3.1.5.5 Storm hazards
Structure of the atmosphere
The atmosphere has four layers:
- The troposphere, that we live in near the surface of the earth
- The stratosphere, that houses the ozone layer
- The mesosphere, a colder and lower density layer with about 0.1% of the atmosphere
- The thermosphere, the top layer, where the air is hot but very thin
The troposphere and tropopause:
Most of our weather and climate is found in the layer of turbulent air known as the troposphere. The
temperature decreases as you go higher into the troposphere because its mains source of heat is the
warm ground.
The tropopause acts like a lid on weather systems in the troposphere. Without it, climatic hazards
might be more powerful/ bigger.
,Mechanisms of weather (temperature, pressure and humidity and inextricably linked with each
other)
Temperature:
The sun’s incoming variable radiation causes differences in temperature. This causes imbalances in
pressure.
The sun heats the earth’s surface (not the air). The air is heated (sensible heat = heat you can feel)
by heat from the ground and expands. Warm air is less dense, rises, cools and condenses to form
clouds and rain (low pressure).
Pressure
Different air masses with different temperatures have different pressures. The units of pressure are
millibars (mb).
High pressure air flows to low pressure air, which causes global wind patterns. The bigger the
difference in high and low pressure, or the closer they are in distance, the stronger the winds over a
certain distance (pressure gradient). The global wind patterns affect local wind patterns.
Cold air contracts and sinks, causing high pressure on the Earth’s surface.
In areas of low pressure, warm air expands, rises, cools, condenses and forms clouds, causing wind
and rain. The lower the pressure the worse the weather will be.
When warm air rises in one place, cool air sinks somewhere else at areas of high pressure. As the
cold air sinks it warms up. No water vapour condenses and no clouds form. The sky stays clear. High
pressure gives the hottest summer weather and coldest winter weather.
When there is high pressure in summer:
- There are no clouds so the sun is strong.
- Since there is no cloud to trap heat in, the evenings can be cool.
- No clouds means the ground gets cold at night. Water vapour condenses on grass to form
dew.
- No clouds means no rain so there may be drought in some places.
- Inland on very hot days, the hot air may rise rapidly, cool and form huge black clouds which
can cause thunderstorms, causing heaving rain and flooding.
When there’s high pressure in the winter:
- There is no cloud to act as a blanket. The days are clear, cold and bright.
- With no cloud the ground cools fast at night and cools the air above it. Water vapour
condenses and freezes on cold surfaces, giving frost.
- It also condenses on dust and other particles in the air, giving fog. This makes driving
dangerous.
- Ice and frost mean animals have trouble finding food.
- Pipes may burst and homes may get flooded.
Isobars join lines of equal pressure (the weight of the air measured in millibars) at 4mb intervals.
- 1013mb is average pressure at sea level.
- 1000mb and below is low pressure, causing wind and rain.
, - 1020mb and above is high pressure, causing dry and calm conditions.
- Tropical storms (980mb) and strong hurricanes (950mb).
Isobars that are close together bring stronger winds, isobars that are far apart bring gentler winds.
Humidity:
Humidity is given as relative humidity (as a percentage). It measures moisture levels in the air, i.e.
the amount of water vapour in the air.
Humidity influences the stability of air masses (air mass = a large block of air). Dry air is more stable
than moist air.
Humidity also controls rainfall patterns and heat transfer from the ground level to the atmospheric
level.
When moist air rises, the water vapour in the air will cool and condense as latent heat (stored in the
water vapour) is released. As condensation takes place, heat is released into the atmosphere. This
makes the surrounding air less dense and the air continues to rise, cool, condense, etc.
Air with a high water content (high humidity) such as air masses over oceans will experience much
condensation. This will release more heat, which encourages more further convection with further
condensation, further heat release and yet more convection. This causes large-scale convectional
storms (such as tropical storms/ hurricanes). A storm will continue to grow if it has a supply of moist
air flowing into the system. Warm air from the ocean takes the place of the rising air.
When air rises faster due to increased heating, the pressure gradient increases, leading to faster
winds and so more water vapour enters the system.
Weather/ radar maps
