Fundamentally speaking, everything that happens in the atmosphere is a function of three basic factors: temperature, pressure, and moisture. This concept also applies to an air mass, which is a large body of air that generally has the same temperature and moisture characteristics throughout.
Together, these characteristics—warm or cold, dry or moist—determine the kind of weather the air mass will produce and govern its interactions with other air masses.
To help you anticipate weather conditions, keep in mind that the geographical origin of an air mass determines its basic temperature and moisture characteristics.
Continental Polar—Air masses that move south from Canada are usually very dry and cold.
Maritime Polar—Air masses that move south from the oceans bring moist, cold air.
Continental Tropical—Air masses that move north from Mexico are usually dry and hot.
Maritime Tropical—Air masses that move north from southern oceans bring moist, warm air.
As they move, air masses take on the temperature and moisture characteristics of the surface beneath them. A dry air mass traveling over the Great Lakes can pick up enough moisture to produce large amounts of “lake effect” snow. Conversely, moist air off the Pacific Ocean tends to dry out as it passes over the inland deserts.
TIP: Get to know the specific areas you intend to visit. Talk to local pilots, most will gladly share their knowledge.
Three factors determine an air mass temperature:
This is the temperature to which the air would have to be cooled at constant pressure and constant water vapor content to become completely saturated. So, pay attention if a METAR reports temperature and dewpoint within a few degrees of each other: instrument meteorological conditions like low ceilings and visibilities can thwart your flying plans.
Temperature and dewpoint determine an air mass’s stability.”
See what happens when temperature and dewpoint converge.
The amount of moisture in the air compared with the amount of moisture at the current temperature is reported as the relative humidity percentage. Higher relative humidity often means lower visibility and more cloud coverage.
Fog can form quickly after sundown or at sunrise when the temperature-dewpoint spread decreases as the temperature drops at night and very early in the morning.
Temperature and dewpoint determine an air mass’s stability. Stable air resists vertical motion and tends to bring benign (though not necessarily good) weather, while unstable air can bring turbulence and severe weather.
| Stable Air | Unstable Air | |
|---|---|---|
| Clouds | Layered clouds or fog | Puffy, with extensive vertical development |
| Precipitation | Small drops in fog and low-level clouds; large drops in thick, stratified clouds; widespread rain or snow | Large drops in heavy rain showers; showers usually brief; hail possible |
| Icing | Rime, freezing rain, or clear ice | Moderate to heavy clear ice |
| Visibility | Restricted for long periods | Good, outside areas of precipitation |
| Other | Frost; dew; temperature inversions; little or no turbulence | High and/or gusty winds; potential for turbulence; lightning |
On a grand scale, air masses are set in motion by uneven heating of the Earth’s surface, which causes atmospheric circulation that creates variations in density and pressure.
For example, air near the equator receives more heat from the sun than air at the poles. In the Northern Hemisphere, as the air near the equator warms, its density decreases, and it begins to rise and flow away from the equator toward the north pole. This creates a low pressure area near the equator. Once the rising air cools, it becomes denser and begins to descend, eventually flowing back toward the low pressure area near the equator. The air warms again and the same convection pattern repeats.
The Earth’s rotation causes Coriolis force, which affects the direction of wind flow and counteracts the tendency of air to flow directly from high to low pressure areas.
The spinning Earth influences atmospheric circulation as it creates three convective cells north (and three convective cells south) of the equator. In the northern hemisphere’s northernmost cell, mid-latitude air rises and migrates north to the Arctic, where it becomes a cold, dense, high pressure air mass. This mass moves south, displacing other air masses (and growing warmer) along the way.
TIP: Air temperature, density, and pressure affect aircraft performance. Expect diminished takeoff and climb performance when operating in warm, less dense air and (to a lesser extent) in low pressure areas.
At a given location, the actual speed and direction of the wind is determined by a combination of three things working together at the same time:
Following the path of least resistance, high pressure air flows toward areas of lower pressure. Dramatic pressure differences between two areas can signify stronger winds.
On weather maps, the letter “H” represents the center of a high pressure area, while an “L” shows the center of a low. Lines called isobars connect areas of equal pressure. Where isobars are close together, differences in pressure occur over shorter distances. Wind tends to be stronger in these areas.
