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 2024-02-25.

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	25% 8% 3 6% 5% 3 4 4 5% 6% 30%	77990
	Night	30% 9% 3 6% 5% 3 4 4 4 5% 28%	81242
Transparency	Day	37% 38% 10% 10% 4 1	77701
	Night	41% 36% 8% 10% 4 1	81111
Seeing	Day	27% 10% 21% 25% 13% 3	14284
	Night	33% 7% 17% 23% 17% 2	22738

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% 13% 4 6% 6% 3 4 3 4 5% 10%	2219
	11th to 20th	Day	30% 12% 4 6% 5% 3 4 2 3 6% 26%	2207
	21st to end	Day	56% 11% 3 5% 4 2 2 2 3 3 9%	2808
February	1st to 10th	Day	35% 11% 3 8% 6% 3 5% 3 4 6% 16%	1710
	11th to 20th	Day	32% 9% 4 7% 7% 3 4 3 3 6% 21%	1665
	21st to end	Day	26% 13% 4 6% 7% 4 4 4 4 5% 21%	1392
March	1st to 10th	Day	31% 11% 4 8% 5% 3 4 5% 4 6% 19%	1849
	11th to 20th	Day	34% 11% 4 6% 6% 3 3 4 3 5% 20%	1980
	21st to end	Day	29% 11% 4 7% 6% 4 5% 5% 4 5% 19%	2240
April	1st to 10th	Day	28% 9% 4 7% 7% 3 5% 6% 5% 6% 20%	2160
	11th to 20th	Day	23% 8% 4 9% 6% 4 6% 5% 6% 5% 24%	2205
	21st to end	Day	23% 10% 4 7% 7% 3 5% 5% 5% 6% 25%	2398
May	1st to 10th	Day	18% 7% 3 7% 6% 3 6% 5% 5% 6% 34%	2520
	11th to 20th	Day	17% 9% 3 6% 5% 4 6% 5% 6% 8% 32%	2477
	21st to end	Day	20% 8% 4 7% 6% 3 5% 5% 5% 6% 32%	2749
June	1st to 10th	Day	22% 8% 4 6% 6% 4 5% 5% 5% 6% 31%	2700
	11th to 20th	Day	22% 8% 4 8% 6% 3 6% 5% 6% 8% 25%	2700
	21st to end	Day	14% 7% 3 6% 5% 4 6% 4 6% 7% 38%	2700
July	1st to 10th	Day	13% 5% 2 5% 5% 2 4 4 5% 7% 50%	2700
	11th to 20th	Day	11% 4 2 5% 4 2 4 4 6% 6% 53%	2700
	21st to end	Day	9% 2 1 3 3 2 2 3 4 6% 63%	2772
August	1st to 10th	Day	10% 5% 2 4 4 2 3 3 5% 5% 56%	2498
	11th to 20th	Day	9% 5% 2 4 4 2 4 4 5% 7% 56%	2298
	21st to end	Day	14% 5% 3 4 5% 3 4 4 5% 8% 45%	2507
September	1st to 10th	Day	15% 7% 2 5% 5% 3 3 3 5% 6% 46%	2160
	11th to 20th	Day	19% 7% 3 7% 4 3 3 4 5% 6% 40%	2160
	21st to end	Day	25% 7% 3 5% 5% 3 4 4 4 6% 36%	1896
October	1st to 10th	Day	25% 7% 2 6% 4 3 4 5% 5% 6% 33%	1800
	11th to 20th	Day	32% 9% 3 7% 4 3 4 4 5% 7% 22%	1800
	21st to end	Day	36% 8% 3 5% 5% 3 4 4 4 6% 22%	1837
November	1st to 10th	Day	40% 8% 3 5% 7% 3 5% 4 5% 5% 16%	1488
	11th to 20th	Day	38% 10% 3 5% 6% 3 4 4 3 4 21%	1520
	21st to end	Day	39% 10% 4 6% 5% 2 4 5% 4 5% 17%	1520
December	1st to 10th	Day	34% 11% 5% 6% 4 3 3 4 3 4 22%	1520
	11th to 20th	Day	36% 14% 4 7% 6% 5% 4 4 5% 3 11%	1693
	21st to end	Day	39% 15% 5% 9% 5% 3 4 3 3 3 10%	2442
January	1st to 10th	Night	40% 13% 4 7% 5% 3 4 4 4 4 12%	4726
	11th to 20th	Night	34% 8% 2 5% 4 2 4 4 5% 8% 25%	4689
