Tropical Storms

What is a tropical storm?

Intense low pressure weather systems that develop in the tropics

What are the different names for tropical storms?

Hurricanes, cyclones, typhoons or willy-willys

What conditions are needed for tropics storms?

• Sea temperature

• Ocean depth

• Location

• Wind sheer

• Sea temperature of ≥27°C

• Ocean depth of at least 60-70m

• A location of at least 5°N/S of the equator

• Low wind sheer

Why is the condition “Sea temperature of ≥27°C” required?

Heat energy (fuel) for the storm

Why is the condition “Ocean depth of at least 60-70m” required?

Rising warm air takes moisture in to the atmosphere from the hot ocean, which then condenses to form huge towering cumulonimbus clouds, which eventually burst, releasing large quantities of rainfall

Why is the condition “A location of at least 5°N/S of the equator” required?

In order for the Coriolis effect to make the storm spin, which is too weak over the equator

Coriolis effect

Created by the Earth’s rotation and makes winds bend

Why is the condition “Low wind sheer” required?

If the winds in the upper atmosphere are too strong (ie strong wind sheer) the rising warm moist air gets blown away / blown off course and the storm cannot form properly

What are the main characteristics of tropical storms?

• Size

• Hazards

• Movement

• Structure

• Where they form

• Direction of movement

• Climax and nadir

• 600/700km across

• They bring heavy rain, high wind speeds, storm surges and landslides

• They spin or rotate as they move. The spin is created by the Earth spinning (Coriolis effect)

• When they reach a certain size, they develop an eye (80 mph). Also consist of eye wall and rain bands

• They develop in tropical areas between 5-20°N/S of the equator

• They move westwards

• Over hot oceans they are their worst, but burn out if they pass over land surfaces or move towards the poles

What are the conditions of the eye like?

Calm, high pressure, descending air

What is the difference between high pressure and low pressure?

Low pressure areas are characterised by rising air and are often associated with cloudy, wet, and windy weather, while high pressure areas are associated with sinking air and typically bring clear, dry, and calm conditions

How do tropical storms form? (sequence of events)

1. A strong upward movement of air draws water vapour up from the ocean surface

2. This evaporated air cools as it rises and condenses to form towering thunderstorm clouds

3. Several small thunderstorms join together to form a giant spinning storm. When it reaches 120km per hour it is classified as a tropical storm

4. The storm develops an eye at its centre where air descends rapidly (high pressure). The outer edge is where the stronger most intense winds and rain are felt

5. As the storm grows it is carried across the ocean it gathers strength

6. On reaching the shore the storms energy supply is cut off and friction from the land slows it down and begins to weaken

What makes the tropical storms spin?

• Wind blows from areas of high pressure to low pressure

• As the Earth rotates it causes the wind to bend and sends warm air rising

At what location is the effect of the Coriolis effect the strongest?

At 5°N/S of the equator the effect of the Coriolis force brings about the maximum rotation of the air

What direction do the storms spin in the different hemispheres?

Storms spin anticlockwise in the Northern Hemisphere, and clockwise in the Southern Hemisphere

How long do tropical storms generally last?

7-14 days

Saffir-Simpson Scale

Estimates storm damage when it hits land, although many storms never make landfall

What wind speeds are classified as a tropical depression?

≤38mph

What wind speeds are classified as a tropical storm?

39-73mph

What wind speeds are classified as Category 1?

74-95mph

What wind speeds are classified as Category 2?

96-110mph

What wind speeds are classified as Category 3?

111-129mph

What wind speeds are classified as Category 4?

130-156mph

What wind speeds are classified as Category 5?

≥157mph

What is a flaw of the Saffir-Simpson scale?

In assessing impact, it does not take into account rainfall and storm surges

What is the frequency of tropical storms?

Tropical storms happen frequently. Over 100 tropical storms are formed each year.

Why do most tropical storms not develop into a major hazard?

Most tropical storms will never reach land, so they do not develop into a major hazard

Hurricane season in the Northern and Southern Hemispheres

Storms are more frequent in the Northern Hemisphere between June and November, and in the Southern Hemisphere between November and April

Storm surge

An abnormal rise of water generated by a storm, over and above the predicted astronomical tides

Storm tide

The water level rise due to the combination of storm surge and the astronomical tide

What does “razed” mean?

To destroy to the ground : demolish

e.g. buildings razed to the ground