Jaffa Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Jaffa Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	5% 9% 3 7% 4 4 4 3 5% 6% 51%	139772
	Night	6% 10% 3 7% 4 3 4 3 5% 6% 50%	138921
Transparency	Day	36% 4 7% 21% 21% 12%	134508
	Night	36% 4 8% 21% 21% 11%	135190
Seeing	Day	13% 18% 34% 26% 8% 1	16637
	Night	12% 10% 26% 32% 17% 3	27224

By Month

Percentage Cloud Cover SW (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	8% 20% 6% 11% 5% 4 3 2 5% 6% 29%	5318
	11th to 20th	Day	4 13% 4 13% 5% 3 3 2 4 5% 44%	5340
	21st to end	Day	4 11% 4 10% 4 3 3 3 4 7% 46%	5844
February	1st to 10th	Day	11% 16% 5% 10% 5% 4 5% 3 3 6% 32%	1890
	11th to 20th	Day	9% 18% 6% 12% 5% 5% 4 3 5% 6% 29%	1922
	21st to end	Day	6% 20% 5% 11% 5% 3 4 4 6% 6% 30%	1730
March	1st to 10th	Day	4 17% 8% 12% 5% 4 4 3 5% 6% 34%	4847
	11th to 20th	Day	4 12% 7% 9% 5% 5% 5% 4 5% 6% 38%	5000
	21st to end	Day	7% 20% 8% 13% 6% 4 3 3 4 4 28%	5486
April	1st to 10th	Day	9% 14% 4 8% 5% 4 4 4 4 6% 38%	2218
	11th to 20th	Day	9% 15% 4 8% 4 3 4 3 5% 5% 39%	2523
	21st to end	Day	9% 12% 2 7% 5% 4 4 3 6% 6% 42%	2600
May	1st to 10th	Day	7% 11% 3 7% 4 4 4 3 5% 6% 45%	5880
	11th to 20th	Day	8% 8% 2 7% 4 4 4 3 6% 7% 46%	5841
	21st to end	Day	5% 7% 2 5% 3 3 4 4 6% 7% 53%	6423
June	1st to 10th	Day	4 5% 2 5% 4 4 4 4 6% 8% 54%	2635
	11th to 20th	Day	5% 5% 2 4 3 3 3 3 4 6% 62%	2770
	21st to end	Day	2 3 1 3 3 3 3 3 4 4 71%	2600
July	1st to 10th	Day	1 2 1 3 3 3 3 3 3 4 73%	5879
	11th to 20th	Day	2 3 1 4 4 3 5% 4 6% 8% 60%	5880
	21st to end	Day	2 2 1 3 3 3 4 3 4 6% 69%	6401
August	1st to 10th	Day	2 2 1 4 3 3 4 3 4 6% 68%	5779
	11th to 20th	Day	2 2 1 3 3 4 3 3 4 5% 69%	5798
	21st to end	Day	3 3 1 5% 4 4 5% 4 6% 6% 58%	6410
September	1st to 10th	Day	3 3 1 3 3 3 4 4 4 6% 68%	2256
	11th to 20th	Day	4 4 1 4 3 3 4 4 5% 5% 62%	2200
	21st to end	Day	7% 3 1 3 2 2 3 2 4 6% 67%	2200
October	1st to 10th	Day	5% 5% 2 3 3 3 2 3 4 4 67%	3438
	11th to 20th	Day	8% 7% 1 4 2 2 3 3 4 8% 57%	3250
	21st to end	Day	7% 9% 2 5% 3 4 3 2 4 5% 56%	3532
November	1st to 10th	Day	9% 11% 3 5% 4 3 3 3 4 6% 47%	1854
	11th to 20th	Day	12% 10% 3 7% 3 4 3 3 4 6% 44%	1890
	21st to end	Day	7% 14% 4 8% 5% 4 4 2 4 5% 43%	1752
December	1st to 10th	Day	7% 8% 3 5% 3 3 3 2 4 8% 55%	2700
	11th to 20th	Day	8% 13% 4 10% 5% 5% 4 3 6% 10% 32%	3106
	21st to end	Day	11% 15% 6% 11% 5% 4 4 3 4 6% 29%	4580
January	1st to 10th	Night	7% 17% 6% 13% 5% 3 3 3 4 6% 32%	7605
	11th to 20th	Night	7% 14% 3 10% 3 2 2 2 3 5% 49%	7650
	21st to end	Night	3 9% 4 9% 4 3 3 3 5% 7% 49%	8370
February	1st to 10th	Night	9% 13% 5% 9% 5% 4 4 4 5% 5% 36%	2730
	11th to 20th	Night	7% 15% 3 9% 6% 4 4 3 4 6% 38%	2584
	21st to end	Night	6% 14% 5% 11% 4 3 4 4 5% 6% 39%	2076
March	1st to 10th	Night	4 17% 6% 10% 5% 4 5% 4 5% 6% 36%	5092
