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 2025-09-19.

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	9% 9% 3 9% 6% 6% 6% 5% 7% 8% 33%	130760
	Night	8% 11% 4 10% 6% 6% 5% 5% 6% 8% 32%	126973
Transparency	Day	46% 8% 15% 21% 9% 1	130437
	Night	48% 8% 15% 20% 9% 1	126873
Seeing	Day	14% 13% 29% 26% 14% 4	17817
	Night	13% 12% 30% 29% 13% 2	25630

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	5% 14% 7% 13% 7% 7% 6% 4 8% 8% 22%	2719
	11th to 20th	Day	5% 11% 4 11% 7% 8% 8% 7% 10% 8% 21%	2822
	21st to end	Day	6% 10% 4 8% 6% 6% 6% 5% 7% 8% 34%	3608
February	1st to 10th	Day	7% 12% 5% 12% 7% 7% 4 5% 7% 9% 25%	1600
	11th to 20th	Day	3 15% 6% 13% 8% 4 5% 5% 6% 8% 27%	1644
	21st to end	Day	5% 14% 5% 12% 8% 8% 7% 4 7% 7% 24%	1540
March	1st to 10th	Day	4 7% 4 15% 10% 9% 7% 5% 6% 7% 26%	4518
	11th to 20th	Day	6% 15% 5% 12% 6% 5% 5% 4 7% 8% 27%	4727
	21st to end	Day	6% 12% 5% 12% 6% 5% 5% 4 6% 7% 31%	5925
April	1st to 10th	Day	4 10% 4 10% 8% 6% 8% 5% 7% 9% 29%	2400
	11th to 20th	Day	7% 13% 5% 10% 6% 5% 5% 5% 6% 8% 31%	2578
	21st to end	Day	10% 11% 4 10% 6% 6% 5% 5% 6% 8% 29%	2800
May	1st to 10th	Day	7% 8% 3 8% 6% 5% 6% 5% 8% 9% 35%	6320
	11th to 20th	Day	7% 10% 4 11% 7% 7% 7% 5% 8% 9% 26%	6832
	21st to end	Day	12% 8% 3 9% 6% 6% 6% 6% 7% 9% 28%	7907
June	1st to 10th	Day	13% 8% 3 8% 6% 5% 5% 4 6% 7% 35%	3200
	11th to 20th	Day	17% 10% 3 8% 6% 5% 5% 5% 5% 7% 27%	3200
	21st to end	Day	10% 7% 3 8% 6% 6% 6% 5% 7% 8% 34%	3200
July	1st to 10th	Day	11% 5% 2 7% 5% 5% 5% 4 6% 8% 42%	7240
	11th to 20th	Day	8% 6% 2 7% 5% 5% 5% 4 6% 8% 44%	7240
	21st to end	Day	12% 5% 2 6% 5% 4 5% 4 6% 9% 42%	7595
August	1st to 10th	Day	10% 6% 2 7% 5% 4 4 3 5% 6% 48%	4541
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 4 7% 9% 38%	4470
	21st to end	Day	13% 6% 2 7% 5% 4 4 4 6% 7% 41%	4747
September	1st to 10th	Day	12% 6% 3 6% 5% 5% 4 3 6% 8% 43%	2400
	11th to 20th	Day	12% 7% 3 6% 4 4 5% 4 6% 7% 41%	2368
	21st to end	Day	10% 8% 2 7% 5% 5% 5% 5% 7% 8% 38%	2047
October	1st to 10th	Day	12% 10% 3 10% 6% 5% 5% 5% 7% 7% 31%	3180
	11th to 20th	Day	10% 10% 3 9% 6% 6% 6% 5% 7% 7% 31%	2938
	21st to end	Day	10% 12% 4 8% 4 4 5% 4 7% 9% 33%	2989
November	1st to 10th	Day	13% 17% 4 9% 5% 5% 4 3 6% 7% 28%	1523
	11th to 20th	Day	9% 16% 5% 11% 7% 4 4 4 5% 6% 27%	1400
	21st to end	Day	8% 15% 5% 10% 8% 6% 4 4 5% 7% 28%	1328
December	1st to 10th	Day	11% 18% 5% 11% 6% 6% 6% 4 5% 5% 22%	1960
	11th to 20th	Day	10% 16% 5% 10% 6% 7% 7% 4 6% 7% 21%	2262
	21st to end	Day	4 8% 3 10% 6% 5% 5% 5% 7% 8% 40%	2992
January	1st to 10th	Night	4 13% 6% 15% 8% 5% 5% 5% 7% 8% 24%	7226
	11th to 20th	Night	4 10% 5% 11% 7% 8% 7% 6% 10% 9% 22%	7107
	21st to end	Night	8% 15% 4 7% 5% 5% 6% 5% 6% 7% 31%	7347
February	1st to 10th	Night	7% 16% 5% 11% 6% 6% 5% 4 5% 7% 27%	2800
	11th to 20th	Night	5% 13% 4 12% 7% 6% 5% 5% 7% 7% 28%	2640
