Weather

Understanding all kinds of weather

When you set out to master a complex topic such as the meteorology a pilot needs to know, an overview can be a huge help.

The points below hit some of the highlights of what pilots should know to better understand weather, both to fly safely and to answer FAA knowledge questions correctly.

In three decades of writing about weather for people who aren’t meteorologists, I’ve found some concepts that tend to trip up understanding of some aspects of weather. Look for my “metmets” for alerts about a possibly confusing meteorological concept.

UNEQUAL HEATING OF EARTH. Solar radiation supplies almost all of the Earth’s heat that makes life possible and drives the weather. While the sun warms our planet, the Earth is always giving up heat that radiates into space as infrared energy. The balance between incoming and outgoing heat energy keeps the Earth in a temperature range that allows life as we know it to exist.

Earth has seasons because its axis is tilted in relation to the planet’s yearly path around the sun. During the Northern Hemisphere winter, the North Pole is tilted away from the sun; in summer, it’s tilted toward the sun, resulting in the hemisphere’s long summer days, when the sun is higher in the sky.

Solar energy reaching the regions for a few hundred miles around the North and South poles does not replace all the infrared energy escaping into space there.

In contrast, in the tropics—the region a few hundred miles north and south of the equator—infrared energy escaping back into space can’t keep up with all of the arriving solar energy.

Yet, the tropics don’t continue warming until all the water evaporates; the polar regions don’t continue to cool until they reach absolute zero. That’s because winds and ocean currents move heat from the tropics into the middle latitudes and polar regions, while at the same time other winds and currents move cool air and water from the polar regions into the middle latitudes and tropics.

These movements begin with solar energy warming land and water, which in turn warm the layer of air next to them. As air warms, it becomes less dense and rises. In the polar regions, the land and oceans grow colder as infrared energy radiates away, cooling the air next to them, making it more dense and causing it to sink and spread out, heading toward the tropics.

Metmet: When taking knowledge tests, remember that all weather, such as changing altimeter settings, starts with unequal heating of the Earth and involves a heat exchange.

EARTH’S ROTATION GETS WEATHER GOING. On a global scale, warm air rises in the tropics and cold air tends to sink in the polar regions. If the Earth were not rotating, this would set up a relatively simple global wind pattern with rising tropical air flowing aloft directly to the polar regions, sinking, and flowing directly back toward the tropics near Earth’s surface.

Since the Earth is rotating, high-altitude winds heading away from the tropics and low-level winds blowing from the poles toward the equator trace curved paths across the planet. Scientists call the effect of the Earth’s rotation on winds and ocean currents the Coriolis force.

This force works with other forces that affect the winds to create huge wind spirals we see in large storms, and global-scale winds both at the surface and aloft. These global-scale winds include the generally west-to-east flow of winds in the middle latitudes, including the high-altitude jet stream winds (see “Weather: Is the Force with You?” October 2015 Flight Training).

To sum up: Swirling storms and upper-air winds that swoop toward the equator and back toward the poles help ocean currents keep the Earth’s supply of heat in balance by moving warm air toward the poles and cold air toward the equator.

HOW AIR PRESSURE DIFFERENCES FORM. Rising warm air is one way in which the atmosphere creates areas of low atmospheric pressure at the surface. Surrounding air—where the pressure is higher—flows toward the low-pressure areas as wind, replacing the rising air. Sinking air creates an area of high atmospheric pressure at the surface, where air flows outward toward areas of lower pressure.

The formation of the cumulus clouds that can grow into thunderstorms and tropical storms—which are organized collections of thunderstorms—begins with warm air rising. The formation of low-pressure areas that become extratropical storms is more complicated.

The curving paths of winds aloft, including jet streams, help create areas of high and low atmospheric pressure at the surface by encouraging air to rise from the surface in some areas, creating low pressure, and to sink toward the surface in other places, to form surface high-pressure areas.

