Robertson Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Robertson Observing Site clear sky chart from 2005-11-06 to 2024-12-02.

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% 8% 3 8% 5% 5% 5% 4 6% 8% 36%	105706
	Night	8% 9% 3 9% 6% 5% 5% 4 6% 8% 36%	102455
Transparency	Day	43% 8% 17% 23% 8%	105450
	Night	43% 8% 16% 24% 9% 1	102358
Seeing	Day	13% 9% 28% 29% 17% 5%	15844
	Night	12% 6% 28% 36% 15% 2	25095

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% 15% 7% 16% 7% 6% 5% 4 5% 6% 24%	2219
	11th to 20th	Day	5% 9% 3 11% 8% 11% 10% 6% 7% 8% 23%	2199
	21st to end	Day	5% 10% 5% 14% 8% 6% 6% 5% 5% 7% 30%	2513
February	1st to 10th	Day	7% 12% 5% 11% 9% 6% 7% 6% 6% 8% 23%	1610
	11th to 20th	Day	6% 13% 5% 12% 6% 6% 5% 5% 7% 7% 27%	1665
	21st to end	Day	5% 9% 5% 14% 8% 7% 8% 5% 8% 7% 25%	1413
March	1st to 10th	Day	6% 12% 4 11% 8% 7% 7% 5% 6% 7% 26%	3144
	11th to 20th	Day	6% 12% 4 10% 6% 4 4 4 7% 9% 34%	3489
	21st to end	Day	6% 10% 5% 10% 7% 6% 6% 4 6% 7% 35%	3979
April	1st to 10th	Day	8% 11% 4 8% 6% 6% 5% 4 6% 8% 35%	2280
	11th to 20th	Day	11% 12% 4 9% 6% 4 5% 5% 7% 7% 29%	2384
	21st to end	Day	13% 9% 3 9% 5% 5% 6% 4 7% 8% 32%	2631
May	1st to 10th	Day	17% 11% 4 7% 4 4 5% 4 5% 7% 32%	4430
	11th to 20th	Day	9% 7% 3 10% 7% 6% 6% 4 7% 8% 34%	4433
	21st to end	Day	12% 9% 3 9% 6% 6% 6% 5% 6% 8% 29%	5255
June	1st to 10th	Day	13% 7% 2 7% 5% 5% 6% 5% 7% 8% 33%	3040
	11th to 20th	Day	21% 9% 4 8% 5% 4 5% 4 6% 8% 27%	3040
	21st to end	Day	11% 7% 3 7% 5% 6% 5% 5% 8% 8% 35%	3040
July	1st to 10th	Day	8% 4 1 5% 4 5% 6% 5% 8% 11% 44%	4915
	11th to 20th	Day	7% 3 2 6% 4 5% 5% 5% 8% 11% 43%	4740
	21st to end	Day	5% 4 1 4 4 4 4 5% 7% 10% 51%	5117
August	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 6% 8% 44%	4412
	11th to 20th	Day	8% 5% 2 6% 4 4 4 4 7% 11% 46%	4229
	21st to end	Day	10% 6% 2 5% 4 4 4 3 6% 8% 48%	4466
September	1st to 10th	Day	9% 6% 2 6% 4 4 5% 4 6% 8% 47%	2280
	11th to 20th	Day	13% 7% 3 8% 4 4 4 4 5% 8% 38%	2280
	21st to end	Day	10% 6% 2 5% 4 4 5% 3 6% 7% 50%	2009
October	1st to 10th	Day	8% 8% 2 6% 4 5% 4 4 5% 9% 45%	3170
	11th to 20th	Day	6% 5% 2 7% 5% 5% 5% 4 8% 13% 40%	3119
	21st to end	Day	5% 9% 4 8% 5% 6% 6% 5% 7% 8% 37%	2926
November	1st to 10th	Day	14% 14% 4 10% 6% 5% 4 3 5% 7% 28%	1568
	11th to 20th	Day	9% 16% 5% 12% 7% 6% 4 5% 6% 6% 24%	1600
	21st to end	Day	7% 13% 6% 14% 6% 6% 5% 5% 6% 6% 27%	1436
December	1st to 10th	Day	7% 13% 5% 11% 9% 8% 4 3 7% 7% 26%	1146
	11th to 20th	Day	7% 14% 4 12% 7% 6% 6% 4 6% 8% 27%	1439
	21st to end	Day	3 9% 4 10% 5% 4 4 4 6% 8% 42%	2090
January	1st to 10th	Night	6% 14% 6% 15% 8% 6% 6% 4 6% 8% 22%	4726
	11th to 20th	Night	4 8% 4 14% 10% 11% 9% 5% 6% 8% 21%	4699
	21st to end	Night	7% 12% 4 11% 7% 6% 6% 5% 7% 7% 28%	5144
February	1st to 10th	Night	6% 12% 5% 12% 8% 6% 6% 5% 6% 8% 25%	2588
