This report describes the typical weather at the OR Tambo International Airport (Johannesburg, South Africa) weather station over the course of an average May. It is based on the historical records from 1994 to 2012. Earlier records are either unavailable or unreliable.
Johannesburg has a mild temperate climate with dry winters and warm summers. The area within 25 mi of this station is covered by grasslands (90%) and built-up areas (10%).
The month of May is characterized by falling daily high temperatures, with daily highs decreasing from 68°F to 63°F over the course of the month, exceeding 75°F or dropping below 56°F only one day in ten.
Over the course of May, the length of the day is essentially constant. The shortest day of the month is May 30 with 10:37 hours of daylight; the longest day is May 1 with 11:06 hours of daylight.
The earliest sunrise is at 6:31am on May 1; the latest sunset is at 5:37pm on May 1; the latest sunrise is at 6:46am on May 31; the earliest sunset is at 5:23pm on May 30.
Daylight savings time (DST) is not observed in 2012.
The median cloud cover ranges from 9% (clear) to 22% (mostly clear). The trend is for the sky to be clearing with the median cloud cover decreasing by 13% from the start to the end of the month.
On May 31, the clearest day of the month, the sky is clear, mostly clear, or partly cloudy 60% of the time, and overcast or mostly cloudy 8% of the time.
On May 1, the cloudiest day of the month, the sky is overcast, mostly cloudy, or partly cloudy 23% of the time, and clear or mostly clear 47% of the time.
The probability that precipitation will be observed at this location varies throughout the month. Precipitation is most likely around May 1, occurring in 22% of days. Precipitation is least likely around May 31, occurring in 10% of days.
Throughout May, the most common forms of precipitation are thunderstorms, light rain, and moderate rain.
Thunderstorms are the most severe precipitation observed during 43% of those days with precipitation. They are most likely around May 1, when it is observed during 10% of all days.
Light rain is the most severe precipitation observed during 30% of those days with precipitation. It is most likely around May 1, when it is observed during 6% of all days.
Moderate rain is the most severe precipitation observed during 24% of those days with precipitation. It is most likely around May 1, when it is observed during 5% of all days.
Either snow is exceptionally unlikely to fall at this location in May or this station does not reliably report precipitation types.
The relative humidity typically ranges from 30% (comfortable) to 88% (very humid) over the course of a typical May, rarely dropping below 14% (very dry) and reaching as high as 100% (very humid).
The air is driest around May 31, at which time the relative humidity drops below 38% (comfortable) three days out of four; it is most humid around May 1, rising above 81% (humid) three days out of four.
Dew point is often a better measure of how comfortable a person will find the weather than relative humidity because it more directly relates to whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid.
Over the course of a typical May, the dew point typically varies from 26°F (dry) to 48°F (dry) and is rarely below 12°F (dry) or above 55°F (very comfortable).
Over the course of May typical wind speeds vary from 2 mph to 12 mph (light air to moderate breeze), rarely exceeding 19 mph (fresh breeze).
The highest average wind speed of 8 mph (gentle breeze) occurs around May 31, at which time the average daily maximum wind speed is 12 mph (moderate breeze).
The lowest average wind speed of 7 mph (light breeze) occurs around May 17, at which time the average daily maximum wind speed is 11 mph (gentle breeze).