	21st to end	Night	52% 12% 3 5% 3 2 2 2 3 3 12%	4831
February	1st to 10th	Night	36% 11% 3 6% 4 3 4 3 5% 5% 20%	2470
	11th to 20th	Night	29% 13% 3 6% 6% 2 4 4 4 5% 24%	2402
	21st to end	Night	31% 10% 3 5% 4 3 3 4 4 5% 26%	1957
March	1st to 10th	Night	32% 9% 3 7% 5% 3 4 4 4 5% 24%	2109
	11th to 20th	Night	34% 9% 3 6% 5% 3 4 4 4 5% 22%	1979
	21st to end	Night	34% 11% 4 7% 6% 3 4 4 4 5% 19%	2116
April	1st to 10th	Night	29% 11% 4 6% 7% 3 4 4 5% 5% 23%	1800
	11th to 20th	Night	23% 10% 4 7% 6% 3 4 5% 4 6% 27%	1701
	21st to end	Night	25% 9% 4 7% 6% 3 6% 5% 4 6% 25%	1561
May	1st to 10th	Night	18% 7% 3 5% 5% 3 4 4 5% 6% 40%	1440
	11th to 20th	Night	16% 6% 3 5% 5% 3 6% 5% 8% 8% 36%	1416
	21st to end	Night	17% 7% 2 6% 6% 4 5% 5% 6% 8% 36%	1522
June	1st to 10th	Night	22% 8% 3 6% 6% 3 4 5% 4 8% 32%	1260
	11th to 20th	Night	17% 9% 5% 7% 6% 4 5% 6% 5% 7% 28%	1260
	21st to end	Night	12% 6% 3 5% 5% 3 4 4 6% 7% 44%	1260
July	1st to 10th	Night	11% 5% 2 4 6% 3 3 3 5% 6% 54%	1260
	11th to 20th	Night	8% 4 2 4 4 3 3 3 3 7% 60%	1260
	21st to end	Night	4 2 1 3 3 2 3 4 4 5% 67%	1545
August	1st to 10th	Night	9% 3 1 3 3 2 3 4 4 6% 63%	1440
	11th to 20th	Night	7% 3 1 3 2 2 3 4 4 8% 63%	1602
	21st to end	Night	9% 4 3 5% 6% 3 4 4 6% 10% 47%	1848
September	1st to 10th	Night	10% 4 2 4 3 2 5% 5% 5% 6% 56%	1800
	11th to 20th	Night	19% 7% 2 5% 5% 3 4 3 4 6% 42%	1800
	21st to end	Night	21% 6% 2 6% 5% 3 4 3 4 6% 39%	2060
October	1st to 10th	Night	20% 7% 3 6% 6% 2 5% 4 4 4 39%	2160
	11th to 20th	Night	30% 7% 3 6% 6% 2 4 4 5% 5% 29%	2160
	21st to end	Night	34% 8% 2 5% 4 3 4 3 4 5% 28%	2514
November	1st to 10th	Night	39% 10% 3 6% 5% 3 4 3 4 5% 17%	2590
	11th to 20th	Night	37% 9% 4 5% 4 3 4 3 4 5% 22%	2660
	21st to end	Night	37% 9% 4 6% 7% 3 4 3 3 4 19%	2660
December	1st to 10th	Night	32% 9% 3 6% 5% 3 4 5% 4 5% 24%	2660
	11th to 20th	Night	44% 13% 4 7% 6% 2 3 3 3 3 12%	3510
	21st to end	Night	40% 14% 4 7% 5% 2 3 3 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	62% 28% 6% 3	2219
	11th to 20th	Day	47% 27% 7% 11% 5% 3	2228
	21st to end	Day	66% 24% 4 4 2	2808
February	1st to 10th	Day	47% 38% 8% 5% 2	1710
	11th to 20th	Day	43% 38% 6% 7% 4 1	1674
	21st to end	Day	44% 39% 7% 7% 3	1392
March	1st to 10th	Day	44% 39% 7% 6% 2 1	1849
	11th to 20th	Day	45% 38% 7% 8% 2	1980
	21st to end	Day	44% 39% 8% 8% 1	2240
April	1st to 10th	Day	43% 40% 10% 5% 2	2160
	11th to 20th	Day	37% 39% 8% 8% 6% 1	2205
	21st to end	Day	38% 40% 10% 7% 5% 1	2398
May	1st to 10th	Day	35% 40% 11% 11% 4	2520
	11th to 20th	Day	30% 40% 12% 12% 5%	2480
