Other Depressions — Page 359, Lesson 332

I want to walk you through the section on "Other Depressions," and we're going to start with tornadoes. Tornadoes are invariably associated with cumulonimbus clouds. In some cases, the rotation you see at the bottom of the storm actually extends all the way to the top of the storm cloud. The destructive power of a tornado comes from two things. First, there's a localized reduction in pressure — we're talking a drop of 20 to 200 hectopascals. That pressure drop can cause structures to literally explode as the tornado passes over them, because the pressure inside the building is much higher than the suddenly very low pressure outside. Second, the wind speeds inside the vortex are extremely high — up to 300 knots. That's the same unit we use for aircraft speeds, so you can appreciate how violent that is. Tornadoes usually last only a matter of minutes. Some occasionally last a few hours, and they move across the ground at speeds up to 40 knots. We're increasingly using Doppler radars for tornado detection. Doppler radar doesn't just see where precipitation is — it also measures particle speeds within the vortex. This is making local tornado warnings more reliable, but even with that technology, we still can't give more than about 30 minutes' advance warning. On the radar screen, tornadoes develop a typical 'hook' pattern — that's the radar 'signature' you'd look for. Now, within Europe, tornadoes are much weaker systems. Their maximum diameter is on the order of 100 to 150 metres, but they're usually much smaller than that. Let's move on to a different type: the West African Tornado. Despite the name, these are not true tornadoes in the sense we just discussed. West African tornadoes are associated with the passage of the Intertropical Convergence Zone, or ITCZ, through countries bordering the Gulf of Guinea. They are thunderstorm squall lines — they form in a line oriented north-south and move from east to west. They occur between March and November, and they are most common from March to May and again from October to November. Now, the excerpt ends with a set of practice questions. These are the book's practice questions — let's try them one at a time.