Around the planet we find areas of calm, quiet weather controlled by high pressure, and unsettled or stormy weather controlled by low pressure. When you look at a weather map and see the letter H for high pressure, and the letter L for low pressure, you know that there's more air squeezed around the high pressure than there is around the low pressure. The H is like a mountain of air, or a mass of air. A region of high pressure is a mass of air that is just about the same everywhere, it may be cold or warm, dry, or moist. Between different masses of air are dividing boundaries that are called fronts.
A cold front is the leading edge of cold air, that is moving into warmer air. A warm front is the leading edge of warm air that is moving into cool air. Other boundaries can separate dry air from moist air.
In a mass of high pressure, air is squeezed and crowded together, and it moves toward the L, seeking lower pressure. Moving air is called wind. Because Earth is rotating, in the northern hemisphere, as air leaves high pressure it turns to the right to spin clockwise. As it approaches low pressure, it speeds up and begins to spin counterclockwise.
Even when air is calm where you are, wind is always blowing at different heights, around Earth. The jetstream is one of those winds. At about 6 miles above the ground, the jetstream pushes and guides weather systems around the planet. It can blow between 100 mph and 200 mph. In the winter, when the jetstream encircles the northern hemisphere to divide bitterly cold air in the Arctic from cool air south of it, it is called the polar vortex.
When air warms, it grows, rises, and spreads out. That’s why hot air balloons rise and then float in the sky. When air gets cold, it shrinks, sinks, and takes up less space, to become denser, or heavier to create higher pressure.
Air moves as wind from high pressure to low pressure. In the low pressure, air rises and spreads out above the ground to go back to sink over the high pressure area. Then it starts all over again. When air moves in a circle it is called a circulation.
Near oceans and big lakes, you notice air circulations when the land is hot, and the water is not. Over the land, the air gets warmer and lighter, and it rises. The pressure gets lower. At the same time, over the water the air stays cooler, so it is heavier, and it sinks to create high pressure. The wind blows from the water to the land, and then above the ground from the land back to the water to make a circulation. This is why you feel a cool breeze near large lakes or the ocean in the summer.
Weather forecasting starts with applying the principles of meteorology to what we know about the current weather. Math is used to project how fast a system might form or fade, and how long it would take to get to a new location. For this, meteorologists use computer models. No one weather forecast model is always better than others. Each has strengths which make them better on certain days, seasons, and situations, but the problem in forecasting is even if one model does well for a couple of days it doesn't mean it will continue to do so the next day. Some models are better in some regions than others. Models use calculus, statistics, physics, thermodynamics and hydrodynamics, along with input data to project the future. Online, you can easily find a dozen different models from various countries, agencies, and universities. Private businesses and the military have their own weather models. If you have a strong enough computer, you can actually run your own model.
No model is on target all of the time, so a good forecaster will look at multiple models to get a sense of all possibilities. She or he will use their experience, and knowledge of weather patterns to come up with the most likely scenario. For any forecast, the farther out in time it is, the harder it is to get it correct. That’s just what we see in all sports and all endeavors. The farther away you are from the target, the harder it is to reach it. In weather forecast models, use the trends rather than the actual numbers in an extended forecast. Expect the numbers to change a bit each day. As new data arrives, forecasters see things that were not detectable before.
Online and on TV, you find extended forecasts for a week to 10 days, and you also see weather outlooks. An Outlook is not a specific forecast or a guarantee, but it is what meteorologists use when they see that ingredients for something will be in place. In hurricane season you hear a daily tropical weather outlook, which previews possibilities up to 5 days forward. Seasonal outlooks cover longer periods like months. Particularly in spring, you are presented with periodic outlooks for the threat of severe thunderstorms. These go out as much as 8 days, before the actual threat. For weather threats, there are two terms that are similar to Outlook and Forecast. The first is Watch.
A Watch is like a short-term outlook. A watch means that ingredients for a certain type of hazard will be in place over a general area in the near future, and you should watch for the threat that could develop.
A Warning is more like a forecast. A Warning says that a specific type of threat is very likely or happening in a specific area, and you’ve got to act to protect yourself.
Outlooks, Watches and Warnings, all appear as a on a map as an outline or polygon. All of them can and will change and be adjusted or cancelled, based on data. It is critical to understand that forecasting weather is like identifying what a distant vehicle is when you are driving down the Interstate. The farther away it is, the less certain you are of what it is. It may not be in your lane. It may be parked. It may exit before you get near it.
Many weather apps have more and more data like radar, satellite, storm tracks, pollen counts, lightning, and weather forecasts. Most of the data and forecasts come straight from a computer model, which won’t perfectly match the ones you might see on TV. Many weather apps give raw computer model values for your location, meaning if you drive 5 or 10 miles and look at the app, you’ll likely get different values. Within a single county, weather apps generate hundred of forecast values, something that a human could not do. The numbers may update many times each day so you may see a lot of change. While weather apps look precise with things like rain chances, that rain percentage doesn’t tell you coverage, duration, intensity, or likely impact from rain. For something like that, the weather apps are just a rough guide to trends.
Weather apps have a lot of value in showing you current radar, along with giving you alerts for dangerous conditions. Like all technology, weather apps are not 100% reliable. Sometimes you won’t get an alert for weather hazards, just like sometimes a phone call is dropped or a message comes in without you hearing the phone ring. Get a weather radio as a backup.
It might seem like so many things are happening in weather that never happened before. With 24-hour Internet and cable and satellite and streaming TV, and social media, and the fact that everyone has a camera, we see and hear about every little weather event that happens. Much of this is not new, but it now affects more people simply because the population has grown.
Just the fact that we have so many high-flying jets now results in more cirrus clouds through the process of condensation that follows fuel combustion. As a result of pollution particles, towns that are downwind from big cities get a little more rain than the cities do. We sometimes find dense fog near factories that release pollution and water vapor. These are unintentional, but there is an actual science of weather modification. We can’t create weather from nothing, and we can’t stop storms from happening. The average thunderstorm has more power than a nuclear bomb, and the amount of energy that a hurricane uses everyday is so immense that we can’t come close to matching it. It probably would not be a good idea anyway to try to change nature, in ways that we don’t really understand.
In the science of weather modification, we can sometimes influence or nudge weather that is starting or already occurring. The most common weather modification is cloud seeding where dry ice or chemicals are dropped from aircraft into growing clouds to either weaken them or make them produce more rain. This is done in agriculture in many countries. The science does work, to give farmers a little bit more rain, but in practice, it's not easy to control or be sure that you actually did anything that wasn’t going to happen anyway. Earth’s atmosphere is vast and complicated.
A simpler weather modification is using the downwash from helicopter rotars to clear a thin layer of fog from a runway so airplane traffic can continue.