	21st to end	Day	38% 41% 9% 10% 2	2750
June	1st to 10th	Day	37% 38% 11% 9% 3 2	2700
	11th to 20th	Day	37% 42% 13% 7% 1	2700
	21st to end	Day	29% 40% 13% 12% 5% 2	2700
July	1st to 10th	Day	20% 41% 13% 17% 8% 2	2626
	11th to 20th	Day	20% 39% 13% 21% 7% 1	2619
	21st to end	Day	14% 40% 12% 21% 10% 3	2618
August	1st to 10th	Day	20% 40% 14% 20% 5% 1	2481
	11th to 20th	Day	19% 38% 14% 17% 10% 2	2298
	21st to end	Day	24% 40% 13% 18% 4 1	2507
September	1st to 10th	Day	21% 39% 12% 16% 9% 4	2160
	11th to 20th	Day	27% 39% 10% 13% 9% 2	2160
	21st to end	Day	33% 40% 9% 11% 6% 1	1896
October	1st to 10th	Day	33% 39% 11% 10% 6%	1800
	11th to 20th	Day	38% 41% 11% 8% 2 1	1800
	21st to end	Day	41% 40% 8% 6% 5%	1837
November	1st to 10th	Day	44% 43% 5% 4 2 1	1488
	11th to 20th	Day	45% 41% 6% 5% 2 1	1520
	21st to end	Day	46% 42% 4 5% 2	1520
December	1st to 10th	Day	43% 42% 4 6% 4	1520
	11th to 20th	Day	48% 36% 9% 5% 1 1	1696
	21st to end	Day	58% 31% 6% 3 1	2442
January	1st to 10th	Night	60% 29% 5% 4	4726
	11th to 20th	Night	43% 27% 8% 13% 6% 3	4730
	21st to end	Night	66% 23% 4 5% 2 1	4831
February	1st to 10th	Night	47% 38% 8% 6% 2	2470
	11th to 20th	Night	41% 39% 6% 8% 5% 1	2404
	21st to end	Night	46% 36% 6% 8% 3	1957
March	1st to 10th	Night	42% 39% 9% 8% 1	2109
	11th to 20th	Night	45% 38% 6% 7% 3	1980
	21st to end	Night	46% 39% 8% 7% 1	2116
April	1st to 10th	Night	41% 40% 10% 8% 1	1800
	11th to 20th	Night	37% 39% 7% 10% 5% 2	1701
	21st to end	Night	41% 40% 9% 7% 4	1561
May	1st to 10th	Night	31% 40% 12% 12% 5%	1440
	11th to 20th	Night	27% 41% 12% 13% 6%	1416
	21st to end	Night	35% 40% 12% 10% 3	1522
June	1st to 10th	Night	37% 39% 10% 7% 4 2	1260
	11th to 20th	Night	37% 41% 12% 9% 1	1260
	21st to end	Night	27% 39% 15% 12% 5% 3	1260
July	1st to 10th	Night	21% 39% 9% 19% 8% 3	1225
	11th to 20th	Night	17% 38% 13% 23% 8% 1	1218
	21st to end	Night	11% 39% 9% 22% 15% 5%	1459
August	1st to 10th	Night	18% 38% 13% 21% 9% 1	1432
	11th to 20th	Night	13% 37% 16% 18% 10% 5%	1602
	21st to end	Night	22% 40% 12% 18% 7% 1	1848
September	1st to 10th	Night	15% 39% 12% 20% 11% 4	1800
	11th to 20th	Night	26% 39% 8% 13% 11% 2	1800
	21st to end	Night	30% 40% 9% 13% 6% 2	2060
October	1st to 10th	Night	31% 39% 10% 13% 7%	2160
	11th to 20th	Night	36% 39% 10% 11% 3	2160
	21st to end	Night	38% 40% 8% 8% 5%	2514
November	1st to 10th	Night	44% 42% 5% 6% 2 1	2590
	11th to 20th	Night	43% 43% 7% 5% 2	2660
	21st to end	Night	43% 43% 6% 5% 2	2660
December	1st to 10th	Night	40% 43% 6% 7% 3	2660
	11th to 20th	Night	53% 36% 6% 3 2	3506
