Weather

Atmospheric Pressure

Atmospheric pressure, also known as barometric pressure, is the force exerted by the weight of air molecules pressing down on the Earth's surface. The air around us has weight, and it presses against everything it touches. That pressure is called air pressure.

• Air pressure is highest at sea level, where the density of air molecules is greatest. It decreases with increasing altitude.
• The standard air pressure at sea level is defined as 1013.25 millibars (mb), 29.92 inches of mercury, or 14.7 pounds per square inch (psi).
• Air pressure is measured using an instrument called a barometer, hence why it's also referred to as barometric pressure.

Factors Affecting Atmospheric Pressure

• Altitude: Air pressure decreases with increasing altitude, since there are fewer air molecules pressing down from above the higher you go. Air pressure drops about 3.5 mb for every 30 meters (100 feet) increase in elevation.
• Temperature: Warm air is less dense than cold air. As air warms, the molecules move further apart, lowering the air density and decreasing the air pressure. Conversely, cooling air causes molecules to pack together more closely, increasing air density and pressure.
• Humidity: Moist air is less dense than dry air. Water vapor molecules (H2O) are lighter than nitrogen (N2) and oxygen (O2) molecules that make up most of the dry air. Therefore, humid air will have a lower pressure than dry air.

The Effects of Atmospheric Pressure on Weather

Air pressure is a fundamental factor shaping our weather patterns. Meteorologists analyze air pressure and pressure changes to forecast the weather. Understanding high and low pressure systems is key to predicting wind, temperature, clouds, and precipitation. In general:

• Low pressure systems tend to result in unsettled weather conditions like cloudiness, wind, and precipitation. Rising air motion in lows cools, condenses moisture, and forms clouds and precipitation.
• High pressure systems are usually associated with settled, calm, and fair weather conditions. Sinking air motion in highs warms, evaporates moisture, and creates clear skies.

The movement of air from high to low pressure areas creates wind. The greater the pressure difference between the pressure systems, the stronger the wind. Pressure differences are the driving force of weather.

Many types of severe weather like hurricanes, tornadoes, and severe thunderstorms form in low pressure zones. Daily and seasonal temperature ranges tend to be more extreme under high pressure than low pressure, since clear skies allow more solar heating by day and cooling at night. Rapidly falling air pressure usually indicates an approaching storm, while rising pressure indicates clearing and calmer weather.

Foehn

In the mountains, pressure differences between two sides of a massif result in wind rushing over the terrain. The resulting wind is known as foehn. Before mountain flights, pilots should check for these winds, by reading foehn charts.

Clouds

We can observe clouds to anticipate weather changes and flying conditions.

In short:

• Cirrus-type clouds indicate fair but changing weather.
• Stratus-type clouds generally mean stable air.
• Cumulus clouds indicate thermic activity, meaning instability and turbulence.
• Nimbostratus brings prolonged precipitation.
• Cumulonimbus are the most dangerous due to severe weather hazards.

High-Level Clouds (16,500 - 45,000 ft)

Cirrus

Thin, wispy clouds composed of ice crystals. Cirrus clouds on their own indicate fair weather. However, increasing cirrus can be an early sign of an approaching frontal system or storm in the next 24-36 hours.

Cirrostratus

Thin, veil-like clouds that often cover the entire sky and cause a halo around the sun or moon. May indicate an approaching warm front and rain within 12-24 hours.

Cirrocumulus

Small, white patches or scales. Cirrocumulus mixed with cirrus often precedes a warm front and rain within 24 hours.

Mid-Level Clouds (6,500 - 23,000 ft)

Altostratus

Gray or blue-gray clouds that often cover the entire sky. The sun may be dimly visible as a round disk. Altostratus indicates an approaching warm or occluded front with continuous precipitation, usually beginning in 6-12 hours.

Altocumulus

Gray puffy masses in rows or patches. May indicate an approaching cold front or thunderstorm.

Low-Level Clouds (surface - 6,500 ft)

Stratus

Low, gray, uniform clouds that often cover the entire sky. May produce drizzle or light snow. Indicates stable air.

Stratocumulus

Low, puffy, gray or white clouds in patches or layers with some blue sky visible. Indicates slightly unstable air. Light turbulence possible.

Nimbostratus

Dark gray, wet looking, formless layer of clouds that often bring continuous rain or snow. Indicates stable air and poor visibility.

Cumulus

Detached, puffy white or light gray clouds with flat bases. Fair weather cumulus indicate thermic activity, and therefore unstable air. Turbulence should be expected, especially in the afternoon. Cumulus clouds can grow vertically into towering cumulus which may develop into cumulonimbus. Additionally, a cumulus cloud's position relative to the trigger is a very good indicator for wind speed and direction at altitudes between the trigger and the cloud base.

Cumulonimbus

Heavy, dense, towering vertical clouds with dark bottoms. Associated with thunderstorms, severe turbulence, lightning, hail, and icing. Very dangerous for aviation and should be avoided.

Weather Models

Weather models are software applications that use mathematical equations to simulate the behavior and evolution of the atmosphere. They take in current weather observations from satellites, weather balloons, surface stations, and other sources as inputs. The models then solve the equations to predict how the weather will change over time, producing forecasts of temperature, precipitation, wind, and other variables. Execution of these programs is often distributed across clusters of machines. Below are details of two widely-used weather models.

ECMWF

The European Centre for Medium-Range Weather Forecasts (ECMWF) produces a global weather model considered one of the most accurate in the world. Key facts about the ECMWF model:

• Horizontal resolution of about 9 km globally
• 137 vertical levels up to 80 km altitude
• Produces forecasts out to 10 days, updated twice daily
• Ensemble system produces 51 separate forecasts to quantify uncertainty
• Also used to generate monthly and seasonal forecasts

ICON-EU

ICON (ICOsahedral Non-hydrostatic) is a newer global model developed by the German weather service (DWD). The ICON-EU is a high-resolution regional nest within the global ICON model focused on Europe. Key facts about ICON-EU:

• Horizontal resolution of 6.5 km over Europe
• 60 vertical levels up to 22.5 km altitude
• Forecasts out to 5 days, updated 8 times per day
• Two-way interaction with the global ICON model for better consistency
• Detailed depiction of local weather influenced by Europe's mountains and seas