TIP: When you hear words like high, low, warm, and cold, remember that they’re being used in a relative sense. For example, a cold front in the summer typically brings air that’s warmer than a warm front in the winter.
The Earth’s rotation causes Coriolis force, which affects the direction of wind flow and counteracts the tendency of air to flow directly from high to low pressure areas. In the northern hemisphere, the Coriolis effect deflects the wind to the right and causes air to flow clockwise around highs and counterclockwise around lows.
Counterclockwise Path—Imagine a high pressure area, sitting next to a strong low. Viewed from above, the strong pressure differential pulls the air to the left, toward the low, while Coriolis force tries to pull it to the right (but with less force). Thus, the air still moves toward the low, but not directly toward it. Deflected by Coriolis force, it follows a curved path—a counterclockwise spiral toward the center of the low.
Dramatic pressure differences between two areas can signify stronger winds.
Clockwise Path—Now imagine the same high pressure area sitting next to a weaker low. In this case, the pressure gradient is not very strong, and even though the air is still being pulled toward the low, the Coriolis force is sufficient to overcome it. So, what happens? Instead of being pulled left toward the low, the air is pulled to the right: It follows a curved, clockwise path around the high.
Ground friction, which increases with proximity to the surface, tends to both slow down the air and counteract Coriolis force. Think of it this way: The air trying to move independently from the Earth causes Coriolis force, but the Earth dragging the air along with it causes friction. Increased friction reduces relative air-Earth motion and lessens Coriolis force.
TIP: Friction from terrain and other obstructions slows the wind near the surface and reduces Coriolis force. You’ll notice that, as you climb to a higher altitude, the wind tends to shift a few degrees and increase in speed.
High pressure system—The air in a high pressure system descends and flows outward in a clockwise direction toward the surrounding, lower pressure air. As the air spreads away from the high, air from above descends to replace it. This generally results in light winds and clear skies as the descending air warms and moisture evaporates.nfr
Low pressure system—The air in a low pressure system converges inward and upward in a counter-clockwise direction from the surrounding, higher pressure air. As the air converges it is forced to rise. This generally brings stronger winds, cloudy skies, and precipitation as the rising air cools and the moisture in it condenses.
TIP: The closer you are to the center of a surface low pressure system, the stronger you can expect the wind and any associated weather to be.
Your location relative to high and low pressure systems can give you an idea of the kind of weather to expect.
Cold Front Demarcation—Blue line with triangles
Warm Front Demarcation—Red line with half-moons
Stationary Front Demarcation—Alternating red and blue line with a triangle on the blue portion and half-moon on the opposite red portion
Occluded Front Demarcation—Purple line with alternating triangles and half-moons
A front is the boundary line between two different air masses. The air masses' characteristics and how they move or don’t move determine the type of front. For example, the leading edge of a cold air mass moving toward a warmer air mass is called a cold front.
Winds often converge at frontal boundaries, and temperature and pressure differences can be significant on either side of a front.
Winds often converge at frontal boundaries, and temperature and pressure differences can be significant on either side of a front. So, if a front approaches, you can expect the weather to change. But realize that not every cold or warm front brings the exact same weather.
Fronts are identified by temperature, pressure, moisture, and other factors.
TIP: Cold fronts nearly always extend anywhere from a south direction to a west direction from the center of low pressure areas and never from the center of high pressure systems. Warm fronts also extend from the center of low pressure areas but nearly always on the east side of the low.
When a colder air mass starts to move along the ground, its leading edge is called a cold front. Because the colder, more dense air is heavier than the warmer air it’s displacing, it slides underneath, forcing the warm air to rise. The rising air cools to the dewpoint and forms clouds along the front. Thunderstorms will form if enough moisture and unstable air are present. A fast-moving cold front can also kick up severe squall line thunderstorms along or ahead of the actual front. Cold fronts typically move at 25 to 30 mph, but some can reach 60 mph.
A fast-moving cold front can also kick up severe squall line thunderstorms along or ahead of the actual front.
TIP: In most areas of the United States, cold fronts bring thunderstorms and other severe weather. In drier regions, the lack of moisture inhibits the development of severe weather, and cold front passage typically results in a wind shift and cooler temperatures.
