Hythe Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hythe Observatory clear sky chart from 2005-11-06 to 2024-01-12.

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	11% 9% 4 8% 7% 4 6% 6% 6% 8% 33%	74527
	Night	9% 11% 5% 8% 7% 4 6% 6% 6% 7% 32%	75200
Transparency	Day	48% 9% 16% 20% 7% 1	74190
	Night	50% 8% 14% 19% 8% 1	75055
Seeing	Day	15% 13% 29% 25% 14% 3	15381
	Night	14% 12% 30% 29% 13% 2	23553

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	7% 14% 7% 11% 8% 4 4 6% 7% 8% 25%	1139
	11th to 20th	Day	7% 12% 5% 10% 9% 4 7% 7% 7% 9% 24%	1138
	21st to end	Day	8% 15% 7% 8% 8% 4 6% 5% 6% 6% 27%	1426
February	1st to 10th	Day	8% 13% 5% 10% 8% 5% 5% 6% 6% 10% 24%	1440
	11th to 20th	Day	4 14% 8% 11% 9% 3 5% 6% 6% 8% 25%	1475
	21st to end	Day	5% 13% 6% 11% 9% 5% 8% 6% 7% 7% 24%	1381
March	1st to 10th	Day	7% 10% 5% 11% 10% 5% 7% 6% 6% 8% 27%	1800
	11th to 20th	Day	8% 14% 5% 9% 6% 4 4 5% 7% 8% 30%	1863
	21st to end	Day	8% 15% 7% 9% 7% 3 6% 6% 6% 8% 24%	2358
April	1st to 10th	Day	5% 9% 5% 9% 9% 5% 7% 7% 7% 9% 28%	2160
	11th to 20th	Day	8% 13% 5% 9% 7% 3 5% 6% 6% 8% 30%	2315
	21st to end	Day	12% 10% 4 8% 7% 4 6% 5% 6% 8% 29%	2520
May	1st to 10th	Day	12% 9% 4 8% 8% 5% 6% 6% 5% 7% 31%	2520
	11th to 20th	Day	10% 8% 2 6% 6% 4 6% 6% 7% 8% 36%	2682
	21st to end	Day	16% 7% 3 7% 6% 4 6% 6% 6% 7% 32%	3111
June	1st to 10th	Day	14% 6% 3 6% 7% 3 6% 6% 6% 8% 34%	2880
	11th to 20th	Day	17% 10% 3 8% 7% 4 5% 7% 5% 7% 27%	2880
	21st to end	Day	10% 6% 3 7% 7% 4 6% 6% 7% 9% 35%	2880
July	1st to 10th	Day	14% 6% 3 5% 5% 4 6% 6% 6% 8% 37%	2880
	11th to 20th	Day	12% 6% 3 6% 7% 4 6% 5% 7% 9% 34%	2880
	21st to end	Day	10% 5% 3 6% 6% 3 6% 6% 6% 9% 41%	3031
August	1st to 10th	Day	11% 5% 2 6% 7% 4 6% 5% 6% 8% 39%	2507
	11th to 20th	Day	9% 5% 2 5% 5% 4 6% 6% 7% 8% 44%	2475
	21st to end	Day	12% 7% 3 6% 7% 3 6% 6% 6% 8% 37%	2628
September	1st to 10th	Day	13% 6% 3 5% 6% 4 5% 4 5% 8% 41%	2160
	11th to 20th	Day	13% 7% 3 5% 5% 3 4 6% 6% 7% 41%	2152
	21st to end	Day	10% 8% 3 5% 6% 3 5% 6% 7% 8% 39%	1940
October	1st to 10th	Day	12% 10% 3 9% 7% 3 5% 5% 6% 6% 33%	1800
	11th to 20th	Day	12% 9% 3 7% 7% 4 6% 6% 8% 6% 31%	1674
	21st to end	Day	11% 12% 4 8% 6% 4 5% 4 5% 8% 32%	1722
November	1st to 10th	Day	13% 16% 5% 8% 6% 4 4 5% 6% 7% 28%	1446