	11th to 20th	Night	4 15% 5% 7% 4 5% 5% 4 6% 7% 38%	5000
	21st to end	Night	8% 19% 5% 7% 4 3 4 3 4 5% 37%	5489
April	1st to 10th	Night	11% 13% 3 6% 3 3 4 3 4 6% 45%	2180
	11th to 20th	Night	7% 11% 3 6% 4 4 5% 3 6% 7% 44%	1820
	21st to end	Night	10% 11% 2 7% 5% 3 5% 3 5% 5% 45%	1800
May	1st to 10th	Night	7% 11% 2 4 4 3 4 3 5% 7% 48%	4100
	11th to 20th	Night	6% 6% 2 5% 4 4 3 3 5% 7% 55%	4074
	21st to end	Night	7% 5% 1 5% 3 4 3 4 5% 7% 57%	4474
June	1st to 10th	Night	7% 5% 2 5% 4 3 3 3 6% 7% 55%	1750
	11th to 20th	Night	5% 4 1 4 2 3 3 3 5% 6% 64%	1612
	21st to end	Night	3 4 2 3 2 2 4 2 6% 7% 66%	1800
July	1st to 10th	Night	3 3 2 4 4 3 3 2 3 3 70%	4100
	11th to 20th	Night	2 4 2 6% 4 4 4 5% 7% 7% 55%	4100
	21st to end	Night	2 3 1 4 3 3 4 3 4 6% 65%	4472
August	1st to 10th	Night	2 2 1 4 3 3 4 4 5% 7% 66%	4033
	11th to 20th	Night	3 2 2 5% 4 4 4 3 4 5% 64%	4051
	21st to end	Night	3 5% 2 6% 4 5% 4 4 5% 5% 57%	4553
September	1st to 10th	Night	3 4 1 4 3 4 3 3 3 6% 66%	2140
	11th to 20th	Night	5% 4 1 4 3 4 3 3 5% 7% 64%	2200
	21st to end	Night	6% 4 1 3 2 2 3 2 3 4 70%	2200
October	1st to 10th	Night	5% 4 1 4 2 2 3 2 3 4 70%	3755
	11th to 20th	Night	9% 6% 2 4 3 3 3 3 4 5% 59%	3898
	21st to end	Night	7% 8% 2 5% 3 3 4 3 5% 7% 53%	4264
November	1st to 10th	Night	9% 10% 2 5% 3 3 3 3 6% 7% 49%	2665
	11th to 20th	Night	10% 8% 2 5% 3 4 3 4 5% 8% 47%	2730
	21st to end	Night	8% 12% 3 5% 4 3 4 2 5% 7% 46%	2842
December	1st to 10th	Night	7% 7% 2 5% 2 3 3 2 4 8% 56%	4710
	11th to 20th	Night	9% 13% 4 7% 4 4 4 3 5% 7% 41%	5321
	21st to end	Night	10% 14% 4 11% 6% 4 4 3 5% 6% 32%	6681

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% 4 13% 16% 8% 2	5321
	11th to 20th	Day	45% 3 8% 21% 17% 6%	5340
	21st to end	Day	40% 5% 12% 21% 17% 5%	5844
February	1st to 10th	Day	54% 5% 12% 18% 10% 2	1890
	11th to 20th	Day	57% 5% 10% 18% 10% 1	1931
	21st to end	Day	52% 6% 9% 21% 11% 2	1730
March	1st to 10th	Day	50% 6% 6% 20% 13% 5%	4847
	11th to 20th	Day	46% 5% 8% 29% 11% 1	5000
	21st to end	Day	59% 5% 13% 16% 6% 1	5486
April	1st to 10th	Day	48% 4 9% 21% 14% 4	2218
	11th to 20th	Day	46% 5% 10% 21% 16% 2	2526
	21st to end	Day	43% 4 8% 26% 18% 1	2600
May	1st to 10th	Day	38% 6% 6% 25% 21% 4	5880
	11th to 20th	Day	36% 4 4 30% 24% 3	5844
	21st to end	Day	28% 6% 4 31% 28% 3	6424
June	1st to 10th	Day	28% 4 6% 24% 29% 9%	2635
	11th to 20th	Day	25% 3 4 20% 29% 19%	2770
	21st to end	Day	18% 3 1 12% 33% 33%	2600
July	1st to 10th	Day	16% 2 2 6% 21% 53%	5816
	11th to 20th	Day	23% 3 2 13% 31% 28%	5811
	21st to end	Day	18% 3 3 9% 30% 37%	6217
August	1st to 10th	Day	23% 3 3 23% 27% 21%	4208
	11th to 20th	Day	22% 3 3 18% 30% 24%	4197
	21st to end	Day	19% 3 3 15% 36% 24%	4617
September	1st to 10th	Day	18% 4 4 18% 29% 26%	2256
	11th to 20th	Day	24% 4 5% 19% 29% 19%	2200
