Strathmore
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Strathmore clear sky chart from 2005-11-06 to 2025-02-25.

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	8% 8% 3 8% 5% 5% 5% 4 6% 8% 39%	112137
	Night	6% 9% 4 9% 6% 5% 5% 4 6% 8% 37%	115182
Transparency	Day	41% 6% 15% 25% 11% 2	111881
	Night	44% 6% 15% 23% 11% 2	115085
Seeing	Day	12% 7% 26% 31% 19% 6%	15660
	Night	11% 6% 30% 36% 16% 2	24979

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	3 9% 7% 15% 8% 8% 6% 3 6% 6% 29%	3179
	11th to 20th	Day	3 11% 4 11% 6% 4 6% 6% 7% 7% 34%	3177
	21st to end	Day	3 7% 4 9% 6% 5% 5% 4 6% 8% 44%	4004
February	1st to 10th	Day	5% 13% 5% 13% 9% 7% 6% 4 6% 7% 27%	1616
	11th to 20th	Day	6% 13% 4 12% 6% 5% 6% 6% 8% 7% 28%	1755
	21st to end	Day	4 13% 5% 11% 7% 7% 6% 4 8% 7% 29%	1541
March	1st to 10th	Day	3 12% 5% 13% 7% 5% 7% 4 5% 6% 31%	3138
	11th to 20th	Day	7% 9% 2 6% 4 4 5% 3 5% 9% 45%	3483
	21st to end	Day	10% 21% 6% 13% 5% 4 5% 3 4 6% 24%	3841
April	1st to 10th	Day	9% 13% 5% 9% 6% 5% 5% 5% 7% 7% 29%	2351
	11th to 20th	Day	11% 12% 4 9% 5% 5% 5% 4 6% 8% 31%	2440
	21st to end	Day	13% 11% 4 8% 6% 5% 4 4 6% 8% 31%	2470
May	1st to 10th	Day	19% 13% 4 8% 5% 5% 4 3 5% 6% 29%	4335
	11th to 20th	Day	10% 7% 3 9% 6% 6% 6% 5% 8% 9% 32%	4694
	21st to end	Day	15% 8% 3 9% 6% 6% 6% 5% 6% 7% 27%	5161
June	1st to 10th	Day	16% 7% 2 6% 5% 5% 6% 5% 7% 9% 32%	2850
	11th to 20th	Day	18% 9% 3 6% 6% 5% 5% 5% 6% 8% 29%	2850
	21st to end	Day	8% 5% 2 5% 5% 5% 6% 4 8% 10% 43%	2850
July	1st to 10th	Day	7% 3 1 5% 4 4 4 4 7% 8% 52%	4740
	11th to 20th	Day	6% 3 1 4 3 3 4 4 7% 10% 54%	4740
	21st to end	Day	3 2 1 3 3 3 4 3 6% 9% 61%	5062
August	1st to 10th	Day	8% 6% 2 6% 5% 4 6% 5% 7% 9% 44%	4401
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 7% 11% 46%	4033
	21st to end	Day	10% 4 1 4 2 4 4 4 5% 7% 54%	4521
September	1st to 10th	Day	10% 5% 3 6% 3 3 3 2 5% 7% 52%	2365
	11th to 20th	Day	14% 7% 2 5% 4 4 4 3 5% 7% 44%	2090
	21st to end	Day	10% 8% 2 6% 5% 4 4 3 6% 8% 45%	2090
October	1st to 10th	Day	8% 6% 2 7% 5% 4 6% 4 7% 7% 43%	3312
	11th to 20th	Day	7% 7% 2 6% 4 5% 5% 4 6% 9% 46%	2950
	21st to end	Day	7% 8% 2 7% 5% 5% 6% 4 5% 5% 45%	3101
November	1st to 10th	Day	10% 15% 5% 9% 6% 4 4 4 6% 8% 30%	1613
	11th to 20th	Day	7% 12% 3 8% 7% 6% 5% 4 5% 6% 37%	1554