	11th to 20th	Day	9% 15% 5% 9% 8% 3 5% 6% 5% 7% 28%	1330
	21st to end	Day	9% 12% 5% 10% 9% 4 6% 4 5% 7% 29%	1262
December	1st to 10th	Day	10% 12% 5% 9% 7% 4 6% 6% 5% 5% 32%	1140
	11th to 20th	Day	12% 13% 4 9% 8% 4 7% 6% 5% 8% 25%	1442
	21st to end	Day	5% 8% 3 7% 6% 4 5% 7% 7% 9% 38%	2090
January	1st to 10th	Night	7% 14% 6% 10% 10% 4 6% 5% 6% 7% 25%	3026
	11th to 20th	Night	7% 11% 6% 9% 8% 5% 8% 6% 7% 7% 26%	2879
	21st to end	Night	8% 16% 7% 8% 7% 4 6% 5% 6% 8% 25%	2929
February	1st to 10th	Night	8% 15% 5% 9% 7% 5% 5% 5% 5% 8% 27%	2520
	11th to 20th	Night	6% 13% 5% 10% 9% 5% 7% 6% 7% 7% 26%	2369
	21st to end	Night	4 10% 7% 11% 8% 6% 8% 6% 5% 8% 28%	1875
March	1st to 10th	Night	5% 11% 5% 11% 8% 5% 5% 5% 5% 8% 32%	2160
	11th to 20th	Night	9% 12% 5% 9% 7% 4 5% 5% 6% 8% 31%	2092
	21st to end	Night	7% 13% 6% 9% 8% 4 5% 5% 7% 8% 28%	1998
April	1st to 10th	Night	4 9% 5% 9% 9% 5% 8% 7% 7% 8% 29%	1800
	11th to 20th	Night	10% 9% 5% 8% 6% 4 6% 7% 6% 8% 32%	1595
	21st to end	Night	11% 11% 5% 7% 6% 4 6% 6% 6% 5% 34%	1440
May	1st to 10th	Night	12% 9% 5% 7% 6% 4 6% 5% 5% 7% 35%	1440
	11th to 20th	Night	10% 9% 4 5% 6% 4 6% 5% 6% 8% 37%	1207
	21st to end	Night	15% 8% 4 7% 6% 2 4 6% 5% 7% 35%	1164
June	1st to 10th	Night	11% 8% 5% 6% 7% 4 6% 5% 6% 6% 36%	1080
	11th to 20th	Night	20% 6% 3 7% 5% 3 6% 6% 9% 9% 26%	1080
	21st to end	Night	11% 6% 2 6% 6% 3 6% 6% 7% 8% 39%	1080
July	1st to 10th	Night	13% 5% 3 4 6% 3 6% 6% 6% 9% 39%	1080
	11th to 20th	Night	12% 5% 3 7% 6% 3 5% 6% 9% 8% 36%	1080
	21st to end	Night	8% 5% 3 4 5% 4 6% 6% 7% 9% 42%	1282
August	1st to 10th	Night	9% 5% 2 5% 5% 3 5% 6% 7% 10% 42%	1432
	11th to 20th	Night	6% 3 2 5% 6% 4 6% 7% 7% 8% 46%	1424
	21st to end	Night	12% 6% 3 6% 7% 3 5% 6% 7% 8% 36%	1727
September	1st to 10th	Night	13% 6% 3 7% 5% 4 4 5% 6% 7% 40%	1800
	11th to 20th	Night	11% 8% 2 5% 4 2 4 5% 6% 8% 43%	1808
	21st to end	Night	11% 7% 3 6% 6% 4 6% 6% 7% 7% 38%	2020
October	1st to 10th	Night	13% 10% 4 7% 6% 4 5% 5% 5% 8% 33%	2160
	11th to 20th	Night	10% 9% 4 7% 8% 5% 6% 5% 6% 7% 34%	2286
	21st to end	Night	12% 13% 4 8% 7% 4 5% 6% 6% 6% 31%	2632
November	1st to 10th	Night	13% 14% 5% 8% 7% 3 6% 5% 5% 6% 27%	2613
	11th to 20th	Night	9% 14% 5% 9% 7% 4 5% 5% 6% 7% 28%	2850
	21st to end	Night	9% 14% 5% 9% 9% 4 4 5% 6% 7% 28%	2916