	21st to end	Night	57% 28% 6% 7% 2	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% 16% 26% 16% 8% 1	304
March	1st to 10th	Day	34% 15% 23% 20% 7% 1	420
	11th to 20th	Day	37% 12% 18% 19% 12% 1	420
	21st to end	Day	39% 14% 18% 20% 8% 1	462
April	1st to 10th	Day	34% 13% 23% 19% 10% 1	420
	11th to 20th	Day	28% 13% 25% 21% 10% 3	412
	21st to end	Day	26% 14% 28% 19% 12% 1	420
May	1st to 10th	Day	21% 10% 25% 31% 11% 2	423
	11th to 20th	Day	23% 7% 21% 29% 15% 5%	448
	21st to end	Day	22% 11% 27% 24% 12% 4	511
June	1st to 10th	Day	23% 11% 26% 27% 11% 2	630
	11th to 20th	Day	24% 12% 27% 27% 8% 1	630
	21st to end	Day	19% 9% 23% 29% 18% 3	630
July	1st to 10th	Day	15% 6% 26% 32% 17% 3	621
	11th to 20th	Day	10% 8% 24% 38% 16% 4	618
	21st to end	Day	9% 4 19% 37% 23% 7%	518
August	1st to 10th	Day	14% 6% 17% 33% 21% 9%	447
	11th to 20th	Day	9% 3 17% 38% 25% 8%	437
	21st to end	Day	14% 10% 23% 30% 17% 6%	495
September	1st to 10th	Day	19% 8% 24% 30% 15% 4	450
	11th to 20th	Day	23% 9% 18% 28% 20% 3	450
	21st to end	Day	24% 10% 20% 23% 17% 5%	450
October	1st to 10th	Day	30% 7% 19% 21% 18% 5%	450
	11th to 20th	Day	38% 8% 16% 18% 16% 4	450
	21st to end	Day	40% 12% 19% 16% 11% 3	363
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	35% 10% 23% 22% 9% 1	610
March	1st to 10th	Night	35% 10% 21% 20% 13% 1	720
	11th to 20th	Night	41% 9% 19% 22% 9% 1	720
	21st to end	Night	41% 8% 18% 22% 10% 2	792
April	1st to 10th	Night	35% 8% 19% 24% 12% 2	720
	11th to 20th	Night	28% 9% 25% 25% 12% 2	633
	21st to end	Night	33% 6% 21% 27% 13% 1	481
May	1st to 10th	Night	20% 7% 17% 31% 24% 2	360
	11th to 20th	Night	20% 4 9% 33% 31% 4	354
	21st to end	Night	21% 4 12% 33% 27% 2	389
June	1st to 10th	Night	27% 4 14% 29% 24% 2	360
	11th to 20th	Night	22% 7% 19% 29% 20% 3	360
	21st to end	Night	15% 4 15% 29% 33% 3	360
July	1st to 10th	Night	13% 3 11% 31% 39% 4	350
	11th to 20th	Night	11% 4 9% 32% 37% 6%	348
	21st to end	Night	5% 1 9% 27% 50% 8%	394
August	1st to 10th	Night	13% 1 6% 22% 47% 10%	366
	11th to 20th	Night	7% 2 6% 28% 46% 10%	534
	21st to end	Night	13% 3 12% 30% 38% 4	660
September	1st to 10th	Night	14% 2 11% 38% 31% 4	720
	11th to 20th	Night	22% 4 14% 22% 33% 6%	720
	21st to end	Night	27% 5% 14% 29% 24% 2	720
October	1st to 10th	Night	26% 5% 15% 27% 24% 3	720
	11th to 20th	Night	37% 6% 15% 23% 16% 2	720
	21st to end	Night	38% 8% 20% 23% 9% 2	792
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 2024-02-22 03:05:43. Version ac07

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