	21st to end	Night	4 11% 6% 12% 7% 8% 5% 4 6% 8% 28%	2090
March	1st to 10th	Night	2 8% 3 14% 9% 7% 7% 5% 6% 8% 30%	5376
	11th to 20th	Night	6% 13% 5% 12% 6% 6% 6% 4 7% 9% 27%	5268
	21st to end	Night	6% 11% 4 9% 5% 4 4 3 5% 8% 40%	5050
April	1st to 10th	Night	4 10% 4 10% 8% 7% 7% 5% 7% 7% 30%	2000
	11th to 20th	Night	8% 9% 4 9% 5% 5% 6% 5% 7% 8% 33%	1771
	21st to end	Night	11% 11% 4 8% 6% 5% 6% 5% 7% 5% 34%	1600
May	1st to 10th	Night	6% 10% 3 8% 6% 5% 6% 5% 6% 7% 39%	3640
	11th to 20th	Night	8% 9% 4 10% 8% 6% 6% 5% 7% 9% 30%	3057
	21st to end	Night	9% 7% 3 9% 6% 5% 6% 6% 8% 10% 32%	2968
June	1st to 10th	Night	11% 8% 5% 7% 7% 6% 5% 4 5% 7% 36%	1200
	11th to 20th	Night	19% 9% 3 8% 4 5% 5% 5% 8% 9% 26%	1200
	21st to end	Night	11% 6% 2 8% 6% 5% 5% 5% 7% 8% 38%	1200
July	1st to 10th	Night	11% 5% 3 6% 5% 5% 5% 5% 6% 7% 41%	2720
	11th to 20th	Night	9% 6% 2 7% 5% 4 6% 4 6% 7% 44%	2720
	21st to end	Night	10% 5% 2 6% 5% 5% 5% 5% 7% 9% 42%	3244
August	1st to 10th	Night	9% 5% 2 6% 4 4 5% 4 6% 8% 49%	2609
	11th to 20th	Night	11% 5% 2 7% 5% 5% 5% 4 7% 7% 42%	2577
	21st to end	Night	10% 5% 2 7% 6% 6% 6% 5% 7% 9% 38%	3145
September	1st to 10th	Night	12% 6% 3 7% 4 5% 4 4 6% 7% 42%	2000
	11th to 20th	Night	11% 8% 3 6% 4 4 4 4 6% 8% 42%	1989
	21st to end	Night	11% 8% 3 7% 5% 6% 5% 5% 7% 7% 38%	2133
October	1st to 10th	Night	11% 9% 3 9% 6% 6% 5% 5% 6% 7% 32%	3800
	11th to 20th	Night	7% 7% 3 8% 5% 6% 6% 4 6% 8% 40%	3993
	21st to end	Night	9% 12% 3 10% 6% 6% 6% 5% 6% 7% 31%	4564
November	1st to 10th	Night	12% 15% 5% 9% 6% 5% 6% 4 5% 6% 27%	2755
	11th to 20th	Night	9% 16% 5% 10% 6% 6% 4 4 6% 7% 28%	3000
	21st to end	Night	8% 17% 5% 12% 7% 4 4 4 6% 7% 27%	3070
December	1st to 10th	Night	9% 13% 5% 10% 8% 6% 5% 4 6% 7% 28%	5220
	11th to 20th	Night	9% 18% 5% 12% 7% 6% 5% 3 5% 6% 24%	5930
	21st to end	Night	4 11% 5% 11% 6% 6% 6% 5% 6% 7% 33%	7964

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	56% 6% 12% 16% 10% 1	2719
	11th to 20th	Day	54% 7% 13% 14% 10% 1	2840
	21st to end	Day	45% 9% 21% 16% 8%	3608
February	1st to 10th	Day	54% 9% 13% 11% 13% 1	1600
	11th to 20th	Day	55% 8% 14% 15% 8% 1	1652
	21st to end	Day	57% 7% 13% 16% 7% 1	1540
March	1st to 10th	Day	53% 8% 9% 21% 9%	4518
	11th to 20th	Day	51% 8% 12% 19% 10%	4727
	21st to end	Day	49% 8% 13% 19% 11%	5925
April	1st to 10th	Day	48% 9% 15% 19% 9% 1	2400
	11th to 20th	Day	49% 7% 14% 19% 10% 1	2578
	21st to end	Day	50% 9% 12% 22% 7% 1	2800
May	1st to 10th	Day	42% 8% 11% 23% 14% 2	6320
	11th to 20th	Day	48% 9% 14% 20% 7% 1	6836
	21st to end	Day	47% 9% 15% 18% 10% 1	7908
June	1st to 10th	Day	47% 7% 14% 21% 10% 1	3200
	11th to 20th	Day	55% 7% 12% 18% 8%	3200
	21st to end	Day	43% 10% 16% 21% 9%	3200
July	1st to 10th	Day	38% 7% 15% 21% 18% 2	7161
	11th to 20th	Day	37% 7% 15% 27% 13% 1	7153
	21st to end	Day	36% 8% 22% 27% 7% 1	7381
August	1st to 10th	Day	36% 7% 16% 32% 9%	4524