December	1st to 10th	Night	7% 10% 5% 9% 9% 5% 5% 5% 5% 7% 32%	3040
	11th to 20th	Night	9% 16% 5% 9% 8% 4 7% 5% 6% 7% 25%	3750
	21st to end	Night	4 10% 4 9% 8% 5% 7% 6% 6% 7% 34%	5566

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% 6% 14% 18% 4 1	1139
	11th to 20th	Day	56% 9% 12% 15% 5% 2	1138
	21st to end	Day	57% 8% 11% 17% 7%	1426
February	1st to 10th	Day	54% 9% 13% 12% 11% 1	1440
	11th to 20th	Day	57% 8% 14% 14% 7%	1483
	21st to end	Day	57% 7% 13% 15% 6% 1	1381
March	1st to 10th	Day	53% 9% 11% 18% 8% 1	1800
	11th to 20th	Day	51% 8% 13% 19% 8% 1	1863
	21st to end	Day	56% 10% 16% 14% 5%	2358
April	1st to 10th	Day	49% 9% 16% 17% 8% 1	2160
	11th to 20th	Day	52% 7% 15% 18% 8% 1	2315
	21st to end	Day	50% 10% 12% 21% 7%	2520
May	1st to 10th	Day	50% 9% 14% 17% 9% 1	2520
	11th to 20th	Day	43% 9% 14% 21% 12% 1	2686
	21st to end	Day	48% 8% 14% 20% 9% 2	3112
June	1st to 10th	Day	47% 8% 15% 21% 9%	2880
	11th to 20th	Day	55% 8% 12% 18% 8%	2880
	21st to end	Day	42% 10% 16% 22% 9%	2880
July	1st to 10th	Day	41% 10% 17% 23% 8% 1	2801
	11th to 20th	Day	43% 11% 20% 21% 5%	2793
	21st to end	Day	36% 11% 23% 24% 6%	2862
August	1st to 10th	Day	40% 10% 22% 23% 4 1	2490
	11th to 20th	Day	35% 9% 22% 26% 7%	2475
	21st to end	Day	43% 10% 18% 23% 5%	2628
September	1st to 10th	Day	42% 9% 21% 22% 6% 1	2160
	11th to 20th	Day	41% 9% 18% 25% 7%	2152
	21st to end	Day	42% 9% 17% 22% 9% 1	1940
October	1st to 10th	Day	50% 7% 16% 21% 6%	1800
	11th to 20th	Day	49% 9% 17% 20% 5%	1674
	21st to end	Day	51% 6% 18% 19% 6%	1722
November	1st to 10th	Day	55% 8% 13% 18% 5% 1	1446
	11th to 20th	Day	55% 7% 16% 15% 7%	1330
	21st to end	Day	56% 7% 13% 16% 7% 2	1262
December	1st to 10th	Day	53% 8% 12% 16% 11% 1	1140
	11th to 20th	Day	57% 9% 12% 16% 6%	1444
	21st to end	Day	39% 9% 14% 24% 11% 2	2090
January	1st to 10th	Night	58% 8% 12% 15% 5% 2	3026
	11th to 20th	Night	55% 8% 14% 14% 7% 2	2879
	21st to end	Night	57% 7% 12% 13% 10%	2929
February	1st to 10th	Night	56% 6% 15% 13% 9%	2520
	11th to 20th	Night	55% 9% 12% 19% 5%	2371
	21st to end	Night	54% 7% 13% 18% 7% 1	1875
March	1st to 10th	Night	50% 8% 15% 19% 8% 1	2160
	11th to 20th	Night	50% 6% 14% 22% 8% 1	2093
	21st to end	Night	52% 8% 13% 19% 8%	1998
April	1st to 10th	Night	49% 8% 15% 18% 9% 1	1800
	11th to 20th	Night	49% 8% 13% 20% 9% 1	1595