Metmet: Not all surface low-pressure areas are warmer than the surrounding air. Tropical storms such as hurricanes, which form over warm water, are warmer than the surrounding air. Extratropical storms, which do not form over land or cool water, have centers that are cooler than the surrounding air.

AIR PRESSURE DECREASES WITH ALTITUDE. The air’s pressure, which creates forces acting in all directions, depends on the weight of all the air above it that’s being accelerated by gravity toward the Earth’s center. If air isn’t moving up or down, the air pressure at any altitude represents a balance between the weight of the air above and the pressure of the air below pushing up.

Air pressure decreases with height, which affects aircraft performance. Also, aircraft altimeters use the regular decrease of air pressure with altitude to indicate height above sea level.

RISING AIR COOLS, SINKING AIR WARMS. Any time air moves into an area of lower pressure, such as by rising, its pressure decreases, which cools the air. If air moves into an area of higher pressure, such as by sinking from aloft, its pressure increases, which warms the air.

Thus, when anything forces air to rise, such as heat from the warm ground or wind blowing over a mountain, the pressure of the rising air decreases at a regular rate that is not affected by the temperature of the surrounding air. As the pressure decreases, the air cools. When air descends into higher air pressure below, it’s compressed and warmed, also at a regular rate.

RISING AIR BRINGS CLOUDS, PRECIPITATION. Unless the air is very dry, rising air creates clouds and maybe precipitation. As rising air grows colder, the water vapor in it begins condensing into tiny cloud drops. If the air rises high enough, ice crystals begin forming. When conditions are right, including enough water vapor in the air, clouds can produce precipitation such as rain or snow.

Sinking air warms, which prevents clouds from forming or evaporates clouds. This is why places on weather maps marked with an “H” tend to have generally clear skies.

Metmet: You sometimes read that rising air grows colder with altitude because the surrounding colder air aloft cools it. The rising air cools as its pressure decreases to match the pressure of the surrounding air. This is a key to mastering the concept of atmospheric stability, which is very important in understanding certain kinds of weather, including thunderstorms (see “Weather: Stable or Unstable?” April 2014 Flight Training).

Understanding all kinds of weather

When you set out to master a complex topic such as the meteorology a pilot needs to know, an overview can be a huge help.

The points below hit some of the highlights of what pilots should know to better understand weather, both to fly safely and to answer FAA knowledge questions correctly.

In three decades of writing about weather for people who aren’t meteorologists, I’ve found some concepts that tend to trip up understanding of some aspects of weather. Look for my “metmets” for alerts about a possibly confusing meteorological concept.

UNEQUAL HEATING OF EARTH. Solar radiation supplies almost all of the Earth’s heat that makes life possible and drives the weather. While the sun warms our planet, the Earth is always giving up heat that radiates into space as infrared energy. The balance between incoming and outgoing heat energy keeps the Earth in a temperature range that allows life as we know it to exist.

Earth has seasons because its axis is tilted in relation to the planet’s yearly path around the sun. During the Northern Hemisphere winter, the North Pole is tilted away from the sun; in summer, it’s tilted toward the sun, resulting in the hemisphere’s long summer days, when the sun is higher in the sky.

Solar energy reaching the regions for a few hundred miles around the North and South poles does not replace all the infrared energy escaping into space there.

In contrast, in the tropics—the region a few hundred miles north and south of the equator—infrared energy escaping back into space can’t keep up with all of the arriving solar energy.

Yet, the tropics don’t continue warming until all the water evaporates; the polar regions don’t continue to cool until they reach absolute zero. That’s because winds and ocean currents move heat from the tropics into the middle latitudes and polar regions, while at the same time other winds and currents move cool air and water from the polar regions into the middle latitudes and tropics.

These movements begin with solar energy warming land and water, which in turn warm the layer of air next to them. As air warms, it becomes less dense and rises. In the polar regions, the land and oceans grow colder as infrared energy radiates away, cooling the air next to them, making it more dense and causing it to sink and spread out, heading toward the tropics.

Metmet: When taking knowledge tests, remember that all weather, such as changing altimeter settings, starts with unequal heating of the Earth and involves a heat exchange.