	11th to 20th	Night	6% 10% 4 10% 6% 6% 6% 6% 7% 7% 31%	2402
	21st to end	Night	3 10% 4 11% 8% 6% 6% 5% 7% 8% 33%	2041
March	1st to 10th	Night	7% 11% 4 12% 7% 6% 7% 6% 7% 9% 25%	3719
	11th to 20th	Night	7% 10% 4 9% 6% 5% 5% 5% 7% 10% 32%	3490
	21st to end	Night	7% 8% 2 9% 6% 5% 6% 6% 8% 9% 34%	3655
April	1st to 10th	Night	5% 10% 4 7% 7% 5% 5% 5% 6% 7% 40%	1900
	11th to 20th	Night	10% 11% 4 9% 6% 5% 5% 5% 7% 7% 30%	1743
	21st to end	Night	15% 8% 2 9% 6% 5% 3 4 5% 7% 36%	1548
May	1st to 10th	Night	16% 12% 4 8% 5% 4 4 3 5% 7% 31%	2540
	11th to 20th	Night	11% 7% 3 7% 5% 5% 6% 4 6% 7% 38%	2468
	21st to end	Night	14% 8% 2 8% 6% 5% 5% 4 5% 9% 33%	2318
June	1st to 10th	Night	10% 5% 3 9% 4 5% 5% 6% 7% 8% 38%	1140
	11th to 20th	Night	19% 8% 2 6% 4 4 6% 5% 6% 8% 31%	1140
	21st to end	Night	8% 5% 3 5% 4 5% 4 5% 7% 10% 44%	1140
July	1st to 10th	Night	7% 4 2 6% 4 4 5% 5% 9% 10% 45%	2028
	11th to 20th	Night	6% 3 2 5% 4 3 5% 4 6% 9% 53%	2220
	21st to end	Night	3 3 1 3 3 3 3 3 6% 9% 63%	2435
August	1st to 10th	Night	7% 4 1 7% 4 5% 5% 4 6% 7% 51%	2528
	11th to 20th	Night	6% 4 2 6% 5% 5% 6% 5% 8% 9% 44%	2606
	21st to end	Night	9% 5% 2 5% 4 3 4 4 6% 7% 52%	3200
September	1st to 10th	Night	8% 4 1 5% 4 4 4 3 7% 10% 50%	1900
	11th to 20th	Night	13% 6% 2 6% 4 4 5% 4 5% 8% 44%	1900
	21st to end	Night	7% 6% 2 5% 4 4 4 4 6% 7% 50%	2167
October	1st to 10th	Night	9% 7% 3 6% 4 4 4 3 5% 7% 48%	3800
	11th to 20th	Night	5% 6% 2 7% 5% 3 5% 4 5% 7% 51%	3778
	21st to end	Night	8% 9% 3 8% 5% 5% 5% 4 6% 9% 40%	4619
November	1st to 10th	Night	15% 14% 5% 9% 6% 5% 5% 3 5% 6% 28%	2730
	11th to 20th	Night	9% 12% 5% 14% 7% 5% 5% 4 6% 7% 26%	2800
	21st to end	Night	6% 13% 6% 12% 6% 5% 5% 4 6% 8% 29%	2959
December	1st to 10th	Night	6% 12% 4 11% 8% 7% 6% 4 6% 8% 27%	3069
	11th to 20th	Night	7% 14% 6% 13% 8% 6% 5% 5% 7% 9% 21%	3753
	21st to end	Night	3 7% 3 7% 6% 5% 5% 4 6% 7% 48%	5562

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	60% 8% 12% 13% 7%	2219
	11th to 20th	Day	56% 10% 12% 16% 5% 1	2220
	21st to end	Day	50% 10% 13% 20% 7%	2513
February	1st to 10th	Day	56% 7% 13% 16% 8%	1610
	11th to 20th	Day	52% 7% 14% 17% 9% 1	1674
	21st to end	Day	52% 10% 12% 18% 8%	1413
March	1st to 10th	Day	51% 10% 17% 13% 8%	3144
	11th to 20th	Day	45% 11% 21% 17% 6%	3490
	21st to end	Day	46% 8% 13% 21% 12%	3979
April	1st to 10th	Day	46% 8% 15% 20% 11%	2280
	11th to 20th	Day	51% 8% 13% 18% 10% 1	2384
	21st to end	Day	46% 8% 16% 21% 8% 1	2631
May	1st to 10th	Day	49% 7% 15% 20% 9%	4430
	11th to 20th	Day	42% 10% 12% 23% 12% 1	4437
	21st to end	Day	46% 13% 15% 16% 9% 1	5256
June	1st to 10th	Day	44% 8% 18% 20% 9% 1	3040
	11th to 20th	Day	54% 8% 14% 17% 6% 1	3040
	21st to end	Day	41% 11% 19% 21% 7%	3040
July	1st to 10th	Day	32% 10% 24% 28% 6%	4840
	11th to 20th	Day	30% 8% 18% 38% 6%	4659
	21st to end	Day	26% 8% 21% 38% 8%	4955
August	1st to 10th	Day	36% 8% 21% 29% 6%	4396