	21st to end	Day	5% 12% 4 10% 6% 6% 5% 4 7% 9% 33%	1400
December	1st to 10th	Day	4 13% 5% 11% 5% 6% 6% 4 6% 6% 32%	2290
	11th to 20th	Day	5% 11% 5% 11% 7% 6% 6% 4 6% 7% 31%	2642
	21st to end	Day	3 9% 4 8% 5% 5% 5% 5% 6% 8% 43%	3498
January	1st to 10th	Night	3 15% 6% 14% 8% 7% 5% 4 6% 7% 25%	6766
	11th to 20th	Night	3 10% 5% 12% 6% 6% 6% 5% 6% 7% 34%	6739
	21st to end	Night	3 10% 5% 10% 6% 4 4 4 5% 7% 42%	6980
February	1st to 10th	Night	5% 11% 4 14% 7% 6% 6% 5% 6% 7% 30%	2783
	11th to 20th	Night	5% 13% 5% 10% 6% 5% 5% 6% 7% 7% 29%	2532
	21st to end	Night	4 10% 5% 11% 7% 6% 6% 4 7% 8% 33%	2004
March	1st to 10th	Night	3 10% 5% 14% 9% 7% 5% 5% 7% 8% 26%	3736
	11th to 20th	Night	8% 8% 2 6% 5% 4 5% 4 6% 8% 43%	3495
	21st to end	Night	9% 21% 7% 13% 7% 4 5% 4 5% 6% 18%	3809
April	1st to 10th	Night	9% 13% 4 9% 6% 6% 5% 4 7% 9% 29%	1829
	11th to 20th	Night	9% 12% 5% 9% 6% 6% 5% 4 6% 7% 31%	1691
	21st to end	Night	11% 11% 4 7% 6% 5% 5% 4 8% 8% 31%	1710
May	1st to 10th	Night	19% 11% 4 8% 5% 5% 5% 4 5% 5% 30%	2628
	11th to 20th	Night	9% 7% 3 8% 5% 6% 6% 5% 7% 11% 33%	2199
	21st to end	Night	15% 9% 3 10% 7% 6% 6% 5% 6% 7% 26%	2414
June	1st to 10th	Night	14% 9% 2 8% 6% 4 6% 5% 8% 7% 31%	1330
	11th to 20th	Night	15% 8% 4 7% 5% 5% 6% 6% 7% 8% 30%	1330
	21st to end	Night	8% 6% 2 5% 5% 4 3 6% 8% 8% 44%	1330
July	1st to 10th	Night	6% 4 2 6% 5% 5% 6% 5% 6% 8% 47%	2220
	11th to 20th	Night	4 3 2 5% 4 4 5% 5% 9% 12% 47%	2220
	21st to end	Night	4 3 1 5% 3 4 5% 4 8% 8% 56%	2475
August	1st to 10th	Night	8% 7% 2 6% 5% 5% 5% 5% 7% 9% 43%	2529
	11th to 20th	Night	6% 5% 2 7% 5% 5% 5% 4 8% 12% 42%	2806
	21st to end	Night	9% 3 1 4 4 3 5% 4 6% 9% 53%	3146
September	1st to 10th	Night	9% 6% 3 5% 4 4 5% 4 5% 7% 49%	1811
	11th to 20th	Night	13% 7% 2 5% 4 4 3 3 5% 7% 47%	2090
	21st to end	Night	9% 5% 2 5% 3 4 5% 5% 6% 8% 48%	2090
October	1st to 10th	Night	8% 6% 3 6% 4 4 4 4 6% 9% 47%	3662
	11th to 20th	Night	7% 7% 2 6% 4 4 5% 4 7% 8% 46%	3962
	21st to end	Night	5% 8% 2 7% 5% 5% 6% 5% 7% 7% 44%	4457
November	1st to 10th	Night	9% 13% 4 8% 5% 6% 5% 4 5% 8% 34%	2666
	11th to 20th	Night	5% 12% 3 8% 7% 6% 5% 5% 6% 8% 36%	2845
	21st to end	Night	5% 12% 4 9% 6% 5% 5% 4 6% 9% 35%	3000
December	1st to 10th	Night	4 12% 5% 12% 7% 7% 7% 5% 7% 7% 29%	4890