When a warmer air mass moves, the leading edge is called a warm front. Because the warmer air is less dense, it travels up and over the cooler air below. This causes a temperature inversion with very stable, smooth air ahead of the front. If there’s enough moisture in the air, expect widespread stratus cloud coverage, steady rain, and icing in areas that are below freezing. A warm front has a gradual slope over colder air which can extend hundreds of miles ahead of the warm front’s surface position. Warm fronts typically move slower than cold fronts, at around 10 to 25 mph.
TIP: Weather charts only depict the front’s location at the surface. When you fly toward an approaching warm front, you may encounter the warmer, less dense air more than 100 miles from where it is charted on the surface.
When a faster moving cold front catches up with a slower moving warm front, an occluded front forms. An occluded front can have the weather characteristics of a cold or warm front, depending on the relative air temperature ahead of each front. Hazardous weather is likely if there’s enough moisture in the air.
Cold front occlusion—Occurs when the air associated with a fast-moving cold front is colder than the air ahead of a slow-moving warm front. The cold air replaces the cooler air and pushes the warm front aloft. The associated weather is typically a mixture of both warm and cold front weather.
Warm front occlusion—Occurs when the air ahead of a warm front is colder than the air ahead of a cold front. The cold air is forced aloft by the warm front. The associated weather can be severe if the air forced aloft is unstable.
When air masses have very different moisture levels, the boundary between them is called a dryline.
When opposing air masses have relatively equal pressure, they stop moving and the line tween them becomes a stationary front. Stationary fronts often bring cloudy, wet weather that can last a week or more.
TIP: When planning a trip, watch for stationary fronts forecast along your route. You could get delayed for several days!
When air masses have very different moisture levels, the boundary between them is called a dryline. This is common at the narrow boundary found in the Plains region between moist air from the Gulf of Mexico and dry continental air from the west. It is most often present during the spring, where it is often the site of thunderstorm development. Typically, the dryline advances eastward during the day and retreats westward at night. The imbalance of dry and moist air at the dryline can trigger severe thunderstorms and even tornadoes.
TIP: Pay close attention to ATIS and ASOS/AWOS reports ahead when you approach a dryline, as the weather can change quickly.
Cold Front—Gusty winds, turbulence, and thunderstorms
Warm Front—Widespread low stratus clouds, steady rain, and icing conditions
Stationary Front—Cloudy and rainy weather that may last several days
Occluded Front—A mixture of cold and warm front weather that is dependent on temperature
TIP: You can see how the weather is changing by looking at METARs from airports on either side of a front.
Different regions in the United States have unique weather characteristics. These are the most common, prevalent weather patterns and potential flying hazards that can be expected in each.
Spring and summer—Prevalent high pressure systems bring good flying weather.
Fall, winter, and spring— Onshore flow of moisture can bring severe icing conditions.
Winter—Moisture off the Pacific Ocean causes rain on the windward side of the mountains and snow on the leeward side. The coast can be cool and wet with fog and low ceilings.
Northern Rockies—Tend to have strong winds and turbulence. Summers can produce thunderstorms and prolonged heat waves. High density altitude is a major concern. Fall offers the best flying weather, though the snow tends to begin in mid-October.
Rising hot air causes many strong low pressure systems to form in this region and move east.
Summers—Generally warm and dry in the desert, although thunderstorms and hail may be encountered. High density altitude is a major concern. Moisture from the Pacific Ocean, the Gulf of California, and the Gulf of Mexico triggers rain in the mountains. Many of these thunderstorms can be "dry thunderstorms" that produce lightning but no rain. In some cases these can produce nasty dry microbursts.
Desert winters—Cool and dry.
Mountain winters—Expect strong downslope winds, mixed icing in clouds, and blizzards.
California—Coastal fog forms over land in the winter and migrates from the ocean in the summer.
Southern California—Has smog, onshore flow, and temperature inversions. October to April is rainy. Fall and spring bring the Santa Ana winds, which are a turbulent line between moist oceanic air and dry desert air.
