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## How do geologists divide Earth's past into time periods?

Lucio Margherita
Featured 5 months ago

By major worldwide geological events.

#### Explanation:

Such events however, have to leave a discernible trace of their occurrence all over the Earth's crust. The best ones, and most often used, are the rises and falls of the sea level.

When these increases and decreases of the available ocean water occur, they do simultaneously and worldwide. Yes, I know, tides are not simultaneous, but this is so because the amount of ocean water remains the same. The extra liquid available here is missing somewhere else. And, in any case, I am not referring to daily or even monthly cycles. We are speaking of geological time where the smallest time unit is closer to 1,000 years.

Over that time frame not only the phenomenon manifests itself all over the world, but it modifies the environment so as to leave a specific imprint on the subjacent rocks. What might have been a beach or a prairie becomes the seafloor with different conditions and denizens. This phenomenon is called a "transgression".
When the sea regresses under similar conditions, the movement is called a "regression"

Transgressions and regressions are not the only method to separate geological periods. The boundary between the Mesozoic and the Tertiary eons is clearly visible everywhere by a thin layer of rock rich in Iridium. This otherwise uncommon radioactive element (Ir) was possibly spread out by a planetary catastrophic event that might have caused the prompt demise of a number of animal and vegetal species, including the dinosaurs.

But a geological epoch is not only characterised by its limits. What happens between those limits is also relevant. The Tertiary was a mountain building period, the Cretaceous saw the sky full of birds, As for the inhabitants of the Earth, they may also be used to distinguish different epochs of its existence.

I am certain that you are aware that the Jurassic was the era of the dinosaurs; you might have seen fossils of trilobites, they characterise the Cambrian and Ordovician periods.
Mammals appear in the late Mesozoic and flourish in the Cainozoic; and, speaking of flowers, the angiosperms (or flourishing plants) made their appearance in the Tertiary.

Generally speaking, each geological period is delimited by major traumatic events and characterised by conditions extended over a long given period.

## What generates weather and climate patterns on Earth?

Fletcher
Featured 3 months ago

Difference in temperatures and humidity around the Earth!

#### Explanation:

Different areas of Earth get different amounts of sunlight, therefore they get different amounts of radiation and heat. This causes air currents, and the hot air tries to move to the cold air. This is what causes weather.

Our atmosphere basically becomes a heat engine. High and low pressure areas, wind, clouds, and precipitation systems are all caused, either directly or indirectly, by this uneven heating and the resulting heat redistribution processes.

This is all dependent on the availability of water. If the air is very dry the change of temperature will not result in changes to weather. Temperatures that would create a hurricane over the ocean do almost nothing over the Sahara desert.

## In what biome do we live in?

Kate M.
Featured 4 months ago

Humans can be found living in virtually all types of biomes.

#### Explanation:

Humans can be found living in virtually all types of terrestrial biomes.

Due to technological advancements, humans have been able to modify their environment and adapt to many different places. Clothing allows us to stay comfortable in areas with cooler temperatures. Building shelter allows us to stay cooler in dry, hot areas. In areas where the amount of vegetation is limited, tools have allowed us to hunt.

However, certain biomes are more popular for human habitation than others. Very few people live in the tundra . The map below shows population density from 1990 to 2015 with areas in bright green having higher density, or more people, than areas in blue.

Humans in Meghalaya, India have created living bridges out of rubber tree roots because other materials rot so quickly due to the extreme precipitation of the region (see here). The Nenets live within the Arctic Circle, herding reindeer and relying on them for food, shelter and clothing (see here)

## Why are wind speed and direction important for understanding changing weather?

James J.
Featured 2 months ago

Two reasons.

#### Explanation:

The first reason is it is if you know the wind speed and direction you will know what direction the weather is coming from and how fast it is moving. For example, if it is raining at a town 80 nautical miles (wind is measured in knots official) to the west of a second town, and the radar returns show that the precipitation is moving to the east at a speed of 20 kts, a relatively accurate prediction for the second town is that in 4 hours time it will be raining.

More importantly, wind direction and speed helps to plot the atmospheric pressure. The Buy Ballot law states that in the Northern Hemisphere, if the wind is at your back the area of low pressure is to your left. When you plot many wind points you get a pattern that illustrates the pressure pattern and when you look at speed you get an idea how far away the location is from the center of the pressure center.

http://cstar.cestm.albany.edu/PostMortems/CSTARPostMortems/2007/Mar_2_2007/2march2007storma.htm

This map shows how plotting the wind can show you the location of the pressure center. The thing to remember is that sometimes measuring stations are more than 100 miles apart, so plotting the wind like this can really help.

## How are typhoons formed?

James J.
Featured 2 months ago

The same way as a hurricane since they are the same thing.