	11th to 20th	Night	4 11% 6% 13% 7% 6% 6% 5% 7% 9% 27%	5550
	21st to end	Night	4 9% 4 8% 5% 5% 5% 4 6% 7% 42%	7458

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	54% 5% 10% 20% 9% 1	3179
	11th to 20th	Day	45% 6% 12% 16% 17% 3	3198
	21st to end	Day	37% 5% 14% 31% 12% 1	4004
February	1st to 10th	Day	55% 6% 14% 15% 9% 2	1616
	11th to 20th	Day	51% 7% 13% 20% 8% 1	1764
	21st to end	Day	51% 6% 12% 18% 12% 1	1541
March	1st to 10th	Day	50% 7% 13% 18% 12% 1	3138
	11th to 20th	Day	35% 7% 18% 26% 13% 1	3484
	21st to end	Day	62% 6% 12% 15% 5%	3841
April	1st to 10th	Day	51% 8% 14% 18% 8% 1	2351
	11th to 20th	Day	49% 6% 12% 22% 10% 1	2440
	21st to end	Day	50% 6% 13% 21% 10% 1	2470
May	1st to 10th	Day	55% 6% 12% 22% 5%	4335
	11th to 20th	Day	42% 9% 13% 25% 10%	4698
	21st to end	Day	51% 9% 13% 20% 6% 1	5162
June	1st to 10th	Day	46% 8% 15% 21% 9% 1	2850
	11th to 20th	Day	50% 8% 14% 19% 7% 2	2850
	21st to end	Day	33% 8% 20% 28% 9% 2	2850
July	1st to 10th	Day	27% 6% 19% 29% 16% 3	4666
	11th to 20th	Day	25% 6% 19% 36% 13% 1	4659
	21st to end	Day	19% 5% 13% 30% 24% 8%	4902
August	1st to 10th	Day	36% 6% 15% 34% 8% 1	4384
	11th to 20th	Day	32% 6% 23% 31% 8% 1	4072
	21st to end	Day	29% 5% 19% 34% 13% 1	4520
September	1st to 10th	Day	33% 5% 15% 27% 15% 6%	2365
	11th to 20th	Day	40% 5% 14% 26% 12% 2	2090
	21st to end	Day	36% 6% 15% 28% 12% 3	2090
October	1st to 10th	Day	39% 5% 18% 30% 8% 1	3312
	11th to 20th	Day	34% 5% 18% 27% 13% 2	2950
	21st to end	Day	39% 5% 9% 33% 13% 1	3101
November	1st to 10th	Day	52% 6% 11% 21% 8% 1	1613
	11th to 20th	Day	47% 5% 14% 24% 9% 1	1554
	21st to end	Day	45% 8% 16% 22% 8% 2	1400
December	1st to 10th	Day	52% 5% 14% 22% 6% 2	2290
	11th to 20th	Day	51% 5% 17% 21% 5% 1	2644
	21st to end	Day	37% 6% 11% 25% 18% 3	3498
January	1st to 10th	Night	56% 6% 11% 20% 5% 2	6766
	11th to 20th	Night	48% 5% 12% 17% 14% 3	6780
	21st to end	Night	41% 5% 15% 27% 12% 1	6980
February	1st to 10th	Night	50% 6% 12% 19% 12% 1	2783
	11th to 20th	Night	49% 6% 12% 20% 11% 1	2533
	21st to end	Night	45% 7% 12% 22% 11% 2	2004
March	1st to 10th	Night	50% 8% 12% 21% 8% 1	3736
	11th to 20th	Night	36% 9% 18% 24% 12% 1	3495
	21st to end	Night	65% 6% 12% 12% 4	3809
April	1st to 10th	Night	49% 7% 13% 19% 11%	1829
	11th to 20th	Night	49% 8% 12% 21% 9% 1	1691