	21st to end	Night	48% 9% 10% 22% 9% 1	1440
May	1st to 10th	Night	48% 7% 14% 20% 11% 1	1440
	11th to 20th	Night	43% 8% 11% 22% 15% 1	1207
	21st to end	Night	46% 7% 14% 21% 10% 1	1164
June	1st to 10th	Night	48% 6% 12% 24% 10%	1080
	11th to 20th	Night	49% 9% 16% 20% 5% 1	1080
	21st to end	Night	40% 10% 19% 21% 11%	1080
July	1st to 10th	Night	38% 9% 17% 27% 8% 1	1050
	11th to 20th	Night	42% 11% 18% 23% 5%	1044
	21st to end	Night	33% 11% 25% 26% 4	1211
August	1st to 10th	Night	34% 11% 23% 24% 7%	1424
	11th to 20th	Night	31% 11% 21% 28% 8% 1	1424
	21st to end	Night	42% 10% 18% 23% 6% 1	1727
September	1st to 10th	Night	43% 9% 19% 22% 6% 1	1800
	11th to 20th	Night	39% 7% 18% 27% 8%	1808
	21st to end	Night	43% 10% 13% 22% 11%	2020
October	1st to 10th	Night	49% 8% 16% 20% 6% 1	2160
	11th to 20th	Night	47% 9% 16% 23% 4	2286
	21st to end	Night	52% 7% 14% 17% 9%	2632
November	1st to 10th	Night	56% 7% 12% 18% 5% 1	2613
	11th to 20th	Night	55% 8% 13% 16% 8%	2850
	21st to end	Night	55% 8% 12% 17% 6% 2	2916
December	1st to 10th	Night	51% 8% 13% 16% 11% 1	3040
	11th to 20th	Night	58% 8% 12% 15% 7%	3747
	21st to end	Night	47% 8% 13% 23% 8% 2	5566

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	12% 14% 30% 28% 12% 4	180
	11th to 20th	Day	18% 15% 28% 27% 10% 2	163
	21st to end	Day	15% 15% 35% 20% 13% 2	176
February	1st to 10th	Day	14% 18% 32% 21% 13% 2	160
	11th to 20th	Day	14% 15% 32% 28% 9% 2	235
	21st to end	Day	10% 16% 34% 27% 12% 1	302
March	1st to 10th	Day	11% 15% 32% 26% 12% 2	420
	11th to 20th	Day	10% 14% 35% 27% 11% 3	420
	21st to end	Day	16% 16% 31% 23% 13% 1	462
April	1st to 10th	Day	10% 20% 33% 24% 10% 3	420
	11th to 20th	Day	15% 16% 30% 24% 13% 2	412
	21st to end	Day	19% 17% 30% 22% 10% 2	420
May	1st to 10th	Day	18% 15% 32% 23% 9% 2	423
	11th to 20th	Day	16% 8% 26% 29% 18% 3	652
	21st to end	Day	16% 11% 24% 29% 17% 4	874
June	1st to 10th	Day	15% 16% 31% 24% 11% 2	810
	11th to 20th	Day	21% 11% 26% 24% 15% 3	810
	21st to end	Day	13% 8% 28% 26% 19% 7%	810
July	1st to 10th	Day	14% 8% 27% 28% 20% 4	796
	11th to 20th	Day	14% 8% 29% 26% 18% 5%	792
	21st to end	Day	12% 10% 26% 31% 16% 5%	777
August	1st to 10th	Day	14% 11% 23% 28% 16% 8%	447
	11th to 20th	Day	11% 17% 32% 20% 14% 4	437
	21st to end	Day	15% 14% 32% 23% 12% 5%	495