	11th to 20th	Day	39% 9% 18% 24% 11%	4512
	21st to end	Day	41% 8% 22% 23% 5%	4746
September	1st to 10th	Day	38% 8% 20% 26% 6% 1	2400
	11th to 20th	Day	41% 8% 18% 26% 7%	2368
	21st to end	Day	42% 9% 17% 22% 9% 1	2047
October	1st to 10th	Day	50% 6% 19% 20% 5%	3180
	11th to 20th	Day	50% 7% 15% 24% 4	2938
	21st to end	Day	48% 6% 25% 18% 4	2989
November	1st to 10th	Day	55% 8% 13% 19% 5% 1	1523
	11th to 20th	Day	56% 7% 15% 15% 7%	1400
	21st to end	Day	57% 7% 12% 16% 7% 2	1328
December	1st to 10th	Day	64% 6% 13% 10% 6%	1960
	11th to 20th	Day	60% 8% 14% 13% 4	2264
	21st to end	Day	39% 7% 15% 28% 10% 1	2992
January	1st to 10th	Night	56% 7% 10% 17% 10% 1	7226
	11th to 20th	Night	53% 7% 13% 14% 12% 1	7151
	21st to end	Night	49% 9% 18% 14% 9%	7347
February	1st to 10th	Night	56% 6% 13% 13% 11%	2800
	11th to 20th	Night	53% 8% 11% 20% 7% 1	2642
	21st to end	Night	53% 7% 13% 19% 7% 1	2090
March	1st to 10th	Night	49% 7% 13% 16% 14%	5376
	11th to 20th	Night	50% 7% 16% 18% 9%	5269
	21st to end	Night	40% 8% 12% 25% 15%	5050
April	1st to 10th	Night	47% 8% 14% 20% 10% 1	2000
	11th to 20th	Night	46% 8% 13% 21% 10% 1	1771
	21st to end	Night	47% 9% 10% 23% 9% 1	1600
May	1st to 10th	Night	42% 6% 12% 24% 13% 2	3637
	11th to 20th	Night	46% 10% 13% 23% 8% 1	3057
	21st to end	Night	42% 10% 19% 17% 9% 3	2968
June	1st to 10th	Night	47% 6% 11% 24% 11% 1	1200
	11th to 20th	Night	51% 8% 15% 20% 5% 1	1200
	21st to end	Night	41% 9% 18% 21% 10%	1200
July	1st to 10th	Night	40% 7% 16% 24% 13% 1	2690
	11th to 20th	Night	37% 7% 13% 29% 14%	2684
	21st to end	Night	35% 8% 22% 28% 6%	3153
August	1st to 10th	Night	31% 8% 20% 35% 6%	2601
	11th to 20th	Night	37% 9% 16% 27% 11% 1	2601
	21st to end	Night	40% 8% 25% 21% 5% 1	3145
September	1st to 10th	Night	41% 8% 19% 23% 7% 2	2000
	11th to 20th	Night	39% 7% 18% 28% 8%	1989
	21st to end	Night	44% 10% 13% 21% 12% 1	2133
October	1st to 10th	Night	49% 7% 17% 19% 8% 1	3800
	11th to 20th	Night	40% 7% 16% 30% 7%	3993
	21st to end	Night	51% 7% 17% 18% 6%	4564
November	1st to 10th	Night	56% 7% 13% 18% 5% 1	2755
	11th to 20th	Night	55% 8% 13% 16% 8%	3000
	21st to end	Night	56% 7% 12% 17% 6% 2	3070
December	1st to 10th	Night	55% 7% 16% 15% 6%	5220
	11th to 20th	Night	63% 6% 15% 12% 5%	5927
	21st to end	Night	48% 7% 13% 25% 6% 1	7964

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% 29% 13% 4	200
	11th to 20th	Day	16% 17% 29% 28% 10% 2	200
	21st to end	Day	14% 15% 38% 19% 13% 2	220
February	1st to 10th	Day	13% 14% 31% 24% 17% 3	200
	11th to 20th	Day	11% 13% 30% 30% 13% 2	284
	21st to end	Day	11% 16% 35% 26% 11% 1	359
March	1st to 10th	Day	10% 14% 32% 28% 13% 2	500
	11th to 20th	Day	11% 15% 33% 27% 11% 3	500
	21st to end	Day	15% 16% 31% 23% 13% 1	550
April	1st to 10th	Day	9% 20% 35% 24% 9% 3	500
	11th to 20th	Day	13% 18% 31% 23% 12% 3	492
	21st to end	Day	17% 17% 30% 23% 10% 3	500