	21st to end	Night	46% 8% 13% 22% 9% 2	1710
May	1st to 10th	Night	55% 5% 12% 21% 6% 1	2628
	11th to 20th	Night	40% 10% 13% 27% 10% 1	2199
	21st to end	Night	53% 8% 16% 17% 5% 1	2414
June	1st to 10th	Night	47% 9% 13% 20% 9% 1	1330
	11th to 20th	Night	48% 9% 17% 17% 7% 2	1330
	21st to end	Night	34% 9% 21% 27% 9% 1	1330
July	1st to 10th	Night	33% 6% 18% 24% 18% 1	2185
	11th to 20th	Night	26% 7% 18% 37% 8% 4	2178
	21st to end	Night	23% 7% 12% 30% 22% 6%	2397
August	1st to 10th	Night	36% 6% 15% 30% 13% 1	2521
	11th to 20th	Night	33% 6% 23% 28% 9% 1	2833
	21st to end	Night	28% 5% 19% 34% 12% 3	3146
September	1st to 10th	Night	33% 6% 14% 26% 15% 6%	1811
	11th to 20th	Night	37% 5% 15% 26% 13% 4	2090
	21st to end	Night	32% 7% 17% 27% 15% 4	2090
October	1st to 10th	Night	33% 5% 15% 29% 16% 2	3662
	11th to 20th	Night	34% 5% 17% 27% 15% 2	3962
	21st to end	Night	37% 5% 12% 33% 12%	4457
November	1st to 10th	Night	48% 6% 12% 21% 10% 2	2666
	11th to 20th	Night	45% 6% 15% 20% 12% 1	2845
	21st to end	Night	45% 7% 17% 20% 8% 2	3000
December	1st to 10th	Night	53% 5% 17% 18% 6% 2	4890
	11th to 20th	Night	53% 5% 20% 17% 5%	5547
	21st to end	Night	40% 5% 10% 26% 16% 3	7458

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	6% 4 30% 35% 17% 9%	200
	11th to 20th	Day	7% 7% 32% 32% 19% 5%	200
	21st to end	Day	8% 8% 31% 35% 15% 3	220
February	1st to 10th	Day	13% 8% 25% 38% 12% 5%	208
	11th to 20th	Day	8% 6% 36% 32% 14% 3	300
	21st to end	Day	6% 8% 32% 35% 15% 4	329
March	1st to 10th	Day	10% 10% 31% 32% 13% 5%	460
	11th to 20th	Day	9% 10% 30% 33% 14% 5%	460
	21st to end	Day	12% 11% 34% 22% 16% 6%	506
April	1st to 10th	Day	13% 10% 32% 28% 14% 2	460
	11th to 20th	Day	12% 9% 29% 27% 18% 5%	452
	21st to end	Day	15% 14% 27% 25% 16% 4	460
May	1st to 10th	Day	18% 9% 22% 24% 21% 6%	529
	11th to 20th	Day	14% 6% 20% 32% 23% 5%	675
	21st to end	Day	16% 7% 20% 28% 23% 8%	734
June	1st to 10th	Day	14% 8% 24% 31% 19% 4	680
	11th to 20th	Day	17% 8% 28% 30% 15% 2	680
	21st to end	Day	10% 4 23% 33% 23% 8%	680
July	1st to 10th	Day	10% 4 17% 35% 26% 9%	671
	11th to 20th	Day	10% 4 18% 35% 25% 8%	668
	21st to end	Day	5% 4 20% 33% 27% 10%	699
August	1st to 10th	Day	8% 4 22% 34% 22% 9%	487
	11th to 20th	Day	8% 1 19% 34% 29% 8%	477
	21st to end	Day	10% 6% 19% 30% 21% 12%	539