	11th to 20th	Day	31% 7% 23% 34% 5%	4271
	21st to end	Day	34% 6% 22% 32% 6%	4465
September	1st to 10th	Day	34% 8% 17% 25% 13% 2	2280
	11th to 20th	Day	43% 7% 19% 24% 6% 1	2280
	21st to end	Day	32% 6% 18% 31% 11% 2	2009
October	1st to 10th	Day	37% 5% 25% 23% 10%	3170
	11th to 20th	Day	35% 6% 20% 34% 6%	3119
	21st to end	Day	44% 7% 19% 26% 4	2926
November	1st to 10th	Day	56% 5% 15% 16% 7%	1568
	11th to 20th	Day	60% 10% 13% 13% 4	1600
	21st to end	Day	56% 8% 15% 17% 3	1436
December	1st to 10th	Day	55% 8% 13% 16% 7%	1146
	11th to 20th	Day	54% 9% 17% 13% 7%	1440
	21st to end	Day	38% 7% 15% 18% 18% 3	2090
January	1st to 10th	Night	58% 8% 14% 16% 4	4726
	11th to 20th	Night	58% 7% 14% 15% 6%	4740
	21st to end	Night	50% 10% 15% 18% 6%	5144
February	1st to 10th	Night	55% 6% 13% 19% 7%	2588
	11th to 20th	Night	47% 8% 14% 20% 10%	2403
	21st to end	Night	45% 8% 15% 22% 9%	2041
March	1st to 10th	Night	50% 10% 16% 18% 5%	3719
	11th to 20th	Night	44% 9% 20% 19% 7%	3490
	21st to end	Night	39% 10% 15% 23% 12%	3655
April	1st to 10th	Night	41% 8% 14% 21% 15% 1	1900
	11th to 20th	Night	48% 7% 14% 19% 11% 2	1743
	21st to end	Night	46% 7% 13% 23% 11% 1	1548
May	1st to 10th	Night	50% 7% 12% 20% 10% 1	2540
	11th to 20th	Night	39% 10% 13% 21% 15% 1	2468
	21st to end	Night	44% 10% 18% 20% 7% 1	2318
June	1st to 10th	Night	39% 9% 17% 25% 9% 2	1140
	11th to 20th	Night	47% 9% 14% 19% 9% 1	1140
	21st to end	Night	33% 10% 22% 25% 10%	1140
July	1st to 10th	Night	30% 9% 25% 29% 7% 1	1994
	11th to 20th	Night	26% 6% 17% 43% 9%	2178
	21st to end	Night	19% 6% 22% 41% 11% 1	2358
August	1st to 10th	Night	31% 7% 21% 32% 8%	2520
	11th to 20th	Night	32% 8% 21% 29% 10% 1	2630
	21st to end	Night	30% 6% 20% 34% 9%	3200
September	1st to 10th	Night	30% 7% 17% 30% 12% 4	1900
	11th to 20th	Night	38% 7% 15% 29% 10% 1	1900
	21st to end	Night	32% 5% 16% 31% 13% 3	2167
October	1st to 10th	Night	37% 4 19% 33% 6%	3800
	11th to 20th	Night	33% 5% 14% 35% 13%	3778
	21st to end	Night	41% 6% 20% 28% 5%	4619
November	1st to 10th	Night	57% 6% 12% 18% 7% 1	2730
	11th to 20th	Night	56% 9% 15% 15% 4 1	2800
	21st to end	Night	53% 8% 17% 18% 4	2959
December	1st to 10th	Night	53% 7% 15% 18% 6% 1	3069
	11th to 20th	Night	57% 9% 13% 14% 5%	3751
	21st to end	Night	34% 6% 14% 27% 16% 3	5562

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	14% 9% 27% 29% 15% 7%	180
	11th to 20th	Day	13% 7% 28% 32% 18% 3	180
	21st to end	Day	12% 11% 25% 34% 13% 5%	220
February	1st to 10th	Day	13% 7% 32% 36% 10% 2	270
	11th to 20th	Day	11% 8% 29% 32% 16% 4	270
	21st to end	Day	5% 6% 31% 36% 19% 4	225
March	1st to 10th	Day	13% 9% 24% 36% 16% 3	273
	11th to 20th	Day	11% 11% 35% 28% 12% 3	460
	21st to end	Day	13% 6% 35% 31% 12% 3	506
April	1st to 10th	Day	11% 12% 35% 28% 9% 4	460
	11th to 20th	Day	14% 16% 29% 29% 9% 3	452