Observing weather patterns for several days before you fly helps to provide the big picture on how frontal and pressure systems expect to be moving.Summer—High heat and humidity mixed with heavy rain showers, thunderstorms, squall lines, and possibly tornadoes. Drylines can form when moist Gulf of Mexico air and dry, hot desert air meet.
Summer and fall—Threat of hurricane landfall.
Winters—Near the Gulf, are mild with rain and morning fog due to low temperature-dewpoint spreads.
Summers—Warm and dry. Moisture from the Gulf of Mexico triggers occasional thunderstorms.
Winters—Very cold and dry. Blizzards are common due to cold air from the north and moisture from the Gulf of Mexico mixing in this region.
Midwest/Ohio Valley
Low pressure systems form on the lee side of the Rocky Mountains and grow as they head east and are fed by moisture from the Gulf of Mexico.
Summers—Usually hot, humid, and rainy with air mass thunderstorms. The southern portion of this region is part of Tornado Alley.
Fall— High dewpoints cause night and morning fog.
Winter—Can be cold with lake effect snow near the Great Lakes. A low pressure system can bring heavy rain in the southern portion of the region and heavy snow in the northern portion.
Summers— Moisture from the Gulf of Mexico and the Atlantic Ocean causes hot and humid weather with rain and thunderstorms. Thunderstorms are typically pulse-type (single cell) that do not produce severe weather and usually last 20 to 30 minutes.
Summer and Fall—Hurricanes can pose a threat.
Winters—Cool and mild with occasional heavy rain showers.
Florida—Surrounding water triggers morning fog, afternoon thunderstorms, sea breezes, and turbulence. Some of the Gulf Coast states and Florida can have thunderstorms triggered by sea breeze fronts.
New England/Mid-Atlantic
This region’s weather is influenced by its location between mountains and the ocean.
Summers—Warm and humid with heavy rain showers and thunderstorms. Expect low visibility due to haze. Morning fog forms due to easterly flow off the Atlantic Ocean and up the rising terrain of the Appalachian Mountains.
Winter—Can bring heavy snow and rain due to upper-level lows from the west and south. Instrument meteorological conditions and icing are common. Areas near the Great Lakes experience lake effect snow.
ATC, flight service, and onboard technology like weather radar and datalink are excellent in-flight weather resources to help assess if and how conditions are changing.Alaska Southeast/Gulf of Alaska (Juneau, Ketchikan): Plenty of moisture along the coast causes low ceilings and lots of rain the entire year.
Alaska Cook Inlet/Kenai Peninsula (Anchorage, Kenai, Homer): VFR conditions are common year-round, although occasional coastal morning fog and/or low ceilings could be encountered in summer. In winter, expect occasional ice fog and blowing snow. In the spring and fall, look for icing in clouds, freezing rain, and snow. Occasional radical pressure changes will cause high winds.
Central Alaska (Fairbanks): Summer features extended daylight hours, very high temperatures, haze, and thunderstorms with hail. In the fall, look for snow, freezing rain, and ice fog as the temperatures begin to drop. Winter brings very little sunlight, extremely low temperatures, and more ice fog. The snow, freezing rain, and ice fog continue into the spring months.
Seward Peninsula (Nome, Kotzebue, Bethel): High humidity brings low clouds during summer months. In the fall, look for more low clouds along with ice fog, blowing snow, and freezing rain. These conditions continue into winter with colder temperatures. Rising temperatures in the springtime add freezing rain to the mix of snow and low clouds.
Western Aleutian Island Chain (Cold Bay, Dutch Harbor, Adak): Low clouds and windy conditions throughout the year.
Far North (70 degrees latitude, Barrow): Cold conditions, low clouds, ice fog, and blowing snow most of the year.
Forecasts are imperfect, but you can extract a lot of truth from them to make the best go/no-go decisions. When flying, compare the forecast to actual conditions and pay attention to trends. ATC, flight service, and onboard technology like weather radar and datalink are excellent in-flight weather resources to help assess if and how conditions are changing. Depending on your datalink provider and in-cockpit equipment, you can pull up graphical and textual weather that offer a clear picture of wind shifts, pressure changes, and temperature changes at altitude and at your destination. This together with what you hear from ATC or flight service and what you see outside the cockpit, greatly improves your situational weather picture aloft.
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