	21st to end	Day	21% 3 4 23% 33% 16%	2200
October	1st to 10th	Day	25% 2 3 19% 36% 15%	3438
	11th to 20th	Day	30% 2 5% 32% 26% 6%	3250
	21st to end	Day	33% 3 7% 30% 24% 3	3532
November	1st to 10th	Day	39% 5% 10% 23% 19% 4	1854
	11th to 20th	Day	42% 4 9% 29% 15% 1	1890
	21st to end	Day	45% 4 9% 27% 13% 3	1752
December	1st to 10th	Day	31% 3 11% 37% 16% 2	2700
	11th to 20th	Day	50% 4 12% 21% 12% 1	3104
	21st to end	Day	55% 5% 8% 16% 14%	4580
January	1st to 10th	Night	55% 4 13% 16% 10% 3	7605
	11th to 20th	Night	41% 3 7% 25% 18% 6%	7650
	21st to end	Night	37% 4 12% 19% 21% 7%	8370
February	1st to 10th	Night	48% 5% 12% 21% 11% 2	2730
	11th to 20th	Night	49% 3 9% 21% 16% 2	2585
	21st to end	Night	45% 5% 8% 25% 13% 4	2076
March	1st to 10th	Night	48% 5% 6% 21% 14% 6%	5092
	11th to 20th	Night	45% 4 8% 27% 15% 1	5000
	21st to end	Night	50% 5% 12% 19% 13% 1	5489
April	1st to 10th	Night	41% 5% 10% 21% 17% 6%	2180
	11th to 20th	Night	39% 5% 8% 23% 21% 4	1820
	21st to end	Night	42% 4 9% 24% 20% 1	1800
May	1st to 10th	Night	32% 7% 8% 23% 27% 3	4097
	11th to 20th	Night	29% 4 4 31% 27% 4	4075
	21st to end	Night	26% 4 5% 29% 32% 2	4474
June	1st to 10th	Night	28% 4 6% 23% 30% 9%	1750
	11th to 20th	Night	22% 4 5% 18% 30% 21%	1612
	21st to end	Night	19% 3 1 10% 31% 35%	1800
July	1st to 10th	Night	21% 2 1 6% 16% 53%	4055
	11th to 20th	Night	28% 3 2 10% 27% 30%	4046
	21st to end	Night	22% 2 3 10% 27% 37%	4344
August	1st to 10th	Night	24% 3 5% 18% 26% 24%	2928
	11th to 20th	Night	28% 3 4 13% 29% 24%	2928
	21st to end	Night	21% 2 4 10% 34% 27%	3277
September	1st to 10th	Night	22% 3 4 15% 26% 30%	2140
	11th to 20th	Night	23% 2 5% 20% 28% 22%	2200
	21st to end	Night	22% 2 3 19% 34% 21%	2200
October	1st to 10th	Night	21% 2 4 19% 35% 18%	3755
	11th to 20th	Night	31% 2 3 30% 26% 8%	3898
	21st to end	Night	33% 3 5% 29% 25% 5%	4264
November	1st to 10th	Night	34% 4 8% 25% 22% 6%	2665
	11th to 20th	Night	36% 4 12% 28% 18% 3	2730
	21st to end	Night	39% 4 9% 29% 16% 4	2842
December	1st to 10th	Night	29% 3 9% 35% 21% 4	4710
	11th to 20th	Night	45% 3 15% 25% 12% 1	5322
	21st to end	Night	53% 5% 12% 17% 12% 2	6681

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	27% 17% 26% 20% 10%	330
	11th to 20th	Day	15% 22% 33% 20% 8% 2	330
	21st to end	Day	15% 18% 35% 24% 8% 1	363
February	1st to 10th	Day	24% 22% 32% 16% 6% 1	330
	11th to 20th	Day	20% 24% 35% 16% 4 1	407
	21st to end	Day	16% 26% 37% 16% 4	446
March	1st to 10th	Day	19% 24% 36% 16% 5% 1	509
	11th to 20th	Day	18% 20% 36% 21% 5%	500
	21st to end	Day	22% 25% 33% 17% 2	550
April	1st to 10th	Day	18% 24% 31% 21% 5% 1	500
	11th to 20th	Day	21% 27% 29% 17% 5% 1	492
	21st to end	Day	19% 25% 28% 20% 7% 1	500
May	1st to 10th	Day	15% 24% 32% 21% 7% 1	503
	11th to 20th	Day	13% 25% 32% 21% 7% 2	528
