Author: Weatherman

Synoptic Map: Friday 11 August at 06:00. Barometric Pressure displayed as surface level isobars. Source: South African Weather Service The SA Weather Bureau’s standard early-morning synoptic map offers a good starting point to interpret what has happened during the week. Note that the isobars are based on surface readings and does not make provision for differences in pressure that arise from elevation. Over land, the readings from actual weather stations will be slightly less than the values indicated on the map. The first feature of note is the position and strength of the southern Indian high pressure cell located...

Satellite image: Infrared view of Southern Africa at 11:00 CAT, Friday 04 August 2017, taken from a position over 0° longtitude. Source: Eumetsat   An intricate system of high pressure cells controlled local conditions for the duration of the week. Early in the week, the South Atlantic high made landfall between Oranjemund and Saldanha Bay, slowly migrating across southern Africa from west to east. The southern Indian high was more or less stationary south-east of Madagascar although its core, at a strong 1028 mB, was displaced northward by about 600 km. By the end of the week, the southern...

  Map: Isobars at surface level and the vertical distance between 1000 mB (sea level) and 500 mB (around 18,000 feet) in colour shaded areas. Forecast for Sunday 30 July 2017. Lighter areas indicate a lower 500 mB surface (thinner atmosphere) while darker colours indicate a higher 500 mB surface (thicker atmosphere). Source: GrADS/Cola.   The main feature of this week’s weather is the extended high pressure cells around the southern hemisphere. There are still only four major core areas with relatively large distances between the cores. This space has been invaded by the outer fringes of the high...

Map: Synoptic map for surface conditions on Friday 21 July 2017 06:00 for southern Africa and the southern seas. Source: South African Weather Service   High pressure control on the surface and in the alto levels is still the order of the week. A fairly static weather pattern continued for the duration of the week with high pressure cells on either side of the sub-continent, and a relatively strong high also present over a very large area of southern Africa. This is typical for the winter period after the solstice. As can be seen from the synoptic map, the...

Map: Wind direction, temperature and relative humidity near surface (ground) level on the interior plateau (850mB) for Sunday 16 July 2017. The map is relevant for the 12-hour period preceding the time marker at the bottom of the image which indicates midnight Sunday/Monday. Source: wxmaps.org, GrADS/COLA The dominant feature of the week’s weather is the extensive ridging in the upper air, covering most of Namibia and in fact, very large areas of southern Africa. Ridging indicates upper air high pressure control originating from the strong high pressure conditions prevalent on the surface over the eastern half of the sub-continent....

Map: Barometric pressure at sea level (colour lines) and vertical distance between 1000 mB and 500 mB contours expressed in 10-metre units Source: wxmaps.org, GrADS/COLA   While both the southern Indian and the South Atlantic high pressure cells behaved as expected, a very prominent trough that reached some 3000 km from north to south, lay between the two highs and had a remarkable impact on both. At the bottom (southern) end of the trough, at about 20°S, a very substantial vortex with a cone pressure of only 956 mB was driving the active frontal system that siphoned away the...

Mean Temperature Anomalies for the period Friday 30 June to Saturday 08 July 2017 Source: wxmaps.org, GrADS/COLA The three weeks after the winter solstice are known as the coldest part of the southern African winter. There is little agreement amongst scientists why this happens, but meteorologists agree that it has much to do with the relative latitude and strength of the South Atlantic and the southern Indian high pressure cells. The map above shows that for the next week, although it is winter, mean temperatures are expected to be higher than normal, meaning no frost over Namibia. The synoptic...

500 mB Height and Vorticity for Tuesday 27 June 2017 Source: wxmaps.org, GrADS/COLA After several days of generally lower pressure over most of the sub-continent, the South Atlantic high pressure cell made landfall during Wednesday. This saw a minor drop in night temperatures across Namibia south of Grootfontein. A bigger drop and much colder nights were experienced in the Karasburg and Keetmanshoop districts with the cold coming from the east. The week’s local weather was marked by several intrusions of moisture in the middle layers of the atmosphere roughly between 13,000 and 18,000 feet. Surface level humidity mirrored this...