May	1st to 10th	Day	18% 15% 33% 23% 9% 2	503
	11th to 20th	Day	15% 10% 25% 29% 18% 3	757
	21st to end	Day	15% 11% 25% 28% 16% 4	1006
June	1st to 10th	Day	14% 18% 32% 24% 11% 2	930
	11th to 20th	Day	21% 11% 26% 23% 15% 3	930
	21st to end	Day	13% 7% 26% 27% 20% 7%	930
July	1st to 10th	Day	14% 8% 26% 29% 20% 3	916
	11th to 20th	Day	13% 9% 28% 28% 18% 5%	912
	21st to end	Day	12% 11% 25% 30% 17% 5%	896
August	1st to 10th	Day	14% 10% 22% 29% 17% 7%	527
	11th to 20th	Day	11% 16% 32% 21% 15% 4	517
	21st to end	Day	14% 13% 30% 24% 12% 7%	583
September	1st to 10th	Day	15% 13% 27% 24% 14% 6%	530
	11th to 20th	Day	15% 13% 27% 28% 12% 4	523
	21st to end	Day	14% 21% 33% 18% 11% 2	490
October	1st to 10th	Day	20% 18% 27% 21% 11% 3	490
	11th to 20th	Day	21% 12% 30% 24% 11% 2	357
	21st to end	Day	16% 16% 32% 23% 11% 3	295
November	1st to 10th	Day	20% 14% 34% 23% 10% 1	200
	11th to 20th	Day	25% 12% 36% 20% 7% 1	200
	21st to end	Day	14% 18% 30% 26% 9% 4	200
December	1st to 10th	Day	9% 20% 30% 30% 6% 6%	200
	11th to 20th	Day	14% 19% 29% 28% 10% 2	200
	21st to end	Day	10% 12% 31% 30% 14% 4	220
January	1st to 10th	Night	12% 12% 30% 31% 12% 2	1100
	11th to 20th	Night	12% 18% 30% 30% 10% 1	1063
	21st to end	Night	17% 20% 32% 23% 7% 1	966
February	1st to 10th	Night	16% 14% 25% 24% 16% 4	800
	11th to 20th	Night	12% 14% 30% 29% 14% 2	784
	21st to end	Night	8% 13% 31% 29% 16% 3	660
March	1st to 10th	Night	8% 14% 31% 29% 15% 3	800
	11th to 20th	Night	13% 11% 31% 31% 12% 2	800
	21st to end	Night	13% 8% 30% 30% 18% 2	880
April	1st to 10th	Night	9% 10% 35% 30% 13% 3	800
	11th to 20th	Night	13% 10% 31% 32% 13% 2	583
	21st to end	Night	13% 13% 31% 27% 15% 1	400
May	1st to 10th	Night	12% 13% 29% 33% 12% 1	400
	11th to 20th	Night	11% 10% 32% 32% 15% 1	394
	21st to end	Night	15% 7% 27% 29% 20% 2	433
June	1st to 10th	Night	14% 11% 30% 29% 16% 1	400
	11th to 20th	Night	21% 6% 23% 30% 19% 2	400
	21st to end	Night	13% 5% 27% 32% 20% 2	400
July	1st to 10th	Night	13% 4 28% 35% 19% 1	390
	11th to 20th	Night	14% 6% 27% 28% 21% 3	388
	21st to end	Night	9% 4 25% 37% 22% 4	438
August	1st to 10th	Night	9% 6% 20% 35% 26% 4	398
	11th to 20th	Night	7% 8% 32% 32% 19% 2	396
	21st to end	Night	13% 7% 30% 29% 20% 1	599
September	1st to 10th	Night	15% 8% 26% 34% 15% 2	800
	11th to 20th	Night	15% 8% 25% 31% 19% 2	792
	21st to end	Night	14% 15% 32% 26% 12% 2	760
October	1st to 10th	Night	18% 12% 31% 27% 11% 1	760
	11th to 20th	Night	14% 11% 35% 29% 10% 1	760
	21st to end	Night	19% 16% 32% 25% 7% 1	836
November	1st to 10th	Night	18% 14% 32% 24% 10% 1	805
	11th to 20th	Night	17% 16% 35% 26% 5% 1	1000
	21st to end	Night	15% 15% 34% 26% 9% 1	1035
December	1st to 10th	Night	11% 17% 33% 29% 8% 1	1100
	11th to 20th	Night	14% 14% 31% 28% 12% 2	1100
	21st to end	Night	13% 11% 31% 33% 11% 1	1210

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