September	1st to 10th	Day	18% 15% 28% 22% 13% 4	450
	11th to 20th	Day	17% 14% 29% 25% 11% 4	450
	21st to end	Day	15% 19% 32% 20% 12% 2	450
October	1st to 10th	Day	21% 18% 27% 20% 12% 3	450
	11th to 20th	Day	20% 13% 30% 23% 11% 2	324
	21st to end	Day	17% 16% 31% 23% 10% 3	266
November	1st to 10th	Day	19% 14% 32% 23% 11% 1	180
	11th to 20th	Day	24% 13% 35% 21% 6% 1	180
	21st to end	Day	15% 19% 29% 26% 8% 3	180
December	1st to 10th	Day	8% 20% 29% 30% 7% 6%	180
	11th to 20th	Day	14% 19% 27% 29% 9% 1	180
	21st to end	Day	11% 12% 32% 29% 14% 3	198
January	1st to 10th	Night	12% 13% 30% 31% 12% 2	1040
	11th to 20th	Night	13% 18% 30% 29% 10% 1	959
	21st to end	Night	18% 19% 30% 25% 7% 1	861
February	1st to 10th	Night	17% 15% 26% 23% 15% 4	720
	11th to 20th	Night	13% 15% 30% 27% 13% 2	704
	21st to end	Night	8% 12% 30% 30% 18% 3	592
March	1st to 10th	Night	9% 14% 29% 29% 15% 3	720
	11th to 20th	Night	12% 11% 31% 31% 12% 2	720
	21st to end	Night	13% 9% 30% 29% 18% 2	792
April	1st to 10th	Night	9% 10% 35% 30% 14% 3	720
	11th to 20th	Night	14% 8% 30% 32% 14% 2	527
	21st to end	Night	14% 14% 30% 27% 15% 1	360
May	1st to 10th	Night	14% 12% 28% 34% 12% 1	360
	11th to 20th	Night	11% 9% 32% 32% 14% 1	354
	21st to end	Night	16% 7% 25% 30% 20% 2	389
June	1st to 10th	Night	15% 9% 31% 28% 17% 1	360
	11th to 20th	Night	21% 5% 23% 30% 18% 2	360
	21st to end	Night	13% 5% 27% 32% 21% 2	360
July	1st to 10th	Night	14% 3 27% 37% 18% 1	350
	11th to 20th	Night	15% 5% 27% 28% 21% 3	348
	21st to end	Night	9% 4 24% 39% 21% 4	394
August	1st to 10th	Night	9% 6% 19% 34% 26% 5%	358
	11th to 20th	Night	5% 8% 34% 31% 20% 2	356
	21st to end	Night	15% 7% 30% 28% 19% 1	539
September	1st to 10th	Night	16% 8% 28% 32% 15% 2	720
	11th to 20th	Night	16% 8% 25% 30% 18% 2	720
	21st to end	Night	14% 15% 31% 26% 12% 2	720
October	1st to 10th	Night	18% 12% 31% 27% 11% 2	720
	11th to 20th	Night	14% 11% 34% 29% 10% 1	720
	21st to end	Night	20% 16% 31% 25% 8% 1	792
November	1st to 10th	Night	18% 13% 32% 25% 11% 1	763
	11th to 20th	Night	17% 17% 35% 26% 5% 1	950
	21st to end	Night	15% 16% 34% 26% 8% 1	981
December	1st to 10th	Night	11% 17% 33% 29% 8% 1	1040
	11th to 20th	Night	14% 15% 32% 26% 12% 2	1040
	21st to end	Night	13% 12% 31% 33% 10% 1	1144

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