September	1st to 10th	Day	12% 6% 23% 31% 21% 6%	490
	11th to 20th	Day	18% 6% 20% 29% 21% 6%	490
	21st to end	Day	12% 12% 31% 28% 13% 5%	490
October	1st to 10th	Day	17% 8% 20% 29% 19% 7%	390
	11th to 20th	Day	10% 7% 34% 26% 18% 6%	300
	21st to end	Day	13% 14% 31% 29% 10% 2	330
November	1st to 10th	Day	14% 10% 31% 29% 12% 3	300
	11th to 20th	Day	14% 10% 37% 26% 13% 1	276
	21st to end	Day	6% 8% 36% 30% 12% 10%	200
December	1st to 10th	Day	6% 9% 37% 32% 13% 4	200
	11th to 20th	Day	7% 12% 33% 34% 14% 1	200
	21st to end	Day	7% 5% 25% 32% 26% 4	220
January	1st to 10th	Night	8% 7% 34% 36% 12% 2	1000
	11th to 20th	Night	9% 7% 37% 34% 12% 1	1000
	21st to end	Night	11% 8% 34% 32% 12% 2	895
February	1st to 10th	Night	9% 7% 31% 34% 16% 3	800
	11th to 20th	Night	10% 5% 32% 37% 14% 2	784
	21st to end	Night	7% 4 30% 34% 19% 4	646
March	1st to 10th	Night	9% 4 31% 36% 17% 3	760
	11th to 20th	Night	9% 7% 35% 35% 13% 2	760
	21st to end	Night	14% 6% 28% 34% 15% 2	836
April	1st to 10th	Night	13% 7% 29% 35% 15% 1	689
	11th to 20th	Night	15% 6% 31% 35% 13%	563
	21st to end	Night	15% 7% 31% 37% 10%	570
May	1st to 10th	Night	17% 7% 26% 34% 16% 1	508
	11th to 20th	Night	12% 3 25% 38% 20% 1	374
	21st to end	Night	18% 6% 23% 35% 17% 1	411
June	1st to 10th	Night	17% 7% 24% 37% 13% 2	380
	11th to 20th	Night	14% 6% 31% 33% 15% 1	380
	21st to end	Night	11% 3 23% 39% 23% 1	380
July	1st to 10th	Night	10% 4 13% 45% 27% 1	370
	11th to 20th	Night	8% 3 20% 44% 23% 2	368
	21st to end	Night	4 3 22% 43% 26% 2	416
August	1st to 10th	Night	9% 3 16% 44% 25% 2	378
	11th to 20th	Night	8% 4 18% 47% 22% 1	559
	21st to end	Night	10% 4 22% 39% 25% 1	627
September	1st to 10th	Night	11% 3 22% 40% 22% 2	638
	11th to 20th	Night	16% 4 18% 38% 23% 1	760
	21st to end	Night	10% 5% 29% 38% 16% 2	760
October	1st to 10th	Night	15% 5% 24% 38% 16% 2	760
	11th to 20th	Night	13% 5% 29% 37% 14% 3	760
	21st to end	Night	10% 8% 34% 34% 13% 1	836
November	1st to 10th	Night	15% 6% 35% 33% 9% 1	911
	11th to 20th	Night	10% 8% 42% 29% 10% 2	1000
	21st to end	Night	11% 5% 36% 35% 12% 2	1000
December	1st to 10th	Night	10% 7% 32% 34% 15% 2	1000
	11th to 20th	Night	8% 6% 36% 37% 11% 2	1000
	21st to end	Night	12% 5% 30% 36% 14% 2	1100

Page generated at 2025-02-22 04:51:37. Version ac07

Copyright © 2025 Attilla Danko.