#### Explanation:

Start with an unstable air mass formed over a dry area (like Australia or Africa)

The air mass moves over warm ocean water. Warm as in at least 26.5 C or 80 F.

As the air mass is unstable it will cause convergence in the lower part of the atmosphere., over the warm water. As the air comes together it will rise.

Condensation occurs aloft, forming convective cloud. This occurs to a great extent forming cumulonimbus clouds. The air continues to rise but because the water below is so warm, more moisture gets sucked up.

It is usually said that the winds aloft are light. This is only sort of true. What it should include is no wind sheer aloft. This means that the rising air is allow to continue to rise in the same column, over the warm water. If the winds are too strong or there is a wind sheer (change in direction and strength) the rising air will move away and the developing storm will "fall over" for a lack of a better term.

This occurs between 5 and 20 degrees latitude. At those latitudes the Coriolis effect is light but not non-existent. Light Coriolis allows the convergence to continue, whereas a stronger Coriolis will slow the convergence as it deflects the air due to the rotation of the Earth.

So the air keeps rises and forms more cloud, but because you have a whole ocean of warm water underneath the storm keeps sucking up moisture. This forms a multitude of cumulonimbus clouds that form together into a tropical storm and eventually a typhoon or hurricane.

http://expeditieaarde.blogspot.ca/2013_03_01_archive.html

This image simplifies it but it has the general point.

## What climate type is found in Los Angeles and describe its major characteristics?

Featured 1 month ago

A Mediterranean climate.

#### Explanation:

Los Angeles is classified as a warm Mediterranean climate. A Mediterranean climate is a specific type subtropical climate characterized by a dry summer, with a rainy season in the winter, and moderate changes in temperature between the seasons (you won't need a winter coat).

The summer months in LA are typically hot and very dry (it usually doesn't rain during the summer and temperatures exceed ${80}^{o} F$ [${27}^{o} C$]).

Winter months are mild and snow is incredibly rare however temperatures usually fall below freezing on at least one night per year.

Despite the fact that winter is LA's rainy season, LA averages only 15 inches (381 millimeters) of rainfall annually (to put things in perspective the average annual rainfall for all US cities is 30.2 inches [767 millimeters]).

Below is an image of an abating storm -red sky at night, sailors' delight!

Image is my own work; feel free to reuse in any way except for commercial purposes

LA also experiences a weather phenomenon known as the Santa Ana winds (the locals call them "Santa Anas").

Santa Anas are strong (40 mile per hour [or 64 kilometers per hour] plus), hot, dry winds that blow from east to west (us weather nerds like to call them katabatic winds). These winds are the result of cold air from Canada moving into the high desert regions of the Great Basin (Nevada and Utah) which displace the hot dry air to southwest into the lower-lying region of Southern California. These weather events usually last for about a week or so.

This isn't Los Angeles (it's San Diego a large city about 100 miles south of LA) but the same concept applies, this was taken during a rare wintertime Santa Ana.

Image is my own work; feel free to reuse in any way except for commercial purposes

Santa Anas typically occur, but aren't specifically relegated to, the final months of the summer and are common through the end of autumn (though they can occur at any time of year). These winds result in a dramatic increase in temperature (in the summer temperatures can reach ${110}^{o} F$ [${43}^{o} C$] and in the rare event that one occurs in the winter temps can exceed ${90}^{o} F$ [${33}^{o} C$]). Along with an increase in temperature Santa Ana's cause a drop in humidity (typically below 10%, it literally feels like you're getting sandblasted).

Another picture showing the effect of Santa Anas the change in the sky's color is actually due to the particulate matter being blown around (ironically this picture was taken on the same night as the previous one)

Image is my own work; feel free to reuse in any way except for commercial purposes

The image of the palm tree in the article I wrote here.

Thanks to the dry climate wildfires are a constant threat to Los Angeles and Southern California which can easily get out of hand. Luckily Southern California has firefighting crews who work tirelessly to combat and prevent them from occurring and spreading.

LA has variable terrain which aids in the creation numerous microclimates throughout the Los Angeles region. In the summer months the inland valleys to north (such as Laurel Canyon and Studio City) can be as much as ${30}^{o} F$ (${17}^{o} C$) warmer than the coastal regions (such as Santa Monica) due to the lack of onshore flow (winds that blow from the ocean). The converse is typically true for the winter months where the temperatures in the coastal regions are typically warmer than those in the inland valleys. These valley areas are typically dryer than the coastal regions year-round because the surrounding hills can inhibit fog, which is also common not just in LA but in all of coastal California, from settling in the region as readily as in coastal regions.

The two images below are prime examples of the oh-so-ubiquitous California fog (I wasn't driving, don't use your phone while driving, for cereal don't).

Image is my own work; feel free to reuse in any way except for commercial purposes

Image is my own work; feel free to reuse in any way except for commercial purposes

I know this is a lot of info but I hope it helps!

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