EARTH’S ROTATION GETS WEATHER GOING. On a global scale, warm air rises in the tropics and cold air tends to sink in the polar regions. If the Earth were not rotating, this would set up a relatively simple global wind pattern with rising tropical air flowing aloft directly to the polar regions, sinking, and flowing directly back toward the tropics near Earth’s surface.

Since the Earth is rotating, high-altitude winds heading away from the tropics and low-level winds blowing from the poles toward the equator trace curved paths across the planet. Scientists call the effect of the Earth’s rotation on winds and ocean currents the Coriolis force.

This force works with other forces that affect the winds to create huge wind spirals we see in large storms, and global-scale winds both at the surface and aloft. These global-scale winds include the generally west-to-east flow of winds in the middle latitudes, including the high-altitude jet stream winds (see “Weather: Is the Force with You?” October 2015 Flight Training).

To sum up: Swirling storms and upper-air winds that swoop toward the equator and back toward the poles help ocean currents keep the Earth’s supply of heat in balance by moving warm air toward the poles and cold air toward the equator.

HOW AIR PRESSURE DIFFERENCES FORM. Rising warm air is one way in which the atmosphere creates areas of low atmospheric pressure at the surface. Surrounding air—where the pressure is higher—flows toward the low-pressure areas as wind, replacing the rising air. Sinking air creates an area of high atmospheric pressure at the surface, where air flows outward toward areas of lower pressure.

The formation of the cumulus clouds that can grow into thunderstorms and tropical storms—which are organized collections of thunderstorms—begins with warm air rising. The formation of low-pressure areas that become extratropical storms is more complicated.

The curving paths of winds aloft, including jet streams, help create areas of high and low atmospheric pressure at the surface by encouraging air to rise from the surface in some areas, creating low pressure, and to sink toward the surface in other places, to form surface high-pressure areas.

Metmet: Not all surface low-pressure areas are warmer than the surrounding air. Tropical storms such as hurricanes, which form over warm water, are warmer than the surrounding air. Extratropical storms, which do not form over land or cool water, have centers that are cooler than the surrounding air.

AIR PRESSURE DECREASES WITH ALTITUDE. The air’s pressure, which creates forces acting in all directions, depends on the weight of all the air above it that’s being accelerated by gravity toward the Earth’s center. If air isn’t moving up or down, the air pressure at any altitude represents a balance between the weight of the air above and the pressure of the air below pushing up.

Air pressure decreases with height, which affects aircraft performance. Also, aircraft altimeters use the regular decrease of air pressure with altitude to indicate height above sea level.

RISING AIR COOLS, SINKING AIR WARMS. Any time air moves into an area of lower pressure, such as by rising, its pressure decreases, which cools the air. If air moves into an area of higher pressure, such as by sinking from aloft, its pressure increases, which warms the air.

Thus, when anything forces air to rise, such as heat from the warm ground or wind blowing over a mountain, the pressure of the rising air decreases at a regular rate that is not affected by the temperature of the surrounding air. As the pressure decreases, the air cools. When air descends into higher air pressure below, it’s compressed and warmed, also at a regular rate.

RISING AIR BRINGS CLOUDS, PRECIPITATION. Unless the air is very dry, rising air creates clouds and maybe precipitation. As rising air grows colder, the water vapor in it begins condensing into tiny cloud drops. If the air rises high enough, ice crystals begin forming. When conditions are right, including enough water vapor in the air, clouds can produce precipitation such as rain or snow.

Sinking air warms, which prevents clouds from forming or evaporates clouds. This is why places on weather maps marked with an “H” tend to have generally clear skies.

Metmet: You sometimes read that rising air grows colder with altitude because the surrounding colder air aloft cools it. The rising air cools as its pressure decreases to match the pressure of the surrounding air. This is a key to mastering the concept of atmospheric stability, which is very important in understanding certain kinds of weather, including thunderstorms (see “Weather: Stable or Unstable?” April 2014 Flight Training).