	21st to end	Day	15% 10% 33% 25% 14% 2	621
May	1st to 10th	Day	13% 10% 25% 30% 16% 6%	653
	11th to 20th	Day	9% 10% 23% 33% 19% 5%	675
	21st to end	Day	13% 10% 25% 28% 17% 6%	734
June	1st to 10th	Day	14% 11% 29% 30% 14% 3	680
	11th to 20th	Day	22% 11% 28% 22% 14% 4	680
	21st to end	Day	14% 6% 23% 26% 22% 9%	680
July	1st to 10th	Day	10% 5% 21% 34% 24% 6%	671
	11th to 20th	Day	9% 6% 24% 29% 24% 7%	668
	21st to end	Day	6% 7% 24% 30% 24% 9%	739
August	1st to 10th	Day	10% 7% 21% 29% 26% 8%	676
	11th to 20th	Day	10% 5% 23% 31% 24% 6%	603
	21st to end	Day	13% 7% 24% 31% 20% 5%	539
September	1st to 10th	Day	11% 10% 30% 28% 17% 3	490
	11th to 20th	Day	20% 7% 29% 28% 13% 2	490
	21st to end	Day	16% 9% 29% 25% 19% 2	374
October	1st to 10th	Day	17% 5% 29% 29% 16% 4	300
	11th to 20th	Day	11% 10% 31% 29% 16% 3	300
	21st to end	Day	11% 14% 36% 25% 12% 2	330
November	1st to 10th	Day	21% 5% 34% 27% 11% 2	300
	11th to 20th	Day	16% 13% 33% 27% 9% 2	300
	21st to end	Day	11% 11% 33% 33% 11% 2	285
December	1st to 10th	Day	13% 4 22% 36% 16% 9%	182
	11th to 20th	Day	10% 11% 35% 36% 9%	180
	21st to end	Day	10% 4 31% 22% 26% 7%	198
January	1st to 10th	Night	11% 7% 31% 35% 14% 1	950
	11th to 20th	Night	11% 8% 33% 37% 11% 1	950
	21st to end	Night	14% 8% 33% 35% 10% 1	1045
February	1st to 10th	Night	10% 6% 28% 37% 16% 2	878
	11th to 20th	Night	11% 4 31% 37% 15% 3	744
	21st to end	Night	5% 4 22% 41% 24% 4	628
March	1st to 10th	Night	10% 5% 29% 38% 15% 2	760
	11th to 20th	Night	10% 6% 32% 38% 13% 1	760
	21st to end	Night	12% 5% 27% 39% 14% 3	740
April	1st to 10th	Night	8% 7% 32% 37% 14% 2	570
	11th to 20th	Night	15% 9% 28% 33% 13% 2	563
	21st to end	Night	18% 8% 29% 33% 11% 1	570
May	1st to 10th	Night	14% 6% 29% 32% 17% 1	570
	11th to 20th	Night	12% 6% 27% 35% 18% 2	538
	21st to end	Night	16% 5% 22% 35% 19% 2	411
June	1st to 10th	Night	12% 7% 25% 35% 18% 2	380
	11th to 20th	Night	20% 7% 23% 32% 16% 2	380
	21st to end	Night	9% 7% 18% 40% 24% 2	380
July	1st to 10th	Night	6% 4 18% 45% 25% 1	370
	11th to 20th	Night	10% 4 24% 42% 18% 1	368
	21st to end	Night	5% 4 22% 38% 27% 4	425
August	1st to 10th	Night	6% 4 22% 39% 25% 4	567
	11th to 20th	Night	8% 3 18% 42% 27% 2	564
	21st to end	Night	8% 6% 24% 37% 23% 1	627
September	1st to 10th	Night	10% 4 25% 40% 19% 2	570
	11th to 20th	Night	16% 5% 20% 43% 16% 1	570
	21st to end	Night	9% 8% 30% 37% 14% 2	722
October	1st to 10th	Night	16% 6% 26% 38% 13% 1	760
	11th to 20th	Night	10% 6% 30% 36% 15% 2	760
	21st to end	Night	16% 8% 34% 33% 9% 1	1045
November	1st to 10th	Night	23% 7% 27% 34% 8% 1	975
	11th to 20th	Night	15% 7% 30% 34% 12% 2	1000
	21st to end	Night	12% 9% 34% 35% 8% 2	1000
December	1st to 10th	Night	11% 6% 28% 35% 17% 3	960
	11th to 20th	Night	12% 8% 32% 34% 13% 1	950
	21st to end	Night	10% 4 29% 35% 20% 3	1045

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