% 3 3 3 4 5% 24%	2402
	21st to end	Night	31% 12% 2 6% 4 3 3 3 4 6% 26%	2015
March	1st to 10th	Night	28% 16% 3 8% 4 4 4 3 4 5% 20%	3646
	11th to 20th	Night	23% 8% 3 6% 4 4 4 5% 7% 8% 28%	3489
	21st to end	Night	32% 14% 5% 8% 5% 3 3 3 3 5% 20%	3715
April	1st to 10th	Night	28% 11% 3 7% 6% 4 4 3 5% 5% 23%	1900
	11th to 20th	Night	22% 10% 3 8% 5% 4 4 4 5% 5% 28%	1796
	21st to end	Night	25% 10% 3 8% 5% 5% 5% 4 4 6% 25%	1647
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	20% 7% 3 7% 6% 4 5% 3 4 4 38%	2280
	11th to 20th	Night	30% 7% 2 7% 5% 3 4 3 5% 5% 29%	2280
	21st to end	Night	33% 8% 3 6% 3 4 4 3 4 5% 28%	2575
November	1st to 10th	Night	39% 10% 3 7% 4 5% 4 3 4 5% 17%	2590
	11th to 20th	Night	37% 10% 3 6% 3 3 4 2 4 5% 22%	2660
	21st to end	Night	37% 10% 4 7% 6% 4 4 3 3 4 19%	2660
December	1st to 10th	Night	32% 10% 3 7% 5% 4 4 4 4 5% 24%	2660
	11th to 20th	Night	44% 14% 3 8% 5% 3 3 2 3 3 12%	3510
	21st to end	Night	40% 15% 4 8% 4 3 3 2 2 3 16%	5214

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	47% 41% 6% 3 1 1	2219
	11th to 20th	Day	38% 37% 6% 11% 5% 3	2228
	21st to end	Day	55% 40% 2 3	2808
February	1st to 10th	Day	26% 65% 2 2 3 2	1710
	11th to 20th	Day	25% 65% 3 4 2 1	1674
	21st to end	Day	22% 65% 4 5% 2 1	1438
March	1st to 10th	Day	36% 44% 10% 7% 3	3226
	11th to 20th	Day	26% 44% 9% 20% 2	3490
	21st to end	Day	37% 44% 10% 6% 3	3938
April	1st to 10th	Day	23% 67% 4 5% 1	2280
	11th to 20th	Day	22% 66% 4 5% 2 1	2330
	21st to end	Day	23% 66% 4 5% 2	2532
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	23% 60% 6% 8% 3	1900
	11th to 20th	Day	28% 60% 4 6% 3	1900
	21st to end	Day	24% 61% 4 6% 4	1881
November	1st to 10th	Day	29% 60% 5% 5% 1	1488
	11th to 20th	Day	24% 61% 5% 5% 3 2	1520
	21st to end	Day	24% 61% 5% 5% 5% 1	1520
December	1st to 10th	Day	26% 60% 3 6% 4	1520
	11th to 20th	Day	37% 51% 6% 3 2 1	1696
	21st to end	Day	48% 43% 5% 3 1 1	2442
January	1st to 10th	Night	47% 41% 5% 5% 1 1	4726
	11th to 20th	Night	33% 39% 8% 13% 6% 2	4730
	21st to end	Night	54% 40% 2 3	4831
February	1st to 10th	Night	25% 65% 3 3 3 2	2470
	11th to 20th	Night	26% 64% 3 4 2 1	2404
	21st to end	Night	19% 65% 5% 7% 3 1	2015
March	1st to 10th	Night	37% 45% 7% 7% 5%	3646
	11th to 20th	Night	22% 44% 13% 17% 3	3490
	21st to end	Night	42% 44% 8% 5% 1	3715
April	1st to 10th	Night	22% 66% 4 6% 1	1900
	11th to 20th	Night	21% 66% 4 5% 3 1	1796
	21st to end	Night	22% 66% 4 5% 3	1647
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	21% 61% 7% 8% 3	2280
	11th to 20th	Night	26% 60% 4 7% 3	2280
	21st to end	Night	23% 61% 4 8% 4	2575
November	1st to 10th	Night	29% 59% 6% 5% 1	2590
	11th to 20th	Night	25% 60% 4 5% 3 2	2660
	21st to end	Night	27% 61% 3 3 5% 1	2660
December	1st to 10th	Night	25% 61% 3 6% 5% 1	2660
	11th to 20th	Night	41% 52% 3 2 1 1	3506
	21st to end	Night	47% 40% 5% 6% 1	5214

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% 19% 12% 8% 4	180
	11th to 20th	Day	47% 13% 14% 16% 8% 2	180
	21st to end	Day	44% 14% 17% 14% 10% 1	212
February	1st to 10th	Day	43% 11% 20% 18% 6% 2	370
	11th to 20th	Day	37% 13% 21% 20% 8% 1	365
	21st to end	Day	34% 17% 25% 14% 8% 1	320
March	1st to 10th	Day	34% 15% 21% 19% 9% 1	460
	11th to 20th	Day	35% 12% 18% 19% 14% 2	460
	21st to end	Day	38% 15% 19% 19% 8% 1	506
April	1st to 10th	Day	32% 14% 23% 19% 10% 1	460
	11th to 20th	Day	27% 13% 26% 21% 10% 3	452
	21st to end	Day	28% 14% 27% 18% 12% 2	460
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	381
November	1st to 10th	Day	39% 16% 16% 18% 11%	180
	11th to 20th	Day	47% 8% 14% 11% 14% 6%	180
	21st to end	Day	46% 8% 22% 7% 9% 8%	180
December	1st to 10th	Day	44% 14% 13% 12% 11% 5%	180
	11th to 20th	Day	51% 10% 16% 15% 7% 1	180
	21st to end	Day	49% 10% 17% 19% 5% 1	198
January	1st to 10th	Night	49% 13% 16% 14% 7% 1	850
	11th to 20th	Night	46% 9% 15% 22% 8%	845
	21st to end	Night	43% 11% 16% 21% 9%	836
February	1st to 10th	Night	42% 11% 22% 18% 6%	760
	11th to 20th	Night	37% 10% 21% 24% 7%	744
	21st to end	Night	36% 10% 23% 21% 9% 1	628
March	1st to 10th	Night	35% 11% 20% 20% 13% 1	760
	11th to 20th	Night	40% 8% 19% 22% 10% 1	760
	21st to end	Night	40% 9% 18% 22% 10% 1	836
April	1st to 10th	Night	34% 9% 18% 26% 12% 2	760
	11th to 20th	Night	27% 9% 25% 25% 12% 2	668
	21st to end	Night	33% 7% 20% 27% 12% 1	507
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	38% 8% 20% 24% 9% 2	812
November	1st to 10th	Night	46% 9% 20% 22% 4	740
	11th to 20th	Night	45% 8% 22% 16% 9% 1	760
	21st to end	Night	43% 11% 18% 16% 10% 1	760
December	1st to 10th	Night	37% 8% 21% 23% 10%	760
	11th to 20th	Night	51% 11% 20% 14% 3	845
	21st to end	Night	49% 12% 22% 14% 3	935

Page generated at 2024-10-22 17:21:00. Version ac07

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