	21st to end	Day	12% 18% 32% 26% 10% 2	573
June	1st to 10th	Day	6% 16% 39% 29% 7% 2	565
	11th to 20th	Day	9% 18% 34% 27% 12% 1	700
	21st to end	Day	5% 18% 39% 32% 5% 1	530
July	1st to 10th	Day	3 11% 45% 33% 7% 1	526
	11th to 20th	Day	4 12% 41% 32% 9% 2	524
	21st to end	Day	3 8% 43% 35% 10% 2	575
August	1st to 10th	Day	7% 11% 42% 32% 8% 1	527
	11th to 20th	Day	4 10% 36% 37% 9% 3	517
	21st to end	Day	6% 12% 34% 36% 9% 3	583
September	1st to 10th	Day	5% 10% 33% 37% 13% 3	530
	11th to 20th	Day	7% 17% 33% 32% 9% 2	530
	21st to end	Day	9% 14% 29% 35% 12% 2	530
October	1st to 10th	Day	11% 20% 30% 27% 11% 2	433
	11th to 20th	Day	11% 18% 31% 26% 12% 2	330
	21st to end	Day	12% 21% 34% 25% 8% 1	363
November	1st to 10th	Day	18% 21% 28% 26% 7% 1	330
	11th to 20th	Day	17% 19% 35% 18% 9% 2	330
	21st to end	Day	17% 14% 29% 28% 10% 2	330
December	1st to 10th	Day	20% 13% 32% 24% 9% 2	330
	11th to 20th	Day	21% 13% 35% 25% 6% 1	330
	21st to end	Day	26% 18% 27% 23% 6%	363
January	1st to 10th	Night	19% 9% 28% 25% 15% 3	840
	11th to 20th	Night	16% 9% 28% 30% 14% 4	840
	21st to end	Night	15% 8% 27% 31% 15% 3	924
February	1st to 10th	Night	17% 10% 30% 29% 10% 3	840
	11th to 20th	Night	16% 13% 29% 29% 12% 2	824
	21st to end	Night	15% 13% 33% 26% 11% 1	692
March	1st to 10th	Night	16% 15% 29% 27% 10% 3	808
	11th to 20th	Night	13% 12% 28% 30% 14% 3	800
	21st to end	Night	15% 17% 26% 30% 9% 2	880
April	1st to 10th	Night	18% 14% 28% 28% 11% 2	780
	11th to 20th	Night	13% 13% 30% 31% 11% 2	593
	21st to end	Night	14% 12% 25% 33% 14% 3	600
May	1st to 10th	Night	12% 9% 30% 34% 13% 2	600
	11th to 20th	Night	10% 11% 25% 32% 19% 4	591
	21st to end	Night	11% 10% 26% 31% 19% 4	650
June	1st to 10th	Night	7% 11% 22% 38% 20% 2	600
	11th to 20th	Night	7% 11% 28% 37% 15% 2	600
	21st to end	Night	5% 5% 26% 39% 20% 5%	600
July	1st to 10th	Night	3 8% 24% 34% 28% 3	585
	11th to 20th	Night	5% 4 24% 43% 21% 3	582
	21st to end	Night	4 4 19% 39% 32% 2	657
August	1st to 10th	Night	4 6% 25% 39% 24% 2	597
	11th to 20th	Night	5% 5% 22% 40% 26% 3	594
	21st to end	Night	4 6% 23% 42% 22% 3	660
September	1st to 10th	Night	5% 6% 18% 43% 24% 4	740
	11th to 20th	Night	6% 7% 22% 36% 24% 5%	800
	21st to end	Night	8% 7% 20% 31% 26% 9%	800
October	1st to 10th	Night	9% 8% 28% 33% 19% 4	800
	11th to 20th	Night	11% 6% 24% 34% 21% 4	800
	21st to end	Night	12% 9% 26% 31% 18% 4	880
November	1st to 10th	Night	15% 10% 24% 30% 17% 4	820
	11th to 20th	Night	14% 10% 29% 28% 14% 5%	840
	21st to end	Night	14% 10% 27% 30% 15% 4	952
December	1st to 10th	Night	17% 10% 26% 30% 15% 1	1050
	11th to 20th	Night	21% 10% 29% 29% 10% 2	1050
	21st to end	Night	24% 12% 25% 29% 9% 1	955

Page generated at 2026-02-27 23:40:25. Version ac07

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